Mg2Si-Mg2Sn混溶隙及具有共格纳米结构的Mg2Si-Mg2Sn热电材料的研究

Mg2Si-Mg2Sn based thermoelectric materials are promising for power generation in intermediate temperature range because of their advantages of high performance, non-toxicity, and abundant constituent elements. The ZT value of bulk Mg2Si-Mg2Sn can be greatly enhanced via embedded nanostructures with coherent interfaces that scatter phonons dramatically without interrupting the carrier transportation. The miscibility gap in the Mg2Si-Mg2Sn pseudo-binary system opens the door for preparing coherently nanostructured Mg2SixSn1-x composites，but published Mg2Si-Mg2Sn phase diagrams reveal large discrepancies in the gap edge. The goal of this project is to obtain a reliable miscibility gap of Mg2Si-Mg2Sn, and thus to introduce coherent nanostructures in Mg2SixSn1-x based alloys via in-situ phase separation near the gap edge. Various Mg2SixSn1-x alloys with stoichiometric compositions will be prepared from high purity Mg2Sn and Mg2Si single phases. Analysis of XRD, SEM, EDX, and TEM will be performed to observe the microstructure and to determine the compostion of the samples. Dedicated DTA/DSC experiments and in-situ high temperature X-ray diffraction analysis are to be carried out to investigate decisive phase transformation points. By comparing our data with those from literatures, a more reliabe miscibility gap of Mg2Si-Mg2Sn system will be obtained. And hence coherently nanostructured Mg2SixSn1-x based nanocomposites can be achieved by proper processing and choosing compositions near the gap edge. The microstructure, doping conditions, band structure, and thermoelectric properties of all samples will be investigated and the relations between each other will be analyzed. Results of this project may offer a promising way for microsture controllation and properies optimization to get highly effective thermoelectric materials.

Mg2Si-Mg2Sn基热电材料具有高效、无毒、原料丰富等优点，在中温发电领域具有重要的应用前景。在Mg2Si-Mg2Sn体材料中引入具有共格界面的纳米结构是实现声子、载流子的独立调控并提高ZT的有效途径。Mg2Si-Mg2Sn赝二元体系的混溶隙是制备具有共格纳米结构的Mg2SixSn1-x合金的关键基础，但现有的Mg2Si-Mg2Sn相图对混溶隙的边界存在很大争议。本项目拟利用高纯Mg2Si、Mg2Sn实现Mg2SixSn1-x体系的可控制备，采用DTA/DSC等热分析方法，结合高温原位X射线衍射、SEM、EDX、TEM等实验手段，对Mg2Si-Mg2Sn相图进行补充修正，获取准确的混溶隙；选取混溶隙附近的成分，设计合理实验流程以实现共格界面Mg2SixSn1-x基热电材料的可控制备；研究微观结构、掺杂条件、能带以及热电性能间的作用规律，为高效热电材料微结构设计及性能优化提供方向和指导。

在Mg2Si-Mg2Sn体材料中引入具有共格界面的纳米结构是实现声子、载流子的独立调控并提高ZT的有效途径。.本课题利用非平衡凝固和长时间退火的方法，在Sb或Bi掺杂Mg2SixSn1-x (x=0.3-0.7)热电材料中同时引入调制掺杂结构和共格纳米结构，实现了对声子和电子的独立调控。非平衡凝固使材料内形成层状梯度结构，每层具有不同Si/Sn比，Sb或Bi更多分布于Si含量较低的区域，形成调制掺杂结构。在该结构中，载流子从富Sn层向富Si层移动，迁移率升高的同时不影响总体电子浓度，使样品具有非常高的功率因子。Sb掺杂Mg2SixSn1-x合金获得了高达6.8×10-3 W/mK2的功率因子。退火处理使层状结构内部析出大量弥散分布的、晶界与基体共格的纳米晶粒。共格纳米结构能散射中低频声子从而降低晶格热导率，但电子传输不受影响。Bi掺杂Mg2SixSn1-x合金获得了低达0.4 W/mK的晶格热导率。两种结构的共同作用使材料的ZT值得到大幅度提高。Sb掺杂、Bi掺杂Mg2SixSn1-x的最高ZT分别达到 1.5、1.45，超过文献报道的同类材料最高值。.为研究Mg2Si-Mg2Sn的混溶隙，采取两类方法获得平衡成分，一是将凝固样品进行超长时间退火（超过90h），二是将凝固样品高能球磨后热压。只要样品中不出现过量Sn，所获得Mg2SixSn1-x（0.1≤x≤0.9）的XRD都显示为相应的单相固溶体，再次退火或长时间存放后样品性能稳定。一旦出现过量Sn，则样品分解为多相，长时间存放会分解为粉末。样品的稳定与否与配比成分x无关，而取决于气氛、水分、设备等因素。因此我们认为Mg2Si-Mg2Sn赝二元体系中可能不存在混溶隙，文献报道的关于Mg2Si-Mg2Sn混溶隙的不同结果，最根本原因在于该体系成分的难于控制。.利用高能球磨和射频感应热压方法将上述具有调制掺杂结构和共格纳米结构的Mg2SixSn1-x合金制成均匀的纳米晶材料。一方面，高能球磨引入的大量纳米晶对中长程声子形成有效散射而使热导率大幅度降低，但同时散射电子导致电导率降低。球磨时间分别为1小时、3小时、12小时的Bi掺杂Mg2SixSn1-x热压样品的最高ZT值分别为1.3、1.1和0.9。高能球磨后热压得到的纳米晶材料具有非常好的稳定性。.本课题的研究成果，为高效热电材料微结构设计及性能优化提供了方向和指导。