GIBBS ENERGY OF FORMATION OF SIC - A CONTRIBUTION TO THE THERMODYNAMIC STABILITY OF THE MODIFICATIONS

The Gibbs energy of formation Delta(f)G(0) of hexagonal alpha-SiC was determined by electromotive force (emf) measurements between 1200 and 1300 K using the galvanic cell Si, SiO2\Th(Y)O-2\SiO2, alpha-SiC, C wh ich gives (T in K): Delta(f)G(0) [alpha-SiC] =-94770+24.24.T J/mol. Th e third-law enthalpy of formation was calculated as Delta(f)H(298)(0)[ alpha-SiC] =-74.4 kJ/mol at 298 K. In order to examine more closely th e relative stability of alpha-SiC and cubic beta-SiC, Gibbs energy of transformation Delta(tr)G measurements were made by the same method be tween 1100 and 1300 K using the cell beta-SiC, C, SiO2[Th(Y)O-2]SiO2, C, alpha-SiC. An emf of about 20 mV of the metastable cell was measure d up to 5 h cell operation. This observation implies that a-SiC is the more stable modification in the investigated temperature range yieldi ng Delta(tr)G[beta-->alpha-SiC] approximate to-8 kJ/mol at 1200 K.