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 (TiO1.1) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity sigma of TiO1.1 increases to 9000 S/m at 800 degrees\C showing semiconducting behavior. The Seebeck coefficient alpha of TiO1.1 shows a general trend in which the value increases gradually from 0.380 mV/K at 300 degrees C to 1.01 mV/K at 950 degrees C. As a consequence, the power factor alpha(2)sigma reaches 8.61 x 10(-3) W/(m.K-2), the largest value for all reported oxide materials. The thermal conductivity kappa of TiO1.1 increases with temperature, from 1.26 W/(m.K) at 300 degrees C to 7.12W/(m.K) at 950 degrees C. In spite of the considerably large values Of K, the figure of merit Z = alpha(2)sigma/kappa reaches 1.59 x 10(-3) K-1 for TiO1.1 at 700 degrees 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 degrees 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 degrees C demonstrates the usefulness of nonstoichiometric titanium oxides for high-temperature thermoelectric conversion.