Microstructures and Thermoelectric Properties of Sintered Ca3Co4O9-Based Oxide

Abstract

Zn- and Ti-doped thermoelectric misfit cobalt oxides Ca3Co4−x−y Zn x Ti y O9 (x = 0 to 0.5, y = 0 to 0.5) were prepared by solid-phase reaction, followed by uniaxial compression molding and sintering at 1173 K for 20 h. Powder x-ray diffraction (XRD) data suggest that Zn and Ti dopants substitute in the rocksalt layer rather than in the CoO2 layer for x ≤ 0.1 and y ≤ 0.1, respectively. In Zn and Ti single-doped samples for x ≤ 0.1 and y ≤ 0.1, ZT at room temperature increased with x and y through an increase in the absolute Seebeck coefficient despite a decrease in electrical conductivity. In Zn and Ti double-doped samples, minor phases other than the misfit oxides were observed at approximately x + y ≥ 0.1. At x ≤ 0.1 and y ≤ 0.1, double doping improved the thermoelectric properties. ZT at room temperature reached a maximum value of 0.035 at (x, y) = (0.1, 0.03).