Optimization of Thermoelectric Performance of CaTiO₃-Based Materials by Dy/Nb Co-Doping

CaTiO₃-based compounds emerge as a promising thermoelectric material due to their environmentally benign, thermally stable, and cost-efficient merits. Nonetheless, pristine CaTiO₃ manifests inherently inferior electronic transport properties. In this paper, Ca_1–xDy_xTi_0.95Nb_0.05O₃ (x=0–0.15) bulk ceramics were prepared via solid-phase sintering combined with hot-pressing sintering, and the composition, microstructure, and thermoelectric properties were analyzed. The results show that Dy and Nb doping can significantly increase the carrier concentration and effectively improve the electronic transport properties. Also, the lattice thermal conductivity is drastically reduced due to the introduction of large mass-field strain and stress-field strain, thus scattering high-frequency phonons. Ca_0.85Dy_0.15Ti_0.95Nb_0.05O₃ bulk ceramic has a zT maximum of 0.29 at 1073 K, showing that the CaTiO₃-based materials have a promising prospect for thermoelectric applications.

Ca_1–xDy_xTi_0.95Nb_0.05O₃ (x=0–0.15) bulk ceramics were synthesized by solid-phase sintering and hot pressing. First, powders of CaCO₃, Nb₂O₅, Dy₂O₃, and TiO₂ were mixed and ground in a planetary ball mill for 12 h, then dried and cold-pressed. The pre-burned samples were ground for 6 h and the dried powders were cold-pressed again. The pressed samples were then placed in a graphite mold with a diameter of 13 mm for hot pressing at 1573 K for 1.5 h to obtain dense disks with a thickness of 2 mm.

The phase composition of the bulk samples was performed by a model EMPYREAN X-ray diffractometer (XRD, PANalytical Co., the Netherlands). The elemental distribution of the samples was measured by a model JXA-8530F electron probe X-ray micro-analyzer (EPMA, JEOL Co., Japan). The Seebeck coefficient and resistivity of the samples were measured simultaneously by a model LSR-3 device (Linseis Co., Germany). The samples used for the test were long strips with the sizes of approximately 11.0 mm × 2.0 mm × 2.5 mm. The carrier concentration and Hall mobility of the system were measured by a model 8400 Hall measurement system (Lake Shore Co., Ltd., USA). The thermal conductivity of the system was measured by a model LFA-457 laser flash thermal conductivity meter (Netzsch Co., Germany), and the specific heat capacity of the bulk samples was calculated according to the Debye-Dulong law.

The polycrystalline Ca_1–xDy_xTi_0.95Nb_0.05O₃ (x=0–0.15) samples with a single phase were prepared. The main phase of the synthesized samples is an orthorhombic structure, with a space group of Pnma. Dy³⁺ and Nb⁵⁺ occupy Ca²⁺ and Ti⁴⁺ sites in the matrix, respectively, causing the lattice expansion and introducing the donor impurity levels, transforming CaTiO₃ from an insulating wide-bandgap semiconductor to a good electrical conductivity thermoelectric material. The actual amounts of elements Dy and Nb in the bulk samples are basically consistent with the nominal doping concentrations, indicating that elements Dy and Nb are incorporated into the CaTiO₃ matrix effectively. The electrical conductivity decreases with increasing temperature, while the absolute value of the Seebeck coefficient increases with increasing temperature, showing typical characteristics of degenerate semiconductor electron transport. At high temperatures, the lattice vibrations are violent, and the acoustic wave scattering dominates the carrier transport, with other scattering mechanisms having little effect on the electronic transport properties. As Dy concentration increases, the effective mass of the density of states decreases. When x=0.15, the maximum zT value can reach 0.29 at 1073 K, which is comparable to the others reported CaTiO₃ based thermoelectric performances in the literature.

Ca_1–xDy_xTi_0.95Nb_0.05O₃ (x=0–0.15) samples with a single phase were prepared by solid-phase reaction combined with HP sintering. Dy and Nb doping could significantly increase the carrier concentration and effectively improve the electronic transport performance. Meanwhile, the high-frequency phonons were scattered due to the large mass difference and covalent radius difference between Dy³⁺ and Ca²⁺, resulting in a substantial reduction in the lattice thermal conductivity of the system. Ca_0.85Dy_0.15Ti_0.95Nb_0.05O₃ achieved the maximum zT value of 0.29 at 1073 K due to the simultaneous optimization of electrical and thermal properties, indicating that CaTiO₃ oxide thermoelectric materials could have a promising prospect for thermoelectric applications.