强磁场辅助碲化铋系热电材料的熔炼法制备、微结构调控及热电性能研究

Single crystalline Bi2Te3-based thermoelectric materials are prone to cleave along the weak van der Waals bonding planes.Furthermore,the process of single crystal growth is quite energy-consuming and takes a very long time. Recently, some powder metallurgical (PM) methods, such as mechanical alloying (MA) combined with spark plasma sintering (SPS), melt spinning combined with spark plasma sintering (SPS), have been developed for preparing polycrystalline Bi2Te3-based materials, and some exciting progress has been reported. However, PM methods need more complicated facilities and are not suitable for large scale production yet. Melting -solidification (MS) method, which is one of the most traditional and efficient methods for large scale production of many metallic materials, is employed to prepare polycrystalline Bi2Te3-based thermoelectric materials in this project. To manipulate the solidification process, strong magnetic field is introduced during the process of solidification, and some refractory intermetallic nanoparticles are added into the melt of Bi2Te3-based materials. Thermodynamics and kinetics of nucleation and growth of Bi2Te3-based materials are supposed to be altered, and the microstructure of the solidified Bi2Te3-based materials will be adjusted accordingly, therefore their comprehensive properties can be improved and optimized. In this project, the nucleation and cyrstallization process of the melt are monitored by DTA, and the structure, morphology and composition of the solidfied Bi2Te3-based thermoelectric materials are characterized by many testing techniques,such as XRD,DTA,SEM,TEM, EDS and EPMA. Based on the investigation of the project, the law of microstructure control and thermodynamics and kinetics of nucleation and crystal growth of Bi2Te3-based materials under magnetic field will be unveiled. The research results of this project will be of both scientific and practical application value for the optimization of thermoelectric properties and large scale production of Bi2Te3-based and other thermoelectric materials.

碲化铋系单晶材料机械性能差、生产周期长，基于粉末冶金的一些多晶材料新工艺近年来取得了长足进展，但在规模化和单位成本方面仍难以和单晶材料抗衡。熔炼-凝固法是大规模制备传统材料的最有效方法之一，本项目将熔炼凝固法用于碲化铋系多晶块体材料的制备，在熔体中添加纳米形核剂以提高其形核率，并在凝固过程中引入强磁场，为体系引入除温度、压力之外的新变量 - 磁场强度，从而可以通过调节磁场参量，改变体系的热力学状态，实现对碲化铋系材料凝固过程和微结构的有效调控，进而优化其综合性能。本项目首次将强磁场与熔炼法结合用于碲化铋系多晶材料的制备，探索强磁场环境下碲化铋系材料凝固结晶的热力学、动力学规律及其微结构调控机理，这在国际国内都是一项有意义的探索和创新。对揭示强磁场条件下热电材料的形核结晶长大规律、丰富热电材料微结构和性能的调控优化途径，推动碲化铋系材料的高效规模化制备和应用推广具有重要的科学意义和应用价值。

本项目将熔炼凝固法用于碲化铋系多晶块体材料的制备，以P型碲化铋为研究对象，通过在其熔炼凝固过程中引入强磁场，充分研究了冷却速度、磁场强度及磁性和非磁性元素掺杂对材料的晶体取向、微结构、机械性能、电热输运性能和热电性能的影响等，有效提高了材料的热电性能和机械性能，为高性能多晶碲化铋基热电材料的高效制备和大规模应用奠定了重要基础。主要研究结论如下：.1. 采用真空熔炼法可以高效制备P型Bi0.5Sb1.5Te3块体热电材料，耗时约5小时，无需二次成型。.2. 冷却方式（炉冷、空冷、水冷和液氮淬火）可有效调控无磁场下制备的Bi0.5Sb1.5Te3块体材料的微结构及热电性能，其中空冷试样的ZT值在323K可达1.05。.3. 添加形核控制剂MoSi2可有效提高材料的热电性能和机械性能，其中MoSi2含量为0.2%wt的试样的ZT值在373K可达1.33，抗弯强度为28.0MPa。.4. 冷却速度对磁场下熔炼凝固制备Bi0.5Sb1.5Te3块体热电材料的热电性能和机械性能也具有较大的影响，随着冷却速度的提高，材料的热电优值ZT逐渐上升，其中冷却速度为16K/min的p型Bi0.5Sb1.5Te3在323K时具有最高的ZT值1.23，抗弯强度为20.7MPa。.5. 随着磁场强度的增加，Bi0.5Sb1.5Te3的过冷度逐渐减小，材料晶体的c轴朝垂直磁场方向转动，富Te条纹逐渐变小变长，并形成大量的BSTⅡ纳米棒，从而大幅度降低材料的热导率，提高材料的热电性能。磁场强度为2T时材料的ZT值在323K可达1.71..6. 磁性元素Fe、Ni掺杂对材料的生长具有强烈的扰乱作用，导致富BSTⅡ纳米棒散乱排列，生成Te纳米晶，从而有效降低材料的热导率。其ZT值在323K可达1.60。. 通过项目研究，丰富了块体多晶碲化铋热电材料的高效制备手段，掌握了冷却方式、添加剂对材料热电性能的影响规律，探索了磁场强度、冷却速度和掺杂元素对磁场下制备的碲化铋材料微结构和电热输运性能的影响，获得了高热电优值的块体多晶碲化铋热电材料。在该项目的资助下，在Adv.Energy Mater. Nano Energy、J. Mater. Chem. A、Intermetallics等知名学术期刊上发表SCI论文26篇，申请发明专利2项，培养毕业硕士/博士研究生13名，较好地完成了项目的研究任务。