【作者】 章根强；

【导师】 李晓光；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2009， 博士

【摘要】 寻找缓解能源危机的途径已经成为二十一世纪人类亟待解决的重大问题之一。热电材料具有将热能与电能直接相互转换的本领,基于该类材料的器件可有效地回收利用热能,提高能源利用效率,被视为能够有效缓解能源危机的有效手段之一。然而,目前热电领域的关键问题仍是如何提高材料的热电优值,实现其推广应用。理论研究表明,低维材料体系,特别是一维纳米结构材料的热电性能将能得到显著的提高。而实验结果也证实了超晶格薄膜、纳米晶镶嵌体以及纳米线等纳米结构的热电性能得到了有效的优化。然而,目前热电纳米材料的合成方法,特别是液相方法的发展仍不完善,导致了部分被理论预言具有巨大应用前景的热电纳米结构,包括核壳异质结纳米线和单晶纳米管等至今仍难以获得,这无疑将大大抑制热电领域的研究进展。本文围绕完善液相反应法合成热电纳米材料这一主旨,针对国际上关于热电纳米材料研究中的挑战性问题,开展了一系列研究工作并取得了一定的研究成果,其主要内容概括如下:第一章综述了热电材料的研究历史和研究进展及其相关器件应用的展望。主要分析了热电材料性能的影响因素及改进方法,介绍了块材热电材料的种类和基本性能以及纳米热电体系的近期重要进展,详细分析了纳米热电材料的合成途径及性能研究,并在此基础上总结了热电纳米材料研究中存在的一些问题。第二章中,我们发展了一种基于无毒有机溶剂乙二醇的简单液相方法,成功的实现了中温区热电材料PbTe花状等级结构、一维纳米线以及尺寸可调的纳米立方块这三种不同纳米结构的选择控制合成。通过详细研究体系中表面活性剂以及NaOH对产物形貌的影响,系统的分析了不同形貌的晶体的形成过程和可能的生长机理,并在此基础上,对实验结果给出了合理的解释。第三章发展了一种基于乙二醇的溶剂热方法,实现了Ⅴ-Ⅵ族A₂B₃型二元、三元化合物六角形片状纳米结构的普适合成。在详细研究有机添加剂PVP K-30和NaOH浓度对产物形貌的影响后,确定了PVP K-30的作用为稳定剂,而NaOH对六角形纳米片的形成起着关键作用。进一步细致研究六角形纳米片制备过程中各阶段中间产物的形貌、微结构与成分分析结果后,确定“定向附着”机理为单晶六角形纳米片形成的反应机理。该方法提供了一种设计并获得具有相似晶体结构的材料体系纳米结构的新思路,为纳米材料合成途径新方法探索提供一个很好的参考。第四章中,我们发展了一种基于乙二醇溶剂的简单液相外延方法,首次合成Ⅴ-Ⅵ族热电材料体系Te/Bi和Te/Bi₂Te₃核壳异质结纳米线。通过详细研究异质结纳米线的界面处的高分辨晶格条纹相结构得出,内核Te纳米线与包裹层Bi或Bi₂Te₃的外延关系可描述为:Te（100）//Bi或Bi₂Te₃（100）,Te＜100＞//Bi或Bi₂Te₃＜100＞。热电性能研究表明,Te/Bi核壳异质结纳米线块材复合物的热导率和Seebeck系数都得到了大幅改善,有助于材料热电性能的优化。然而,其电阻率大大高于Bi块材样品,需要进一步改进块材复合物的质量。该方法中所涉及的合成思路可以推广应用到其它材料体系的核壳异质结纳米线结构。第五章中,我们将纳米尺度的Kirkendall效应引入了基于乙二醇为溶剂的简单液相方法当中,成功的合成了尺寸可调的PbSe、NiSe₂空心球结构以及Bi₂Te₃纳米管结构。通过细致研究这些空心纳米结构形成过程中不同反应时间的中间产物的形貌变化,确认了Kirkendall效应为导致空心结构形成的反应机理。相关性能研究表明,PbSe的光学带隙、目趋NiSe₂的磁性质均表现出明显的尺寸效应。而Bi₂Te₃纳米管块材复合物的Seebeck系数和热导率都得到了大幅度的优化,表明了一维空心纳米热电材料具有的优异热电性能。第六章中,发展了一种不同分布的外加磁场辅助下的简单液相方法,实现了具有“刺猬”状结构和一维链状纳米线结构的铁磁性金属Ni纳米晶体的控制合成。通过对中间产物的形貌变化分析,提出了“刺猬”状纳米结构的形成过程。对产物的磁性质研究表明,样品的微结构对其磁性质具有重要的影响。本章中首次研究了非均匀分布磁场对于铁磁性材料的生长过程的影响,将对磁场诱导合成方法的发展提供重要的参考。第七章中,基于Pechini过程,我们发展了一种改进的溶胶凝胶方法成功的降低了YBa₂Cu₃O₇的成相温度,根据对该溶胶-凝胶过程中的化学反应分析可知,所发展的溶胶凝胶方法具有很好的普适性,可拓展应用于降低其它材料体系的成相温度。在此基础上,结合多孔氧化铝模板合成了YBa₂Cu₃O₇纳米线阵列。在纳米线制备过程中,我们采用多次重复灌注有效的提高了填充效率。经退火后,YBa₂Cu₃O₇纳米线具有超导电性,其转变温度为92K。更多还原

【Abstract】 How to release the energy crisis has been one of the most important problems for the 21^st century.Thermoelectric（TE） devices that can directly generate electric power by thermal energy and vice versa,have been considered as an efficient approach for improving the fuel efficiency and relieving the world’s energy crisis.Nonetheless,the central issue in TE area is still to enhance the TE figure of merit（ZT） of the materials. Theoretical calculations indicate that the TE performance will be remarkably enhanced in low dimensional systems due to the sharper density and enhanced phonon scattering,which have been proved by studying the TE properties of superlattice films, bulk materials with embedded nanocrystals,nanowires and so on.However,it is still underdeveloped on the effective routes,especially the solution phase methods,for the synthesis of TE nanomaterials,therefore,it is still challenging works to obtain various promising TE nanostructures,including core/sheU heterostructure nanowires,large scale single crystalline nanotubes and so on,which will undoubtedly limit the advance of the TE area.This dissertation focuses on developing the solution phase synthesis of TE nanostructures and aims to solve some challenging topics in TE areas.The main contents in the dissertation are presented as follows:In chapter 1,we introduced the history and research progresses of TE materials, as well as the potential applications.The main contents include:the dominating factors and the possible routes for improving TE performance,the state-of-art bulk TE materials and the most important advances and synthetic routes for the TE nanostructures.According to the general analyses,several open questions are proposed at the end of this chapter.In chapter 2,we carried out the selective synthesis of PbTe hierarchical flower-like crystals,one-dimensional nanowires and size controllable nanocubes through a facile solution phase method using non-toxic ethylene glycol as solvent.On the basis of the research on the influence of NaOH concentration,surfactant and the systematic analysis on the shape evolution of the time-dependent products,we proposed a plausible growth mechanism of the products with different morphologies.In chapter 3,we developed a facile solvothermal route for the general synthesis ofⅤ-Ⅵalloy hexagonal platelets.Based on various control experiments,it can be concluded that the organic additive acts as the stabilizing agent and the NaOH plays a critical role during the formation of hexagonal platelets.The formation mechanism can be confirmed as the oriented attachment aider the systematic analyses on the time-dependent products.This work may provide a new rationale pertaining to the design of the solution synthesis of nanoarchitectures for materials possessing similar intrinsic crystal symmetry and will contribute to the development on exploring the novel synthetic routes for nanostructures.In chapter 4,we obtained Te/Bi and Te/Bi2Te3 core/shell heterostructure nanowires,for the first time,through the epitaxial growth involved in an ethylene glycol mediated facile solution phase method.The epitaxial relationship between Te and Bi or Te and Bi₂Te₃ can be described as:Te（100）//Bi or Bi₂Te₃（100）, Te<100>//Bi or Bi₂Te₃<100>.The largely enhanced Seebeck coefficient and greatly lowered thermal conductivity of the heterostructure nanowire composites strongly contribute to the improvement of the thermoelectric performances,while the electrical conductivity of the nanowire composites greatly reduced compared with that of bulk Bi,where much effort should be devoted to enhance the quality of the bulk composite. This facile and general strategy provides an alternative route to synthesize 1D core/shell heterostructures and could be extended to other material systems with low lattice mismatch.In chapter 5,by introducing the nanoscale Kirkendall effect into an ethylene glycol mediated facile solution phase method,we successfully obtained PbSe and NiSe₂ hollow spheres,and Bi2Te3 hollow tubular nanostructures.It is confirmed that the Kirkendall effect could be the involved formation mechanism during the formation of these hollow structure based on the observation of the time-dependent products.The optical band gap of the PbSe and the magnetic property of the NiSe₂ hollow spheres both exhibit notable size effect.In addition,the Seebeck coefficient and thermal conductivity of the Bi₂Te₃ nanotube composites are greatly optimized, which indicates that the one dimensional tubular structure may possess excellent TE performance.In chapter 6,we investigated the influence of the extemal magnetic field distribution on the morphology of the ferromagnetic Ni crystal during the growth based on a facile solution phase method and carried out the controllable synthesis of three dimensional urchin-like crystals and one dimensional chain-like nanowires. Through the analysis of the shape evolution process of the intermediate products,we proposed the formation process of the urchin-like Ni nanostructure.It is indicated that the magnetic properties are quite dependent on the microstructure of the products. This work figures out the effect of non-uniform magnetic field on the growth process of the ferromagnetic metal and may benefit the advance of the magnetic-field assisted synthetic approaches.In chapter 7,we developed an optimized sol-gel process based on Pechini method which successfully lowered the crystallization temperature of the YBa₂Cu₃O₇ phase.According to the chemical reaction involved in this sol-gel process,it could be extended to other material systems.Furthermore,we obtained the highly ordered YBa₂Cu₃O₇ nanowire arrays through the sol-gel template route with the multi-step evacuation perfusion process in order to enhance the filling efficiency.The superconducting transition temperature（T_c） of the YBa₂Cu₃O₇ nanowires annealed in oxygen is about 92 K.更多还原