Thermoelectric properties of delafossite-related layered metal oxides

Thermoelectric properties of layered metal oxides Na_xCoO₂ in a delafossite (CuFeO₂)-related crystal structure are investigated, and a possible mechanism of p-type metallic conduction leading to outstanding thermoelectric performance as an oxide is discussed in terms of crystal chemistry. Sintered samples of Na_xCoO₂ (x≈0.5) were prepared from Na₂CO₃ and Co₃O₄ through the conventional solid state reaction by firing in air. Probably due to losses of Na during firing, electrical properties of the oxides are revealed to be sensitive to the heating atmosphere. The oxides show almost temperature-independent electrical conductivities of >10² S cm^-1 up to 1000 K, being accompanied by the positive Seebeck coefficients of 100 - 200 μV K^-1 which increase linearly with temperature. Although a poor sinterability of our samples at the present state limits the conductivity to the values lower than those previously reported, the oxide shows a power factor value of >6×10^-4 W m^-1K^-2 at 1000 K. Since the thermal conductivity of the oxides was measured as 1.5 - 2.5 W m^-1K^-1 at room temperature, and is usually expected to become lower at higher temperatures, the maximum figure of merit is anticipated to reach >0.4×10^-3 K^-1. The oxide is thereby confirmed to be the most promising p-type oxide candidate so far investigated.