【作者】 倪华良；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2007， 博士

【摘要】 热电材料是实现热能和电能直接转换的材料，可用于半导体制冷和发电。Bi₂Te₃基化合物是室温下性能最好的热电材料，经过几十年的研究，块体Bi₂Te₃基材料的最高热电优值ZT一直徘徊在1左右。随着纳米技术的兴起，近年来有关在低维材料中取得高热电优值的报道不断出现。将材料的晶粒细化到纳米级，可以增加对载流子和声子的散射，从而提高Seebeck系数，降低热导率，提高热电性能。本文采用水热法合了不同成分的纳米结构Bi₂Te₃基化合物，采用真空热压法制备了块体纳米复合材料，对合成机理和性能作了系统研究。本文系统研究了过量Te和掺杂Ag对性能的影响。研究发现，改变Bi和Te的相对含量可控制Bi₂Te₃中的施主掺杂浓度，当名义成份为Bi₂Te_x（x=2.85～3.45）时，材料的室温电导率在8.0×10³～1.03×10⁵ Sm^-1的宽区间内变化；在水热合成的粉末中，用葡萄糖还原沉积不同数量的Ag，也可以在宽区间内调节材料的电导率。这表明，可以通过调节原料中Bi、Te配比和Ag掺杂，实现Bi₂Te₃基纳米复合材料的载流子浓度和电学性能的优化。三元合金对短波长声子的散射较强，可进一步降低热导率，材料的能带结构也得以优化，因此三元合金具有更高的热电性能。本文用水热法成功地合成了Bi₂（Te，Se）₃三元合金，并对其相组成进行了表征分析。研究发现当名义成份为Bi₂Te_xSe_0.45（x=2.40～3.00）时，产物为结构相同，晶格常数存在差异的两相Bi₂（Te，Se）₃合金。热压后Bi₂Te_2.70Se_0.45和Bi₂Te_3.00Se_0.45接近单相。对材料电学性能分析表明，Te的相对含量可以调节掺杂浓度，当材料接近单相时，载流子受到的散射较强，Seebeck系数较高。在（Bi，Sb）₂Te₃合金中，随着Sb含量增加，形成单相合金越来越困难。在Bi_1.8Sb_0.2Te₃中，可以用水热法直接得到单相合金，在BiSbTe₃中，热压之后可得到单相合金，在Bi_0.5Sb_1.5Te₃中无法得到单相合金。以水热合成的Bi₂Te₃、Sb₂Te₃和Bi₂Se₃纳米粉末为原料，采用复合热压法制备了单相的Bi₂Te₃基三元合金。结果表明，用该方法可在整个Bi₂Te₃-Bi₂Se₃赝二元系内合成单相的Bi₂（Te，Se）₃合金。但是在整个Bi₂Te₃-Sb₂Te₃赝二元系内均无法合成单相合金。其原因是水热合成的Bi₂Te₃和Bi₂Se₃粉末晶粒尺寸都在几十纳米左右，容易通过原子扩散形成单相合金。水热合成Sb₂Te₃的晶粒尺寸分布不均，大的晶粒尺寸在微米级，因而无法形成单相合金。水热合成的纳米材料具有大量晶界，构成对载流子和声子的强烈散射，可提高Seebeck系数，降低热导率。但是大规模制备纳米材料，存在技术和成本上的困难。本文将不同温度下合成的Bi₂Te₃纳米粉末掺入到工业粉末中，通过真空热压法制备微纳结构Bi₂Te₃基热电材料，期望用少量的纳米材料来提高工业材料的性能。结果表明纳米粉末的晶粒尺寸对性能有重大影响。纳米粉末的晶粒尺寸越小，对载流子和声子的散射越强。在没有掺杂优化的情况下，在120℃合成的纳米粉与N型工业粉的复合材料中取得最高ZT值为0.83。本文研究了Bi₂Te₃基区熔材料和热压材料的微区Seebeck系数分布。在热电材料研究中，一般认为整块试样的性能是均匀的。用常规手段测得的材料的热电性能，如Seebeck系数、电导率和热导率等都是材料的整体性能。本文分析了Bi₂Te₃基材料的微区Seebeck系数分布，发现材料中Seebeck系数的分布是不均匀的，高低相差40μV／K以上。Seebeck系数的微区分布，与成分、晶体取向和形貌无关，可能与材料中的缺陷有关。余热发电是热电材料的一个重要应用，由于它的转化效率较低，发电装置必须具有很低的制造、安装、运行和维护成本，才能使其实用化。工业上的大型烟囱是一个重要的余热源。针对这一余热源，设计了位于烟囱内部的无动力冷却热电发电装置。该装置对烟囱结构影响很小，安装方便，降低了发电成本，采用无动力水冷，有助于增加总的能量输出，简化结构，进一步降低成本。实验表明，水循环在没有驱动的情况下可以持续进行，在40mm×40mm面积上的输出功率为0.027W。本文制作了电学性能测量仪，可在-100～1000℃范围内测量材料的Seebeck系数和电导率。编写了测量控制程序，整个测试过程由计算机控制，操作方便。制造了热压设备，最高工作温度为1000℃，最大压力20T。更多还原

【Abstract】 Thermoelectric materials interconvert heat and electricity directly. They are used in power generation and cooling. Bi₂Te₃ based materials are the best thermoelectric material at room temperature. The figure of merit ZT of Bi₂Te₃ is about unit for decades. In recent years, the research of thermoelectric material has made new progress by nanotechology. High figure of merits over unit are reported. Nanostructured materials have a large amount of boundaries that will strongly scatter the phonons and carries. Therefore the Seebeck coefficient is improved, the thermal conductivity reduced, and the thermoelectric properties improved. In this paper, various Bi₂Te₃ based alloys were hydrothermally synthesized and hot pressed to make bulk samples. The mechanisms and properties are systematically investigated.In this paper, the influences of excess Te and doped Ag on the properties are systematically studied. The result shows that the doping levels can be optimized by changing the proportion of Bi and Te. The conductivities are 8.0×10³ to 1.03×10⁵ Sm^-1 for the nominal compositions of Bi₂Te_x（x=2.85～3.45）. The conductivities also can be optimized by coating Ag on the hydrothermally synthesized nanopowders by glucose.Ternary Bi₂Te₃ based alloys have the lower thermal conductivities due to the stronger scattering to the short wave phonons, and the optimized electronic structures. Therefore, the ternary alloys have the higher properties than binary alloys. In this paper, ternary Bi₂Te₃ based alloys were synthesized by hydrothermal method. The strucucture analysis shows that the products are composed of two phases, both of which of are Bi₂（Te, Se）₃ alloys with different lattice constants. Sample Bi₂Te_2.70Se_0.45 and Bi₂Te_3.00Se_0.45 are nearly single phase. The property analyses show that changing the Te content can optimize the doping levels. The samples close to single phase have the stronger scattering. In （Bi, Sb）₂Te₃ alloys, it become difficult to form single phase alloys with increasing the Sb content. Bi_1.8Sb_0.2Te₃ is single phase after hydrothermal synthesis. BiSbTe₃ is single phase after hot pressing. But Bi_0.5Sb_1.5Te₃ can not form single alloys.Single phase ternary Bi₂Te₃ based alloys are synthesized by synthesizing binary alloys of Bi₂Te₃, Bi₂Se₃ and Sb₂Te₃ followed by hot pressing. The results show that single phase alloys were formed in the whole Bi₂Te₃-Bi₂Se₃ quasi binary series. However no single phase alloys were formed in the whole Bi₂Te₃-Sb₂Te₃ quasi binery series. The crystal sizes of Bi₂Te₃ and Bi₂Se₃ were both tens of nanometers, so it is easier to form single phase alloys by diffusion,while the sizes of part of Sb₂Te₃ crystals were on the micrometer level, so it is difficult to form single phase.The hydrothermally synthesized powders have a large amount of boundaries that strongly scatter the phonons and carriers. Seebeck coefficient is improved and thermal conductivity reduced. However it is difficult to industrially produce nanomaterials because of the technical and commercial barriers. In this paper, micro-nano-composite Bi₂Te₃ alloys are synthesized by mixing the nanopowders hydrothermally synthesized at 120-250℃ into the industrial materials and hot pressing. The crystal sizes of the nanopowders have a great influence on the properties. The smaller is the nanopowders, the stronger is the scattering. The sample hot pressed with nanopowders synthesized at 120℃ and N type industrial powders has the highest ZT value of 0.83.The micro zone distribution of Seebeck coefficient was analyzed. The distributions of Seebeck coefficient are generally believed to be even. The usually tested are the average value. However, the result here shows that the distribution of Seebeck coefficient in the samples is unhomogeneous, with a fluctuation of 40 μVK^-1. The distribution has no relationship to the composition, morphology and crystal orientation.Waste heat power generation is an important thermoelectric use. Because of its low efficiency, the thermoelectric generation device should be of low cost in production, installation, running and maintenance. Industry chimney is an important waste heat source. To the heat source, an inside thermoelectric generator without cooling driver is made. The device has low effect to the chimney structure, that is easy for installing, and reduces the cost. The device is cooled without a driver, which simplifies the structure and further reduces the cost. The water-cooling runs continuously and the output power is 0.027 W in testing.更多还原