Synthesis of c-axis Oriented NaxCo2O4 Thermoelectric Oxide by the Citric Acid Complex Method

Abstract

The citric acid complex (CAC) process and subsequent pressureless sintering were explored towards the synthesis of a c-axis oriented NaxCo2O4 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-NaxCo2O4 phase, reflecting its layered crystal structure. During compacting and sintering of this powder, the plate-like grains were well aligned along the pressed plane because of their specific shape, resulting in the sintered body with large crystallographic anisotropy. The CAC method was also effective for synthesizing the sample with compositional homogeneity and fine microstructure. The electrical resistivity of the CAC sample was almost the same as that of the sample prepared by the conventional solid state reaction method in spite of the fine microstructure, because of its high degree of c-axis orientation. The Seebeck coefficient was significantly improved by adopting the CAC method, resulting in the large enhancement of the power factor.