First-principles design of strongly anharmonic crystalline solids with ultra-low lattice thermal conductivity

该奖项支持计算研究和教育，以推进绝缘和半导体材料热传输特性的预测建模。化石燃料（石油、煤和天然气）占世界能源使用的绝大部分。不到1/3的总能量被有效利用，而其余的主要以废热的形式被拒绝。仅在美国，这种被浪费的能量就相当于4000亿加仑汽油所含的能量。许多提高能源效率或废热再利用的技术都依赖于隔热陶瓷和半导体的可用性。例如，热电通过一种叫做塞贝克效应的现象将热转化为电，这种现象在热电极和冷电极之间发生作用。高性能热电器件需要导热性能差但导电效率高的材料。目前，这种材料已知的很少，这阻碍了这种节能技术的广泛采用。同样，更好的陶瓷隔热材料可以通过提高内燃机的工作温度来节省能源。非金属晶体中的热是通过原子振动传导的，比如声波。传统的半导体（如硅）是很差的隔热体，因为这些波彼此独立传播，遇到的阻力很小。降低热导率的经验方法旨在阻止振动波的向前运动。然而，这些方法需要复杂的制备方法，而且效率往往有限。PI和他的团队将设计本质上具有低导热性的材料，因为原子振动的热传导波彼此散射。在某些固体中，这种散射可以变得如此强烈，以至于热流达到与玻璃和非晶态固体相关的最低可能值。该项目将推进计算技术和量子力学概念，以识别和故意创造具有所需性能的隔热固体。拟议研究的更广泛影响是多方面的，包括研究成果的广泛传播，在丰富的多学科环境中对研究生的教育，以及向本科生介绍现代计算方法。该研究项目的成功完成将有助于开发新型隔热材料，如热电材料和涂层，在节能方面具有潜在的优势。计算出的热性能将通过与现有的计算材料数据在线数据库合作，向科学和技术界传播。该奖项支持研究和教育，通过开发降低晶格导热率的计算方法和理论概念，推进晶体绝缘体和半导体热传输特性的预测建模。PI和他的团队主要对强非调和固体感兴趣，其中固有声子-声子相互作用将热导率限制在接近无定形极限的值；这类材料在节能和废热回收方面具有重要意义，如陶瓷涂层和热电材料。PI基于量子力学的设计策略包括使用具有非调和键的孤对电子、具有强p-d电子杂化效应的过渡金属的氧化物和氧硫化物、具有接近电子Jahn-Teller不稳定性的离子的高对称性固体，以及具有受挫结构配位的化合物。这些方法是基于基本原理，通过一种独立于加工、杂质和晶粒结构的机制增强晶体材料中的本征声子散射。这将允许在大块氧化物和半导体中获得最小的晶格热导率。为了精确计算热性能，该小组将进行以下理论发展：(1)基于压缩传感的构建组成无序材料晶格动力学哈密顿量的方法；(2)计算强非调和固体低温热输运性质的有效路径积分分子动力学技术；(3)处理部分填充d和f电子壳层中具有竞争轨道有序态的固体绝热晶格动力学的电子结构技术。这些方法将形成一套全面的计算技术，用于非调和固体中热输运的第一性原理研究。本文的工作将有助于提高对强非调和材料热输运性质的基本认识，推进固体热输运性质建模的理论和软件工具，并为低导热材料的合理设计提供理论依据。拟议研究的更广泛影响是多方面的，包括研究成果的广泛传播，在丰富的多学科环境中对研究生的教育，以及向本科生介绍现代计算方法。该研究项目的成功完成将有助于开发新型隔热材料，如热电材料和涂层，在节能方面具有潜在的优势。计算出的热性能将通过与现有的计算材料数据在线数据库合作，向科学和技术界传播。

NONTECHNICAL SUMMARYThis award supports computational research and education to advance predictive modeling of thermal transport properties of insulating and semiconducting materials. Fossil fuels (petroleum, coal, and natural gas) account for the vast majority of the energy use in the world. Less than 1/3 of the total energy content is used productively, while the rest is rejected, mainly in the form of waste heat. In the US alone, the amount of this wasted energy is roughly equivalent to that contained in 400 billion gallons of gasoline. Many technologies for improving energy efficiency, or reusing the waste heat, depend on the availability of thermally insulating ceramics and semiconductors. For instance, thermoelectrics convert heat into electricity via a phenomenon called the Seebeck effect, which operates between a hot and a cold electrode. High-performance thermoelectric devices require materials that are poor conductors of heat but efficient conductors of electricity. Currently, few such materials are known, which prevents a wider adoption of this energy-saving technology. Similarly, better ceramic thermal insulators would save energy by enabling increased operating temperatures in combustion engines.Heat in nonmetallic crystals is conducted by atomic vibrations, such as sound waves. Conventional semiconductors (e.g. silicon) are poor thermal insulators because these waves propagate independently of each other and encounter little resistance. Empirical methods for decreasing thermal conductivity aim to impede the forward movement of vibrational waves. However, these approaches require complex preparation methods and are often limited in efficiency. The PI and his group will design materials that have intrinsically low thermal conductivity because the heat-carrying waves of atomic vibrations scatter off of each other. In select solids, this scattering can become so strong that the flow of heat reaches the lowest possible value associated with glasses and amorphous solids. The project will advance computational techniques and quantum mechanical concepts for identifying and deliberately creating thermally insulating solids with desired properties.The broader impacts of the proposed research are several-fold, involving broad dissemination of research results, education of graduate students in a rich multidisciplinary environment, and introduction of undergraduate students to modern computational methods. Successful completion of this research program will contribute to the development of new thermally insulating materials as thermoelectrics and coatings with potential benefits in energy conservation. The calculated thermal properties will be disseminated to the scientific and technical community by partnering with existing online databases of computed materials data.TECHNICAL SUMMARYThis award supports research and education to advance predictive modeling of thermal transport properties of crystalline insulators and semiconductors via the development of computational methods and theoretical concepts for lowering lattice thermal conductivity. The PI and his group are mainly interested in strongly anharmonic solids where intrinsic phonon-phonon interactions limit thermal conductivity to values near the amorphous limit; such materials are of interest in energy conservation and waste-heat recovery as ceramic coatings and thermoelectrics. The PI's quantum-mechanics-based design strategy involves the use of lone-pair electrons that host anharmonic bonds, oxides, and oxosulfides of transition-metals that host strong p-d electronic hybridization effects, high-symmetry solids with ions near electronic Jahn-Teller instabilities, and compounds with frustrated structural coordination. These approaches are based on fundamental principles for enhancing intrinsic phonon scattering in crystalline materials via a mechanism that is independent of processing, impurities, and grain structure. This will allow the attainment of minimal lattice thermal conductivity in bulk oxides and semiconductors.To enable accurate computation of thermal properties, the group will pursue the following theoretical developments: (1) compressive-sensing-based methods for building lattice dynamical Hamiltonians for compositionally disordered materials, (2) efficient path integral molecular dynamics techniques for calculating low-temperature thermal transport properties of strongly anharmonic solids, and (3) electronic structure techniques for treating adiabatic lattice dynamics of solids with competing orbital ordering states in partially filled d-and f-electron shells. These methods will form a comprehensive suite of computational techniques for first-principles studies of thermal transport in anharmonic solids.The proposed work is expected to improve fundamental understanding of the thermal transport properties of strongly anharmonic materials, advance the theory and software tools for modeling thermal transport properties of solids, and provide a theoretical basis for rational design of materials with low thermal conductivity. The broader impacts of the proposed research are several-fold, involving broad dissemination of research results, education of graduate students in a rich multidisciplinary environment, and introduction of undergraduate students to modern computational methods. Successful completion of this research program will contribute to the development of new thermally insulating materials as thermoelectrics and coatings with potential benefits in energy conservation. The calculated thermal properties will be disseminated to the scientific and technical community by partnering with existing online databases of computed materials data.