【作者】 张杰；

【导师】 刘惠军；

【作者基本信息】 武汉大学 ， 材料物理与化学， 2016， 博士

【摘要】 全球范围内新兴能源的生产、保护和管理需求促进了更有效的能量转换方式的发展。热电材料可以实现热能和电能之间的直接转换,其中的热能可以来自化石燃料的燃烧、汽车尾气所排放的废热或者各种化学反应过程放出的热量。因此,热电材料在发电和节能方面起着重大的作用。用热电材料制作的器件具有体积小、可靠性高、使用寿命长、无噪音、无污染、无运动部件、免维护等突出优点。热电材料的转化效率是由无量纲的热电优值(ZT＝S2σT/κ)来决定,由于ZT表达式中的三个输运系数(S、σ和κ)是相互关联的,并且都和材料的晶体结构、电子特性、载流子浓度有关,要使材料的ZT值大幅度提高往往非常困难。尽管材料的ZT值没有理论极限,但是在上世纪90年代以前,一些传统的块体热电材料如Bi2Te3、PbTe和Si1-xGex的最优ZT值一直徘徊在1.0左右,无法与传统的发电或制冷方式相媲美。直到1993年,Hicks等人的理论工作指出低维结构有可能获得比块体材料高出很多的ZT值。其原因是量子限域效应增加了费米能级附近的电子态密度从而提高了材料的功率因子,同时材料中出现的大量表面/界面增强了声子散射使得热导率显著降低。随后这种理论预测由实验所证实,例如Bi2Te3/Sb2T3超晶格、PbSeTe/PbTe量子点、Si纳米线和BiSbTe纳米复合材料的ZT值都取得突破,达到2.0左右。尽管如此,以Bi、Sb、Pb、Te等为组元的具有较高热电性能的材料在低维化方面还存在一些困难,主要是缺乏工艺简单、成本低廉的合成手段,因而并不适合于大规模制备应用；而且这些材料基本上都是由价格昂贵或有毒的元素所组成,所有这些无疑制约了它们的实际应用。最近,黑磷和磷烯在热电材料方面的潜在应用引起了人们的关注,它们具有组成元素含量丰富、价格低廉、对环境没有污染等优点。本文结合密度泛函理论、电子和声子Boltzmann输运理论以及分子动力学方法研究了黑磷及相关低维材料的结构、电子、声子以及热电输运性质,探索它们在高性能热电材料方面的潜在应用。我们首先研究了块体黑磷的热电性能。计算发现黑磷的电热输运都表现出很强的各向异性。虽然在扶手椅型方向黑磷具有较高的晶格热导率,然而由于较大的功率因子,使得黑磷在800 K时的ZT值仍然可以优化到1.1。我们还考察了Sb掺杂对黑磷热电性能的影响,发现等电子替换不仅能通过增大费米面附近的态密度来改善黑磷的电输运性质,而且可以引入质量差来降低体系的晶格热导率,从而使得掺杂后的黑磷(P0.75Sb0.25)在800 K时的ZT值提高了4倍之多。在低维结构方面,我们首先研究了五种磷烯同素异形体(α-、β-、γ-、δ-和ζ-相)的热输运性质。对于a-磷烯,由于结构的特殊性,导致其热输运各向异性。而对于其它四种磷烯,热输运对方向的依赖性不明显,它们热导率的大小关系是β-相>δ-相>γ-相>ζ-相。在五种磷烯同素异形体中,我们发现具有更复杂结构的ζ-相的热导率最低,主要归因于其较低的声子弛豫时间和较高的三声子散射相空间。结合其优异的电输运特性,我们预测ζ-磷烯是一种有前途的高性能热电材料。我们还研究了具有五个原子层厚度的Bi2Te3片层结构的热电性能。计算表明,由于量子限制效应,片层结构的带隙大于相应的块体结构。结合玻尔兹曼输运理论和弛豫时间近似,我们计算了Bi2Te3片层的电输运系数。结果表明在合适的载流子浓度下体系的Seebeck系数可以达到583 μV/K。通过拟合第一性原理的总能计算,我们确定了描述该片层结构Morse potential中的势参数,并由分子动力学模拟得出了体系的晶格热导率。通过优化载流子浓度,Bi2Te3片层结构的室温ZT值可以达到较高的水平；而且热电性能表现出强烈的温度依赖关系,当温度为800 K时,体系的最优ZT值可以达到2.0。Sb掺杂可以进一步降低体系的晶格热导率,从而将ZT值增大到2.2。我们最后研究了不同宽度和不同边缘构型的磷烯纳米带的电、热输运性质。计算发现所有扶手椅型的纳米带都表现出半导体特性,而所有锯齿型的纳米带都表现出金属性。氢原子的钝化可以将金属性的纳米带变成半导体,同时增大半导体性纳米带的带隙。所有纳米带的带隙都是随着带宽的增加而减小。由于半导体性的纳米带都具有较大的带隙,使得它们在费米面附近表现出很大的Seebeck系数。热输运的研究表明扶手椅型的纳米带具有极低的晶格热导率。结合其较优异的电输运特性,可以将扶手椅型纳米带的室温ZT值优化到较高水平。更多还原

【Abstract】 The emerging global need for energy production, conservation, and management has promoted in the more effective means of power generation. A potential energy is from waste heat by using thermoelectric materials. The heat can come from the combustion of fossil fuel, automobile exhaust, and the process of chemical reaction. For this point, the thermoelectric materials play an important role in the power generation and energy conservation. Thermoelectric devices are scalable, reliable, quiet operation, no moving parts and maintenance-free. The efficiency of a thermoelectric material is determined by the dimensionless figure of merit (ZT= S2σT/κ). However, these transport parameters (S、σ and κ) are closely interrelated and related to the crystal and electronic structure, as well as the carrier concentration. As a result, the ZT value of some traditional bulk thermoelectric materials such as Bi2Te3, PbTe and Si1-xGex has remained at about 1.0 for several decades, whereas a ZT～3 is necessary to compete with conventional refrigerators or power generators. In 1993, the theoretical work of Hicks et al. reported that low-dimensional structures could exhibit significantly higher ZT values because of improved power factors on account of quantum confinement and decreased thermal conductivity caused by phonon boundary scattering. Since then, many efforts have been made to synthesize low-dimensional or nanoscale materials, such as Bi2Te3/Sb2Te3 superlattice structure, PbSeTe/PbTe quantum-dot superlattice, Si nanowires and nanostructured BiSbTe bulk alloys. However, the above mentioned materials synthesized by complicated fabrication technologies usually contain the elements are either expensive or toxic, which are not suitable for large-scale applications. Recently, the possibility of using black phosphorus (BP) and its low-dimensional structures as thermoelectric materials has attracted growing interest. Moreover, the phosphorus element is earth-abundant, low-cost and environmentally friendly. In this dissertation, we use a multiscale approach which includes density functional theory (DFT), semiclassical Boltzmann theory for electrons and phonons, and classical molecular dynamics (MD) simulations to investigate the structural, electronic, phonon, and transport properties of black phosphorus and related low-dimensional structures, and explore their possibility as high-performance thermoelectric materials.We first estimated the thermoelectric properties of bulk black phosphorus. It is found that the electronic and thermal transport in BP exhibit strong orientation dependence. Although the lattice thermal conductivity is large along the armchair direction, the thermoelectric efficiency can reach 1.1 at 800 K since we can get a large power factor. We also show the effect of substitution of P atom with Sb atom on the thermoelectric properties of BP. It is found that Sb substitution can not only enhance the electronic transport of BP by increasing the DOS around the Fermi level, but also suppress the lattice thermal conductivity because of the mass difference between Sb and P atom. As a result, the optimal ZT value of the substituted system (P0.75Sb025) is 4 times higher than that of pristine BP at 800 K.As for the low-dimensional system, we first studied the thermal transport properties of five phosphorene allotropes (α-, β-, γ-,δ- and ζ-phase). It is found that the a-phosphorene exhibits considerable anisotropic thermal transport, while it is less obvious in the other four phosphorene allotropes. The highest thermal conductivity is found in the β-phosphorene, followed by δ-, γ- and ζ-phase. The much low thermal conductivity of ζ-phosphorene is consistent with its relatively complex atomic configuration and can be attributed to a low phonon relaxation time and high scattering phase space. Combined with good electronic transport properties, it is very promising to use ζ-phosphorene as high-performance thermoelectric material.We also investigated the transport properties of Bi2Te3 consisting of one quintuple layer (QL). It is found that the band gap of an isolated QL is considerably larger than that of bulk Bi2Te3. The electronic transport of the QL is then evaluated using the semiclassical Boltzmann theory within the relaxation time approximation. By fitting the energy surface from first-principles calculations, a suitable Morse potential is constructed and used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. By optimizing the carrier concentration of the system, the ZT of Bi2Te3 QL can be enhanced to a relatively high value. Moreover, the ZT value exhibits strong temperature dependence and can reach as high as 2.0 at 800 K. This value can be further increased to 2.2 by the substitution of Bi atoms with Sb atoms, giving nominal formula of (Bi0.25Sb0.75)2Te3.Finally, we studied the electronic and thermal transport properties of phosphorene nanoribbons with different width and edge configurations. It is found that the armchair phosphorene nanoribbons are semiconducting while the zigzag nanoribbons are metallic. By passivating the edge phosphorus atoms with hydrogen, the zigzag series also become semiconducting, while the armchair series exhibit a larger band gap than their pristine counterpart. The band gaps of semiconducting PNRs decrease monotonically with increasing ribbon width. Due to a relatively large band gap, the Seebeck coefficient of semiconducting PNRs exhibits a large value around the Fermi level. With the help of phonon Boltzmann transport equation, we found the armchair nanoribbons exhibit a lower lattice thermal conductivity than that of zigzag nanoribbons. Combined with good electronic transport properties, it is very promising to use APNR as high-performance thermoelectric material.更多还原