【作者】 肖承京；

【导师】 杨君友；

【作者基本信息】 华中科技大学 ， 材料学， 2010， 博士

【摘要】 作为室温附近热电性能最好的热电材料之一,Bi2Te3系材料在国防、医疗、微电子、航空航天等诸多领域有广泛应用前景。但传统的Bi2Te3系块体材料的热电性能一直在一个比较低的水平徘徊。随着纳米技术的飞速发展,热电材料正面临新的发展机遇。研究表明,Bi2Te3系纳米热电材料,包括纳米量子点、纳米线、纳米薄膜和块体纳米材料,热电性能可望大幅度提高。在对Bi2Te3系热电材料的国内外研究现状、发展趋势详细调研的基础上,提出了本文的研究目的、意义及研究内容,拟从纳米颗粒、纳米薄膜以及块体材料三个方面研究多尺度Bi2Te3系热电材料的制备工艺及其热电性能。采用电化学原子层外延(ECALE)、微波辅助湿化学方法(MAWCS)、机械合金化(MA)结合等离子活化烧结(PAS)和热压烧结(HP)方法分别了制备纳米薄膜、纳米颗粒和块体Bi2Te3系热电材料。通过X射线衍射分析(XRD)、场发射扫描电镜(FE-SEM),能谱分析(EDS)，高分辨透射电镜(HRTEM)等多种分析测试手段研究了材料的成分及组织结构；通过对带隙、Seebeck系数、电阻率、载流子浓度、迁移率及热导率等材料性能的测试考察了工艺条件对Bi2Te3系热电材料性能的影响,在此基础上优化了其热电性能。考察了Bi、Se在不同衬底及相互之上的欠电位沉积(UPD)特性,确定了Bi2Se3化合物在多晶Pt和单晶Au衬底上的ECALE沉积工艺,在上述衬底上成功沉积了Bi2Se3纳米薄膜。组织结构分析表明：在多晶Pt衬底上经400个稳定ECALE循环获得了平整的正交结构Bi2Se3薄膜；单晶Au衬底上200个循环获得菱方Bi2Se3薄膜,薄膜由大量垂直衬底的纳米片构成。二者的物相和形貌差异主要源于衬底表面状态和单循环沉积量。红外光谱分析发现Au衬底上沉积的Bi2Se3薄膜能隙发生蓝移。利用微波辅助湿化学法成功合成了二元Sb2Te3纳米片、Sb2Se3纳米棒。探讨了微波合成条件对产物组成的影响,分析了微波辅助合成中的反应机制及合成粉体微观形貌的生长和调控机制。Sb2Te3化合物为六角形纳米片,TEM和SAED及HRTEM分析证实纳米片为菱方结构,纳米片沿垂直于z轴的六个边向外扩张生长。Sb2Se3化合物呈纳米棒状,正交结构Sb2Se3晶体三个方向上的生长速率不一致,导致纳米棒沿[001]方向进行一维生长。采用MAWCS方法首次合成了Bi0.4Sb1.6Te3和Bi2Te2.5Se0.5三元化合物固溶体纳米粉体,探讨了反应环境中碱性强度和还原剂对合成反应的影响,发现在一定的KOH与KBH4配合量下可以得到目标三元化合物。利用表面活性剂可获得Bi0.4Sb1.6Te3纳米片,Bi2Te2.5Se0.5在无表面活性剂条件下也能获得六角形纳米片结构。将MAWCS合成的Bi0.4Sb1.6Te3纳米片掺入MA合成的Bi0.4Sb1.6Te3粉体进行纳米复合后热压成型,研究了不同纳米片掺入比例对材料热电性能的影响。纳米片的加入能显著降低声子热导率,加入7.5wt.%纳米片后样品热导率比未复合样品下降7.2%,声子热导率下降9.8%,室温下7.5wt.%掺入比例的样品有最大ZT值(1.31)。采用MA-PAS制备了P型Bi0.4Sb1.6Te3块体材料。探讨了PAS工艺对材料性能的影响,发现平行压力方向的热电性能要优于垂直压力方向。考察了不同烧结温度对材料热电性能的影响,653K烧结的样品在323K下具有最大功率因子和Seebeck系数,分别为5.7x10-3W·m-1·K-2和244.8μV·K-1,653K烧结样品具有最大的ZT值1.42(T=323K)。TEM观察到PAS烧结样品中存在孪晶,晶粒内部具有明暗相间的层片条纹。HRTEM观察条纹处存在晶格扭曲和晶格缺陷,说明这些层片条纹是由晶内微观应力造成的剧烈晶格畸变所导致的衬度差别。晶格畸变、晶内缺陷及纳米非晶区使晶格热传导受到强烈干扰,导致热导率下降,热电性能提高。更多还原

【Abstract】 As one of the most frequently used thermoelectric materials near room temperature, Bi2Te3-based compounds have been broadly applied in many fields, such as minicooler for micro-and opto-electronics, medical device, electronic consumables, and so on. However, the thermoelectric performance of traditional Bi2Te3-based bulk materials has been stagnated for a long time. With the development of nanotechnologies, thermoelectric materials are facing new opportunities and rejuvenation. As pointed out by previous researchers, thermoelectric performance of nano-materials could be greatly improved owing to the strong quantum confinement effect. Based on a detailed review to the research status and developing trend of thermoelectric materials, the research topic of this thesis is focused on Bi2Te3-based multi-dimensional thermoelectric materials, the preparation techniques of multi-dimensional bismuth telluride based thermoelectric materials were explored and their thermoelectric performance was optimizedElectrochemical atomic epitaxy(ECALE), microwave assisted wet chemical synthsis (MAWCS), mechanical alloying plus hot pressing (MA-HP) and mechanical alloying plus plasma activated sintering (MA-PAS) were subjected to fabricate the nano-films, nano-particles and Bi2Te3 bulk materials respectively. FE-SEM, EDS, XRD and HRTEM were performed to characterize the morphology, composition and structure of Bi2Te3-based multi-dimensional materials respectively. Energy band gap and transport properties, such as Seebeck coefficient, electrical resistivity, charge carrier concentration, mobility and thermal conductivity, were measured to study the effect of processing conditions on thermoelectric performance of Bi2Te3-based compounds. Based on these results, thermoelectric performance of the materials was optimized.Electrochemical aspects of bismuth and selenium on different substrates and each other are carefully investigated. The optimal deposition conditions are determined for the ECALE process of Bi2Se3 on polycrystalline Pt substrate and single crystal Au substrate. A 400 ECALE-cycle and 200 ECALE-cycle Bi2Se3 thin film deposits were grown on polycrystalline Pt substrate and single crystal Au substrate respectively. The 400-cycle nanofilm on polycrystalline Pt substrate shows an orthorhombic structure and a smooth morphology. While a rhombohedral Bi2Se3 film, was obtained on the single-crystal Au substrate. The different structure of Bi2Se3 deposits should be ascribed to the crystal structure and surface situation of the substrates. Due to the quantum confinement effect, the band gap of the 200-ECALE-cycle Bi2Se3 film on Au substrate is blueshifted.Sb2Te3 and Sb2Se3 binary compounds were synthesized successively by the means of microwave assisted wet chemical method (MAWCS). Effect of synthesizing conditions on the composition of the reaction products was studied, and the reaction mechanisms of microwave assistant synthesis are analyzed. Hexagonal Sb2Te3 nano-plates with rhombohedral structure were synthesized by MAWCS; while Sb2Se3 compound with orthorhombic structure, which has a nanorod morphology, was obtained by MAWCS. The unique morphology of the compound is determined by the growing speed difference between different crystal planes.Bio.4Sb1.6Te3 and Bi2Te2.5Se0.5 ternary compounds were synthesized via MAWCS for the first time in this work. The strength of KOH and KBH4 plays a critical role in the synthesis of Bi0.4Sb1.6Te3 and Bi2Te2.5Se0.5. The morphology of Bi0.4Sb1.6Te3 powders was tuned from irregular nano-particles to nano-plates structure with addition of a certain amount of sodium dodecyl sulfonic (SDS) to the solution system; while Bi2Te2.5Seo.5 hexagonal nano-plates could be obtained without addition of any surfactant.The Bi0.4Sb1.6Te3 nano-plates synthesized by MAWCS are mixed with the as-MAed Bi0.4Sb1.6Te3 powders and then consolidated with hot press. Effect of the content of nano-plates on thermoelectric properties of the bulk (Bi,Sb)2Te3 alloys was investigated. When the doping content is 7.5wt.%, the total thermal conductivity and the phonon thermal conductivity of the sample reduced 7.2% and 9.8% respectively than those of the sample without addition of nano-plates. The maximum figure of merit was achieved as ZT=1.31 at room temperature with addition of 7.5wt.% nano-plates.P-type Bio.4Sb1.6Te3 bulk thermoelectric materials were fabricated by MA-PAS. Effects of processing conditions of PAS were investigated. Thermoelectric performance parallel to the pressure direction outweighs that of perpendicular to the pressure direction of the samples. The dependence of thermoelectric performance on sintering temperature and current were researched, and the maximal Seebeck coefficient of 244.8μV·K-1was obtained at 323K in the sample sintered at 653K, and it also has the maximum power factor of 5.7×10-3Wm-1K-2. The maximum ZT of 1.42(T=323K) was achieved in the sample sintered at 653K.TEM observation shows that there are twin crystals and layered stripes within the grains of the samples prepared by MA-PAS, and further HRTEM observation reveals confirmed crystal lattice distortion and crystal defects exist around those stripes. The results of TEM and HRTEM observation demonstrate the layered stripes are caused by diffractioin contrast due to dramatic crystal lattice distortion and lattice defects. Crystal lattice distortion, crystal defects within the grains, together with nano non-crystal areas greatly depress crystal lattice thermal conduction, which results in decrease of thermal conductivity and improvement of thermoelectric performance.更多还原