Synthesis of Ca–Co hydroxides and their use in facile fabrication of textured Ca_yCoO₂ thermoelectric ceramics

A brucite-type hydroxide of Ca_zCo_1−z(OH)₂ with a high Ca content was synthesized and utilized as a single-source precursor for the fabrication of bulk ceramics of a semiconducting layered calcium cobaltite Ca_yCoO₂. Plate-like Ca_zCo_1−z(OH)₂ particles with z up to 0.22 were reproducibly obtained by a reverse co-precipitation method using KOH as an alkaline source. Uniaxial pressing of the plate-like Ca_zCo_1−z(OH)₂ particles (z = 0.22), which was calcined at 373 K beforehand, at a pressure as high as 500 MPa produced a highly dense green pellet where the plate-like particles were stacked along the pressing direction. The green pellet was then converted by a heat treatment at 923 K into an oxide ceramic of preferentially (010)-oriented Ca_yCoO₂ via a topotactic-like pyrolytic reaction maintaining edge-sharing CoO₆ octahedra. Although the ceramic also contained a minor insulating Co₃O₄ phase (16.7 ± 0.7 mol% estimated from thermal analysis), the electrical measurements proved its p-type semiconducting behavior resulting from the Ca_yCoO₂ phase with enhanced Seebeck coefficient exceeding 160 μV/K at room temperature.