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

The thermoelectric properties of a mixed oxide (Zn1-xAlx)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, sigma, 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)A(lx))O (x>0) shows a general trend in which the absolute value increases gradually from ca.-100 mu V K-1 at room temperature to ca. -200 mu V K-1 at 1000 degrees C. As a consequence, the power factor, S-2 sigma, reaches ca. 15 x 10(-4) W m(-1) K-2, th, largest value of all reported oxide materials. The thermal conductivity, kappa, 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 kappa. In spite of the considerably large values of kappa, the figure of merit, Z=S-2 sigma/kappa, reaches 0.24x10(-3) K-1 for (Zn0.98Al0.02)O at 1000 degrees C. The extremely large power factor of(Zn1-xAlx)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 (Zn0.98Al0.02)O at 1000 degrees C demonstrates the potential usefulness of the oxide.