Phonon Engineering of Nanocomposite Thermoelectric Materials

热电（TE）是由热机或加热装置发电的过程。现代TE应用的例子包括便携式冰箱，饮料冷却器，电子元件冷却器，红外传感等。TE器件可能的未来应用包括将废热有效地转化为可用能源，提高光伏电池的效率等。由于其能源效率，TE材料已经被研究了几十年。热电材料的效率由质量量z来定义。习惯上用无因次量ZT来表示热电材料的有用性，其中T是温度。较大的ZT值要求高塞贝克系数S、高导电性和低导热性。热导率可以由载流子和声子共同贡献。S的增加通常意味着电导率的降低，而后者的增加通常意味着载流子导热系数的增加。因此，在典型的TE材料中增加Z是非常困难的。显然，对于更高的Z值，我们需要具有电子散射效率较低和声子散射效率较低的材料。过去几十年的研究表明，SiGe和双硫族化合物分别是良好的高温和低温TE材料。人们认识到，选择降低维数的材料是增加ZT相对于体积值的谨慎策略。这是因为低维提供了四个新特征，每个特征都有助于增加ZT:(1)增加费米能级附近态的电子密度；(2)各向异性有效质量张量和多个费米能量椭球的贡献；(3)通过增加声子在势垒-阱界面的散射来降低热导率，而界面上的电子散射没有那么大的增加；(4)满足量子约束条件时，在一定载流子浓度下增加了载流子迁移率，从而可以在一定程度上利用调制掺杂和δ掺杂。使用特征(2)增加ZT的尝试需要以一种最好地利用有效质量张量的各向异性性质的方式实现低维增长，并且可能不容易控制。相比之下，特征(1)和(3)被认为是可控的，对增加ZT非常有效。考虑特征(1)所提供的量子尺寸效应将增加费米能级上的电子密度，而考虑特征(3)所利用的边界散射效应将大大降低热导率，而不会对电导率造成太大损失。本提案旨在通过在宽温度范围内对硅基纳米复合热电材料的增强ZT进行系统的最先进的理论研究，确定发展声子工程的关键参数。提议的研究将基于四个关键的理论成分，所有这些都是由PI开发的：平衡几何，电子态和声子的第一原理方法（使用伪势和密度泛函理论）；大胞元系统中声子的绝热键荷模型一个处理声子-声子相互作用的非调和弹性连续体模型以及一个声子电导率理论模型。研究的系统将是：(i)薄Si/Ge和Si/SiGe超晶格，（ii）嵌入Ge和SiGe矩阵的Si纳米线，以及（iii）嵌入Ge或SiGe矩阵的Si纳米点。我们的研究还将帮助我们确定维度在大温度范围内增强热力学优值图中的作用。

Thermoelectricity (TE) is the process of generating either electricity from heat engines or heating devices from electricity. Examples of modern TE applications include portable refrigerators, beverage coolers, electronic component coolers, infrared sensing, etc. Possible future applications of TE devices include efficient conversion of waste heat into usable energy, in improving the efficiency of photovoltaic cells, etc. TE materials have been investigated for several decades due to their energy efficiency. The efficiency of TE materials is defined by the figure of merit quantity Z. It is customary to express the usefulness of a thermoelectric material in terms of the dimensionless quantity ZT, where T is temperature. Larger values of ZT require high Seebeck coefficient S, high electrical conductivity, and low thermal conductivity. The thermal conductivity can be contributed by both carriers and phonons. An increase in S normally implies a decrease in electrical conductivity, and an increase in the latter normally implies an increase in carrier thermal conductivity. Thus, it is very difficult to increase Z in typical TE materials. Clearly, for higher values of Z, we require materials which are characterised by less efficient scattering of electrons and efficient scattering of phonons. Research over the past several decades has shown that SiGe and Bi-chalcogenides are good high-temperature and low-temperature TE materials, respectively. It is being realised that material choice with reduced dimensionality is a prudent strategy for increasing ZT relative to bulk values. This is because low dimensionality provides four new features, each of which can be helpful in increasing ZT: (1) increase in electronic density of states near Fermi level; (2) anisotropic effective mass tensor and contribution from multiple Fermi-energy ellipsoids; (3) reduction in thermal conductivity by increasing phonon scattering at the barrier-well interfaces, without as large an increase in electron scattering at the interface; and (4) increased carrier mobility at a given carrier concentration when quantum confinement conditions are satisfied, so that modulation doping and delta-doping can be utilised to some extent. An attempt to increase ZT using feature (2) requires achieving low-dimensional growth in a manner that makes best use of the anisotropic nature of the effective mass tensor, and may not be easily controllable. In contrast, features (1) and (3) are considered to be controllable and very effective in increasing ZT. The quantum size effects offered by the consideration of feature (1) will increase electron density of states at Fermi level, and exploiting the boundary scattering effects by the consideration of feature (3) will substantially reduce the thermal conductivity without much loss to the electrical conductivity. This proposal seeks to identify the key parameters for developing the phonon engineering of Si-based nanocomposite thermoelectric materials by undertaking a systematic state-of-the-art theoretical study of their enhanced ZT over a wide temperature range. The proposed research will be based on four key theoretical ingredients, all of which have been developed by the PI: a first-principles approach (using the pseudopotential and density-functional theories) for equilibrium geometry, electronic states, and phonons; the adiabatic bond charge model for phonons in systems with larger unit cells; an anharmonic elastic continuum model to treat phonon-phonon interactions; and a model phonon conductivity theory. The systems are investigations will be: (i) thin Si/Ge and Si/SiGe superlattices, (ii) Si nanowires embedded in Ge and SiGe matrices, and (iii) Si nanodots embedded in a Ge or SiGe matrix. Our study will also help us identify the role of dimensionality in the enhancement of the thermodynamic figure of merit over a large temperature range.