【作者】 张艳华；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料学， 2005， 硕士

【摘要】 热电材料是一种能够实现热能和电能直接相互转换的功能材料,在温差发电和热电制冷等领域具有重要的应用价值和广泛的应用前景。Bi₂Te₃基化合物的最高无量纲热电优值ZT接近于1,是目前室温附近应用的最好的热电材料。制备纳米和低维的热电材料有助于提高材料的热电性能。 本文采用溶剂热/水热合成方法,以BiCl3、Te粉或Te化物以及稀土元素化合物为反应前驱体,采用不同的合成工艺路线,合成了多种含稀土元素的Bi₂Te₃基合金纳米粉及Bi₂Te₃纳米粉,应用XRD、TEM、SEM等手段对合成粉末进行了物相成分和微观形貌结构的分析;实验研究和具体讨论了Bi₂Te₃水热合成过程中的化学反应机理和形核长大机制。最后采用真空热压技术,对部分Bi₂Te₃基合金纳米粉末进行热压,并测试了其热电性能。本文主要取得以下研究结果。 1.采用溶剂热合成方法制备了颗粒尺寸在30nm左右的LaxBi_2xTe3（x≤1）热电材料纳米粉末。并研究了反应温度和反应时间对产物成分和微观结构的影响,研究表明,含稀土元素La的Bi₂Te₃化合物与标准Bi₂Te₃有类似的晶体结构和晶胞常数,晶相La的含量随合成温度升高而明显提高,而反应时间对La_xBi₄Te₆中La含量无显著影响,并初步判断La在Bi₂Te₃相晶格中占据Bi原子的位置。合成产物的几何形态与Bi₂Te₃基化合物的层状晶体结构有关,同时与晶体生长中温度对离子扩散速度的影响有关。在较低温度下合成的产物以不规则多面体纳米颗粒为主,随温度升高合成产物形貌以弯曲的薄片状为主,并存在少量纳米管。 2.采用水热合成方法,在添加适当含量EDTA和强碱性条件（添加NaOH）下,制备了La_xBi_2xSe_yTe_3y四元合金,并主要研究了合成过程中强碱性条件和添加EDTA对反应产物的影响,研究表明,添加足够量的碱性添加剂,是合成单相Bi₂Te₃基合金的必要条件;添加适量EDTA使得溶液中纳米薄片“自组装”连接形成一种纳米片状晶相互平行排列独特的花瓣状组织。但是过量EDTA的加入将抑制合成反应进行,形成颗粒状Bi和杆状TeSe固溶体。 3.采用水热法,分别合成制备了含稀土元素La的Bi₂Te₃基三元合金和四元合金（另外掺Se）化合物。并主要研究了产物的晶体结构特征和热电性能参数随温度的变化。研究发现,Bi₂Te₃基三元或四元合金其晶体结构同标准的Bi₂Te₃基本相似,均为六面体层状结构,稀土元素La在晶胞中的占位是随机取代了Bi₂Te₃晶格中6c位的Bi原子,四元合金中Se原子取代大部分3a位和少部分6c位的Te原子。根据占位数对其合成产物实际成分进行计算,认为与设计含量基本吻合。通过对Bi₂Te₃基三元或四元合金试样热电性能的分析,发现Bi₂Te₃基三元和四元合金热电材料均为n型半导体,说明置换Bi更多还原

【Abstract】 Thermoelectric （TE） materials are a kind of semiconductor functional materials, which can be used to convert directly heat energy to electricity or reversely. They are of interest for applications in TE cooling devices and power generators. Bi₂Te₃ and its alloys, which have the highest dimensionless figure of merit close to unity, are known as the best TE materials currently available near room temperature. The figure of merit of TE materials could be significantly improved if the materials were nanostructured, since the thermal conductivity could be decreased more significantly than the electric conductivity of the materials.In the present work, various Bi₂Te₃ based nanopowders have been solvothermally and hydrothermally synthesized from the precursors LaCl₃, BiCl₃ and Te powder or telluride via various chemical routes. The structures and morphologies of the nanopowders were investigated by XRD, SEM and TEM. Chemical reactions, nucleation and crystal growth mechanism during the synthesis of Bi₂Te₃ have been experimentally investigated and detailedly discussed. The bulk materials of Bi₂Te₃ based nanopowders have been prepared by vacuum hot pressing. The TE transport properties of hot pressed samples have been measured. Some important results of the present work are listed as fellows.1. Rare-earth element contained thermoelectric La_xBi_2-xTe₃ （x ≤1） nano-sized powders about 30nm have been prepared by solvothermal synthesis using ethanol as the solvent. The temperature and time effect of solvothermal synthesis on the particle sizes and compositions of La_xBi_2-xTe₃ nanopowders has been investigated. It was found that the ternary compounds La_xBi_2-xTe₃ have the same crystal structure and similar lattice parameters as those of binary Bi₂Te₃. The lanthanum contents in La_xBi_2-xTe₃ increase with the synthesis temperature and are independent of the reaction time. This indicates that La atoms would be seated at the substitutional positions of Bi in the Bi₂Te₃ crystal cells.The structures and morphologies of the powerd are related to the layered crystal structure of Bi₂Te₃ based compounds and the temperature effect on the ionic diffusion in the crystal growth. The powders synthesized at low temperatures have an irregular polyhedral morphology, while those synthesized at high temperatures tend to form thin sheets and nanotubes.2. Nanostructured monophase La_xBi_2-xSe_yTe_3-y alloys have been synthesized with a hydrothermal route, NaOH and ethylenediaminetetraacetic disodium salt （EDTA） as the additives. The NaOH and EDTA effects on the particle sizes and compositions of La_xBi_2-xSe_yTe_3-y, nanopowders during the hydrothermal synthesis have been investigated. It was found that an alkaline additive is necessary for the synthesis of a monophase Bi₂Te₃ based alloys. EDTA acts as a soft template for the lateral growth of the nano-sheets and the self-assembly of the petal-like structure by the parallel side-by-side arrangement of the nano-sheets. It was found, however, that the synthesis reaction could be hindered by excessive EDTA.3. La contained Bi₂Te₃ TE ternary and quarternary compounds have been prepared via the hydrothermal synthesis using BiCl₃, LaCl₃, tellurium and selenium powders as the precursors. The crystal structures and TE properties of Bi₂Te₃ alloy compounds have beenstudied. XRD Rietveld refinement results indicated that the ternary and quarternary compounds have the same crystal structure and similar lattice parameters as those of binary Bi2Te3. Se and La atoms would respectively be seated at substitutional positions of Te （3a and 6c positions） and Bi （6c position） in Bi2Te3 crystal cells. It was found that both ternary and quarternary Bi2Te3 compounds are n-type, suggesting La elements are donors in the semiconductors. The quarternary Bi2Te3 compounds exhibit higher electrical conductivities and Seebeck coefficients than the ternary Bi2Te3 compounds, but its high electrical conductivities and relative density result in the high thermal conductivities. The maximum dimensionless figures of merit of 0.58 and 0.51 have been obtained for ternary and quarternary Bi2Te3 compounds - samples at about 450K.4. Nanostructured thermoelectric Bi2Te3 powders with various morphologies were hydrothermally synthesized in the temperature range between 120°C and 250°C, or using different routes and precursors. The effects of the different conditions of hydrothermal synthesis on the polyhedral morphology and composition of Bi2Te3 nanopowders have been investigated. It was found that the sizes of the polyhedral particles increase with the increase in the synthesis temperature due to particle coarsening, but little growth could be observed for the nanorods or nanosheets, suggesting that the latter originate from the fragments fallen from the tellurium precursor. The formation of various morphologies of Bi2Te3 nanopowders was discussed. Various nucleation mechanisms, including molecule growth model, continuous nucleation model and nucleus saturation model, have been proposed for the different synthesis conditions. It was experimentally found that lateral growth, including "surface-nucleation lateral growth", "spirally lateral growth" and "twin-crystal lateral growth", dominates the Bi2Te3 crystal growth during the hydrothermal synthesis due to the anisotropic lattice structure.更多还原