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

The thermoelectric properties of a nonstoichiometric titanium oxide (TiO 1.1 ) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity σ of TiO 1.1 increases to 9000 S/m at 800 °C showing semiconducting behavior. The Seebeck coefficient α of TiO 1.1 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 α 2 σ reaches 8.61 × 10 -3 W/(m·K 2 ), the largest value for all reported oxide materials. The thermal conductivity κ of TiO 1.1 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 =α 2 σ/κ reaches 1.59 × 10 -3 K -1 for TiO 1.1 at 700 °C. The extremely large power factor of TiO 1.1 compared to other metal oxides can be attributed to the optimal carrier density. The dimensionless figure of merit Z T of 1.64 attained by TiO 1.1 at 800 °C is the largest value for all reported thermoelectric materials in this temperature region. Moreover, the fact that TiO 1.1 has Z T 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.