In Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques. I. Thermochemical Modeling

Abstract

To fabricate macro-structural SiC components containing an In Situ SiC/C thermocouple using an integrated SALD and SALDVI technique, thermodynamic analyses on the involved reactant gases have been performed with the CET89 code based on the minimization of the system free energy. The gaseous precursors considered include tetramethylsilane (TMS) and methyltrichlorosilane (MTS) for the deposition of silicon carbide, and methane, ethylene and acetylene for the deposition of carbon. Reactions between disilane and acetylene and between TMS and ammonia have also been thermodynamically calculated for the deposition of silicon carbide and silicon nitride (for use as an insulation layer between the thermocouple and the matrix), respectively. Based on these analyses, four characteristic temperature zones have been defined for the decomposition of silicon carbide from TMS. A silicon nitride deposition map has been built for the TMS and ammonia system. The deposition temperature range of silicon nitride is found to increase with the total pressure of TMS plus ammonia and the addition of hydrogen, and be affected by the ratio of TMS to ammonia. The addition of hydrogen also introduces a stable silicon carbide and silicon nitride mixture zone that otherwise does not exist. The co-deposition of graphite with silicon carbide and silicon nitride is found in the TMS-containing systems at certain conditions. However, the threshold temperature at which graphite co-deposition occurs can be increased by the addition of hydrogen, thereby eliminating or reducing the graphite co-deposition. Based on these thermodynamic analyses, the gaseous precursors for the deposition of silicon carbide, silicon nitride and carbon have been selected for further experimental evaluation, the result of which is reported in part II of this series.