Thermodynamic approach to the vaporization and growth phenomena of SiC ceramics. I. SiC and SiC–SiO2 mixtures under neutral conditions

Vaporization of mixed SiC powders. Partial pressures and grain morphology changes under vacuum conditions

“…Consequently, this mixture remained a good reference for comparison of pressure variations. The partial pressures remained very low compared to the calculated thermodynamic (equilibrium) pressures from Honstein et al 6 -a factor 80 for SiO(g) and 5 for CO(g) -and the ratio p(SiO)/p(CO) was <1 contrary to the expected value 3.76 for congruent vaporization in the SiC-SiO 2 pseudobinary system. An oxygen diffusion process in the silica layer at the surface of the SiC grains was responsible for the hindered (or also so-called "retarded") vaporization as already proposed by Honstein et al 15 for SiC-SiO 2 mixtures while carbon diffusion was still faster.…”

“…• their pressures moved with effusion time toward the twophase SiC-C pressures due to the preferential loss of silicon, i.e., non-congruent vaporization of the SiC compound as already explained by Honstein et al 6 • at the moment all the silica has disappeared/vaporized -when Si(g) took the place of CO(g) in the spectrum (mass 28) -the partial pressures of all Si-C system species were within the non-stoichiometric range of the SiC compound.…”

“…The retained main reaction was finally SiC(s) + 2SiO 2 (s) = 3SiO(g) + CO(g) (1) that would start at the SiC/SiO 2 interface for local pressure more that 1 bar. Honstein et al 6 recently proved by thermodynamics that the present proposed main reaction cannot lead to any pure SiC growth process but more certainly to black-glass formation. From a micro-structural point of view, Kingery and Berg 7 made investigations on the initial stages of sintering of two spheres in relation with the mass transport.…”

“…At the beginning of the experiments, pressure differences between the powder samples for SiO(g) and CO(g) are attributed to a kinetic barrier for vaporization since • these pressures were far below those calculated by thermodynamics in the preceding paper by Honstein et al 6 • the effective vaporizing surface area of the different samples influences the partial pressures.…”

“…The present experimental work is a follow-up to the two theoretical thermodynamic analyses performed to identify vaporization and condensation flows of silica and silicon carbide and their mixtures by Honstein et al 6,9 Preceding investigations dealing with active or passive oxidation domains as summarized by Vaughn and Mass 10 and recently by Honstein et al 9 let suppose that one of these two domains may be the only convenient conditions for growth and the only way to check for these conditions is to measure vapor pressures during the SiC powders consolidation heat treatments associated with post experimental micro-structural characterizations.…”