Thermoelectric properties of c-axis oriented Ni-substituted NaCoO 2 thermoelectric oxide by the citric acid complex method

Abstract

The citric acid complex (CAC) process and subsequent pressureless sintering was explored towards the synthesis of the c-axis oriented Ni-substituted NaCoO 2 thermoelectric polycrystal. The effects of the CAC process on the shape of powder precursor, crystallographic anisotropy of a sintered compact and its thermoelectric properties were investigated. The CAC process provided a plate-like powder precursor composed of the γ-Na x CoO 2 phase, reflecting its layered crystal structure. During compacting and sintering of this powder, the plate-like grains were well aligned along the pressed plane, resulting in the sintered body with large crystallographic anisotropy. With the Ni substitution, the electrical resistivity increased and thermal conductivity decreased mainly because of a decrease in grain size. On the other hand, the Ni substitution led to a significant increase in the Seebeck coefficient. The highest calculated dimensionless figure of merit ZT = 0.176 was obtained for NaCo 0.9Ni 0.1O 2 sample at 673 K.