Most of the ceramic ionic conductors typically conduct one ion. For example, AgI conducts Ag⁺ ion, Sodium β”-alumina conducts Na⁺ ion and Yttria-stabilized zirconia (YSZ) conducts O^2- ions. Some selected materials such as alkaline earth cerates or zirconates conduct both protons and O^2- ions. In such materials also, conductivity of one species is not independent of the other species. Thus, temperature and atmosphere determine the total conductivity. All of such materials are single phase materials. We have been investigating two phase materials such that one phase conducts one ion (Na⁺) while the other phase conducts another ion (O^2-). The materials in such cases is a composite with the two phases forming an interpenetrating network. The volume ratios of the two phases can be adjusted as necessary. Since the two phases essentially constitute a mixture, conduction of one species occurring through one phase is independent of the conduction of the other species through the second phase. Yet, the condition of electroneutrality is applicable globally across multiple phases. Transport equations can then be written down in the usual manner but accounting for different volume fractions for the two phases.

A mixed Na⁺ ion and O^2- ion two phase conductor is fabricated by first making a sintered dense composite containing α-alumina and YSZ such that both phases are contiguous. YSZ and α-alumina are immiscible. This allows the fabrication of fine-grained, dense, strong composites. Subsequently, the sample is packed in Na-β”-alumina + YSZ powder and heat treated in air over a range of temperatures from 1200^oC to 1400^oC. During this process, α-alumina is converted into Na-β”-alumina by the incorporation of Na₂O into α-alumina by coupled transport in which Na⁺ transports through the formed Na-β”-alumina and O^2- ions transport through YSZ. The reaction front over time sweeps through the sample forming a two phase composite of Na-β”-alumina + YSZ. Such a material is now capable of conducting two ions; Na⁺ and O^2-. Materials capable of transporting other alkali ions can be made by an ion exchange process in molten salts. For example, by ion exchanging in lithium salts, it is possible to make mixed Li-ion and O^2- ion conducting materials. The oxygen ion conductivity is negligible at low temperatures. Thus, the materials at room temperature are predominantly alkali ion conductors with possible use in all solid state lithium ion and sodium ion batteries.

Another mixed ion conductor was made by a similar process using a composite of Ga₂O₃ and YSZ. It was similarly packed in Na-β”-alumina powder and heat treated. In this case a two phase mixture containing YSZ and sodium zirconium gallate formed, the latter with a chemical formula of Na_0.7Ga_4.7Zr_0.3O₈ with a monoclinic structure. This compound is isostructural with Na_0.7Ga_4.7Ti_0.3O₈ which is known to be one dimensional sodium ion conductor. Measurement of ionic conductivity on Na_0.7Ga_4.7Zr_0.3O₈ + YSZ showed it to be a mixed Na⁺ ion and O^2- ion conductor. Sodium ion conductivity of Na_0.7Ga_4.7Zr_0.3O₈ is much lower than that of Na-β”-alumina. The activation energy for Na⁺ ion transport is much lower than for O^2- ion transport in YSZ. As a result, the two phase mixture is predominantly a Na⁺ ion conductor at low temperatures and an O^2- ion conductor at elevated temperatures.

Both Na-β”-alumina + YSZ and Na_0.7Ga_4.7Zr_0.3O₈ + YSZ are mixed Na⁺ ion and O^2- ion conductors. This allows one two explore electrochemical transport of two species and examine the applicability of Goldman-Hotchkin-Katz trype of an equation, routinely used in aqueous electrochemistry, for solid-state systems. Preliminary results of experiments conducted by maintaining different chemical potentials of Na₂O at the two electrodes and different chemical potentials of oxygen (by testing in a fuel cell mode) will be described. Possible degradation of Na-β”-alumina in reducing atmosphere will also be addressed.

Acknowledgements: This work was supported by the National Science Foundation under Grant Number DMR-1407048.