710 L. Cheng et al./ Carbon 41(2003)707-711 treatment, the longitudinal thermal diffusivity was found to 13-00020exp(-14×10-272)+00130 be increased and could be well fitted by the following (7) 1549 19.38 0.0020·exp(-14×10-2723)+0.0163 709+00396 The experimental curve of the thermal diffusivity obtained by the second measurement was coincident with that Certainly, the microstructure change affected the trans- obtained by the third measurement. As a result, the thermal diffusivity could not be increased by further treatment any verse appeared in Eqs. (7) and = ne threshold more, and then it could be given by the same function. The exponential term disappeared in this function, and the first curve intersected the second curve at 1400C which was tudinal thermal diffusivity increased the treatment temperature. These would not be difficult to increasing temperature than the transverse one. Therefore, understand if the microstructure change in the composit the former was more sensitive to the microstructure change caused by the heat treatment was non-reversible in the composites than the latter The transverse thermal diffusivity of the SiC/c/Sic Because ascs was large acs, the thermal dif. obtained by the first measurement was compared with that fusivity of the coating wa obtained by the second measurement in Fig. 7. Similarl the fibers. and then it could the thermal diffusivity the thermal diffusivity after the treatment could be wel of the composite. Consequently, the effect of the coating fitted by the following function on the transverse thermal diffusivity of the C/Sic wa contrary to that of longitudinal one 25.76 +0.0167 Figs. 4 and 5 showed that the longitudinal thermal diffusivity of the composite was much larger than trans- For the same reason, the exponential term disappeared in verse one. This is reasonable because the axial thermal this function, and the first curve intersected the second At roo ity of fibers is half an order larger than radical one. curve at 1400C. Although the thermal diffusivity was At room temperature, both the longitudinal and transverse increased, the difference of the transverse thermal dif- thermal diffusivity was a little lower than the reported data. fusivity produced by the treatment was much smaller than that of the longitudinal one over the whole temperature 3.4. Effect of heat treatment on the thermal diffusivity range. This should be easy to understand because the increase of the former produced by the exponential term The longitudinal thermal diffusivity of the SiC/C/Sic was much smaller than that of the latter obtained by the first measurement was compared with The thermal diffusivity of the CVD Sic was not those obtained by the second and the third measurement in increased by the treatment(Fig. 1), and that of the Fig. 6. The first measurement acted on the second one as a would not be changed by the treatment also heat treatment, so did the second on the third. After the 1400 C. Consequently, the microstructure change of the Pyc interlayer produced by the treatment should 0,16 p3 D-SiC/C/SiC‖(g) o 3D-siC/C/(g) o 30./C/SiC= 005 F0.01 0 1c00 1500 500 Fig. 6. Effect of the heat treatment on the longitudinal thermal Fig. 7. Effect of the heat treatment on the transverse thermal diffusivity of the Sic/C/SiC composite diffusivity of the SiC/C/SiC composite710 L. Cheng et al. / Carbon 41 (2003) 707–711 treatment, the longitudinal thermal diffusivity was found to 5 2 10.60 ] 72 23 a CS 5 2 ] 0.0020 ? exps d 2 1.4 3 10 T 1 0.0130 be increased and could be well fitted by the following 1.15 T function (7) i 15.49 a 9 5 1 ]] 0.0396 (9) SCS 0.9 5 2 19.38 72 23 T a SCS 5 2 ]] 0.0020 ? exps d 2 1.4 3 10 T 1 0.0163 1.1 T The experimental curve of the thermal diffusivity obtained (8) by the second measurement was coincident with that obtained by the third measurement. As a result, the thermal Certainly, the microstructure change affected the trans- diffusivity could not be increased by further treatment any verse thermal diffusivity, and then an exponential term more, and then it could be given by the same function. The appeared in Eqs. (7) and (8) with the same threshold exponential term disappeared in this function, and the first temperature and activation energy. However, the longi- curve intersected the second curve at 1400 8C which was tudinal thermal diffusivity increased more rapidly with the treatment temperature. These would not be difficult to increasing temperature than the transverse one. Therefore, understand if the microstructure change in the composite the former was more sensitive to the microstructure change caused by the heat treatment was non-reversible. in the composites than the latter. 5 5 The transverse thermal diffusivity of the SiC/C/SiC Because a was larger than a , the thermal dif- SCS CS obtained by the first measurement was compared with that fusivity of the coating was larger than the transverse one of obtained by the second measurement in Fig. 7. Similarly, the fibers, and then it could increase the thermal diffusivity the thermal diffusivity after the treatment could be well of the composite. Consequently, the effect of the coating fitted by the following function on the transverse thermal diffusivity of the C/SiC was 5 25.76 contrary to that of longitudinal one. a 9 5 1 ]] 0.0167 (10) SCS 1.2 Figs. 4 and 5 showed that the longitudinal thermal T diffusivity of the composite was much larger than trans- For the same reason, the exponential term disappeared in verse one. This is reasonable because the axial thermal this function, and the first curve intersected the second diffusivity of fibers is half an order larger than radical one. curve at 1400 8C. Although the thermal diffusivity was At room temperature, both the longitudinal and transverse increased, the difference of the transverse thermal dif￾thermal diffusivity was a little lower than the reported data. fusivity produced by the treatment was much smaller than that of the longitudinal one over the whole temperature 3 .4. Effect of heat treatment on the thermal diffusivity range. This should be easy to understand because the increase of the former produced by the exponential term The longitudinal thermal diffusivity of the SiC/C/SiC was much smaller than that of the latter. obtained by the first measurement was compared with The thermal diffusivity of the CVD SiC was not those obtained by the second and the third measurement in increased by the treatment (Fig. 1), and that of the fibers Fig. 6. The first measurement acted on the second one as a would not be changed by the treatment also below heat treatment, so did the second on the third. After the 1400 8C. Consequently, the microstructure change of the PyC interlayer produced by the treatment should be Fig. 6. Effect of the heat treatment on the longitudinal thermal Fig. 7. Effect of the heat treatment on the transverse thermal diffusivity of the SiC/C/SiC composite. diffusivity of the SiC/C/SiC composite