MgAgSb化合物的晶体结构及热电性能

发布时间：2018-03-26 08:07

  本文选题：热电材料　切入点：MgAgSb　出处：《浙江大学》2017年博士论文

【摘要】：MgAgSb合金是近年来新开发的近室温高性能热电材料,具有组成元素储量丰富的优点。但是仍存在许多问题,譬如由于存在多晶型转变以及Mg的高活性,高性能纯相α-MgAgSb难以获得;电学性能尚未优化,MgAgSb基热电材料具有较低热导率然而内在物理机制尚不明确;高温相理论预测与实验结果不相符等。本文重点研究纯相α-MgAgSb的制备,电学性能优化,本征低热导率的内在机理及不同晶型MgAgSb的晶体结构与热电性能,取得如下成果:(1)通过控制制备工艺,成功获得纯相α-MgAgSb,并通过In掺杂调节体系的载流子浓度并研究了该体系基本物理特性。利用第一性原理计算了 α-MgAgSb的能带结构,其为窄禁带间接带隙半导体,禁带宽度Eg～0.26eV,价带顶具有高能谷简并度NV=8。采用SPB模型计算得到该体系的态密度有效质量m*= 2.0me,单带有效质量mb*=0.5me。利用变形势理论结合实验结果得到该体系的变形势E=20.0eV。In掺杂的α-MgAgSb在525K获得最高zT～1.1,分析得到该体系的最优载流子浓度为8～9×1019cm-3。(2)基于晶格动力学和声子传输理论,从化学键角度解释了 α-MgAgSb本征低晶格热导率的起因,即全局和局域弱化学键共同存在是非笼状结构Nowotny-Juza化合物α-MgAgSb本征低晶格热导的物理机制。低价电子数(VEC=8)引起α-MgAgSb全局弱键,表现为低声速;α-MgAgSb扭曲立方结构中独特的Mg-Ag-Sb三中心局域弱键引起低频光学支的共振散射。低温热性能测量和模型分析也证明爱因斯坦低频振动模的存在。这类分级化学键特征可以推广到其他Nowotny-Juza化合物,晶格动力学计算表明Nowotny-Juza化合物CdCuSb和CaAgSb也具有低声速和低频光学声子的特点。(3)通过变温高分辨同步辐射粉末X射线衍射(SR-PXRD)分析了 MgAgSb的多晶型晶体结构特征与相变。Sb和Ag3Sb杂质出现在400-600 K的高温下,含量随温度升高而升高。MgAgSb结构对称性随温度升高。两种MgAgSb多晶型物(β-MgAgSb和γ-MgAgSb)在700K下共存,但在高温(800-1000K)下仅发现立方结构γ-MgAgSb相。y-MgAgSb相的结晶度随温度增加而降低,样品在1000 K时晶体仅为43.8%。杂质液化很大程度上影响了 y-MgAgSb的稳定性。γ-MgAgSb的高分辨率粉末数据可以使用Ⅰ型(Mg,Ag和Sb分别占在4b,4c和4a位置)和Ⅱ型(Mg,Ag和Sb分别占在4a,4b和4c位置)的half-Heusler晶体结构模型。对精修的结构因子进行的最大熵法表明,Ⅱ型给出了更多的物理声学电子密度。热电性能测量γ-MgAgSb具有半导体特征,与Ⅱ型结构模型结果更加相符合。
[Abstract]:MgAgSb alloy is a newly developed thermoelectric material with high performance near room temperature in recent years, which has the advantage of abundant element reserves. However, there are still many problems, such as the existence of polycrystalline transformation and the high activity of mg, which makes it difficult to obtain high performance pure phase 伪 -MgAgSb. The electrical properties of MgAgSb based thermoelectric materials have not been optimized but the intrinsic physical mechanism is not clear. The theoretical prediction of high temperature phase does not accord with the experimental results. In this paper, the preparation of pure phase 伪 -MgAgSb, the optimization of electrical properties, etc. The intrinsic mechanism of intrinsic low thermal conductivity and the crystal structure and thermoelectric properties of different crystalline MgAgSb have been obtained as follows: 1) by controlling the preparation process, The pure phase 伪 -MgAgSb was successfully obtained, and the carrier concentration of the system was adjusted by in doping and the basic physical properties of the system were studied. The band structure of 伪 -MgAgSb was calculated by first principle, which is a narrow gap band gap semiconductor. The band gap is 0.26eV, and the valence band top has high energy valley degeneracy (NVN 8). The effective mass of the system is calculated by SPB model, and the effective mass of the system is calculated by SPB model, and the effective mass of the system is 0.5me. by using the theory of variable situation and the experimental results, the variable state E=20.0eV.In of the system is obtained. The highest concentration of 伪 -MgAgSb was obtained at 525K, and the optimum carrier concentration of the system was 89 脳 1019cm-3.2) based on lattice dynamics and phonon transport theory. The cause of the intrinsic low lattice thermal conductivity of 伪 -MgAgSb is explained from the point of view of chemical bond, that is, the global and local weak chemical bonds exist together, the physical mechanism of 伪 -MgAgSb intrinsic low lattice thermal conductivity of non-cage structure Nowotny-Juza compound 伪 -MgAgSb is explained. The global weak bond of 伪 -MgAgSb is induced by low valence electron number (VEC8). The resonance scattering of the low frequency optical branch caused by the unique local weak bond of Mg-Ag-Sb in the twisted cubic structure of 伪 -MgAgSb. The low temperature thermal performance measurement and model analysis also proved the existence of Einstein's low frequency vibration mode. Chemical bond characteristics can be extended to other Nowotny-Juza compounds, Lattice dynamics calculations show that Nowotny-Juza compounds CdCuSb and CaAgSb also have the characteristics of low sound velocity and low frequency optical phonons. The crystal structure and phase transition of MgAgSb have been analyzed by means of high resolution synchrotron radiation powder X-ray diffraction (SR-PXRD). The impurities of Sb and Ag3Sb appear at 400-600K high temperature. The content of MgAgSb polycrystalline compounds (尾 -MgAgSb and 纬 -MgAgSb) coexisted at 700K, but the crystallinity of cubic structure 纬 -MgAgSb phase 路y-MgAgSb phase decreased with the increase of temperature. The crystal size of the sample at 1000 K is only 43.8%. The liquefaction of impurity greatly affects the stability of y-MgAgSb. The high resolution powder data of 纬 -MgAgSb can be used in the positions of 4b 4c and 4a for 纬 -MgAgSb) and 4a4b and 4a for type 鈪，

本文编号：1667088