Thermoelectric Power Measurements in Wustite

High temperature thermoelectric power measurements were performed on single crystal samples of wustite in equilibrium with a carbon dioxidecarbon monoxide atmosphere. These measurements were made using a "heat pulse" technique which allowed the thermoelectric power of a sample to be determined in a matter of seconds. The short time required for this measurement precluded any possibility of ionic diffusion in the thermal gradient during the test. Measurements made in this manner were compared with measurements made on samples in a steady-state thermal gradient. This comparison indicated that after a sufficiently long residence time (~0.5-2.0 hours) in a thermal gradient, a sample's thermoelectric power is altered by the diffusion of ions in the thermal gradient. However, in a carbon dioxide-carbon monoxide atmosphere, this is a small effect. No significant effects of grain boundaries or crystal orientation were observed in this study. The results of this study are similar to those obtained by earlier workers in that the measured thermoelectric power was anomalously low and changed sign from positive to negative with increasing defect concentration. A defect model for wustite is proposed to explain these results. This model assumes a defect structure dominated by clusters of four vacancies coordinated around a trivalent iron cation in a tetrahedral position. Due to the high negative charge of such a cluster, electron holes are trapped in octahedral sites adjacent to the cluster vacancies. Conduction occurs through the thermally activated hopping of these trapped holes between cluster near-neighbor sites. The high defect concentrations in wustite result in these near-neighbor sites being shared between different clusters. It is therefore possible for a given electron hole to hop through the crystal on a continuous path of near-neighbor sites. A modified Heikes-type equation is used to show that such a model is consistent with the measured values of the thermoelectric power. The proposed model is qualitatively consistent with other studies of x-ray and neutron diffraction, electrical conductivity, and diffusion. Thermoelectric power measurements were also performed at low temperatures (90*-370*K) on wustite single crystals that had been quenched from a high temperature equilibrium state into liquid nitrogen. Above 120 K, these measured results are consistent with the proposed model if a significant aggregation of the defect clusters occurs during the quench. Estimates of defect diffusion distances during a quench show that such aggregation is possible. Below 120*K, the measured thermoelectric power drops sharply and changes sign from positive to negative. Similar behavior has been observed in magnetite. These results are explained in terms of the ordering of trivalent and divalent cations in the vicinity of the aggregated clusters. This ordering creates narrow 3d and 3d5 bands in which both electrons and electron holes are mobile. Thesis Supervisor: Dr. H. Kent Bowen Title: Professor of Ceramics and Electrical Engineering