Improving the thermoelectric performance and thermal stability of Ca₃Co₄O_9 + δ ceramics by sintering in oxygen atmosphere

We report on the influence of sintering gas atmosphere on the thermoelectric (TE) performance of Ca₃Co₄O_9 + δ ceramics made from precursor powders synthesized using chemical sol–gel route. Two sets of polycrystalline pellets were sintered in air and oxygen atmosphere, respectively. Sintering gas atmosphere and sintering temperature were found to have great influence on the Ca₃Co₄O_9 + δ crystal orientation and formation of the Co₃O₄ secondary phase. All samples, regardless of sintering atmosphere, had similar Seebeck coefficients S that increased with measurement temperature. By contrast, the electrical resistivity ρ changed dramatically as the sintering temperature and atmosphere changed. For samples sintered in air, when the sintering temperature is above 1193 K, ρ increased as the sintering temperature increased. For samples sintering in oxygen, ρ decreased monotonically as the sintering temperature increased up to 1253 K. Benefited from the reduced ρ, the electrical power factor of the sample sintered in the oxygen at 1253 K is 0.539 mW m⁻¹ K⁻², and 40% greater than that from the sample sintered at 1193 K in air. Dimensionless figure-of-merit ZT of the sample sintered at 1253 K in oxygen is 0.28 at 1073 K, which is 20% greater than that of samples sintered in air at 1193 K. Overall, the highest TE performance was obtained for Ca₃Co₄O_9 + δceramics sintered in oxygen at 1253 K, which is significantly higher than the Ca₃Co₄O_9 + δ decomposition temperature of 1199 K in air.