Latent Property of Defect-Controlled Metal Oxide: Nonstoichiometric Titanium Oxides as Prospective Material for High-Temperature Thermoelectric Conversion

Abstract

The thermoelectric properties of a nonstoichiometric titanium oxide (TiO1.1) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity σ of TiO1.1 increases to 9000 S/m at 800 °C showing semiconducting behavior. The Seebeck coefficient α of TiO1.1 shows a general trend in which the value increases gradually from 0.380 mV/K at 300 °C to 1.01 mV/K at 950 °C. As a consequence, the power factor α2σ reaches 8.61×10-3 W/(m·K2), the largest value for all reported oxide materials. The thermal conductivity κ of TiO1.1 increases with temperature, from 1.26 W/(m·K) at 300 °C to 7.12 W/(m·K) at 950 °C. In spite of the considerably large values of κ, the figure of merit Z=α2σ/κ reaches 1.59×10-3 K-1 for TiO1.1 at 700 °C. The extremely large power factor of TiO1.1 compared to other metal oxides can be attributed to the optimal carrier density. The dimensionless figure of merit ZT of 1.64 attained by TiO1.1 at 800 °C is the largest value for all reported thermoelectric materials in this temperature region. Moreover, the fact that TiO1.1 has ZT values of nearly unity or greater in the range from 500 to 1000 °C demonstrates the usefulness of nonstoichiometric titanium oxides for high-temperature thermoelectric conversion.