【作者】 樊东晓；

【导师】 赵新兵； 朱铁军；

【作者基本信息】 浙江大学 ， 材料科学与工程， 2011， 硕士

【摘要】 热电材料可以实现电能和热能之间的相互转换,在热能发电和电能制冷领域有其独特的优势。高效的热电器件同时需要性能良好的n型和p型热电材料。本论文致力于研究环境友好、成本经济的p型硅化物热电材料,主要选取Mg2Si和高锰硅材料,从制备工艺和元素掺杂方面探索提高其热电性能的方法。针对Mg2Si热电材料难以稳定制备的难题,本文采用了固相反应制备Mg2Si材料,探索出较为稳定的固相反应的参数,以此工艺制备了成相较好的纯Mg2Si,热电性能测试显示其为n型半导体,在725K时出现最高ZT值0.28。n型的Mg2Si热电材料性能较好且研究广泛,而同一种体系的n型和p型热电材料制作的热电器件具有更佳的应用优势。因此研究Mg2Si的p型改性就具有重要意义。本文采用固相反应法制备了以硅化锂(Li13Si4)作为Li供体的Li掺杂Mg2Si材料Mg2-xLixSi,并研究了其热电性能。实验中所有的Li掺杂试样都成功转变为p型半导体。其电导率相对于未掺杂试样有所降低,而热导率则有所升高,理论配比为Mg1.97Li0.03Si的试样在725K出现最高的ZT值0.12。高锰硅是性能优异的p型硅化物热电材料,适当的元素掺杂和制备工艺的改进可以进一步提高其性能。本文采取悬浮熔炼、甩带和放电等离子烧结等工艺制备Ge取代的高锰硅试样Mn (Si1-xGex)1.733。XRD分析表明：实验所得高锰硅试样中存在两种非公度相,其中以Mn15Si26为主,快速凝固能够减少MnSi金属相的含量；Ge对Si位的取代使得衍射峰向低角区偏移。SEM分析研究了甩带所得快凝合金薄片的贴辊面和自由面的微观形貌差异,并利用形核理论对此差异的出现做出解释。研究了高锰硅熔锭和快凝合金中的MnSi金属相在SPS前后形态与分布的变化。测试并比较了悬浮熔炼组和快速凝固组试样的热电性能。结果显示：Ge取代可以优化材料的电学输运性质,快速凝固则有效地降低了材料的热导率,同时提高了材料的电导率,有助于提高材料的热电性能。SEM分析显示,快速凝固试样中的MnSi金属相呈聚集分布,有一定的连通性,此结构特征也有助于提高材料的电导率。实验范围内,当Ge取代量x=0.010时,ZT值最高,悬浮熔炼试样在850K时ZT值为0.53,快速凝固试样在750K时ZT值达到0.55。此外,本文还采用B203做助熔剂的方法制备了Ga掺杂的Mg2Ge0.4Sn0.6系列试样。XRD分析表明产物成相良好,助熔剂法可以有效地制备Mg-Ge-Sn三元单相固溶体。多数掺杂试样表现出p-n型转变的特征。更多还原

【Abstract】 Thermoelectric materials can convert heat energy to electricity directly or vice versa, which have unique advantage and various applications in thermoelectric power generation and cooling. A good thermoelectric device necessarily requires both n-type and p-type thermoelectric materials with good performance. This thesis is mainly focused on the study of environmentally friendly and cost-effective p-type thermoelectric silicides such as Mg2Si and higher manganese silicide (HMS), having explored the ways to enhance their thermoelectric performance by some certain kinds of elements doping and the optimization in their fabrication methods.Due to the existing fact that Mg-Si based thermoelectric materials are quite tricky to prepare, solid state reaction method employed in the thesis to prepare Mg2Si was carefully controlled and repeatedly tested to find the stable and repeatable reaction conditions. Well-formed pure Mg2Si sample was prepared under the fixed reaction conditions. It is revealed in the measurements of thermoelectric properties that pure Mg2Si sample is n-type semiconductor. The highest ZT value 0.28 has been obtained at 725K.N-type Mg2Si enjoys a relatively good thermoelectric property and has been intensely studied. But due to the fact that the thermoelectric devices with n-type and p-type terminals of the same material system will generate higher efficiency, it is also necessary and meaningful to modify Mg2Si into p-type semiconductor. Solid state reaction method was adopted to prepare Li-doped Mg2Si compound Mg2-xLixSi, with Li13Si4 as a donator of lithium. The thermoelectric properties of Li-doped and undoped Mg2Si samples were measured and compared. All the Li-doped samples were successfully modified into p-type. Compared with the undoped sample, the Li-doped Mg2Si samples have relatively higher thermal conductivity and lower electrical conductivity. The maximum ZT value in Li-doped Mg2Si samples was 0.12 at 725K, obtained in the sample with the theoretical stoichiometric proportion as Mg1.97Li0.03Si.Higher manganese silicide (HMS) is a p-type silicide material with extraordinary thermoelectric performance. Proper elements-doping and improvement in preparation will further enhance its p-type thermoelectric performance. Levitation melting, melt spinning and spark plasma sintering (SPS) were adopted to prepare Ge-substituted higher manganese silicide thermoelectric alloys Mn(Si1-xGex)1.733. XRD patterns showed that there were two incommensurate phases in the obtained HMS samples and the main incommensurate phase was Mn15Si26, and that rapid solidification could reduce the content of MnSi metallic phase. Lattice distortion was produced by the substitution of Ge for Si, rendering a gradual shift of diffraction peaks to low-angle region in XRD patterns. The micro-morphologic differences of the near-roller surface and the free surface in the flake alloys obtained by melt spinning were analyzed by SEM. This kind of micro-structural difference was explained in the thesis with nucleation theory. The shape and distribution of MnSi metallic phase in the samples before and after SPS were also analyzed by SEM. Spark plasma sintering was employed to prepare high density bulk alloy samples from both the levitation melted powder and the melt-spun powder. Thermoelectric performances of these samples were measured and compared. Results revealed that the thermal conductivity decreased in melt-spun samples and the electrical transportation properties optimized because of Ge doping. Optimization in these two aspects would do good to the enhancement of the dimensionless figure of merit ZT. The aggregation of MnSi metallic phase in the rapid solidified samples was observed with SEM analysis, helping to explain the increase of electrical conductivity in these samples.Within the range of Ge substitution ratios in the experiment, the highest ZT value has been obtained when x=0.010. For the levitation melted samples, the maximum ZT value was 0.53 at 850K, and for the melt-spun samples,0.55 at 750K.Flux synthesis method with B2O3 as fluxing agent was employed to prepare Ga-doped Mg2Ge0.4Sn0.6 samples. XRD patterns showed that the single-phased three-component sosoloid was obtained. Thus flux synthesis method proved to be an effective way to prepare Mg-Ge-Sn sosoloid. Most Ga-doped samples exhibited a p-n type conversion phenomenon.更多还原