Heats and Free Energies of Formation of Two Sialons by Mass Spectrometry

Abstract
The heats of formation at 298 K of SiAl2O2N2(c) (β'‐sialon) and SiAl4O2N4(c) (15R‐sialon) were determined as −413±6 and −592±8 kcal/mol by the third‐law method. Second‐law values were −414±4 and −595±5 kcal/mol. A tentative method of predicting the heats is given. The major vapor species observed were SiO and N2. It was found that the normal valences of the elements in these two sialons are not rigorously maintained during decomposition in vacuum.