Effect of Partial Substitution of 3d Transition Metals for Co on the Thermoelectric Properties of Na<SUB>x</SUB>Co<SUB>2</SUB>O<SUB>4</SUB>

Abstract

Layered P2 type Na x Co 2 O 4 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 NaCrO 2 + Co 2 CrO 4 phases. On the other hand, the X-ray diffraction patterns of the Mn-substituted samples showed a single P2 type γ-Na x Co 2 O 4 phase. The Fe- and Ni-substituted samples were mostly composed of the Na x Co 2 O 4 phase with small amounts of Co 3 O 4 and CoNiO 2 phases. In the case of the Zn-substituted samples, only the Na x (Co 0.95 Zn 0.05 ) 2 O 4 was mostly composed of the Na x Co 2 O 4 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 x Co 2 O 4 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-y Fe y ) 2 O 4 , the Seebeck coefficient was enhanced by the partial substitution, and in the case of Na x (Co 1-y Fe y ) 2 O 4 , 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 0.95 Zn 0.05 ) 2 O 4 above 550 K and the maximum dimensionless figure-of-merit ZT, 0.68, was obtained at 965 K.