Thermodynamic stability of K β-alumina

The activity of K2O in a mixture of α-alumina and Kβ-alumina has been determined using the solid state galvanic cell: Ta, Bi-5 mol pct K // α-alumina + Kβ-alumina // In + In2O3, Ta in the temperature range 600 to 1000 K. The cell is written such that the right hand electrode is positive. The solid electrolyte consisted of a dispersion of α-alumina (~15 vol pct) in a matrix of Kβ-alumina. The emf of the cell was found to be reversible and to vary linearly with tem-perature. From the emf and auxiliary data on In2O3 and K2O from the literature, the activity of K2O in the two-phase mixture is obtained as log aK2O = 2.368 -20,850/T(K) (±0.015) The standard free energy of formation of Kβ-alumina from component oxides is given by K2O (s) + 9.5 β-Al2O3 (s) ΔK2O·9.5A12O3 (s) ΔG° = -398,920 + 45.01 T(K) (±1000) J mol-1