【作者】 邱安宁；

【导师】 吴建生； 张澜庭；

【作者基本信息】 上海交通大学 ， 材料学， 2009， 博士

【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。随着热电材料研究的逐步深入,材料的晶体结构越来越复杂,掺杂的成分越来越多,给实际研究工作带来较大的困难。固体能带理论是凝聚态物理最成功的理论之一,固体的许多基本性质,如磁性质、电学特性等,都与固体的电子结构密切相关。因此对某些复杂体系,通过探索其电子结构,利用固体能带理论来研究热电传输问题,可以深入理解材料的热电行为,找出晶体结构与热电传输性质的演变规律。本论文采用基于密度泛函理论的第一性原理计算方法对几种具有特殊晶体结构的热电材料(ReSi1.75,β-Zn4Sb3和Rh3ScSi7)的晶格缺陷和电子结构(如态密度、能带结构、成键性质、有效质量等)进行了研究,并研究了掺杂工艺对这几种材料晶体结构和电子结构的影响。利用半经典的玻耳兹曼传输理论,在能带结构的基础上对这几种热电材料的传输性能进行了解析,并结合实验数据和计算结果进行比较和分析,解释了材料特殊传输性能的原因,预测了进一步提高材料热电性能的手段。研究结果表明,ReSi1.75为窄能隙半导体。能带中价带顶是一条平坦的能带,而导带底为具有抛物线形状的能带。ReSi1.75中Re原子的d态电子与晶格中的硅空位缺陷之间形成悬挂键,使得ReSi1.75表现半导体性质。ReSi1.75在[001]方向上空穴具有较大的有效质量,而在[100]和[010]方向上电子具有较大的有效质量。掺杂Al和Mo后ReSi1.75的费米能级向价带移动。沿[100]方向p型掺杂的ReSi1.75和沿[001]方向n型掺杂的ReSi1.75会有更好的热电性能。β-Zn4Sb3为窄能隙p型半导体,其电子结构对晶体结构并不敏感。β-Zn4Sb3中某些Zn-Zn键键长很短,这在能量上不稳定。驰豫后,Zn-Zn键键长显著增大,而Zn-Sb键键长却增大不多。这是由于β-Zn4Sb3中Zn-Zn键为较弱的共价键,而Zn-Sb键为较强的共价键。掺杂对β-Zn4Sb3的Seebeck系数和电导率影响趋势相反,直接通过元素掺杂对于提高β-Zn4Sb3热电材料的电性能优势并不明显。Rh3ScSi7是一种半金属材料,最高的价带穿过费米能级进入到导带中,使得Rh3ScSi7以空穴载流子传输为主。Rh3ScSi7是一种具有传输各向异性的热电材料。掺杂Al使得Rh3Sc(Si0.98Al0.02)7在(0001)晶面上的功率因子明显提高,最高达将近50%。Rh3ScSi7和Rh3Sc(Si0.98Al0.02)7的ZT值都随温度升高而增大。Al掺杂能够显著提高(0001)晶面上的热电性能。更多还原

【Abstract】 Thermoelectric (TE) materials, which can convert heat and electricity directly and reversely, are a new class of functional materials. There are of great important and potential application values in TE power generators and cooling devices. With the further research of TE materials, their crystal structures become more and more complex and the doped compositions become more and more complicated, which have brought great difficulties in practical research work. The band theory of solids is one of the most successful theories of condensed matter physics. Many basic properties of solids, such as magnetic properties, electrical properties and so on, are all closely related to the electronic structures of solids. Therefore by using the band theory of solids and exploring the electronic structure to study the thermoelectric transport problems of complex systems, the TE behavior could be understood and the evolution laws between the crystal structures and TE performances could be discovered.In this thesis, the first principles calculation method was used based on the density functional theory to study and reveal the relations between the specific crystal defects and the electronic structures (such as density of states, band structure,bonding character,effective mass and so on) of three TE materials (ReSi1.75,β-Zn4Sb3 and Rh3ScSi7). Then the effects of doping to the crystal and electronic structures were analyzed. The transport properties of these materials were also resolved based on the band structure and semi-classical Boltzmann transport theory. The calculation results were compared and analyzed with the experimental data. The reason for the specific anisotropic transport properties was explained in detail, thus the means of improving the TE performance were predicted and proposed.The results show that ReSi1.75 is a narrow gap semiconductor. The valence band maximum is a flat band; whereas the conduction band minimum is a parabolic band. The dangling bonds are formed between the Re d electrons and the Si vacancy defects, which makes ReSi1.75 show semiconductor behavior. The effective mass of holes along [001] direction is comparatively large; whereas the effective masses of electrons along [100] and [010] directions are comparatively large. The TE performance should be much more excellent along [100] direction for p-doped ReSi1.75 and [001] direction for n-doped ReSi1.75.β-Zn4Sb3 is a p-type narrow gap semiconductor, whose electronic structure is not sensitive to its crystal structures. The bond lengths of some Zn-Zn bond are exceptionally short, which are not stable in energy. After fully relaxed, the bond length of Zn-Zn bond increases larger significantly whereas the bond length of Zn-Sb bond increases slightly. The reason is that the covalent Zn-Zn bonds are weak and the Zn-Sb bonds are rather strong. The doping techniques cause contrary effects on the Seebeck coefficient and electrical conductivity ofβ-Zn4Sb3. Therefore, the advantage to improve TE performance ofβ-Zn4Sb3 by element doping was discovered not to be significant.Rh3ScSi7 is a semi-metal, the valence band maximum crosses the Fermi level and enters the conduction bands, which makes the hole carriers play a dominant role in Rh3ScSi7. Rh3ScSi7 is an anisotropic TE material. Al doping can increase the power factors on (0001) plane of Rh3Sc(Si0.98Al0.02)7 obviously; a maximum of 50% increase can be achieved. The figure of merits ZT of both Rh3ScSi7 and Rh3Sc(Si0.98Al0.02)7 increase with the rising temperatures. The thermoelectric performance ZT on (0001) plane of Rh3ScSi7 could be significantly improved Al doping.更多还原