On the electrolytic nature of molten aluminum and rare earth oxides

Abstract

The electrolytic cell method and its application for Gibbs energy measurement in high temperature, concentrated ionic melts was investigated. Previous challenges related primarily to signal interpretation during decomposition voltage measurement have hindered determination of Gibbs energy. An electrolytic cell method is proposed herein utilizing the sensitivity of large amplitude alternating current voltammetry, which enabled precise measurement of chemical potential during electrolytic decomposition. A novel, containerless approach for electrochemical study of high temperature, reactive electrolytes in a molten pendant droplet is described. For the first time, melts of pure alumina, lanthana and yttria were electrolytically decomposed to metal alloy and oxygen gas using iridium electrodes. The method was validated in molten alumina. Systematic investigation of the half-cell reactions corresponding to oxygen evolution and aluminum deposition revealed their electrochemical nature. Measurements of the chemical potential and partial molar entropy of aluminum in an iridium-rich, binary alloy liquid were obtained in close agreement with previous predictions. The method was extended to the pseudo-binary system molten lanthana-yttria. The results revealed selective extraction of lanthanum and indicated that molten lanthana-yttria does not exhibit ideal mixing behavior, contradicting previous predictions. However, data interpretation in multicomponent electrolyte compositions were challenged primarily by a lack of thermodynamic data for the attendant alloy system and influence of dissolved oxygen on the observed alloy composition. This work represents a pioneering effort for electrochemical operation in molten refractory oxides at temperatures above 2000 K.