【作者】 李鑫；

【导师】 李双明；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2018， 博士

【摘要】 热电材料是一种可以实现热能和电能的相互转换的新型功能材料,由于其无污染、无噪音、可靠性高和反应灵敏等优点在航空航天、温差发电和制冷等领域有广泛的应用前景。Mg₂（Si,Sn）中温热电材料的成本低廉且无毒无污染,近年来受到极大的关注,但由于Mg元素极易挥发和氧化,以及原料中各元素极大的熔点差导致其合成和制备非常困难。针对这一问题,本文采用部分熔化的高温度梯度区熔定向凝固方法进行探索,在平界面凝固下获得高质量的单相Mg₂（Si,Sn）晶体,并结合第一性原理计算对载流子输运特性进行研究并预测最优热电性能,为制备高性能的Mg₂（Si,Sn）基热电材料提供理论依据和可靠方法。研究中先对简单的二元Mg₂Sn合金定向凝固工艺参数、凝固机理、晶体生长特性和微观组织形貌进行探讨,同时结合第一性原理计算对Mg₂Sn单晶的电子结构、力学性能、热力学和动力学参数,以及其传导方式的各向异性进行研究。基于二元Mg₂Sn合金的研究结果,对不同成分下Mg₂Si_1-xSn_x单晶的能带收敛效应及其对本征热电性能的影响进行讨论,在考虑Seebeck系数各向异性的基础上对热电性能进行优化,所得的结论主要有:在高温度梯度(200 Kcm^-1)和1μms^-1的抽拉速率下得到单相的Mg₂Sn晶体;与传统Bridgman法和Radio-frequency（RF）感应熔炼法制备的结果相比,晶体的热电性能得到了显著提升,Seebeck系数最大值可达-261μVK^-1,高的Seebeck系数值使得功率因子提高了近3倍,达到0.3 mWm^-1K^-2,并且由于去除了第二相Sn的影响,热导率也有了显著的降低。通过第一性原理计算对Mg₂Sn晶体进行基础研究,表明本征条件下晶体以n型传导为主,沿[111]方向上原子排布方式为Sn-Mg-Mg-Sn,其中Mg-Sn键为较强的共价键,而较弱的Mg-Mg则依靠范德瓦尔斯力连接,这种复杂的化学键排布方式有利于获得更高的热电性能。基态下Mg₂Sn表现出非金属特性,为脆性材料,弹性常数各向异性因子的计算结果表明,Mg₂Sn晶体沿[111]方向有望获得更好的机械性能。高温度梯度定向凝固法制备的Mg₂Sn单晶沿生长方向[111]和其垂直的11 0和112方向的Seebeck系数表现出很强的各向异性,存在传导方式随取向变化的OCSR（Orientation-dependent Conduction Sign Reverse）现象,而电导率和热导率则表现为各向同性。第一性原理计算还表明三元Mg₂Si_1-xSn_x固溶体由二元合金的间接能隙半导体转变为直接能隙半导体,价带的最高点和导带的最低点均落在第一Brillouin区的中心G点,随着Sn含量的增加带隙逐渐减小;导带底部靠近Fermi能级处的两条导带随着Sn含量的增加发生相对移动,其间距ΔE值逐渐减小,当Sn含量x=0.625时,ΔE达到最小值0.008 eV,两条导带发生收敛,且能带的收敛有利于Seebeck系数的提高,理论计算预测结果表明,其最大值在n型和p型传导条件下分别可达-520μVK^-1和345μVK^-1,对应的最大PF值分别为6.7 mWm^-1K^-2和4.5 mWm^-1K^-2,因此Mg₂Si_1-xSn_x固溶体更适合n型掺杂。高温度梯度定向凝固法制备的Mg₂Si_1-xSn_x固溶体的固溶区间为x≥0.6,成分为x=0.65和0.75时,得到了均匀的单相组织,未掺杂条件下的Mg₂Si_1-xSn_x单晶Seebeck系数最高可达-518μVK^-1,功率因子值为2.56 mWm^-1K^-1,相比固相反应法制备的纳米材料提高了2.5倍;Sb掺杂的Mg₂Si_0.35Sn_0.65单晶沿生长方向[111]的Seebeck系数值在测试温度范围内要高于其垂直方向[11 0],其最大值可达-265μVK^-1,Seebeck系数大小的的各向异性主要受载流子有效质量的影响,[111]方向上掺杂浓度为1.25at%Sb的单晶在600 K时功率因子最大值可达5.56 mWm^-1K^-2,这一结果与之前报道的最高值相比提高了20%,对应的ZT值在730K时为1.3,研究结果表明通过制备单晶材料,利用Seebeck系数各向异性来提高功率因子是有效的优化Mg₂Si_1-xSn_x热电材料性能的方式。更多还原

【Abstract】 Thermoelectric materials can provide the energy conversion between heat and electricity.This new type of functional material has a broad application prospects in the fields of aerospace,power generation,and refrigeration,because of its pollution-free,noiseless,reliable,and responsive.In recent years,medium temperature thermoelectric material Mg₂（Si,Sn）has received great attention due to its low cost and non-toxic.However,the chemical activity and volatile of Mg,as well as the great melting point difference of the elements,results in the difficulty in synthesis and preparation.Concerning this issue,high temperature gradient directional solidification（HGDS）method is used in this paper to prepare high quality Mg₂（Si,Sn）crystal by controlling the planar interface growth.First principles calculation is employed to research the carrier transport properties and predict the optimal thermoelectric performance.The results provide theoretical basis for the preparation of high performance Mg₂（Si,Sn）thermoelectric materials.Mg₂Sn binary alloy is firstly researched including directional solidification process parameters and mechanism,properties of crystal growth,and microstructure morphology.Combining with the first principles calculation,the electronic structure,mechanical properties,thermodynamic and kinetic parameters of Mg₂Sn single crystal are studied.Based on the results of Mg₂Sn,the influence of band convergence in Mg₂Si_1-xSn_x single crystal is discussed.Additionally,thermoelectric performance is optimized on the basis of the anisotropic Seebeck coefficient.We obtained the following results:At high temperature gradient(200 Kcm^-1),and the single-phase Mg₂Sn crystal is obtained at 1μms^-1.The thermoelectric performance is dramatically improved comparing with the crystals prepared by conventional Bridgman and Radio-frequency（RF）induction melting method.The maximum of Seebeck coefficient is-261μVK^-1,and the increased values result in the power factor enhanced about 3 times.The thermal conductivity also significantly reduced without the influence of the second phase Sn.First principles calculation is employed to make a basic research for Mg₂Sn crystal.The dominated conduction type is electron in the intrinsic condition for Mg₂Sn.The atoms arrangement in[111]orientation is Sn-Mg-Mg-Sn,including the strong Mg-Sn covalent bonds and weaker Mg-Mg bonds.This complex arrangement of chemical bond is helpful to obtained higher thermoelectric performance.Under the ground state,the brittle Mg₂Sn crystal shows the non-metallic features.The anisotropic factor of elastic constants indicates that,the crystal along[111]orientation is expected to get better mechanical properties.The Seebeck coefficient of Mg₂Sn single crystal prepared by HGDS shows a strong anisotropy in the direction of growth direction[111]and its perpendicular direction11 0 and 112,and appears the phenomenon of orientation-dependent conduction sign reverse（OCSR）.The electrical and thermal conductivities are isotropic.In addition,first principles calculation results indicates that,ternary Mg₂Si_1-xSn_x solid solutions are direct bandgap semiconductors,and the extreme points of valence band and conduction band fall on the G point of the first Brillouin zone.The distance between the two lowest conduction bandsΔE is changed with the variation of Sn contents.The minimum ofΔE is 0.008 eV at x=0.625,which indicates convergence of the two conduction bands.The convergent band is beneficial to the improvement of Seebeck coefficient.Prediction results of theoretical calculation indicate that,the maximum of Seebeck coefficient for n and p type conduction is-520μVK^-1 and 345μVK^-1,respectively.The corresponding value of power factor is 6.7 mWm^-1K^-2 and 4.5mWm^-1K^-2,and the n-type doping is more suitable for the Mg₂Si_1-xSn_x solid solutions.The single-phase Mg₂Si_1-xSn_x single crystals are obtained at x=0.65 and 0.75 by HGDS method.The maximum of Seebeck coefficient and power factor for undoped Mg₂Si_1-xSn_x single crystal is-518μVK^-1 and 2.56 mWm^-1K^-1,respectively.It increase more than 2.5 times compared with the nanocrystalline prepared by solid state reaction method.The Seebeck coefficients of Sb-doped Mg₂Si_0.35Sn_0.65 single crystals in growth direction[111]are higher than its perpendicular direction[11 0]in the measured temperature range,and the maximum is-265μVK^-1.The top value of power factor of1.25 at%Sb doped crystal is 5.56 mWm^-1K^-2 at T=600K,and this result improved about 20%comparing with the maximum of previous reports.The corresponding value of ZT is 1.3 at730 K.The results of research show that,the method of growing single crystal using the anisotropic Seebeck coefficient is a effective way to improve the performance of Mg₂Si_1-xSn_x thermoelectric materials.更多还原