Thermoelectric properties of Al-doped ZnO as a promising oxide material for high-temperature thermoelectric conversion

The thermoelectric properties of a mixed oxide (Zn_1-xAl_x)O (x = 0, 0.005, 0.01, 0.02, 0.05) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity, σ, of the oxide increases on Al-doping by more than three orders of magnitude up to ca. 10³ S cm^-1 at room temperature, showing metallic behaviour. The Seebeck coefficient, S, of (Zn_1-xAl_x)O (x > 0) shows a general trend in which the absolute value increases gradually from ca. -100 μV K^-1 at room temperature to ca. -200 μV K^-1 at 1000°C. As a consequence, the power factor, S²σ, reaches ca. 15 × 10^-4 W m^-1 K^-2, the largest value of all reported oxide materials. The thermal conductivity, κ, of the oxide decreases with increasing temperature, owing to a decrease in the lattice thermal conductivity which is revealed to be dominant in the overall κ. In spite of the considerably large values of κ-, the figure of merit, Z = S²σ/κ, reaches 0.24 × 10^-3 K^-1 for (Zn_0.98Al_0.02)O at 1000°C. The extremely large power factor of (Zn_1-xAl_x)O compared to other metal oxides can be attributed to the high carrier mobility revealed by the Hall measurements, presumably resulting from a relatively covalent character of the Zn-O bond owing to a fairly small difference of the electronegativities of Zn and O. The dimensionless figure of merit, ZT, of 0.30 attained by (Zn_0.98Al_0.02)O at 1000°C demonstrates the potential usefulness of the oxide.