Electrical properties of Ga and ZnS doped ZnO prepared by mechanical alloying

A series of n-type ZnO alloys doped with Ga and ZnS were prepared by mechanical alloying. Densities of 95% to 98% of theoretical density were achieved by hot pressing the milled powders at 1000 and 1200 °C, respectively. The electrical resistivity and Seebeck coefficient of alloys containing 0.25–3.0 at. % Ga were characterized between 22 and 1000 °C. The magnitude of the resistivity and Seebeck coefficient at 22 °C ranged from 0.2 mΩ cm and −25 μV/°C for the most heavily doped specimen to 1.1 mΩ cm and −70 μV/°C for the lightly doped material. The alloys exhibit a positive temperature coefficient of resistivity and Seebeck coefficient with a nearly constant slope over the temperature range studied. Thermal diffusivity measurements on a specimen containing 1.0 at. % Ga were performed over the same temperature range. The thermal conductivity appears to follow a T−1 dependence, decreasing from 180 mW/cm °C at 22 °C to 82 mW/cm °C at 1000 °C. An estimate of the maximum dimensionless thermoelectric figure of merit, ZT, in this system at 1000 °C gives a value of 0.26, a factor of three to four less than current state-of-the-art materials such as Si–Ge. A significant reduction in thermal conductivity would be required to make these alloys competitive with existing thermoelectric power generation materials.