Synthesis of NaxCo2O4 thermoelectric oxide with crystallographic anisotropy by chemical solution process

Abstract

The synthesis of the NaxCo2O4 with a layered crystal structure was attempted by using the polymerized complex (PC) method or citric acid complex (CAC) method, both of which are a kind of chemical solution process. The effects of these chemical solution processes on the crystallographic anisotropy, microstructure and thermoelectric properties of the sintered NaxCo2O4 were investigated and compared to those of the conventional SSR method. In the case of the PC method, the sintered body with higher chemical homogeneity and finer microstructure than those of the SSR sample was obtained, resulting in the high thermoelectric performance. However, the crystallographic orientation was almost the same as that of the SSR sample. On the other hand, in the case of the CAC method, a flaky powder precursor was formed by grain growth of the NaxCo2O4 phase along the c-plane. The NaxCo2O4 sample sintered from this powder precursor showed a high crystallographic anisotropy and good chemical homogeneity, so the CAC process was found to be quite effective for synthesizing a crystallographically oriented NaxCo2O4 polycrystal with high thermoelectric performance.