Effect of the Addition of Copper Particles on the Thermoelectric Properties of the Ca₃Co₄O_{9 + δ} Ceramics Produced by Two-Step Sintering

Abstract—Composite thermoelectric materials based on layered calcium cobaltite Ca₃Co₄O_{9 + δ} doped with copper particles were synthesized by two-step sintering, and their microstructure, and electrotransport and thermoelectric properties were studied. It was determined that the introduction of copper particles into the ceramics improves their sinterability at moderate sintering temperatures (T_sint ≤ 1273 K), leading to a decrease in the porosity of the samples and an increase in their electrical conductivity and power factor, whereas the oxidation of copper to less conductive copper(II) oxide significantly decreases the electrical conductivity and power factor of the ceramics sintered at elevated temperatures (T_sint ≥ 1373 K). The power factor is maximum for the Ca₃Co₄O_{9 + δ} + 3 wt % Cu ceramic sintered at 1273 K (335 μW/(m K²) at a temperature of 1100 K), which is by a factor of 2.3 higher than the power factor of the base material Ca₃Co₄O_{9 + δ} with the same thermal history (145 μW/(m K²) at 1100 K) and more than 3 times higher than the power factor of the Ca₃Co₄O_9+δ ceramic synthesized by the conventional solid-phase method.