Enhancement of Electrical Conductivity in Thermoelectric [Ca₂CoO₃]_0.62[CoO₂] Ceramics by Texture Improvement

Abstract

In the preparation of textured ceramics of [Ca 2 CoO 3 ] 0.62 [CoO 2 ] (CCO-3-layer) by the reactive-templated grain growth (RTGG) method using β-Co(OH) 2 templates, we have examined the effect of processing conditions on the orientation degree and thermoelectric properties of the resultant ceramics. All the RTGG-prepared ceramics showed a high degree of orientation with the Lotgering's factor reaching approximately 1. It was found, however, that texture differences detected by rocking curve measurement (in terms of the (004) plane of the CCO-3-layer) significantly affect the in-plane electrical conductivity, σ a b . An increase in the orientation degree of β-Co(OH) 2 templates in a green compact increased the orientation degree of the resultant CCO-3-layer ceramic that is formed on the templates by heat treatment. Furthermore, an increase in pressure and duration time of uniaxial pressing during heat treatment was found to increase the orientation degree of the CCO-3-layer ceramic. As a result, a ceramic with improved texture exhibited a rocking curve FWHM (004) of ∼6.0° (FWHM: full width at half maximum), and σ a b (at 1060 K) of 2.61 ×10 4 S/m, which is approximately 1.7 times as large as that of the RTGG-prepared ceramic before refining the processing conditions.