CARBON PERGAMON Carbon41(2003)707711 Thermal diffusivity of 3d C/ Sic composites from room temperature to1400°C Laifei Cheng", Yongdong Xu, Qing Zhang, Litong Zhang State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi an Shaanxi 710072, China Received 1 April 2002; accepted 17 October 2002 Abstract A 3D C/SiC composite and a bulk CVd SiC material were prepared. The effects of the CVD SiC coating and the heat reatment on the longitudinal and transverse thermal diffusivity of the C/Sic composites were investigated. The thermal diffusivity of the C/SiC composites could be well fitted by a multinomial function from room temperature to 1400C which ncludes a power term, an exponential term and a constant term. The exponential term affected the thermal diffusivity and led to its increase above 1200C with activation energy of 77 kcal/ mol. The microstructure change in the composites was the reason that the thermal diffusivity was increased above 1200C. The longitudinal thermal diffusivity of the composite was twice or more than the transverse one and increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it. The heat treatment could increase the thermal diffusivity and make the exponential term disappeared in the functions. The functional curve before the treatment intersected that after the treatment at the treatment temperature C 2002 Elsevier Science Ltd. All rights reserved Keywords: A. Carbon composites; D. Thermal diffusivity; D. Microstructure 1. Introduction where p(kg m)is the density and c,o Kgk )the specific heat at constant pressure. Because the SiC matrix, Carbon fiber reinforced silicon carbide composites(C/ PyC interlayer, fibers and pores are included in a 3D SiC)have been developed and tested for thermostructural C/SiC composite, its thermal diffusivity can be calculated plications such as the components of turbine engines, the by [4 reentry thermal protection system of spacecraft, ultra-light- of the most important properties for all these application Although some data is reported in literatures on thermal where u, is the volume fraction of any phase, a" is the diffusivity of C/SiC composites [3], it has not been thermal diffusivity of the composite and a is that of the The thermal diffusivity (ms)of a material is direction of the thermal flow. The value of m s Adthe systematically investiga phase, n is related to the preform structure an ween related to its thermal conductivity k(wmk )by the -1 and ollowing equation [4 The thermal diffusivity of a 3D C/SiC composite is very sensitive to changes of its microstructure and composition. k t is possible to determine the mechanisms for the micro- structural and compositional changes of the composite quantitatively by systematically measuring its thermal diffusivity. Obviously, this is significant to microstructure R Corresponding author. Tel: +86-29-849-4616; fax:+86-29. analysis of the composite. In this paper, a 3D C/Sic composite and a bulk CVD SiC material were prepared E-mail address: chenglfnwpu. edu.cn(L. Cheng). The effects of the Cvd Sic coating and the heat treatment 0008-6223/02/S-see front matter 2002 Elsevier Science Ltd. All rights reserved doi:1o.1016S0008-6223(02)00382-2Carbon 41 (2003) 707–711 T hermal diffusivity of 3D C/SiC composites from room temperature to 1400 8C Laifei Cheng , Yongdong Xu, Qing Zhang, Litong Zhang * State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an Shaanxi 710072, China Received 1 April 2002; accepted 17 October 2002 Abstract A 3D C/SiC composite and a bulk CVD SiC material were prepared. The effects of the CVD SiC coating and the heat treatment on the longitudinal and transverse thermal diffusivity of the C/SiC composites were investigated. The thermal diffusivity of the C/SiC composites could be well fitted by a multinomial function from room temperature to 1400 8C which includes a power term, an exponential term and a constant term. The exponential term affected the thermal diffusivity and led to its increase above 1200 8C with activation energy of 77 kcal/mol. The microstructure change in the composites was the reason that the thermal diffusivity was increased above 1200 8C. The longitudinal thermal diffusivity of the composite was twice or more than the transverse one and increased more rapidly by the exponential term. The former was decreased by the CVD SiC coating, but the latter was increased by it. The heat treatment could increase the thermal diffusivity and make the exponential term disappeared in the functions. The functional curve before the treatment intersected that after the treatment at the treatment temperature.  2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon composites; D. Thermal diffusivity; D. Microstructure 23 21 21 1. Introduction where r (Kg m ) is the density and c (J Kg K ) the p specific heat at constant pressure. Because the SiC matrix, Carbon fiber reinforced silicon carbide composites (C/ PyC interlayer, fibers and pores are included in a 3D SiC) have been developed and tested for thermostructural C/SiC composite, its thermal diffusivity can be calculated applications such as the components of turbine engines, the by [4] reentry thermal protection system of spacecraft, ultra-light- n n weight mirrors and so on [1,2]. Thermal diffusivity is one a 5 Ov a (2) C i i of the most important properties for all these applications. n where v is the volume fraction of any phase, a is the Although some data is reported in literatures on thermal i C n thermal diffusivity of the composite and a is that of the diffusivity of C/SiC composites [3], it has not been i phase, n is related to the preform structure and the systematically investigated. 2 21 direction of the thermal flow. The value of n is between The thermal diffusivity a (m s ) of a material is 21 21 21 and 11. related to its thermal conductivity k (W m K ) by the The thermal diffusivity of a 3D C/SiC composite is very following equation [4] sensitive to changes of its microstructure and composition. k It is possible to determine the mechanisms for the micro- a 5 ] (1) rc structural and compositional changes of the composite p quantitatively by systematically measuring its thermal diffusivity. Obviously, this is significant to microstructure *Corresponding author. Tel.: 186-29-849-4616; fax: 186-29- analysis of the composite. In this paper, a 3D C/SiC 849-4620. composite and a bulk CVD SiC material were prepared. E-mail address: chenglf@nwpu.edu.cn (L. Cheng). The effects of the CVD SiC coating and the heat treatment 0008-6223/02/$ – see front matter  2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0008-6223(02)00382-2