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

Abstract

The thermoelectric properties of a mixed oxide (Zn _1-x Al _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. 10 ³ S cm ^-1 at room temperature, showing metallic behaviour. The Seebeck coefficient, S, of (Zn _1-x Al _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, S ² σ, 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=S ² σ/κ, reaches 0.24×10 ^-3 K ^-1 for (Zn _0.98 Al _0.02 )O at 1000 °C. The extremely large power factor of (Zn _1-x Al _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 _0.98 Al _0.02 )O at 1000 °C demonstrates the potential usefulness of the oxide.