Gibbs energy of formation of sic: A contribution to the thermodynamic stability of the modifications

The Gibbs energy of formation Δ_fG^o of hexagonal α-SiC was determined by electromotive force (emf) measurements between 1200 and 1300 K using the galvanic cell Si, SiO₂|Th(Y)O₂|SiO₂, α-SiC, C which gives (T in K): Δ_fG^o (α-SiC) =-94770+24.24·T J/mol. The third-law enthalpy of formation was calculated as Δ_fH^o₂₉₈ (α-SiC)=-74.4 kJ/mol at 298 K. In order to examine more closely the relative stability of α-SiC and cubic β-SiC, Gibbs energy of transformation Δ_trG measurements were made by the same method between 1100 and 1300 K using the cell β-SiC, C, SiO₂|Th(Y)O₂|SiO₂, C, α-SiC. An emf of about 20 mV of the meta-stable cell was measured up to 5 h cell operation. This observation implies that α-SiC is the more stable modification in the investigated temperature range yielding Δ_trG (βα-SiC) ≈︁ −8 kJ/mol at 1200 K.