Potentiometric determination of activities in the two-phase fields of the system Na 2O(α)Al 2O 3

Abstract

Three compounds have been found to be stable in the pseudobinary system Na 2O(α)Al 2O 3 between 825 and 1400 K; two nonstoichiometric phases, β-alumina and β″-alumina, and NaAlO 2. The homogeneity of β-alumina ranges from 9.5 to 11 mol% Na 2O, while that of β″-alumina from 13.3 to 15.9 mol% Na 2O at 1173 K. The activity of Na 2O in the two-phase fields has been determined by a solid-state potentiometric technique. Since both β- and β″-alumina are fast sodium ion conductors, biphasic solid electrolyte tubes were used in these electrochemical measurements. The open circuit emf of the following cells were measured from 790 to 980 K: ▪ ▪ ▪ The partial molar Gibbs' energy of Na 2O relative to γ-Na 2O in the two-phase regions can be represented as: Δ Ḡ Na 2O (α- + β-alumina) = −270,900 + 24.03 T, Δ Ḡ Na 2O (β- + β″-alumina) = −232,700 + 56.19 T, and Δ Ḡ Na 2O (β″-alumina + NaAlO 2) = −13,100−4.51 T J mol −1. Similar galvanic cells using a Au Na alloy and a mixture of Co + CoAl 2+2 x O 4+3 x +(α)Al 2O 3 as electrodes were used at 1400 K. Thermodynamic data obtained in these studies are used to evaluate phase relations and partial pressure of sodium in the Na 2O(α)Al 2O 3 system as a function of oxygen partial pressure, composition and temperature.