Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers
Hierarchical thermoelectrics: crystal grain boundaries as scalable phonon scatterers
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DOI：
10.1039/c5nr05279c
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发表时间：
2016-02-14
期刊：
影响因子：
6.7
通讯作者：
Leoni, Stefano
中科院分区：
文献类型：
--
作者：
Selli, Daniele;Boulfelfel, Salah Eddine;Leoni, Stefano
热电材料对于可持续发展具有战略价值，因为它们可以从废热中产生电能。近年来，几种策略在改善热电性能方面表现出一定的效率。掺杂剂影响载流子浓度，而热导率可以受到合金化和纳米结构化的影响。纳米尺度的特征对散射声子有积极的贡献，但是那些具有长平均自由程的特征仍然难以改变。在这里，我们使用分级纳米晶粒的概念，以证明在岩盐铅硫族化合物的热导率降低。我们证明，可以通过利用重建沿着相变路径，连接岩盐结构，其高压形式的优势，获得颗粒。由于晶粒特征随尺寸的变化而自然变化，因此它们会影响不同长度尺度上的热导率。为了理解这种效应，我们使用先进的分子动力学技术相结合的工程颗粒和评估的PbSe的热导率。通过仅影响晶粒形态，即在恒定的化学性质下，出现两种不同的效果：相对于完美晶体，晶格热导率显著降低，并且其温度依赖性显著抑制。这是由于在不同的尺寸尺度上的晶界的低频声子的散射增加。沿着这条线，我们提出了一个可行的过程，通过施加压力，通过机械负载或冲击波作为一种新的材料设计的范例，生产分层热电材料。
Thermoelectric materials are strategically valuable for sustainable development, as they allow for the generation of electrical energy from wasted heat. In recent years several strategies have demonstrated some efficiency in improving thermoelectric properties. Dopants affect carrier concentration, while thermal conductivity can be influenced by alloying and nanostructuring. Features at the nanoscale positively contribute to scattering phonons, however those with long mean free paths remain difficult to alter. Here we use the concept of hierarchical nano-grains to demonstrate thermal conductivity reduction in rocksalt lead chalcogenides. We demonstrate that grains can be obtained by taking advantage of the reconstructions along the phase transition path that connects the rocksalt structure to its high-pressure form. Since grain features naturally change as a function of size, they impact thermal conductivity over different length scales. To understand this effect we use a combination of advanced molecular dynamics techniques to engineer grains and to evaluate thermal conductivity in PbSe. By affecting grain morphologies only, i.e. at constant chemistry, two distinct effects emerge: the lattice thermal conductivity is significantly lowered with respect to the perfect crystal, and its temperature dependence is markedly suppressed. This is due to an increased scattering of low-frequency phonons by grain boundaries over different size scales. Along this line we propose a viable process to produce hierarchical thermoelectric materials by applying pressure via a mechanical load or a shockwave as a novel paradigm for material design.