Negative Thermal Expansion Materials under Pressure: a New Route for High Performance Thermoelectrics

在全球不断增长的能源需求格局中，热电技术为解决一个重要的利基问题提供了一条有吸引力的途径，因为它们可以将热量，特别是废热转化为电能。在过去的几十年里，以声子玻璃电子晶体的概念为指导，寻找高效率的热电材料。尽管在这一概念下已经取得了显著的进展，但能量转换性能仍然远远低于热电学与传统发电方法竞争所需的性能，这主要是由于电子和声子输运之间强耦合的性质。该提案旨在探索一个新的领域，当负热膨胀材料，压力和热电结合在一起时，电子和声子输运可以解耦。负热膨胀材料具有固有的性质，即大量低频声子与柔性较差的材料相比具有较强的声子非谐波性，导致其本征导热系数很低，这对热电器件是有利的。然而，到目前为止，由于通常发生负热膨胀的低热功率或极低的温度范围，它们尚未被广泛研究用于热电学。在此，我们提出了一个新的概念，通过施加压力来提高热电系数，利用独特的特点，即电和声子输运性质可以在很大程度上以相反的方式调谐。本项目的总体目标是推进具有代表性的负热膨胀材料在机械压缩下的电和声子输运特性的基础科学研究，并合理地进行掺杂和合金化，作为高效热电材料的新策略。提出了基于第一性原理和玻尔兹曼输运方程的组合非调和晶格动力学作为实现这一目标的方法。这些研究的结果很可能为基于负热膨胀材料的热电材料的合理优化提供重大进展，并有可能为未来的能源需求做出明确的贡献。本课题的新颖之处在于，对负热膨胀材料施加压力，不仅可以通过增加声子的非谐性来降低晶格热导率，而且可以大大提高其热功率，将负热膨胀范围向上推至更高的温度，从而实现热电体中电与声子输运的解耦，从而显著提高热电性能。这种相反的效应在传统的优化热电材料的方法中从未实现过，在传统的热电材料中，电和声子输运总是强相关的，并且是一致的。

Thermoelectrics offer an attractive pathway for addressing an important niche in the globally growing landscape of energy demand, since they can convert heat and, in particular waste heat, to electricity. In the past decades, searching for high efficiency thermoelectrics has been guided by the concept of phonon glass electron crystal. Despite remarkable progress have been made under this concept, the energy conversion performance is still well below what is needed for thermoelectrics to compete with traditional electricity producing methods, primarily due to the nature of strong coupling between electronic and phononic transport. This proposal aims at exploring a new area where the electronic and phononic transport can be decoupled, when negative thermal expansion material, pressure, and thermoelectrics meet together.Negative thermal expansion materials have inherent nature that a large number of low frequency acoustic phonons possess strong phonon anharmonicity compared with less flexible materials, leading to very low intrinsic thermal conductivity, which is beneficial for thermoelectrics. However, they have not been extensively studied for thermoelectrics so far, due to the low thermopower or extremely low temperature range that negative thermal expansion normally occurs. Herewith we propose a new concept of applying pressure to enhance the thermoelectric coefficient by taking advantage of the unique feature that the electrical and phononic transport properties can be largely tuned in an opposite way.The overall goal of this project is to advance the fundamental science underlying the electrical and phononic transport properties of some representative negative thermal expansion materials under mechanical compression and rationally performed doping and alloying as novel strategy for high efficiency thermoelectrics. Combined anharmonic lattice dynamics based on first principles and Boltzmann transport equation are proposed as approaches to this end. The result of these investigations is likely to provide a major advancement to rational optimizing of thermoelectrics based on negative thermal expansion materials, with the potential to make a clear contribution to the energy needs of the future.The novelty of this project manifests itself in that, applying pressure to negative thermal expansion materials not only reduces the lattice thermal conductivity by augmenting phonon anharmonicity, but also largely enhances their thermopower and pushes the negative thermal expansion range upward to higher temperatures, which in turn achieves decoupling the electrical and phononic transport in thermoelectrics and results in significant improvement in thermoelectric performance. Such opposing effects have never been realized in traditional methods for optimizing thermoelectric materials, where the electrical and phononic transport is always strongly correlated and acts in unison.