Thermoelectric properties of nonstoichiometric TiO as a promising oxide 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 up to ca. 9000 S/m at 800 °C, is showing semiconducting behavior. The Seebeck coefficient, α, of TiO_1.1 shows a general trend in which the absolute value increases gradually from ca. 0.4 mV/K at 300°C to ca. 1.0 mV/K at 950°C. As a consequence, the power factor, α²σ, reaches ca. 8.6×10^-3 W/(K²·m), the largest value of all reported oxide materials. The thermal conductivity, κ, of TiO_1.1 increases with temperature, from ca. 1.3 W/(K·m) at 300°C to ca. 7.1 W/(K·m) at 950°C. In spite of the considerably large values of κ, the figure of merit, Z=α²σ/κ, reaches 1.6×10^-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 large carrier density. The dimensionless figure of merit, ZT, of 1.64 attained by TiO_1.1 at 800°C is the largest value of all reported other thermoelectric materials in this temperature region. And that TiO_1.1 has ZT values of nearly unity or greater in the range of 500°C to 1000°C, demonstrates the usefulness of the nonstoichiometric titanium oxides for high-temperature thermoelectric conversion.