硅酸盐学报

CaTiO₃热电材料因具有环境友好、良好的热稳定性和成本低等优点而受到广泛关注和研究。本征CaTiO₃具有大的能带带隙，表现出不良的载流子输运性能。通过采用固相烧结结合热压烧结工艺制备了n型Dy和Nb双掺杂CaTiO₃基热电材料Ca_1–xDy_xTi_0.95Nb_0.05O₃ (x=0～0.15)，并对样品的组成、微观形貌、热电性能进行了表征与分析。Dy和Nb掺杂不仅可以显著增加载流子浓度，提高电输运性能，同时由于引入大的质量场和应力场波动，显著散射高频声子，体系的晶格热导率大幅度降低。由于电性能和热性能的协同优化，在1073 K时，Ca_0.85Dy_0.15Ti_0.95Nb_0.05O₃获得最大zT值约0.29，表明CaTiO₃氧化物热电材料具有很大的应用潜力。

Introduction 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 z T maximum of 0.29 at 1073 K,showing that the CaTiO₃-based materials have a promising prospect for thermoelectric applications.Methods 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,Nb_2O₅,Dy_2O₃,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.Results and discussion 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.Conclusions Polycrystalline 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 z T 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.