High temperature mass spectrometric study of the vaporization behaviour of SiC–SiO2 system

The study of the gaseous phase of different SiC–SiO2 powder compositions with a high temperature mass spectrometer and a multiple cells device provides information about the evaporation behaviour of silicon carbide which serves as material for production of fine particle filters for diesel engines. We obtained a variation of the partial pressure values with a maximum for samples containing 30–45 mol% of SiO2 in contrast to thermodynamic predictions. Some SiC powders were subjected to a heat treatment in order to obtain an oxide layer on the grain surfaces. Different times of heat treatment resulted in different CO(g) and SiO(g) pressures which at high temperature converged to the same values. Surprisingly, an excess of SiO2 provoked a decrease of the SiO(g) pressure which was explained by taking into account two steady-state congruent vaporization processes in the effusion cell, one from SiC + SiO2 and one from SiO2.