Prospects for Electrical Performance Tuning in Ca3Co4O9 Materials by Metallic Fe and Ni Particles Additions

Electro-Thermo-Magnetic Effect-Induced Large Thermoelectric Performance of Calcium Cobaltite Nanocomposites

As attractive thermoelectric oxides, Ca3Co4O9-based materials have been intensively studied for their applications in recent years. However, their thermoelectric performance is enormously limited due to the contradiction of electrical resistivity and thermal conductivity. Herein, BaFe12O19 nanospheres were introduced into the Ca3Co4O9 matrix. The metallic Ag, ferrites, and matrix phase survived together, and a high density of nanoscale BaFe12O19 precipitation was observed. The reduction of work function could lead to band bending and form an interface potential due to the electro-thermo-magnetic effect contributing to the hole migration. As a result, a huge ZT value of 0.51 for the 8 wt % BaFe12O19/Ca3Co4O9 nanocomposites was obtained at 1073 K, accompanied by a low electrical resistivity of 6.7 mΩ·cm and a high Seebeck coefficient of 217.5 μV/K. In addition, a significant reduction of thermal conductivity (1.11 W/(m·K)) occurred, which was due to the nanoscale ferromagnetic phase effectively scattering the mid- and short-wavelength heat-carrying phonons. The synergistic enhancement of thermoelectric performance confirmed that the electro-thermo-magnetic effect is an effective way to solve the challenging problem of performance deterioration in oxide thermoelectric materials.

Effect of the Addition of Copper Particles on the Thermoelectric Properties of the Ca3Co4O9 + δ Ceramics Produced by Two-Step Sintering

Abstract

Composite thermoelectric materials based on layered calcium cobaltite Ca3Co4O9 + 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 (Tsint ≤ 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 (Tsint ≥ 1373 K). The power factor is maximum for the Ca3Co4O9 + 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 Ca3Co4O9 + 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 Ca3Co4O9+δ ceramic synthesized by the conventional solid-phase method.