MgAgSb基合金的组织结构与热电性能

发布时间：2018-07-07 08:55

本文选题：热电材料 + MgAgSb合金　；参考：《哈尔滨工业大学》2017年博士论文

【摘要】：MgAgSb合金在300~548K温度区间具有较低的本征热导率,但载流子浓度较低,尚未达到最佳范围,功率因子和热电优值仍有较大提升空间。此外,该合金的电热输运机制尚不清楚,制约进一步的性能优化。本文主要通过元素掺杂增大MgAgSb基合金的载流子浓度,提高功率因子和热电优值。利用X射线衍射仪、透射电子显微镜、Seebeck系数/电导率综合测试系统、闪光导热仪和综合物性测量系统等手段主要研究了烧结温度、合金成分和Mg位掺杂(Li、Ca和Yb)对MgAgSb基合金的微观组织、输运特性和热电性能的影响规律,采用单能带抛物线理论结合声学支散射模型和Debye-Callaway模型及结构分析查明电热输运机制,揭示掺杂改性机理。研究发现,烧结温度对MgAg_(0.97)Sb_(0.99)合金的晶粒尺寸、相组成和热电性能有明显影响。当烧结温度不高于573K时,MgAg_(0.97)Sb_(0.99)合金为纯相,晶粒尺寸较小(约150nm);当烧结温度为593K时,晶粒尺寸显著增大(1~5μm);当烧结温度为613K时,出现微量第二相。随烧结温度升高,载流子浓度先降低后上升,功率因子先减小后增大,总热导率单调增大,热电优值下降。通过优选合金成分,即增加Sb含量,增大了MgAg_(0.97)Sb_(0.99+x)合金的载流子浓度,提高了功率因子。当x不高于0.005时,合金为纯相;当x为0.01时出现Sb纳米颗粒。Sb含量增加,载流子浓度与电导率先显著增大后略减小,功率因子单调升高,其中室温值由18.2μWcm-1K-2升至22.9μWcm-1K-2。同时总热导率也增大,热电优值未见提高,但高功率因子有利于提高热电器件的输出功率密度。采用理论模型计算确定MgAg_(0.97)Sb_(0.99)合金的最高功率因子对应的最佳载流子浓度为9.0×1019cm-3,未掺杂合金的载流子浓度仅为2.7×1019cm-3,通过Li掺杂大幅度增大Mg1-xLixAg_(0.97)Sb_(0.99)合金的载流子浓度,显著提高功率因子。随Li掺杂量增加,功率因子先升高后降低,当x=0.01时出现峰值,室温为24.0μWcm-1K-2;同时晶格热导率先减小后增大,最终平均热电优值(300~548K)略有提高,从1.0(x=0)升至1.1(x=0.01)。采用Ca或Yb掺杂增大Mg_(1-x)M_xAg_(0.97)Sb_(0.99)(M=Ca或Yb)合金的载流子浓度,降低晶格热导率,提高热电优值。Ca掺杂量增加,Mg1-xCaxAg_(0.97)Sb_(0.99)合金的载流子浓度单调增大,功率因子升高;产生晶格畸变,降低晶格热导率。Yb掺杂量增加,Mg1-xYbxAg_(0.97)Sb_(0.99)合金的载流子浓度先升高后基本不变,功率因子先升高后略降低;晶格热导率先显著降低后升高,热电优值在x=0.005时达最大,最高热电优值在548K为1.4。相比Ca掺杂,相同量Yb掺杂造成更强烈的晶格畸变,显著降低晶格热导率,明显提高热电优值。采用高能球磨结合快速烧结方法制备出纳米结构的MgAgSb合金,通过引入高密度的空位晶界、层错、位错等缺陷,大幅度增强了声子散射概率,降低了晶格热导率。MgAg_(0.97)Sb_(0.99)合金的本征载流子浓度远低于理论最佳值,主要通过元素掺杂增大载流子浓度,提升功率因子,同时晶格畸变降低晶格热导率,提高热电优值。
[Abstract]:The MgAgSb alloy has a lower intrinsic thermal conductivity at the temperature range of 300~548K, but the carrier concentration is low, and it has not reached the optimum range. The power factor and the thermoelectric merit still have a larger lifting space. In addition, the electrothermal transport mechanism of the alloy is not yet clear, which restricts the further performance optimization. This paper mainly increases the MgAgSb bonding through the doping of elements. X ray diffractometer, transmission electron microscope, Seebeck coefficient / conductivity comprehensive testing system, flash thermo thermal conductivity meter and comprehensive physical measurement system are used to study the sintering temperature, alloy composition and Mg doping (Li, Ca and Yb) on the microstructure of MgAgSb based alloys. The influence laws of transport characteristics and thermoelectric properties are investigated by using the single energy band parabola theory, the acoustic support scattering model and the Debye-Callaway model and the structural analysis to find out the mechanism of the electric heat transport and reveal the mechanism of doping modification. It is found that the sintering temperature has a significant influence on the grain size, phase composition and thermoelectric properties of MgAg_ (0.97) Sb_ (0.99) alloy. When the sintering temperature is not higher than 573K, the MgAg_ (0.97) Sb_ (0.99) alloy is a pure phase, and the grain size is smaller (about 150nm). When the sintering temperature is 593K, the grain size increases significantly (1~5 m). When the sintering temperature is 613K, the second phase appears. With the sintering temperature rising, the carrier concentration decreases first and then rises, the power factor decreases first and then increases, the total thermal conductivity is increased. The total thermal conductivity is increased. By optimizing the alloy composition, that is, increasing the Sb content, increasing the carrier concentration of MgAg_ (0.97) Sb_ (0.99+x) alloy and increasing the power factor. When x is not higher than 0.005, the alloy is pure phase, and when x is 0.01, the Sb nanoparticles.Sb content increases, and the carrier concentration and electrical conductivity first increase significantly after a significant increase in the carrier concentration and conductivity. The power factor increases monotonously, in which the room temperature rises from 18.2 to 22.9 Wcm-1K-2 to 22.9 Mu and the total thermal conductivity increases, and the thermoelectric merit is not improved, but the high power factor is beneficial to the increase of the output power density of the thermoelectric device. The optimum carrier of the maximum power factor of MgAg_ (0.97) Sb_ (0.99) alloy is determined by theoretical model calculation. The concentration is 9 x 1019cm-3, the carrier concentration of the undoped alloy is only 2.7 x 1019cm-3. The carrier concentration of Mg1-xLixAg_ (0.97) Sb_ (0.99) alloy is greatly increased by Li doping, and the power factor is significantly increased. With the increase of Li doping, the power factor rises first and then decreases, when the X =0.01 is at the peak value and the room temperature is 24 um Wcm-1K-2; and the lattice heat is at the same time. The ultimate average thermoelectric value (300~548K) increases slightly from 1 (x=0) to 1.1 (x=0.01). The carrier concentration of Mg_ (1-x) M_xAg_ (0.97) Sb_ (0.99) (M=Ca or Yb) alloy is increased by Ca or Yb doping, the thermal conductivity of the lattice is reduced, the doping amount of the thermoelectric value increases, and the carrier concentration of the alloy (0.97) (0.99) is increased. With the increase of degree monotonicity and the increase of power factor, lattice distortion and lattice thermal conductivity.Yb doping increase, the carrier concentration of Mg1-xYbxAg_ (0.97) Sb_ (0.99) alloy increases first, and the power factor rises first and then decreases slightly; the thermal conductivity of lattice is the first to decrease, and the thermoelectric value reaches the maximum at x=0.005, and the highest thermoelectric merit is at the highest value. 548K doping with the same amount of Yb leads to more intense lattice distortion of the same amount of Yb, which significantly reduces the thermal conductivity of the lattice and obviously improves the thermoelectric value. The nano structure MgAgSb alloy is prepared by high energy ball milling and rapid sintering. By introducing high density vacancy grain boundary, stacking error, dislocation and other defects, the phonon scattering is greatly enhanced. The intrinsic carrier concentration of.MgAg_ (0.97) Sb_ (0.99) alloy is much lower than that of the theoretical optimum. The carrier concentration increases mainly through the doping of the element, and the power factor is increased. The lattice distortion reduces the thermal conductivity of the lattice and improves the thermoelectric value.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG146.22

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