ELECTRICAL-PROPERTIES OF CA12AL14O33 - EFFECT OF HYDROGEN REDUCTION

Complex changes in the electrical properties of the oxide ion-conducting solid electrolyte Ca12Al14O33 were observed on heating in reducing atmospheres. On gentle reduction the high oxide ion conductivity of Ca12Al14O33 Was gradually destroyed, due to the formation and growth of resistive grain boundary and surface components. An interesting effect is the thickening of the grain boundary layer on reduction as indicated by capacitance data; the grain boundary appears to form a shell around the residual, oxide ion conducting core of the individual grains. Similar impedance responses were observed in samples annealed below 700-degrees-C and in samples slow cooled from higher temperatures. Under more severe reduction, at temperatures above 800-degrees-C, the surface layer was no longer observed and the conductivity of both the shell and core increased and appeared to become electronic. This behaviour could be retained to lower temperatures by quenching. These observations are consistent with two reduction processes: the mobile oxide ions are effectively trapped as immobile hydroxides at lower temperatures, whereas more extensive reduction at high temperatures yields conductive electronic behaviour.