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Fibres for use at highest temperatures.

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Die Angewandte Makromolekulare Chemie 22 ( 1 9 7 2 ) 159-179 ( N r . 305)
Aus der Sigri Elektrographit GmbH, Meitingen b. Augsburg
Fibres for Use at Highest Temperatures
Von D. OVERHOFF*
(Eingegangen am 6. Juli 1971)
SUMMARY :
Materials having gained importance over the past decade are - in addition to
the developments of high temperature-resistant polymer fibres - various types of
fibrous materials, being distinguished by their resistance to temperatures exceeding
1000°C. These materials are in general inorganic compounds formed into fibres by
special shaping methods. The distinguished properties of some of these fibre types
are high mechanical strength, oxidation resistance, electrical conductivity and light
weight, apart from their temperature resistance.
The main processes used for their manufacture are: drawing of very h e filame:nts, vapour deposition of compounds in the form of fibres, and spinning of
inorganic filaments by the viscose process.
This paper discusses metal fibres and fine filaments as well as oxide, carbide and
nitride filaments of the third and fourth group of the periodic system of elements,
as far as they are of importance.
Also discussed will be fibres of boron, silicon carbide, silicon nitride and boron
carbide being representative of the vapour deposition process and also fibres of
aluininium oxide, titanium oxide, silicon dioxide being representative of the viscose
process, as well as other types.
Only a small amount of fibres are being produced by the mentioned processes.
Carbon fibres are made in general by full thermal deconiposition of infusible, organic fibres in inert atmosphere. Their manufacture will be detailed together with
their properties and various fields of application.
Concepts of the chemistry of thermal decomposition will be illustrated and also
the connections between the structure of the basic materials and those of carbon
filaments.
ZUSAMMENFASSUNG :
M’ahrend der letzten 10 Jahre haben einige Fasermaterialien an Bedeutung gewonnen, die im Unterschied zu den hochtemperaturbestandigen Polymerfaden
durch ihre Anwendbarkeit bei Temperaturen uber 1 000 “C hervorstechen. Chemisch gesehen handelt es sich um anorganische Stoffe. Ihre Fasergestalt erhalten
sie durch einige spezielle Formgebungsverfahren.
*
Presented a t the Scientific Symposium “The Physics and Chemistry of Fibre
Materials”, Miinchen, 3rd and 4th June 1971.
169
D. OVERHOFF
Neben der Temperaturbestllndigkeit zeichnen sie sich durch hohe mechanische
Festigkeit, Oxidationsbestandigkeit,elektrische Leitfiihigkeit und geringes spezifisches Gewicht aus.
Die haufigsten Formgebungsverfahren sind das Ziehen von sehr feinen Drahten,
die Abscheidung von fliichtigen Verbindungen aus der Gasphase auf einem Substratfaden sowie die Herstellung anorganischer Faden nach dem ViscoseprozeB.
Der Beitrag behandelt Metalldrahte und feinste Faden sowie Oxid-, Carbid- und
Nitrid-Faden der Elemente der 3. und 4. Gruppe des Periodensystems der Elemente,
soweit diese von Wichtigkeit und technischeni Interesse sind.
Als Vertreter der aus der Gasphase abgeschiedenen Faden werden Bor-, Siliziumcarbid-, Siliziumnitrid- und Borkarbidfaden diskutiert. Der ViskoseprozeB
und die danach hergestellten Produkte sind durch Aluminiumoxid-, Titandioxidund Siliziumdioxidfaden vertreten.
Der thermische Abbau von nichtschmelzenden Polymerfaden fiihrt zu Kohlenstoffaden, die im Gegensatz zu den obenerwahnten Materialien in groBeren Mengen
hergestellt werden. Deren Herstellung aus Polyacrylnitril, Rayon, Pechen und
bituminosen Riickstllnden wird besprochen. Die Eigenschaften von Kohlenstofffaden werden deren heutigen und zukiinftigen Anwendungen gegeniibergestellt.
The fibres and monofilaments which will be discussed in this paper, have in
comparison to organic polymer fibres, a high tensile strength and a high
YOUNG’S
modulus. Their main application lies in the construction of composite materials. Another distinguishing feature of these materials is that they
are of inorganic structure.
The title “Fibres for Use a t Highest Temperatures” could therefore be
said to be misleading and a more exact title would read ”Inorganic fibres, filamoduli for
ments and monofilaments with high tensile strength and YOUNG’S
use in high temperature applications and in the construction of light weight
composite materials”.
The main interest in the development of several of the materials under
discussion was not a search for materials resistant a t high temperatures but
the knowledge with relation to the mechanical properties of single crystal
“whiskers”. The main incitement to the development of these fibrous materials was the demand for extremely strong and s t s light weight composite
materials. Other governing factors were, the need of oxidation resistance,
electrical and thermal conductivity etc.
The production of the fibres as well as their application distinguishes them
from organic fibres.
I n Table 1 some facts about the temperature resistance of the materials
under discussion are shown. The fibrous materials are temperature resistant
to a t least 400°C in air. I n inert atmospheres some of these fibres are resistant
to over 2000 “C. The limitation of their use is controlled by the commencement
of evaporation, melting or softening as well as by the commencement of
160
Fibres for Use at Highest Temperatures
oxidative degradation. Table 2 will give some impression of the mechanical
properties to be found in these materials.
Table 1. Temperature and oxidation resistance of fibres and filaments.
material
I
atmosphere
B/W
SiC/B/W
RiC/W
B4C
BN
SiOa( +C)
Carbon
Table 2.
air
inert
air
inert
air
inert
air
inert
air
inert
I temperature ("C)
400
700
700
700
900
1500
1100
2 100
900
2 500
air
inert
air
inert
1500
500
3 000
Mechanical properties of filaments and fibres.
Filaments
A1
Ti
Fe
w
B/W
SiC/B/W
SiC/W
TiBz/W
B4C
BN
SiOz
A120 3
ZrOz
CHT
CHM
1 I
density
specific
'Icrn3
(km)
2.7
4.5
7.9
19.3
4-6
20
50
25
specific
2 800
2 900
2 500
1900
2.6
2.8
3.5
4.8
2.2
1.9
145
140
96
31
14 200
13 700
13 000
10 200
150
116
12 500
4 800
2.34
4.0
4.9
1.7
1.9
30
18
14
180
120
5 000
5 500
7 100
13 000
22 000
25 000
161
D. OVERHOFF
The density is an important additional property. I n comparison to steel, an
everyday construction material, all fibres possess a much lower density. The
combination of the high mechanical strength and stiffness with low density
makes these fibrous materials extremely attractive in the field of reinforcement
of plastics and light metals.
Remarkable is the specific strength of boron, silicon carbide, boron carbide
and carbon fibres: They have a specific strength three times higher than light
metal and iron wires. Even more interesting are the different specific YOUNG’S
moduli: ceramic and carbon fibres are up to 10 times stiffer in comparison to
metals. The specific YOUNG’Smodulus of metal wires is independent of the
t,ype of metal of approximately the same size. Therefore metal wires are not
suitable for use in the construction of stiff composite materials, owing to the
fact that their YOUNG’Smoduli cannot be increased by either construction
of alloys or other treatments. You can see the extremely high specific YOUNG’S
moduli possessed by boron, silicon carbide, boron carbide and carbon fibres.
As reinforcement of epoxy resins their stiffness is in first approximation proportional to their volume content as shown in Table 3.
Table 3. Specific properties of composites.
material
steel
aluminium alloy
titanium alloy
glassfibre/epoxyVf
= 0.5
boronlepoxy Vf = 0.5
Borsic/aluminium Vp = 0.5
Borsicjtitanium Vr = 0.5
siliconcarbidelepoxy Vf = 0.6
carbon/epoxy Vf = 0.5
density
specific
strength
specific
modulus
(km)
7.9
2.9
4.3
2.0
2.1
2.8
3.6
2.5
1.5
18
25
21
50
75
48
34
42
70
2 450
3 250
2 450
2 800
11 300
7 800
6 600
11 700
22 000
By embedding fibres in a plastic matrix one can exploit their good mechanical
properties as the loads are transferred by the matrix to the reinforcement.
Advantageous in this application is the fact that the fibres remain elastic
right up to the breaking point which is in contrary to organic fibres.
The high YOUNG’S
modulus and the resulting low breaking strains do not
facilitate the handling of these materials. Therefore the normal textile processes
of weaving, knitting and sawing present difficulties.
162
Fibres for Use at Highest Temperahreg
By making this detour into the field of fibrous composite materials it must
be pointed out that because of their properties these fibres are used in applica.tions different to the textile polymer fibres which you are mostly concerned
with.
I n Table 4 the fibres which will be discussed in this paper are listed according to their production methods :
1 . The drawing of extremely fine metallic monofilaments.
2. The production of monofilaments by the vapor deposition process.
3. The chemical conversion of fibres through a gaseous reaction agent.
4. The spinning of inorganic fibres as in the viscose process, and
5. Thermal decomposing of organic fibres into carbon.
Table 4. Fibres and fibre-forming-methods.
~
fibre forming
method
I
fibrous material
I
remarks
#drawingof very
fine wires
Al, Nb, Ta, Ti, Fe,
Fe-Ni alloys W etc.
TAYLOR-process
chemical vapor
deposition
B/W
SiCjBjW
SiCjW
W-wire as hot
precursor conversion
C t o B4C
Bz03 t o BN
rayon spinnerette
and extrusion
SiOz ( + C )
A h 03
ZrOz
CaO, ZrOz
ZrOz
1 % HfOz
ZrOz
HfOz
t,hermal degradation
of organic precursors
Carbon
+
+
+
substrate
+ SiOz
+ A1203
precursor :
polyacrylonitrile
Rayon, pitch
Metallic Monofilaments
I n Table 4 the drawing of metallic monofilaments is listed because their
mechanical properties lay between those of polymer fibres and high modulus
fibres. The possible applications for these materials are not numerous, owing
to the fact that metal fibres possessing a high strength have also a high density,
whereas light weight metal fibres have a low strength. Therefore these materials will not be discussed in detail.
163
D. OVERHOFF
Filaments produced by vapour deposition
A more interesting production process is achieved by thermal decomposing
organic silicon, boron, titanium etc. compounds on filamentary substrat,esl,2 , 3 .
Fig. 1 shows the principal of a deposition device for the production of boron
mono-filaments4. The substrate consists of an electrically conductive wire
which has a diameter of approximately 10 microns. The decomposition t,emperature of 900 to 1400°C is reached by passing an electric current through
the wire. The gas normally used is a mixture of an halogen compound and
hydrogen. The fibre deposition speed is regulated in the way to get a microcrystalline structure. An optimum structure is normally obtained when the
growth rate is slowed down. Too high deposition speed results in the formation of cracks and a nonuniform deposite. The deposition temperature is of
great importance. That is to say micro-crystalline depositions can only be
obtained a t the lowest possible decomposition temperatures. In commercial
production the filament growing rate is of the order of 1 to 3 microns per
second. The end thickness of the filaments is approximately 100 microns
whereas the dimensions of the crystallites are approximately 20 to 50 Angstrom units. The crystals do not possess any preferred orientations.
Electric P o w e r
Subr t r a t e
I
Outpar
- - Electric
Power
Plate
c
Filament
Take-up
BC5 -Hz
Fig. 1.
Schematic diagram of boron deposition process.
Boron filaments are the most important product by this manufacturing
process. They are formed on a tungsten wire either by decomposing boron
chloride in a hydrogen stream or boranes a t temperatures below 1400“C. In
Fig. 2 the morphology of a boron filament with its typical “corn-bob’’ structure is s h 0 ~ 5 The
.
nodular surface layer is extremely thin as can be assertained by etching. Beneath this layer the structure is completely homogeneous.
The surface layer contributes a relatively small amount to the strength of the
whole structure. Fig. 3 shows a cross section of a boron fibres. The substrate
is clearly distinguishable against the boron deposite. Even though the sub164
Fibres for Use at Highest Temperatures
strate is only 1.7% of the volume its weight percentage is 12.5 owing to the
tungstens high density which is 19.3 g/cm3.
Fig. 2 .
Morphology of the surface of a boron fibre (“Corn bob structure”).
Fig. 3.
Cross section of a boron fibre.
Up until now experiments to replace the tungsten with a lighter wire for
example graphit coated glass or carbon fibre have not met with success owing
to :
a ) pour electrical conductivity or
b) difficulties involved with handling these materials.
165
D. OVERHOFF
Filaments of silicon carbide, titanium boride and boron carbide can be
made under similar conditions but only silicon carbide filaments are of importance owing to their good oxidation resistance a t high temperatures. As to be
seen in Table 5 the boron and silicon carbides tensile strength is extremely
moduli are only comparable with those of carbon fibres
high. The YOUNG'S
and whiskers, which are not listed here.
Table 5. Mechanical properties of filaments by chemical vapor deposition.
filament
tensile strength
(kglmm2)
YOUNG'S
modulus
(106 kp/cm2)
Fibres produced through chemical precursor conversion
As shown in Table 5 one method of producing boron carbide monofilaments
is vapor phase deposition. However it is also possible to produce this material
by the conversion of suitable precursor filaments by reaction with a gaseous
component. This process should be explained by two examples : boron carbide
and boron nitride filaments. The former are produced from a carbon fibre by
reaction with boron chloride6, the latter with boric oxide fibres and ammonia?.
Depending on the reaction temperature boric oxide reacts with ammonia to
form additional compoufids which are converted into boron nitride in various
stages. The maximum process temperature accedes 1800°C.
( B ~ 0 3 )NH3
~.
NH3
(BN)x(Bz03)y. (NH3)z
T > 350 "C
X, y, z = f (T)
+
Depending on the form of the raw material it is possible to manufacture boron
nitride and boron carbide as fibres, filaments, tows, yarn and also as cloth. The
tensile strength of boron carbide processed in this manner is up t o 170 kp/mm2
166
Fibres for Use at Highest Temperatures
and the YOUNG'S
modulus up to 3.5 x 106 kp/cm2. The mechanical properties
of boron nitride fibres are somewhat lower, as shown in Table 6. Their low
density of 1.7-1.9 g/cm3 and extreme temperature resistance makes them
very attractive for many applications. Owing to their good thermal conductivity boron nitride fibres can be used in a woven form for the construction
of high speed, hot running friction bearings.
The structure of boron nitride fibres is in many ways closely related to that of
ca.rbon fibres. The main difference being the high electrical resistivity of
more than 101052 cm possessed by boron nitride fibres. The reason for this
difference is that in boron nitride the electrons are highly localised whereas the
n-electrons in carbon are completely unlocalised. Owing to the layer lattice
structure which is similar to graphite the anti friction properties of boron nitride are extremely good.
Tttble 6 . Properties of boron nitride fibres.
property
tensile strength
YOUNG'S
modulus
density
temperature resistance
oxidation resistance
electrical resitivity
thermal conductivity
1
magnitude
100-1 50 kp/cm2
0.3-7 . 106 kp/cm2
1.7-1.9 g/cm3
up t o 2 100 "C
up t o 900°C
no (e < 10+10.f2 cm)
Yes
Fibres produced by the viscose spinning process
By adding to a viscose solution a considerable quantity of highly hydrated
ceramic materials which can either be dissolved or dispersed and by following
the viscose spinning process it is possible to produce ceramic fibres, filaments,
tows and yarns899. After the spinning process the fibre (which has been filled
with ceramic) is then thermally treated, thus either carbonising or burning
the organic binder. The main use for this process is the production of silica
fibres and multicomponent oxide fibres in combination with carbon. I n the
same way one can also produce fibres of alumina and oxide mixtures of for
example: zirconia with calcia, zirconia with silica and zirconia with hafnia
etc. The firing and sintering takes place a t over 1500"C.
The mechanical properties of materials produced in this way are somewhat
lower than those of the materials previously discussed. This is mainly due to
the fact that they are produced by sintering and that it is very difficult to
167
D. OVERHOFF
controll the graingrowth in this process. However the most interesting feature
of the oxide fibres is their high temperature resistance combined with their
extremely high oxidation resistance.
H20
chemicals
cake yarn t o
wet processing
r
V
Fig. 4.
Rayon spinning process flow diagram.
Fibres produced by thermally decomposing a polymeric precursor
During the thermal decomposition of organic material two reactions take
place, either simultaneously or alternatively. The fibre decomposes either under
depolymerisation whereby only a small amount of carbon residue is obtained
or the thermal energy causes cross-linking. When this cross-linking a t higher
temperatures is not followed by a further depolymerisation this results in a
large amount of carbon residue. This is the first criteria, which must be considered in the production of carbon fibre. A further requirement is that one
must select an organic precursor which will not melt during decomposition
that means it retains its fibrous structure. These two requirements greatly m i t e
the number of potential polymer precursors. Therefore today only cellulose
and polyacrylonitrile are of commercial interest for the production of carbon
fibres. Apart from these two raw materials pitch and bituminous residues are
currently coming in interest. These materials are spun a t 400 "Cand thus obtain
a fibrous structure.
It is extremely difficult to formulate pyrolytical decomposition in the manner
which most organic chemists adopt, because it takes place as numerous single
reactions occurring simultaneously and successively. Most of these reactions are
168
Fibres for Use at Highest Temperatures
condensation ones which lead after a short time to highly condensed ring
systems, making exact analyses extremely difficult. This reduces the possibility
of examination to the first stages of the reaction.
The PAN process
The thermal decomposition of polyacrylonitrile PAN is initiated in two ways :
1. through a cyclisation of the acrylonitrile units by formation of dihydronaphthyridine rings and
2. by the cross-linking of the acrylonitrile chains.
Both of these reactions take place in the presence of oxygenlo.
\
CHI
c*N
The first step is the formation of a hydroperoxide on the a-hydrogen activated
C-atom followed by the formation of imino groups, cyclisation and dihydro169
D. OVERHOFF
genation through a carbonium ion leading to the formation of a ladder polymer with conjugated C-N
double bondslo. The condensation of the nitrile
groups can take place intermolecularily and intramolecularily and is influenced
mainly by a statistic of the nearest neighbour. Infrared analysis indicates the
presence of the groups postulated.
After this introduction to the somewhat untransparent chemistry of the
thermal degradation of PAN, Fig. 5 shows a flow diagram with the three
stages : thermal pretreatment, carbonisation and high temperature treat-
I
PAN fibres
-thermal
I
degradation
up to 1600°C
1
high temperature
treatment
1
high modulus
carbon fibres
high strength
Fig. 5. Production of PAN based carbon fibres.
ment. I n the pretreatment the precursor is stabilised under the influence of
air. That is t o say it is rendered flameproof. This technique making flameproof
textiles was discovered shortly after the development of PAN fibre&. When
it was discovered that by placing the fibres under tension during this process
modulus were influenced, this was considered to be
the strength and YOUNG’S
of the atmost importance for the production of high strength and high modulus
carbon fibre@. Fig. 6 shows how the shrinkage of PAN fibres is influenced by
different loading. As can be seen it is possible to overcome the tendency the
fibres have to shrink. Fig. 7 shows how after carbonisation and heat treatment
modulus is influenced by the change in length which takes place
the YOUNG’S
during pretreatment. As is to be seen in Fig. 8 the tensile strength and
stiffness are also influenced by the temperature a t which the high temperature
treatment takes placel3.
170
Fibres for Use at Highest Temperatures
I
10
3 *- L
a
&
O
4
*
l0Og
n
&
*
c
CI
5
I
I
1
I
I
2
3
time in hours
I
4
5
Pig. 6. Length changes of PAN fibres during thermal pretreatment at 22OOC.
-40 -30 -20 -10
0 +K) +2O +3O +4O
length change after thermal pretreatment at 220°C
Fig. 7. Length changes after thermal pretreatment and YOUNG’S
moduli of carbon
fibres.
The cellulose process
Cellulose mainly decomposes by dehydration. Approximately four carbon
atoms remain in each ring unit forming a graphitelike structure in the carbon
fibrel4. Owing to the fact that no change in length under loading is possible
during the pyrolysis of cellulose it is also not possible to influence the orien171
D. OVERHOFF
.2
ul
-1
100-
c
Y
P
cn
c
3
Y
0
I
I
I
1
I
1
Fig. 8. Effect of heat treatment temperature on the mechanical properties of
carbon fibres.
tation in the fibres. However it can be seen in Fig. 9 one can take advantage
of the fact that carbon begins to soften a t approximately 2500°C and can
therefore be stretched when in this stagel5. In Fig. 10 a laboratory equipment
designed to stretch carbon fibres a t high temperatures is shown. I n Fig. 11
the effect is to be seen which this stretching has on the strength and modulus
of carbon fibres.
chemical dehydration
200-400oc
UD
t0 1200
Ihigh temperature treatment I
I
stmtching
I no stretching J
hlgh modulus
Fig. 9.
172
Production of Rayon based carbon fibres.
carbon fibres
Fibres for U5e at Highest Temperatures
Fig. 10. Laboratory equipment for stretching of carbon fibres at high temperatures.
e
'9
400-
45
e
T
E
E 300a
Y
3 :Y
ii200-
2 f3
E
Y
(I)
C
0
0
t
L
E
ul
1
.- 100-
E
I
I
10
Fig. 11.
I
I
I
20
30
40
elongation (7.)
I
I
50
Effect of high temperature elongation upon YOUNG'S
modulus and tensile
strength.
Carbon fibres made by the PAN process obtain their high tensile strength
when heated to 1400 to 1800°C. There is evidence of an optimum between
the stretching in the pretreatment stage and the tensile strength after car173
D. OVERHOFF
bonisation, whereas fibres produced by the cellulose process show an increase
modulus with increased stretching and stretin tensile strength and YOUNG’S
ching temperature.
T h e structure of carbon fibres
Owing to the fact that the orientation always dictates the fibres YOUNG’S
modulus now the correlation between the structural and mechanical properties of carbon fibres should be discussed. The graphite modification of carbon
possesses a layerlike structure in which the carbon atoms are connected by
strong covalent bonding forces within a hexagonal network. The forces between the layers are poor. I n a well crystallised structure the layers have a
regular repetition; however in many types of carbon one can observe irregnlarities in the stacking sequence. I n extreme cases one notices that even though
regular arrangement exists within the layers, no correlation between the
layers is in evidence. Whereas in well ordered graphites all the expected X-ray
lines can be detected the greatly disordered “non graphitising carbons” show
diffuse halos and broad asymetrical interferences. The former are produced
by the statistical arrangement of the layers whereas the latter represent the
layer size.
It is necessary to make these comments as carbon fibres show all the structural properties of ”non graphitising carbons“. As with all orientated polymer
fibres carbon fibres possess a greater or lesser degree of anisotropy. Depending
on the orientation treatment the layers are parallel to the fibre axes to a
greater or smaller extent. This brings up to the X-ray phenomena as shown in
Fig. 12. By orientating the layers more and more into a parallel position to
the fibre axis one observes how the arcs representing the layer stacking become
more and more focused and even though the layers are highly orientated to
the fibre axis the asymetrical interferences representing the crystallite dimensions within the layers show no dependence of scattering density on
scattering direction. But there is evidence for a different line width.
Therefore this explains that the YOUNG’Smodulus is dependent on the
extent of the orientation to the fibre axis of the graphit like layers as shown
in Fig. 13. One can note how the width of the scattering arcs influences the
YOUNG’S
modulus.
Another interesting structural feature is the fibrillar arrangement of carbon
fibres. As with polymer fibres it is possible to detect this using electron miwork16 especially shows
croscopy and low angle X-ray scattering. RULANDS
that basic structural units exist which are ribbon shaped. These ribbons have
a width of about 60 A and a length of several thousand 8. A certain number
174
Fibres for Use at Highest Temperatures
6
E= 1,60.10
kp.cm-2
E= 2,54.106 kp-cm- 2
-
E= 3,94 106 kp.cm-2
Fig. 12.
Orientation and modulus of carbon fibres.
(x)
(0)
PAN based fibres
RAYON based fibres
1
2
Fig. 13.
4
6
8
Orientation parameter
10
(ZO)
Effect of crystal orientation upon YOUNG’S
modulus of different carbon
fibres.
176
D. OVERROFF
of the ribbons run parallel forming a micro fibril. The micro fibrils have a
preferred orientation t o the fibre axis. Because their packing is imperfect, a
number of voids are decernable a t the bounderies of the micro fibrils. These
voids are long and thin and their preferred orientation follows closely that of
thc straight parts of the ribbons. The low angle scattering of X-rays produced
by different orientated void systems is shown in Fig. 14 for different orientated
E= 1,60 * lo6 kp.cm-2
E = 2,OS * 106 kp.cm-2
E= 2,54406 kp.cm-2
E=3,32 *lo6 kp.cm-2
*
E=3,94 .lo6 kp.cme2
Fig. 14.
Low angle scattering and modulus of carbon fibres.
carbon fibres. It is also possible to detect this fibrillar structure by examining
the broken surface of a highly orientated carbon fibre using high resolution
scanning electron microscopy as shown in Fig. 15. However the most convincing argument concerning the existence of this structure is an examination
of a dark field electron micrograph a s shown in Fig. 16. It is possible to detect
a moire structure in the circled region of the (002) dark field electron micrograph. This results from the superposition of two clearly visible micro fibrils.
176
Fibres lor [Jse at Highest Temperatures
Fig. 15. Broken surface of a carbon fibre under the high resolution scanning electron microscope.
Fig. 16.
Dark field electron micrograph of a carbon fibre.
177
D. OVERHOFB
Summing up the major points, there are a number of inorganic fibres a n d
filaments possessing extraordinary mechanical and thermal resistant properties. Owing t o these properties t,heir main application lies in the field of the
composite materials requiring high stiffness and high temperature resistance.
Comparing for example the foreseen development of carbon fibres t o the now
common glass fibres it can be truly said, t h a t these newly developed fibres
have a promising future.
Discussion
H. CHERDRON,
FrankfurtlHochst : A new polymeric precursor for carbon fibres,
namely the polyacetylenes, has been recently announced. It is said that these precursors have mainly two advantages : (a) a very high carbon content so that only
5% of hydrogen has to be removed in the carbonizing step; (b) polyacetylenes are
highly oriented, which results in carbon fibres of extraordinarily high mechanical
properties. Have you made a comparison of this process with the existing ones?
D. OYERHOFF
:Up to now I have not heard about this process but I would agree
with you that this process would also be of commercial interest if the precursor
material is not too expensive.
W. HOPPE,
Miinchen: Is there any hope to synthesize carbon fibres with diamondlike structure ?
D. OVERHOFF
: Since carbon fibres are produced by the conversion of carbonaceous
material a t high temperatures it is improbable that diamond structures are formed
because they are not stable a t these temperatures.
W. RULAND,
Brussels: As far as the mechanical properties are concerned, diamond fibres would not be preferable to carbon fibres with highly oriented graphitic
layers since the YOUNG’S
modulus of the latter along the fibre axis is nearly as high
as that of a diamond fibre. This is due to the fact that the theoretical YOUNG’S
modulus of a graphitic layer parallel to the layer plane is about as high as that of
diamond. Perpendicular to the fibre axis the modulus of carbon fibres is much
lower. This anisotropy of the modulus is a desired property for the use of fibres
in composite materials. Diamond fibres would not show this anisotropy and would
thus be less interesting for this application.
W. 0. STATTON,
Utah: It was shown recently (WATTand JOHNSON
in England)
that tension on PAN fibres during the high temperature treatment eliminates the
maximum in tensile strength, so that the PAN and rayon fibres become more
similar.
D. OVERHOFF
: This is true : the dependence of tensile strength and modulus upon
the heat treatment temperature is similar for PAN and cellulose when the fibres
are tensioned under the same conditions.
W. RULAND,
Brussels: I think it is not useful to discuss changes in tensile strength
and YOUNG’S
modulus as functions of heat-treatment temperature or stretching.
modulus is uniquely deterThe results of our studies have shown that YOUNG’S
178
Fibres for Use at Highest l’emperature8
mined by the preferred orientation of the graphitic layers. Since preferred orien’S
will
tation increases with heating as well as with stretching, Y O ~ N G modulus
increase with both treatments. The variation of tensile strength with these treatments is, however, more complex. For ideal structures one would expect the tensile
strength to increase proportionally with increasing YOUNG’S
modulus for a given
type of material. However, the proportionality between tensile strength and YOUNG
modulus is affected by structural changes differently from changes of the preferred
orientation. A better understanding of these problems would be achieved if one
would make a detailed study of the ratio of tensile strength to YOUNG’S
modulus
which is, in the case of purely elastic deformation as in carbon fibres, equal to the
extension on fracture as function of heating and stretching. From the results so far
obtained one can conclude that the increase in YOUNG’S
modulus obtained by heating
and stretching is always accompanied by a loss in the extension to fracture.
C. P. TALLEY,
J. appl. Phys. 30 (1959) 1114.
J. C. WITHERS,L. C. Mc CANDLESS,
and B. A. MACKLIN, in “Proceedings of the
Conference on Chemical Vapor Deposition of Refractory Metals, Alloys and
(ed.) p. 315, American Nuclear Society, Inc.,
Compounds”, A. C. SCHAFFHAUSER,
Hinsdale, Illinois 1967.
3 G. H. MILLER et al., Texaco Experiment Incorporated, unpublished work, 1967.
Work supported by NASA Lewis Research Center under Contract NAS 3-7948.
4 A. H. LASDAY
and C. P. TALLEY,in “Advanced Fibrous Reinforced Composites”, Vol. 10, paper D-I, Society of Aerospace Material and Process Engineers.
1966.
5 H. FENNINGER,
VDI-Z. 113 (1971) 354.
6 D. I. BEERNTSEN,
W. D. SMITH,and I. ECONOMY,
Appl. Polym. Symp. 9 (1969)
365.
7 I. ECONOMY,
R. V. ANDERSON,
and V. I. MATKOVICH,
Appl. Polym. Symp. 9
(1969) 377.
1. WIZON,Appl. Polym. Symp. 9 (1969) 395.
9 B. H. HAMLING,
A. W. NAUMANN,
and W. H. DRESHER,
Appl. Polym Symp.
9 (1969) 387.
10 B. DANNER
and J. MEYBECK, “Mechanisms of the Thermal and Alkaline Degradation of Polyacrylonitrile”, International Conference on Carbon Fibers,
their Composites and Applications, London 1971, Paper No. 6.
11 W. G. VOSBURGH,
“The Heat Treatment of Orlon Acrylic Fiber to Render it
Fireproof” Text. Res. J. 1960, 882.
l2 W. WATTand W. JOHNSON,
Amer. Chem. SOC.Meeting, Atlantic City, September (1968) ; Polymer Preprints p. 1245.
l 3 R. MORETON,
W. WATT,and W. JOHNSON,
Nature 213 (1967) 690.
l4 R. BACON
and M. M. TANG,
Carbon 2 (1964) 211.
l5 R. BACONand W. H. SMITH,“Tensile Behavior of Carbonized Ray on Filaments at Elevated Temperatures”, Second Conf. on Ind. Carbon and Graphite,
London, April 1965, SOC.Chem. Ind. [London] 1966, 203.
l6 A. FOURDEUX,
R. PRRRET,and W. RULAND,“General Structural Features of
Carbon Fibers”, Int. Conference on Carbon Fibers, their Composites and Applications, London 1971, Paper No. 9.
1
2
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179
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