Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered α-titanate materials, NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na0.4Ni0.2Ti0.8O2 were ca. 7×103 S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered α-titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm−1 K−1 at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na0.4Ni0.2Ti0.8O2 and Na0.4Co0.2Ti0.8O2 was about 1.9×10−4 and 1.2×10−4 K−1 around 700 °C, respectively.