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Microwave processing of ceramics: Modeling, characterization and application

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O rd e r N u m b e r 0311753
M icrowave processing of ceramics: M odeling, characterization
and application
Yu, Xiang Dong, Ph.D.
The Pennsylvania State University, 1992
UMI
300 N. Zeeb Rd.
Ann Arbor, MI 48106
The Pennsylvania State University
The Graduate School
Department of Engineering Science and Mechanics
MICROWAVE PROCESSING OF CERAMICS:
MODELING, CHARACTERIZATION AND APPLICATION
A Thesis in
Engineering Science and Mechanics
by
Xiang Dong Yu
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Doctor of Philosophy
December 1992
Date of Signature
We approve the thesis of Xiang Dong Yu.
_____________
Vasundara V. Varadan
Distinguished Alumni Professor of Engineering
Science and Mechanics and Electrical Engineering
Thesis Advisor
Chair of Committee
Vij^y K. Varadan
Distinguished Alumni Professor of Engineering
Science and Mechanics and Electrical Engineering
A-
Sridhar Komameni
Professor of Clay Mineralogy
Deepak Ghodgaonkar
Assistant Professor of Engineering Science
and Mechanics
Richard P. McNitt
Professor of Engineering Mechanics
Head of the Department of Engineering
Science and Mechanics
10 ( W /*? 2-
ABSTRACT
This thesis presents the results of modeling, characterization and application of
microwave energy to ceramic processing. Microwave processing of materials provides
several advantages that in many cases help improve product quality, uniformity of grain
structure, and yield. The ability of the microwave energy to penetrate and, hence, heat from
within the product, helps reduce processing time, costs, and in some cases reduce the
sintering temperature. There is also some evidence that microwave processing of materials
actually provides improved microstructure and other properties. Microwave processing
makes it possible to quickly remove binders, sinter ceramics without rupture or cracking,
reduce internal stress, lower thermal gradients, and control the state of oxide. However,
microwave heating of material and sintering of ceramics is a complex process that is not
understood completely and there are many remaining problems that still need to be solved
before this new technique can be transferred from the research laboratory to actual
industrial use.
The objectives of this thesis are:
1. To establish the basic equations for describing microwave processing of ceramics,
namely sintering and heating.
2. To use the established equations for obtaining microwave deposition and temperature
distribution considering dielectric properties variation with temperature and heat transfer
boundary condition such as radiation and convection.
3. To use the Finite Difference Time Domain (FDTD) method to simulate a single mode
cavity and related thermal insulation problems.
4. To establish a single mode high power microwave heating system for in situ microwave
processing and characterization.
5. To establish a method of measuring the reflection coefficient using an impedance
analyzer and calibrating the detectors on the impedance analyzer.
6. Use the microwave heating and sintering system as well as measurement technique to
heat, sinter and characterize densified and green ceramic rod specimens.
7. Use the single mode high power microwave heating devise for ceramic processing
application.
The basic equations describing the interactions of ceramics with electromagnetic
fields are derived first. Modeling the green ceramic during microwave sintering as a
deformable dielectric, a continuum mechanics model is used to describe the interaction of
electromagnetic waves with the deformable thermoelastic body. Combined with the thermal
and mass diffusion equations into the consideration, such a description is unique, efficient
and complete in modeling microwave processing macroscopically. A further application of
this theory would enable the prediction of the stress fields generated in the ceramic body if
temperature gradients occur. Using those equations, microwave heating of ceramics is
modeled. The Finite Difference Time Domain (FDTD) method is also used to simulate
heating of ceramics in a single mode cavity and analyze the insulation scheme. To further
understand microwave sintering and heating of ceramics, an experimental study using a
single mode high power microwave heating device was conducted. This device would
make it possible to simultaneously heat and sinter ceramics and characterize the process
with the same source. In this experimental work, a ceramic rod in the microwave cavity is
modeled by an equivalent T network. The reflection caused by the ceramic rod, coupling
aperture and variable short is measured by a modified reflectometer attached to the
transmission line. The dielectric property of the ceramic rod is the function of the measured
reflection coefficient. An inversion technique would allow for retrieving corresponding
dielectric properties of the ceramics during heating or sintering. The ceramic rod samples
are either a densified product obtained commercially or a green coupon obtained through an
extrusion process. The single mode high power microwave cavity was also used in the
ceramic processing.
TABLE OF CONTENTS
Page
LIST OF FIGURES............................................................................................................. ix
LIST OF TABLES...............................................................................................................xii
ACKNOWLEDGEMENTS........................................................................
xiii
Chapter 1 INTRODUCTION..............................................................................................1
1.1
Scope of the Thesis............................................................................................ 1
1.2
Statement of Problem..........................................................................................1
1.3
Ceramic Processing............................................................................................4
1.4
Microwave Processing of Ceramics...................................................................7
1.5
Thesis Organization.....................................................................
13
Chapter 2 BASIC EQUATIONS TO DESCRIBE MICROWAVE HEATING OF
CER A M IC S.................................................................................................... 15
2.1
Introduction...................................................................................................... 15
2.2
Equations for Microwave Material Interaction................................................ 15
2.2.1 Maxwell’s Equations............................................................................... 15
2.2.2 Boundary Condition............................................................................... 18
2.3
Theory of Electromagnetic Field Interactions with Deformable Dielectrics... 19
2.4
Thermal Diffusion Equation............................................................................ 22
2.4.1 Mass Conservation Equation.................................................................. 23
2.4.2 Momentum Conservation Equation........................................................24
2.4.3 Moment of Momentum...........................................................................25
2.4.4 Energy Conservation Equation.............................................................. 27
2.4.5 Boundary Condition...............................................................................30
2.5
Mass Diffusion Equation.................................................................................32
2.6
Conclusion and Discussion.............................................................................. 35
Chapter 3 MODELING MICROWAVE PROCESSING OF CERAMICS..................... 37
3.1
Introduction......................................................................................................37
3.2
Theory.............................................................................................................. 38
3.3
Description of the Model.................................................................................. 40
3.4
Impedance Method...........................................................................................40
3.5
Computing Procedures..................................................................................... 44
3.6
Results and Discussion....................................................................................44
3.6.1 Microwave Energy Absorption by Ceramics......................................... 44
3.6.2 Simulating Microwave Heating of Ceramics......................................... 46
3.7
Conclusion....................................................................................................... 49
Chapter 4 MODELING SINGLE MODE CAVITY WITH FINITE DIFFERENCE
TIME DOMAIN METHOD (FDTD)................................................................51
4.1
Introduction......................................................................................................51
4.2
Finite Difference Time Domain Method...........................................................53
4.2.1 Introduction.............................................................................................53
4.2.2 FDTD Method Formulation....................................................................55
4.2.3 Outer Radiation Boundary Condition (ORBC)..................................... 57
4.3
Structure of Model............................................................................................ 60
4.4
Computation Procedures.................................................................................. 63
4.5
Results and Discussion....................................................................................65
4.6
Conclusion....................................................................................................... 66
Chapter 5 EXPERIMENTAL SYSTEM.......................................................................... 71
5.1
Introduction......................................................................................................71
5.2
Magnetron.........................................................................................................71
5.3
Circulator.......................................................................................................... 73
5.4
Directional Coupler.......................................................................................... 75
5.5
Impedance Analyzer.........................................................................................75
5.6
The 4-Stub Tuner.............................................................................................77
5.7
Iris
5.8
Resonator Cavity.............................................................................................83
................................................................................................................ 80
Chapter 6. IN SITU MICROWAVE HEATING, SINTERING AND
CHARACTERIZATION........................................................................................... 88
6.1
Introduction..................................................................................................... 88
6.2
Historical Background.....................................................................................88
6.3
Mathematical Model for the Characterization.................................................. 90
6.4
Model Description........................................................................................... 95
6.5
Impedances of the Iris and the Variable Short.................................................97
6.6
Measurement of the Reflection Coefficient.....................................................98
6.7
Effect of Iris on the Reflection Coefficient Measurement............................ 103
6.8
Green Ceramic Rod Preparation.................................................................... 105
6.9
Experimental Set-up and Characterization Procedures..................................108
6.10 Results and Discussion.................................................................................. 109
6.11 Conclusions.................................................................................................... 119
Chapter 7 APPLICATION OF MICROWAVE ENERGY
TO CERAMIC PROCESSING......................................................................122
7.1
Introduction.................................................................................................... 122
7.2
Insulation Considerations in Microwave Processing................................... 123
7.3
Temperature Measurement.............................................................................125
7.3.1 Introduction............................................................................................125
7.3.2 Pyrometer Measurement.......................................................................127
7.3.3 Thermocouple Measurement................................................................. 127
viii
7.3.3.1 Introduction............................................................................. 127
7.3.3.2 The Principle of the Thermocouple........................................ 128
7.3.3.3 Theory of Thermoelectricity.....................................................128
7.3.3.4 Thermocouple in the TE103 Cavity..................................................130
7.3.3.5 Summary........................................................................................... 133
7.4
Binder Burn-out of Tape-Casted Ceramics by Microwave Energy............... 133
7.4.1 Introduction........................................................................................... 133
7.4.2 Microwave Processing......................................................................... 134
7.4.3 Ceramic Tape Preparation..................................................................... 135
7.4.4 Characterization Method...................................................................... 135
7.4.5 Results and Discussion......................................................................... 135
7.4.6 Conclusion............................................................................................ 139
7.5
Microwave Sintering of A^Oj/c-ZrC^ Composites..................................... 140
7.5.1 Background........................................................................................... 140
7.5.2 Microwave Sintering............................................................................141
7.5.3 Sample Preparation...................................................................
142
7.5.4 Mechanical Properties.......................................................................... 142
7.5.5 Conclusion............................................................................................ 149
Chapter 8 CONCLUSIONS AND FUTURE W ORK..................................................152
REFERENCES ................................................................................................................ 155
APPENDIX EXPRESSION FOR q and Dj j ................................................................172
ix
LIST OF FIGURES
Figure
Eagg
2.1
Differential Control Volume, dxdydz, for Species Diffusion Analysis................. 33
3.1
Ceramic Slab............................................................................................................ 41
3.2
Microwave Power Absorption for Slabs with Different Loss Factors.................. 45
3.3
Microwave Power Absorption for Slabs with Different DielectricProperties... .45
3.4
Procedures for Simulating Microwave Heating of Ceramics................................ 47
3.5
Dynamic Temperature Profile 0.01 inside the Slab from the Incident Plane
3.6
Temperature Distribution over the Thickness of the Slab....................................... 48
3.7
Total Microwave Energy Absorbed by Slab vs. Temperature................................ 50
4.1
Yee Cell for Finite Difference Time Domain M ethod.............................................54
4.2
Single Mode Cavity for Material Processing...........................................................62
4.3
Structure of Ceramic Rod and Surrounding Microwave Susceptor...................... 62
4.4
Truncated Gauss Pulse............................................................................................64
4.5
Frequency Response of an Empty Cavity...............................................................67
4.6
Convergence Analysis of the FDTD Method......................................................... 67
4.7
Field Distribution in the Cavity with a Ceramic Rod.............................................. 69
4.8
Microwave Energy Absorbed by a Ceramic Rod in the Cavity............................. 69
4.9
Microwave Energy Absorption by Both Ceramic Rod and
48
Microwave Susceptor at Room Temperature.......................................................... 70
4.10
Microwave Energy Absorption by Both Ceramic Rod and
Microwave Susceptor at lOOO'C...........................................................................70
5.1
Single Mode High Power Microwave
Processing and Characterization System................................................................72
5.2
Multicavity Oscillator Magnetron.............................................................................74
5.3
A 3-Port Circulator...................................................................................................76
X
Directional Coupler.................................................................................
..76
Impedance Analyzer................................................................................
..78
A 4-Stub Tuner........................................................................................
..79
Characteristics of One Stub.....................................................................
..79
Three Coupling Iris for Material Processing Cavity..............................
..81
Electric and Magnetic Polarization in the Iris.........................................
..82
Field Distribution in TE103 Cavity........................................................
..84
Equivalent Circuit of a Ceramic Rod in the Transmission Line.............
..92
Equivalent Circuit of a Ceramic Rod in the Cavity.................................
..96
A Vector Network Analyzer.....................................................................
..99
Measurement of the Impedance of the Iris..............................................
100
Measurement of the Impedance of the Variable Short............................
100
Three Detector Reflectometer...................................................................
102
Input Microwave Power vs. Output DC Voltage from Detector.............
104
Thermal History of Microwave Heating and Characterization..............
111
Position of the Variable Short vs. Time.................................................
111
Reflection Coefficient vs. Time..............................................................
112
Measured Dielectric Properties of Coors AD-998..................................
113
Microwave Sintering of Alumina Ceramic Rod History..................... .
115
Density vs. Sintering Tim e.....................................................................
116
Variation of the Ceramic Rod Diameter with Sintering Tim e.................
117
Variation of Dielectric Properties of the Green Ceramics
During Microwave Sintering..................................................................
118
Insulation Scheme for Microwave Processing.......................................
126
Measuring Temperature with Thermocouple in the Cavity.....................
131
Ceramic Tape Manufacturing Flow Chart...............................................
136
Temperature as a Function of Time for Microwave Binder Bum O ut..
137
Temperature as a Function of Time for Conventional Binder Bum Out.
137
Dielectric Properties of Ceramic Tapes at Different Temperatures........
138
xi
7.7
Conventional Sintering Schedule...........................................................................143
7.8
Microwave Sintering Schedule...............................................................................143
7.9
Diametral Compression Test...................................................................................148
7.10
Diametral Compression Strength...........................................................................148
7.11
Micrograph of Conventional Sintered Alumina-4% Zirconia................................150
7.12 Micrograph of Microwave Sintered Alumina-4% Zirconia...................................150
7.13
Micrograph of Conventional Sintered Alumina-10% Zirconia............................. 151
7.14
Micrograph of Microwave Sintered Alumina-10% Zirconia................................151
xii
LIST OF TABLES
Table.
Page
4.1
Resonant Frequencies for Different Cavity Structures........................................... 68
6.1
Characteristics of Non-Propagating Mode in the Waveguide............................... 107
6.2
Real Part of the Dielectric Constant of Teflon and Quartz................................ 120
7.1
Comparison of Results from Microwave and Conventional Sintering................144
7.2
Vickers Hardness of the Sintered Ceramics..........................................................147
ACKNOWLEDGEMENTS
The author would like to take this opportunity to sincerely thank Dr. Vasundara V.
Varadan, thesis advisor, and Dr. Vijay K. Varadan for providing the most important source
of support and strength throughout the course of this work as well as during graduate
study. Special thanks also go to the author’s doctoral committee of Drs. Sridhar Komameni
and Deepak Ghodgaonkar for their valuable instruction and suggestions throughout this
study.
The author also would like to thank the colleagues in the Research Center for the
Engineering of Electronic and Acoustic Materials for their help during his stay at the center.
Finally, the author thanks his parents, BinQi Yu and ShuFang Chen, and his wife,
Jenny, for their patience and encouragement.
1
Chapter 1
INTRODUCTION
1.1 Scope of the Thesis
This thesis attempts to address three areas which need to be researched in detail
before microwave processing can be widely used in the ceramic industries. They are
modeling, characterization and application. In the modeling part, the equations that describe
microwave processing, modeling microwave heating of ceramics and application of finite
time domain method to simulate a single mode microwave cavity are given. In the
characterization part, an in situ microwave heating and characterization system was
established, densified as well as green ceramics are simultaneously either heated or sintered
and characterized. In the application part, processing considerations for insulation design
and temperature measurements with thermocouple in the microwave field are presented.
Also, applications of microwaves processing to binder burn-out and sintering of
AI2 Q 3/Z1O 2 arc studied.
1.2 Statement of Problem
Microwave processing is a typical interdisciplinary area for researching.
Knowledge of both microwave fields and components and ceramic processing techniques
is necessary for success. Although both microwave technology and ceramic processing are
mature fields by themselves, combining them to form a new technology is not an easy
task. Today, almost all the work done in the area of microwave processing of ceramics is at
a small scale at various laboratories. The experimental results have shown that microwave
processing has many advantages over conventional methods. These advantages include
lowered sintering temperature, reduced activation energy, accelerated diffusion rate, fine
2
microstructure, etc. The application of such a technique to a large scale production line
requires more detailed research in the area of modeling, characterization and engineering
design.
Microwave processing of ceramics is a complicated process. It is necessary,
however, to theoretically understand such a physical process to guide industrial design and
application. Another reason is that experimental research work consumes time and could be
very costly in practice. The interactions between microwave fields and ceramics cause an
energy transformation from microwaves to heat Such a thermal energy would rapidly raise
the temperature of the ceramic materials. At high temperatures, various diffusion processes
will occur and ceramics are densified. For the green ceramics, which are homogeneous
mixture of ceramic powders and pores, a complete description of microwave interactions
lends itself to the theory of wave propagation in a random medium, a topic well understood
only by the specialists. To overcome the difficulty in describing this complicated process,
an effective medium approach would be the first choice to be considered. With such a
choice, ceramic materials, which are to be processed in a microwave field, can be seen as a
continuum medium with properties equivalent to that of the mixtures. The densification
process can be treated as the interactions of microwave fields with deformable dielectrics.
Coupled with appropriate thermal transfer and mass diffusion equations, a complete
description of the microwave process can be arrived. For densified ceramics, a regular
approach can be used for modeling which only requires consideration of dielectric property
changes with temperatures. For a complicated structure such as the case involving
insulation, ceramic samples and microwave susceptor, numerical methods have to be
employed to obtain fields distribution and more importantly microwave energy absorbed by
the ceramic sample.
In developing microwave processing of ceramics, problems
such as basic
scientific studies on microwave-materials interactions and loss mechanisms need to be
solved. There is also a critical need for a broad data base on dielectric properties of
materials at high temperature over different frequencies. Recently, dielectric properties of
ceramics has been studied via a so-called free space method. However, the free space
3
method suffers from long characterization times that are needed for the large planar sample
to reach thermal equilibrium. Heating at high temperature for such a period of time may
very well alter the microstructure of the material. More importantly, because the material
being characterized was heated in the conventional furnace rather than microwaves, the
microwave-material interaction mechanisms were not revealed. Contrary to the free space
characterization method which requires heating of large planar specimens, the in situ
microwave heating and characterization method uses microwaves to heat a thin ceramic rod
and the same field is also used to detect the effect of the dielectric property change with
temperature. Such a technique can be used to continuously measure dielectric permittivity
as a function of temperature and offers a unique approach for understanding the interactions
between microwaves and ceramics. It is applicable to both densified as well as green
ceramic specimen.
The use of microwave energy for processing of ceramics is still veiy much in its
infancy. Preliminary investigation has shown that it is applicable to drying, slip casting,
calcination, sintering, joining, plasma assisted sintering and chemical vapor deposition. Its
potential is considerable since a number of very distinctive advantages have been claimed.
Since ceramic processing usually requires high temperature, insulation is necessary to
avoid any temperature gradients which may cause fracture of the ceramic specimen. For
microwave processing, insulating materials have to be microwave transparent as well as
possess good insulation properties. For the ceramic materials which have very low loss at
microwave frequency, initial heating has to be provided for smooth operation. Those
arrangements should not block penetration of microwaves into the ceramic sample.
An important issue in microwave processing of ceramics is the proper measurement
of temperature. Because of microwave interference, temperature measurements in the
electromagnetic field with a thermocouple have been difficult in most cases, which
prevents proper assessment of the effect of microwaves on ceramic processing.
Since microwave processing is a very fast densification process, its mechanical and
microstructural properties have to be evaluated so that a complete processing technique can
be developed to obtain products with the desired quality.
4
1.3 Ceramic Processing
Ceramics are defined as inorganic and nonmetallic materials. One important
characteristic of ceramics is that it is basic to the operation of many industries. For
example, refractories are basic components for the metallurgical industry. Abrasives are
essential to the machine tool and automobile industry. Glass products are essential to the
automobile as well as to the architectural, electronic and electrical industries. Cements are
essential to the architectural and building industry. Various special electrical and magnetic
ceramics are essential to the development of computers and many other electronic devices.
Recently developed structured ceramics and their composites are promising materials for
engine application operated at high temperature. As a matter of fact, almost every industrial
production line, office and home depend on ceramic materials. A major characteristic of
ceramics familiar to everyone is that they are brittle and fracture with little or no
deformation. This behavior is in contrast to metals, which yield and deform. As a result,
ceramics cannot be formed into shapes by the normal deformation process used for metals
or plastics. There are two basic processes used in the ceramic industry for shaping
ceramics. One is to use fine ceramic particles mixed with a liquid or binder or lubricant or
pore spaces, a combination that has Theological properties which permit shaping. Then by
heat treatment the fine particles are agglomerated into a cohesive useful product. The
essentials of this procedure is first to find or prepare fine particles, shape them, and then
stick them back together by heating. The second basic process is to melt the material to
form a liquid and then shape it during cooling and solidification; this is most widely
practiced in forming glass. Since the emphasis of this research is in the polycrystal
ceramics, a little more review is given on the first method.
Ceramics, depending on their chemical composition which determine their
properties, are used in electronic devices, structure materials, chemical processing
components, refractory structures, construction materials, and domestic products.
Generally, ceramic products are obtained through raw powder material processing, forming
5
and sintering. Although recent development in raw material processing and forming has
tremendously increased the engineering control of the raw material property and forming
technology, the final stage of ceramic processing, i.e., sintering of ceramics, is still done
using the old technique, where green ceramic products are placed in the furnace and the
temperature is slowly increased to one-half to two-thirds of the melting point. Such a
process is slow and often needs a long time for a complete cycle which results in waste of
time and cost of energy.
In processing of polycrystal ceramics, mixed-oxide industrial chemical are
commonly produced by calcining a mixture of carbonates, hydroxides, sulfates, nitrates,
acetates, oxalates, alkoxides, and so on. In general, the reaction produces an oxide and a
volatile reaction product (e.g., C 0 2, S i0 2, H 2 0,...). During calcination process, the
reaction may be controlled ( 1 ) by the reaction rate at the reaction surface, (2 ) by gas
diffusion or permeation through the oxide product layer, or (3) by heat transfer. In general,
the calcination reaction is heterogeneous. Hence, the reaction occurs at a sharply defined
reaction interface. Drying is the removal of organic binder or liquid from a porous material
by means of its transport and evaporation into a surrounding area. It is an important
operation prior to firing in processing bulk raw materials. Drying cost is a significant factor
in the selling price of industrial minerals.
In drying ceramic ware, the initial drying rate is independent of the water content
and depends solely on the temperature, humidity, and rate of movement of the air over the
surface of the ware. The rate of drying is equal to the rate from a free water surface. If an
enlarged cross section of the ware is observed, it appears that there is a continuous film. At
a water content such that the particles just come in contact, this water film disappears from
the surface and the rate of drying suddenly decreases. The lower rate of drying is due to the
resistance to the flow of liquid to the surface or may be caused by vaporization in the
interior and diffusion of the vapor out to the surface. Measurements of shrinkage during
drying process indicate that the major part of shrinkage occurs during the constant rate
period. The shrinkage is essentially completed during the constant drying period. The water
films decrease in thickness until at the critical point, at which the rate of drying and also the
6
rate of shrinkage sharply change, the particles have just come in contact. This is the end of
the shrinkage and the beginning of a lower rate of drying.
Sintering is the term used to describe the consolidation of the product during firing
where the temperature in the product exceeds one half to two thirds of the melting
temperature, which is sufficient to cause significant atomic diffusion for solid state
sintering or viscous flow when a liquid phase is present or produced by a chemical
reaction. Consolidation implies that within the product, particles have joined together into
an aggregate that has strength. Sintering is often interpreted to imply that shrinkage and
densification have occurred.
The driving force for sintering is the reduction in the total free energy AGT of the
system
AGt = AGv + AGb +AG s
(1.1)
where AGy, AGB and AGS represent the changes in free energy associated with the
volume, boundaries, and the surface of the grains, respectively. The major driving force in
conventional sintering is AGS, but the other terms may be significant in some stages for
some material systems.
The mechanism for transport during sintering are surface diffusion, evaporation
condensation, boundary diffusion, lattice diffusion, viscous flow and plastic flow. Surface
diffusion is a general transport mechanism that can produce surface smoothing, particle
joining, and pore rounding, but it does not produce volume shrinkage. In materials where
the vapor pressure is relatively high, sublimation and vapor transport to the surface of
lower vapor pressure also produce these effects. Diffusion along the grain boundaries and
diffusion through the lattice of the grains produce both neck growth and volume shrinkage.
The mechanism of bulk viscous flow and plastic deformation may be effective when a
wetting liquid is present and mechanical pressure is applied, respectively.
Microstructure change during sintering could be classified into initial, intermediate
and final stages. In the initial stages, particles become smooth, grain boundaries form and
7
neck grow and interconnected. Open pores will be rounding, active and segregated dopant
start defuse, the porosity decreases about 12%. Shrinkage of open pores intersecting grain
boundaries, significant mean porosity decreases and slow grain growth are the main
phenomena in the intermediate stage. During final stage, closed pores containing kiln gas
when density is less than 92%. Closed pores intersect grain boundaries, others shrink to a
limited size or disappear. Pores larger than grains shrink relatively slowly, grains of much
larger size appear rapidly. Pores within large grains shrink relatively slowly.
1.4 Microwave Processing o f Ceramics
Most of the early work on using microwaves to process materials dates back to the
early 1960’s and has led to some commercial applications in foundry and investment
casting industry such as work done by Schoroeder and Hackett [1971], Valentine [1973,
1977], Stengel [1974]. The application of microwaves to the ceramic processing started in
late 1960’s, when Tinga and Voss [1968] and Tinga [1969] published papers on theories of
using microwaves to heat and sinter ceramics. Berteaud and Badot [1976] were able to use
microwaves to heat refractory materials. Interests grew in the early 1980’s. Schubring
[1983] successfully sintered an alumina spark-plug insulator. Johnson and Brodwin
[1984] constructed a single mode cavity to sinter ceramics and subsequently used it for
characterizing a densified ceramic rod. Roy et al. [1985] discovered that microwaves could
be used to melt gels for ceramic powder processing. The research work on the subject of
microwave processing of ceramics has been booming from the 1980s to the present. To
accommodate these developments and for better communications among researchers, the
Material Research Society and the American Ceramic Society organized four symposia in
1988, 1990, 1991 and 1992 which resulted in four proceedings. Those symposia dealt
extensively with theoretical modeling, dielectric property characterization, equipment
design and laboratory experiments on the microwave processing of ceramics and their
composites in the area of calcining, drying, sintering, etc.
Microwave processing has many advantages over the conventional method. Tian
8
and Johnson [1988] have demonstrated that microwave processing ceramics can produce
ultra fine microstructure. The ceramic material they used is AI2 Q 3 . Jenny and Kimrey
[1988] have sintered AI2 O 3 + 0.1% MgO in a vacuum using 28GHz microwave power
instead of the most common 2.45GHz, the results suggested that more power can be
absorbed by the ceramic compact because of the high value of the loss factor at 28GHz.
They also reported that microwave heating lowered the sintering temperature and lowered
the activation energy and increased the diffusion rate during microwave processing. To
explain the observed phenomena, Meek et al. [1991] suggested that the electric field and
power density are greatly intensified in the neck region between the grains; such a high
power density could induce extreme high temperature in at neck region. This high
temperature at the grain boundary could slightly melt ceramics there, which would change
solid state sintering to liquid phase sintering; therefore, sintering rate is enhanced. Further
work by Katz et al. [1991] indicated that relaxation type loss mechanism may be operable
in the microwave frequencies during microwave heating of ceramics. That loss mechanism
increases the correlation factor for diffusion. Later, Booske et al. [1992] proposed an ionic
motion model to account for non-thermal effects during microwave heating of crystalline
solid. Theoretical prediction of the microwave absorption during microwave sintering was
also done by Varadan et al. [1988] by using rigorous multiple scattering theory.
Similar advantages can be obtained for other ceramics with microwave processing.
Krage [1981] sintered fenite with microwaves and found that the properties obtained from
the microwave energy are comparable to the conventional. Desgardin et al. [1986] did
microwave sintering of BaTiOs
based ceramics. Aliouat et al. [1990] have
used
microwaves to sinter spinel-type oxides such as LiFe3 Og. By comparing mechanical
properties of between microwave and conventionally processed alumina, Patterson et al.
[1991] found that microwave processed alumina has a higher toughness and lower
hardness. Noncrystalline T i0 2 was also sintered with microwaves by Eastman et al.
[1991]. Mcmahon et al. [1990] tried a variety of ceramic materials with the microwave
sintering technique.
The microwave sintering technique was also used to process ceramic composites
9
such as sintering of partially stabilized zirconia by Wilson and Kunz [1988], 50% dense
alumina compact plus 10% vol. SiC whisker by Meek et al. [1987c], A12 0^ plus 10% vol.
SiC whisker in the 2.45 GHz cavity by Katz et al. [1988a] and titanium diboride by Katz et
al. [1989] and Holcombe and Dykes [1991], zirconia-toughned alumina composite by
Kimrey et al. [1990] and Patil et al. [1991], Yttria-2wt.% Zirconia by Holcombe et al.
[1988]. Macdowell [1984] used microwaves to heat glass-ceramic composite and found
that sodium nepheline was useful in converting microwave energy to heat. By using a gas
pressurized cavity at about IMPa, Tian and Johnson [1988] sintered A l2O3-30% wt. TiC
composite and obtained 95% density. Those composites are generally difficult to densify by
conventional sintering methods. By applying microwave energy to internally combustible
material, Ahmad et al. [1991] fabricated A12 Oj /TiC composites from Compacts of T i0 2 +
Al + C.
Microwave energy used for preparing fine ceramic powder has been demonstrated
by the work of Komameni et al. [1988] and Kladnig and Horn [1990]. Singh et al. [1991]
used microwave plasma to synthesize several non-oxide ceramic powders.
Johnson [1991] has demonstrated that microwave generated plasma can be used to
sinter ceramics. He also gave some considerations on the possible mechanisms of enhanced
diffusion.
Microwave heating is also applicable to melting and making fine ceramic powders
from ceramic gel as shown by Roy et al. [1985], Komameni and Roy [1986] and
Surapanani et al. [1991].
Sintering of non-oxide ceramic materials with microwave energy was studied by
Katz et al. [1988b] on Br2 C, Tiegs et al. [1991,1991], Ferber. et al. [1991] and Kiggans
et al. [1991] on SiN, Kumar et al. [1991] on SiC. Holcombe and Dykes [1990,1991] have
developed a casket scheme for ultra high temperature microwave sintering of general non­
oxides.
Effect of particle size on microwave processing of alumina was studied by Arindam
et al. [1990]; it was found that microwave heating culminates in accelerated densification
10
with a better uniformity and homogeneity of microstructural vis-a-vis conventional fast
firing,
Ahmad et al. [1988] used microwaves to calcine, sinter and anneal Y I^C ujG y.*
super conducting pellets. The microwave processed pellets have more refined
microstructure, low porosity, improved oxygen content and higher super conducting
transaction temperature over conventional processed samples. Aliouat et al. [1990] and
Hyoun et al. [1991] also used microwave energy sintered Y I ^ C ^ C ^ .
Microwave processing is also applicable to glass processing. Hassler and Johansen
[1988] have used microwaves to heat fused quartz in the optical fibers fabrication process.
Kao and Mackenzie [1991] used magnetite to act as a microwave susceptor during
microwave sintering of soda-lime glass, the finished product has a function of microwave
shielding and absorbing effects. Pope [1991] constructed a microwave furnace to sinter
sol-gel derived silica glass.
Walkiewicz [1988] used microwaves to selectively heat minerals. Hamlyn and
Bowden [1992] have applied microwaves to process earthware ceramics. Wright et al.
[1989] used microwaves to process ilmenite and titania-doped hematite. In Standish and
Womer [1990] study, microwave energy was also used in the reduction of metal oxides
with carbon.
In fact, microwave heating technique is applicable to all phases of ceramic
processing, i.e., drying, calcining, binder burnout and sintering, as demonstrated by
Harrison et al. [1988]. Similar work was also done by Selmi et al. [1992] on Barium
Strontium Titanate. In Selmi’s work, it was observed that microwave calcination enhanced
the solid state reaction and achieved required calcination much faster than the conventional
method because of the high microwave power absorption ability of the carbonate.
Application of microwaves to ceramic processing in a large scale was studied by
Katz and Blake [1991].
A variety of material systems have been investigated, together with a number of
different approaches. The microwave applicator used during microwave processing could
be both multimode such as work done by Harrison et al. [1988], Ahmad et al. [1988],
Katz et al. [1988a], Krage [1981] and single mode such as work done by Tian and
Johnson. [1988]. Patil et al. [1991] used circular waveguide cavity to sintering alumina
rod, the resonant mode in the circular cavity can be changed to accommodate samples of the
different shape. To process ceramics composites, Tian et al. [1988] used a closed cavity
with gas pressure to aid microwave sintering. Jow et al.[1987], Kimrey and Janney [1988]
have discussed the design principle for high power microwave cavities.
One parameter which is important in designing and applying microwave energy to
ceramic processing is the dielectric property of ceramic materials. Early work was done by
\b n Hippie [1954] and his co-workers at the MIT Laboratory for insulation research.
Subsequent work was continued at the same institution by Westphal [1975,1977,1980].
Their monumental work has established the database for microwave dielectric properties.
To satisfy recent interests in the area of microwave processing of ceramics, dielectric
properties of the materials at broader band of frequencies and higher temperatures have to
be obtained to provide bases for designing and applying microwave heating for industrial
applications. Recently, Fuller et al. [1984], Ho [1988] and Varadan et al. [1991] have
established methods to characterize ceramic dielectric properties at high temperature using
either free space or cavity method.
While most of the research work in microwave processing of ceramics is done in
experimental form, only few of them are on the modeling of this process. It is necessary,
however, to theoretically understand such a physical process to guide industry design and
practical application. Another reason is that experimental research work consumes time and
could be very costly in practice. A relatively new approach which is being taken by some
researchers is the construction of models which attempts to address the use of microwaves
in the processing of materials from the theoretical point of view. With regard to microwave
processing of ceramics, these models currently fall into two main approaches; an atomistic
approach where the interactions between microwaves and materials are considered in terms
of the effect on parameters such as atomic diffusion as done by Kenkre et al. [1990] and
Kenkre [1991], Bykov et al. [1991], Meek [1987a], Meek et al. [1988,1991], Katz et al.
[1991] and Gupta and Evens [1991] and a ‘micro’ approach where the material is
12
considered a homogeneous continuum and power deposited is set against heat losses as
done by Iskander[1990], Smyth [1990], Iskander et al. [1991], Chatterjee and Misra
[1991], Eugene and Snider [1991], Kriegsmann [1991], Ultimately, these two approaches
will be combined.
The interactions between microwaves and materials are strongly materials-property
dependent and most of the ceramics are low loss materials at room temperature. Three
approaches have involved the use of coupling aids, microwave susceptor and high
microwave frequencies. The concept of trying to rind a second phase which will aid
coupling of the green body with the incident microwaves appears to be fundamentally
sound. The additives must aid coupling and the densirication process but not be detrimental
to the properties of the sintered body, for example, by leaving residual glassy grain
boundary phase. Roy et al. [1985], Meek et al. [ 1987b], Komameni and Roy [1986]
have suggested and used some additives for microwave processing.
The use of a microwave susceptor to reduce the severity of the inverse temperature
gradient with microwave heating alone attract more and more interests in the designing of
microwave processing. Research work by Humphrey [1980], Krage [1981], Wilson and
Kunz [1988], Harrison et al. [1988] and Janney et al. [1992] have shown microwave
susceptor provides a degree of conventional radiant heating so that a uniform heating can be
achieved and fast heating rate can be realized. An inherent disadvantage is that microwave
susceptor reduces the efficiency of the process by absorbing some of the incident energy.
To obtain consistent microwave sintering results, Aliouat et al. [1990] have
published results on using a control algorithm for microwave sintering in resonant system.
The use of high frequency microwaves ( typically in the 20-40GHz range) has
resulted in the ability to dissipate high level power within ceramic materials. Results
presented by Jenny and Kimrey [1988] showed that the material has a higher microwave
absorbing ability for it has a high loss tangent and more uniform heating rate due to shorter
wavelength. However, the depth of penetration is reduced at high frequencies. This could
lead to reduced uniformity of heating at high temperature and for large sample.
Ultimately, the deciding factor for microwave processing will depend upon
13
economics. Studies have been performed in this area by Jolly [1972], Das and Curlee
[1987], Schmidt [1986], Patterson [1975], Sanio and Schmidt [1988] and Some
conflicting results have been obtained. One of the key reasons for the uncertainty is that a
number of initial assumptions have to be made at present because of absence of suitable
data. In Das’s study, it is concluded that microwave sintering of ceramics will not conserve
energy, when the conversion of fuels to electricity and the conversion of electricity to
microwave energy are considered. According Paterson’s[1975] study, microwave sintering
results in an energy saving of as much as 90% over conventional electric furnace
techniques.
1.5 Thesis Organization
This thesis has eight chapters.
In the first chapter, the scope of the thesis and the problems addressed in this thesis
are mentioned. The historical backgrounds regarding conventional and microwave
processing of ceramics are also reviewed.
Chapter 2 presents the equations describing microwave processing of ceramics.
Ceramics undergoing microwave processing are treated as deformable dielectrics. Heat
transfer and mass diffusion equations are also included in the description.
To understand microwave processing of ceramics, chapter 3 is devoted to simulate
microwave irradiation of a ceramic slab. To account for the temperature dependence of the
dielectric properties of the material, the impedance method is used to determine the
electromagnetic energy absorbed by the ceramics; direct time integration is used to treat the
nonlinearity of the problem.
In chapter 4, the method of finite difference time domain method is used to simulate
the structure of a single mode cavity. In the simulation, a complicated structure of insulation
material, microwave susceptor and ceramic sample are considered.
Chapter 5 details the high power microwave heating system which is used for
characterization as well as processing of ceramics. The system consists of a magnetron,
14
circulator, directional coupler, impedance analyzer, 4-stub tuner, iris, variable short and
microwave resonator cavity.
Chapter 6 describes the method of in situ microwave heating and characterization of
ceramics. The ceramic rod in the microwave resonant cavity is represented by an equivalent
T-circuit. The variation of the dielectric properties of the ceramic rod is sensed by
measuring the reflection coefficient through a reflectometer modified from an impedance
analyzer. Both densified and green ceramic rods are used for characterization.
Chapter 7 shows the application of microwaves to ceramic processing. Qualitative
analysis of insulation scheme, temperature measurement with pyrometer and thermocouple
are given. The advantages of the microwave processing method over the conventional one
are discussed. In this chapter, microwave-assisted binder burn-out, microwave sintering
of zirconia toughened alumina and resulted structural and microstructural properties are
presented.
Chapter 8 gives the conclusion and the work that needs to be done in the future.
15
Chapter 2
BASIC EQUATIONS TO DESCRIBE MICROWAVE
HEATING OF CERAMICS
2.1 Introduction
As stated in the previous section, sintering is a complicated process where green
ceramics densify through various diffusion paths at high temperature. Micro or nano scale
description of the sintering is still an active research subject in the material research
community. For the problem to be solved here where the knowledge of temperature and
microwave power absorption are important, the densification process can be described as
the interactions between electromagnetic fields and deformable dielectric body. In this
section, the basic equations which could properly describe microwave processing of
ceramics are derived. By including mass diffusion equation into the consideration, a
complete description of microwave processing of ceramics can be done.
2.2 Equations for Microwave Material Interaction
y
2.2.1 M axwell’s Equations
It is known that ceramics are dielectric materials independent of whether are in the
green or densified form. The equations describing interactions between microwaves with
ceramics are commonly known
as Maxwell’s equations. It
was the genius of
J.C.Maxwell in 1865, who corrected the inconsistencies of Ampere’s Law and thus
enabled to study the propagation of electromagnetic waves. Although there is no difference
in using integral or differential form of Maxwell’s equations in general, the integral form is
preferred here to account for the densification process of the green ceramics during
microwave processing of ceramics. The integral form of Maxwell’s equations are
q dV ( Coulomb's Law)
h
-
l
dS = 0 (Absence of Free Magnetic Poles)
1
B
j E dl = -
| H dl = j J
(2.1)
B dS ( Faraday's Law)
dS +
D dS ( Ampere's Law)
(2.2)
(2.3)
(2.4)
Using the international system of units, E and H are electric and magnetic field intensity
and have the units of Volt/meter (V/m) and Ampere/meter (A/m). D and B are electric field
flux and magnetic field flux and have the units of Coulomb/met2 (C/m2) and Weber/meter2
(Wb/m2). J and p are electric current density and electric charge density. The units for J
and p are Ampere/meter3 (A/m3) and Coulombs/meter3 (C/m3), respectively.
Considering that the densification process occurs during microwave sintering of
ceramics, the derivative with respect to the time in the above listed equations should be
taken as a material derivative. Therefore, the differential form of Maxwell’s equations to
describe the field variation during microwave processing of ceramics is as follows
17
V x H =J + ^
at
+ (V-D)v
(2 .8)
with local velocity v coupled in the Maxwell’s equation, it is necessary to consider the
thermal diffusion and mass diffusion equations in order to solve this problem. In practice,
an iterative method has to be used to solve those complicated and coupled equations. At
each iterative step, it is assumed that microwave would not see the movement of the body
but would see the different boundary with different electromagnetic properties in different
iterative steps. In so doing, the regular differential form of Maxwell’s equations are
recovered. For a complete description of the behavior of the medium under influence of the
fields, the constitutive relations which were obtained from experiments need to be used
D = £r£oE
B = p rp 0 H
(2.9)
.1 = a E
where £q and |i 0 are the permittivity and permeability of free space. They are 109/36ti
(F/m) and 4rcxl0 ' 7 (H/m), respectively. ^ and |ir are the relative dielectric constant and
relative permeability, a is the conductivity of the material and has a unit of mho/meter (Cl
/m).
After some mathematical manipulations, the vector Helmholtz equations used to
describe electromagnetic wave propagation in the medium can be obtained as follows
V2 E + k 2 E = — - iw J
(2.10)
V2 H + k2 H = - Vx J
(2.11)
£
For the case considered here, there would be no source charge or the electric current in the
region of considerations.
18
2.2.2 B oundary C ondition
At the surface which separates two regions with different physical properties, the
electric and magnetic fields will satisfy the boundary conditions which can be derived from
Maxwell’s equations.
(1) At the boundary where both 1 and 2 are dielectric media, the boundary conditions are
n * ( D i -D 2 ) = qs
n ■( B i -B 2 ) = 0
(2 . 12)
n x ( Ei -E 2 ) = 0
nx(Hi-H2 )=Js
where qs and J s are the surface charge and surface current
(2) At the boundary where 1 is the perfect conductor and 2 is the dielectric medium, the
boundary conditions are
nD
2
=qs
n • ( B i -B 2 ) = 0
(2.13)
n x ( E i -E2 ) = 0
n
X
H2 = J s
In using microwaves to process ceramics, the amount of microwave energy absorbed by
the materials is important to know. The time instantaneous Poynting vector is the flux of the
electromagnetic power through the medium, which is defined as
P = E x H*
(2.14)
The time average Poynting vector is thus found to be
p(t)d(cot)
the net time average power flux enter a close surface S is then
(2.15)
By using Maxwell’s equations
| (E x H*) • dS = j ( o j [ nolir H H * . e 0 e' E • E*]dV
(2.17)
+ | |e oeoe'
e EE E ¥* dV + | | J • E dV
In the above equation, the imaginary part is the energy stored in the electric and magnetic
field, while the real part is the energy which will be transformed to the heat. So the power
absorbed by the dielectric medium per unit volume is
Pabsorb = ^ ( Coe" (o E- E* + J • E )
(2.18)
= i-EoCefftl) ( E E * )
2.3 Theory of Electrom agnetic Field Interactions
w ith D eform able Dielectrics
To model microwave processing of ceramics, theory of electromagnetic fields
interacting with a deformable dielectric need to be reviewed for better understanding. In this
section, a basic theory of electromagnetic fields interacting with deformable dielectrics
especially with ceramics is presented.
According to the response to the electromagnetic field, ceramics can be classified as
ferroelectric, diamagnetic, ferromagnetic and ferromagnetic materials, etc. In modeling
microwave processing of ceramics, all the interacting mechanisms of microwave with
20
ceramics have to be studied. A complete description of those mechanisms is obviously
beyond the scope of this thesis. For simplicity, the structure of material which will interact
with electromagnetic fields can be envisioned as a body containing the following classes of
charges and current distributions.
1. A free charge q per unit volume and a free current J, which consists of either charges
and or currents from an exterior source or conduction current and space charges which
belong to the material.
2. Bound point charges 8 qa located at xa and moving with velocities v a . The bound
charges are distributed in such a way that over any finite material volume the average
charge vanishes:
(2.19)
a
3. Microscopic current loops 8 jp, which cause the magnetization of the body. Over the
closed Loops lp it follows that
(2 . 20)
when the material with above mentioned electric structure is exposed to an electric field E
and a magnetic field B, it will experience a force f(em), a torque l(em), and a rate of energy
change w(em) per unit volume given by
f(em)dV = ( qE + JxB)dV + £ {8qaE(x«) + 6qavax B(xa)}
a
(2 .21)
l(em)dV =[qx xE + xx ( Jx B )] dV + 2
a
(&qaxx E(xa) + 5qaXaX[vaxB(xa)]|
21
+2
I Xp X [8 jpX B( xp) ]dl
(2.22)
ph
W(em)dv = J-Edv + X
8
qa v„-E( xa) + X I % E ( x p ) d l
P h
(2.23)
The summations inthe abovethree equations are over the bound charges and microscopic
current loops that arecontained in the material volume dV. If x is used to indicate the
position of the centroid of dV and
and 1;^ are defined such that
Xoc= x + S<a)
(2.24)
xp= x + S(p)
(2.25)
then by employing a formal power series expansion for the values of the fields at x
E(*«).E(*)+E,i(x)4)
(2-26)
B(xp) = B(x) +Bfi(x) Sip,
(2.27)
and by defining the polarization P and the magnetization U as
Pdv = X
8
qa^(a)
(2.28)
a
Udv = £ ( 8 j i) X ^ )dl
P
(2.29)
the force f(em), the moment l(em), and the rate of energy production w(em) as given by eq.
2.21,2.22 and 2.23,can be written in the first approximation (by neglecting second-order
terms in £(a), etc.) as
f<em) i = qE i + e ijkJjBk + E i,kPk + ®ijk (Pj + PjVl.l) Bk
+ ejjkVjBkiiPi + UkBki
l(em) = XX f(em) + P x £ + U XB
(2.31)
22
W(em) = V- f(em) + p £ • fr - U B + J E
(2.32)
C = E + vxB
(2.33)
where C, is defined as
■fr is the polarization density per unit mass
pfr = P
(2.34)
and the dot above a quantity indicates material differentiation
f ■ $ >
*Pj-v
(2.35)
in the derivation of above equation, the following relation has been employed :
( P + P d iv v ) = X ^(a)5q«
(2.36)
a
which follows from the fact that the summation is over all particles. If the above forms for
f(em), l(em)
, and w(em) are accepted, the balance laws of a continuum interacting with
electromagnetic fields can be obtained as demonstrated in the following sections.
In the case where free space charges, microscopic current is neglected and the
dielectric is not deformable, the work done by the electromagnetic field on the dielectric is
the same as obtained before which is the last term of eq. 2.32.
2.4 Thermal Diffusion Equation
To predict the behavior of ceramics during microwave processing, the temperature
distribution has to be known. Basic equations such as mass, momentum, energy
conservation and related boundary conditions are found to be useful. Combined with the
Maxwell’s equations as well as the mass diffusion equation, the microwave sintering
process can be completely described.
The goal of this section is to use a general set of basic principles and establish
23
thermal diffusion equations to model thermal behavior of ceramics in the microwave fields.
2.4.1 Mass Conservation Equation
A green ceramic sample, which is formed by a static press, is a homogeneous
mixture of ceramic particles and pores. If the pores are considered massless, the total
mass of the ceramic green sample and the densified product are the same, then the mass
conservation equation of the classical mechanics can be invoked. Consider an arbitrary
volume V fixed in space, bounded by surface S, if continuous medium of density
p(t,x,y,z) fills the volume at time t, the total mass in V is
(2.37)
In the sintering process, the density depends on the position and time. The rate of mass
increase in the volume is
(2.38)
If the mass of air pores and the evaporation of the ceramics at high temperature are
neglected, the total mass will not be created or destroyed inside volume V, this must also
be equal to the rate of inflow of mass through the surface. Since the integral vanishes for
arbitrary choice of the volume V, it follows that the integrand must vanish at each point of a
region in which no mass is created or destroyed. The resulting equation, a consequence of
the conservation of mass, is known as the continuity equation.
^ +V(pv)=0
(2.39)
24
2.4.2 Momentum Conservation Equation
The momentum conservation equations are actually the realization of Newton’s
second law of motion. For a differential control volume in the green ceramic body, this
states that the sum of all forces acting on the control volume must equal to the net rate at
which momentum leaves the control volume.
At an instant of time t, the linear momentum of all particle contained in a domain V
is
111 = jv P V'
dV
(2.40)
if the body is subjected to surface tractions T;, surface pressure p and body force per unit
volume fj, the resultant force is
F i= ((T i-p )d V + ' ( fi + f(em)i)dv
I
(2.41)
£ n i = Fi
(2.42)
Newton's law states that
Hence, according to eq 2.40 and 2.41, it is found that
pui dV = |
cn- P) dS + I
( f(em) i +fi) dV
(2.43)
where p is the mass density, pv; the total momentum density, T; the total stress vector
acting on the body, fj the applied body force density per unit mass due to exterior sources
( assumed independent of the electromagnetic fields).
According to Cauchy's formula, the surface traction may be expressed in terms of
the stress field <rij, so that T^OjjVj, where Vj is the unit vector along the outer normal to the
boundary surface S of the domain V. a,j is the stress field. On substituting Gj jVj for T, and
25
transforming the surface integral into a volume integral by gauss’ theorem, eq. 2.43
becomes
I.
T r ^ (pv‘vJ)]dv = / J § L- £ +f<“ H
dv
(2 -44>
since above equation must hold for an arbitrary domain V, the integral on the two side
must be equal. Thus
dpvj
d
i r 4| : ,pViVj)=§ i - | i +f''”>i+f
(2.45)
The left-hand side of eq. 2.45 is equal to
T?l
3vj
dvj
+P at +VjaXj
(2.46)
The quantity in the first parentheses vanishes according to the equation of continuity, while
the second is the acceleration dv/dt. Hence the celebrated Eulerian equation of motion of a
continuum is obtained.
P d t ” dxj
3xj
(em)l + t
(2.47)
2.4.3 Moment of Momentum
Newton’s law also states that the rate of change of moment of momentum is equal
to the total applied torque about the origin. The moment of momentum of a body
occupying region V of space with boundary S at an instant of time is
Hi = j ( a + eijkXjpDk) dV
(2.48)
where Qj is the spin density. If the body is subjected to a surface traction Tj, surface
pressure p,
body force per unit volume fj surface torque A,; and applied body couple lj
of non-electromagnetic origin, the resultant moment about the origin is
26
= j^ (®ijkxj[fk
(®ijkxj[fk + ^(em)
id
—
f(em) id
l(em),i +
+ ^|
) ddV
V +
ejjk[XjTk
e]jk
- XjVfc ] + A.[ dS
(2.49)
Euler’s law states that, for any region V,
^ « i = Li
(2.50)
so the integral form of Euler’s law is
& j ( oi + eijkxjpuk) dV
=
J
^
(2.51)
(®ijkxj[fk "**■
+ l(em)i ++ lil i ) dV
+ I 6^ijklxj
jjk[XjTk - XjVk ] + A<i dS
■f(em) k] +
dV +
For the problem considered here, it is assumed that there is no spin density, the
surface torque and the applied body torque are identically zero:
Ci = 0
M O
h=0
(2.52)
Introducing Cauchy's formula into the the first term at the right side of equation 2.51 and
transforming the result into a volume integral by Gauss’s theorem. It can be shown that
^ I eijk xj pvk dV =
(2.53)
'v
{ eijk (xj^lk),l" ®ijk(xjp),k + eijk xj ( f(em) k +fk) + l(em)i }dV
I
Evaluating the material derivative and using eq. 2.53, following equation can be found
eijk xj|(p v k ) + ^ - ( e ijk xj pvkvi)
(2.54)
1
= eijk (xjCTlk),l“ eijk(xjp),k +eijk xj ( f(em) k +fk) + l(em)i
The second term in the above equation can be written as
eijk pVjVk +eijk xj
pvkvi) = 0 + eijk xj
pvkvi)
(2.55)
27
hence, eq. 2.56 becomes
The sum in the square bracket vanishes by the equation of motion hence, eq. 2.56 reduces
to
(2.57)
The effect of the electromagnetic fields causes the asymmetry of the stress tensor.
2.4.4 Energy Conservation Equation
The law of conservation of energy is the first law of thermodynamics. Its
expression for a continuum can be derived as soon as all forms of energy and work are
listed. There are three forms of energy: the kinetic energy K, the internal energy E and the
gravitational energy G.
The kinetic energy contained in a regular domain V at time t is
(2.58)
The internal energy is written in the form
(2.59)
The gravitational energy depends on the distribution of mass and may be written as
(2.60)
Where E is the internal energy per unit mass. The first law of the thermodynamics states
that the energy of a system can be changed by absorption of heat Q and by work done on
the system. Expressing this in term of rates, it is found that
28
^ (K + E + G) = 0 + W
(2.61)
The heat input into the body must be imparted through the boundar. Let dS be a surface
element in the body with unit outer normal Vj. The heat transferred to the body is assumed
to be representable as hjVjdS. If the medium is moving, it is assumed that the surface
element dS be composed of the same particles. The rate of heat input is, therefore
(2.62)
!= - | hiV*dS
The work done on the dielectrics by the body force per unit volume fj and f(em)i in V,
surface traction T and surface pressure p on S
and the energy inputted by the
electromagnetic field w(em) and exterior source q can be expressed as
W = | { ( fi +f(em)i ) Vj + W(em) + q }dV + J ( T; - pV; ) Vi dS
(2.63)
Using eq. 2.61, it is obtained that
A
dt
^pVjVi + p£ ] dV
N
(2.64)
=Jf(fjVi + f(em) jvj + w(em) + ( OijvOj - f~ +<i)dV
Using the formula to compute the material derivative, it is easy to obtain the following
result after some calculations
2
dt
2
dt
+ ^rP
2
pd3l +
K dt
v+
dt
dt
+^
a ~ + p £ div v +
dt
^2 5 5
)
+d)p div v
K
= fiVi + f(em) iVi + W(em) + ( OijvOj -
+q
The above equation can be simplified greatly if the equations of continuity and motion are
used. Here, f ; is the total body force per unit mass. The difference between fj and Fj is the
gravitational force, by definition
29
(2.66)
since
dt
at
(2.67)
ldxi
and for a gravitational field that is independent of time, where
(2.68)
at
Combining eq. 2.39 and eq. 2.47, eq. 2.65 becomes
+ p t*
= FiVi+f(™>i V i + w < e m ) + ( ^
' ^
?
+<i
(2.69)
But
Ipvi^XL = lp S lv i
T 1 dt
¥ dt
(2.70)
and if heat transfer obeys Fourier’s law,
.
. aT
hi=-k^ r
(2.71)
where k is the thermal conductivity and T is the absolute temperature. Let
E = cT
(2.72)
where c is the specific heat, then eq. 2.69 becomes
p ( l F +Vi div (cT ■*
=^
(k^ )
+ FiVi + f(em) iVi + W(em) + (aijV i)|j ” I k f * + **
(2.73)
30
2.4.5 B oundary Condition
The heat conduction equation is the second-order in space and requires known
conditions on either T or its normal derivative at every boundary point. At the boundary,
heat will transfer between ceramics body and environment due to temperature difference.
According to the physical mechanisms that underlie the heat transfer modes, different
boundary conditions have to be constructed to account for the heat transfer at the boundary.
1. Known temperature: In the conventional heating of ceramics, the furnace is set to a
certain temperature. The temperature at the boundary is therefore a constant. Hence
(2.74)
Tb = T0
2. Insulated Walls: If ceramics are completely insulated by the material which is microwave
transparent, the heat transfer at the boundary in the normal direction is zero. Therefore
(2.75)
3. Conduction: Conduction may be viewed as the transfer of energy from the more
energetic particles to the less energetic ones of a substance due to interactions between
particles. If the heat flux at the boundary is known, the heat transferred is
■k — lb • n = q0
(2.76)
dn
4. Convection: Convection is defined as the conveying of heat through a liquid or gas by
motion of its parts. The convection heat transfer is comprised of two mechanisms. In
addition to energy transfer due to random molecular motion, there is also energy being
transferred by bulk, or macroscopic motion of the fluid. This fluid motion is associated
with the fact that, at any instant, large number of molecules are moving collectively or as
aggregates. Such a motion, in the presence of the temperature gradient, will give rise to
heat transfer. Because the molecules in the aggregate retain their random motion, the total
heat transfer is then due to a superposition of energy transport by the random motion of the
molecules and by the bulk motion of the fluid.
Regardless of the particular nature of the convection heat transfer mode, the
appropriate rate is of the form
9T
- k — lb - n = h«, ( Tb - T „ )
dn
(2.77)
where, the convective heat flux is proportional to the difference between the surface and
fluid temperature, Tb and T, respectively
5.Radiation
Thermal radiation is the energy emitted by matter that is at a finite temperature.
Radiation is an electromagnetic phenomenon and which travels easily through a vacuum at
the speed of light. The energy of the radiation field is transported by electromagnetic
waves. While the transfer of energy by conduction or convection requires the presence of a
material medium, radiation does not.
According to the Stefan-Boltzmann law, the maximum flux at which radiation may
be emitted from a surface is given by
dT
- k — lb n = o R Ti>
dn
(2.78)
where Tb is the absolute temperature of the surface and o R is the Stefan-Boltzman constant.
Such a surface is called an ideal radiator or blackbody. The heat flux emitted by a real
surface is less than that of ideal radiator and is given by
dT
- k — lb n= o r F r T{
dn
(2.79)
where FR is a radiative property of the surface and is called emissivity. This property
indicates how efficient the surface emits compared with an ideal radiator. In a lot of
situations as well as in the microwave heating of ceramics, the ceramic body which has a
small surface is completely surrounded by a much larger surface. The net rate of radiation
heat exchange between the surface and its surrounding can be expressed as
32
Mewing all the boundary conditions, it is obvious that convection and radiation heat
transfer are the two main heat loss mechanisms in the microwave processing of ceramics.
Since the temperature for processing of ceramics is often very high, radiation radiation heat
loss becomes a dominant heat transfer mechanism
because of the fourth-power
relationship. Since there is almost no fluid flow inside microwave furnace or single mode
cavity, radiation is still a significant heat transfer mechanism even it is close to room
temperature. For the reason mentioned above, the radiation boundary condition is applied
in modeling microwave processing of ceramics.
2.5 Mass Diffusion Equation
In microwave sintering of ceramics, diffusion will occur in the homogeneous
mixture of ceramic powders and pores. The pores will be diffused out during solid state
reaction. The diffusivity of the mixture depends on the temperature, pressure, external
electromagnetic field and material to be sintered. Such a phenomenon can be veiy well
described by the mass diffusion equation. The mass diffusion equation is analogous to the
heat equation. Consider a homogeneous medium that is a binary mixture of ceramics
powder and pores and stationary. That is, the mass average velocity is everywhere zero
and mass occurs only by diffusion. The resulting equation can be solved for the species
concentration. Applying this equation to microwave sintering process, the entire
densification process can be described.
Allowing for concentration gradients in each of the x,y and z coordinate directions,
it is wise to define a differential control volume dxdydz, as shown in figure 2 . 1 , within the
medium and consider the process that influences the distribution of ceramics. With
concentration gradients, diffusion must result in the transport of ceramics through the
control surfaces. The conservation equation can be written as
Mc,in +>*C,g-Mc.out = d! £1='toc,st
(2.80)
According to the Fick’s law of diffusion, the species transport rates at opposite
33
n"
A,Z+dz
A,y+dy
_lt
,, x+ dx
A, x
II
n A, z
Figure 2.1 Differential Control Volume, dxdydz, for Species Diffusion Analysis
34
surface is related by
d nr Ydydz
nC,x+dxdydz=nC,xdydz +-1 — T
dx
(2.8X.a)
d nr vdxdz
nC y+dydxdz= nC)ydxdz +-±— ^ ------ -
(2 .8 1 . b)
n
ir
d n r .dxdy
nc,z+dzdxdy=nc,zdxdy +- L—r — <2-8 1-c)
dz
In addition, there may be volumetric chemical reactions occurring through the medium. The
rate at which the ceramic is generated within the control volume due to such reactions may
be expressed as
Mc=ricdxdydz
(2.82)
Where M "c is the rate of increase of the mass of ceramics per unit volume of the mixture.
Finally, these processes may change the mass of ceramics stored within the control volume,
and the rate of change is
Me st = —^-^dxdydz
dt
(2.83)
The net mass that flows in must equal to the rate of storage of ceramics plus their
generation within the volume. Using eq. 2.80, it can be seen that
. ! * £ . ^ a + lic = i?PC
dx
dy
dz
dt
(2.84)
using the definition
nC = PC V c
The conservation equation becomes
(2.85)
35
dt
(2.86)
+ “ (PCUc) +~{pCvc ) + ~(pCW c) = nc
dx
dy
dz
The species velocity can be expressed by the bulk velocity, so that
L2
- .2
- .2
dy2
dz2
,
8 p c +i P c +l £ c '
dt
dx
dz
dy
dx2
(2.87)
For a stationary medium, the mass average velocity V is zero. Hence the final mass
diffusion equation is as follows when mass generation is neglected
dpC
= Dcp
d2pC , d2pc | ^ P C
dx2
dt
dy2
'
(2.88)
dz2
where D™,
up is the diffusion coefficient.
Boundary Condition
1. Known concentration condition
2
Me, st = -^ d x d y d z
dt
(2.89)
-C D cp^ L |x=o = Jc s
dx
(2.90)
. Constant Species Flux
when Jc,s= 0 , this condition is called impermeable surface condition.
3. Initial Condition
pc( 0 ,x,y,z)=po
(2.91)
2.6 Conclusion and Discussion
In this chapter, a set of equations which could be used to completely describe
microwave processing of ceramics are derived. They include Maxwell’s equations, thermal
and mass diffusion equations. It is believed that such a description is complete. However,
it is difficult to solve them without making many assumptions, for there are many
parameters that need to be experimentally obtained. Theses parameters are diffusion
coefficient and elastic coefficients. Equations 2.5, 2.6, 2.7, 2.8, 2.73 and 2.88 are the
core of the descriptions. If the diffusion coefficient and elastic constants which relate the
deformation and stress are known, those equations can be solved with the finite element
method as shown by Lewis and Schrefler [1987].
For a basic understanding of the microwave processing of ceramics, a simplified
problem is considered in the next section. There, a ceramic slab radiated by a TE plane
wave is modeled to simulate microwave heating of ceramics.
37
C hapter 3
M ODELING MICROWAVE PROCESSING O F CERAM ICS
3.1 Introduction
The use of microwave energy is a new and exciting approach in ceramic
processing. Since microwave heating is a volumetric process, it could provide uniform
heating so that the temperature gradient which is observed in conventional rapid heating
method can be avoided. Rapid and uniform heating are important in sintering of ceramics.
On the contrary, non-uniform heating is often observed during this thesis research with
microwave sintering. Therefore, it is of practical interest to simulate the phenomenon of
microwave heating for better control and more efficient use. In spite of the significance of
the problem, there is no comprehensive analysis available which would describe the
behavior of ceramic materials exposed to electromagnetic radiation. Research by Iskander
[1988] and Watters et al. [1988] has revealed some of the mechanisms of microwave
heating of ceramics. However, the simulation of microwave heating of ceramics with a
temperature dependent dielectric property is still lacking. In this chapter, a method of
simulating microwave heating of ceramics with temperature dependent dielectric properties
is developed. The impedance method is used to find the microwave energy absorbed by
ceramics. A non-linear finite element method is developed to determine the dynamic
temperature profile in the ceramics during microwave heating. Using this method, the
thermal runaway phenomenon in microwave heating of ceramics is successfully simulated.
With detailed analysis of the microwave energy absorption pattern in the ceramics, the
effects of dielectric properties on microwave energy absorption by ceramics are
discussed. The causes of non-uniform heating by using microwave energy alone are also
investigated. In doing so, a better understanding of microwave heating of ceramics is
realized.
38
3.2 Theory
In order to simulate microwave heating of ceramics, it is necessary to find the
electric and magnetic fields strength inside ceramics and absorbed microwave energy.
Electric and magnetic Helds are linked by Maxwell's equations, a group of linear
differential equations. Assuming an e 'i£0t harmonic time dependence, Maxwell's equations
can be expressed as follows,
V (eoerE) = pe
(3.1)
(3.2)
V x E = j n o n rH
(3.3)
V xH = c E - j t o e e rE
(3.4)
where e0 and |X0 are the permittivity and permeability in the vacuum, e,. and (Xj. are the
relative permittivity and permeability of the material, o is the conductivity of the material. E
and H are the electric and magnetic field strength, respectively. The propagation of the
energy in the electromagnetic fields can be deduced from this equation system and leads to
Poynting's theorem
(3.5)
which states that the mean energy, P, flowing into a surface, S, depends on the amplitude,
distribution and prevailing phase of the electric and magnetic field.
By using Gauss' law, equation 3.5 can be converted into the volume integral
which can then be resolved into three single integrals
39
P = jo) I (ioP^H • H*)dv - jcoj eoe^E’E*)dv
(3.6)
+ col £()£r(E 'E*)dv
V
The first two integrals take account of the magnetic and electric fields respectively while the
third represents the energy dissipation in the dielectric in a general form. Therefore, the
energy that is converted into heat by the alternating field is
(3.7)
It increases with the frequency, the square of the electric field strength and the imaginary
part of the dielectric constant. Once the profile of e/' as a function of temperature and the
electric field strength in the homogeneous body are known, it may be possible to study the
thermal runaway conditions through the source-incorporated heat-diffusion equation. The
diffusion of thermal energy in a homogeneous bounded volume V is determined by the
partial differential equation
pCp
Kh V2T = P
(3.8)
where p, Cp and Kh are the mass density, the specific heat, and the thermal conductivity of
the material, respectively. P is the microwave energy density absorbed by the material. At
the boundary of the volume V, the boundary condition
K h n VT = h(T-T0) + F ^ T
4
- Tq)
(3.9)
must be satisfied. In which h is the heat convection coefficient, Fr and or are the emissivity
of the material and Stefan-Boltzmann constant. T 0 is the ambient temperature. The initial
condition is
T(r,0) = Ti
This heat diffusion equation is analogous to the forced Fisher equation
(3.10)
40
Tt = Txx + G(T)
(3.11)
which is known to have chaotic behavior for specific initial and boundary conditions as
investigated by Fisher [1937] and Rothe [1981].
For the densified ceramics to be considered here, the diffusion equation is not
needed.
3.3 Description of the Model
For the simplicity, a ceramic slab with Finite thickness under plane wave radiation
is considered here, as depicted in figure 3.1. The incident electric field is a monochromatic
plane wave propagating in the z-direction and is polarized along the y-axis. To account for
material non-linearity during microwave heating, the slab is further divided into layers so
that finite element method can be used accurately. It is assumed that each element will have
the same material properties during microwave heating process at each iterative step. Since
the ceramic slab is assumed to be very large, the problem becomes one-dimensional. In the
following discussion, layers with smaller thicknesses will be considered as different media
since they may have different material properties such as dielectric constant and loss factor
which are functions of temperature during the microwave heating process.
To simulate microwave heating of ceramics, the microwave energy absorbed by
the ceramic slab must first be calculated. Therefore, the electric and magnetic fields in the
ceramic slab have to be determined. The electromagnetic field inside a dielectric body of
arbitrary shape is difficult to be determined. For the model considered here, an impedance
method which is given by Varadan and Varadan [1988] is applied and proven to be effective
to account for material non-linearity.
3.4 Impedance Method
The system of figure 3.1 is a ceramic slab discretized into n regions for the
consideration of dielectric properties change with temperature. The ceramic slab is radiated
41
k-1
Z2
Zk-1
Zk
k+1
n-1
Zk+1
Zn-1
Incident
Wave
Reflected
Wave
To
sources
Slab
Figure 3.1 Ceramic Slab
42
by a normally incident, time harmonic wave in region 1. There are forward- and
backward-travelling Helds for each region. The total electric field for each region can be
written as
E|c(z) = E£e-Ykz + Ejje+Tfcz = Efc-Tkzj i + p e +2 TfkzJ = Eje-7kz(l+ r(z))
(3.12)
where / ( z) is defined as the reflection coefficient at any location in the the region and it is
the complex ratio of the reflected wave to the incident wave
Hz) =3f€+2Tkz
(3.12)
The corresponding total magnetic field is
Hr(z) = Hje-Vkz + Hfce+Ykz = E^
Z| l -
=^
^
(l- T[z))
(3.14)
A total field impedance Z(z) is defined at any location z by the ratio of the electric field to
the total magnetic field
kt )
1
- Tkfz)
A reverse expression can be obtained for T(z) in terms of Z(z) by solving for T(z)
Ik(z)
' T1k
Zk(z) + Tik
(3.16)
The reflection coefficient T(z) at location z can be obtained in terms of reflection coefficient
H z') at location z' in the same region, i.e.
H z) =F(z')e2Y(z-z’)
(3.17)
At any interface separating two regions, T(z) and Z(z) satisfy the following two conditions.
1. The total field impedance Z(z) is continuous across the interface; that is, at an interface
separating kth and k+lth layer
Zk{zk) = Zk+i(0)
(3.18)
2. The reflection coefficient is discontinuous across the interface.
In practice, the reflection coefficient f n(0) in the region n and the incident field
E+j in the region 1 are usually known. The total field impedance for the nth region is
(3.19)
1
• UO)
Since
Z„.i(zn. 1) = Zn(0)
(3.20)
the reflection coefficient can be obtained at Z=zn . 1
rr n-l(z)-=
. i ~ \ — Zn-l(zn-l)~ fln-1
— 7 -— r— —
^
(3.21)
Zn-l(zn-l)'*'^ln-l
At Zn.j = 0 the reflection coefficient is
r n-i(0) =r(zn.i)e-2 Vn-i(zn-i)
(3.22.a)
the iteration procedure from eq.3.19 to eq.3.22 can be used n-1 time, so that the reflection
coefficient in each region and at any position can be found.
At the region 1 , the incident field is known so that the reflected field is
E i= E |ri(0)
(3.22.b)
Using the tangential electric field continuous condition, it is easy to find that
E i(0)=E2(0)
(3.23)
E+=e { [ 1 1 M ]
\ i+ r 2(o)/
(3.24)
E^E ^O )
(3.25)
and
again, using iteration procedure from eq.3.22(a) to eq.3.25e n-1 time, the field distribution
for each region can be obtained.
For the lossy dielectric material, the energy absorption in each region is
44
3.5 Computing Procedures
When the microwave energy absorbed by the ceramic slab is known, the heat
diffusion equation can be used to calculate the temperature variation with time and position.
Since the dielectric constant and loss factor are functions of temperature, the microwave
energy absorbed by the ceramic slab is also a function of temperature. Hence, the heat
diffusion equation becomes non-linear. A non-linear finite element method is therefore
needed to find the dynamic temperature distribution profile. At the surface of the ceramic
slab, radiation link elements are used to account for radiation heat loss. Conduction loss is
neglected since the radiation loss is the prime heat loss at high temperature. The detailed
implementation of the non-linear analysis is as follows. The time step to do the non-linear
analysis is designated first. The temperature at the end of time step is then estimated. The
material properties at the middle of the temperature increment are used for each media. The
microwave energy absorbed by each media with different dielectric properties is calculated
according to the technique described above. The temperature distribution is then obtained
by using power absorption data. The computed results will be compared with the prior
estimated temperature. Such an iteration procedure will continue until the difference
between the estimated and calculated temperature reaches a prescribed value.
3.6 Results and Discussion
3.6.1 Microwave Energy Absorption by Ceramics
By using the technique stated above, the effects of dielectric constant and loss
factor on the power absorption by ceramics are considered. Figure 3.2 gives the
comparison of power absorption by ceramic slabs with the same dielectric constant and
different loss factors. The increase in loss factor will dramatically increase the ability of the
Normalized Power Absorption Density
45
e = ( 9.0, 0.01 )
o.o.
OX)
3.1
0.2
0.3
0.4
0 .5
0.6
Distance From Incident Plane
( Incident W avelength )
Normalized Power Absorption Density
Figure 3.2. Microwave Power Absorption for
Slabs with Different Loss Factors.
—
e = ( 10.0,0.1 )
e = ( 9.0, 0.01 )
I I I I | IT I I | I I I I | 1 I 1 I | I I I I | I I I I
.0
0.1
0.2
0.3
0.4
0.5
Distance From Incident Plane
( Incident Wavelength )
Figure 3.3. Microwave Power Absorption for Slabs
with Different Dielectric Properties.
0.6
46
ceramic slab to absorb microwave energy. Figure 3.3 shows that with the increase of both
dielectric constant and loss factor, ceramic absorbs more microwave power and the
distribution of the absorbed microwave energy becomes more uniform.
3.6.2 Simulating Microwave Heating of Ceramics
The developed non-linear finite element method is used in this case to find the
dynamic temperature profile of a ceramic slab under plane wave radiation considering
changes in both dielectric constant and loss factor with temperature. The analysis
procedure is displayed in figure 3.4. The data of dielectric constant and loss factor change
with temperature are taken from Fukushima et al.[1987]. The incident microwave power
flux is taken to be 30kw/m2.
The mass density, specific heat and thermal conductivity are taken to be 4g/cm3,
1.125 j/g°C and 10w/cm2, respectively. The thickness of the plate is taken to be 5.08 cm.
The finite element analysis routine on ANSYS is implemented in the calculation. The
calculation is done on a VAX-11/780 computer. The CPU time is 79 seconds.
Figure 3.5 gives the temperature variation with time at 0.01m inside the slab
from the edge of ceramic slab at the microwave incident side. The temperature increases
slowly at the beginning and rapidly after 600°C.
Figure 3.6 displays the temperature profile over the thickness of the slab. An
appreciable temperature gradient is observed. This temperature gradient is caused by nonsymmetric microwave radiation of the ceramic slab and radiation heat loss at the boundary
which subsequently results in a non-uniform power absorption by the ceramic slab. If
symmetric radiation is realized, i.e., microwave radiation is from both sides of the slab,
the uneven heating will result only from boundary radiation heat loss and the center of slab
will have the highest temperature. Hence, the radiation heat loss at the boundary is the
major cause of the non-uniform heating with microwaves observed in the early research
where the ceramic sample is melted at the center while the boundary is still intact. In order
to prevent this effect, good insulation must be used at the boundary. Also, in practice, the
47
Input
geometry data &
:reate finite element
N . mesh
Input initial temperature
and physical parameters
Obtain microwave
energy absorbed by each
v
element
Prescribe AT & estimate
A t' obtain complex
dielectric constant at
T+AT/2
Calculate T + AT' N
adjust At =AT/AT'*At'
recalculate temperature
distribution T + AT y
Calculate
electromagnetic
field strength inside
the slab
Obtain complex dielectric
constant according to
calculated temperature
distribution
STOP
Figure 3.4. Procedures for Simulating Microwave Heating of Ceramics.
48
TEMPERATURE
( °C
)
2000
1000-
0. 0
0.1
0.2
0.4
0.3
TIME ( hour)
Figure 3.5. Dynamic Temperature Profile 0.01m
inside the Slab from the Incident Plane.
2000
18.480 (min.)
1600
17.701 (min.)
1200
16.312 (min.)
800
13.301 (min.)
U
o
2
a
2
8.
E
Q)
H
12.000 (min.)
400
7.988 (min.)
0.761 (min.)
0
0
1.3
2.5
3.8
5.1
Distance From Incident Plane (cm)
Figure 3.6. Temperature Distribution over the Thickness of the Slab.
49
ceramic sample needs to be rotated continuously to prevent any uneven radiation. Figure
3.7 shows the variation of total microwave energy absorbed by the ceramic slab against the
temperature variation at 0 .0 1 m inside the slab from the edge of the ceramic slab at incident
side. It indicates that as the ceramic becomes hot, its energy absorption ability is increased.
Therefore, thermal runaway is simulated in microwave heating of the ceramics.
3.7 Conclusion
A method of modeling microwave heating of ceramics is developed here. The
results show that increasing the dielectric constant could increase microwave power
absorption uniformly while increasing the loss factor could increase the material's ability to
absorb microwave energy. It is found that non-uniform heating observed in the laboratory
can be caused by boundary radiation loss and non-uniform radiation by the microwave
source. Through this research, it is observed that the dielectric property of a material at
elevated temperature has a very important role in designing microwave processing. In
microwave sintering of ceramics, the green sample changes its microstructure during
microwave heating. Therefore, the dielectric properties change not only with temperature
but also with microstructure. Hence, the characterization of the dielectric property of
ceramics during microwave sintering is very important. In doing so, it would be able to
control the sintering process and understand thoroughly the mechanism of microwave
sintering. Therefore, it is necessary to develop a method to dynamically characterize the
dielectric properties of ceramics during processing. Chapter 6 will introduce a system for in
situ microwave heating, sintering and characterization. It is also useful to model microwave
heating for the ceramic samples of complicated shapes. For the case where green ceramic
is surrounded by insulation material and placed in a microwave applicator, the impedance
method to find electromagnetic fields distribution is no longer applicable. The numerical
approach is desirable for predicting field distribution in the ceramic specimen as well as in
insulation material, so that uniform temperature distribution can be achieved during
microwave processing of ceramics.
50
1000
O
£
2 0 0 0
Temperature ( ° C )
Figure 3.7. Total Microwave Energy Absorbed by Slab vs. Temperature.
51
Chapter 4
MODELING SINGLE MODE CAVITY WITH
FINITE DIFFERENCE TIME DOMAIN METHOD (FDTD)
4.1 Introduction
Theoretically, microwave heating is a volumetric process so that uniform and rapid
heating is possible. However, it is shown in chapter 3 that nonuniform heating can occur in
practice because of nonuniform field distribution and heat loss at the boundary. It is
therefore necessary to provide suitable insulation to avoid any temperature gradient Also,
initial heating is necessary for ceramics which are low loss material at room temperature. A
popular method of obtaining both initial heating and good insulation is to place some
microwave susceptor around ceramic specimen. To understand such a complicated
structure, a numerical technique has to be employed to find fields strength and energy
absorption by the ceramic specimen as well as microwave susceptor. In this chapter, FDTD
is introduced and used to analyze a single mode cavity for ceramic processing. In doing so,
the design of insulation scheme will be provided and full advantages of microwave heating
can be utilized.
Microwave processing of materials provides several advantages that in many cases
help improve the products quality, uniformity of grain structure, and yield. The ability of
the microwave energy to penetrate and hence, heat from within the product, helps reduce
processing time, costs, and in some cases of ceramic processing, reduce the sintering
temperature. There is also evidence that microwave processing of materials actually
provides improved microstructure and other reported advantages of microwave processing
of ceramics include removal of water, binders and gases without rupture or cracking,
reduction of internal stress, lowering of thermal gradients, and possibility of controlling the
state of oxidation.
There are several research and development activities in the area of microwave
52
processing of materials. Those activities show that much progress has been made. Some
experiments and studies that support some of the anticipated advantages of microwave
processing have been successfully completed. However, these studies suggest that
microwave heating is a complicated process and that there are many remaining problems
that still need to be solved before this new processing technique can be fully understood
and/or optimized. For example, some basic interaction mechanisms that may have resulted
in the apparent reduction in activation energy of the reported lowering in sintering
temperature need to be studied in more depth. The role of frequency and the sample
insulation combination on sintering results also need to be better understood. From the
engineering and technology point of view, other types of problems such as the design of
optimum processing parameters for improved quality, yield, and economics also need to be
studied. To this end, it is believed that computational techniques and numerical methods can
play an important role in the future research and development activities in the area of
microwave processing of materials.
Computational techniques may be used to calculate and predict microwave power
deposition patterns in materials. These calculations may be made for a wide variety of
heating technique and more cost effective computer modeling and sample-insulation
combinations. Computer modeling and computational techniques can also be used to
calculate temperature distribution and heating patterns in processed samples and may help
identify critical parameters in controlling heating process.
Single mode microwave resonant cavity has a simple structure and a known field
distribution, It has received a lot of attention from those who are interested in using it as a
microwave applicator. However, the field distribution, when a dielectric body is inserted
into the cavity, is still unknown especially when microwave susceptor is used for
preheating. By not knowing such a field distribution and power absorption by the dielectric
materials, thermal runaway and nonuniform heating could occur. In this chapter, efforts
are made to simulate a single mode cavity to be used for material processing by using the
finite difference time domain method. By analyzing different insulation schemes, a better
understanding of interactions between microwaves and ceramics is achieved. In the initial
53
simulation, the resonating frequencies of the cavity with different structures are obtained to
analyze the effect of the cavity structure on the resonance frequency and to validate the
applicability of the FDTD method to the current problem. During subsequent calculation,
the cavity was subjected to a continuous incident wave. The field distribution in the cavity
and energy absorption by the inserted dielectrics are obtained to analyze the current
microwave processing insulation scheme.
4.2 Finite Difference Time Domain Method
4.2.1 Introduction
The finite-difference time-domain method is a technique that has recently generated
a great deal of interests and has been widely applied to 3D dielectric objects. The solution
procedure is based on expressing the time-domain Maxwell’s curl equation in terms of their
finite-difference representations. Such a representation has been made elegantly in terms of
Yee’s [1966] cell as shown in figure 4.1. In general, FDTD method is useful and attractive
in the area of microwave processing, because ( 1 ) in the microwave processing, the
computational domain is physically enclosed by the conducting walls of the cavity. Hence,
most of the computational difficulties and the inaccuracies are eliminated and replaced by
“solid,” well defined boundary conditions on perfectly conducting walls. (2) The FDTD
method involves the development of the stead-state solution of the field quantities iteratively
and as a function of time. Hence, for the microwave processing of materials research where
the electric characteristics of the materials are constantly changing with temperature, the
varying properties of materials may be included in the FDTD calculations. (3) The FDTD
method may be used to calculate the number of modes with and without the sample, the
field distribution in each case, and may even provide some guidelines to the optimum size
of the sample that may be heated uniformly, without cracks, and in the shortest period of
time. (4) The FDTD solution has the unique advantage of dealing accurately and routinely
with dielectric interfaces and inhomogeneities in the properties of materials to be microwave
Figure 4.1 Yee Cell for Finite Difference Time Domain Method
55
processed. The technique simply utilizes different finite-difference equations that explicitly
involve the properties of materials at the dielectric interfaces. Hence, the FDTD solution
procedure may be used to model various insulation-sample combinations that provide
optimum sintering results using microwave heating.
In recent publications, Sheen et al. [1990] and Moore and Ling [1990] have shown
that FDTD method can be used to model microstripline structure. Alinikula and Kunz
[1991] and Kunz et al. [1991] have also used FDTD in predicting aperture coupling for a
shield wire and wave guide problems. Those computations show that FDTD can be used
for analyzing aperture coupled cavity problems.
4.2.2 FDTD Method Formulation
For Maxwell’s equation, given in chapter 2, when all three constitutive parameters,
e, jx, o, are present the curl equations for isotropic material with no electric sources can
be written as,
dH
= --MV xE)
dt
|i
(4.1)
dE
= _ £ E + I ( V x H)
dt
e
e
(4.2)
By using separate field formalism, the total field can be expressed as
£_£total = gincident +£scattered
(4 3 )
jj_ |jto ta l = [^incident + |jscattered
(4 4)
The incident fields satisfy the free space Maxwell’s equations and can be specified
analytically throughout the problem space while the scattered fields are found
computationally and only the scattered fields need to be absorbed at the problem space
faces.
In the media of the scatter, the total fields satisfy
56
VxEt0,al =
^
(4.5)
dt
VxHtolal = e -E^ tal- + oE t0tal
dt
(4.6)
while the incident fields traversing the media satisfy free space conditions
VxEinc = - n o ^ .
(4.7)
VxH“c = eo^=r—
■
dt
(4.8)
rewriting the total field behavior as
VxfEinc+Escal) = -p d(H,nc+HScat)
(4.9)
V x jH ^ + H 8031) = e ^ Em-C^ ESCa-) + a (Einc+EScat)
(4.10)
ot
by subtracting the incident fields above to obtain the equations governing the scattered
fields in the media
VxEscat = - p ^ f ^ ^ dt
,
v d H inc
(^"^°)“ a T
VxH scat = E ^ ^ - + oE inc ( e - e o j ^ - + o E inc
dt
L'
outside the scatter in free space the scattered fields are
(4.11)
(4.12)
muscat
VxEscat = -|i 0 —3 :—
dt
(4.13)
VxHKat = E o ^ ^
(4.14)
the above equations can now be rearranged so that the time derivative of the fields are
expressed as a function of the remaining terms for ease in generating the appropriate
difference equations.
dfjscat _ - (|i-|io)dHinc UVxFScat)
dt "
|i
dt V
]
^ SCat_ O g scat. f fg in c . ( E - e o ^ + i ( V x H scat)
dt
E
E
(4.15)
(4.16)
57
Substituting derivatives with corresponding time and position differences and using Yee’s
cell as reference, the difference form for the Ex and Hx can be written as
| H § q j,K )n -Hg(I,JtK -l)n J
Az
At
'e +cAt
H |(I,J,K ) n + 1 =H|(I,J,K)n - j ^ ) ( H lx(IJ,K )n+1 -H jl(I,J,K)n)
(4.18)
n \^ ( I J ,K ) n+2 -E |(I,J-l,K ) n + 2 , / nEf(I,J,K )n+2 -E ^(I,J,K -l ) n + 2
Ay
vM1/
Az
Both E and H are evaluated over successive cycles where E is evaluated first at n+1/2 and
H at n+1. This interleaves E and H temporally and results in a temporal center difference or
“leapfrog” approach when coupled with the spatial central difference. The above notation
shows E at N=n+l/2 updated from its prior value at n-1/2 and the curl of H at n. Similarly
H is evaluated at N=n+1 from its earlier value at n and the curl of E at n+1/2.
4.2.3 Outer Radiation Boundary Condition (ORBC)
Since FDTD technique uses a finite problem space, there must be components on
the surface of the problem space. These components, unlike the remaining interior
components, are not completely surrounded by their neighbors. As a result, when
calculating the update of the surface components according to the formulation given in
previous section, there is not enough information to correctly calculate the updated value.
An outer radiation boundary condition must be employed to approximate the missing field
components. If the approximation were perfect, then the outermost component would be
updated as if the scattered field passed through the surface component's location were
58
unaffected by the truncation of the problem space. Any real ORBC is an approximation. It
will approach the ideal and will generate some reflected radiation, typically small, from the
outmost or outer boundary components.
There have been a number of ORBC developed prior to the now widely used Mur’s
radiation condition. Taflove and Brodwin [1975] used the conducting layer approach in his
early work with FDTD technique. Gilbert [1976] have examined the legendre polynomial
analytic continuation approach. Mur [1981] derived an outer radiation boundary as well as
more generalized treatment starting with the wave equation.
Following Mur’s treatment, the wave equation for a single field component W may
be written as
(4.19)
where c0 is the velocity of propagation. Assuming that the FDTD problem space is located
at x>0 , there is a boundary at x= 0 that a scattered wave will reach and be reflected unless
an ORBC is imposed.
The ORBC is found from the above wave equation. The scattered wave has velocity
components V x, Vy, V z such that V x 2 + V y 2 + V z 2 = C02. It is also possible to define
inverse velocity components Sx, Sy, Sz where
C — 1
Ox ~
c _
i Oy —
1
c
, Oz —
1
(4.20)
(4.21)
The scattered wave can be approximated by a plane wave where
W = Re (\|/(t + Sxx +Syy +Szz))
(4.22)
and, by expressing Sx as
(4.23)
59
the wave equation becomes
W= Re
(4.24)
when
(4.25)
i.e. Sx k 0 the wave is traveling in the -x direction at some unspecified angle i.e. Sy and Sz
are not specified. This wave satisfies, as can be seen by direct substitution, the first order
boundary condition at the boundary x = 0
(4.26)
The W satisfying the above equation is therefore a wave traveling in the -x direction
and is outgoing and may therefore be characterized as absorbed. Since Sy and Sx are not
specified, a solution is not determined for the above first order boundary condition.
Approximations of the first and second order will allow W to be found at the boundary
x=0. The first order approximation is that
( l -(C g S y )2 - (C q S z )2 j>2 -
1 + 0 (C Q S y J2 + 0 ( 0 0 8 2 ) ? a
1
(4.27)
so that the first order boundary condition is approximated to first order by
(4.28)
The second approximation is
60
( l - ( C g S y ) 2 - (C g S z
)2}i -
o ((( ( C o S y )2
1-
(C g S y )2 -K C q S z)2 ) +
~
2^ (c0^yF "KcqSz)2)
- K c q S z J 2 ) )^ )
(4.29)
so that the second order boundary condition is approximated as
— - 1- i( (CQSyf2-H coS^ Wlx=0= 0
dx
1
(4.30)
dt
By taking the time derivative of the above equation and multiplying by l/c 0, the second
approximation becomes
The approximate boundary condition for FDTD application can be obtained through the
appropriate discretization.
4.3 Structure of Model
The cavity to be modeled, as shown in figure 4.2, is a rectangular cavity which is
used in this thesis research for processing of ceramics such as sintering, joining and
simultaneous microwave heating, sintering and characterization of ceramics. The resonant
cavity is primarily resonated in TE103 mode at 2.45GHz. The cross section of the cavity
has a dimension of WR430. The length of the cavity can be adjusted so that the cavity will
always be in resonance at 2.45 GHz with the insertion of different dielectric materials in
practice. A section of the waveguide is connected ahead of the cavity which acts as a
transmission line for sending the incident wave.
The computing mesh is constructed as follows. The cubic cell used for modeling
has a dimension of 3.413mmx 3.413mm x3.413mm. The thickness of the cavity wall is
61
molded by four nodes. The cavity is assumed to be made of copper. The Mur [1981]
absorbing boundary condition is used at the incident side of the buffer to limit the
calculation space. The position of the variable short can be readily adjusted by a parameter
specified in the program. In doing so, the cavity can be tuned to the resonant position. The
iris used in processing of materials could be rectangular or circular. In this calculation,
rectangular iris is used for the easy implementation. Circular iris has to be approximated by
many rectangular.
Three particular structures are considered in this analysis. The first one is an iris
coupled empty cavity. Its resonance characteristics is analyzed. In doing so, the
applicability of the FDTD method to the current problem can be validated and its accuracy
can be estimated. The effect of the iris on the resonance of the cavity can also be analyzed.
In the second case, a ceramic rectangular rod is inserted into the cavity alone which is
simulating the case where simultaneous microwave heating, sintering and characterization
are researched. The ceramic rod can be either densified or green. In the third case, the
ceramic rod is surrounded by four SiC rods as shown in figure 4.3. These SiC rods are
used as the microwave susceptor in microwave sintering of ceramics for initial heating
purpose because of their high microwave absorbing ability. Such an arrangement has been
experimentally proved to be essential in achieving uniform temperature distribution during
microwave processing of materials. At room temperature, most of the ceramic materials
have very low dielectric loss. Their ability to absorb microwave energy is very little. When
dielectric material which has low loss is inserted into the cavity, it can cause large reflection
and low heating rate. Such a large reflected microwave energy can be detrimental to other
microwave components such as the magnetron, etc. Also, uniform temperature distribution
can not be realized with dielectric material alone because of heat loss at the boundary.
Holcombe and Dykes [1991] and Janney et al. [1992] have done research work to
construct the right insulation scheme for providing initial heating and maintaining uniform
temperature distribution for the materials to be processed. Theoretical investigation is still
lacking for analyzing those structures in the guiding of practical design.
62
Iris
Ceramic Rod
Variable Short
Figure 4.2. Single Mode Cavity for Material Processing
Ceramic Rod
Microwave
Susceptor
Figure 4.3. Structure of Ceramic Rod and Surrounding Microwave Susceptor
63
4.4 Computation Procedures
In the initial calculation, the response of the cavity in the frequency domain is
needed to determine the frequency at which the cavity is resonating. To do so, a truncated
electric Held pulse which polarized in the Y direction and has a Gauss form is used to
illuminate the cavity. Mathematically, the pulse has the form
iL _
y(t) = e | (Beto*DTj2 |
(4.32)
Figure 4.4 illustrates three pulses for different values of Beta. DT is the time step for the
iteration. The response of the cavity is recorded on time domain. Such a time domain
response is then transformed to the frequency domain by using the Fast Fourier
Transformation method. In the subsequent simulation, the continuous incident
electromagnetic waves have to be used for the calculation. Since the frequency for
microwave heating is designated to be 2.45 GHz. The mode of wave propagation in
WR430 waveguide can only be TE10, while other modes will be evanescent. With such an
argument, the precise incident wave distribution in the waveguide ahead of the cavity can
be specified. For the case where a TE10 mode of waves propagating in Z-direction and
polarizing in Y-direction, the electric field has the form of
Ey = Eyo sin (®*-) cos (cot - pg z)
(4.33)
where a is the width of the waveguide, pg is the phase constant in the waveguide. The field
distribution inside the cavity will reach harmonic state after a sufficient number of iteration
steps. Hence, the field distribution will have the form Exye"^03t. To find the spatial field
distribution, the following method is used. Since
1.00-1-
0.75—
Beta= 125
Beta= 100
0 .5 0 —
Beta= 75
0
.
0 . 00- -
-0 .2 5
t— i—i— i—I— i—i— i—i— |—i— i—i— i—|— r —i— i— r
- 1 .0
-
6.5
0 .0
0.5
Time ( nano-second)
Figure 4.4 Truncated Gauss Pulse
1'.0
Assume the iteration time step is At, then let
N =^4At
(4.36)
N
E (x ,y )= fflJ Ey( nAt )
(4.37)
n=0
The microwave energy absorbed by the lossy dielectric material will be
P Absorption = 2 ^
(4.38)
4.5 R esults an d Discussion
Figure 4.5 shows the frequency response of a closed cavity when it is illuminated
by a microwave pulse. Each maxima gives a resonant mode. Figure 4.6 gives the
frequency response of a closed cavity which is designed to be resonant at 2.45GHz in
TE103 mode after different number of iteration steps. The results in figure 4.6 illustrate
that more accurate results could be realized with increased number of iteration steps.
Comparing theoretical and FDTD results, it is found that the relative error is about 0.1%
when 8192 iteration steps are used. Table 4.1 tells the resonant frequencies for different
cavity structures. The number of iteration steps used here is 8192. Lower resonant
frequency is obtained when dielectric rod is inserted into the cavity. Such a result is
consistent with the phenomena observed when the cavity is examined by connecting it to a
vector network analyzer. This result would also be useful for calibrating a very popular
dielectric measurement technique, that is, the cavity perturbation method for measuring
dielectric properties of various materials. More importantly, the knowledge of the frequency
response would enable us to adjust the position of the short so that the cavity is always in
66
the resonant state at 2.45GHz.
Figure 4.7 shows the electric field distribution inside the cavity when a ceramic
rectangular rod is inserted into the cavity. It is obvious that the field distribution inside the
cavity has the same shape as that of the empty cavity. The field distribution in the dielectric
region is essentially suppressed due to the ceramic material which has a higher dielectric
constant. At the region close to the iris, the field distribution is disturbed by the presence of
the iris as compared to the cavity with no iris. Figure 4.8 gives the microwave energy
absorbed by the dielectric ceramic rod. Since the ceramic rod has low loss, the energy
absorbed by the ceramic rod is smaller despite it is located at the maximum electric field
region. Such a power absorption knowledge can be used to obtain the temperature
distribution in the ceramic rod.
Figures 4.9 and 4.10 show the microwave energy absorbed by the ceramic and SiC
rods at two temperatures. The result for the energy absorption at room temperature shows
the SiC rod absorbing much more microwave power than the ceramic rod. Hence at the
beginning of microwave processing of ceramics, the ceramic rod was mainly heated by the
radiation from the SiC rods. At 1000*C, the ceramic rod absorbs almost the same amount
of microwave energy as the SiC rod and is heated mainly by transforming absorbed
microwave energy. The microwave energy absorbed by SiC rod is decreased because SiC
rod has large conductivity at high temperature which prevents microwave penetration. The
heat generated by SiC rods will be continuously radiating to the alumina rod for the
insulation purpose rather than heating. In so doing, uniform and volumetric heating
process can be realized.
4.6 C onclusion
It is shown here that the finite difference time domain method is applicable for
analyzing aperture coupled single mode cavity for microwave processing of ceramics. It not
only indicates the working condition of the cavity but also gives the specific
electromagnetic field distribution and energy absorption by material to be processed.
67
Ey
750_: “
(v/m) 500
40
2.0
Frequency ( GHz)
Figure 4.5. Frequency Response of an Empty Cavity
2.50
Resonant Frequency
2 .45 -
2 .40 ■
9
Calculation
Theoiy
2.35 -
2.30
0
5000
10000
15000
20000
Num ber of Iteration Steps
Figure 4.6. Convergence Analysis of the FDTD Method
68
Table 4.1 Resonant Frequencies for Different Cavity Structures
"v^^sM O D E
TE101
TE102
TE103
THEORY RESULT
FOR PERFECT
CAVITY
1.529
1.926
2.448
CLOSED COPPER
CAVITY
1.532
1.922
2.447
1.542
1.932
2.435
1.319
1.877
2.249
1.319
1.877
2.136
COPPER
CAVITY WITH
IRIS
CLOSED CAVITY
& DIELECTRIC
ROD
COPPER
CAVITY & IRIS
& DIELECTRIC
Figure 4.7. Field Distribution in the Cavity with a Ceramic Rod
Figure 4.8 Microwave Energy Absorbed by a Ceramic Rod in the Cavity
70
- 25000
P(w)
Figure 4.9 Microwave Energy Absorption by Both Ceramic Rod
and Microwave Susceptor at Room Temperature
Figure 4.10 Microwave Energy Absorption by Both Ceramic Rod
and Microwave Susceptor at 1000°C
71
Chapter 5
EXPERIMENTAL SYSTEM
5.1 Introduction
The main equipment used for this thesis research is the single mode high power
microwave heating system as shown in figure 5.1. It consists of a 120v-high voltage
power supply, a magnetron, a circulator, a directional coupler for power measurements, an
impedance analyzer, a 4-stub tuner, a section of three quarter waveguide, a variable short,
a pyrometer and attached computer data acquisition machine. Other equipments used are a
8510 network analyzer and its related free space set-up and waveguide measurement
system, a scanning electron microscopy, a X-ray diffractometer, a multi-mode microwave
oven and a conventional electric furnace. Since the single mode high power microwave
heating system has a direct effect on the characterization and processing, a brief
explanation of the function of each component is given in this chapter.
5.2 Magnetron
The continuous wave magnetron is the microwave tube most commonly used in the
microwave processing system. The first magnetron was developed prior to World War n in
the late 1930s and early 1940s. A concentrated effort was made to advance the technology
during World War II because of the need for higher frequency operation, primarily for radar
systems.
Compared with klystron, a magnetron has limited electronic tuning and
modulation capabilities. Because of its low cost and high efficiency on the order of 80
percent, it has being increasingly used in industry heating, diathermy equipment,
microwave ovens, etc.
A schematic diagram of a multicavity magnetron oscillator is shown in figure
VAX
CLUSTER
HP
9000 PC
I
HP 3497A DATA
ACQUISITION SYSTEM
POWER
SOURCE
CONTROL
UNIT
DUMMY
LOAD
TEMPERATURE
LINEAR
VOLTA<3E
DISPLAYCEMEN1
TRANSI)UCER
POWER
METER
PYROMETER
i
T T
-55 dB
JMAGNETROI'
HEAD
0TO 3K W
2.45 GHZ
CIRCULATO
-55 dB
J
WAVEGU1DI
COUPLER
'
HIGH POWEF
PHASE
IMPEDANCE
ANALYZER
HIGH VSWR
4 -STUB
TUNER
IjADJUSTABLE
^W A V E G U ID E
| j SHORT
VARIAB
IRIS
CAVITY
APPLICATOR
HIGH
VOLTAGE
SUPPLY
Figure 5.1 Single Mode High Power
Microwave Processing and Characterization System
CERAMIC
SAMPLE
-j
to
73
5.2.(a). As may be seen from the figure, the magnetron consists of a multicavity anode
block, a coaxial cathode, means of coupling the generated microwave power to the outside
and a permanent magnet to provide a magnetic field along the axial length of the cathode
and at right angle to the dc electric field in the radial direction. When the anode is at ground
potential, a negative high voltage is applied to the cathode. Electrons are emitted from the
heated cathode and, without the presence of a magnetic field, the electrons would travel
radially to the anode. When a magnetic field is applied in a direction parallel to the tube's
axis, the electrons follow a curved path to the anode and form a rotating electron cloud as
shown in figure 5.2.(b). If the magnetic field is strong enough, the electrons are prevented
from reaching the anode and the magnetron current is cut off.
Anode current flow excites oscillations in the resonant cavities which, in turn,
influence the shape of the rotating electron cloud. Some electrons are slowed down and
others are speeded up, depending on the direction of the electric field on the circuit as the
electrons pass through it. The electrons with increased velocity return to the cathode and
release secondary electrons as they strike the cathode. The slowed down electrons, which
have lost most of their energy to the resonant circuits along the way, eventually end up
impinging on the anode at low velocities. The rotating electron cloud assumes a spoke
shape and rotates at a constant velocity, giving up large amounts of energy to the
microwave field. The microwave energy is coupled by means of a probe from one of the
resonant cavities into an output coupling or antenna where it is launched into a transmission
line, usually waveguide or coaxial line.
5.3 Circulator
Operation of a magnetron at higher than specified VSWR may result in unstable
performance and possible damage to the tube. A Circulator is used when necessary
between the generator and the load to reduce the power reflected back to the generator from
a mismatched load and thus reduce the VSWR at the magnetron output.
The circulator is a three or four port device which has the unique ability to couple
74
Magnetic field
Anode
Microwave
Output
Cathode
Figure 5.2 Multicavity Oscillator Magnetron
75
energy between adjacent ports on one direction only and isolate between non-adjacent
ports. Referring to figure 5.3, energy entering port 1 exits only at port 2 and energy
entering port 2 exits only at port 3. For the three port device, energy entering port 3 exits
only at port 1. Thus, the energy circulates around the device in only one direction. Port 1 is
the input, port 2 is the output, and port 3 is terminated with a matched dummy load which
absorbs energy reflected from output port 2. Very little energy is coupled from port 3 to the
input port 1. The circulator, like the isolator, relies on the non-reciprocal properties of
ferrite for its operation.
5.4 Directional Coupler
The directional coupler is a calibrated power sampler which has the ability to
distinguish between two directions of power flow. The directional coupler consists of a
main line and an auxiliary line that are separated from each other except in the coupling
region where some of the energy in the main line is coupled in one direction into the
auxiliary line. In figure 5.4, energy entering terminal 1 of a coaxial directional coupler exits
at terminal 2 except for a small amount of energy coupled into the auxiliary line and
appearing at terminal 3. Energy applied to terminal 2 appears at terminal 1 and almost no
energy at terminal 3. Dual directional coupler for simultaneous sampling of power in both
directions consists of two directional couplers back to back in one package. Coupling is
defined as the ratio of main input power to auxiliary line output power. Directivity is
defined as the ratio of auxiliary line power as a result of incident main power to the
auxiliary line power as a result of reflected main power. Values of coupling are normally
anywhere from 3dB to 50 dB. Directivity is usually in the order of 30 or 40 dB.
5.5 Impedance Analyzer
The function of the phase/impedance analyzer is to provide additional degree of
freedom in matching difficult load situation where the load impedance can change with
76
Dummy
Load
Mismatched
Termination
Power
Source
Figure 5.3 A 3-Port Circulator
Pi
Auxiliary Line
Main Line
Figure 5.4 Directional Coupler
77
power level, power source frequency. A multistub is often used
in high power
impedance matching where reflected power is utilized to determine stub insertions. Such
an approach could cause performance difficulty such as in case where there is more than
one tuner solution and the solution selected is the one with most sensitivity to load
impedance fluctuations or the load impedance might be such that the reflected power has
to be increased before the optimum tuner solution can be found. Therefore, using only
reflected power as the tuning indicator can be difficult
The impedance analyzer, which equipped with phase sensitive detectors that are
inserted in the waveguide, is located between the power source and the tuner and is
connected to a suitable oscilloscope where the load impedance is displayed in a polar
format identical to the Smith chart. With this display, the action of the tuner can be
precisely interpreted and the optimum tuner position can be obtained. Figure 5.5 shows the
construction of the impedance analyzer.
5.6 The 4-Stub l\in er
In operating the single mode microwave heating device, it is important to match an
arbitrary load impedance to the transmission line. In doing so, maximum power transfer
can be obtained and the standing wave which could appeared in the transmission line will
be eliminated. Although impedance matching could be realized by functions of the variable
iris and the variable short, ability to provide extra matching tuning is always necessary
when fixed iris is used or difficult matching load is encountered. The 4-stub tuner is
utilized in this set-up to tune single mode cavity during microwave processing of ceramics.
The construction of the 4-stub tuner is illustrated in figure 5.6. It consists of four
variable depth screws mounted on a fixed carriage free to move longitudinally to have
various penetration. The screw will act as a shunt capacitance for 1/b < 3/4 and a shunt
inductance for l/b>3/4 which is shown in figure 5.7. Therefore, the movable screw has the
same effect as the short circuited stub used in the low power microwave circuit for the
tuning purpose. The matching mechanism for a short circuited stub can be found in basic
Figure 5.5 Impedance Analyzer
79
3/8 A,g
Figure 5.6 A 4-Stub Tuner
B'
0.5
Figure 5.7 Characteristics of One Stub
80
electromagnetic book. Theoretically, three stubs can match arbitrary load in the
transmission line while four stubs are more than enough for all the matching purposes.
5.7 Iris
For a metallic enclosure to act as a microwave applicator where microwave can
resonant and materials can be heated in it, microwaves have to be coupled into the cavity.
Basically, three coupling methods are usually used in the microwave engineering: (1) probe
coupling, (2) loop coupling, and (3) iris coupling. In the high power microwave sector, iris
coupling is often accepted to avoid high current in the loop or the probe. According to the
shape and the position of the iris at the common boundary of the cavity and transmission
line, the effect of the iris can be inductive or capacitive. Some of them are illustrated in
figure 5.8. The effect of the iris can be to the first approximation equivalent to an electric
dipole normal to the aperture and have a strength proportional to the normal component of
the exciting electric field, plus a magnetic dipole in the plane of the iris and have a strength
proportional to the exciting magnetic field as shown in figure 5.9. The constants of
proportionality are parameters that depend on the iris size and shape. These constants are
often referred to as electric and magnetic polarizability of the iris and characterize the
coupling or radiating properties of the iris. For the iris used in this thesis, it is circular and
located at the transverse wall of the transmission waveguide, its normalized inductive
susceptance is
=
(5.1)
8 r 3 P„
where a is the dimension of the broad side of the waveguide, b is the short side of the
waveguide, r is the radius of the aperture and (3g is the wavenumber in the waveguide.
81
Inductive Iris
L
b
V '/////////////////a
*------ a------ ►
Capacitive Iris
Figure 5.8 Three Coupling Iris for Material Processing Cavity
82
Electric Polarization
i >i
■§)
Magnetic Polarization
Figure 5.9 Electric and Magnetic Polarization in the Iris
83
5.8 Resonator Cavity
Essentially, a resonant cavity, or a heater, consists of a metallic enclosure into
which a launched microwave signal of the correct electromagneticfield polarization will
suffer from multiple reflections between preferred directions. The superposition of the
incident and reflected wave is very well defined in the space. The precise knowledge of
electromagnetic field configuration enables the dielectric material under treatment to be
placed in the position of maximum electric field for optimum transformation of the
electromagnetic energy to heat. Inside the cavity, large stored energy will be transformed to
heat via displacement and conduction currents flowing through the dielectric material.
In this thesis research, the rectangular single mode cavity is used. Its resonant
mode is TE103. Therefore, the electromagnetic fields distributions in the cavity with no
dielectric material inserted are
Ey = r ? A K lQ3z Q a Sin 30L sin 3SZ.
y
71
a
d
Hx =
sin ^ cos
H x = -2 jA s i n c o s
a
( 5 .2 )
(5,3)
d
(5-4)
The geometry of the rectangular cavity and its electric field distribution are shown in
figure 5.10. Where a and b are the broad and narrow sides of the waveguide cross section
and d is along the Z direction and represents the length of the cavity. An important
parameter specifying the performance of the resonator cavity is the quality factor Q. In
general, the quality factor is defined as
q
_ 0 ) (time-average energy stored in system)
energy loss per second in system
^
^
When a cavity is at resonance state, the time average electric and magnetic energy stored in
the cavity are equal. The average stored electric energy is given by
84
TE103
Figure 5.10 Field Distribution in TE103 Cavity
85
W' = f
L ‘ L
j.EyEjdxdydz
(5.6)
=-^-aHdk5o3 4|A|2
The magnetic energy stored in the cavity is
wrn= ^ | 0' J0l
(hxH]i +HZHz)dx d y d z = We
(5.7)
In order to determine the Q of the cavity, the loss caused by the finite conductivity of walls
must be evaluated. For small losses, the surface currents are essentially those associated
with loss-free field solution. Thus the surface current is given by
Js= n x H
(5.8)
where n is a unit normal to the surface and directed into the cavity. Hence the power loss in
the walls is given by
P, = ^
[
Js J s* ds = 5m I IH tan I2 dS
J will*
(5.9)
/ W illi
where Rm= 1/ g 8s is the resistive part of the surface exhibited by the conducting wall
having a conductivity a and for which the skin depth is 8 S = ( 2/o)(ia )
1 /2
Htan is the
tangential magnetic field at the surface of the cavity walls. Substituting eq. 5.2,5.3,5.4 to
eq. 5.7, a straightforward calculation gives
Pl = (A) Rm2a3b + 2d3b + adL+da3
(5.10)
d2
By using eq. 5.5, the Q of the cavity is given by
Q - “ (We + W.)
(5 U )
When a ceramic rod, which has a relative permittivity e = e ’ - j e” is placed into the
cavity, the lossy dielectric has an effective conductivity toe”, and hence the power loss in
the dielectric is
86
Jfj
Pid =
dV =
= i2 I J • E
E **dV
=
I | E l 2 dV
(5.12)
Considering both dielectric and wall loss, the quality factor is then
q > (W ^ W .)
P| + Pid
In practice, energy is coupled to a resonator structure by means of an iris which can
be inductive and capacitive as discussed before. The iris used in this thesis research has
circular shape and it is therefore inductive. The admittance of such an iris is given by eq.
5.1.
For an empty cavity where only the circular iris and variable short are controlling its
behavior, the input admittance at the plane of the iris is
Yo
+ ----- j-1------ r
tanh (ot + j (ij
(5.14)
When a cavity is in resonance, the imaginary part of the input admittance is zero.
When it is matched, the real part of the input admittance will be unity. When both resonance
and match are realized, the cavity is said to be critically coupled. At that time, there will be
no reflection from the cavity and energy conversion efficiency is the highest. Hence, by
solving equations in eq. 5.15, it would be able to find the required radius of the iris and
the position of the variable short for critical coupling, i.e.,
I m a g .( ^ ) = 0
v Yo
R e .( ^ ) = l
(5.15)
When the dielectric material is inserted into the cavity, the equivalent impedance of
the inserted dielectrics usually cannot be expressed analytically. Such an impedance,
however, can be obtained through a measurement of the reflection coefficient may using
the afore mentioned reflectometer. A control algorithm can be designed for coupling and
matching. Correspondingly, the related control equipment can be arranged so that an
87
automatic control system can be made for optimal transferring microwave to heat for
material processing with best efficiency.
88
Chapter 6
IN SITU MICROWAVE HEATING, SINTERING
AND CHARACTERIZATION
6.1 Introduction
In developing microwave processing of ceramics, some problems need to be
solved such as basic scientific studies on microwave-materials interactions and loss
mechanisms. There is also a critical need for a broad data base on the dielectric properties
of materials at high temperature over different frequencies. In situ heating and
characterization of ceramics can be used to continuously measure material permittivity and
offer unique approach for understanding the interactions between microwaves and
materials. In this chapter, a method of using a high power single mode microwave heating
cavity to simultaneously heat or sinter and characterize densified or green ceramics is
described. Sintered ceramics rod was heated and characterized so that its dielectric property
at high temperature was retrieved. A green ceramic rod, which was produced by an
extrusion process, was simultaneously microwave sintered and characterized.
6.2 Historical Background
One function of in situ microwave heating and characterization of ceramics is that it
can be used to obtain the permittivity of materials. Knowledge of the permittivity of
materials at elevated temperature is generally needed to develop industrial application. Such
data is needed to develop process model that will predict the internal fields and the heating
pattern and rates, so that optimum processing parameters can be developed to meet material
and product requirements. Conventionally, dielectric properties of material at high
temperature are often obtained through a free space measurement technique as done by
Breeden [1969], Ho [ 1981], Gangnon et al. [1986] as well as Varadan et al. [1991]. In
89
the free space measurement technique, samples are positioned in the path of the incident
beam and the complex transmission and reflection coefficients are measured by two
identical receiving antenna suitably aligned with respect to the incident beam and the
sample. The dielectric properties are then deduced from observed transmission and
reflection coefficient. Through years of development, free space method seems to be a
widely accepted technique. The advantage of using the free space method is that with one
planar specimen, dielectric property of ceramic materials can be acquired for a wide range
of frequencies. However, the free space method suffers from a long period of
characterization time, due to the time needed to reach thermal equilibrium for a large planar
sample. Such a long time heating at a high temperature may very well alter the
microstructure of the material. More importantly, since the specimen being characterized
was heated by conventional methods rather than by microwaves, the microwave-material
interaction mechanisms were not revealed.
Contrary to the free space technique with conventional method of heating of large
planar specimen, in situ microwave heating and characterization method uses microwaves
to heat thin ceramic rod and that microwaves are also used to detect the variation of the
dielectric properties of ceramic rod with temperature. In some cases, another microwave
source which doesn’t interfere with heating source is also used as detecting signal. The in
situ microwave heating and characterization method has the advantages of obtaining a high
heating rate and uniform heating pattern due to volumetric nature of microwave heating.
Therefore, the required characterization time can be substantially reduced.
Couderc et al. [1973] was the first to conduct the research on in situ heating and
characterization. He used a cavity resonator which has two dominant resonating modes.
One of those resonances is used for heating the sample at frequency of 2.45 GHz where the
second resonances is used for measuring the dielectric properties. The shape of the test
specimen could be either spherical or rod. The highest temperature reached in his
experiment was 600°C. In 1984, Areneta et al. used an equivalent circuit to represent
dielectric rod in the waveguide. The iris and the in-perfect movable short which are part of
the cavity were represented by the impedances arrived from either analytically or
90
experimentally. As the temperature increases, the reflection coefficient will be changed due
to the change of the dielectric properties of the ceramic rod. This change was monitored by
a VSWR device. It is actually moving on a slotted line. During characterizing process, the
VSWR meter need to be constantly moved mechanically along slotted line. The dielectric
properties of the ceramic rod were deduced from measured reflection coefficient. The
characterization errors may have been accumulated during experiment because of the
movement of VSWR meter. Fukushima et al. [1987] used a rectangular cavity to heat and
characterize ceramic material with microwave energy. A precise control of the iris size and
the position of the variable short was adopted in their experiment to maintain the cavity in
the critical coupling state. The dielectric properties of the sample was measured by detecting
both variable iris size and position of the variable short which are controlled to give the
resonance and critical coupling. The highest temperature achieved was 1800°C.
Another application of the in situ microwave heating and characterization is to use
the technique to actually sinter and characterize ceramics. In doing so, on line temperature
and dielectric properties versus time profiles were measured during the sintering process.
Therefore, the structure variation versus microwave absorptivity of the green sample during
sintering can be determined. The mechanisms of microwave sintering of ceramics can be
fully understood. However, such an important experiment has not been realized at present.
Similar work has been done to the curing of resin and heating oil with microwaves. Jow et
al. [1987], Hu [1983] observed fast changing dielectric constant in resin curing and oil
shaking due to high power microwave irradiation.
6.3 Mathematical Model for the Characterization
The capability of in situ characterization when sintering is very useful in providing
insights into the various dynamic process associated with microwave sintering. In general,
in-situ characterization is important in giving insights into the nature of physical properties
and processing of the rod being heated.
The approach used in analyzing the cavity and rod is to model them by their
equivalence transmission line circuit representation and use circuit theory to determine the
pertinent equation for characterization.
In lieu of the above mentioned approach, it is important to have an accurate
equivalent circuit model for the rod in the microwave cavity. In this section, a brief review
of the development of such an equivalent circuit is given.
Cylindrical obstacles in a rectangular waveguide are used in many microwave
devices. Considerable efforts have been made by many authors to investigate such
structures. Applying Schwinger’s [1968] variational formula for the equivalent circuit,
Marcuvitz [1986] gave a model represent the rod by a T-equivalent circuit as displayed in
figure 6.1. To improve the approximation give by Marcuvitz, Nelson [1969] described a
numerical technique which removes the limitation on the rod diameter imposed by
Marcuvitz’s formulation. By calculating the reflection and transmission coefficient of a rod
in an infinite waveguide. The T-equivalent circuit is derived from those reflection and
transmission coefficients. Araneta et al. [1984] took a further step in M arcuvitz’s
derivation; he used more terms in the expansion which allowed more accuracy.
The pertinent expression given by Schwinger’s variational approach is as follow:
f r, . ry v
Ml + Ml ) 2j
(
{ ET - l ) k 2
—
—
( 6 . 1)
|
<J>o(x,z) dS - (e? -
<J>e(x,z) Ge (x,Z x’,z) d>e(x', z') dS'dS
Ceramic
Rod
Z ll - Z 1 2
Z ll - Z 12
Z12
Figure 6 .1 Equivalent Circuit of a Ceramic Rod in the Transmission Line
93
2j(eM)k2 _
ka(Z n - Z 1 2 )
(6.2)
<I>o(x,z)
c,z)dS
dS -(ej
- (ej -1)
- l) kk2^
21 JI (j)o(x,z) Go(x,z x',z')•J^x', z') dS'dS
OoVodS
where the integration is over the cross-section of the rod and,
G: = - -r- sin
sin
( sjn kI z- z'I ± sin kI z+z'l)
1
ka
a
a v
7
+
j ^
i
y
sin 21®25-sin-11®24- /
3
(6.3)
x
3 IgjlCol Z - Z I + gjKnl Z +z1ij
I
_______ ___ ______ ___
a n=2
K"
where the plus is used when i = e and the minus sign is used when i = o; and
<(>(x,z) = total electric field intensity
(x,z) = incident electric field intensity
k = 2 tiA
k2 = k2 . (mLj2 , Kl = k
(6.4)
(6.5)
e? = complex dielectric constant of the rod
K=
, X.g = wavelength in the waveguide
A.*
the subscripts e and 0 , respectively, denote even and odd symmetry about the reference
located at z = 0. The coordinate used is shown in figure 6.3.1. The width of the broad side
of the waveguide is “a” and the axis of the rod is the line ( x0, yo> 0 ). Eq. 6.1 and 6.2 can
94
be evaluated in conjunction with solution of the wave equation,
(v 2 + e*k2) $ =
(6 .6)
0
inside the rod.
r , e ) = £ ( A 2m cos m0 J 2 m(lcr) + A2 m+i sin (2m+l) 0 J 2 m+i(k,r)}, even
(6.7)
m=0
<»o(r, 0 ) = X
( f i2m+i
cos ( 2m+l)0 J 2 m+i(k'r) + B2 m sin 2m0 J 2 m(k'r)}, odd (6 .8 )
m=0
where
(k ? = e,* k 2
when the rod is at the middle of the waveguide, its axis is at x=a/2, z= 0. In this case,
Equation 6.7 and 6 . 8 may be simplified by symmetry consideration about the =a/2. Thus
(6.9)
r, 0) = X A 2 m cos m0 J 2 m(k'r)
m=0
e)= X
A 2m COS
(6 . 10)
m0 j 2 m(k'r)
m=0
In the derivation of the Marcuvitz model, ( Zj 1 + Z 1 2 ) and ( Zj 1 - Z 1 2 ) were
approximately by using only the first term of the <j>e and <}>0i in eq. 6.7 and 6 .8 . The results
are
j ( Zu + Z 12 ) = Ka c s c ^ f l X sin2
n2 -
}2 -1
11=2
- i - In (kac sin
- 1 dn27CX° +
2n \ k
» > it
a
4
(6 . 11)
Y° ^ ' a J o(P)Y i(«) }
a J 0(p )Ji(a )-p Ji(p )Jo (a ) _
95
s in ®
l + _La iM >
I 4n
Ji(ot) pj0((J)J1(ct) - otJi(p) J»(a)
Z n - Z12
(6. 12)
Considered the next term in the expansion of <J>e and (|>0,Areneta et al. [1984]
obtained another formula for the equivalent circuit for the case where rod is atthe center of
the waveguide. The form of <[>e and <f»0 used are as follows
(r, 0) = A0 Jo(kr) + A2 cos 20 J2 (kr)
<j>0 (r, 0 ) = Bi cos 0 Ji(k'r) +B 3 cos 30 J3 <kr)
(6.13)
(6.14)
Substituting eq. 6.13 in 6 .1 and eq. 6.14 in 6.2, it is obtained
1
_ : (e? - 1 ) k2 Cq - C0 C2 (Dq2 - D 20 ) / D 22 + C2D 00/D 22
(ZH + Z 12) J
ka
D 2 2 - (D 0 2 D 2 0 /D 2 2 )
_ ;{e? - 1 ) k2 Ci - C 1C3 (D 13 - D 31 ) / D 33 + C3D 11/D 22
{Zll + Z i 2 ) = j
D u -{D 13D 31/D 33 )
ka
(6.15)
(6.16)
All the unknown coefficients “ C’s “ and “ D’s ” are listed in the appendix.
6.4 Model Description
The model for in situ microwave heating, sintering and characterization is depicted
in figure 6.2. As given by Marcuvitz [1986], the effect of the ceramic rod in a waveguide is
modeled by the electric elements Z 1 2 andZ jj-Z ^- In figure 6.4.1, Z s is the impedance of
the variable short, whereas Z l is the impedance of the iris. PI and P2 are the distances from
the center of the ceramic rod to the position of iris and variable short, respectively.
96
Iris
I
I
r,z
Short
Ceramic
Rod
-| Zll - Zl 2
- H . Z11-Z12
Zl
Zs
I
J
Figure 6.2 Equivalent Circuit of a Ceramic Rod in the Cavity
97
The measured reflection coefficient I \ and resulting impedance, Z, can be expressed in
terms of ZI, ZS, Z ^ a n d Z ^ -Z ^ , i.e,
( 6 - 1 7 )
where
z -Z o a n y i)
Zo - Z tan^yPl)
^
v
'
(6l9)
Zl21 = Zn- Z12 + Z|2(f7“
~2 ‘^ +r Sc)
(Z ll + ZSC)
+ z otany 2)
ZSc“ Z°Z0 + ZStanh(7P2)
=
z
q
z
s
(6-20)
^ 6
2
1
)
In the case when the dielectric constant and loss tangent of the ceramic rod are
small, which is true for many ceramics, Zt l~Zl2 is also veiy small and can be neglected in
the modeling. Hence
Zc
ZIc
+^L_ + ^L_
ZSC Z 1 2
(6.21)
By solving the complex equation,
(6 2 2 )
the complex permittivity ( e'-je") can be deduced.
6.5. Impedance of the Iris and the Variable Short
The impedance of a iris with circular shape made from a thin metal plate can be
modeled by a inductive susceptance element in the transmission line as given by eq. 5.1.
98
Since the iris used here is made of an aluminum plate which is not a perfect conductor, its
value has to be characterized.
Ideally, the impedance of the short is zero and it reflects all the energy incident on to
it. For the variable short used here, which is constructed in a way to prevent sparking at
high microwave power, its impedance is not zero.
It is necessary therefore that the impedances of both iris and variable short have to
be characterized. To do so, a network analyzer system as shown in figure 6.3 has to be
used for the measurement. To obtain the impedance of the iris, the set up in figure 6.4(a) is
used. Its equivalent circuit is shown in figure 6.4(b).
The measurement of the impedance of the short is accomplished by using the set up
as shown in figure 6.5(a). The equivalent circuit is displayed in figure 6.5(b).
The general procedures for obtaining impedance value of the iris and variable short
is to get reflection first and then to convert this reflection coefficient to appropriate
impedance value according to the given equivalent circuit.
Before making any measurement, the network analyzer system has to be calibrated
by using a TRL calibration technique as shown by Ghodgaonkar et al. [1989].
6.6 Measurement of the Reflection Coefficient
Analyzing the characterization model stated in section 6.4, it is easy to see that an
accurate measurement of the reflection coefficient in the transmission line is crucial for the
entire characterization process. Traditionally, the slotted line has been the instrument for
measuring reflection coefficient. The measurement technique with slotted line needs to
move detector mechanically, so it is not ideal for automatic system and also the
measurement may suffer from such a mechanical move. In this section, a generalize
multiprobe reflectometer is used so that direct connection between measuring devices and
digital computer via analog-to-digital converter can be realized.
The idea of using multiprobe reflectometer to measure reflection coefficient has
HP Vectra PC-308
Technical
Computer
Synthesized
Sweeper
0.01-40 GHz
HP 8510B
Microwave
Network Analyzer
HP 7440A
Plotter
S - Parameter
Test - Set
HP 82906A
0.045 - 40 GHz
Printer
Port 1
Coaxial to
Waveguide
Transition
l i l t
waveguide
Coaxial to
Waveguide
Transition
Figure 6.3 A Vector Network Analyzer
100
Iris
Zo
Zo
Waveguide Transition
(a) Iris Impedance Measurement
(b) Equivalent Circuit
Figure 6.4 Measurement of the Impedance of the Iris
Zo
Waveguide Transition Variable Short
(a) Variable Short
Impedance Measurement
(b) Equivalent Circuit
Figure 6.5 Measurement of the Impedance of the Variable Short
101
been studied by Caldecott [1973]. Here the multiprobe reflectometer is a modification of the
impedance analyzer introduced in chapter 5. As stated in chapter 5, the impedance analyzer
has four detectors evenly spaced with a spacing of 3A,g/8.
The details on how to obtain the reflection coefficient by using the multiprobe
reflectometer, as shown in figure 6.6, are discussed as follows. Since the operating
frequency of the system is known, only the incident power and the phase and the amplitude
of the reflection coefficient are not known. There are three unknowns and information
obtained from the three probes is in general sufficient to evaluate them. Let V be the
incident voltage and Vn the standing wave voltage on the transmission line at the location of
the nth probe. Following Caldecott's [1973] treatment,
V„ = V{ 1 + pexp 0(9-<}>„)])
(6.23)
N 2 = V2 { 1 + p2 + 2pcos ( 0 - (J)„))
(6.24)
where p and 0 are the magnitude and the phase of the reflection coefficient of the load and
<|)n is the phase shift corresponding to the distance from the probe to the load and back,
where <(>„ is taken as positive and must be accurately measured. The power at each probe
position is linearly proportional to the I Vn I2. Therefore,
P„= P { 1 + p2 + 2pcos ( 0 - <>„)}
= P ( 1 + p2 + 2p (sin0sin <(>„ + cos0cos<t>n) }
(6.25)
let
A = 2Pcos 0
B = 2P sin 0
D= P ( 1+p2 )
P n = |V „ P
(6.26)
Source
load
Figure 6.6 Three Detector Reflectometer.
103
After some mathematical manipulation, the final results are
0 =arctanfiA
(a 2
"
p
B2)
on
2P
4-
(6.27)
2
= A-( d + ( d 2 - a 2 - b 2 £1
(6.28)
(6.29)
The DC voltage outputted from detector and input microwave power satisfies the
square law relationship when input microwave power is small. For the experimental work
performed here, the detectors are not operated in such a square law region because high
microwave power has to be used to heat the ceramic rod in the cavity. It is therefore
necessary that the relationship between output DC voltage from detector and large input
microwave power has to be established. To do so, a dummy load which has the same
impedance as the transmission line is attached to the reflectometer. Since the attached load
is matched, there would be no reflection, i.e. p = 0 and the power at each detector position
is same. By adjusting input microwave power to the transmission line and measuring the
corresponding DC voltage form each detector, it is easy to establish corresponding relation
between input microwave power and DC voltage from each detector. Although it is
necessary to carry out such a calibration every time the characterization process is carried
out, the general trend of the DC voltage and input microwave power is depicted in the
figure 6.7. It shows that the relation is cubical. To acquire desired reflection data, the
measured DC voltage output results will be transformed to the power by using that
established relation. Subsequently, those transformed data will be substituted to the
equation 6.28 to get reflection coefficient
6.7 Effect of Iris on the Reflection Coefficient Measurement
In the in situ microwave heating and characterization set-up, the impedance analyzer
is located immediately ahead of the rectangular cavity which is excited by a circular iris. To
104
DC Voltage From
Detectors { V )
0.4
0.3
0.2
u—
•—
**"
0.1
detector 1
detector 2
detectors
detector 4
0.0
0
1000
2000
3000
Input Microwave Power ( w )
Figure 6.7 Input Microwave Power vs. Output DC Voltage from Detector
105
satisfy the boundary condition, higher microwave modes have to present in the region of
the iris. Those modes are the so-called evanescent modes, i.e., they will be attenuated
along transmission line. Since impedance analyzer is just ahead of the cavity, the effect of
those modes need to be considered. According to the microwave theory, the cutoff
frequency for for both TE and TM wave is
(6.30)
the wavelength at the cutoff frequency is
t, -
A.
(6.31)
The corresponding propagation constant is therefore
Yg = Vkc2 ' k2
■
w
(6.32)
w
So the electric field will have the form
E = Ebe'Y«z
(6.33)
and the time average power has the form
p = p0 e-2Y*z
(6.34)
Few higher modes which could effect the measurement are listed in table 6.1.
According to the dimension of the impedance analyzer, first crystal detector is
8A,g/3 at 2.45 GHz. So the possible effects of the higher modes are minimum. Hence, the
value obtained from the crystal will not be biased from those degenerated modes.
6.8 G reen C eram ic Rod P reparation
Green ceramic rod is needed for performing in situ microwave sintering and
characterization. Such a green ceramic rod is rarely available in the market due to its fragile
106
nature in the green state. On the other hand, the desired size and particular composition
make it even harder to obtain green ceramic rod samples. Therefore, it is necessary to find a
way to make such a specimen.
In general, green ceramic rod can be made with various processing methods, such
as pressing, extrusion, slip casting and injection molding, etc. Among those processing
methods, extrusion has proven to be an economical and often a necessary way to produce a
large piece of ceramic ware of either regular or irregular cross section. With this
consideration and the availability of the processing machine, the extrusion method is
adopted to make the green ceramic rod.
Extrusion is the shaping of the cross section of a cohesive plastic material by
forcing it through a rigid die. Products formed by extrusion include structural refractory
products, hollow furnace tubes, honeycomb catalyst supplies, transparency alumina tubes
for lamps and flat substrates and tile products. In the extrusion process, the feed material is
usually plastic which is commonly formed by directly batching and mixing the raw material
and additive in a high shear mixer.
Since the green alumina rod is needed in the experiment, alumina powder with
small particle size is selected. The alumina powder used is Baikalox CR30 alumina with
average particle size of 0.1 micron. The powder has a major alpha phase above 85%. The
content of sodium, potassium, silicon, iron and calcium are less than 40PPM, so the purity
of alumina is better than 99.99%, Alumina powder doped with 0.3% wt. MgO was ball
milled in methanol for two hours in a plastic jar. The mill ball used is made of zirconia.
The mixed slurry is then dried in a glass tray for about 12 hours at a temperature of 70°C.
To make the plastic body for the extrusion processing, the alumina powder must be mixed
with certain binder so that the slurry would have desired rheological properties. In this
study, the composition of the binder and the ceramic powder is taken from the research
results of Bruch [1972]. In their research study, similar particle size of alumina powder
were used. Therefore, the 20% wt. PVA ( polyvinyl Alcohol) is first mixed with distilled
water in a glass beaker. Then 50 vol.% binder and 50 vol.% of alumina powder are
mixed in a shear mixer. The mixer has two mixing heads. The rotation speed of the mixing
107
Thble 6.1 Characteristics of Non-Propagating Mode in the Waveguide
Frequency
k (m m )
Power
Field
Y(l/mm) Attenuation (db) Attenuation (db)
TE20,TEoi
109.2
0.02599
38.47
19.23
TEn.TM n
97.67
0.03877
25.79
12.89
TE21.TM21
84.28
0.05406
18.49
9.24
72.8
0.06938
14.41
7.20
TE30
108
heads is 20 rpm. To reduce the heat generated by the friction between the powder and the
mixing heads which are made of stainless steel, the mixer is also cooled by pressured air.
After the powder and binder are mixed together for about two hours, the mix can be fed to
the extruder. Samples are collected through a glass tray. The relative density of the green
alumina rod is about 41%.
6.9 Experimental Set* up and Characterizing Procedures
A schematic diagram of the automated dielectric characterization system is shown in
figure 5.1.1. As a main controller, the combination of a HP9826 personal computer and a
HP3494 data acquisition system is used. The output voltages from crystal detectors are
related to the input microwave power. The voltage from the temperature controller is
linearly related to the temperature. The voltage from the linear voltage displacement
transducer indicates the position of the variable short. Those voltages will be collected by a
HP3494 data acquisition system controlled by an algorithm written in HP BASIC. The
geometric data of the ceramic rod, waveguide dimensions and operating frequency are
imputed in advance to the data file. To increase the experimental accuracy, the EIP578
frequency counter is connected to the characterizing system to monitor operating frequency.
Upon turning on the microwave generator, the variable short is adjusted to the
resonating position so that the microwave power can be absorbed by the ceramic rod
efficiently. The output voltages from three detectors, the voltage from the temperature
controller and the voltage from the linear voltage displacement transducer were acquired by
the data acquisition system at a prescribed temperatures. These raw data were then
transformed to the input microwave power at each detector. The magnitude and the phase
of the reflection coefficient were found by solving the nonlinear eq. 6.28. Eq. 6.23 is used
to set up a complex nonlinear equation to find real and imaginary parts of the complex
dielectric constant.
When solving a complex equation, the real and imaginary parts of the function
F(xj, x2) are assumed to be fj and f2. The complex equation therefore becomes
109
ftfxi, x2) = Re(
=0
(6.35)
f2(*i, *2 ) = Im ( F{x1? x2)) = 0
(6.36)
F ( x i , x 2 ))
Let X denote the vector of value ( xl, x 2 ) then, in the neighborhood of X, each of the
function f j can be expanded by using Taylor’s series
2 "jr
fi( X + 8X) =fi( X )+ £
8x: + O (8 X2 )
(6.37)
j=l dxj
By neglecting terms of order dX2 and higher, a set of linear equations for the corrections
dX that moves each function closer to zero can be obtained. They are
X otij 8xj = f t
i=i
(6.38)
where
aij ^
(6.39)
9xj
Pi = -fi
(6.40)
Matrix equations can be solved by Krammer's rule. The corrections are then added to the
solution vector,
xjiew = x9ld+8xi , i= 1,2
(6.41)
and the process is iterated to convergence.
6.10 R esults and Discussion
1. Simultaneous Microwave Heating rod Characterization
A Coors
AD-998 alumina rod with a diameter of 0.8cm was used in the
characterization experiment. Figure 6.8 gives the thermal historical during microwave
heating and characterization. To exclude any microwave susceptor effects, the alumina rod
was heated with microwave alone. Temperature was measured with a pyrometer. Since the
110
work range of the pyrometer is at 500-1 SOOT, the thermal history under SOOT was not
given. Since the pyrometer can only measure surface temperature, the highest temperature
measured is only 1200T. During the characterization process, the position of the variable
short has to be adjusted so that the microwave cavity can be always in resonant status.
Figure 6.9 gives the relationship between position of the variable short and the time. It is
easy to see that the position of the variable was gradually moved towards the iris plate with
increasing time of heating. The energy was absorbed by the ceramics by properly adjusting
the variable short. This is an expected result. As the temperature goes up, so do the
dielectric constants of the alumina rod, which essentially shorten the length of the cavity for
resonant. The magnitude of the reflection coefficients measures by the impedance analyzer
are plotted against the time as shown in figure 6.10. It is seen that the magnitude of the
reflection coefficients are very close to 0.3 during entire characterization process. The
energy reflected was only around 10%. The dielectric constants, the real part and the
imaginary part, are given in figure 6.11. In figure 6.11, the results from Fukushima et al.
[1987] were also plotted. The material used here is essentially 94% alumina while
Fukushima’s is 92% alumina. It is easy to see that the results obtained here are compatible
to that of Fukushima’s. Generally speaking, both the real and imaginary parts of the
dielectric constants of AD-998 have smaller value at high temperature than that of 92% at
same temperature. At room temperature, AD-998 has a larger value that that of 92%
alumina.
2. In Situ Microwave Sintering and Characterization
Alumina green ceramic rods made from the extrusion process as stated in 6.8 are
used in this experiment. Unlike densified ceramics, green ceramic rod which is not
densified will be sintered or consolidated in the microwave heating process. Sintering is
usually finished in such a way that the temperature of the green ceramics is raised to half or
two-thirds of the melting point of the material and kept there for a certain amount of time;
the green ceramics will be densified during this dwell time. In order to be able to get the
1200
1000
u
TJBT
-
800600-
12
400-
I
£
200-
0
10
20
30
Time ( min.)
Figure 6.8 Thermal History of Microwave Heating and Characterization
Short Position (m)
.012^ . 011-
.007-
Time (min.)
Figure 6.9 Position of the Variable Short vs. Time
112
1.0-r*
Reflection Coefficient
0 .8- -
0.0
Time ( min.)
Figure 6.10 Reflection Coefficient vs. Time
113
2.0
1.5
e
"
1.0
0.5
0.0
0
250
500
750
1000 1250 1500 1750 2000
18
AD-998
92%ALUMINA
16
14
e'
12
10
8
0
1000
2000
Temperature ( 0 C )
Figure 6.11 Measured Dielectric Properties of Coors AD-998
114
information about microwave sintering, the whole experimental procedure can be divided
into two stages, similar to the conventional process. The first stage is the initial heating
where the green ceramic rod is heated to 1200°C. The temperature is measured by using a
pyrometer described in section 7.2, the measured temperature is actually the surface
temperature. The inside temperature of the ceramic rod would be much higher. A higher
surface temperature is found to be harmful to the sample.
Figure 6.12 shows the history of the initial heating and subsequent sintering time.
At the preheating stage, no densification occurs. The second stage is the sintering stage,
where green ceramics will start to sinter. Since the information about the diameter and the
density of the ceramic rod is needed, ceramic rods are heated to 1200°C and sintered
according to a prescribed schedule. Figure 6.13 gives the densification history of the
ceramic rod after it reaches 1200°C. Figure 6.14 indicates the diameter of the ceramic rod at
different sintering stages. The variation of the diameter of the ceramic rod will be coupled
in to the computer program in retrieving dielectric data. Figure 6.15 demonstrates the
variation of dielectric properties of the green ceramic rod during microwave sintering
process. The variation of the dielectric constants reflect the microstructure change in the
microwave sintering.The real part changes very little in the initial heating stage, where only
natural physical property changes of the ceramics are displayed. In the sintering stage, the
ceramic material is becoming densified and therefore the dielectric constant is also changing
more or less according to the mixing rule. For the imaginary part of the dielectric constant,
it changes rather rapidly in the initial stage where it is believed that intrinsic conduction
increases loss rapidly with increasing temperature. In the sintering stage, the variation of
the imaginary part of the dielectric constant was quite slow compared with the initial stage
of the heating. This is due to two mechanisms. On the one hand, the mixing rule is still
valid so the dielectric loss would increase with densification. On the other hand, the
reduction of the pore size and number would decrease the multiple scattering loss in the
ceramics.
It is observed in this characterization process that microwave sintering and
characterization is more complicated than microwave heating and characterization. In the
1400-r-
Temperature ( ° C )
1200—
1000-
-
800—
600—
400
Time ( Min.)
Figure 6.12 Microwave Sintering of Alumina Ceramic Rod History
116
Density of the Green Ceramic Rod ( %)
0.60
0.55 -
0.50
0.45
0.40
0
10
20
30
TIME ( m in.)
Figure 6.13 Density vs. Sintering Time
40
50
117
Diameter of the Green Ceramic Rod (cm)
0.94
0.92
0.90
0.68
.86
°
Column 3
0.84
0.82
0.80
0
10
20
30
40
50
TIME ( min. )
Figure 6.14 Variation of the Ceramic Rod Diameter with Sintering Time
118
5.0
4.5
i
4.0
3.5
3.0
0
10
20
30
40
50
40
50
TIME ( m in.)
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
10
20
30
TIME ( m in.)
Figure 6.15 Variation of Dielectric Properties of the Green Ceramics
During Microwave Sintering
119
sintering process, the ceramic rod not only densities in the axial direction but also in the
longitudinal direction such that green ceramics are continuously brought to the cavity which
creates densification gradients. The heat loss at both ends of the ceramic rod will attribute
to the nonuniform densification. Hence, the density of the ceramic rod could not be
brought to a higher value. The obtained dielectric data is an average one.
There are many factors could effect the accuracy of this characterization technique
such as the mathematical model, the impedance of the iris and variable short, the
imperfection of the cavity which act as a transmission line and measurement of the
reflection coefficient, etc. Although every effort has been made to enhance the accuracy of
this characterization technique, there is still room for improvement To be able evaluate this
procedure, some standard samples were measured for comparison with known results from
Ghodgaonkar et al. [1989]. Table 6.2 gives the results for both Teflon and Quartz rods. It
is seen that the results obtained from this technique are quite close to the known results.
For the real part of the dielectric constant, it is observed that the error is about 6.8%. The
imaginary part of the dielectric constant was not able to compare.
6 .1 1 Conclusions
A system has been established for microwave heating, sintering and characterizing
of both densified and green ceramics. Mathematical model and experimental procedures are
presented for such a characterization. The system is useful in revealing the interactions
between ceramics and microwaves. The method is convenient and the speed of
characterization has been dramatically increased. The established method is especially
promising for characterizing microwave sintering of ceramics.
Because of heat loss at surface and both ends of the sample, the temperature
measured by the pyrometer at surface is less than that of inside. Therefore, the measured
dielectric values are average properties of the materials.
The appearances of the temperature gradients at surface and both ends of sample
made the characterization process very difficult. The relationships among diameter of the
120
Table 6.2 Real Part of the Dielectric Constant of Teflon and Quartz
^ ' s N Material
Methoa'V^
Teflon
Quartz
This Research
2.158
4.01
Published
2.02
3.82
121
rod green sample, density and time are difficult to obtain. A sophisticated experimental
set-up has to be developed, which would be able to rotate and move the sample
longitudinally, to guarantee no temperature distribution gradient A good insulation material
which is microwave transparent has to be used to reduce the temperature distribution in
axial direction. On the other hand, the resonator, iris and variable short have to be finely
made so that their properties are closer to that of the ideal elements.
122
Chapter 7
APPLICATION OF MICROWAVE ENERGY
TO CERAMIC PROCESSING
7.1 Introduction
The use of microwave energy for the processing of ceramics is a relatively new
concept which is still very much in its infancy. Preliminary investigation has shown that it
is applicable to drying, slip casting, calcination, sintering, joining, plasma assisted
sintering and chemical vapor deposition. Its potential is considerable since a number of
very distinct advantages have been claimed for the technology. The volumetric nature of
power deposition and the subsequent inverse temperature profile appears to offer four main
advantages over the conventional sintering. Those may be summarized as i) the possibility
of heating large shapes very rapidly and uniformly; ii) a reduction in thermal stresses which
lead to cracking during processing; iii) the potential for sintering ceramics “from the inside
out”; and iv) since the absorption of microwave energy varies with the composition and
structure of different phases, selective heating is possible, creating the possibility of
generating new structure with useful properties. A further possible benefit, which still
requires verification, is the possibility that microwave radiation results in “ non-thermal”
effects during the processing of materials. With respect to the sintering of ceramics, this
has manifested itself as apparent reduction in activation energies for the atomic movement
processes involved. The evidence for this phenomenon consists of reduced time and/or
temperature requirements; however, the results are as yet inconclusive since there are still
many difficulties associated with temperature measurement during microwave processing.
The two principle reasons for this are i) the effects of microwaves on thermocouple-they
induce currents, and ii) the existence of the inverse temperature profile which needs to be
corrected by setting up proper insulation scheme. Such an inverse temperature profile
means that the surface temperature, which most temperature measurement devices have
recorded, is lower than the internal temperature, which controls most diffusive processes.
123
Nevertheless, there is now evidence from a growing number of sources that activation
energies might be affected in some ways.
In lieu of the practical needs in applying microwave heating to ceramic processing,
this chapter will be devoted to the discussion of some fundamentals in microwave
processing, such as insulation and temperature measurements, and to giving some practical
applications. The insulation schemes used in microwave processing to guarantee uniform
temperature distribution are reviewed first. The principles of temperature measurements
with pyrometer and thermocouple are stated. Special attention is paid to the temperature
measurement with thermocouple in a single mode cavity used in this thesis research for
microwave heating. In doing so, the effect of microwave on the temperature measurements
is revealed. Finally, microwave energy is applied to binder burnout for ceramic tape and
sintering alumina/c-zirconia composites. The results show that microwave processing has
many advantages over conventional processing.
7.2 Insulation Considerations in Microwave Processing
For microwave processing of ceramics, uniform electric field distribution, or
practically no temperature gradient, is needed to insure uniform microstructure in the entire
ceramic product. By analyzing the mechanism of microwave and conventional heating, it is
not difficult to see that using microwave alone will not guarantee a uniform temperature
distribution throughout the sample because of the heat loss due to radiation and convection
at the boundary. Conventional heating will slow down the heating rate because ceramics
are not good heat conductors. Naturally, the hybrid heating technique is considered to
provide initial and uniform heating during microwave processing.
Levinson [1972] suggested a method for heating ceramic article that is not self­
heating in a microwave furnace by surrounding the articles with finely divided particles of
materials that do couple strongly with microwave fields. In particular, he cited the use of
finely divided carbon powders, iron ore, magnetite, and radio-frequency ferrite powders.
Paterson [1975] dealt explicitly with the problem of “runaway” heating of material when
124
using microwaves. In his case, the material used is nylon, which exhibits “a rapid change
in the rate of heating takes place beyond a specific temperature with the rate of temperature
rise being exponential.” The poor thermal conductivity of nylon coupled with the
“runaway” heating behavior leads to localized melting and generally poor control of
thermal processing. The solution to this problem proposed by Peterson was to coat the
nylon article being heated with a second material. Initially, the coating is heated
preferentially to the nylon by microwave. Because the coating does not undergo thermal
runaway, heating is accomplished in a controlled manner. Methods specific to the heating of
ceramics were also proposed in the last decade.
Nishitani [1979] reported incorporating lossy, conductive particles in a dielectric
material to effect microwave heating. The only requirement he cited is that the bulk
conductivity of lossy material must be > 103£}/cm at high temperature and that the diameter
of the particle must be within S to 10 time the skin depth of the bulk conductor. Sutton and
Johnson [1980] reported additional findings using “noncoupling” oxides and “strong”
coupling materials. They claimed a range of additive between 1% and 90%, which covers a
much large range of additive than that claimed by Nishitani. Holcombe and Dykes [1990]
worked specifically with zirconia, yttria and alumina and combined both external and
internal indirect heating methods. They described the heating of the ceramic article as
occurring in three stages. Initially, the microwaves couple to the zirconia insulation
surrounding the article, which transfer the heat to the part by conduction; Holcombe and
co-workers called this the “electric blanket effect” and referred to the zirconia insulation as a
microwave “Pump.” At 700°C to 1000"C, the second stage of heating is initiated. The third
stage of heating starts when the major-phase material begins to couple, which for most
ceramics occurs above 1000*C. Recently, Janney et al. [1992] used a so called “picket
fence” arrangement for sintering zirconia. They argued that a “picket fence” provides
uniform field distribution and uniform heating. The arrangement of the picket fence
consists of five SiC rods surrounding the zirconia part to be microwave sintered. Zirconia
bulk fiber is placed between and around the parts, and the entire construction is enclosed in
an alumina fiber crucible. They further said that such an arrangement would facilitate
125
controlled indirect microwave heating at low temperature and controlled direct heating at
high temperature. The application of a hybrid heating technique was extensively
investigated by Arindam et al. [1990]. The arrangement used by Arindam is similar to
Janny’s. The ceramic sample is surrounded by the microwave transparent insulation
material. Several SiC rods are placed around the whole structure. By using this kind of
construction, they found that microwave hybrid heating results in an improved parity in
temperature across specimen cross-section vis-a-vis conventional fast firing and stand­
alone microwave sintering. This enhanced parity in temperature is said to be responsible for
the better microstructure homogeneity and improved mechanical properties relative to the
conventional fast firing. They also found that for large ceramic specimens, microwave
hybrid heating has many advantages over conventional fast firing and stand-alone
microwave sintering in terms of microstructure uniformity and mechanical properties
relative to smaller sample.
Following the above analysis and modeling results of Chapter 3 and Chapter 4, the
insulation scheme used in this thesis research is shown in figure 7.1.
7.3 Temperature Measurement
7.3,1 Introduction
Temperature measurement is an important issue in microwave processing of
ceramics. An accurate measurement of temperature is vital for correctly assessing the
effects of microwaves on ceramic processing such as enhanced diffusion, lowering
sintering temperature. Currently there are two measurement techniques employed by
researchers in the microwave processing research community. One is to use a pyrometer,
which is primarily dependent on the radiation of the infrared light of the ceramic body at
high temperature. Another is to use a regular thermocouple with proper electromagnetic
shielding. Because both radiation and microwaves could affect those measurements, it is
important to understand those measuring mechanisms so that proper correction can
126
Insulating
Material
Microwave
Susceptor
Ceramic Sample
Thermocouple
Insulating Material
Figure 7.1 Insulation Scheme for Microwave Processing
127
be made for obtaining accurate temperature measurements.
7.3.2 Pyrometer Measurement
The energy radiated by any object at a particular wavelength is strongly dependent
on the absolute temperature, a dependence described by Plank’s equation. A measurement
of emitted radiation from a target at certain defined wavelengths allows one to make a
calculation of absolute temperature. Typically, an optical filter is chosen which limits the
radiation collected by the detector to a certain selected range of wavelengths. This allows
for tailoring the characteristics of the detector and surface properties of the target material.
The pyrometer sensor is essentially a small telescope, designed to gather light from
an incandescent target at greater distances and with finer spectral resolution. Major
advantages also include simplicity of installation, greater ease of high temperature
measurement and again, no microwave interference in the measurement. However, the
variation of surface structure during microwave processing of ceramics could change the
emissivity of the matter to be measured which in turn effects the temperature measurement.
To solve this problem, the emissivity of the ceramics during different processing stages
needs to be stored so that correct measurements can be made.
7.3.3 Thermocouple Measurement
7.3.3.1 Introduction
In practice, sparking is often encountered when a thermocouple is used directly to
measure temperature in the presence of the electromagnetic fields. In this thesis research
work, it is found that sparking is more likely to happen in the multimode commercial
microwave oven than in the single mode cavity where the thermocouple is inserted into the
cavity through a non-radiating hole in the short side of the waveguide wall as illustrated in
figure 7.2. The detailed effect of the electromagnetic field on measuring temperature with
128
thermocouple is still quite unknown. Here, some qualitative analysis is given to explain
often observed phenomena which may provide some insights to temperature measurements
with a thermocouple.
7.3.3.2 The Principle of the Thermocouple
A thermocouple is a device that converts a temperature difference into an
electromotive force called Seebeck voltage. Thermocouples are usually made of two
dissimilar metal wires connected so that one junction is held at a reference temperature and
the other junction serves as the temperature sensing device. Thermoelectric measurements
then require (1) a sensing element connected through a reference junction by (2) electrical
lead wires to a (3) voltage measuring instrument.
The operation of the thermocouple measuring temperature relies on the Seebeck
voltage, which was discovered by Thomas Johann Seebeck in 1821 in Germany. He
discovered that when two wires of different compositions were connected at their ends
only, an electrical current would flow in the circuit if one of the connections was heated.
Efforts were also made by Jean Charles Athanase in 1824 in France, A.C. Becquerel in
1826 in Paris, and Lord Kelvin in 1847 in England. Henri Le Chatelier, in 1885 in France,
proposed a thermocouple with pure platinum as one leg and platinum 90, rhodium 10 as the
other, making it possible for a thermocouple to be used for practical applications. In 1927 it
was adopted as the sensing element for Range III of the IPTS and is still used for that
purpose.
7.3.3.3 Theory o f Thermoelectricity
Most of the thermocouples are based on metal conductors, although ceramic
conductors are also used. The theory of metallic conduction is a reasonable base for the
qualitative discussion of thermoelectric measurement.
The band theory of metal is based on the concept of metal nuclei arranged in a
129
periodic crystalline array in such a way that the outer electrons of each atom come so close
together that the Pauli exclusion principle requires them to be arranged in a quasi-free
electron cloud around the nuclei. The energy levels of these electrons are degenerate; that
is, they are collected together in bands of energy so that any electron in the band can have a
particular allowed energy state. Because every metal or alloy has a unique electronic and
crystalline structure, the allowed energy states and their electronic population will also be
unique. Therefore, when two metals come in contact, the electrons in the metal with higher
energy will flow into the one with lower energy. This will occur at the junction of the two
metals until the excess electrons in the metal of lower energy build up a reverse EMF
which, if current is allowed to flow by making a second connection at a different
temperature, can deliver heat at the first junction equal to the product of current and EMF;
that is the heat equals the electrical energy, or
dQ = 7t I d0
(7.1)
where dQ is the heat delivered at a junction in time dt by a current I. The Peltier EMF
depends only on the temperature and the two junction materials. It is fundamental potential
responsible for the Seebeck voltage. Thus the Petilier voltage for two materials, A and B,
with junctions at temperature T1 and T2 when current flow is zero, is
«ABT i -T2 = {*AB>ri -
(7.2)
The Seebeck voltage is the net voltage for such a junction. It includes another term,
which also depends on the electronic structure of A and B. It depends on the two Thomson
voltages, which are fundamental reversible thermodynamic quantities that can deliver heat
if current is allowed to flow. The Thomson voltage depends on the way the Fermi energy
of each conductor varies with temperature. In simplest form it can be written
lomson
where a is an empirically determined Thomson coefficient. Then the first law of the
thermodynamics requires that
130
dQ r = vTT dO =
adT ID 6
(7.3.b)
1 .
The direction of the heat flow depends on the direction of current flow, being delivered in
the direction the electrons travel. The Seebeck voltage, then, is the sum of two reversible
thermodynamic electropotentials at open-circuit conditions: the Peltier potential and the
Thomson potential. The latter is the net Thomson potential depending on the difference in
Ef for the temperature range imposed. In equation form,
VS= VP + VT
(7.4)
The Seebeck coefficient a s is often called thermodynamic power and is determined, for
materials A and B, as
dVs = ocab dT
(7.5)
7.3.3.4 Thermocouple in the TE103 Cavity
The arrangement of a thermocouple in the cavity for measuring temperature is
shown in figure 7.2. The effect of the electromagnetic field on the function of the
thermocouple can be analyzed as follows. The field distributions in a TE103 mode cavity
are
(7.6)
(7.7)
131
Iris
v
CeramicRod Variable Short
Non-Radiating Hole
Thermol Couple
Figure 7.2 Measuring Temperature with Thermocouple in the Cavity
132
E y =
Ei03y sin ( ^ ) sin (3az)
(7.8)
according to the boundary condition at an interface, the electrical field which is
perpendicular to the conductor could generate electric charge and the magnetic field which
is parallel to the metal conductor surface could create surface current These currents may
prevent normal measurement of the temperature. In the case of TE103 mode cavity, the
magnetic field which is parallel to the thermocouple direction is very close to the minimum
position and will create negligible currents on the thermocouple.
On the other hand, the thermocouple can be seen as a coaxial probe which will
couple electromagnetic energy to the cavity. The thermocouple wire that is the conductor
forms the center conductor of the coaxial line. According to the waveguide exciting theory
of Montgomery et al. [1948], such a probe can only excite the TE01 mode where the
electric field is parallel to the wide side of the waveguide rather than to the short side.
However, such a mode cannot exist in the cavity because of the frequency of the
microwaves and the shape of the waveguide. By using the reciprocity theory, it is easy to
see that such a setup will not couple any energy to the coaxial line because the mode of the
cavity is TE103.
From another point of view, any waveguide mode that has a nonzero electric field
along the probe will excite currents on the probe according to the coupling theory. Since the
resonant mode used in the cavity is TE103, there is no electric field parallel to the
thermocouple direction. The coupling of microwave energy to the coaxial line is therefore
minimum. On the other hand, the electric field distribution has to be disturbed by insertion
of the thermocouple which could be seen as a metal wire. Such a disturbance could
generate some other modes different from the TE10 mode and have electric fields parallel to
the thermocouple direction. Those parallel electric fields could excite electric current on the
thermocouple which would effect the temperature measurement Some of the electric fields
other than TE10 mode which could be generated in the cavity are listed in the table 6.1.
The small leakage observed in the practice suggested that some high mode of
microwaves which favor the coupling of the energy to the coaxial line occurs in the cavity
to satisfy the boundary condition created by the insertion of the thermocouple. The leakage
133
is relatively large when the cavity is not in resonating mode and will be negligible when the
cavity is in resonance.
7.3.3.5 Summary
In practice, the single mode cavity will not be suitable for processing large pieces of
ceramics which are embedded in certain crucible insulations. In order to avoid the effect of
electromagnetic fields on the temperature measurements, a metal foil sheath is often used.
Some researchers have suggested the use of ceramic material to cover the thermocouple, so
that sparking could be prevented, for it reduces the magnitude of the electromagnetic
fields.
In conclusion, both the pyrometer and the thermocouple have advantages and
disadvantages. On the other hand, according to Meek [1991], correct temperature,
measurement can never be made during microwave sintering of ceramics because of the
temperature difference between ceramic grain and grain boundary after some experimental
observations. However, such an argument was rejected by Johnson [1991] after some
simple calculations. At the same time, Johnson’s calculation was not adequate for
calculating the field distribution around the contacting region of ceramic particle during
microwave heating.
7 . 4 Binder Burnout of Tape-Casted Ceramics by Microwave Energy
7 .4.1.In trod u ction
Ceramic materials are good for their ability to resist corrosion and maintain strength
at high temperature, and for their varied electrical properties. Recently, plastic forming
methods applied to the making of the complex-shaped green ceramic product are receiving
a lot of interest. Plastic forming methods include extrusion, injection molding, slip casting
and tape casting. In those forming methods, a large volume of binder, up to 50%, has to be
134
added to the ceramic powders to achieve appropriate rheologic properties for processing.
This large volume of binder has to be removed before final sintering can take place.
Methods of binder burnout include evaporation by Wei et al. [1988], thermal by Calvert
and Cima [1990], degradation by Mutsuddy [1987], and solvent extraction by Watson and
Smith [1989]. The conventional furnace heating method requires a very low heating rate to
complete the binder burnout process. If a high heating rate is used, it will cause a large
thermal gradient which may generate cracks in the green sample. A long period of time is
often required to remove the last traces of residue. Reduction in time and energy required
by the debinding step could significantly enhance the economics and productivity of the
manufacturing process. Microwave heating, known to be a volumetric heating process, is
the subject of this investigation for binder burnout. In this investigation, both conventional
and microwave fired tapes are characterized for their dielectric properties in a waveguide.
These dielectric properties are effective for determining completeness of firing and
understanding the firing process. The measurement of the dielectric properties can be easily
expanded to a free space characterization method for online process monitoring, as shown
by Varadan et al. [1991].
7.4.2 M icrowave Processing
Use of microwaves as a heating source was developed after World W ar II.
Applications of microwaves to material processing started in 60’s. In the last few years,
tremendous interest has been generated in the material research community in the use of
microwave energy to process materials such as ceramics, polymers and their composites,
as surveyed by Sutton et al. [1988]. Applications of microwave processing of ceramics
have shown good promise in such areas as fine microstructure, lower sintering temperature
and reduced sintering time, as well as energy savings. However, use of microwave power
for the binder bumout process has not received much attention. Harrison et al. [1988] is
the only one who tried to use microwave to binder bumout, but the study was not
systematic.
135
7.4.2 Ceramic Tape Preparation
To make ceramic tape for the microwave and conventional binder bumout
experiments, a tape casting technique is used as shown in figure 7.3. Barium strontium
titanate was mixed with 28 w t% of binder (B73210 from Polamar-MSI) and ball-milled in
a plastic jar with zirconia media for 24 hours. The slurry was then deaired and tape casted.
The tape was dried, cut and pressed at a pressure of 35 MPa and at a temperature of 6070°C for 15 minutes.
7.4.3 Characterization Method
To understand both microwave and conventional binder bumout processes, the
ceramic tapes were characterized at room temperature by measuring their dielectric
properties at 15GHz after different stages of firing at room temperature. The samples were
heated to various temperature by exposing them to the microwave fields as shown in figure
7.4. Samples were also heated in a conventional furnace and the time history is as recorded
in figure 7.5. The tapes are then sized to fit into a rectangular waveguide for
characterization. To guarantee intimate contacts between sample and waveguide wall, the
edges of the specimen were coated with a thin layer of silver paint. The measurements
were done using a HP-8510 network analyzer and the data acquisition system as depicted
in figure 6.3. Prior to the measurement, a 2-port TRL calibration procedure as shown by
Ghodgankar et al. [1989] was performed. The complex reflection coefficient from the
metal-backed sample was measured. The dielectric constant is obtained from the reflection
coefficients and the permeability of the material is assumed to be unity.
7.4.5 Results and Discussion
The values of dielectric constants of both microwave and conventionally fired
ceramic tapes at different stages are plotted in figure 7.6. The change in dielectric constant
OROANICS
BALL MILL
FOR 24 HOURS
DE-AIR
TAPECAST
LAMINATE
35MP»
00-70 «C
BINDER
BURNOUT
Figure 7.3 Ceramic Tape Manufacturing Flow Chart
137
700.
600500o
400-
z
s
300-
<5
o.
§
200 -
H
100 -
0
30
20
10
Time ( Minute )
Figure 7.4 Temperature as a Function of Time for Microwave Binder Burnout
1000
800
600
400
0
,
200
100
200
300
400
Tim e ( M in. )
Figure 7.5 Temperature as a Function of Time for Conventional Binder Burnout
138
60
conventional
m icrow ave
Dielectric Constant
50-
40-
30 -
20
0
200
400
600
800
1000
Temperature ( ° C )
Figure 7.6 Dielectric Properties of Ceramic Tapes at Different Temperatures
139
with temperature explains the phenomena during the binder burnout process. Before Bring,
the green tape can be considered as a homogeneous mixture of ceramic powders and
binder. The binder can be considered as the matrix phase where no connectivity between
the ceramic particles exists. The dielectric constant of the green tape can be easily calculated
by using a mixing rule. The value of the dielectric constant was around 55. When the
temperature of the tape reaches around 200°C, the binder starts to leave the sample
gradually. The dielectric constant is the result of the coexistence of ceramic powders, binder
and air pores. Since the dielectric constants of the binder and air pores are equal to 2.5 and
1, respectively, the dielectric constant reduces when the binder leaves the tape. At the end
stage of binder burnout, the tape contains no binder and the dielectric constant is the
compositive property of the ceramic powder with weak connectivity and pores. The weak
connectivity between ceramic particles contributes to the large variation of the dielectric
properties. After exposure to high temperatures in both the microwave and
the
conventional heating process, the dielectric constants of the tapes are increased.
Comparison of dielectric properties of microwave and the conventionally heated
tape shows that microwave heating completes the binder burnout process much faster and
earlier than that of the conventional process with no damage to the sample. It is not clear at
this moment whether this difference in the results is due to different heating mechanisms or
the different temperature at the ceramic boundary and inside the ceramic grain, which is
suggested by Meek [1987a] in observation of microwave sintering of ceramics. Meek has
suggested that the enhanced diffusion in microwave sintering of ceramics is due to the
different temperature distribution at the ceramic grain boundary and inside the ceramic
grain.
7.4.6 C onclusion
It has been shown here that green ceramics made from the plastic forming process
can be prefired by using a microwave heating technique. Dielectric characterization is a
useful method of monitoring binder burnout process. Microwave assisted binder burnout
140
was completed at a lower temperature and in a shorter time than that of conventional
heating.
7.5 Microwave Sintering o f Al20 3/ c-Z r02 C om posites
7.5.1 Background
It has been the object of much research that the tetragonal to monoclinic Z r02 phase
transformation can be utilized to increase the fracture toughness of ceramics. In zirconia
toughened alumina (ZTA), retention of >10 vol.% metastable, tetragonal Z r0 2 in the
alumina matrix is the key to obtain increased room-temperature fracture toughness.
However, to retain the metastable tetragonal Z r0 2 it is essential that the Z r0 2 grain size be
less than some critical size which is reported to be in the range of 0.5 to 0.8|im. Above the
critical size, the Z r0 2 grain size transforms to the monoclinic form. Garvie [1984] has
discussed the benefits of having Z r02 present in both the monoclinic and tetragonal forms
for cutting tool applications. One method of controlling the volume retention of tetragonal
Z i0 2 grains is to partially stabilize the zirconia with MgO or Y 20^. However, the high
temperature required for ZTA densification usually results in significant grain coarsening
and the inability to control the desired zirconia volume concentration and the phase form.
To meet the requirement, numerous processing approaches have been investigated
to achieve lower densification temperature and homogeneous distribution of Z r0 2 in an
alumina matrix. These methods have included attrition milling by Claussen and Ruhle
[1981], colloidal processing by Aksay et al. [1983], chemical vapor-co-deposited aluminaZr02 powders with a surfactant by Hori et al. [1981], co-pyrolyzed solutions by Sproson
[1984], hydrothermal reaction of Al-Zr alloys by Somiya et al. [1984], hydrolysis of a
zirconium alkoxide in the presence of A12O j particles by Fegley et al. [1985],
polymer/powder flocculation by Moffatt et al. [1988], and sol-gel processing by Becher
[1981], Most of these powder processing approaches require hot pressing because there is
excessive Z r0 2 grain growth at the elevated temperature (>1600) which is required for
141
densification. Recently, Sproson and Messing et al. [1984] used seeded boehmite gels for
the controlled synthesis and relatively low-temperature sintering of zirconia-toughened
alumina was realized.
7.5.2 Microwave Sintering
Sintering of zirconia toughened alumina with microwave energy has been studied
by several researchers. Kimrey et al. [1990] used both 2.45GHz and 28GHz microwave
furnaces to sinter alumina/zirconia system containing 10-70wt.% Z r0 2- It is found that
microwave enhanced diffusion was more easily observable at higher frequency. Microwave
sintering needs as low as 500°C compared with the conventional sintering process. No
mechanical properties were reported with both conventional and microwave sintering
results. They concluded that the microwave effect is the function of the frequency and that
the reduction in sintering temperature results in a significant decrease in the grain size in the
microstructure. Patil et al. [1991] have used microwaves to sinter alumina powders mixed
with 15 vol.% zirconia in a single mode cavity. In their study, the general results indicate
that microwave sintering is superior to the conventional sintering; sometimes it is even
comparable to the high gas pressure sintering. However, the temperature measurement was
very questionable in his study and the mechanical property comparisons between results of
both conventional and microwave sintering were not given.
Hence, microwave sintering
can be used to substitute for the gradually popular hot isostatic press sintering method
which is effective but expensive for densifying materials that are usually difficult to sinter
with conventional methods.
In the following study, both conventional and microwave methods are used to
sinter alumina/zirconia composites containing Owt.%, 4wt.% and 10wt% zirconia. The
densified samples are characterized with scanning electron microscope to observe
microstructure. The density, hardness, elastic properties and diametral compression
strength are obtained for comparison.
142
7.5.3 Sample Preparation
Ceramic materials in the Z r0 2/ A12O j system containing Owt.%,4 wt.% and
10wt.% Z r0 2 were chosen for this study. Proper combination of alumina, zirconia and
0.2wt% MgO powders were mixed together by the ball milling process in a plastic jaw
using zirconia ball for 3 hours in the isopropanol. The slurry was sonicated for 10 minutes
and then dried in a glass plate. The dried cake was crushed by using a food processor.
Specimens were uniaxially pressed in a cylindrical die, which has a diameter of 1.27-cm, at
a pressure of 30 MPa. The green specimen has a cylindrical shape and has a diameter of
1.27cm and a height around 0.4cm. The specimens were prefired in a electrical furnace at
600°C for two hours to bum out binders.
The conventional sintering was performed in an electrical furnace according to the
sintering schedule described in figure 7.7. The conventional sintering time and temperature
were taken from French et al. [1992]. The microwave sintering was performed in a single
mode high power microwave heating device introduced in chapter 5. The microwave
sintering schedule is shown in figure 7.8. Such a sintering temperature was taken to
compare with a commercial product.
7.5.4. Mechanical Properties
In this section, mechanical properties were obtained to compare conventional and
microwave sintering results. Table 7.1 gives sintering time, total cycling time, sintering
temperature, and final density, as well as elastic properties.
1. Density
Sample density is obtained by using Archimedes’ rule. It is obvious that microwave
processing needs lower temperature, and shorter times and reaches a higher density than
that of conventional processing method. Therefore, microwave processing enhances
diffusion and lowers sintering temperature. Since there are only a small quantity of
samples used in both microwave and conventional processing, the economic aspects of
143
Temperature ( ° C )
2000
1000 -
0
10
20
Time ( hour )
Figure 7.7 Conventional Sintering Schedule
Temperature ( 0 C )
2000
temperature
0
100
200
Time (min.)
Figure 7.8 Microwave Sintering Schedule
144
Table 7.1 Comparison of Results from Microwave and Conventional Sintering
Samples
Sintering
Method
Sintering
Time( hour)
Sintering
Temp. ( °C )
Relative
Density (%)
Total Cycle
Time (hour)
A
A
mw
conv.
mw
conv.
2
3
2
9
1530
1600
1530
1600
97.91
96.63
2.66
16.6
2.66
2.66
16.6
365.2
387.3 393.5 277.9
282.6
.228
0.262
0.302
Young's (GPa) 376.8
Modulus
Poisson's
0.199
Ratio
ZTA4 ZTA4 ZTA 10 ZTA1C
mw
2
conv.
9
1530
1600
99.99 99.01 99.98
99.89
16.6
0.242
0.305
145
the two processing techniques is not given here.
2. Elastic Properties
Longitudinal and transverse elastic wave velocities were measured from polished
samples made from either conventional or microwave sintering. The elastic properties
(Young's modulus and Poisson ratio) were then calculated from these values using the
standard relationship from Papadakis [1967]. Table 7.1 shows the behavior of Young’s
modulus for the composites. It can be seen that Young’s modulus decreases with increasing
c- Zr02 content. It can be seen that composites with 0 % wt.of Z r0 2 from both microwave
and conventional processing techniques have a lower value, which is probably due to a
slightly higher value of porosity content in those samples. For ZTA4 and ZTA 10, the
samples produced from microwave processing have slightly smaller values of Young’s
modulus and Poisson ratio.
3. Hardness
For each sample, 8 to 10 room-temperature hardness measurements were made
using Vickers indenter with 2.0392kg load and 10-s dwell time. Hardness values were
determined from the equation
H = 2PSin(e/2)/d2
(7.9)
where H is the hardness number, P is the indentation load (kg), 0 is the angle between
opposite faces of the indenter which is 136° here, and d is the indentation diagonal length
(mm). An indentation load of 2.0392kg was chosen to produce impressions significantly
larger than the grain size, thus providing an adequate sampling of the microstructure. Table
7.2 shows the behavior of the composite samples. It is seen that in general products made
from microwave processing have produced higher hardness than that of conventional
methods which may be due to a slight higher density of the composites produced from
microwave processing.
4. Diametral Compression Test
The diametral compression test is based upon the state of stress developed when a
cylindrical specimen is compressed between two diametrically opposite surfaces. This ideal
146
loading, shown in figure 7.9, produces a biaxial stress distribution within the specimen.
The stresses at any point in a cross-section can be calculated by the elastic theory. Of the
primary interests here are the maximum tensile stresses, which act across the loaded
diameter and have the constant magnitude:
where:
a = maximum tensile stress
P= applied load
D= specimen diameter, and
t = specimen thickness
During the course of testing, proper load is obtained by placing a narrow pad of suitable
material between the specimen and the loading plates. The pad will be soft enough to allow
distribution of the load over a reasonable area and yet narrow or thin enough to prevent the
contact area from becoming excessive. The average maximum tensile stresses for different
specimens are shown in figure 7.10. It is seen that microwave processed samples of ZTA4
and ZTA 10 have a lower diamentral compression strength than that processed from
conventional methods; even microwave processed samples have a higher density. The
differences may be caused by high heating rate or local densification due to non-uniform
temperature distribution, which could cause internal crack during microwave processing.
S.Characterization - SEM Observation
Both conventional and microwave sintered samples were polished to lum finish
using standard metallographic techniques. Polished surfaces were thermally etched in air at
1450°C for 90 minutes to reveal the grain boundaries. Scanning Electron
Microscope(SEM) was used to observe the microstructure. The specimens were sputter
coated with Au-Pd to prevent surface charging during SEM observation. Figure 7.11 to
figure 7.14 gives the microstructure of the both conventional and microwave sintered
147
Table 7.2 Vickers Hardness of the Sintered Ceramics
Sample
Processing
Method
Vicker's
Hardness
A
MW
1961.97
A
CONV.
1883.22
ZTA4
MW
1953.06
ZTA4
CONV.
1867.22
ZTA10
MW
1786.92
ZTA10
CONV.
1607.45
148
1
T
Diametral Compression Strength ( M P a)
Figure 7.5.4 Diametral Compression Test
400
300
Microwave
Conventional
200
100
Weight Concentration of Zirconia (%)
Figure 7.10 Diametral Compression Strength
149
results. It is easy to see that microwave processing yields a more fine and uniform
microstructure than that of conventional processing.
7.5.5 Conclusion
It is easy to see that microwave sintering of ceramics is applicable to the ceramic
processing. Although it is still not known how much microwaves have effected ceramic
densification and microstructure development during ceramic sintering, it is shown here
that microwaves have helped to reduce the sintering temperature, shorten sintering time and
yield fine and uniform microstructure. Since microwave sintering of ceramics is a new
field, the effect of microwave sintering on the strength of the final product is not known
and there are not many data available for comparison. Therefore systematic research is
needed for providing the right schedule to achieve required product quality.
150
Figure 7.11 Micrograph of Conventional Sintered Alumina-4%Zirconia
[
00F
0 11
Figure 7.12 Micrograph of Microwave Sintered Alumina-4%Zirconia
151
Figure 7.13 Micrograph of Conventional Sintered Alumina-10%Zirconia
Figure 7.14 Micrograph of Microwave Sintered Alumina-10%Zirconia
152
Chapter 8
CONCLUSIONS AND FUTURE WORK
Microwave processing, where microwave energy acts as an energy source, has
been successfully applied to paper manufacturing and food processing, etc. Its application
to the ceramic processing is still new. Although preliminary research has shown that it
provides some nonthermal effects when it is applied to the sintering of ceramics, the
mechanisms of microwave heating, especially the nonthermal effects, are still not fully
understood. Research work has been undertaken in many laboratories, but a systematic
research effort is still lacking. Most of the research results which have been published are
on very small scale. It is necessary for the entire process to be modeled, characterized and
subsequently applied to practical application. The focus of this study is therefore directed
to those three areas. The achievements and conclusion made in this thesis research are
summarized as follows:
1.The equations to describe microwave processing of ceramics are derived. In the
description, the ceramics under microwave sintering are treated as deformable dielectrics.
The treatment o f ceramics in the microwave sintering as deformable dielectrics enables
considering the effects of moving charges and currents. Therefore, a complete description
of interactions between microwaves and ceramics is obtained. To predict temperature
distribution in the ceramics, a thermal diffusion equation has to be added the picture. To
account for the sintering process where ceramics are densified, a mass diffusion equation
is also needed. It is concluded that those equations can be solved by using a finite element
method once all parameters are determined through either experiment or postulation. Those
experiments will be very time consuming. It is hoped that such a formulation can be used
in future studies.
2. In order to have some insights about microwave heating, the microwave heating
of a ceramic slab is modeled. The modeling results show that ceramics can be heated with
microwaves to the desired temperatures for processing. It is also found that nonuniform
153
heating can be resulted from nonuniform power absorption and heat loss at boundaries.
Therefore, it is important to design a microwave applicator which would distribute
microwave power evenly into the samples. Insulation is also extremely important to ensure
that heat losses at boundaries are minimum. More work is also needed to model microwave
heating of samples of complicated shape and structure where insulating materials are used.
To do so, a Finite Difference Time Domain method is used to model a single mode cavity
used for ceramic processing and characterization. The results show that the FDTD method
is applicable to the modeling of complicated structures comprised of the ceramic sample,
insulation material and microwave susceptor. The electromagnetic field distribution in the
cavity and the power absorption by the ceramics sample, insulation material and microwave
susceptor are obtained. Those results are useful for the designing of industrial microwave
processing.
3. For the purpose of experimental microwave processing and characterizing of
ceramics, a single mode high power microwave heating system is established. It can be
used not only to process ceramics but also to do in situ microwave heating, sintering and
characterization. The acquisition of the data is computerized with a computer and a digital
and analog converter. This system can be further improved so that the operation of the
variable short and variable iris would be automated. In doing so, critical coupling that is
essential to the energy efficiency can be readily obtained.
4. To understand both microwave heating and sintering, the established single mode
microwave heating system is used to characterize microwave heating and sintering of
ceramic rods. It is found that material properties can be revealed by using such a system.
The technique of in situ microwave sintering and characterization is extremely interesting to
investigate the microwave sintering process. The results tell that the real part of the
dielectric constant indicates the densification more than the imaginary part, while the
imaginary part of the dielectric constant mainly shows that the material properties change
with temperature. The mechanisms of dielectric property changes during sintering can be
generally explained by mixture theory and multiple scattering theory. However, more
rigorous work is needed to detail those mechanisms. On the other hand, it is concluded that
154
the microwave components, such as cavity, iris and variable short, have to be carefully
made so that they will behave like the ideal elements.
5. To demonstrate the applicability of microwave heating to the ceramic processing,
the single mode microwave heating system is used for binder burnout and sintering of
ceramics. The results show that microwaves bum out binder in less time and lower
temperature than conventional ones. In using microwaves to sinter alumina/c-zirconia
composites, it is found that microwave sintering results in lower sintering temperature,
higher density and shorter time than conventional sintering.
The initial intent to model, characterize and apply microwave energy to the ceramics
processing is accomplished, although there are problems to be solved before this technique
can be fully applied to the industrial application. It is shown that microwaves can be used to
heat ceramics for all processing purposes.
155
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APPENDIX
EXPRESSIONS FOR C ; AND Du
The expression for the Cj’s and Dy’s are
p
_
-Qo
(A.l)
( r f ■l)k»
_ _ - Qi(K/k)
(A.2)
02
(A.3)
(e?- l)k a
C3 = ■~ ^3
3 ( e r - l) k a [>
' trfl
(A.4)
where
(A.5)
p j„ (a )J„ .,(p ) • a Jn-l (a)Jn (p)
for n= 1,2,3,4.
The Djj’s are given by
Doo = -(e; - l ) '1 <X-2 { C^r-(0,0) + i-Q o t P Y0 (ocPi (p) - a Y, (oc)Jo(p)] )
(A.6)
D „ = - (ej - 1)'•' o J {(fij* [ | + ( f f ( log, [ ( 2 w ) -2] + )]
+
£
(A.7)
|,-(ri2 -w 2 P -3 p
n=3, odd
+1 q , [ p Y! (o)Io (p) - a Y0 (a(J, (p) ] |
D33 = -(e? -1} 1 a-2 ( ( ^ ) ? [ r„ (0 ,0 ) + (A ) r ^ o , o ) ^
IV ^O .O )
(A.8)
-^•Q 3 [ p Y 3 (ot)J2(p) - a Y2( a p 3 ( p ) ] )
173
(A.9)
0,3=031=Ernwlr“’(0’0)+©w H
(A. 10)
0,3 =°31 =(e M ^ [r“ (0'0) +© W
(A. 11)
H
f e ) 4 < 0 ,0 ) = W'2 [ I + ( f f ( loge [ ( 2 w ) -2] +
(A. 12)
+
X
n - (n 2 - w 2)1/2- 2 £ )
n=3, odd
^ , , ( 0 . 0 ) = w '2 [ 1 - ( if C log, [ ( 2 w ) -2] + >
(A. 13)
+
£
n - n2(n2 - w2)"122 + - ^
11=3, odd
11
W I W 0 , 0 ) = » - 4 ( M . + »d<log.2 -2 )-1 7 /1 6
(A. 14)
+
^
n^n2 - w2)-122 - n3 +
w3 + ^ d )
n=3, odd
( * ) W O , ® = w •M § -
^
2 ( S f ( 2- log,2 )
(A. 15)
+
X
n5_ n^n2 - w2)'1/2 - 111 w2 - n & i
n=3, odd
2
8
3w6i
48n
VITA
Xiang Dong Yu was born in Nanton, China. He graduated from Dong An High
School, QiDong, China, in July 1977. He received a Bachelor of Science degree in
January 1982 in the Mechanical Engineering Department at the East China Institute of
Technology, Nanjing, China.
After spending three and a half years as an instructor in his Alma Mater, he came to
the United States to pursue further education in August 1985.
He studied in the
Mathematics Department for one year and subsequently transferred to the Civil and
Engineering Mechanics Department at the Southern Illinois University at Carbondale, IL.
He obtained his Master of Science Degree in May 1988. He came to Penn State in August
1988 and joined the Research Center for the Engineering of Electronic and Acoustic
Materials. Since then, he has been a research assistant in the Department of Engineering
Science and Mechanics.
Xiang Dong Yu is a member of the American Ceramic Society, the American
Society of Mechanical Engineering, the Society of Advanced Material Processing
Engineering, and the Society of Engineering Education.
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