Elucidation of the Formation Mechanism of a Textured Ceramic of a P-type Thermoelectric Layered Oxide [Ca2CoO3]0.62[CoO2]

The reactive-templated grain growth (RTGG) method is a powerful fabrication technique for producing textured ceramics having enhanced performance compared to those of conventionally prepared non-textured ceramics, for various functional materials. Its wide applicability is demonstrated by the fact that the RTGG method using β-Co(OH)2 templates gave textured ceramics of p-type thermoelectric layered cobaltites having various compositions. The orientation degree of a prepared ceramic, which influences its performance, depends on the composition of the ceramic. Thus, in order to determine general guidelines for the production of a highly textured ceramic, we analyzed the formation mechanism of the model system [Ca2CoO3]0.62[CoO2] (CCO: Ca2CoO3 layer + CoO2 layer) ceramic on β-Co(OH)2 templates by using high-temperature X-ray diffraction (XRD), pole figure, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We demonstrated that a textured CCO ceramic is formed through a series of in-situ topotactic conversions via intermediate phases with a preserved CoO2 layer of β-Co(OH)2 templates. In general, we showed, for the first time, that 'a reaction design with partially preserved crystallographic similarities' is essential for the fabrication scheme of a highly textured ceramic with enhanced performance.