Effect of Partial Substitution of 3d Transition Metals for Co on the Thermoelectric Properties of Na_xCo₂O₄

Abstract

Layered P2 type Na_xCo₂O₄ thermoelectric oxides partially substituted by 3d transition metal (up to 15 at%: Cr, Mn, Fe, Ni, or Zn) for Co were synthesized through calcination and sintering in air. The Cr-substituted samples were mostly decomposed into the NaCrO₂+Co₂CrO₄ phases. On the other hand, the X-ray diffraction patterns of the Mn-substituted samples showed a single P2 type γ-Na_xCo₂O₄ phase. The Fe- and Ni-substituted samples were mostly composed of the Na_xCo₂O₄ phase with small amounts of Co₃O₄ and CoNiO₂ phases. In the case of the Zn-substituted samples, only the Na_x(Co_0.95Zn_0.05)₂O₄ was mostly composed of the Na_xCo₂O₄ phase with a small amount of the NaO phase. The effects of partial substitution of these 3d transition metals for Co except for Cr in the layered Na_xCo₂O₄ on thermoelectric properties were investigated. The electrical resistivity of these partially substituted samples significantly increased over the entire temperature range, as compared to the non-substituted sample, indicating reduction in carrier concentration. Except for Na_x(Co_1−yFe_y)₂O₄, the Seebeck coefficient was enhanced by the partial substitution, and in the case of Na_x(Co_1−yFe_y)₂O₄, it is suggested that the decrease in the Seebeck coefficient is associated with reduction in the electron-electron correlation due to the substitution. For the Mn-substituted samples, a small grain size and many pores compared to the non-substituted sample were effective for reducing the thermal conductivity. The figure-of-merit Z was enhanced in the case of the Na_x(Co_0.95Zn_0.05)₂O₄ above 550 K and the maximum dimensionless figure-of-merit ZT, 0.68, was obtained at 965 K.