【作者】 姜洪义；

【导师】 张联盟；

【作者基本信息】 武汉理工大学 ， 材料学， 2003， 博士

【摘要】 本论文在综合评述国内外对Mg-Si基热电材料研究进展的基础上，选择低温固相反应合成Mg-Si基热电化合物、采用放电等离子烧结（SPS）工艺制备Mg-Si基热电材料。系统研究了n型、p型Mg-Si基热电化合物的合成方法，合成品质及其添加物的固溶行为与控制方法，测定并分析了两类热电材料的热电性能及其与组成、掺杂之间的关系，得到了若干研究结果。 论文应用热力学理论对Mg-Si基热电化合物固相反应过程中涉及的Mg-Si-O-C体系进行了热力学计算，探讨了固相反应发生的可能性及工艺制度对材料组成、性能的影响。结果表明：Mg颗粒与Si颗粒之间的反应是固相反应，杂质的引入是由于Mg元素高度的活泼性能而混入的MgO、Mg₃C₂等气相反应的产物。因此必须对工艺过程加以控制以避免杂质的产生和生成的Mg₂Si被氧化。同时由于Mg极易蒸发，其蒸发量与掺杂元素的含量相当，只要控制样品配料时考虑蒸发的Mg，就可以保证产物的化学计量。 应用差热分析、X射线衍射、电子探针元素分析、X射线荧光分析等手段详细探索了固相反应法制备Mg-Si基热电化合物的工艺制度。结果表明：通过固相反应，Mg₂A（A=Si；Ge）单相化合物可以在823K反应温度下保温8小时获得；各种组分的Mg₂Si_1-xGe_x（0＜x＜1）产物不仅晶型完整、无碳化、无氧化、分布均匀，而且颗粒大幅度细化。在1073K反应温度下再次保温8小时后，可以得到在全组成范围内连续变化的Mg₂Si_1-xGe_x固溶体。 探讨了反应温度、保温时间、掺杂的先后次序对样品热电性能的影响。结果表明：在823K下进行固相反应并保温8小时是合适的，对Mg₂Si这种n型半导体材料而言，Te和Sb的掺入顺序对其热电性能几乎没有影响。 选择Te、Sb、Ag元素作为掺杂对象，研究了掺杂元素种类和掺杂量对MR₂Si基热电材料热电性能的影响。结果表明：掺杂Te、Sb元素得到的Mg₂Si基热电材料仍为n型半导体；掺杂Ag元素得到的Mg₂Si基热电材料转变为p型半导体。在Mg₂Si热电材料基体中掺杂Te、Sb元素后，在结构中引入了缺陷，增大了体武汉理工大学博士学位论文系中载流子有效质量，提高了Seebeck系数;降低了体系导电活化能，提高了电导率，同时也降低了热导率。所以能够大幅度提高Mg一Si基热电体系的整体热电性能。 掺入4000PpmTe的样品的优值系数在55oK时达到0.69 xlo一3K一’，无量纲优值zT为0.35;而掺入s000ppmsb的样品在65oK达到了1 .02 x 10一，K“，无量纲优值ZT为0.66。分别是未掺杂的MgZSi试样的1 .8倍和3 .1倍尽上。尤其是掺入S000ppmsb的样品，其性能己经达到现有报导的最高值。 论文还首次在Mg一Si基热电材料中引入BN纳米粒子第二相，结果表明:在Mg一Si基热电材料体系中引入BN纳米粒子第二相能有效降低材料的热导率，提高热电材料的优值系数。更多还原

【Abstract】 On the basis of the main progress about Mg-Si-based thermoelectric materials, solid-state reaction was selected in the present paper to synthesis Mg2Si-based compounds at low temperatures followed by SPS method for the preparation. The synthesis method and properties of n-type and p-type Mg2Si-based thermoelectric materials as well as the solid solution behavior and control method of the dopants. were studied systematically. The thermoelectric properties of two type materials were mainly tested and analyzed, and some innovative results were then obtained.At first, basic thermodynamic theories were introduced to calculate the Gibbs function changes of Mg-Si-O-C system, which was an effective tool for verifying the variation of composition during the solid-state reaction process. The possibility of the occurrence for solid-state reaction was discussed. The influence of processing parameters on the compositions and properties of the products was explored too. The results showed the reaction between the Mg and Si powders was solid-state reaction and impurities such as MgO, Mg3C2: were usually produced resulting from the reactions with air or graphite crucible since Mg was very active. So the synthesis process must be optimized to avoid the formation of the above impurities and the oxidation of the synthesized Mg2Si. Furthermore, since Mg had great evaporation ability and the total evaporation amount of Mg metal was accessing to the amount of dopants, it must be taken into account when the raw powders were weighted.DTA, XRD, EPMA. X-ray fluorescence analysis and SEM were adopted in the evaluation of structure, composition and morphology. The processing parameters were then decided by the measured results. The results showed that Mg2A (A=Si or Ge) compounds as well as Mg2Si1-xGex solid solution without any carbonization or oxidation were successfully synthesized at 823K for 8hrs under a high purity Ar atmosphere. The products were integrated crystals and their particle sizes were distributed uniformly. After a further reaction at 1073K for 8hrs, Mg2Si1-xGex (x changed from 0 to 1) solid solutions were obtained. The influences of holding time, reaction temperature, particle size of the products and doping sequence on thermoelectric properties of Mg2Si were discussed in detail.Te, Sb and Ag were selected as the dopants in the synthesizing process of Mg2Si, and the influence of dopants and dopants’ amount on thermoelectric properties of Mg2Si specimen was studied. The results showed that the Mg2Si-based thermoelectric material doped with Te and Sb was still n-type, but that doped with Ag changed into p-type. With the introduction of the dopants, the effective mass of carriers was changed and the Seebeck coefficient was increased. At the mean time, the dopants reduced the forbidden energy gap, which changed the carrier concentration and thus increased electrical conductivity. By calculating the forbidden energy gap and electrical conductivity of Mg2Si specimen doped with different amount Sb, the mechanism of transference changed abruptly at 625K. The value of the gap decreased with the increasing of dopants, so the energy needed for the electron transference was reduced. As a result, the electric conductivity of Mg2Si was increased with the increasing of the dopant.The figure of merit of the Mg2Si samples doped with 4000ppn Te was 0.69 X 10-3K-1 at 550K, while that of the samples with 5000ppm Sb reached a peak of 1.02 X 10-3K-1 at 650K, which seemed to be the best results ever reported for Mg-Si system till now.In addition, the nanometer-size particles of BN was doped into the Mg-Si-based thermoelectric materials as the second-phase for the first time. The result showed that the introduction of nano BN could decrease the thermal conductivity of the Mg2Si-based thermoelectric material and as a result its power factor were improved.更多还原