Thermoelectric properties of La3+ and Ce3+ co-doped CaMnO3 prepared by tape casting

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.

Enhancement in the electrical transport properties of CaMnO3via La/Dy co-doping for improved thermoelectric performance

The untiring endeavour towards green energy is a trending research among the research community. Thermoelectric materials are of vital importance here owing to their emission-free operation. As a righteous candidate, calcium manganate materials are being explored to increase its figure of merit. In this study, the structural, microstructural, electrical transport, and high-temperature thermoelectric measurements of La_xDy_xCa_1-2xMnO₃ {x = 0.025 (L25D25), 0.05 (L50D50), 0.075 (L75D75), and 0.1 (L100D100)} were systematically performed. The structural confirmation of the synthesised sample was validated using X-ray diffraction, which also revealed the orthorhombic (space group: Pnma) crystallisation of co-doped samples with no traces of secondary peaks. A significant increase in the unit cell volume was observed with rare earth substitutions. The morphological studies revealed that the prepared samples were highly dense and the grain size was reduced with rare earth concentration. The substitution of La and Dy enhanced the conductivity values of pristine CMO by two orders of magnitude due to the high concentration of charge carriers and the presence of Mn³⁺ ions due to rare earth doping. The conductivity increased with rare earth concentrations but diminished for x = 0.1 due to the localization of charges. The Seebeck coefficient values were negative for all the prepared samples, indicating electrons as the predominant carriers over the entire operating range. A minimum κ of 1.8 W m^-1 K^-1 was achieved for La_0.1Dy_0.1Ca_0.8MnO₃ and the maximum value zT obtained was 0.122 at 1070 K for La_0.075Dy_0.075Ca_0.85MnO₃.

The effect of Sr substitution on the structural and physical properties of manganite perovskites Ca1-xSrxMnO3-δ (0 ≤ x ≤ 1)

Calcium manganite (CaMnO_3-δ) has been extensively utilized in many applications due to its unique physical and chemical properties. In this study, the effect of Sr-substitution at the Ca-site on the structural, magnetic, electronic and electrical properties of CaMnO₃ manganite perovskites is investigated in detail. The perovskite compounds Ca_1-xSr_xMnO_3-δ (x = 0, 0.25, 0.5, 0.75 and 1) were synthesized through the sol-gel method at 1200 °C. From the patterns of X-ray diffraction, it was observed that all of the synthesized compounds show a pure perovskite phase at room temperature. The refinement results of the perovskite series suggest that a structural transformation from an orthorhombic (Pnma) to a hexagonal (P6₃/mmc) system occurred for 0.50 < x ≤ 0.75. We note however that the sample with the composition x = 0.50 showed a phase mixture of orthorhombic (Pnma) and hexagonal (P6₃/mmc). Based on DFT calculations, we have demonstrated the energetic stability of all compounds by negative formation energy and confirmed the semiconductor behavior by the presence of a band gap. The change in the band gap value with the Sr content suggests the potential tuning of the electronic behavior of CaMnO₃-SrMnO₃ solid solution. Furthermore, as the temperature increases from 300 to 1000 K, the electrical resistivity exhibits a reduction while the Seebeck coefficient (S) shows an augmentation. The negative values of S indicated the n-type-semiconductor nature of all compounds. The obtained values of the activation energy from thermal evolution of resistivity suggested that the electrical transport behavior of all the compounds followed the mechanism of small polaron hopping. Power factor is greatly affected by the Sr amount and reached a maximum value at x = 0.50. Overall, introducing Sr into the CaMnO_3-δ matrix improved the power factor and reduced electrical resistivity. According to the obtained results, the studied manganite perovskites could be proposed as suitable materials for photocatalytic and thermoelectric applications.

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.

Microwave Synthesis and Enhanced Thermoelectric Performance of p-Type Bi0.90Pb0.10Cu1-xFexSeO Oxyselenides

BiCuSeO oxyselenide, one of the best oxygen-containing thermoelectric materials, is promising with great potential applications. In this work, we present a high ZT of >1.3 in Bi_0.90Pb_0.10Cu_0.96Fe_0.04SeO fabricated via microwave synthesis and subsequent spark plasma sintering (SPS). We added 3-4 atom % Fe to the Pb-doped BiCuSeO to regulate the hole carrier concentration and mobility to 0.8-1.0 × 10²⁰ cm^-3 and ∼40 cm² V^-1 S^-1, respectively, achieving moderate electrical conductivity, high Seebeck coefficient, and low carrier thermal conductivity simultaneously in a dual-doped sample. Under the synergistic enhancement by stress field, dislocation, and nanophase, the lattice thermal conductivity of Bi_0.90Pb_0.10Cu_0.96Fe_0.04SeO is limited to 0.24-0.49 W m^-1 K^-1 at 300-873 K. The development of efficient preparation methods for high-performance thermoelectric materials is significant to promote the application of thermoelectric conversion technology.