High temperature sintering of SiC with oxide additives: II. Vaporization processes in powder beds and gas-phase analysis by mass spectrometry

The liquid phase sintering process of SiC using Al2O3–Y2O3 oxide mixture is performed in a temperature range where some matter loss occurs by vaporization. When using a buffer bed of SiC + Al2O3 powder mixture, this matter loss cannot be entirely counteracted and some transfer of matter—alumina—is observed between the green pellets and the beds. The present mass spectrometric study is aimed to the identification of the gaseous species that vaporize from such a bed placed in Knudsen cells in the 1350-1750 K temperature range. The Al(g), Al2O(g), SiO(g) and CO(g) species are identified and their relative pressures measured as a function of alumina composition, crucible materials and excess of C or Si. Differences show that some non-equilibrium vaporization processes occur. In addition, the vaporization of impurities as well as C, Si or SiO2 excess leads to a rapid decrease of their content and influence during the heating first stage before sintering.