Electrical Conduction in Co1-χMgχO

Electrical conductivity and thermoelectric power were measured on Co_1-χMg_χO (0.16χ 0.87) at 900° to 1200°C. Electrical conductivity σ decreased and the activation energy increased with increasing MgO content. Seebeck coefficient α appeared to be positive for all χ. The difference in temperature dependence of σ and α was 0.2 to 0.5 eV and the hole mobility decreased with increasing MgO content. The results were interpreted as showing that electron holes were localized to cation clusters and their thermally activated hopping conduction was operative in the solid solution. The dominant lattice defect changed from singly to doubly ionized cobalt vacancy with increasing MgO content.     

Gas Sensing Characteristics of Porous ZnO and Pt/ZnO Ceramics

Changes in resistivity and chemical changes in reducing gases were measured for porous zinc oxide ceramics with and without a platinum catalyst at 300° and 400°C to examine the gas sensing mechanism and the effect of platinum additions. Reducing gases were oxidized to CO₂ and Hz₂ on the sensor surfaces. Platinum addition promoted the oxidation of reducing gases but did not lead to an increase in the resistivity change at 400°C. The reaction sequences for the gas sensing process are proposed, taking into account partially oxidized intermediates of hydrocarbons and oxidation on platinum without an electron transfer process.     

Electrical Conduction in Pure and Li-Substituted Co3O4

Electrical conductivity and thermoelectric power measurements on pure and Li-substituted Co₃O₄ were performed at 200° to 900°C. Impurity-induced p-type semiconduction was observed at low temperatures and intrinsic behavior at high temperatures. A hopping scheme for electron hole conduction was confirmed, and site exchange of cations in the spinel structure was proposed to explain the electrical conduction at high temperatures.     

Evaporation of Zinc Oxide from Spinel Solid Solutions in Vacuum

The mechanism for the evaporation of ZnO from powders of (Zn₀₂Co_0.8)O·Al₂O₃ (ZCA) and (Zn_0.2Ni_0.8)O·Al₂O₃ (ZNA) spinels was studied at 1335° to 1500°C in vacuum of 3×10⁻⁵ to 10⁻⁴ torr. The evaporation of ZnO occurred in two stages: at a constant rate in the first and a decreasing rate in the second. The rate-determining process was analyzed as a decomposition reaction at the first stage and as the conjugated process of decomposition reaction and diffusion in the solid at the second. The evaporation rate constant and surface emissivity showed a similar dependence on temperature. The ratio of the concentration of ZnO at the surface to the initial concentration was =0.7 when the first stage of evaporation changed to the second.