The effects of dual-doping and fabrication route on the thermoelectric response of calcium cobaltite ceramics

Abstract

Calcium cobaltite (Ca3Co4O9) is an attractive p-type thermoelectric oxide for energy harvesting. Our work provides insights into the way microstructure-property relationships in calcium cobaltite based ceramics can be controlled by dopants and fabrication routes. Ca2.63Bi0.3M0.07Co3.92O9+δ (M = Sr and Ba) ceramics were prepared by solid state reaction (SSR) and spark plasma sintering (SPS) routes. The roles of dopants were clearly identified; Bi and Sr entered the crystal lattice, whilst Ba mainly participated in forming Ba-rich secondary phases. Successful Bi/Sr dual-doping gave rise to pronounced lattice expansion, which could induce strain fields and atomic mass variations, thereby leading to enhanced carrier mobility and phonon scattering. In spite of an increase in porosity, the annealed SPS-processed Ca2.63Bi0.3Sr0.07Co3.92O9+δ ceramic showed enhanced electrical conductivity with minor changes to Seebeck coefficients, due to reduction of secondary phases, increased grain size and improved texture. By contrast, the thermal conductivity of SSR-processed Ca2.63Bi0.3Ba0.07Co3.92O9+δ ceramic was significantly reduced although the electrical conductivity was limited by the high porosity. As a result, a maximum ZT value (//ab) of 0.14 was achieved in both the solid state synthesized Bi/Ba dual-doped samples and annealed SPS-processed Bi/Sr dual-doped samples at 800 K.