Relation Between Thermolectric Properties and Phase Equilibria in the \(ZnO–In_2O_3\) Binary System

Abstract

The electrical conductivities, Seebeck coefficients and thermal conductivities across the ZnO–In(_2)O(_3) binary system are reported and related to the phase compositions and microstructures present at 1150 and 1250 °C. The ZnO–In(2)O(3) binary system is of particular interest as it contains a variety of different types of phases, superlattice (modular) phases, solid solutions, two-phase regions and crystallographic features. Throughout much of the phase diagram, the thermal conductivities are less than 2 W m(^{−1}) K(^{−1}), being limited by both solid solution disorder and thermal resistance due to the presence of InO/ZnO interfaces. Across the phase diagram, irrespective of the actual phases, the materials behave at high temperatures (800 °C) as free-electron conductors with the Seebeck coefficient and electron conductivity satisfying the Jonker’s relationship. In the two-phase regions of the phase diagram, the values of the power factor and figure of merit (ZT) are consistent with a simple law of mixtures, weighted according to the volume fractions of the two phases. Although the largest values of electrical conductivity and Seebeck coefficient occur over a range of composition centered at 40 m/o InO({1.5}), the maximum ZT and power factors are observed at k = 4 (33 m/o InO({1.5})). In contrast to the other modular phases at 1250 °C and below, this phase is hexagonal rather than rhombohedral.