【作者】 司海刚；

【导师】 王渊旭；

【作者基本信息】 河南大学 ， 理论物理， 2012， 硕士

【摘要】 能源是现代工业社会经济发展的基础，能源供给关系着人们的日常生活、国家的经济发展和国家安全。就目前来看，石油、天然气、煤炭等化石能源仍是能源的主要来源。由于化石能源不可再生、分布不均衡以及经济的高速发展对能源需求在逐步增加，对能源的争夺已经成为引发国际形势紧张的重要因素。此外，化石能源的大量使用还会引起温室气体效应和严重的环境危机。为此，新能源的开发和新型能源转化材料研究成为各国科学家关注的热点。热电材料是其中一种新型能源材料，它可以实现热能和电能之间的直接相互转化，能有效利用大量的工业废热、汽车废气、地热、太阳能以及实现无机械制冷。热电材料还具有无污染、无噪声、无可传动部分、体积小、反应快、易于维护、安全可靠等优点，有着极其广泛的应用前景。热电材料是利用固体中载流子和声子的输运及其相互作用，实现热能和电能之间的直接转化。目前较低的热电转换效率是限制热电材料广泛使用的主要原因。热电材料的转化效率（热电优值）高低由无量纲的ZT=S₂σT/（k_e+k_l）值来表示，它是由材料的塞贝克系数S、电导率σ和热导率k=k_e+k_l共同决定。在块材料中的这三个参数是相互联系、相互制约的，这成为限制块材料热电效率提高的主要因素。在纳米材料等低维材料中，塞贝克系数、电导率和热导率这三个参数之间相互关系发生改变，为纳米材料热电效率大幅提供了可能。纳米材料的热电效率相对于块材料大幅提高已经得到了实验上的验证，例如p型Bi_xSb_2-xTe₃纳米化合物，p型Si₈₀Ge₂₀纳米化合物和n型Si₈₀Ge₂₀纳米化合物的最大ZT值分别达到1.4，0.95，1.3，相对于它们分别相应的块材料ZT值1.0，0.65，0.9有较大提高。以Bi原子为基础的低维材料如Bi_1-xSb_x薄膜、纳米线和纳米管已经成功合成，并有较好的热电效率。从相应的块体热电材料In₄Se₃中提取出来的InSe纳米管可能有好的热电表现。在本论文工作中，我们主要目标是研究探索相对于块体In₄Se₃的低维系统的结构和热电性质。我们探讨了InSe纳米管（InSeNTs）的结构、电子结构和热电性质。我们应用第一性原理的计算方法和玻尔兹曼理论预测了InSeNTs的结构并探索了InSeNTs的热电性质。经过充分地优化，（2,2）型被证明是最稳定的一种结构。（3,3）、（4,4）、（6,6）、（8,0）型InSeNTs是半导体性的，其它是金属性的。Se原子4p轨道的饱和度是决定InSeNTs导电性的重要因素。Se原子在半导体InSeNTs中与两个In原子成键，而在金属性InSeNTs中与三个In原子成键。（2,2）型纳米管费米面附近的功率因子是所研究InSeNTs中最大的，并且几乎是BiSb纳米管的十倍。费米面附近重带和轻带的同时出现会引起塞贝克系数和电导率的同时增大。本研究为新型高效热电材料的设计提供了一类极有希望的纳米结构材料。此外，在下一步的工作中还可以通过管大小、掺杂和表面结构调制进一步提高其热电效率。对In₄Se_3-x的研究多数集中在对多晶In₄Se_3-x材料的研究，一方面是由于多晶的界面效应可以有效降低材料的热导率，另一方面是多晶In₄Se_3-x材料的制备相对于单晶的生长更容易。实验上对Se原子缺陷的具体位置不能有效的控制，只能利用浓度变化来探究Se原子缺陷对晶体热电性质的影响。本文的主要目的是通过第一原理的计算方法对Se原子缺陷的具体位置、浓度对材料热电性质的各向异性、变化趋势、绝对大小以及正负塞贝克系数不同影响规律进行研究，并计算了相同缺陷浓度不同缺陷位置的态密度和能带结构，对缺陷具体位置影响因素进行了分析。更多还原

【Abstract】 Energy is the basis for development of modern industrial society, energy supply relates to the people’sdaily life, the national economic development and the national security. As matters stand, fossil fuels suchas petroleum, natural gas and coal are still our major source of enegy. Due to the fossil fuels’unregeneration, uneven distribution and the very strong demand for it, energy is the main cause ofinternational tensions. The extensive use of fossil fuels is leading to greenhouse effect and environmentalcrisis. Therefore, exploration for new energy and research of new energy materials are a focus for concern.Thermoelectric material is a new kind of energy material, it can convert energy between thermal energy andelectrical energy. Home heating, automotive exhaust, and industrial processes all generate an enormousamount of unused waste heat that could be converted to electricity by using thermoelectrics. Asthermoelectric generators are solid-state devices with no moving parts, they are silent, reliable and scalable,making them ideal for small, distributed power generation. Thermoelectrics have long been too inefficientto be cost-effective in most applications.The efficiency of thermoelectric materials is given by the dimensionless figure of merit ZT=S₂σT/（k_e+k_l）, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature,and keand klare the electric and lattice thermal conductivity, respectively. Since the three physicalparameters S, σ, and K are coupled with each other, it remains a major difficulty to further enhance theZT value in bulk thermoelectric materials. High ZT has been achieved in many thermoelectricnanocomposites. For example, peak ZT is increased from1.0in bulk to1.4in p-type Bi_xSb_2-xTe₃nanocomposites, from0.65in bulk to0.95in p-type Si₈₀Ge₂₀nanocomposites, and from0.9in bulk to1.3 in n-type Si₈₀Ge₂₀nanocomposites. Bi-based low-demensional materials such as Bi_1-xSb_xthin film,nanowires, and nanotubes have been successfully produced, and enhanced thermoelectric performanceshave been found in them. InSe nanotubes extracted from corresponding bulk In₄Se₃may also have highthermoelectric performance. In this work, we aim to achieve a better thermoelectric performance forInSeNTs by contacting the low-demensional system of In₄Se₃. We investigate the structural, electric, andthermoelectric properties of InSeNTs. We have employed first-principles calculations and Boltzmanntransport theory to predict InSeNT structures and expore their thermoelectric properties. After fullrelaxation,（2,2） InSeNT is the most stable one among the studied InSeNTs.（3,3）,（4,4）,（6,6）, and （8,0）InSeNTs are semiconducting. Other studied InSeNTs are metallic. The filling degree of the Se4p orbitalplays a key role in determining whether these studied InSeNTs are semiconducting or metallic. The Seatom in the semiconducting InSeNTs is coordinated by two In atoms and that in the metallic InSeNTs is bythree In atoms. The S₂σ/τ of （2,2） InSeNT is much larger than that of other InSeNTs and is nearly10times larger than that of BiSb nanotubes around the Fermi level. It is found that an appearance of light andheavily bands around the Fermi level results in its high Seebeck coefficient and reasonable electricalconductivity. The current research proposes new types of nanotubes to design high-performancethermoelectric materials. Moreover, it is possible to further improve the thermoelectrial performance ofInSeNTs by optimization of tube sizes, doping, or tuning the surface structure in future work.Recently, most of the research efforts about thermoelectric material In₄Se₃crystal focus onSe-deficient polycrystalline compounds of In₄Se_3-x. First, interface efforts of polycrystal can effectivelyreduce thermal conductivity. Second, the fabrication of polycrystalline In₄Se_3-xcompounds is easier thangrowth of Se-deficient In₄Se_3-xsingle crystal. It is difficult to control the position of Se-deficient byexperiment, only to research how does the Se defects concentration affect the thermoelectric properties of polycrystalline compounds of In₄Se_3-x. The main purpose of this paper is to study the influence of positionof Se defects and defects concentration on the anisotropy and the trend of changes of thermoelectriccoefficients. We studied the difference of change regularity of positive and negative Seebeck coefficients.We also calculated the density of states, electronic band structure of In₄Se_3-xsingle crystal with same defectconcentration and different Se-deficient position, and analyse effects of Se-deficient positions.更多还原