Journal of Materials Science, Vol.51, No.21, 9865-9878, 2016
Pre-ceramic polymer-derived open/closed cell silicon carbide foam: microstructure, phase evaluation, and thermal properties
The presented article describes a simple technique to fabricate open/closed cell silicon carbide (SiC) foams. The SiC foams, especially pyrolyzed either at 1200 A degrees C (PSiC_12) or 1500 A degrees C (PSiC_15), offered the best thermophysical properties suitable for advance thermal management systems. Both PSiC_12 and PSiC_15 possessed almost equal concentrations of open and closed pores (similar to 22-24 vol%). On the contrary, the 1800 A degrees C-pyrolyzed specimen (PSiC_18) had 40 vol% open pores and only similar to 9 vol% closed pores. X-ray Micro-CT observation also revealed that the PSiC_15 had significant concentration of closed pores having pore volume in the range of (2-20) x 10(-4) mm(3). While PSiC_15 contained small beta-SiC crystallites of similar to 13-nm size, PSiC_18 possessed similar to 55 nm crystallites of alpha-SiC. From room temperature to 1000 A degrees C, thermal conductivity values of PSiC_12 and PSiC_15 varied from 0.5 to 1.6 W/m K. Both these specimens also offered outstanding oxidation resistance at 1000 and 1500 A degrees C for up to 100 h. Extremely low thermal conductivity, the presence of closed pores that restricted inward diffusion of oxygen even at the elevated temperatures, and the formation of impervious oxide scale at the initial oxidation stage were the key factors behind obtaining such excellent oxidation resistance. On the contrary, due to the presence of much higher channel pores and relatively higher thermal conductivity (similar to 3 W/m K), oxidation of PSiC_18 was found to be much exaggerated resulting in a few orders of magnitude higher oxidation weight gain compared to those obtained for PSiC_12 and PSiC_15.