Person:

Liang, Xin

Compositionally dependent superlattices, (In_2O_3) ((ZnO) _k), form in the (ZnO)-rich portion of the (ZnO-In_2O_3) phase diagram, decreasing thermal conductivity and altering both the electron conductivity and Seebeck coefficient over a wide range of composition and temperature. With increasing indium concentration, isolated point defects first form in (ZnO) and then superlattice structures with decreasing interface spacing evolve. By fitting the temperature and indium concentration dependence of the thermal conductivity to the Klemens-Callaway model, incorporating interface scattering and accounting for conductivity anisotropy, the Kapitza resistance due to the superlattice interfaces is found to be (5.0 ± 0.6 × 10^{−10} m^2K/W). This finding suggests that selecting oxides with a compositionally dependent superlattice structure can be a viable approach, unaffected by grain growth, to maintaining low thermal conductivity at high temperatures.

The present dissertation investigates the relationship between the structure and thermoelectric properties of ZnO based materials, with a focus on trivalent element doping on engineering the microstructure and altering the electrical and thermal transport properties. Within the solubility range, the addition of trivalent elements, such as In3+, Fe3+ and Ga3+, is observed to increase the electrical conductivity of ZnO and decrease the thermal conductivity.

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.