Relation between thermolectric properties and phase equilibria in the ZnO–In2O3 binary system

The electrical conductivities, Seebeck coefficients and thermal conductivities across the ZnO–In2O3 binary system are reported and related to the phase compositions and microstructures present at 1150 and 1250 °C. The ZnO–In2O3 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 InO1.5, the maximum ZT and power factors are observed at k = 4 (33 m/o InO1.5). In contrast to the other modular phases at 1250 °C and below, this phase is hexagonal rather than rhombohedral.