Two–step processing of thermoelectric (Ca0.9Ag0.1)3Co4O9/nano–sized Ag composites with high ZT

Regulating Multiscale Defects to Enhance the Thermoelectric Performance of Ca0.87Ag0.1Dy0.03MnO3 Ceramics

Achieving high thermoelectric properties of CaMnO₃ ceramics is significant for its applications at high temperature. Herein, Ca_0.87Ag_0.1Dy_0.03MnO₃ ceramics with plate-like template seeds additives were prepared by using a solid-state reaction method. The multiscale defects, including grain boundaries, oxygen defects, and Ag nanoprecipitations, which were regulated by the different sintering atmospheres, were beneficial for electron transport and phonon scattering. The grain boundaries as coherent interfaces could act as an alternative phonon scattering source. Oxygen vacancies coupled with Ag nanoprecipitations were verified by geometric phase analysis and annular bright-field analysis. The decrement in oxygen vacancies concentration strongly depended on the enriched oxygen environment, which could reduce electrical resistivities. Compared to the sample sintered at Ar atmosphere, a 17.5 times increment in power factor and a 20.1% reduction of the total thermal conductivity were obtained for the sample sintered at O₂ atmosphere. As a result, the maximum ZT value of 0.22 was obtained at 500 °C. It is an effective way for improving the thermoelectric performance of oxide-based thermoelectric materials.

Influence of ceramic particles additions on the properties of Ca3Co4O9

Abstract

Ca3Co4O9 + x wt% B4C, AlN, TiC, TiB2, or TiN (x = 0.0, 0.25, 0.50, and 0.75) samples were prepared by the conventional solid-state route. In all samples, only the Ca3Co4O9 phase was identified by powder XRD. Nevertheless, microstructural studies have shown that most of the additives have reacted with air and Ca3Co4O9 phase on their surfaces, producing new phases. Moreover, it seemed that grain sizes were, at least, slightly reduced. On the other hand, while nearly no modification of the Seebeck coefficient has been observed, independently of the added compound and proportion, electrical resistivity decreased in all cases, when compared to the pristine sample. Consequently, the power factor of samples with additions was higher than the one determined for the pure sample. Linear thermal expansion also decreased with these additives, pointing out to the formation of relatively strong grain boundaries which can improve the carrier mobility and decrease the thermal expansion. The lowest thermal expansion value has been measured in 0.25 wt% B4C samples, being only around 20% higher than that of Al2O3, which can help to reduce the differential thermal expansion in thermoelectric modules working at high temperatures; these results may be very interesting for applications prospects.

Article highlights

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.

Boosting the Thermoelectric Performance of Calcium Cobaltite Composites through Structural Defect Engineering

Misfit-layered Ca₃Co₄O₉ as a p-type semiconductor is difficult to commercialize because of its relatively poor performance. Here, Ca_2.7-xLa_xAg_0.3Co₄O₉/Ag composites prepared by spark plasma sintering were systematically investigated in terms of La³⁺ dopant levels and nano-sized Ag compacts. Multiscale microstructures of stacking fault, dislocation, and oxygen vacancy-linked defects could be recognized as an effective strategy for tuning the transport of charge carriers and phonon scattering. An increasing concentration of charge carriers was caused by the introduction of nano-sized Ag particles at the grain boundary. The multiscale structural defects served as phonon scattering centers to reduce the thermal conductivity. Finally, the Ca_2.61La_0.09Ag_0.3Co₄O₉/Ag sample exhibited a maximum ZT of 0.35 at 1073 K. The results suggest that the interplay of structural defects provides an impetus for a huge improvement in thermoelectric performance.