(V) Porous Silicon Carbide Synthesized from Coal

This paper provides results of a three year research effort to synthesize porous silicon carbide (SiC) from coal that would be utilized as a phase change material substrate for high temperature thermal energy storage applications. The coal provides the carbon reactant with a silicon source to form silicon carbide. Various silicon sources were investigated that includes silica, preceramic polymers, i.e. silane and siloxane types, and silicon metalloid, respectively.

Bituminous coal as determined by low, medium, and high volatile (rank) and impurity content were investigated. Mix formulation and critical process conditions were defined that ultimately enabled the manufacture of large production scale billets. For example, prototype panels were molded to 12” diameter by 1” thick and 24” length by 24” width by 1” thick form factor, respectively.

Our results revealed that beta phase SiC (beta-SiC) was produced from virtually all formulations, but only particular formulations yielded high quantity of alpha phase SiC (alpha-SiC). Consequently, the thermal conductivity increases from a low conductor beta-SiC indicating 1-5 W/m·K to a high conductor indicating 13 W/m·K with high weight fraction alpha-SiC, respectively. This is comparable to Hastelloy C (12 W/m·K) and Inconel (15 W/m·K) that are typically used in high temperature heat exchanger applications. It was also determined that proper formulations and molding practices yield net shape parts with as much as 60% porosity and nominal 1500 psi, peak at 1900 psi. These strengths are comparable to carbon foam typically used as structural core materials in fiber reinforced sandwich structures.