Thermoelectric properties of nitrogen-doped TiO_2-x compounds

The thermoelectric properties of nitrogen-doped TiO_2−x containing oxygen defects have been investigated. The oxygen-defective TiO_2−x was synthesized by a solid-state reaction of TiO₂ and titanium mononitride, TiN. By inducing a sufficient amount of oxygen defects, a sintered sample consisting of the Magneli phase titanium oxide Ti_nO_2n-1 was obtained. While TiO₂ is an intrinsically insulating material, the electrical resistivity was reduced by the formation of oxygen defects, which introduce electron carriers. Although the Seebeck coefficient also decreased with an increase in the amount of oxygen defects, the thermoelectric power factor was improved by the further reduction of electrical resistivity. In addition, the thermal conductivity was strongly reduced by phonon scattering at nanometer-spaced periodic planar defects of the Magneli phase crystallographic structure, resulting in the nitrogen-doped TiO_2−x exhibiting good n-type performance at high temperatures.