Vaporization thermodynamics and kinetics of hexagonal silicon carbide

Mass spectrometer Knudsen effusion vaporization measurements over the temperature range 1835 to 2264K confirm earlier mass spectrometer results that Si(g) is the predominant vapor species above the SiC-C two-phase region. Si2C(g) and SiC2(g) are found to be the next most important vapor species. Total vapor pressures are measured to be a factor of 10 to 15 higher than values previously reported between 1800 to 2200K. Silicon partial pressures lead to a value of -7.5 kcal mol-1 for the enthalpy of formation of Sic(hexagonal) at 298K. Partial pressures of Si(g), SiC2(g), and Si2C(g) lead to third-law enthalpies of formation for SiC2(g) and Si2C(g) which are in accord with previously recommended values. Langmuir vaporization of SiC(hexagonal) (0001) single crystal faces in the mass spectrometer show ion intensities of Si+ and SiC2+ that are time-independent after an initial induction period during which the intensities increase by a factor of 1.5 before attaining steady-state value. The mass spectrometer studies between 2200 to 2500K give activation enthalpies of ΔH*(2350) = 141.6 +- 2.7 kcal mol-1 for Si(g) and ΔH*(2350) = 166.2 +- 3.5 kcal mol-1 for SiC2(g) vaporization from the (0001) crystal face of hexagonal SiC. Constant temperature total mass-loss Langmuir vaporization experiments using SiC (0001) crystal faces show a time-dependent rate of mass-loss. It is believed that this time-dependent behavior is caused by a decrease in the SiC(hexagonal) single crystal surface area and is not due to impedance of silicon vaporization by the graphite layer formed on the crystal surface during decomposition