High-temperature analysis of the thermal degradation of silicon-based materials. I: Binary Si-O, Si-C, and Si-N compounds

The thermal degradation of SiO2, SiC, and Si3N4 compounds has been previously analysed by means of Knudsen effusion mass spectrometry and departures from equilibrium conditions were observed as soon as SiO2/Si, SiC/C, and Si3N4/Si interfaces came into existence, even for very small atomic flow transfers through the interfaces (similar to 10(-14) mol s(-1) cm(-2)). These departures from equilibrium partial pressures are analysed in terms of evaporation coefficients, ie ratios between pressures for free vaporisation and equilibrium pressures. A complementary analysis of the Motzfeld equation that relates the different molecular flows occurring in an effusion cell is carried out, taking account of the condensation coefficient. The mass spectrometric results already obtained during the vaporisation of silicon compounds SiC, SiO2, and Si3N4 are thus re-analysed through this new relationship with a view to calculating a new set of condensation and evaporation coefficients. The evolution of these coefficients with temperature and the analysis of their origin show that they are mainly the consequence of an entropy barrier to the reaction of transfer of the most volatile component through the interfaces between silicon and silicon compounds and weakly related to the desorption process itself.