L. Cheng et al. /Carbon 41(2003)707-71I responsible for the thermal diffusivity increase of the The heat treatment could increase the thermal diffusivity C/SiC composite. It could be inferred that the graphitiza- and make the exponential term disappeared in the func- tion of the PyC interlayer took placed partially in the tions. The functional curve before the treatment intersected composite. Because graphite had a much larger thermal that after the treatment at the treatment temperature diffusivity than both the Pyc interlayer and the fibers, the graphitization could increase the thermal diffusivity of the composite. Unfortunately, no obvious graphitization after Acknowledgements the treatment was observed by the TEM analysis. Thermo- dynamically, the graphitization of the Pyc interlay The authors acknowledge the support of the Chinese possible in the treatment. A reasonable explanation is National Foundation for Natural Sciences under Contract the interfacial stress produced by thermal expansion mis- No.59772023. match of the fibers and the matrix induced the graphitize- tion. The effect of heat treatment on the thermal diffusivity of a C/SiC composite could be further enhanced by References increasing thickness of the PyC interlayer, it is limited owing to its low volume fraction. Although the micro- [1 Jamet JM, Lmicq P, Schiroky GH. Composite thermo- structure change in a C/SiC composite can be confirmed structures: an overview of the French experience. In: Naslain by its thermal diffusivity increase at high temperatures, it R, editor, High temperature ceramic matrix composites, eminds to be explained by further research. Bordeaux: Woodhead Publications, 1993, pp. 215-29 2] Laux T, Ullmann T, Auweter-Kurtz M, Hald H, Kurz A Investigation of thermal protection materials along an X-38 4. Conclusions re-entry trajectory by plasma wind tunnel simulations, Sec- ond Intermational Symposium on Atmospheric Reentry Ve- The longitudinal and transverse thermal diffusivity of hicles and Systems. Arcahon( France): 2001: 1-9 3] Tawil H, Bentsen LD, Baskaran S, Hasselman DPH. Thermal the C/SiC composite with and without a CVd SiC coating diffusivity of chemically vapor deposited silicon carbide could be well fitted by a multinomial function from room reinforced with silicon carbide or carbon fibers. j Mater Sci temperature to 1400C which includes a power term,an 1985;20:3201-12 exponential term and a constant term. 14] Parker WJ, Jenkins R, Butler CP, Abbott GL. Flash method The exponential term affected the thermal diffusivity of determining thermal diffusivity, heat capacity, and thermal and led to its increase above 1200C with activation conductivity. J Appl Phys 1961; 32(9): 1679-84 energy of 77 kcal/mol. The microstructure change in the 5]Kim Y, Zangvil A, Goela JS, Taylor RL. Microstructure composite was the reason that the thermal diffusivity was mparison of transparent and opaque CVD SiC. JAm Ceram Soc1995;78:1571-9 increased above 1200° [6] Goela JS, Pichering MA, Burns LE. Chemical The longitudinal thermal diffusivity of the C/SiC Sited Sic for high heat flux applications omposite was twice or more than the transverse one and 1996;28552-13 increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it.L. Cheng et al. / Carbon 41 (2003) 707–711 711 responsible for the thermal diffusivity increase of the The heat treatment could increase the thermal diffusivity C/SiC composite. It could be inferred that the graphitiza- and make the exponential term disappeared in the func￾tion of the PyC interlayer took placed partially in the tions. The functional curve before the treatment intersected composite. Because graphite had a much larger thermal that after the treatment at the treatment temperature. diffusivity than both the PyC interlayer and the fibers, the graphitization could increase the thermal diffusivity of the composite. Unfortunately, no obvious graphitization after Acknowledgements the treatment was observed by the TEM analysis. Thermo￾dynamically, the graphitization of the PyC interlayer is The authors acknowledge the support of the Chinese impossible in the treatment. A reasonable explanation is National Foundation for Natural Sciences under Contract the interfacial stress produced by thermal expansion mis- No. 59772023. match of the fibers and the matrix induced the graphitiza￾tion. The effect of heat treatment on the thermal diffusivity of a C/SiC composite could be further enhanced by References increasing thickness of the PyC interlayer, it is limited owing to its low volume fraction. Although the micro- [1] Jamet JM, Lmicq PJ, Schiroky GH. Composite thermo￾structure change in a C/SiC composite can be confirmed structures: an overview of the French experience. In: Naslain by its thermal diffusivity increase at high temperatures, it R, editor, High temperature ceramic matrix composites, Bordeaux: Woodhead Publications, 1993, pp. 215–29. reminds to be explained by further research. [2] Laux T, Ullmann T, Auweter-Kurtz M, Hald H, Kurz A. Investigation of thermal protection materials along an X-38 re-entry trajectory by plasma wind tunnel simulations, Sec- 4. Conclusions ond International Symposium on Atmospheric Reentry Ve￾hicles and Systems. Arcahon (France): 2001:1–9. The longitudinal and transverse thermal diffusivity of [3] Tawil H, Bentsen LD, Baskaran S, Hasselman DPH. Thermal the C/SiC composite with and without a CVD SiC coating diffusivity of chemically vapor deposited silicon carbide could be well fitted by a multinomial function from room reinforced with silicon carbide or carbon fibers. J Mater Sci temperature to 1400 8C which includes a power term, an 1985;20:3201–12. exponential term and a constant term. [4] Parker WJ, Jenkins RJ, Butler CP, Abbott GL. Flash method of determining thermal diffusivity, heat capacity, and thermal The exponential term affected the thermal diffusivity conductivity. J Appl Phys 1961;32(9):1679–84. and led to its increase above 1200 8C with activation [5] Kim Y, Zangvil A, Goela JS, Taylor RL. Microstructure energy of 77 kcal/mol. The microstructure change in the comparison of transparent and opaque CVD SiC. J Am composite was the reason that the thermal diffusivity was Ceram Soc 1995;78:1571–9. increased above 1200 8C. [6] Goela JS, Pichering MA, Burns LE. Chemical vapor de￾The longitudinal thermal diffusivity of the C/SiC posited SiC for high heat flux applications. SPIE Proc composite was twice or more than the transverse one and 1996;2855:2–13. increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it