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The temperature dependence of the electrical resistivity of celion carbon fibres
Materials Science and Engineering, 86 (1987) 205-210
205
The Temperature Dependence of the Electrical Resistivity of Celion Carbon Fibres M. J. YASIN, I. EL-RIHAIL*, M. S. AHMAD and A. M. ZIHLIF
Physics Department, University of Jordan, Amman (Jordan) (Received October 25, 1985; in revised form May 12, 1986)
ABSTRACT
The dependence of the electrical resistivity o f Celion polyacrylonitrile-based carbon fibres on test temperature is characterized by a two-stage electrical conduction process and a semiconductor type o f behaviour with a low activation energy. It was found that annealing affects the elastic moduli. The electrical conductivity increases with increasing elastic moduli. This behaviour was analysed in terms o f the electrical anisotropy and the reorientation o f the c axes o f the graphite crystallites existing in the fibre during the heat treatment process.
1. INTRODUCTION Polymers are usually insulators. Recently, efforts have been e xpe nde d to utilize t h e m as conducting materials such as conductor-filled polymers, poly(vinyl chloride) ionic-selective electrodes and carbonized polymers [ 1 - 3 ] . The p o l y m e r polyacrylonitrile (PAN) is used as a base material in the p r o d u c t i o n of carbon fibres. There is a considerable interest in the electrical resistivity of carbon fibres to be used in some composites and structural applications. In previous studies carried out by J o h n s o n and Watt [4], t h e y reported t ha t high modulus carbon fibres have a high electrical resistivity compared with that of graphite single crystals parallel to the basal plane. R e c en t investigations have shown t hat electrical conductivity is affected by strain d e f o r m a t i o n , stress graphitization and microstructure [ 5-7 ]. In the present paper the *Present address: Physics Department, University of Reading, Reading, U.K. 0025-5416/87/$3.50
dependence of the electrical resistivity of Celion carbon fibres on bot h the test temperature and the heat t r e a t m e n t temperature is investigated. High strength Celion 6000 carbon fibres obtained f r o m the Celanese Corporation, U.S.A., were produced from PAN as the raw material.
2. EXPERIMENTAL DETAILS The samples used in this work were cut from a roll 15 cm in length. Each strand was heated for 1 h at the required t em perat ure under atmospheric pressure using a vacuum furnace. The t em perat ure range was 20600 °C. The masses before and after annealing were determined with a sensitive balance. The mass o f the filament obtained was used to estimate the diameter from the following relation [8]: d = - 4p X 10 -4 p m Irp
(1)
where p (g cm -3) is the mass density and p (g cm -z) is the linear density. The results were compared with the values obtained from scanning electron micrographs. Figure 1 shows the dependence o f the fibre diameter on temperature. The fibre diameter remains constant until 500 °C and hence no correction to the diameter was needed in subsequent results. However, the fibre diameter calculated from eqn. (1) was found to be 7 t~m. Measurements of the electrical resistance R as a function o f t e m p e r a t u r e were carried o u t using the two-point m e t h o d represented in Fig. 2. A current generator is used to pass a constant current o f magnitude 1 mA in the circuit. A single fibre with a length of a b o u t 1 cm was placed on a quartz plate. The two ends of the © Elsevier Sequoia/Printed in The Netherlands
206
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i
100
i
i
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200 300 400 TEMPERATURE (°C)
" ~-..~ ~ : _ ~
,
500
600
Fig. 1. Diameter of the carbon fibres v s . heat treatment temperature (heat treated for 1 h).
o
loo
2o0 30O TEMPERATURE (~]
4O0
Fig. 3. Variation in the electrical resistance during heating (o, ) and cooling ( o , - - ' - - ) for as-spun fibres (heat treated for 1 h). F
1 Dc POWER 220 V
~ SUPPLY
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Fig. 2. Circuit for electrical resistance measurements.
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fibre were covered by silver Dag to obtain a good ohmic contact. The fibre assembly was inserted in the channel tube furnace. A c h r o m e l - c o n s t a n t a n t h e r m o c o u p l e (Philips) was placed near the sample to measure the average temperature. A sample was maintained at each test t e m p e r a t u r e for a time of 15 min before the t e m p e r a t u r e was measured. The current and the voltage across the fibre were measured using digital multimeters. Furthermore, resistance measurements were perf o rmed on single fibres annealed for 2 min.
3. RESULTS AND DISCUSSION
3.1. Electrical resistivity as a f u n c t i o n o f test temperature The electrical resistance for a single fibre was calculated f r o m Ohm's law. The results obtained are shown in Figs. 3 and 4. The resistance decreases as the t em pe r at ur e increases and shows a rather abrupt change in a temperature range 1 6 0 - 2 4 0 °C. During cooling, the plot of the electrical resistance versus t e m p er atu r e has an almost linear relationship with a scatter below 200 °C. However, the resistance shows an irreversible hysteresis
(~1320 ?260
lOO
200 TEMPERATURE
300
400
Fig. 4. The temperature dependence of the electrical resistivity: o , - - . , heating; o , - - ' - - , cooling.
behaviour. This m ay be due to the elimination o f some defects associated with the structure o f the fibre or the base material. Table 1 shows the values of the electrical resistivity for various carbon fibres with different base materials. The values obtained for the electrical resistivity of Celion carbon fibres correlate well with these r e p o r t e d values. Celion carbon fibres have a high electrical resistivity com pared with t hat measured parallel to the basal plane o f graphite single crystals. This m ay be attributed to a high defectscattering process t h a t takes place at the grain boundaries [13]. The resistivity of a solid material can be described by the Arrhenius equation as
207 TABLE 1
TABLE 2
Electrical resistivity of natural graphite crystals and high modulus carbon fibres
The activation energy values for electrical conduction during heating and cooling
Material
Electrical resistivity
Reference
(p~2 cm) Carbon fibres (grade A) Carbon fibres (grade B) Graphitized fibres Carbon fibres (PAN based) Carbon fibres (rayon based) Celion carbon fibres Graphite crystals
862 1145 400 2000 2000 1500 40
[9 ] [9] [ 10 ] [10] [ 10] [ 11 ] [ 12 ]
TEMPERATURE 400
240
160
Heating, first region Heating, second region Cooling, first region Cooling, second region
(~3)
100
60
20
-2.82
o -2.B4
/ o
ell
/
/
•
/
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IiIIo
~L _o o
3~
-2.BB
-2 90
1!2
f
?J
118
2:,
3
f
3.6
lo0o T Fig. 5. Variation in the electrical resistivity with the reciprocal of the temperature: ©, - - , heating; e, --'--, cooling.
w h e r e P0 is a material c o n s t a n t , Ea ( = E g / 2 ) is t h e a c t i v a t i o n e n e r g y for t h e electrical c o n d u c t i o n process, Eg is the e n e r g y gap in the conventional semiconductor notation, R (kcal K -1 mo1-1) is t h e universal gas constant and T is the absolute t e m p e r a t u r e . Figure 5 shows t h e t e m p e r a t u r e d e p e n d e n c e o f p. It consists o f a p p r o x i m a t e l y t w o lines
Experimental values E a
Fitted values
(kcal mo1-1)
E a (kcal tool -1)
0.36 0.06 0.25 0.09
0.40 0.06 0.24 0.10
over the t e m p e r a t u r e range 2 0 - 4 0 0 °C. This b e h a v i o u r reveals a t y p e o f s e m i c o n d u c t i v i t y . H e n c e , the activation e n e r g y can be o b t a i n e d f r o m the slope o f log p versus l I T . Table 2 shows the values o b t a i n e d f o r t h e activation energy. T h e values are low (0.06 and 0.4 kcal mo1-1) c o m p a r e d with the values r e p o r t e d f o r crystalline s e m i c o n d u c t o r s . These values o f activation e n e r g y c o r r e s p o n d to e n e r g y gaps o f 5 X 10 .3 eV (2 X 0.06 kcal mo1-1) and 35 X 10 -3 eV (2 X 0.4 kcal mol-1). Similar values were o b t a i n e d b y Mostovoi et al. [14] f r o m a stress r e l a x a t i o n e x p e r i m e n t w h e n fibres were stressed in t h e high t e m p e r a t u r e range. F u r t h e r m o r e , the existence o f t w o linear regions in Fig. 5 m a y be a t t r i b u t e d t o d i f f e r e n t mobilities o f t h e charge carriers during the electrical c o n d u c t i o n process as was observed in natural graphite single crystals [ 12]. T h e X-ray d i f f r a c t i o n p a t t e r n s h o w n in Fig. 6 indicates t h a t the fibre is a semicrystalline material. X-ray f l u o r e s c e n c e analysis [ 15 ] and R u t h e r f o r d backscattering indicate the presence o f h e a v y and light metallic impurities, w h i c h c o n t r i b u t e to the electrical c o n d u c t i o n m e c h a n i s m . T h e r e f o r e , it is reasonable to consider the existence o f t w o d i f f e r e n t activation processes (i.e. t w o diff e r e n t band gaps). 3.2. E l e c t r i c a l resistivity as a f u n c t i o n o f annealing temperature
Figure 7 shows the e f f e c t o f h e a t treatm e n t o n t h e electrical resistivity o f a single c a r b o n fibre. T h e resistivity decreases w h e n the annealing t e m p e r a t u r e is increased above 160 °C and results in a c o n t i n u o u s increase in the electrical resistivity. T h e decrease s h o w n in t h e first stage can be p r e d i c t e d f r o m the
208
Fig. 8. Scanning electron micrograph showing graphite crystallites in PAN-based carbon fibres. (Magnification, 27 000x.)
treatment may force the c axis to rotate away from the fibre direction and hence it decreases the resistivity. At temperatures higher than 160 °C or in the second stage, the rotation of the graphite crystallites is retarded either by the interfibrillar and interelastic forces between the crystallites within the fibre or by oxidation effects. Therefore, the resistivity increases slowly up to 400 °C. Oxidation takes place during annealing above 160 °C and may have a considerable effect on the observed increase in the electrical resistivity. The explanation of the resistivity behaviour under annealing proposed here is based on the structural changes shown by the X-ray diffraction patterns and the variation in the misorientation angle recently reported by Yasin et al. [16].
Fig. 6. X-ray diffraction pattern as-spun fibres.
,_.
o
E
~
~L Q.
_> I--
U.J n-
o
_1 <
_u n-iu u.J w 100
ANNEALING
200
300
TEMPERATURE
400
(°C)
Fig. 7. The electrical resistivity of single carbon fibres vs. the heat treatment temperature.
electrical anisotropy of the distributed graphite crystaUites shown in the scanning electron micrograph in Fig. 8. Since Celion carbon fibres have a high modulus and a high strain, the c axes of the crystallites are expected to be near the perpendicular direction of the fibre axis. Heat
3.3. Correlation o f electrical conductivity with the elastic moduli The elastic moduli, as well as the electrical resistivity, have been found to be governed by the orientation of the graphite crystallites. The relationship between the electrical conductivity o (= i / p ) of the fibres and Young's modulus is shown in Fig. 9. It seems that the two properties are proportional to each other and exhibit almost a linear relationship for the Celion PAN-based carbon fibres. Figure 10 shows the electrical conductivity as a function of the shear modulus. Young's modulus and shear modulus data are taken from references [ 11, 16-18 ]. Both the elastic moduli increase to a m a x i m u m at 160 °C and then start to decrease slowly with increasing annealing tern-
209 ANNEALING
TEMPERATURE (°C)
350
300
creases in both Young's modulus and the conductivity. A similar relationship between Young's modulus and the electrical conductivity has been reported by Sarian and Strong [19] for annealed carbon fibres. They attributed the observed increase in the electrical conductivity to the removal of the structural defects which results in the electron scattering.
25O 2C0
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o ('
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4. C O N C L U S I O N
9 u
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Young's
2'0
220
230
Modulus
240
25C
E (ON m 2)
Fig. 9. Relationship b e t w e e n Young's elastic modulus and the electrical conductivity of heat-treated Celion carbon fibres.
I
TEMPERATURE (°C)
E U
400 i
5-
500
250
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!
200
160
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123 Z 0 (.9
ACKNKOWLEDGMENTS
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U tl.I _J U.l
The resistivity of single carbon fibres of approximately 7 #m diameter has been measured over a temperature range 20-400 °C. The electrical behaviour shows a two-stage conduction process during heating and cooling. The activation energy of the conduction process was calculated using an Arrheniustype equation. The low values obtained for the activation energy suggest that Celion carbon fibres possess a semiconductor-type behaviour. Furthermore, the electrical resistivity measured for the Celion high modulus fibres is high compared with that of natural graphite single crystals. It was found that the electrical resistivity of annealed samples has a minimum at about 160 °C and the electrical conductivity increases with increasing elastic moduli. These observations were attributed to the rotation of the c axes of the graphite crystallites during heat treatment.
600 t
l
I
I
17
20
23
26
SHEAR
MODULUS
The authors are very grateful to Professor R. J. Farris, Polymer Science Department, University of Massachusetts, for providing the Celion carbon fibres and to Dr. M. M. Abdul-Gader for useful discussion.
G ( G N m-2)
Fig. 10. Relationship b e t w e e n the shear modulus and the electrical c o n d u c t i v i t y of heat-treated carbon fibres.
perature. In other words, a maximum orientation corresponding to a maximum conductivity takes place at the optimum heat treatment temperature of 160 °C. Above this temperature, the orientation decreases because of the rotation of the c axes of graphite crystallites towards the fibre direction, resulting in de-
REFERENCES 1 R. D. Sherman, L. M. Middleman and S. M. Jacobs, Polym. Eng. Sci., 23 (1983) 36. 2 M. M. Abu Samrah, R. A. Bitar and A. M. Zihlif, Appl. Phys. Commun., 3 (1983) 225. 3 R. Bacon and W. A. Schalamon, Appl. Polym. Symp., 9 (1969) 285. 4 W. J o h n s o n and W. Watt, Nature (London), 215 (1967) 384. 5 C . N . Owston, J. Phys. D, 3 (1970) 1615. 6 H. E. Ezekiel, J. Appl. Phys., 41 (1970) 5351. 7 D. Robson, F. Y. I. Assabghy, E. G. Cooper and D. J. Ingrain, J. Phys. D, 6 (1973) 1922.
210 8 S. H. Zeronian and M. S. Ellison, J. Appl. Polym. Sci., 24 (1979) 1494. 9 P. N. Conor and C. N. Owston, Nature (London), 223 (1969) 1146. 10 W.N. Reynolds, Chem.Phys. Carbon, 11 (1973) 1. 11 I. EI-Rihail, M.Sc. Thesis, Physics Department, University of Jordan, 1984. 12 L. A. Pendrys, C. Zeller and F. L. Vogel, J. Mater. Sci., 15 (1980) 2103. 13 C. Herinckx, R. Perret and W. Ruland, Nature (London), 230 (1968) 63. 14 G. E. Mostovoi, Yu. N. Rabotnov, L. P. Kobets
15 16 17 18 19
and V. I. Frolov, Mekh. Kompos. Mater., (1) (1980) 3. A. Ahmad, D. Arafa, N. Saleh and A. M. Zihlif, unpublished data (1985). M. J. Yasin, I. E1-Rihail, M. S. Ahmad and A. M. Zihlif, Mater. Chem. Phys., to be published. G. J. Curtis, J. M. Milne and W. N Reynolds, Nature (London), 220 (1968) 1024. W. N. Reynolds and R. Moreton, Philos. Trans. R. Soc. London, Set. A, 294 (1980) 451. S. Sarian and S. L. Strong, Fibre. Sci. Technol., 4 (1971) 67.
205
The Temperature Dependence of the Electrical Resistivity of Celion Carbon Fibres M. J. YASIN, I. EL-RIHAIL*, M. S. AHMAD and A. M. ZIHLIF
Physics Department, University of Jordan, Amman (Jordan) (Received October 25, 1985; in revised form May 12, 1986)
ABSTRACT
The dependence of the electrical resistivity o f Celion polyacrylonitrile-based carbon fibres on test temperature is characterized by a two-stage electrical conduction process and a semiconductor type o f behaviour with a low activation energy. It was found that annealing affects the elastic moduli. The electrical conductivity increases with increasing elastic moduli. This behaviour was analysed in terms o f the electrical anisotropy and the reorientation o f the c axes o f the graphite crystallites existing in the fibre during the heat treatment process.
1. INTRODUCTION Polymers are usually insulators. Recently, efforts have been e xpe nde d to utilize t h e m as conducting materials such as conductor-filled polymers, poly(vinyl chloride) ionic-selective electrodes and carbonized polymers [ 1 - 3 ] . The p o l y m e r polyacrylonitrile (PAN) is used as a base material in the p r o d u c t i o n of carbon fibres. There is a considerable interest in the electrical resistivity of carbon fibres to be used in some composites and structural applications. In previous studies carried out by J o h n s o n and Watt [4], t h e y reported t ha t high modulus carbon fibres have a high electrical resistivity compared with that of graphite single crystals parallel to the basal plane. R e c en t investigations have shown t hat electrical conductivity is affected by strain d e f o r m a t i o n , stress graphitization and microstructure [ 5-7 ]. In the present paper the *Present address: Physics Department, University of Reading, Reading, U.K. 0025-5416/87/$3.50
dependence of the electrical resistivity of Celion carbon fibres on bot h the test temperature and the heat t r e a t m e n t temperature is investigated. High strength Celion 6000 carbon fibres obtained f r o m the Celanese Corporation, U.S.A., were produced from PAN as the raw material.
2. EXPERIMENTAL DETAILS The samples used in this work were cut from a roll 15 cm in length. Each strand was heated for 1 h at the required t em perat ure under atmospheric pressure using a vacuum furnace. The t em perat ure range was 20600 °C. The masses before and after annealing were determined with a sensitive balance. The mass o f the filament obtained was used to estimate the diameter from the following relation [8]: d = - 4p X 10 -4 p m Irp
(1)
where p (g cm -3) is the mass density and p (g cm -z) is the linear density. The results were compared with the values obtained from scanning electron micrographs. Figure 1 shows the dependence o f the fibre diameter on temperature. The fibre diameter remains constant until 500 °C and hence no correction to the diameter was needed in subsequent results. However, the fibre diameter calculated from eqn. (1) was found to be 7 t~m. Measurements of the electrical resistance R as a function o f t e m p e r a t u r e were carried o u t using the two-point m e t h o d represented in Fig. 2. A current generator is used to pass a constant current o f magnitude 1 mA in the circuit. A single fibre with a length of a b o u t 1 cm was placed on a quartz plate. The two ends of the © Elsevier Sequoia/Printed in The Netherlands
206
IO 3 BO
"2
"\
~]70
u%
-'~"
\
:. ~;\.
"-~.~
oN o
i
i
100
i
i
i
200 300 400 TEMPERATURE (°C)
" ~-..~ ~ : _ ~
,
500
600
Fig. 1. Diameter of the carbon fibres v s . heat treatment temperature (heat treated for 1 h).
o
loo
2o0 30O TEMPERATURE (~]
4O0
Fig. 3. Variation in the electrical resistance during heating (o, ) and cooling ( o , - - ' - - ) for as-spun fibres (heat treated for 1 h). F
1 Dc POWER 220 V
~ SUPPLY
--L
' ~GENERATOR
~
L
~
V
I~--I
"~
1
l=II
\\
'\,,
OUARTZPLATE~ , COPPERELECTRODES
Fig. 2. Circuit for electrical resistance measurements.
15oo u 14z~
~
o
138(}
fibre were covered by silver Dag to obtain a good ohmic contact. The fibre assembly was inserted in the channel tube furnace. A c h r o m e l - c o n s t a n t a n t h e r m o c o u p l e (Philips) was placed near the sample to measure the average temperature. A sample was maintained at each test t e m p e r a t u r e for a time of 15 min before the t e m p e r a t u r e was measured. The current and the voltage across the fibre were measured using digital multimeters. Furthermore, resistance measurements were perf o rmed on single fibres annealed for 2 min.
3. RESULTS AND DISCUSSION
3.1. Electrical resistivity as a f u n c t i o n o f test temperature The electrical resistance for a single fibre was calculated f r o m Ohm's law. The results obtained are shown in Figs. 3 and 4. The resistance decreases as the t em pe r at ur e increases and shows a rather abrupt change in a temperature range 1 6 0 - 2 4 0 °C. During cooling, the plot of the electrical resistance versus t e m p er atu r e has an almost linear relationship with a scatter below 200 °C. However, the resistance shows an irreversible hysteresis
(~1320 ?260
lOO
200 TEMPERATURE
300
400
Fig. 4. The temperature dependence of the electrical resistivity: o , - - . , heating; o , - - ' - - , cooling.
behaviour. This m ay be due to the elimination o f some defects associated with the structure o f the fibre or the base material. Table 1 shows the values of the electrical resistivity for various carbon fibres with different base materials. The values obtained for the electrical resistivity of Celion carbon fibres correlate well with these r e p o r t e d values. Celion carbon fibres have a high electrical resistivity com pared with t hat measured parallel to the basal plane o f graphite single crystals. This m ay be attributed to a high defectscattering process t h a t takes place at the grain boundaries [13]. The resistivity of a solid material can be described by the Arrhenius equation as
207 TABLE 1
TABLE 2
Electrical resistivity of natural graphite crystals and high modulus carbon fibres
The activation energy values for electrical conduction during heating and cooling
Material
Electrical resistivity
Reference
(p~2 cm) Carbon fibres (grade A) Carbon fibres (grade B) Graphitized fibres Carbon fibres (PAN based) Carbon fibres (rayon based) Celion carbon fibres Graphite crystals
862 1145 400 2000 2000 1500 40
[9 ] [9] [ 10 ] [10] [ 10] [ 11 ] [ 12 ]
TEMPERATURE 400
240
160
Heating, first region Heating, second region Cooling, first region Cooling, second region
(~3)
100
60
20
-2.82
o -2.B4
/ o
ell
/
/
•
/
//
tj C~ 9-2.B6
IiIIo
~L _o o
3~
-2.BB
-2 90
1!2
f
?J
118
2:,
3
f
3.6
lo0o T Fig. 5. Variation in the electrical resistivity with the reciprocal of the temperature: ©, - - , heating; e, --'--, cooling.
w h e r e P0 is a material c o n s t a n t , Ea ( = E g / 2 ) is t h e a c t i v a t i o n e n e r g y for t h e electrical c o n d u c t i o n process, Eg is the e n e r g y gap in the conventional semiconductor notation, R (kcal K -1 mo1-1) is t h e universal gas constant and T is the absolute t e m p e r a t u r e . Figure 5 shows t h e t e m p e r a t u r e d e p e n d e n c e o f p. It consists o f a p p r o x i m a t e l y t w o lines
Experimental values E a
Fitted values
(kcal mo1-1)
E a (kcal tool -1)
0.36 0.06 0.25 0.09
0.40 0.06 0.24 0.10
over the t e m p e r a t u r e range 2 0 - 4 0 0 °C. This b e h a v i o u r reveals a t y p e o f s e m i c o n d u c t i v i t y . H e n c e , the activation e n e r g y can be o b t a i n e d f r o m the slope o f log p versus l I T . Table 2 shows the values o b t a i n e d f o r t h e activation energy. T h e values are low (0.06 and 0.4 kcal mo1-1) c o m p a r e d with the values r e p o r t e d f o r crystalline s e m i c o n d u c t o r s . These values o f activation e n e r g y c o r r e s p o n d to e n e r g y gaps o f 5 X 10 .3 eV (2 X 0.06 kcal mo1-1) and 35 X 10 -3 eV (2 X 0.4 kcal mol-1). Similar values were o b t a i n e d b y Mostovoi et al. [14] f r o m a stress r e l a x a t i o n e x p e r i m e n t w h e n fibres were stressed in t h e high t e m p e r a t u r e range. F u r t h e r m o r e , the existence o f t w o linear regions in Fig. 5 m a y be a t t r i b u t e d t o d i f f e r e n t mobilities o f t h e charge carriers during the electrical c o n d u c t i o n process as was observed in natural graphite single crystals [ 12]. T h e X-ray d i f f r a c t i o n p a t t e r n s h o w n in Fig. 6 indicates t h a t the fibre is a semicrystalline material. X-ray f l u o r e s c e n c e analysis [ 15 ] and R u t h e r f o r d backscattering indicate the presence o f h e a v y and light metallic impurities, w h i c h c o n t r i b u t e to the electrical c o n d u c t i o n m e c h a n i s m . T h e r e f o r e , it is reasonable to consider the existence o f t w o d i f f e r e n t activation processes (i.e. t w o diff e r e n t band gaps). 3.2. E l e c t r i c a l resistivity as a f u n c t i o n o f annealing temperature
Figure 7 shows the e f f e c t o f h e a t treatm e n t o n t h e electrical resistivity o f a single c a r b o n fibre. T h e resistivity decreases w h e n the annealing t e m p e r a t u r e is increased above 160 °C and results in a c o n t i n u o u s increase in the electrical resistivity. T h e decrease s h o w n in t h e first stage can be p r e d i c t e d f r o m the
208
Fig. 8. Scanning electron micrograph showing graphite crystallites in PAN-based carbon fibres. (Magnification, 27 000x.)
treatment may force the c axis to rotate away from the fibre direction and hence it decreases the resistivity. At temperatures higher than 160 °C or in the second stage, the rotation of the graphite crystallites is retarded either by the interfibrillar and interelastic forces between the crystallites within the fibre or by oxidation effects. Therefore, the resistivity increases slowly up to 400 °C. Oxidation takes place during annealing above 160 °C and may have a considerable effect on the observed increase in the electrical resistivity. The explanation of the resistivity behaviour under annealing proposed here is based on the structural changes shown by the X-ray diffraction patterns and the variation in the misorientation angle recently reported by Yasin et al. [16].
Fig. 6. X-ray diffraction pattern as-spun fibres.
,_.
o
E
~
~L Q.
_> I--
U.J n-
o
_1 <
_u n-iu u.J w 100
ANNEALING
200
300
TEMPERATURE
400
(°C)
Fig. 7. The electrical resistivity of single carbon fibres vs. the heat treatment temperature.
electrical anisotropy of the distributed graphite crystaUites shown in the scanning electron micrograph in Fig. 8. Since Celion carbon fibres have a high modulus and a high strain, the c axes of the crystallites are expected to be near the perpendicular direction of the fibre axis. Heat
3.3. Correlation o f electrical conductivity with the elastic moduli The elastic moduli, as well as the electrical resistivity, have been found to be governed by the orientation of the graphite crystallites. The relationship between the electrical conductivity o (= i / p ) of the fibres and Young's modulus is shown in Fig. 9. It seems that the two properties are proportional to each other and exhibit almost a linear relationship for the Celion PAN-based carbon fibres. Figure 10 shows the electrical conductivity as a function of the shear modulus. Young's modulus and shear modulus data are taken from references [ 11, 16-18 ]. Both the elastic moduli increase to a m a x i m u m at 160 °C and then start to decrease slowly with increasing annealing tern-
209 ANNEALING
TEMPERATURE (°C)
350
300
creases in both Young's modulus and the conductivity. A similar relationship between Young's modulus and the electrical conductivity has been reported by Sarian and Strong [19] for annealed carbon fibres. They attributed the observed increase in the electrical conductivity to the removal of the structural defects which results in the electron scattering.
25O 2C0
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230
Modulus
240
25C
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Fig. 9. Relationship b e t w e e n Young's elastic modulus and the electrical conductivity of heat-treated Celion carbon fibres.
I
TEMPERATURE (°C)
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400 i
5-
500
250
i
!
200
160
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ACKNKOWLEDGMENTS
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The resistivity of single carbon fibres of approximately 7 #m diameter has been measured over a temperature range 20-400 °C. The electrical behaviour shows a two-stage conduction process during heating and cooling. The activation energy of the conduction process was calculated using an Arrheniustype equation. The low values obtained for the activation energy suggest that Celion carbon fibres possess a semiconductor-type behaviour. Furthermore, the electrical resistivity measured for the Celion high modulus fibres is high compared with that of natural graphite single crystals. It was found that the electrical resistivity of annealed samples has a minimum at about 160 °C and the electrical conductivity increases with increasing elastic moduli. These observations were attributed to the rotation of the c axes of the graphite crystallites during heat treatment.
600 t
l
I
I
17
20
23
26
SHEAR
MODULUS
The authors are very grateful to Professor R. J. Farris, Polymer Science Department, University of Massachusetts, for providing the Celion carbon fibres and to Dr. M. M. Abdul-Gader for useful discussion.
G ( G N m-2)
Fig. 10. Relationship b e t w e e n the shear modulus and the electrical c o n d u c t i v i t y of heat-treated carbon fibres.
perature. In other words, a maximum orientation corresponding to a maximum conductivity takes place at the optimum heat treatment temperature of 160 °C. Above this temperature, the orientation decreases because of the rotation of the c axes of graphite crystallites towards the fibre direction, resulting in de-
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15 16 17 18 19
and V. I. Frolov, Mekh. Kompos. Mater., (1) (1980) 3. A. Ahmad, D. Arafa, N. Saleh and A. M. Zihlif, unpublished data (1985). M. J. Yasin, I. E1-Rihail, M. S. Ahmad and A. M. Zihlif, Mater. Chem. Phys., to be published. G. J. Curtis, J. M. Milne and W. N Reynolds, Nature (London), 220 (1968) 1024. W. N. Reynolds and R. Moreton, Philos. Trans. R. Soc. London, Set. A, 294 (1980) 451. S. Sarian and S. L. Strong, Fibre. Sci. Technol., 4 (1971) 67.