Se替代Te对BiCuTeO电热输运性能的影响

层状氧硫族化合物由于其本征的低晶格热导率和可观的热电性能吸引了广泛关注,其中以BiCuSeO化合物的热电性能最为优异.但是,其同晶型化合物BiCuTeO,由于带隙较小且存在大量本征Cu空位,导致载流子浓度较高,热电性能较差,从而研究较少.针对BiCuTeO存在的上述问题,本文利用Se替代部分Te,以期能够展宽带隙并减少Cu空位,提高其热电性能.采用固相反应结合快速热压烧结制备了BiCuTe_(1-x)Se_xO(x=0, 0.1, 0.2, 0.3和0.4)块体热电材料,并系统地研究了该体系的电热输运性能.研究结果表明,利用Se替代Te,可以使BiCuTeO导电层化学键强度增加、带隙增大、载流子有效质量增加以及载流子散射增强,从而导致载流子浓度和迁移率同时降低,进而电导率随着Se含量增加而剧烈降低, Seebeck系数则显著增大.由于综合电输运性能恶化,功率因子随着Se含量增加而减小,导致热电优值zT随着Se含量增加而降低.最终,Se含量为x=0.1的样品,在室温和723 K时的zT值分别达到约0.3和0.7,仍然在较宽温区内保持较高的zT值.由于Se替代Te改变了BiCuTeO的能带结构,通过载流子浓度优化,有望进一步提高其热电性能.

Effects of Se substitution for Te on electrical and thermal transport properties of BiCuTeO

Recently, layered oxychalcogenide has attracted significant scientific attention because of its intriguing electronic properties, intrinsically low thermal conductivity and, correspondingly, outstanding thermoelectric properties, of which the BiCuSeO possesses the best thermoelectric performance ever reported. For instance, the optimized zT value of BiCuSeO system reaches 1.5 at 873 K through dual-doping approach. Such a zT value is comparable to those of the state-of-art p-type lead chalcogenide thermoelectric materials. However, comparing with BiCuSeO compound, little effort has been devoted to the isomorphic analogue BiCuTeO. On the one hand, the BiCuTeO has a pretty small band gap (0.4 eV) which limits its working temperature range. On the other hand, numerous intrinsic Cu vacancies are present in BiCuTeO due to the weak Cu-Te chemical bonding, leading to an excessive carrier concentration. Thus, further increasing carrier concentration through doping will lead to a deterioration of electrical transport properties and thus reduce the zT value. Herein, we choose Se and partially substitute it for Te in the BiCuTeO to enlarge the band gap and reduce intrinsic Cu vacancies by strengthening the chemical bonding in the conductive layers. By combining solid-phase reaction with hot-pressed sintering, the BiCuTe_1–xSe_xO (x=0, 0.1, 0.2, 0.3, 0.4) bulk thermoelectric materials are prepared, and their microscopic morphology and thermoelectric transport properties are systematically investigated. Our experimental results show that the substitution of Se for part of Te results in strengthening chemical bonding in the conducting layer, enlarging the band gap, increasing the carrier effective mass, reducing the carrier concentration, and enhancing the carrier scattering. Therefore, the electrical conductivity dramatically decreases but the Seebeck coefficient significantly increases with Se content increasing, leading to the decrease of thermoelectric power factor. Furthermore, a slight reduction of the total thermal conductivity is realized by Se alloying due to the decrease of the electronic thermal conductivity. Consequently, the dimensionless figure of merit zT decreases with the Se content increasing because electrical transport properties are deteriorated seriously. Finally, the zT value of 0.3 at room temperature and 0.7 at 723 K are achieved for the sample with x=0.1, indicating that the Se substituted BiCuTeO sample can still maintain comparative zT values in a wide temperature range. Considering that the effective mass of BiCuTeO is significantly increased by Se alloying, the thermoelectric performance of BiCuTe_1–xSe_xO compound might be further improved by optimizing the carrier concentration.