Thermoelectric properties of nonstoichiometric TiO as a promising oxide material for high-temperature thermoelectric conversion

The thermoelectric properties of a nonstoichiometric titanium oxide (TiO/sub 1.1/) are investigated in terms of materials for high-temperature thermoelectric conversion. The electrical conductivity, /spl sigma/, of TiO/sub 1.1/ increases up to ca. 9000 S/m at 800 /spl deg/C, is showing semiconducting behavior. The Seebeck coefficient, /spl alpha/, of TiO/sub 1.1/ shows a general trend in which the absolute value increases gradually from ca. 0.4 mV/K at 300/spl deg/C to ca. 1.0 mV/K at 950/spl deg/C. As a consequence, the power factor, /spl alpha//sup 2//spl sigma/, reaches ca. 8.6/spl times/10/sup -3/ W/(K/sup 2//spl middot/m), the largest value of all reported oxide materials. The thermal conductivity, /spl kappa/, of TiO/sub 1.1/ increases with temperature, from ca. 1.3 W/(K/spl middot/m) at 300/spl deg/C to ca. 7.1 W/(K/spl middot/m) at 950/spl deg/C. In spite of the considerably large values of /spl kappa/, the figure of merit, Z=/spl alpha//sup 2//spl sigma///spl kappa/, reaches 1.6/spl times/10/sup -3/ K/sup -1/ for TiO/sub 1.1/ at 700/spl deg/ C. The extremely large power factor of TiO/sub 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/sub 1.1/ at 800/spl deg/C is the largest value of all reported other thermoelectric materials in this temperature region. And that TiO/sub 1.1/ has ZT values of nearly unity or greater in the range of 500/spl deg/C to 1000/spl deg/C, demonstrates the usefulness of the nonstoichiometric titanium oxides for high-temperature thermoelectric conversion.