L. Cheng et al. /Carbon 41(2003)707-71I UOU thermal diffusivity acs of the C/Sic and that a scs of the SiC/C/SiC could be also well fitted by a function over the full temperature range 16.4 00100·cxp(-14×10-7272)+0484 (5) Fig. 2. XRD patterns of the bulk Sic material prepared by ascs =0.9-0.0074exp(-1.4X10-T2)+0.0416 UPCVD (6) It could seen tha a l was a little larger than al This indicated that the diffusivity of the CVD Si coating is lower than longitudinal one of the fibers, and then it could not increase the thermal diffusivity of the composite It should be noted that there appeared an exponential term in Eqs. (5)and(6) compared with Eqs. (3)and (4), which corresponds to a new mechanism and led to the thermal diffusivity increase after rapid decrease with increasing temperature. The exponential term showed that the mechanism affected the thermal diffusivity above 1200C with activation energy of 77 kcal/ mol. The mechanism should be considered to be the microstructure change in the composites at high temperatures, otherwise 39429KVX2,9910yW039 the thermal diffusivity should be monotone decreasing Fig. 3. S.E.M. photograph of the bulk SiC material prepared by UPCVD 3.3. Transverse thermal diffusivity of the composite Fig. 5 showed the relations of the transverse thermal 3. 2. Longitudinal thermal diffusivity of the composite diffusivity of the C/SiC and SiC/C/SiC to temperature Similarly, the thermal diffusivity acs of the C/SiC and Fig. 4 showed the relations of the longitudinal thermal that a scs of the Sic/C/SiC could be well fitted by a diffusivity of the C/SiC and SiC/C/Sic composite to function over the full temperature range temperature. Although they were more complicated, the o 3D-C/SiC= 0.12 o 3D-SiC/C/SiClI(g) g0.08 500 1000 500 1000 Temperature(c) Temperature(C) Fig. 4. The longitudinal thermal diffusivity of the C/SiC and Fig. 5. The transverse thermal diffusivity of the C/SiC and SiC/C/SiC composite as a function of temperature SiC/C/SiC composite as a function of temperatureL. Cheng et al. / Carbon 41 (2003) 707–711 709 i i thermal diffusivity a of the C/SiC and that a of the CS SCS SiC/C/SiC could be also well fitted by a function over the full temperature range 16.46 i ] 272 23 a CS 5 2 ] 0.0100 ? exps d 2 1.4 3 10 T 1 0.0484 0.9 T (5) 15.13 i ] 272 23 Fig. 2. XRD patterns of the bulk SiC material prepared by a SCS 5 2 ] 0.0074 ? exps d 2 1.4 3 10 T 1 0.0416 0.9 T UPCVD. (6) i i It could be seen that a was a little larger than a . CS SCS This indicated that the thermal diffusivity of the CVD SiC coating is lower than longitudinal one of the fibers, and then it could not increase the thermal diffusivity of the composite. It should be noted that there appeared an exponential term in Eqs. (5) and (6) compared with Eqs. (3) and (4), which corresponds to a new mechanism and led to the thermal diffusivity increase after rapid decrease with increasing temperature. The exponential term showed that the mechanism affected the thermal diffusivity above 1200 8C with activation energy of 77 kcal/mol. The mechanism should be considered to be the microstructure change in the composites at high temperatures, otherwise the thermal diffusivity should be monotone decreasing. Fig. 3. S.E.M. photograph of the bulk SiC material prepared by 3 .3. Transverse thermal diffusivity of the composite UPCVD. Fig. 5 showed the relations of the transverse thermal 3 .2. Longitudinal thermal diffusivity of the composite diffusivity of the C/SiC and SiC/C/SiC to temperature. 5 Similarly, the thermal diffusivity a CS of the C/SiC and 5 Fig. 4 showed the relations of the longitudinal thermal that a of the SiC/C/SiC could be well fitted by a SCS diffusivity of the C/SiC and SiC/C/SiC composite to function over the full temperature range temperature. Although they were more complicated, the Fig. 4. The longitudinal thermal diffusivity of the C/SiC and Fig. 5. The transverse thermal diffusivity of the C/SiC and SiC/C/SiC composite as a function of temperature. SiC/C/SiC composite as a function of temperature