Thermoelectric properties of Ca3–xBixCo4O9 + δ (0.0 ⩽ x ⩽ 1.5) ceramics

Abstract

We have studied the thermal, electrical, and thermoelectric properties of Ca_3–xBi_xCo₄O_{9 + δ} (0.0 ≤ x ≤ 1.5) ceramics prepared by solid-state reactions. Phase-pure Ca_3–xBi_xCo₄O_{9 + δ} solid solutions were obtained at x ≤ 0.3. The samples with 0.3 < x < 1.5 consisted of three phases: Ca_2.7Bi_0.3Co₄O_{9 + δ}, Bi₂Ca₂Co_1.7O_y, and Со₃О₄. The sample with the composition Ca_1.5Bi_1.5Co₄O_{9 + δ} consisted of two phases: Bi₂Ca₂Co_1.7O_y and Co₃O₄. The materials are p-type semiconductors with linear thermal expansion coefficients in the range (10.6–12.8) × 10^–6 К^–1. Their electrical and thermal conductivities are nonmonotonic functions of x, and their thermoelectric power increases with increasing x. The mixed-phase ceramics have been shown to have better thermoelectric properties: the highest thermoelectric power factor, ≈0.24 mW/(m K²) at T = 1100 K, is offered by the ceramics with x = 0.9–1.0, which contain nearly equimolar amounts of Ca_2.7Bi_0.3Co₄O_{9 + δ} and Bi₂Ca₂Co_1.7O_y. This power factor is 2.5 times that of the layered calcium cobaltite Ca₃Co₄O_{9 + δ} and solid solutions based on it: 0.094–0.098 mW/(m K²) at the same temperature.