C Sauder et al. / Composites Science and Technology 62(2002)499-50 determined under a 0. 2% strain. The statistical para- 3. 2. Longitudinal thermal expansion meters(Weibull modulus m and scale factor go) were estimated from the statistical distributions of strength In order to check the validity of measurements, the data using a linear regression technique [7] echnique was applied to a tungsten fibre having a 18 um diameter. Fig. 4 shows that the measured thermal expansions coincide with those values derived from the 3. Results and discussion coefficient of thermal expansion available in the litera- ture(SETARAM data) 3. 1. Electrical conductivity Thermal expansions at various temperatures are plot ted in Figs. 5 and 6. It can be noticed that the investi Electrical conductivity measured at various tempera- gated fibres exhibit different behaviors. Expansion of tures on the PAN-based and rayon-based fibres, that the PAN-based fibre is much smaller than that of the had been previously treated at 2200C, are plotted in rayon-based fibre. At temperatures close to ambient Fig 3a and b. The temperature dependence of electrical temperature, the PAN-based fibre contracts. This beha conductivity is typical for these fibres, since it can be vior is comparable to that displayed by graphite single noticed that electrical conductivity increases with tem- crystal. It is characterized by a negative coefficient of perature. This trend characterizes a semiconductor. thermal expansion at 20C. This behavior results from Although data at low temperatures were not deter- the fibre microstructure shown in Fig. 7 [8], which is not mined, it can be considered that this trend is pertinent isotropic but instead involves basal structural units to an extrinsic semiconductor. Indeed, at low tempera-(BSU) with a preferred orientation with respect to the tures, conduction is governed by impurities(extrinsic fibre axis. This organized microstructure contrasts with conductivity), whereas at high temperatures the thermal the one of rayon-based fibre. Fig. 6 also shows that energy is sufficient to induce electronic transitions(strip treatment influences the fibre respon of valence B V. strip of conduction B.C. ) This is the This phenomenon can be attributed to successive domain of intrinsic conductivity for this type of carbon changes in the fibre microstructure. As the treatment microstructure. The energy gap(AE=EcEv) can b temperature increases, the BSU tend to be oriented determined by using the following equation parallel to the fibres axis, which would increase the coherent domains in X-ray diffraction(XRD). On the a=Cexp(-△E/2k contrary, the behavior and the microstructure of the rayon based fibres do not change when the treatment temperature is increased AE=0.12 and 0. 18ev were obtained respectively for the Expansion of graphite single crystal is shown in PAN-based and the rayon based fibres. These values are Fig. 8 [9. It can be noticed that for both fibres, long itudinal thermal expansion results from a combination 10.6 10.5 120 103 11,7 116 10 11,5 0.0 2,0 0,0 2,0 3,0 1「T(101 1「T(10 Fig. 3. Electrical resistivity vs temperature for a PAN-based (a)and a rayon based(b)fibredetermined under a 0.2% strain. The statistical para￾meters (Weibull modulus m and scale factor
0) were estimated from the statistical distributions of strength data using a linear regression technique [7]. 3. Results and discussion 3.1. Electrical conductivity Electrical conductivity measured at various tempera￾tures on the PAN-based and rayon-based fibres, that had been previously treated at 2200
C, are plotted in Fig. 3a and b. The temperature dependence of electrical conductivity is typical for these fibres, since it can be noticed that electrical conductivity increases with tem￾perature. This trend characterizes a semiconductor. Although data at low temperatures were not deter￾mined, it can be considered that this trend is pertinent to an extrinsic semiconductor. Indeed, at low tempera￾tures, conduction is governed by impurities (extrinsic conductivity), whereas at high temperatures the thermal energy is sufficient to induce electronic transitions (strip of valence B.V.!strip of conduction B.C.). This is the domain of intrinsic conductivity for this type of carbon microstructure. The energy gap (E=Ec–Ev) can be determined by using the following equation:
¼ CexpðE=2kTÞ E=0.12 and 0.18ev were obtained respectively for the PAN-based and the rayon based fibres. These values are quite small. 3.2. Longitudinal thermal expansion In order to check the validity of measurements, the technique was applied to a tungsten fibre having a 18 mm diameter. Fig. 4 shows that the measured thermal expansions coincide with those values derived from the coefficient of thermal expansion available in the litera￾ture (SETARAM data). Thermal expansions at various temperatures are plot￾ted in Figs. 5 and 6. It can be noticed that the investi￾gated fibres exhibit different behaviors. Expansion of the PAN-based fibre is much smaller than that of the rayon-based fibre. At temperatures close to ambient temperature, the PAN-based fibre contracts. This beha￾vior is comparable to that displayed by graphite single crystal. It is characterized by a negative coefficient of thermal expansion at 20
C. This behavior results from the fibre microstructure shown in Fig. 7 [8], which is not isotropic but instead involves basal structural units (BSU) with a preferred orientation with respect to the fibre axis. This organized microstructure contrasts with the one of rayon-based fibre. Fig. 6 also shows that treatment influences the fibre response. This phenomenon can be attributed to successive changes in the fibre microstructure. As the treatment temperature increases, the BSU tend to be oriented parallel to the fibres axis, which would increase the coherent domains in X-ray diffraction (XRD). On the contrary, the behavior and the microstructure of the rayon based fibres do not change when the treatment temperature is increased. Expansion of graphite single crystal is shown in Fig. 8 [9]. It can be noticed that for both fibres, long￾itudinal thermal expansion results from a combination Fig. 3. Electrical resistivity vs temperature for a PAN-based (a) and a rayon based (b) fibre. C. Sauder et al. / Composites Science and Technology 62 (2002) 499–504 501