强磁场辅助生长对SnSe热电性能的调控研究

Thermoelectric materials can be utilized to convert thermal energy and electrical energy directly in a single thermoelectric generation system. Thermoelectric figure of merit ZT is defined to determine the thermoelectric performance of the material. The ZT value of a single bulk material is always below 2.5. Recent experiment showed that the ZT value along the specific crystallographic axis direction could reach about 2.6 for SnSe single crystal. So far, it has been the single bulk material with the highest ZT value and has great potential for thermoelectric application. The high ZT value of SnSe single crystal is original from the ultra-low lattice thermal conductivity. However, the power factor of SnSe is not high enough. If we could find an effective method to enhance the power factor of SnSe, it will possess extensive thermoelectric application prospect. In this project, we will use a combined scheme of magnetic ions doping and high-magnetic-field assisted growth to investigate the structure, magnetic, electrical and thermal transport properties of SnSe thermoelectric material. The power factor of SnSe will be enhanced by the combined tunning of charge and spin degree of freedom to optimize its thermoelectric performance. At the same time, we could obtain the universal law of thermoelectric performance tunning by magnetic ions doping and high-magnetic-field assisted growth for the layered SnSe compound. Combining with the theoretical calculation, we will provide some information for the physical origin of thermoelectric performance of SnSe narrow band semiconductors. We hope to provide some guide to optimize thermoelectric performance for the similar system materials.

热电材料可实现热能和电能之间的直接转换，热电优值ZT通常被用来衡量一种材料热电性能的好坏，单一块体材料的ZT值一般都在2.5以下。最近实验发现SnSe单晶沿特定晶轴方向的ZT值在923K可达2.6左右，是目前为止ZT值最高的单一块体材料，具有很大的热电应用潜力。SnSe单晶的高ZT值主要源于极低的晶格热导率，但其功率因子并不优越。若能找到调控该材料功率因子的有效途径，它将拥有广泛的热电应用前景。本项目拟采用“磁性离子掺杂+强磁场辅助生长”的调控方案，系统研究层状SnSe材料的结构、磁、电和热输运等性质，通过电荷和自旋两个自由度的协同作用实现对该材料功率因子的有效调控，从而优化其热电性能，同时获得磁性离子掺杂和强磁场辅助生长对层状SnSe热电性能调控的一般规律。结合理论计算，理解SnSe窄带半导体材料热电性质的微观物理机制，希望为相似体系材料的热电性能优化提供数据积累和理论指导。

热电材料可以实现热能和电能之间的直接相互转换，衡量其性能的物理量是热电优值ZT。实验发现，SnSe单晶沿特定晶轴方向的ZT值在923K可达2.6左右，具有极高的研究和应用价值。SnSe单晶的高ZT值主要源于极低的晶格热导率，但其功率因子并不优越，因此有效调控该材料的功率因子和热电性能，它将拥有广泛的热电应用前景。在本项目执行过程中，我们主要从强磁场下各向异性热电材料的制备条件探索、SnSe热电性能的内场和外场调控三个方面展开研究。首先，我们在强磁场下制备了层状钴基氧化物Bi2Sr2Co2Oy多晶样品，研究了强磁场辅助生长对材料形貌、结构和性能的影响，实现了强磁场对材料热电性能的有效调控，为强磁场下SnSe材料的合成制备奠定了基础。其次，系统研究了制备条件优化（原料纯度、高温热电测试条件、氧化行为、高能球磨）、元素掺杂效应（磁性元素Mn、Co、Cd和碱金属元素Li、Na、K）、两相复合效应（Bi0.89Sb0.11和Ti3C2Tx）、Sn含量调控等对SnSe多晶晶体结构、电/热输运等物性的影响，通过内场调控实现对SnSe多晶材料热电性能的有效调控。另外，在强磁场下制备了SnSe母体及其In或Sb掺杂的单晶样品，发现强磁场下SnSe单晶生长可以大幅度提高材料的导电性。通过本项目的研究，证实了在SnSe材料中通过磁性离子掺杂和强磁场辅助生长进行热电性能调控的可行性，通过多种实验手段实现了对其热电性能的有效调控，加深了对SnSe窄带半导体材料热电性质的理解，同时也为相似体系材料的热电性能优化提供了数据积累。在该项目的资助下，共计发表SCI论文3篇，其中，1篇Journal of Alloys and Compounds, 1篇Journal of the American Ceramic Society, 1篇Current Applied Physics。