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Microwave plasma synthesis of thermobarrier nitride and diboride nanomaterials

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HOWARD UNIVERSITYTITLE PAGE
Microwave Plasma Synthesis of Thermobarrier Nitride and
Diboride Nanomaterials
A Thesis
Submitted to the Faculty of the
Graduate School
of
HOWARD UNIVERSITY
in partial fulfillment of
the requirements for the
degree of
MASTER OF SCIENCE
Department of Chemical Engineering
By
Mulugeta Dessalegn Sida
Washington DC
May 2009
UMI Number: 1464050
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HOWARD UNIVERSITY
GRADUATE SCHOOL
DEPARTMENT OF CHEMICAL ENGINEERING
Committee Approval Sheet ____________________________
Ramesh C Chawla, Ph.D.
Chairperson
____________________________
Joseph Cannon, Ph.D.
____________________________
James W. Mitchell, Ph.D.
____________________________
James W. Mitchell, Ph.D.
Thesis Advisor
Candidate: Mulugeta Dessalegn Sida
Date of Thesis Defense: April 21, 2009
ii
DEDICATION To my wife (Emebet) and my daughter (Ruth),
iii
ACKNOWLEDGEMENTS The writing of this thesis has been one of the most significant academic challenges that I have
ever faced. I would like to acknowledge first and foremost my thesis advisor Dr. James W.
Mitchell for his vital encouragement and support. Without his patience and guidance, this thesis
would not have been completed. I would also like to thank the following individuals who directly
or indirectly contributed to the success of my work;
Ramesh C. Chawla, Ph.D, Professor and Chair for serving on the thesis committee;
Joseph N. Cannon, Ph.D, P.E., and Professor for serving on the thesis committee;
Jason Ganley, Ph.D for allowing me to use his ball mill apparatus for the first trial;
Anthony D. Gomez and Crawford Taylor for their valuable technical support; and
Dr. Zhou Peizhen for generating SEM images and EDAX spectra used in this thesis.
I would like also to thank all CREST center, Howard nanoscale science and engineering facility,
and chemical engineering department faculty members and staff for their technical as well as
moral support during the project.
Most especially to my family and friends
And to God, who made all things possible.
iv
ABSTRACT The fabrication of various nanomaterials that possess desirable properties such as high
temperature stability, extreme hardness, and corrosion resistance has attracted the interest of
various research groups. Among the materials of interest, silicon nitride, and group four
diborides are known for exceptional properties including enhancement of the mechanical and
thermal properties of composites. Conventionally, these materials have been produced in a
traditional chemical vapor deposition (CVD) apparatus that has utilized thermal energy to drive
the reactions at very high temperature. At high temperature, the vaporization of liquid or solid
precursors alters the pressure inside the CVD reactor as the mass of the precursor held in the
sampling boat decreases overtime. This leads to a fluctuation of the precursor concentration in
the reagent stream. As the result of these limitations, it is not possible to consistently fabricate
nanowires of desired composition. Therefore, in this thesis we designed, constructed and
evaluated a microwave plasma enhanced chemical vapor deposition (PECVD) system to
alleviate these problems. This has been achieved by carrying out CVD reactions at relatively low
temperature since the microwave plasma has created dissociated gaseous molecules, excited
state ions, and a mixture of energetic gaseous components that react spontaneously. Fluctuation
of the precursor concentration is eliminated by installing a mass and pressure controller in
tandem. Using the PECVD system, the fabrication of silicon nitride and zirconium diboride
nanowires has been investigated to optimize the processing of scalable quantities of the
materials. Known methods with the highest potential for the synthesis of nanowires were
investigated. These included direct nitridation of silicon substrates and metal catalyzed etched
silicon wafers, thermo-transformation of pre-ceramic polymer, autoclave reactions, and solid
state mechanical alloying. Additionally, a novel method was developed based on the catalyzed
v
conversion of silicon nitride nanopowder into nanowires. Similarly, zirconium diboride
nanowires (rods) have been synthesized by employing solid state mechanical alloying with
subsequent thermo-transformation. Finally, the synthesized nanowires were characterized using
scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy
dispersive X-ray florescence spectroscopy (EDAX).
vi
TABLE OF CONTENTS COMMITTEE APPROVAL SHEET ............................................................................. ii
DEDICATION.................................................................................................................. iii
ACKNOWLEDGEMENTS ............................................................................................ iv
ABSTRACT ......................................................................................................................v
LIST OF TABLES ........................................................................................................... xi
LIST OF ABBREVIATIONS ....................................................................................... xvi
CHAPTER 1 INTRODUCTION ......................................................................................1
1.1
GENERAL APPROACHES FOR FABRICATING
NANOMATERIALS .......................................................................................... 1
1.2
LITERATURE REVIEW .................................................................................. 2
1.2.1. High Energy Solid State Alloying ................................................................ 5 1.2.2. Chemical Vapor Deposition .......................................................................... 7 1.2.2.1. Fundamental Mechanisms of Deposition in CVD ........................................ 7 1.2.2.2. Chemical Vapor Deposition (CVD) Modifications: ..................................... 8 1.2.2.3. Low Pressure CVD (LPCVD) ...................................................................... 9 1.2.2.4. Microwave Plasma Enhanced CVD (PECVD) ............................................. 9 1.2.2.5. Laser Enhanced CVD (LECVD) ................................................................ 10 1.2.2.6. Metallorganic CVD (MOCVD) .................................................................. 10 1.3. Scope of the Study.............................................................................................. 10
CHAPTER 2 DESIGN AND ASSEMBLY OF A MICROWAVE PLASMA ENHANCED
CHEMICAL VAPOR DEPOSITION (PECVD) SYSTEM .................12
2.1. Introduction ........................................................................................................ 12
vii
2.2. Microwave PECVD System Design .................................................................. 12
2.2.1. Prototype PECVD System .......................................................................... 12 2.2.2. Precursor Delivery Section of a PECVD System ....................................... 15 2.2.3. Reaction Chamber ....................................................................................... 17 2.2.4. Split Heating Furnace ................................................................................. 18 2.2.5. Microwave Plasma Unit.............................................................................. 19 2.2.6. Mass Flow and Pressure Controller ............................................................ 21 2.2.7. Pressure Controller...................................................................................... 22 2.2.8. Pumping Station .......................................................................................... 23 2.2.9. Exhaust Treatment System ......................................................................... 24 CHAPTER 3 EXPERIMENTAL SECTION ................................................................26 3.1 Synthesis of Silicon Nitride Nanowires ............................................................. 26
3.1.1 Synthesis of Silicon Nitride Nanowires by Direct Nitridation ................... 26
3.1.2 Synthesis of Silicon Nitride Nanowires and Nanomaterials
from a Pre-Ceramic Polymer Precursor...................................................... 27
3.1.3. High Temperature and Pressure Autoclave Synthesis
of Silicon Nitride Nanowires ...................................................................... 29
3.1.4. Metal Catalyzed Etching of Silicon Wafers................................................ 30
3.1.5. Transformation of Silicon Nitride Nanopowder into Nanowires ............... 31
3.2 Synthesis of Zirconium Diboride Nanowires ..................................................... 32 3.3 Operation of Microwave Plasma CVD (PECVD) System ................................. 34
3.4 High Energy Solid-State Mechanical Alloying .................................................. 35
CHAPTER 4 SCALED PRODUCTION OF NANOWIRE MATRRIX ....................39
viii
4.1. Bulk Synthesis of Nanowires ............................................................................. 39
4.2. Plasma Enhanced Chemical Vapor Deposition Reaction Kinetics .................... 39
4.3. Kinetics of Nanowire Growth via the Vapor–Liquid–Solid Mechanism ........... 45
4.4. Reactor Design for Scaled Production of Nanowire Matrix .............................. 48
4.4.1. Shelf/Bed Reactor ....................................................................................... 50
4.4.2. Fluidized Bed CVD Reactor ....................................................................... 50
CHAPTER 5 RESULT AND DISCUSSION OF GROWTH AND
CHARACTERIZATION OF NANOWIRES ........................................57
5.1. Silicon Nitride Nanowires .................................................................................. 57 5.1.1 Characterization of Nanowires Obtained by FeCl2 Catalyst-Assisted
Direct Nitridation ........................................................................................ 57
5.1.2. Characterization of Nanowires Obtained by Silicon Nitride
Seeding and FeCl2 Catalyed Growth .......................................................... 61
5.1.3. ........................ Characterization of Silicon Wafers Treated by Direct Nitridation
without Catalyst ................................................................................................. 66
5.1.4. ............... Characterization of NanowiresPrepared via Metal Catalyzed Etching
and Nitridation ................................................................................................... 69
5.1.5 Characterization of Silicon Nitride Matrix Nanomaterials prepared
from Pre-ceramic Polymer Precursor ......................................................... 71
5.1.6. Characterization of Nanomaterials Prepared from Silicon Nitride
Nanopowder................................................................................................ 76
5.1.7 ..................... Characterization of Nanomaterials Prepared by Autoclave Reaction
at High Temperature .......................................................................................... 82
5.2 Zirconium Diboride Nanowires ......................................................................... 83
5.2.1 Characterization of Zirconium Diboride Nanowires (Rods) Obtained after
High Energy Ball Milling and Annealing of Hydrated ZrCl4 and Boron .. 83
ix
CHAPTER 6 CONCLUSION AND RECOMMENDATIONS ...................................89 6.1 Summary and Conclusion .................................................................................. 89 6.2 Recommendation and Future Work ................................................................... 91 REFERENCES.................................................................................................................92 APPENDIX A ...................................................................................................................96 A.1 Calculation to Prepare 0.01MFeCl2: ................................................................. 96 A.2. Preparation of 5M of 50 mL of HF acid ........................................................... 96 A.3.Preparation of 0.02 M of Silver Nitrate ............................................................. 96 A.4. Calculation of Boron and Zirconium (IV) Chloride Anhydrous
Milling Mixture .......................................................................................... 97
A.5 Calculation Boron, Magnesium and Zirconium (IV) Chloride
Anhydrous Milling Mixture........................................................................ 97 x
LIST OF TABLES Table
Page No.
5.1 EDAX data of 0.01M FeCl2 catalyzed direct nitridation ..................................................... 60 5.2
Analyzed EDAX data of direct nitridation reaction ............................................................ 61 5.3
EDAX analysis of silicon wafers seeded with Si3N4 powder .............................................. 63 5.4
Analyzed EDAX data of the silicon nitride seeded silicon wafer before the reaction ....... 64 5.5
EDAX analysis of silicon nitride seeded and catalyzed direct nitridation.......................... 64 5.6
Analyzed EDAX data of silicon nitride seeded and catalyzed direct nitridation ............... 65
5.7
EDAX data of nitride silicon wafer without catalyst .......................................................... 67 5.8
Analyzed EDAX data that shows the presence of SiO2 and free silicon ........................... 68 5.9
EDX analysis of polymer precursor cured in a nitrogen atmosphere ................................. 73 5.10 Stoichiometry from EDAX data of the polymer precursor cured in nitrogen ................... 74 5.11 EDAX analysis of polymer precursor cured in air.............................................................. 75 5.12 Analyzed EDAX data for pre-ceramic precursor cured in air ............................................ 76 5.13 EDAX analysis of nanowires and nanomaterials from silicon nitride nanopowder .......... 79
5.14 Analyzed EDAX data that shows the presence of a high percentage of nitrogen. ............. 80 5.15 EDAX analysis of nanowire tip .......................................................................................... 80 5.16 EDAX analysis of zirconium diboride milled and annealed under argon .......................... 85 5.17 Analyzed EDAX data of zirconium boride nanorods milled in argon................................. 86 5.18 EDAX of nanomaterials milled in air and annealed in argon .............................................. 86 xi
LIST OF FIGURES Figure
Page No.
1.1 Schematic of illustration of the working principle of the planetary ball mill apparatus........ 7
1.2 Schematic diagram illustrating the six steps in crystal growth. (1) ....................................... 8 2.1
Schematic diagram for the microwave plasma system ........................................................ 14 2.2 Custom designed reagent delivery system. .......................................................................... 16 2.3
Quartz tube reactor ............................................................................................................... 18 2.4
Mellen SC-12 split heating oven and controller ................................................................. 19 2.5
Microwave generator (31).................................................................................................... 19 2.6
1″ Evenson microwave cavity............................................................................................. 20 2.7
Coaxial RF cable .................................................................................................................. 20 2.8 Mass and pressure controller display unit(32) ..................................................................... 21 2.9 Mass Flow controller (33) .................................................................................................... 21 2.10 Baratron® high temperature absolute capacitance manometers 631B (34) ....................... 22 2.11 All metal control valve 148J (35) ....................................................................................... 22 2.12 Edwards E2M40 vacuum pump .......................................................................................... 23 2.13 Plasma enhanced CVD system ........................................................................................... 25 3.1
Quartz tube sample boat....................................................................................................... 28 3.2
Boron nitride sample boat .................................................................................................... 29 3.3 Annealing process for the pre-ceramic polymer precursor .................................................. 29 3.4
Planetary ball mill apparatus PM200 .................................................................................. 35 3.5
Planetary ball mill PM200 .................................................................................................. 36 3.6
A PM200 controller ............................................................................................................ 36 xii
3.7
Fastening and removing of grinding jars ............................................................................ 37 3.8
Safety precaution display .................................................................................................... 38 4.1 Diffusion from gas to solid surface...................................................................................... 40 4.2 Mass transfer in the PECVD reactor.................................................................................... 42 4.3
Schematics drawing showing the VSL growth of nanowires .............................................. 46 4.4 Schematic illustration of the possible steps in catalyst assisted nanowires growth from a
solid substrate...................................................................................................................... 46
4.5
SEM image of FeCl2 catalyst assisted silicon oxynitride nanowire growth ...................... 47 4.6
EDAX spectrum of tips of nanowires showing the 100% catalyst. ..................................... 47 4.7 Schematic illustration of various reactions taking place in PECVD reactor ....................... 49 4.8
Shelf/bed reactor for large scale production of nanowires ................................................ 50 4.9
Fluidized bed CVD modelling (47) .................................................................................... 52 4.10
Fluidized bed CVD reactor ................................................................................................ 53 4.11 Graph of pressure drop vs. superficial velocity ................................................................. 54 5.1
SEM image of silicon nitride nanowires at low resolution ................................................. 58 5.2
SEM image of silicon nitride nanowires at higher resolution............................................. 58 5.3
SEM image of silicon wafers treated with 0.01 M FeCl2 ethanol solution at lower
resolution............................................................................................................................. 59
5.4
SEM image of silicon wafers treated with 0.01M FeCl2 ethanol solution at higher
resolution............................................................................................................................. 59
5.5
A representative EDAX spectrum obtained for FeCl2 catalyzed direct nitridation ............ 60 5.6
SEM image of silicon wafer treated with 0.01MFeCl2 ethanol solution and
seeded with silicon nitride powder ..................................................................................... 62
5.7
SEM image of silicon wafers treated with 0.01MFeCl2 ethanol solution seeded with
silicon nitride powder, and reacted in nitrogen at 1200 oC ................................................. 62
xiii
5.8
EDAX spectrum of silicon wafer treated with 0.01MFeCl2 ethanol solution and seeded
with silicon nitride powder ................................................................................................. 63
5.9
EDAX spectrum of silicon wafer treated with 0.01MFeCl2 ethanol solution and seeded
with silicon nitride powder ................................................................................................. 65
5.10 SEM image of silicon wafer after direct nitridation without catalyst ................................. 66 5.11 EDAX spectrum of nitrided silicon wafer without catalyst ................................................ 67 5.12 A schematic illustration of the coaxial nanowire (49) ........................................................ 69 5.13 SEM image of silicon nanostructures resulting from metal catalyzed etching ................... 70 5.14
SEM image of metal catalyzed etched on silicon after annealing at high
temperature under nitrogen ............................................................................................... 70 ®
®
5.15 KiON Ceraset polyureasilazane pre-ceramic polymer (50) ............................................. 71 5.16
SEM image of a pre-ceramic polymer precursor cured in nitrogen at ............................. 72 5.17
Higher resolution SEM image of a pre-ceramic polymer precursor cured in
nitrogen at 260 oC and annealed at 1200oC in nitrogen .................................................... 72
5.18
SEM image of pre-ceramic polymer precursor cured in air and annealed
at 1200oC in nitrogen ........................................................................................................ 73
5.19
EDAX spectrum of the polymer precursor cured in nitrogen ............................................ 74 5.20
EDAX spectrum nanowires and nanomaterials from preceramic polymer
precursor cured in air ........................................................................................................ 75
5.21
SEM image of annealed silicon nitride nanopowder with the presence of
iron (II) chloride catalyst .................................................................................................. 77
5.22
TEM image of annealed silicon nitride nanopowder with the presence of
iron (II) chloride catalyst .................................................................................................. 78
5.23
TEM image of individual nanowire which confirm that the nanowire has
crystalline structure. .......................................................................................................... 78
5.24
EDAX spectrum of nanowires and materials from silicon nitride nanopowder ................ 79 5.25
EDAX spectrums of nanowire tip ..................................................................................... 81 5.26
SEM image of products from high temperature and pressure reaction ............................ 82 xiv
5.27
SEM image of zirconium diboride nanowires (rods) milled and annealed in argon ........ 83 5.28
SEM image of zirconium diboride nanomaterials milled in air and annealed in argon ... 84 5.29
SEM image of zirconium diboride powder using magnesium as reducing agent............. 84 5.30
EDAX spectrum of zirconium diboride nanowires (rods) milled in argon ....................... 85 5.31
EDAX spectrum of zirconium diboride nanomaterials milled in air and
annealed in argon .............................................................................................................. 87
5.32 EDAX analysis zirconium diboride powder obtained by using magnesium
as a reducing agent with milling and annealing in argon.................................................. 88 xv
LIST OF ABBREVIATIONS BET
Brunauer, Emmett, and Teller technique
CVD
Chemical vapor deposition
EDAX
Energy disruptive X-ray florescence
LECVD
Laser enhanced chemical vapor deposition
LPCVD
Low pressure chemical vapor deposition
MOCVD
Metallo-organic chemical vapor deposition
PECVD
Plasma enhanced chemical vapor deposition
SCCM
Standard cubic centimeter
SEM
Scanning electron microscopy
TEM
Transmission electron microscopy
xvi
CHAPTER 1 INTRODUCTION 1.1 General Approaches for Fabricating Nanomaterials The use of nanomaterials is not a new phenomenon; in fact their use can be dated back to
the application of nanoparticles as dye materials in ceramics by ancient people (1). The ancient
method for synthesis of nanomaterials utilized one of the two broadly classified methods of
bottom-up or top-down fabrication (2). The bottom-up method focuses on creating the
nanomaterials by arranging atom by atom or molecule by molecule. As an example chemical
vapor deposition (CVD) is a bottom-up approach, whereby the atoms or molecules are reacted
in the gas phase and deposited onto a solid surface to form nanopowders, nanowires or
nanofilms. Conventional thermal CVD methods are widely used for the synthesis of
nanomaterials. Most previously investigated CVD methods have limitations due to the high
temperature needed to drive the reaction. Additionally considerable irreproducibility of the
results occur due to the uncontrolled variation of the concentration of the precursor overtime. In
this thesis we designed and assembled a costumed plasma enhanced chemical vapor deposition
system (PECVD) that precisely controls the supply of the precursor throughout the reaction. In
addition, the PECVD system lowers the temperature of the reaction by generating dissociated
gas molecules, ionized species, and other reactive gaseous mixture.
The top-down approach involves a reduction of size of the bulk material to the nanoscale. Mechanical alloying and etching can be cited as examples of top-down approaches of
nanomaterial fabrication. Although each synthesis route alone is theoretically sufficient to
produce nanomaterials, in practice both methods are often employed together. This situation
1
occurs in the solid state mechanical alloying process where the size of bulk materials is reduced
to the nano-regime, and simultaneously a solid state reaction creates nanomaterial. While size
reduction of the precursor is characteristic of the top-down method, the formation of a
nanomaterial from a solid state reaction afterwards can be considered a bottom–up approach.
This thesis examines the utility of the solid state alloying procedure for the production of
nanomaterials which can withstand exceptionally high temperature, and enhance mechanical
properties when present in nanocomposites.
The synthesis of nanomaterials by previously described methods is relatively
straightforward .However the fabrication of zero dimension nanomaterials is still a challenge
due to the tendency of these materials to form agglomerates as a result of their high surface
area to volume ratio. The process of fabricating nanowires (one dimensional nanomaterial) by
solid state alloying is even more complex, and is not yet well understood. In spite of the
challenges, research has been going on to exploit the novel properties of nanowires which are
expected to have some electronic, optical and mechanical properties that are superior to those of
the bulk counterpart.
In line with this, the objectives of this thesis are to assemble and evaluate a prototype
CVD system before putting together the actual system, design and construct microwave plasma
enhanced CVD system (PECVD), and finally synthesize and evaluate potentially useful
thermobarrier nanowires on substrates and in bulk quantities.
1.2 Literature Review The synthesis of thermobarrier nanomaterials such as silicon nitride, silicon carbide,
zirconium diboride, boron nitride, and aluminum nitride nanowires is investigated widely.
Research involves the fabrication, characterization, and testing of their effects on mechanical
2
properties of the composite that they reinforce. Nanowires such as silicon nitride (3), silicon
carbide (4), aluminum nitride (5), boron nitride (6), and zirconium diboride (7; 8) have been
used as reinforcement in matrix composites having high fracture toughness. Over the past several
years, great efforts have been placed on the synthesis of one-dimensional nanomaterials and
various methods of synthesis have been exploited. Depending on the properties of the
nanowires desired, different routes can be used to produce them.
Nitride wires such as silicon nitride (Si3N4) and aluminum nitride (AlN), which are
outstanding functional materials for many applications because of their good performances such
as their mechanical, chemical, electronic, optoelectronic and thermal properties, were produced
from the following synthesis routes. The first method is direct nitridation. It involves the direct
reaction of nitrogen or ammonia in the presence or absence of catalyst with the metal or nonmetal to form nitride nanowires as shown in the equation 1.1 and 1.2 (9; 10; 11; 12),
aM
xM
yNH
bN
MN
1.1
3y
H
2
M NZ
1.2
where M is Si or Al ; C, d, w, and z are stoichiometric coefficients; a, b, x and y
are numerical coefficients for balancing chemical reaction.
Without a catalyst the direct nitridation reaction is normally feasible in the vicinity of the
melting point of the solid reactant, but the use of a catalyst can reduce this to a much lower
temperature by forming a eutectic point. A plasma enhanced CVD system can alternatively be
used to produce a reactive gas mixture to realize low temperature reactions. Direct nitridation is
usually the first choice to synthesize nitride nanowires as it results in the formation of pure
nitride nanowires and there will be no need for additional product purification. However, it is
3
very difficult to obtain nanowires in fractional gram quantities or more from this procedure as
the nitride nanowires are formed on the surface of the substrate. Recovery of the material
requires them to be scrubbed from the substrate for further use in fabricating composites.
The second method is the reduction of silicon or aluminum compounds to produce nitride
nanowires. A good example is the carbothermic reduction of silica-containing compounds to
grow silicon nitride nanowires (3).
The third route to nitride nanowires growth and one of the most promising methods for
large scale nanowires production is the ball milling and annealing of pre-ceramic polymer
precursor (13; 14). The ball milling is mainly used to activate a solid state mechanical alloying
reaction which normally leads to nanowire growth during subsequent annealing under a nitrogen
or ammonia environment.
Some additional methods such as autoclave reactions which are high temperature and
pressure reactions (15), sol-gel synthesis (16), template assisted (17), and vaporization of
aluminum in ammonia (18) have also been reported for the synthesis of nitride nanowires.
Diborides of group IV transition metals are also thermobarrier materials since they
exhibit exceptionally high mechanical strengths and high melting points. The production of
diboride nanowires (rod) of group IV transition elements such as hafnium diboride has been
published. The method comprises mechanical alloying and annealing at high temperatures (19).
During mechanical alloying, the hydrated tetrachloride of hafnium is mixed with boron powder
and mechanically alloyed for two hours under argon to initiate the solid state reaction. During
the solid state mechanical alloying, diboride powder is formed. Then, the subsequent annealing
process under argon increases the conversion of the diboride powder to nanowires (rods) (19).
4
Silicon carbide is another class of thermobarrier materials and its nanowire production
has been extensively studied due to its promising application as an engineering material. It can
be synthesized via carbothermal reduction of silicon or silica xerogel(20; 21), ball milling and
annealing of pre-ceramic polymer precursor (22), annealing of SiC powder at high temperature
in the presence of an aluminum catalyst (23), catalyst assisted microwave heating(24) ,annealing
of SiC films (25),and metallorganic chemical vapor deposition (MOCVD) (26).
Boron nitride nanowires can be grown by the CVD method (27), and by ball milling and
annealing (28). During the course of this thesis, the synthesis of silicon nitride and zirconium
diboride nanowires were investigated.
Although one of the above mentioned methods can be employed for the preparation of
the nitride, boride and carbide nanowires, the solid-state mechanical alloying (ball milling) and
the subsequent annealing at high temperature is advantageous for large scale production. The
main reason for the scalability of this method is due to the fact that the solid-state mechanical
alloying can be carried out to fabricate nanowires in grams or even kilogram quantities without
affecting the efficiency of the process. For this reason, boron nitride and carbon nanotubes are
already commercially available as result of a solid-state mechanical alloying process and the
subsequent annealing (28). Since one of the aims of this investigation is to synthesize boride and
nitride nanowires and scale it up to the order of a gram or more, a solid-state mechanical alloying
process with the subsequent annealing at high temperatures approach was employed for bulk
production of nanowires.
1.2.1. High Energy Solid State Alloying High energy solid-state mechanical alloying (ball milling) is a top-down technique which
is used to reduce the particle size to the nanoscale. High energy solid-state mechanical alloying is
5
not only used to reduce the particle size to nano-regime, but also to initiate a solid state reaction.
The milling unit operation which produces a wide range of fine particles and initiates a solid
state reaction can be optimized by manipulating parameters such as type of mill, milling
atmosphere (inert gas or air), milling media (liquid, paste or solid), intensity of milling, ball to
powder weight ratio (BPR), milling time, and milling temperature (in some cases heat will be
generated by the reaction during milling which elevates the milling temperature).
Planetary ball mill is used during the experiment to grind the material to submicron and
activate a solid state reaction. The working principle of planetary ball mill is that the grinding jar
rotates about its own axis and, in the opposite direction, around the common axis of the sun
wheel as shown in Figure 1.1. The superimposition of the centrifugal forces produces grinding
ball movements with high pulverization energy. The centrifugal forces acting on the grinding jar
wall initially carry the grinding balls in the direction in which the grinding jar is rotating.
Differences occur between the speed of the grinding jar wall and the balls; this results in strong
frictional forces acting on the sample. As the rotational movement increases, Coriolis forces act
on the balls to displace them from the grinding jar walls. The balls fly through the grinding jar
interior and impact against the sample on the opposite grinding jar wall. This releases
considerable dynamic impact energy. The combination of the frictional forces and impact forces
causes the high degree of size reduction of planetary ball mills (29).
6
Figure 1.1 Schematic of illustration of the working principle of the planetary ball mill
apparatus
1.2.2. Chemical Vapor Deposition Chemical vapor deposition or CVD is a generic name for a group of processes that
involve depositing a solid material from a gaseous phase. This process is similar in some respects
to physical vapor deposition (PVD). PVD differs in that the pre-cursors are solid, with the
material to be deposited being vaporized from a solid target and deposited onto the substrate (29)
whereas chemical vapor deposition (CVD) is a solid deposition from chemical reactions of
vapors phase components. It belongs to the class of vapor transfer processes which are atomistic
in nature; the deposition species are atoms or molecules (29). It is a highly versatile process
which can be used in production of coating, fibers, nanopowder, nanowires, and nanofilms
1.2.2.1. Fundamental Mechanisms of Deposition in CVD A CVD reaction occurs when the vaporized or sublimed precursor is delivered to the high
temperature region of the reactor, reacts in the vapor phase, and then deposits onto the solid
substrate. Due to the presence of vapor and the solid substrate as well as the mechanism of the
7
reaction, the CVD reaction may be a homogeneous or heterogeneous reaction. The latter usually
involves the following steps (1).
1. Diffusion of growth species from vapor phase onto growing surface.
2. Adsorption of growth species onto growing surface or desorption from growing surface.
3. Surface diffusion of adsorbed growth species.
4. Surface growth when species integrated permanently into crystal structure.
5. By-product formed during reaction on the surface would be desorbed from the surface.
6. By products diffused to the bulk which leaves vacant space for further growth.
Figure 1.2 Schematic diagram illustrating the six steps in crystal growth. (1)
1.2.2.2.
Chemical Vapor Deposition (CVD) Modifications:
The traditional CVD is a thermal process where only heat energy is employed in
activating the reaction. Thermal CVD operates at high temperature that is usually maintained by
8
resistance heating, high frequency induction, radiant heating, hot plate heating, or any
combination of these processes. The operating pressure in thermal CVD ranges from milli-torr to
atmospheric pressure. However, there are various modifications of the thermal CVD to optimize
the deposition processes.
1.2.2.3.
Low Pressure CVD (LPCVD)
As it will be discussed in the section on plasma enhanced CVD reaction kinetics, the rate of
gas phase transport of reactants and by-products is inversely proportional to pressure in the
reactor. At low pressure between 1mtorr - 1 torr, the surface reaction is the rate determining step
and the mass transfer becomes less critical than atmospheric pressure. In this kind of system, the
feed feed flux can be controlled to provide a higher initial gas concentration of reactants. This
usually leads to higher deposition on the substrate, better film uniformity, and better film
coverage over steps and fewer defects (30).
1.2.2.4. Microwave Plasma Enhanced CVD (PECVD) A plasma consists of cation, electrons with negative charge, neutral atoms, ionized
molecules, and radicals. Thermally, the temperature of the gas can be equivalent to as high as
5000K (30) . However, it is difficult to operate at this temperature due to the lack of materials
that can withstand this high temperature. A convenient way of generating the plasma is by low
frequency discharge, where increasing the electrical energy of the gas would lead to dissociation
and ionization of molecules.
Plasma reactors use radio frequency (RF), r microwave (MW) energy glow discharge
electrodes, or resonate cavities. A typical microwave wave plasma apparatus operates at a
frequency of 2.45GHz and was employed in this investigation. The microwave plasma is utilized
to dissociate gas molecules, ionize them, generate an electron density sufficient to keep the
9
plasma going and activate the gas mixture. This results in a reactive gas and low temperature
deposition downstream.
1.2.2.5. Laser Enhanced CVD (LECVD) A laser produces coherent, monochromatic, high energy beam of photons which can be
used effectively to activate the CVD reaction (30). This method is also called photo-assisted
CVD. There are two types of processes to achieve laser enhanced surface reactions. It can be
either pyrolytic, where the substrate is heated locally by selected wavelength of laser or
photolytic, where gas phase dissociation occurs by UV laser excitation to enhance reactivity.
During the pyrolytic process, the wave length of the laser is such that little energy is absorbed by
the gas molecules.
1.2.2.6.
Metallorganic CVD (MOCVD) MOCVD is a type of CVD which utilizes metallo-organic compounds as the precursor. It
is also called organometallic vapor phase epitaxy (OMVPE). Metallo-organics are compounds in
which the atom of a metal element is bound to one or more carbon atoms of a hydrocarbon
group. The term metallo-organic is moderately used to include non-metallic compounds such as
silicon, phosphorous or arsenic. Metallo-organic precursors can be deposited at relatively low
temperature due to the fact that they vaporize at low temperature to generate reactive vapor for
CVD reactions (30).
1.3. Scope of the Study Chapter 1 provides an Introduction, review the literature, covers the fundamentals of high
energy solid-state alloying reactions, and covers chemical vapor deposition methods for
deposition of nanowires.
10
Chapter 2 discusses the design and assembly of the microwave plasma chemical vapor
deposition system. Chapter 3 focuses on the experimental aspects of the study and elaborates
upon the materials, condition and methods used. Chapter 4 illustrates the condition and
parameters for the large scale production of nanowires and nanomaterials. The large scale
production of nanowires is still a challenge. The kinetics, mass transport and surface chemistry
considerations that may lead to the design of a scalable reactor are discussed. Chapter 5 focuses
on the discussion of the results obtained from the executed experiments. Finally, Chapter 6
provides conclusion along with recommendations for future work.
11
CHAPTER 2 DESIGN AND ASSEMBLY OF A MICROWAVE PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION (PECVD) SYSTEM 2.1. Introduction Chemical vapor deposition (CVD) in general is a very versatile method for the synthesis
and deposition of materials. The CVD system incorporates various components. When it
integrates a microwave plasma or radio frequency as the energy source as explained in section
1.2.2.4, the method is labeled as plasma enhanced CVD (PECVD). The other components such
as the precursor delivery system, mass and pressure controllers, heating oven or furnace, a quartz
or alumina tube reactor, and an exhaust system are common in all kinds of CVD systems. The
need for reproducible experimental result due to better controlled parameters as well as relatively
low temperature reaction necessitate for the design of custom made PECVD system.
2.2.
Microwave PECVD System Design 2.2.1. Prototype PECVD System The design of a prototype PECVD system provides experimentally desired data that help
to determine the optimum working parameters for the final system. Before designing the final
system, the prototype system was assembled according to the schematic diagram shown in
Figure 2.1. The prototype system was used to experimentally establish the pressure range that
should be maintained to ignite and sustain the microwave plasma. In addition, the effect of the
flow rate of the precursor on the optical intensity of the plasma, and the capacity of the vacuum
12
pump necessary to achieve the desired pressure in the system were investigated using the
prototype system.
After running consecutive experiments and investigation in the prototype PECVD
system, it was confirmed that the microwave plasma could be mostly sustained at pressures
below 100 torr, but the optimum was found to be between 5 to 20 torr. This indicated that the
vacuum pump should have a capacity to maintain the PECVD system below 100 torr, and
possibly in the range of 5 to 20 torr. It was also established that the microwave plasma can be
ignited after tuning the power dial to more than 80% of the maximum power while maintaining
the reactor pressure below 100 torr. Once the microwave plasma was ignited, it was possible to
maximize the forward power and minimized the reflected power by fine tuning the head and side
knobs of the microwave cavity. It was also noted that the gas flow rate has little effect on the
ignition of the microwave plasma as long as the pressure in the system was kept below 100 torr.
Moreover, the prototype system was helpful in establishing the temperature range of various
nitridation experiments which was carried out successfully later in the final PECVD system. The
prototype system was utilized until the final system was assembled.
13
Figure 2.1 Schematic diagram for the microwave plasma system
14
2.2.2. Precursor Delivery Section of a PECVD System The transportation of precursors to the CVD reactor is very critical for efficient
deposition and synthesis of materials. Hence, the plasma reactor system includes the precursor
delivery section, which is essential for controlling the delivery of the gaseous reactants. The
design and selection of the precursor delivery system mainly depend on the type of precursors to
be utilized. Traditionally, the precursors used in CVD processes are gaseous. However, liquid
and solid precursors are not uncommon. A liquid precursor is typically transformed into a vapor;
the vapor then reacts, and deposits a product onto the surface of a substrate. Vaporization can be
done by bubbling a carrier gas through the liquid precursor which draws the vapor from the
bubbler.
A solid precursor is usually poured into a sampling boat and placed inside a reactor
held at a sufficient high temperature where volatile component react with the flowing gas or a
vaporized liquid precursor to give the desired product. If metallorganic precursors or easily
vaporizable solid precursors are used, sublimation results in the delivery of a vapor from a
bubbler. Therefore, liquid and solid precursors need a special delivery system which includes a
bubbler, an online heater to prevent solidification of the precursor before it is delivered into the
reactor, a pressure regulator, and a mass flow controller to meter the flow into the reactor.
However, the precursor in a gaseous state can be fed directly into the reaction chamber by
controlling the mass and pressure.
With the aim of utilizing the plasma system with solid, liquid or gaseous precursors, the
design incorporated a custom made precursor delivery section. This system is shown in Figure
2.2, which was supplied by Lorex Industries (Lorex Industries, NY), integrated a stainless steel
bubbler, a control valve, a mass flow controller for the vaporized precursors, a Piezocon®
acoustical gas concentration controller for precise mass flow control, and a heat jacket to prevent
15
re-condennsation of vaapor inside the
t delivery line. In addiition to the customized
c
L
Lorex
delivery
unit, the plasma systeem also incluudes a flow controlled pathway
p
for either
e
the dirrect inlet of
p
or
o the alternaative flushingg of the PEC
CVD system with an inerrt gas.
gaseous precursors
Figure 2.22 Custom deesigned reageent delivery system
s
16
2.2.3. Reaction Chamber A CVD reactor has a chamber in which gaseous reagents at high temperature and
controlled pressure undergo chemical reactions that result in the deposition of the material onto a
solid surface or the synthesis of gaseous products inside the reactor. Though there are several
types of laboratory scale CVD reactors, only two types will be discussed here. The first one is a
cold wall CVD reactor. In this reactor, the surfaces within of the reaction zone are kept at low
temperature except the substrate which is maintained at the required temperature. This will cause
the deposition to occur on the surface of the substrate rather than all over the surface of the
reactor. A tube reactor is another kind, which is commonly used as a lab scale CVD reactor. In
this case, the gas flows axially downstream over the surface of the substrate. The tube reactor is
often made of fused silica, but an aluminum oxide tube can be also employed for high
temperature applications. In line with this trend, the selected PECVD reactor, which is shown in
Figure 2.3, was a 1″ diameter, 4 ft long quartz tube which was the right size to fit a 1″ Evenson
microwave plasma cavity. The tube also provided sufficient space for housing a substrate or
sample boat. Moreover, a 1″ diameter and 4 ft long aluminum oxide tube reactor was also
selected for possible high temperature reactions. The quartz tube reactor was supplied by
National Scientific Company of Pennsylvania and the aluminum oxide tube reactor was supplied
by Sentro Tech Corporation of Ohio.
17
Figure 2.3 Quartz tube reactor
2.2.4.
Split Heating Furnace A typical thermally induced CVD reaction occurs at a higher temperature than plasma
assisted one. In the latter case the temperature is usually lower than the conventional thermal
CVD, since the microwave plasma breaks down the gaseous precursor, and converts it into a
mixture of reactive atoms, ions, electrons, and intermediates. Nevertheless, most PECVD
depositions and syntheses are realized above room temperatures, which require maintaining a
desired temperature in heating zone. After considering the processing parameters such as furnace
volume, thermal input, and maximum process temperature, the Mellen split heating furnace oven
model SC-12 which is shown in Figure 2.4 was chosen to heat the portion of the reactor to the
deposition temperature. This heating oven operates up to a maximum of 1200oC and also has a
geometry which supports the tube reactor.
18
Figure 2.4 Mellen SC-12 split heating oven and controller
2.2.5.
Microwave Plasma Unit The microwave plasma unit consists of a 2450 MHz microwave generator, a coaxial RF
cable, and a microwave cavity. The microwave generator model MPG-4M, and 1″ Evenson
cavity from Opthos Instrument Inc. shown in Figure 2.5, Figure 2.6, and, Figure 2.7 were
selected for assembling the microwave plasma unit.
Figure 2.5 Microwave generator (31)
19
Figure 2.6 1″ Evenson microwave cavity
Figure 2.7 Coaxial RF cable
The microwave unit is assembled in such a way that the RF coaxial cable connects the
Evenson cavity to the microwave generator, and the 1″ quartz tube reactor is inserted through the
microwave cavity. First, the microwave plasma is initiated by increasing the main power to more
than 80 % of the maximum power which is followed by tickling the cavity with a Tesla coil after
setting the Tesla output between 5 to7. Once the microwave plasma is ignited, the forward power
can be maximized by adjusting the head and side knobs of the microwave cavity shown in
Figure 2.6.
20
2.2.6. Mass Flow and Pressure Controller The mass flow controller measures and controls the flows of gases, and is used in tandem
with the pressure controller to regulate the pressure of the flowing gas stream. Hence, integrated
mass flow and pressure controller help to set the mass of the reactants entering into the reactor
per minute, while the pressure controller monitors and regulates the reactor pressure by opening
and closing the downstream valve. The controllers are very important for accurate measurement
and adjustment of parameters. The MKS instrument four/eight channel mass flow and pressure
controller display unit model 647 CSR1T which is depicted in Figure 2.8 was employed for this
purpose. The controller displays unite connected to the mass flow controller shown in Figure 2.9
and used to set up the flow of reagents through it.
Figure 2.8 Mass and pressure controller display unit(32)
Figure 2.9 Mass Flow controller (33)
21
2.2.7.
Pressure Controller Pressure control was achieved by installing an online upstream and downstream pressure
sensor. Moreover, the control valve was installed downstream to set the reactor pressure to the
desired point. Baratron® High Temperature Absolute Capacitance Manometers 631B (Figure
2.10) integrated with 148J Control valve (Figure 2.11), from MKS Instruments, was used to
measure and control the pressure inside the quartz tube reactor.
Figure 2.10 Baratron® high temperature absolute capacitance manometers 631B (34)
Figure 2.11 All metal control valve 148J (35)
22
2.2.8.
Pumping Station During PECVD reaction, product formed, gaseous by product generated, and reactants
continuously flow into the reactor. This cause pressure build up inside the reactor overtime.
Therefore, the by-product and unreacted gaseous reactants should be exhausted continuously to
keep a constant pressure inside the reactor. For these reasons Edward vacuum pump shown in
Figure 2.12 was used to exhaust the gases and maintained a constant vacuum during the reaction.
Figure 2.12 Edwards E2M40 vacuum pump
23
2.2.9.
Exhaust Treatment System The unreacted reactants, by product and escaped product from PECVD reaction are not
always harmless to be exhausted directly to the atmosphere. In some instances, exhaust gases
may require treatment or conversion to safe or harmless compounds. The exhaust system could
comprise simple water scrubbing, or acid or base neutralization station to scrub the dangerous
gas leaving from PECVD chamber. Since nitrogen and argon were used as reactants during the
experiments, there was no need for exhaust treatment system. However, the PECVD system can
integrate with any kind of scrubber when the need arise.
Once the above listed components, as well as the necessary vacuum fittings and flex
connection joint were acquired, the system was assembled on a custom designed work bench,
tested and made functional. The assembled PECVD system which is shown in Figure 2.13 was
used throughout the investigation of the synthesis of the high temperature nanowires, and
nanomaterials.
24
Figure 2.13 Plasma enhanced CVD system
25
CHAPTER 3 EXPERIMENTAL SECTION 3.1 Synthesis of Silicon Nitride Nanowires 3.1.1
Synthesis of Silicon Nitride Nanowires by Direct Nitridation The synthesis of silicon nitride nanowires from the reaction of a silicon wafer and
nitrogen or ammonia is often referred to as direct nitridation. Direct nitridation results from the
process whereby nitrogen or ammonia reacts with silicon at the surface of the silicon substrate to
generate silicon nitride, as shown in Equations 3.1 and 3.2.
3
3
3.1
2
4
6
3.2
3.1.1.1. Experimental Procedure Silicon (111) wafers with 0.001-0.005 Ω resistivity (University Wafer, South Boston,
MA) were cut into pieces of 1.0 cm x 0.5 cm. The silicon wafers were then cleaned with
methanol, acetone and trichloroethylene to remove grease and other contaminants from their
surfaces. Since thin film layers of silicon oxide are normally present on the surface of the silicon
wafer, wafers were etched with 0.125% HF solution for one minute to generate an oxide free
surface. The etched wafer was then rinsed with ultra-pure water and blow dried with nitrogen.
The dried silicon substrate was then treated with 0.01M FeCl2 ethanol solution for 30 seconds;
put into a ceramic boat; and placed in the central region of the quartz tube reactor. Before
initiating the experiment, the plasma enhanced chemical vapor deposition system shown in
Figure 2.13 was pumped down for 30 minutes and then flushed with nitrogen for an additional
26
30 minutes to remove oxygen. Once the system was flushed with nitrogen, the Microwave
plasma (2450 MHz model MPG-4M Opthos Instruments Inc., Rockville MD) was ignited by
tickling with a Tesla coil while adjusting the main power to 90% of the maximum power. To
sustain the microwave plasma, the forward power was adjusted to 90 watts and the reflected
power to 6 watts while the pressure of the system was maintained between 50 and 70 torr. The
quartz tube reactor was heated within a SC-12 split oven (The Mellen Company, NH) to 1200oC
rapidly, and held at this temperature for 30 minutes. The oven was eventually switched off and
allowed to cool down to room temperature. During the heating and cooling stage, the system
was purged continuously with nitrogen at a rate of 200 sccm. The experiment was then repeated
under different condition including immersing the silicon wafer in 0.01M FeCl2 followed by
seeding with silicon nitride powder , by increasing the concentration of FeCl2 to 0.05M, and
performing the nitridation reaction without the FeCl2 catalyst.
3.1.2
Synthesis of Silicon Nitride Nanowires and Nanomaterials from a Pre­
Ceramic Polymer Precursor This procedure contains three consecutive steps which should be well controlled to
synthesize silicon nitride nanowires (36) . The polymer precursor which is received in
liquid form was first cured under nitrogen for 30 minutes at 260oC, ball milled for 24
hours, and then annealed at 1200oC in a nitrogen environment in order to synthesize
silicon nitride nanowires within silicon nitride nanopowder.
3.1.2.1.
Experimental Procedure The Ceraset® Polyureasilazane (Kion Corporation, Huntingdon Valley PA)
polymer precursor, which is a liquid as received, was poured in a quartz tube boat shown
27
in Figure 3.1 and placed inside a 2″ quartz tube reactor . Then nitrogen was flowed at 200
sccm while the reaction zone was heated in a SC-12 split oven furnace at a temperature
of 260oC for 30 minutes. This resulted in a cured white solid. The obtained solid was then
mixed with FeCl2 (the added FeCl2 was 3% of the mass of cured pre-ceramic polymer)
and ball milled in a Retch Planetary Mill PM 200 apparatus (Retch America, Newton PA)
at a rotational speed of 300 rpm for 24 hours. Out of the milled pre-ceramic polymer,
5-25 mg of it was placed inside a boron nitride sample boat shown in Figure 3.2, inserted
o
to the center of the 1″ quartz tube reactor and heated at a rate of 10oC/ min to 1200 C. The
system was held at this temperature for 2 hours. The annealing temperature curve is shown
in Figure 3.3. Nitrogen at a constant flow rate of 200 sccm was flowing during heating, the
reaction, and cooling of the PECVD system. After 2 hours of reaction at 1200oC, the oven
was cooled down naturally to room temperature in the same gas. The experiment was also
carried out by heating the oven at rate of 40 oC/ min instead of 10oC/ min.
Figure 3.1 Quartz tube sample boat
28
Figure 3.2 Boron nitride sample boat
1400
Temperature oC
1200
1000
800
600
400
200
360
340
320
300
280
260
240
220
200
180
160
140
120
80
100
60
40
20
0
0
Time (min)
Figure 3.3 Annealing process for the pre-ceramic polymer precursor
3.1.3.
High Temperature and Pressure Autoclave Synthesis of Silicon Nitride Nanowires The autoclave reaction as a method of synthesis of silicon nitride nanowires was first
reported by Zou et al (37).
3.1.3.1. Experimental Procedure The executed autoclave reaction can be written as
29
2Mg N
3SiCl
6MgCl
Si N
3.3
The autoclave reaction illustrated in Equation 3.3 above was carried out in a 25 mL stainless
steel reactor, autoclave (SS-4CS-TW-25, 25mL sample cylinder, Swagelok). As per the Zou et
al. (37) procedure, 1 g of Mg3N2 (99.6 %, Alfa Aesar) and 12 mL of silicon tetra chloride
(99.8%, Fischer Scientific) were poured into the 25 mL autoclave. The autoclave then was
tightly sealed and heated inside a tube furnace at a rate of 15oC / min to 600oC, and maintained at
this temperature for 10 hours. After 10 hours of reaction and subsequent cooling to room
temperature, the powdered products were poured into and washed with 0.1M nitric acid,
followed by distilled water, and dried in a glove box maintained in a nitrogen atmosphere.
3.1.4.
Metal Catalyzed Etching of Silicon Wafers This experimental procedure is similar to the one described by Zhu et al (38) to synthesize
silicon nanowires in a Teflon-lined stainless steel autoclave which contained 5 M HF and 0.02M
silver nitrate solution to synthesize silicon nanowires. However, the original experiment was
modified, since a Teflon beaker was used instead of the Teflon lined stainless steel autoclave and
most importantly pure Si (111) was used instead of B-doped Si (111).
3.1.4.1.
Experimental Procedure A Si (111) 0.001-0.005 Ω wafer was cut into 1.0 cm x 0.5 cm pieces and cleaned in
acetone, methanol and tetrachloroethylene. The cleaned wafer was then etched in a solution
containing 5M HF (HF acid 49%, Fischer Scientific) and 0.02M silver nitrate (99.0% SigmaAldrich, USA) for 10 minutes at 60oC. First, the Teflon beaker containing the solution was
30
immersed in a water bath and heated up to 60oC. Then, the wafer was dipped into the solution
and etched for 10 minutes, resulting in the formation of a thick silver film on the silicon wafer
surface. The wafer was scraped using an X-acto knife to remove the thick spongy film, rinsed
with water, and blown dry in nitrogen. The dried and etched wafer was first characterized using
scanning electron microscopy (SEM) to confirm the formation of silicon nanowires, and then
put into the a ceramic boat and placed inside the center of the quartz tube reactor. After
loading the wafer, the PECVD system was pumped down for 30 minutes and flushed with
nitrogen for an additional 30 minutes. Once the oxygen was eliminated from the system, the
microwave plasma was ignited by tuning the main power to 90 % of the maximum power. Then
the forward power and reflected power were optimized to 70 watts and to less than 10 watts
respectively by fine tuning the 1″ Evenson cavity (shown in Figure 2.6) while maintaining the
system pressure below 50 torr. The reactor was heated to 1000oC and kept at this temperature for
30 minutes before being cooled down to room temperature in the same gas. The same experiment
o
o
o
was carried out at 600 C, 700 C and 800 C.
3.1.5.
Transformation of Silicon Nitride Nanopowder into Nanowires One of the challenges of silicon nitride fabrication is the formation of silicon dioxide
since it is thermodynamically more favorable than silicon nitride. As a way of minimizing the
oxidation of the precursor, silicon nitride nanopowder was utilized as a precursor. Conversion of
silicon nitride nanopowder into nanowire is also advantageous as it is a one step process.
3.1.5.1. Experimental Procedure Two grams of silicon nitride nanopowder (<50 nm particle size (BET), ≥98% trace
metals basis, Sigma Aldrich, USA) and 60 mg of FeCl2 (3% of the weight of silicon nitride
31
nanopowder) were mixed and milled in Retch planetary ball mill for 30 minutes. The milling
served only as a means of mixing silicon nitride nanopowder and the FeCl2 catalyst very well for
an efficient reaction during annealing.
After the solid state mixing, 500 mg of the sample was poured in the quartz sampling
boat and placed inside the center of the quartz tube reactor. The PECVD system was then
pumped down for 30 minutes followed by flushing with nitrogen for 30 minutes. After which,
the system was heated at a rate of 10oC/min to 1200oC and kept at this temperature for 2 hours as
nitrogen was flowing at 200 sccm . The system was then cooled down to room temperature in the
same gas. During the entire process, a constant pressure of 150 torr was maintained. Since the
precursor was silicon nitride nanopowder, there was no need for its transformation; hence, the
microwave system was not turned on during this experiment.
3.2
Synthesis of Zirconium Diboride Nanowires 3.2.1.1
Experimental Procedure A Boron ( crystalline, −60 mesh, 99% , Sigma-Aldrich , USA) and anhydrous
zirconium(IV) chloride ( 98% , Fischer Scientific , USA ) were weighed according to the
stoichiometric ratio of
3ZrCl
10B
3
4
3*(233.33) 10*( 10.81 ) 3*(112.8)
3.4
4*(117.31)
First, the anhydrous zirconium (IV) chloride was hydrated for an hour or two by exposing it to
air. The mixture of partially hydrated zirconium (IV) chloride, 50% excess boron and 10% (w/w)
FeCl2 was poured into a 50 mL ZrO2 container and sealed in an argon atmosphere. Once the
32
mixtures as well as the 3mm diameter grinding balls were sealed inside the Retch Planetary
PM200 mill under argon, the solid- state mechanical alloying was carried out for 2 hours at a
rotational speed of 300 rpm. After the solid state reaction, 25 mg of powder was dispensed into
the boron nitride sampling boat and placed inside the 1″quartz tube reactor shown in Figure 2.3.
Before the reaction, the PECVD system (shown in Figure 2.13) was pumped down for 30
minutes and flushed with argon for another 30 minutes. Subsequent to flushing, the quartz tube
reactor was heated rapidly (about 40oC / min ) to 1100oC inside a SC-12 split oven; held at this
temperature for an hour while argon was continuously flowed in to the system at a rate of 200
sccm; and cooled down to room temperature in the same gas. A pressure between 100- 120 torr
was maintained during the experiment. The same experiment was carried out by milling the
mixture in air as described above instead of argon.
3.2.1.2 Experimental Procedure B Boron, anhydrous zirconium (IV) chloride and magnesium (99 % powder, sigmaAldrich) were weighed according the stoichiometric ratio of
ZrCl
2B
2Mg
2
3.5
First, the anhydrous zirconium (IV) chloride was hydrated for an hour or two by exposing it to
moisture or air. The mixture of partially hydrated zirconium (IV) chloride, 50% excess boron,
50% excess magnesium and 10% (w/w) FeCl2 was poured into a
50 mL ZrO2 container and sealed in argon atmosphere. Once the mixture and 3mm diameter
grinding balls were sealed inside the Retch Planetary PM200 mill under argon, the solid state
mechanical activation was carried out for 2 hours at 300 rpm. After the mechanical activation, 25
33
mg of powder was dispensed into a boron nitride sampling boat and placed inside the 1″ quartz
tube reactor set up as shown in Figure 2.3. The PECVD system (shown in Figure 2.13) was
pumped down for 30 minutes and flushed with argon for another 30 minutes. Subsequent to
flushing, the quartz tube reactor was heated rapidly to 1100oC inside a SC-12 split oven furnace
under the flow of argon at 200 sccm; held at this temperature for an hour; and cooled down to
room temperature in the same gas. A pressure of 100 -120 torr was maintained during the
reaction.
3.3
Operation of Microwave Plasma CVD (PECVD) System 3.3.1.1.
Experimental Procedure A PECVD system described in chapter two, and shown in Figure 2.13 was employed to
carry out most experiments. First, the solid precursor or wafer section was placed in the quartz
boat and positioned inside the quartz tube reactor. Then, both sides of the reactor were sealed and
the PECVD system was flushed down to remove oxygen which was followed by the flow of a
gas of interest such as nitrogen and argon for 30 minutes to an hour. Once the reactor was filled
with the gaseous reactant, the pressure of the PECVD system was adjusted by opening and
closing the downstream valve. The microwave plasma was ignited after the pressure was set, and
optimized by tuning the head and side knob of the microwave plasma cavity as shown in Figure
2.6. The reactants feed stream flowed through the system with the microwave plasma ignited.
The reactor was heated inside the tube oven to the desired temperature and maintained at the
desired temperature during the reaction. After the reaction, the system was cooled down to room
34
temperature in the same gas. Finally, the pressure was raised to atmospheric pressure and the
reactor was opened and the product taken out for characterization.
3.4
High Energy Solid­State Mechanical Alloying 3.4.1.1. Experimental Procedure High energy solid-state mechanical alloying was used for activating a solid-state reaction.
Planetary ball mill apparatus PM200 (shown in Figure 3.4) was employed for the high energy
solid state mechanical alloying. First, the chamber was opened by pressing the open key shown
in Figure 3.5.
Figure 3.4 Planetary ball mill apparatus PM200
35
Figure 3.5 Planetary ball mill PM200
Figure 3.6 A PM200 controller
36
After the PM200 chamber was opened by pressing the “open key” shown in Figure 3.6, the
grinding jars were removed by turning the three star grips to the left while pulling the red sleeve
upward as indicated in Figure 3.7.
Figure 3.7 Fastening and removing of grinding jars
Then the material to be mechanically alloyed was poured into the grinding jars up to one third of
its volume and was filled with grinding balls up to a maximum of two thirds of the volume of the
jars. One third of the volume of the grinding jar was left unfilled for efficient mechanical
alloying as the grinding ball and the material to be alloyed need the space to maximize the
friction and impact force. The jar was secured in the PM200 Planetary Ball Mill apparatus by
turning the three star grips to the right while pulling the red sleeve upward as indicated in Figure
3.7. The mechanical alloying chamber was then closed and the control button was set to select
the appropriate program. A grinding speed of 300 rpm was mostly used, but the timing depends
on the material to be mechanically alloyed. For example the duration of the mechanical alloying
of the pre-ceramic polymer precursor was 24 hours. After setting the alloying time and rate of
37
rotation of the PM200, the instrument was started by pressing the start key. When “the start
key” was pressed, the safety system cautions to check if the grinding bowls were properly
clamped as indicated in Figure 3.8. By selecting “yes” the alloying process was initiated.
Figure 3.8 Safety precaution display
At the end of the solid state mechanical alloying, the PM200 chamber opened up automatically
and the grinding jar was manually loosened to take the material out for annealing.
38
CHAPTER 4 SCALED PRODUCTION OF NANOWIRE MATRIX 4.1.
Bulk Synthesis of Nanowires The challenges confronting the production of large scale nanowires is due to the
complex properties of materials with nanometer dimensions, which are significantly different
from those of bulk materials. Nanometer sized materials have a large fraction of surface atoms,
high surface energy, spatial confinement, and reduced imperfections, which do not exist in the
corresponding bulk materials (2).Due to their small dimensions, nanomaterials have extremely
large surface area to volume ratio, which induces a large fraction of the atoms of the materials to
be the surface or interfacial atoms. This creates more “surface” dependent material properties.
Hence, the productions of nanowires tend to be surface reaction dependent since the individual
atoms would be nucleated and organized in the nano regime. For these reasons, the production of
nanowires may require a reactor with large surface area. However, for such a reactor the same
underlying principles can be applied in the analysis of mass transfer, heat transfer, and reaction
kinetics accompanying nanomaterials growth.
4.2.
Plasma Enhanced Chemical Vapor Deposition Reaction Kinetics One type of PECVD reaction utilizes a solid precursor in the form of a powder or a solid
surface which is place inside the reactor in a sample holder. Gaseous components flow through
the reactor, and react with the solid precursor. The mechanism can be simplified as
4.1
39
Where
A (np) nanoparticle
B(g) gaseous reactant
C (NW) Nanowires
D(g) by- product leaving the reactor
Figure 4.1 Diffusion from gas to solid surface
Where
J1 is the molecular flux from the gas phase to the precursor surface.
J2 is the consumption flux A (g) corresponding to the surface reaction.
Then
CG
C
4.2
K C
4.3 (39)
Where
hG is gas diffusion rate constant
CG is gas phase concentration of A
40
CS is surface concentration of A
KS is surface reaction constant
At steady state
CG
C
K C
4.4
CG
J
K
4.5
hG
Growth rate
4.6
Where
is unit volume
r is the rate of the reaction
Limiting Reaction
Case 1
When the mass transfer is rate limiting, then equation (4.4) is simplified to
K
CG
4.7
Case 2
When the surface reaction is rate limiting, then equation (4.4) is reduced to
K
K CG
4.8
Case 1
When mass transfer is rate limiting:
41
Figure 4.2 Mass transfer in the PECVD reactor
Boundary Conditions
Y=0
U=0 (growth surface; no slip boundary condition at solid surface)
Y=D
U=0
For fluid flow through the circular tube, the velocity profile is
P
PL R
4.9 (40)
/R
μL
Where
R is the radius of the reactor
µ is the dynamic viscosity
L is the length of the tube
U is the velocity of the fluid
Po is modified pressure at the inlet of the reactor
PL is modified pressure at the outlet of the reactor
ro is the radial distance from the center of the reactor ; when r=R the boundary conditions
are satisfied. The assumption that the thickness of the solid reactant is negligible compared to the
diameter of the reactor is made to drive the velocity profile.
42
Boundary Layer
The relation of the boundary thickness to the fluid properties and velocity is given by
Von Karman momentum balance (40), which is derived from the equation of continuity and
equation of motion.
(40)
δ
∂u
u ⎞ ρu ⎤
∂ ⎡
δ
2 ⎛
dy ⎥ + (ρvue − ρvu ) 0 + ρ e ue e
⎢ ρ e ue ∫ ⎜⎜1 − ⎟⎟
ue ⎠ ρ e ue ⎦
∂x ⎣
∂x
0⎝
δ
∂τ xy
⎛
ρu ⎞
⎜
⎟
1
dy
−
=
−
∫0 ⎜⎝ ρ eue ⎟⎠
∫0 ∂y dy
δ
4.10
Where u is the velocity of the fluid; ρ is the density of the luid ; τ xy is shear
stress.
Assumption
1. ρ=ρe
u is a function of y /δ, where δ(x) is the boundary layer thickness of :
u
U∞ f
u
U∞
for 0< y < δ(x)
δ
for y> δ
The general solution is
(boundary –layer region)
(potential flow region)
δ x
A .
µ
ρU∞
- B; parabolic dependence
where A,B are constant.
~
Since
~
∞
/
43
4.11
4.12
Where
δ is the boundary layer thickness
P is the total pressure
If the mass transport in the boundary layer proceeds by diffusion alone, then the rate constant
hG can be written as
~
; Where
4.13
(39).
DAB is the diffusion coefficient
Then equation (4.7) can be written as
CG
where CG
PA
For ideal gas CG
4.14
RT
PA ( partial pressure of gaseous species ) ;
Equation 4.1 can be written as
PA
∞
4.15
/
PA
PA .
∞
∞
Equation (4.17) implies that if the mass transfer is the rate limiting, the rate of nanowire
growth can be enhanced by lowering the pressure (P) of the system which explains why most
of the CVD systems operate below atmospheric pressure.
Case 2
When the surface reaction is rate limiting, the rate can be reduced to equation (4.18)
/
K
/
CG
4.16
Where E is the activation energy ; R is the universal gas constant, and
T is temperature in oK and
K
/
This is the modified Arrhenius equation (41),
44
4.17
where
B is a temperature independent constant; n is also a constant.
The overall reaction rate can be written considering both mass transfer and the
4.18
surface reaction as resistances in series
PA .
e
E /RT
where CG
PA
In most cases at high temperature, mass transfer is the rate limiting step whereas at low
temperature the surface reaction is the rate limiting. Graph of ln r vs
ln
∞
can
be drawn to determine the optimum operation of the reactor for the growth of nanowires.
4.3.
Kinetics of Nanowire Growth via the Vapor–Liquid–Solid Mechanism
The growth of nanowires is a multistage process which comprises forming a desired
product and growing the nanowires out of it. For instance, a silicon nitride nanowire from preceramic polymer precursors or silicon substrates requires the conversion of the precursor to
silicon nitride, and eventually the growth of silicon nitride nanowires out of the product. One of
the most accepted mechanisms of nanowire growth in the presence of a catalyst is the vaporliquid-solid (VSL) mechanism (42; 43).
In the VSL mechanism, the catalyst forms a eutectic liquid droplet which becomes a
preferential site for the vapor condensation. As the vapor deposited in this eutectic liquid droplet,
it becomes supersaturated which leads to nucleation and then nanowire grow (42; 43). The
45
precipitation will continue to result in the growth of nanowires until the material is consumed or
the growth conditions change.
Figure 4.3 Schematics drawing showing the VSL growth of nanowires
The growth of silicon nitride nanowires from a polymeric pre-ceramic precursor in the presence
of iron (II) chloride could be explained pictorially as showing in figure 4.4.
Figure 4.4 Schematic illustration of the possible steps in catalyst assisted nanowires growth
from a solid substrate.
46
Figu
ure 4.5 SEM
M image of FeCl
F 2 catalysst assisted silicon
s
oxyn
nitride nanoowire growth
h
Element
Weight%
Atomic%
Fe K
100.00
100.00
Totals
100.00
F
Figure
4.6 EDAX
E
specttrum of tipss of nanowirres showingg the 100% catalyst.
c
47
As the SEM image and EDAX of tips of nanowires indicated the experimentally grown
nanowires , the process of vapor-liquid-solid growth appears to be validated by the position of
the catalyst particle.
4.4.
Reactor Design for Scaled Production of Nanowire Matrix
As discussed in section (4.1) and (4.2), and depicted in Figure 4.7, a PECVD reactor has
a complex coexistence of
processes which consist of mass transfer,homogeneous and
heterogeneous reactions, surface reactions, surface chemistry, and heat transfer. Therefore,
sufficient contact of gaseous and solid reactants is very critical for reasonable conversion of
reactant to products. The following reactor design is suggested to satisfy the above requirements.
48
Figure 4.7 Schematic illustration of various reactions taking place in PECVD reactor
49
4.4.1.
Shelf/Bed Reactor A shelf reactor which is shown in Figure 4.8 is a conceptual reactor which can be used
for production of a few grams up to a few kilograms of nanowires. It will provide the necessary
large surface area for better contact of gaseous and solid reactants.
Figure 4.8 Shelf/bed reactor for large scale production of nanowires
4.4.2.
Fluidized Bed CVD Reactor When a gas flow is introduced through the bottom of a bed of solid particles, it will move
upwards through the bed via the empty spaces between the particles. At low gas velocities, beds
of solid particles do not move, and thus the bed remains in a fixed state. As the velocity
gradually increase, the pressure drop and the drag on individual particle increase, and eventually
the particles start to move and become suspended in the fluid. The result is a turbulent mixing of
gas and solids. At this critical value, the bed is said to be fluidized and will exhibit fluidic
50
behavior. The tumbling action, much like a bubbling fluid, provides more effective chemical
reactions and heat transfer. This type of reactor is used in catalytic cracking in the petroleum
industries, polymer processing, and coal gasification. However, a fluidized bed CVD reactor
which is identical to the fluidized bed reactor would be a relatively new comer as a method for
mass production of nanomaterials.
The fluidized bed CVD (FB-CVD) reactor which is shown in Figure 4.10 could be one of
more promising types of reactors for large scale production of nanowires. It was introduced in
late fifties for coating of nuclear fuel particle for high temperature gas cooled reactors (44).
Catalytic fluidized bed CVD has been investigated and a commercial pilot plant is operational
for mass production of carbon nanotubes (45; 46). The requirment that nanoparticles matter be
presented throughout the reactor continue to be an absolute necessity. The FB-CVD reactor
design can integrate variouse accessaries.The most usual and the simplest one consists of a
fluidization column heated with an electric oven, and equipped with a distributing plate to
support the fluidized solid precursor and catalyst powder. The gas flow crosses the fluidized bed,
and filters are installed at the exit of the reactor to collect fine nanoparticles. The reactor outlet
can be equipped with a cyclone to minimize fine particles being carried away. A preheating oven
can be placed to heat the gaseous reactants before entering fluidized bed CVD reactor. In
addition , a vibration system (vibro-fluidized-bed reactor,) can be attached to FB-CVD reactor
in order to improve the fluidity of beds by breaking channels and cracks (46).
51
Figure 4.9 Fluidized bed CVD modelling (47)
52
Figure 4.10 Fluidized bed CVD reactor
53
The schematic design of a fluidized bed CVD reactor is shown in Figure 4.10. For the CVD
reaction to occur fluidization of the bed is necessary.
Minimum fluidization velocity
Figure 4.11 Graph of pressure drop vs. superficial velocity
At first, when there is no flow, the pressure drop is zero, and the bed has a certain height. As we
proceed along the right arrow in the direction of increasing superficial velocity, tracing the path
ABCD in Figure 4.11, at first, the pressure drop gradually increases while the bed height remains
fixed. This is a region where the Ergun equation for a packed bed can be used to relate the
pressure drop to the velocity. When the point B is reached, the bed starts expanding in height
54
while the pressure drop levels off and no longer increases as the superficial velocity is increased.
This is when the upward force exerted by the fluid on the particles is sufficient to balance the net
weight of the bed and the particles begin to separate from each other and float in the fluid. As the
velocity is increased further, the bed continues to expand in height, but the pressure drop stays
constant. It is possible to reach large superficial velocities without having the particles carried
out with the fluid at the exit. This is because the settling velocities of the particles are typically
much larger than the largest superficial velocities used.
Upward force on the bed
∆
where ∆
4.19
A is cross-sectional area of the bed
For the height of the bed at this point is L and the void fraction is ε, we can write
Volume of particles= 1
Net Weight of the particles
4.20
1
)gL Where
;
At critical superficial velocity the pressure drop across the bed is equal to the weight of the bed
(48).
∆
4.21
g 1
150
Ergun equation ;
1.75
150
Ф
1
1.75
μ
where fp is the friction factor for a pack bed ; ReP is the Reynolds number ,
and Фs is the sphericity ratio given by the surface area of
a sphere(Sp) to the actual surface area of particles.
Ф
where S
π
55
4.22
Typically, for a bed of nanoparticle particles (Dp≤10-6m), the flow conditions at this
stage are such that the Reynolds number is relatively small (Re ≤1) so that we can use
the Kozeny-Carman Equation. This relationship is applicable to the viscous flow regime
for establishing the point of onset of fluidization. This yield
f
Solving ,for small particles
∆PФ D ε
V ρ L 1 ε
Ф
4.23
(48)
4.24
(48)
This equation holds for particle up to 300μm (48) which applicable for
nanoparticle.
Where Vom is the minimum fluidization velocity
The theoretical analysis of scaled up production of nanowire is suggesting that the
optimum fabrication of nanowires is attained by analyzing the graph of
ln r vs
ln
∞
. In the case of shelf bed reactor, the reactor can be place
inside the microwave plasma chamber to facilitate the nanowire growth. The gaseous
reactant delivered to fluidized bed CVD reactor can be converted to reactive gas using
the microwave plasma system and mixed with inert gas to attain the desired fluidization
velocity.
56
CHAPTER 5 RESULT AND DISCUSSION OF GROWTH AND CHARACTERIZATION OF NANOWIRES 5.1. Silicon Nitride Nanowires 5.1.1
Characterization of Nanowires Obtained by FeCl2 Catalyst­Assisted Direct Nitridation 5.1.1.1. Scanning Electron Microscopic (SEM) Results Nanowires are very fine wires with diameters in the nanometer range (10-9m) and a
length of up to several micrometers (10-6m). Since the nanowires are not visible using optical
microscopy, a scanning electron microscope (SEM) should be used to characterize the shape,
diameter and length of nanowires. Figures 5.1 and 5.2 show the SEM images of silicon oxide
nanowires after nitridation of 1.0 cm x 0.5 cm sections of silicon wafers. The SEM image in
Figure 5.1 indicates densely grown nanowires and Figure 5.2 shows the nanowires at higher
magnification which allows an estimation of the diameter of the nanowires. The nanowires have
a diameter that ranges from 100-200 nm and a length of several micrometers. Figure 5.3 and
Figure 5.4, however, show the SEM micrographs of 1.0 cm x 0.5 cm piece of silicon wafers
before reaction. The white droplets shown in the SEM images of Figure 5.3 are residue of
droplets of 0.01M FeCl2 solution in ethyl alcohol. Figure 5.4 further demonstrates the average
size of the droplet of the 0.01M FeCl2 catalyst which is about 50 µm.
57
Figure 5.1 SEM image of silicon nitride nanowires at low resolution
Figure 5.2 SEM image of silicon nitride nanowires at higher resolution
58
Figure 5.3 SEM image of silicon wafers treated with 0.01 M FeCl2 ethanol solution at
lower resolution
Figure 5.4 SEM image of silicon wafers treated with 0.01M FeCl2 ethanol solution at
higher resolution
59
5.1.1.2. Energy Dis
spersive Sp
pectroscopiic (EDAX) R
Results Table 5.1 EDAX data
d
of 0.011M FeCl2 caatalyzed direect nitridatiion
Element
E
Atomic %
O
3
36.53
+ 2.668
Si
6
63.41
+ 2.63
Totals
100
r
ive EDAX spectrum
s
ob
btained for FeCl
F 2 catalyyzed direct
Figgure 5.5 A representati
nitrridation
60
Table 5.2 Analyzed EDAX data of direct nitridation reaction
Element
Atomic %
O
Si
Fe
N
36.53
63.41
0.84
0
Stoichiometry
1.00
1.00
The quantitative analysis of EDAX data confirmed that the nanowires have a composition of
SiO2. However, silicon nitride nanowires were not detected. This could be due to the inability of
the EDAX to penetrate beneath the silicon oxide layer to detect the core of silicon nitride wire.
5.1.2.
Characterization of Nanowires Obtained by Silicon Nitride Seeding and FeCl2 Catalyzed Growth 5.1.2.1. Scanning Electron Microscopic (SEM) Results The direct nitridation experiment was also carried out by seeding the silicon wafer with a
mixture of silicon nitride powder along with the FeCl2 catalyst. Figure 5.6 and Figure 5.7 show
SEM images of the wafer before and after nitridation respectively. This method of nitridation
also produces dense nanowires. Unlike direct FeCl2 catalyzed nitridation, this method’s EDAX
elemental analysis confirmed the presence of nitrogen, as indicated in Figure 5.8.
61
Figure 5.6 SEM image of silicon wafer treated with 0.01MFeCl2 ethanol solution and
seeded with silicon nitride powder
Figure 5.7 SEM image of silicon wafers treated with 0.01MFeCl2 ethanol solution
seeded with silicon nitride powder, and reacted in nitrogen at 1200 oC
62
5.1.2.2. Energy Dis
spersive Sp
pectroscopiic (EDAX) R
Results Table 5..3 EDAX an
nalysis of sillicon waferss seeded witth Si3N4 pow
wder
Elem
ment Atomicc% Stoichioometry N
49.122
3.770
O
15.199
1.000
Si
33.866
1.227
Cll
0.722
Figuree 5.8 EDAX
X spectrum of
o silicon waafer treated
d with 0.01M
MFeCl2 ethaanol solution
and seeeded with siilicon nitrid
de powder
63
Table 5.4 Analyzed EDAX data of the silicon nitride seeded silicon wafer before the
reaction
Element
Atomic %
Stoichiometry
N
O
Si
49.12
15.19
33.86
3.23
1.00
2.23
The wafer seeded with silicon nitride showed the presence of either a mixture of silicon
nitride nanowires as well as silicon dioxide or the silicon oxynitride. The oxide is due to the
oxidation of the silicon nitride powder since the wafer was etched in HF acid to remove oxide
layers from it. The quantitative EDAX data analysis can be expressed as Si2ON3.
Table 5.5 EDAX analysis of silicon nitride seeded and catalyzed direct nitridation
Element
Atomic%
N
10.16 + 0.21
O
55.18 + 4.31
Si
33.84 + 3.87
64
Figure 5.9
5 EDAX spectrum off silicon wafe
fer treated with
w 0.01MF
FeCl2 ethan
nol solution and
seeded with
w silicon nitride
n
pow
wder
Table 5.66 Analyzed EDAX dataa of silicon nitride
n
seed
ded and cataalyzed direcct nitridation
Eleement
N
O
Si
Atomic
A
%
Stoichioometry
10.16
55.18
33.84
1.0
00
4.7
75
1.6
67
A
After
direct nitridation
n
off the silicon nitride
n
powdder seeded siilicon wafer,, the grown
nanowirees are silicon
n oxynitride with a compposition of Si
S 2O5N. Thuss, this experiimental
procedurre resulted in
n the growth of silicon niitride nanow
wires with miixtures of sillicon dioxidee or
silicon oxxynitride nan
nowire.
65
5.1.3. Characterization of Silicon Wafers Treated by Direct Nitridation without Catalyst 5.1.3.1. Scanning Electron Microscopic (SEM) Results The direct nitridation of a silicon wafer without catalyst at a temperature of 1200oC was
done. The SEM micrograph in Figure 5.10 indicates the absence of nanowires but at this
temperature, nanowires were grown with the use of a catalyst as shown in Figure 5.1.
Figure 5.10 SEM image of silicon wafer after direct nitridation without catalyst
66
5.1.3.2.
Energy D
Dispersive Spectrosco
opic (EDAX
X) Results Tablle 5.7 EDAX
X data of nittride silicon
n wafer with
hout catalysst
Element
Atomicc %
O
11.58
Si
88.03
W
0.399
Totals
T
DAX spectru
um of nitrid
ded silicon wafer
w
withoout catalyst
Figgure 5.11 ED
67
Table 5.8 Analyzed EDAX data that shows the presence of SiO2 and free silicon
Element
Atomic %
Stoichiometry
N
O
Si
0
11.58
88.03
1.00
7.60
The EDAX analysis indicated that the surface of the silicon wafer was partially oxidized during
the reaction.
In summary, nanowires were grown for all catalyzed nitridation experiments carried out
at 1200oC, although the oxygen and nitrogen composition of nanowires varies from one
procedure to another. In contrast, the nitridation of a silicon wafer without catalyst at temperature
of 1200oC indicate no production of nanowires. One of the reasons for this contrast is that the
catalyst forms an alloy with a eutectic point with lower melting point. This eutectic becomes the
preferential site for nucleation and growth of the nanowires. Since the melting point of silicon is
higher than 1200oC, the reaction of silicon with nitrogen to form nanowire needs higher
temperature than 1200oC. For the growth of nanowires except direct nitridation with seeded
silicon nitride powder, the EDAX analysis did not confirm the presence of nitrogen in the
nanowires following the plasma enhanced CVD reaction on the silicon wafer. This could be due
to either a thermodynamically favorable silicon dioxide formation or inability of the EDAX to
penetrate beneath silicon and silicon dioxide layers to detect the inner layer of silicon nitride as
per the observation of Wu et al.(49). According to Wu et al. layers of silicon dioxide and silicon
were formed around silicon nitride nanowires as shown in Figure 5.12.
68
Figure 5.12 A schematic illustration of the coaxial nanowire (49)
5.1.4. Characterization of Nanowires Prepared via Metal Catalyzed Etching and Nitridation 5.1.4.1.
Scanning Electron Microscopic (SEM) Results First, silicon nitride nanostructures were formed through a metal catalyzed etching
process as shown in Figure 5.13. The nanostructures when annealed at a temperature of 500oC to
1000oC under nitrogen were removed to reveal transformed the system of porous structure as
shown in Figure 5.14.
69
Figure 5.13 SEM image of silicon nanostructures resulting from metal catalyzed etching
etching
Figure 5.14 SEM image of metal catalyzed etched on silicon after annealing at high
temperature under nitrogen
70
5.1.5
Characterization of Silicon Nitride Matrix Nanomaterials prepared from Pre­ceramic Polymer Precursor 5.1.5.1. Scanning Electron Microscopic (SEM) Results ®
®
The KiON Ceraset polyureasilazane preceramic polymer contains repeating units in
which silicon and nitrogen atoms are bonded in an alternating sequence which is shown in
Figure 5.15. Curing the polymer in nitrogen or air affects the formation of silicon nitride
nanowires. As shown in Figure 5.18, the pre-ceramic polymer should be cured in nitrogen for the
nanowires to grow.
®
®
Figure 5.15 KiON Ceraset polyureasilazane pre-ceramic polymer (50)
71
8/14/2008
Figure 5.16 SEM image of a pre-ceramic polymer precursor cured in nitrogen at
260 oC and annealed at 1200oC in nitrogen
Figure 5.17 Higher resolution SEM image of a pre-ceramic polymer precursor
cured in nitrogen at 260 oC and annealed at 1200oC in nitrogen
72
Figure 5.18 SEM image of pre-ceramic polymer precursor cured in air and
annealed at 1200oC in nitrogen
5.1.5.2. Energy Dispersive Spectroscopic (EDAX) Results Table 5.9 EDX analysis of polymer precursor cured in a nitrogen atmosphere
Element
Atomic%
N
22.00 + 3.83
O
34.34 + 4.54
Si
41.98 + 2.91
73
Figure 5.19 EDAX spectrum of the polymer precursor cured in nitrogen
Table 5.10 Stoichiometry from EDAX data of the polymer precursor cured in
nitrogen
Element
Atomic %
N
O
Si
22.00
34.34
41.98
Stoichiometry
1.00
1.37
1.03
The nanowires containing matrix composition resulting from the pre-ceramic polymer precursor
as per the EDAX analysis is SiNO.
74
Table 5.11 EDAX analysis of polymer precursor cured in air
Element
Atomic%
O
63.4 + 6.1
Si
34.9 + 5.8
Cl
0.11
Fe
0.78
Totals
Figure 5.20 EDAX spectrum nanowires and nanomaterials from preceramic polymer
precursor cured in air
75
Table 5.12 Analyzed EDAX data for pre-ceramic precursor cured in air
Element
Atomic %
N
O
Si
0
63.4
34.9
1.81
1.00
Stoichiometry
The composition of the powder formed as result of curing the polymer in air is found to be SiO2.
In this procedure the nanowires were grown only when the curing of the pre-ceramic
polymer precursor was performed in nitrogen and the resulting product was subsequently
annealed in nitrogen at 1200oC. However, curing it in air with or without ducumyl peroxide as
the curing agent had no impact the growth of nanowires. Therefore, control of the curing
procedure is not only critical for the growth of nanowires but also for the production of
nanowires with the desired composition.
5.1.6.
Characterization of Nanomaterials Prepared from Silicon Nitride Nanopowder Silicon nitride nanopowder was used as a precursor mixed with iron (II) chloride catalyst to
evaluate this procedure for scaled production of silicon nitride nanowires. This investigation is
intended to grow a matrix of the nanowires of the order of fractional gram lots. Most of the
previous investigations showed no sign of nanowires when the mass of the precursor placed
inside the sampling boat was increased above 50 mg, but in this case, silicon nitride nanowires
were able to grown even when the mass of the precursor was raised above 0.50 g. One of the
76
reasons for the success of this method is that most of the previously investigated fabrication
procedures require the conversion of the precursor into silicon nitride and then the subsequent
growth of the nanowires. Since this procedure used silicon nitride nanopowder as the precursor,
the silicon nitride nanowires were able to grow from a lot of material as large as one gram. The
final product which is silicon nitride nanowires with a matrix of silicon nitride powder depends
on the local nucleation and formation the silicon nitride nanowire. In the case where the
precursor needs to react with either nitrogen or ammonia to form silicon nitride, prior to silicon
nitride containing nanowire growth solid state diffusion may precluded 100% conversation.
Therefore, using silicon nitride nanopowder as precursor has advantage of supplying 100%
converted silicon nitride nanopowder to initiate the catalyzed nucleation of nanowires and
growth.
5.1.6.1.
Scanning Electron Microscopic (SEM) Results Figure 5.21 SEM image of annealed silicon nitride nanopowder with the presence of
iron (II) chloride catalyst
77
Figure 5.22 TEM image of annealed silicon nitride nanopowder with the presence of iron
(II) chloride catalyst
Figure 5.23 TEM image of individual nanowire which confirm that the nanowire has
crystalline structure.
78
5.1.6.2. Energy Dispersive Spectroscopic (EDAX) Results Table 5.13 EDAX analysis of nanowires and nanomaterials from silicon nitride
nanopowder
Element
Atomic%
N
48.34 + 4.41
O
18.01 + 4.00
Si
32.71 + 4.94
Figure 5.24 EDAX spectrum of nanowires and materials from silicon nitride nanopowder
79
Table 5.14 Analyzed EDAX data that shows the presence of a high percentage of nitrogen.
Element
N
O
Si
Atomic %
48.34
18.01
32.71
Stoichiometry
2.68
1.00
1.82
The EDAX analysis of the nanowires grown using this method has a composition of Si2ON3. The
nanowires from this method contain the highest percentage of nitrogen compare to previously
investigated methods.
Table 5.15 EDAX analysis of nanowire tip
Element
Atomic%
Fe
100.00
Totals
80
8/14/2008
Figure 5.25 EDAX spectrums of nanowire tip
The tips of the nanowires were analyzed and found to be pure iron which is the catalyst used
during the reaction. This is in agreement with the Vapor–Liquid–Solid Mechanism (VSL) of
nanowires growth that was discussed in section 4.3 of this thesis.
81
8/14/2008
5.1.7 Characterization of Nanomaterials Prepared by Autoclave Reaction at High Temperature 5.1.7.1. Scanning Electron Microscopic (SEM) Results Figure 5.26 SEM image of products from high temperature and pressure reaction
Although the standard procedure for the autoclave reaction was followed as stated in the
experimental section, no nanowires were obtained during this investigation.
82
5.2 Zirconium Diboride Nanowires 5.2.1
Characterization of Zirconium Diboride Nanowires (Rods) Obtained after High Energy Ball Milling and Annealing of Hydrated ZrCl4 and Boron 5.2.1.1. Scanning Electron Microscopic (SEM) Results As mentioned in the experimental section zirconium diboride nanowires (rods) were
synthesized by methods of mechanical alloying in argon followed by annealing at high
temperature under argon gas. The product of the experiment was characterized using SEM and
EDAX which confirmed the presence of nanowires (rods) as indicated in Figure 5.27. The
experimental result from mechanical alloying in air and subsequent annealing in argon is also
shown below for comparison (Figure 5.28).
Figure 5.27 SEM image of zirconium diboride nanowires (rods) milled and annealed
in argon
83
Figure 5.28 SEM image of zirconium diboride nanomaterials milled in air and
annealed in argon
Figure 5.29 SEM image of zirconium diboride powder using magnesium as reducing agent
84
5.2.1.2. Energy Dispersive S
Spectroscop
pic (EDAX) Results Table 5.16 EDA
AX analysiss of zirconiu
um diboridee milled and
d annealed under
u
argon
n
Elemen
nt
Atom
mic %
B
70.884 + 6.29
O
13.885 + 5.35
Fe
3.922 + 6.29
Zr
11.339 + 0.76
Figure 5.30 EDAX speectrum of zirconium diiboride nan
F
nowires (rod
ds) milled in
n
a
argon
85
Table 5.17 Analyzed EDAX data of zirconium boride nanorods milled in argon
Element Atomic%
B
70.84
O
13.85
Fe
3.92
Zr
11.39
Totals
Stoichiometry
5.57
1.48
1.00
Therefore, the composition is ZrB6O2. Most likely it is a combination of B2O3 and ZrB2. Since
excess boron was used during the reaction, it is logical to get unreacted boron in the product. If
purification of unreacted boron was done, a better picture of the product composition might be
obtained.
Table 5.18 EDAX of nanomaterials milled in air and annealed in argon
Element
Atomic%
B
41.21
O
48.53
Al
3.95
Zr
6.31
The mechanical alloying carried out in air resulted in complete oxidation of boron and zirconium
instead of zirconium diboride formation.
86
Figuree 5.31 EDAX
X spectrum of zirconium
m diboride nanomaterials milled in
i air and
anneealed in argo
on
The resullts indicated
d the formatioon of boron oxide and ziirconium oxxide as final products
p
from
milling thhe mixture in
n air and annnealing in arrgon. The aluuminum peaak is observeed due to the
contaminnation during
g milling insside a grindinng jar made of aluminum
m oxide. EDA
AX analysiss
showed high
h
percentage of boronn which can be traced baack to the use of excess boron
b
duringg the
solid statte mechanicaal alloying. Excess
E
boronn can be rem
moved by leaaching with in
i HCl.
The com
mposition fro
om this proceedure can bee written as ZrB
Z 7O8, which has moree oxygen than the
one curedd in nitrogen
n in additionn to the absennce of nanorrods. Since excess
e
boronn is used duriing
the solid state reactio
on, it was dettected in exccess in the prroduct too.
87
Figurre 5.32EDA
AX analysis zirconium diboride
d
pow
wder obtain
ned by usingg magnesium
m as
a red
ducing agent with millin
ng and anneealing in argon
The solidd state mechaanical alloyiing and subssequent anneealing of the mixture of boron,
b
zirconnium
chloride, and magnessium (as reduucing agent)), did not ressult in the forrmation of nanowires.
n
Howeverr, nanopowd
dered zirconium diboridee can be obtaained from thhis procedurre.
88
CHAPTER 6 CONCLUSION AND RECOMMENDATIONS 6.1
Summary and Conclusion The major achievement of this thesis investigation is the design, assembly and evaluation
a plasma enhanced chemical vapor deposition (PECVD) system for the synthesis of
nanomaterials. This plasma system is superior to the traditional CVD system in that it is
equipped with a pressure and mass flow controller for accurate control of the flow of all gaseous
reactants. Additionally, the microwave plasma generator creates an energetic gaseous mixture
which resulted in the reaction being carried out at lower temperature higher efficiency than that
occurring in a thermal CVD system.
Once the final plasma system was operational, various experiments for nanowire synthesis
were carried out. Early experiments included direct nitridation on a silicon wafer to produce
nitride nanowires. Under the conditions of direct nitridation at 1200oC, silicon dioxide
nanowires were produced preferentially to silicon nitride nanowires. In order to overcome the
preferential thermodynamic formation of silicon dioxide, the experiment was carried out by
seeding with silicon nitride powder. This resulted in the formation of a mixture of silicon dioxide
and silicon nitride nanowires (silicon oxynitride).
The successful seeding with silicon nitride powder to induce the growth of nanowires on a
wafer precludes the production of nanowires in gram quantities unless a very large silicon wafer
is used, which was not feasible due to the size of the reactor. The investigation of scale up
methods included the examination of autoclave reactions, which is a high pressure and
temperature reaction as indicated in section 3.1.3. The characterizations of the products from this
89
procedure indicated that no nanowires had grown. Hence, another potential synthesis procedure
for the growth of silicon containing nanowires by thermal transformation of a pre-ceramic
polymer precursor was investigated. Curing of the pre-ceramic polymer precursor in nitrogen
and the subsequent annealing in nitrogen yielded a mixture of silicon dioxide and nitride
nanowires dispersed throughout a silicon oxynitride matrix.
The objective of producing fractional gram quantities of nanowires was accomplished by
scaling up the process previously demonstrated by direct seeding with silicon nitride nanopowder
as the precursor. The method utilized the annealing of a mixture of an iron (II) chloride catalyst
and silicon nitride nanopowder, reacted in a quartz boat at 1200oC in flowing nitrogen. This
catalyst assisted conversion of silicon nitride nanopowder to nanowires enabled the fabrication of
up to 0.5 grams on matrix in silicon oxynitride in a batch.
In related experiments, zirconium diboride was fabricated by the mechanical alloying of
hydrated zirconium tetrachloride and boron with subsequent annealing under argon. This method
led to the growth of zirconium diboride nanorods in a matrix of residual starting materials.
90
6.2
Recommendation and Future Work This thesis investigated the synthesis of silicon nitride and zirconium diboride nanowires.
However, pristine silicon nitride and zirconium diboride could not be produced due to the
gaseous oxygen containing contaminations which led to the formation of the very stable of
silicon dioxide and zirconium dioxide. In order to eliminate oxygen from the system, the
following recommendations are made for future work:
1. Turbomolecular pumps should be used to create ultra high vacuum down to 10-10 Torr so that
the PECVD system can be freed of oxygen before the experiment. This will need the addition
of new vacuum pumping unit which comprises a roughing pump and turbo pump in tandem
in the PECVD system. Ultra high vacuum connection and components must be incorporated
throughout the system.
2. Ultra dry and high vacuum glove box procedure may be needed for preparing pre-ceramic
polymer for solid state mechanical alloying reactions. Acquisition of inert gases filled section
vessels for the solid state reaction is also recommended.
91
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95
APPENDIX A A.1 Calculation to Prepare 0.01MFeCl2: Molecular weight of FeCl2: 126.75 g/mol
Volume ethyl alcohol required: 10 mL (10 mL 0.01 MFeCl2 in 50 mL of beaker is sufficient to
dip 1 cm x 0.5 cm wafers)
Mass of FeCl2requred to prepare 0.01MFeCl2: 0.01M*(126.75 g/mol)*10 mL*(1 L/1000 mL)
=0.01268 g =12.68 mg
A.2. Preparation of 5M of 50 mL of HF acid 49% HF acid as supplied from Fischer Scientific was used for this experiment.
As per the specification 49% HF (w/w) has a specific gravity of 1.175 at 60 oF.
Therefore, 1litre of 49% HF weigh 1175 g of which 575.75 g HF acid and 599.25 g water.
To prepare 5M of 50 mL HF acid
5M =(X moles of HF)/ (50 mL)
X moles of HF (100%) =0.25 moles, but the stock solution is 49% HF.
Hence, the mass of HF needed is (0.25moles x 20.01 g/mole) = 5 g of 100% HF or 10.21 g 49%
HF. Since it is easy to measure volume, the volume required is obtained by dividing with specific
gravity. The required volume is 10.29/1.175 = 8.69 mL of 49% HF acid.
A.3.Preparation of 0.02 M of Silver Nitrate Molecular mass of AgNO3 = 169.88 g/mol
96
To prepare 50 mL of 0.02M AgNO3 , the mass of AgNO3 needed is
= 50 mL *0.02 mol/L *169.88 g/mol = 0.16988 g =169.88 mg Therefore , the etching solution can be prepare by using Teflon beaker filled with 41.31 mL of
water , 8.69 mL of 49% HF acid and 169.88 mg of silver nitrate.
A.4. Calculation of Boron and Zirconium (IV) Chloride Anhydrous Milling Mixture 3ZrCl
10B
3
4
According to the above reaction
36.03 g of boron will react with 233.22 g of Zirconium (IV) chloride to give us total mass of
269.25 g reactant. Boron usually added 50% in excess; hence, the actual mass is 54.05 g which
make the total reactant mass 287.27 g. Since the 50mls of milling bowls hold 10 g of the above
solid mixture.
The mass of reactant is then
Boron crystal = (54.05/ 287.27) * 10 g =1.89 g
Zirconium (IV) chloride anhydrous =(10 g -1.89 g) = 8.11 g A.5 Calculation Boron, Magnesium and Zirconium (IV) Chloride Anhydrous Milling Mixture ZrCl4 + 2B + 2Mg → ZrB2 + 2MgCl2
As per the above reaction
21.62 g boron, 48.62 g magnesium and 233.22 g anhydrous Zirconium (IV) chloride solid
mixture give 303.46 g total reactant mass. With 50 % excess of boron and 50% magnesium,
97
the reactant mass adjusted to 32.43 g boron, 72.94 g magnesium and 233.22 g anhydrous
Zirconium (IV) chloride which makes the total mass of reactant 338.59 g.
For total of 10 g mixture, the mass of each reactant added
Boron = (32.43*10)/ 338.59 =0.96g=960 mg
Magnesium = (72.94*10)/338.59= 2.15 g =2150 mg
Anhydrous Zirconium (IV) chloride = (233.22*10)/338.59 =6.89 g= 6890 mg
98
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