Thermoelectric Properties of Non-stoichiometric Titanium Oxides for Waste Heat Recovery in Steelworks

抄録

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 1073 K, is showing semiconducting behavior. The Seebeck coefficient, α, of TiO_1.1 shows a general trend in which the value increases gradually from 0.4 mV/K at 573 K to 1.0 mV/K at 1223 K. As a consequence, the power factor, α²σ, reaches 8.6×10⁻³ W/(m·K²), the largest value of all reported oxide materials. The thermal conductivity, κ, of TiO_1.1 increases with temperature, from 1.3 W/(m·K) at 573 K to 7.1 W/(m·K) at 1223 K. 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 973 K. 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, ZT, of 1.64 attained by TiO_1.1 at 1073 K 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 773 to 1223 K, demonstrates the usefulness of the nonstoichiometric titanium oxides for high-temperature thermoelectric conversion.