【作者】 姜广宇；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料科学与工程， 2014， 博士

【摘要】 Mg2(Si,Sn)基合金热电材料具有成本低廉、环境友好等特点,是一类本领域同行广泛关注的新型绿色热电材料。但相比较目前性能最佳的热电材料而言,这个体系还存在制备困难、固溶限尚未确定、晶格热导率偏高、P型材料的性能太低等问题。本文重点研究了Mg2(Si,Sn)合金的固溶限、点缺陷和热电性能,取得如下主要成果：(一)确定了不同制备方法下Mg2Si1-xXSnx合金材料中的固溶区间。本文采用助熔剂法和钽管封装法制备了Mg2Sii-xSnx(x=0.2,0.4,0.5,0.6,0.8)的合金试样,根据XRD和EPMA分析,确定了MgzSi-Mg2Sn合金材料在不同制备方法下的固溶限。研究发现MgzSi1-xSnx材料中存在Mg空位和间隙Mg等点缺陷,点缺陷Mg空位的存在导致了第二相的产生,并对材料的热电性能有着显著的影响。(二)在Mg2Si0.4Sn0.6-xSbx体系中提出了点缺陷工程的概念。为了在保持电性能的前提下降低Mg2Si1-xSnx合金材料的晶格热导率,本文首次通过大量掺杂Sb和调节Mg含量,控制Sb掺杂原子、Mg空位和间隙位Mg原子三种点缺陷在合金材料中的协调作用,有效降低了晶格热导率并同时优化了载流子浓度,显著提升了材料的热电性能。研究发现,Mg2Si0.4Sn0.6-xSbx体系中,Mg空位和间隙位Mg原子可以稳定共存,通过调整Sb和Mg含量可以在一定范围内控制Mg空位和间隙位Mg原子浓度。Mg空位是一种有效的声子散射中心,对降低材料声子热导率具有显著作用。通过添加Zn,可以调节材料电子结构、优化材料电学性能。Zn掺杂Mg2Si0.4Sn0.5Sb0.1的ZT值在750K达到1.1以上。(三)通过受主掺杂和赝三元合金化提高P型Mg2X(X=Si,Ge,Sn)合金的热电性能。研究发现, Ag是一种比B更有效的受主杂质。通过控制材料中间隙Mg原子含量、减少材料的少子浓度,将材料的热电优值提高了35%。对Mg2(Si0.33Ge0.33Sn0.33)和Mg2(Si0.2Ge0.2Sn0.6)两个赝三元合金体系的高温霍尔测试结果表明,两个体系中电子/空穴迁移率的比值相差不大,但是Mg2(Si0.33Ge0.33Sn0.33)的电导率和热导率具有相对优势,更适合作为P型掺杂的基体。Ag掺杂Mg2(Si0.33Ge0.33Sn0.33)的ZT值达到0.4左右,是至今报道的P型Mg2Si基材料热电性能的最高值。更多还原

【Abstract】 Mg2(Si,Sn)-based alloy system is a promising mid-temperature TE material and being paid increasing attention because of its low cost, environmental friendliness and high performance. However, compared with the best thermoelectric materials, there are still some problems to be solved in Mg2(Si,Sn)-based materials. Firstly, it’s too hard to synthesize homogeneous and stoichiometric samples, and the miscibility gap of the alloys is still in controversy. Secondly, the high lattice thermal conductivity still suppresses their thermoelectric performance. Thirdly, the unmatched performance of n-type and p-type materials greatly limits their commercial application due to the quite low p-type ZT. Aiming at these problems, we systematically investigated the solid solubility, the point defects of Mg2(Si,Sn) alloys and improved the thermoelectric performance of n-type and p-type alloys. The main results are summarized as follows:(Ⅰ)We employed two preparation methods:B2O3flux-assistant melting and Ta-tube shielded melting followed by quenching to determine the miscibility gap of the solid solutions. Mg2Si1-xSnx(x=0.2,0.4,0.5,0.6,0.8) alloy samples were synthesized, whose XRD patterns showed their phase is strongly dependent on the cooling rate of quenching. Comparing with the previous results, we redetermined miscibility gap of the solid solutions of Mg2S1and Mg2Sn, and their binary phase diagram was reshaped. Two kinds of dominant point defects were found in the system:Mg interstitials and Mg vacancies, which can tune the electrical conductivities and reduce the thermal conductivities, respectively. The two point defects were believed to impose significant and direct impacts on the thermoelectric performance of Mg2(Si,Sn) system, which are more remarkable than those of the phase compositions and microstructures.(Ⅱ) It is first time that the concept of "point defect engineering" is proposed to reduce the thermal conductivities of Mg2Si1-xSnx alloys. We simultaneously controlled the content of defects in the alloys, i.e. Sb substitution, Mg vacancies and Mg interstitials, by adjusting the Mg content and Sb substitution on Si sites. As a result, two desirable effects were realized:the electrical properties were improved due to the optimized carrier concentration and the thermal conductivities were greatly reduced by the enhanced point scattering on phonons. It was proved that unlike Frenkel defects Mg vacancies and Mg interstitials can coexist in Mg2Si1-xSnx solid alloys instead of recombining. In the system, Mg interstitials mainly act as donors to increase carrier concentration while two roles are played by the Mg vacancies, one is to tune the carrier concentration as acceptors and the other one is to reduce the thermal conductivity as point scattering centers. The point defect engineering can be carried out through controlling the content of Sb and Mg. The electrical properties can be optimized by Zn doping and the maximum state-of-the-art figure of merit ZT>1.1was attained at750K in Mg2Si0.4Sn0.5Sb0.1specimen.(Ⅲ) We improved the thermoelectric performance of P-type Mg2X(X=Si,Ge,Sn) based materials by two methods:acceptor doping and pseudo-ternary alloying. It was found that Ag is a more effective acceptor dopant than B. Mg content was controlled to reduce the electron concentration. The ZT of p-type was increased by35%. The results of high temperature Hall measurements showed that the mobility ratio of electrons to holes of Mg2(Si0.33Ge0.33Sn0.33) and Mg2(Si0.2Ge0.2Sn0.6) pseudo-ternary alloys is almost the same. Mg2(Si0.33Ge0.33Sn0.33) is a better P-type thermoelectric system, which exhibits lower thermal conductivity and higher electrical properties. The reduction in thermal conductivities of Ag doped Mg2(Si0.33Ge0.33Sn0.33) resulted in an increase in ZT and the maximum figure of merit was0.4, which was the highest value ever reported.更多还原