Vibrational Structure and Thermal Transport in Statically and Dynamically Disordered Crystals

非技术总结该奖项支持研究和教育活动，旨在促进我们对固体材料热输运的基本理解。热传输在从节能建筑到冷却电子产品再到将废热转化为电力的各种应用中都是一个关键的考虑因素。导热系数是一种描述热如何在材料中传递的特性。预测固体导热系数的标准方法是基于其原子要么完全有序，要么完全无序的假设。然而，许多固体的原子结构既有有序元素，也有无序元素。例如，一类被称为金属卤化物钙钛矿的材料可以将阳光转化为电能，它含有处于规则有序位置的分子，但分子旋转，这会造成无序。它们的导热系数必须是已知的，才能确定设备的运行温度，而不能使用标准方法进行预测。该项目首先寻求建立进行此类计算的理论工具和计算框架。然后，该框架将被用来理解金属卤化物钙钛矿和钠超氧化物中共存的有序和无序如何影响热导率，后者是钠-空气电池的重要组成部分。这些工具和框架将适用于研究任何含有有序和无序元素的固体中的热输运。参与研究的研究生和本科生将进行跨越材料科学、物理和机械工程的尖端、跨学科的研究。将开发一个模块化的、开放获取的分子模拟课程，并分发给科学界。一项探索有序-无序光谱的推广活动将被开发出来，并向匹兹堡地区的中学生和高中生展示。技术总结该奖项支持研究和教育活动，旨在促进我们对静态和动态无序晶体中热传输的基本理解。这种材料具有明确的晶格和基团，在较高的温度下会出现无序元素。在具有静态无序的晶体中，多势垒势能面导致晶胞之间的差异(例如，氧二聚体在超氧钠中的取向)。在具有动态无序的晶体中，某些原子不会围绕平衡位置(例如，卤化物钙钛矿中旋转的甲基铵离子)振动。中心假设是，静态和动态无序晶体中的热输运可以通过声子、扩散子(离域、非传播振动模)和缺陷的综合处理来描述。我们将建立一个综合了有限温度力常数、晶格动力学、玻尔兹曼输运方程、Allen-Feldman理论和微扰理论的计算框架来预测模式相关性质和热导率。新的贡献将包括首次实现用于热导预测的旋转晶格动力学，用于模拟静态无序散射的虚拟晶体近似方法的公式，以及使用不稳定平衡结构来包括动态无序散射。然后，该框架将用于了解超氧化物钠和卤化物钙钛矿中的静态和动态无序如何影响导热系数。参与研究的研究生和本科生将开展跨越材料科学、物理和机械工程的前沿、跨学科研究。将通过NanHUB开发和分发一个模块化的、开放访问的分子模拟课程。匹兹堡地区的初中生和高中生将开展一项探索秩序障碍谱的推广活动。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARYThis award supports research and educational activities with an aim to advance our fundamental understanding of thermal transport in solid materials. Thermal transport is a critical consideration in applications ranging from energy-efficient buildings to cooling electronics to converting waste heat into electricity. Thermal conductivity is a property that describes how heat moves through a material. Standard approaches for predicting the thermal conductivity of a solid are based on an assumption that its atoms are either perfectly ordered or fully disordered. The atomic structure of many solids, however, has elements of both order and disorder. For example, a class of materials, known as metal halide perovskites which can convert sunlight into electricity, contain molecules at regularly ordered positions, but the molecules rotate, which creates disorder. Their thermal conductivities, which must be known to determine device operating temperatures, cannot be predicted using standard approaches. This project first seeks to build theoretical tools and a computational framework for performing such calculations. The framework will then be applied to understand how thermal conductivity is impacted by co-existing order and disorder in metal halide perovskites and sodium superoxide, which is an important component of sodium-air batteries. The tools and framework will be suitable for studying thermal transport in any solid that contains elements of order and disorder.The participating graduate and undergraduate students will carry out cutting-edge, interdisciplinary research that spans materials science, physics, and mechanical engineering. A modular, open-access molecular simulation course will be developed and distributed to the scientific community. An outreach activity that explores the order-disorder spectrum will be developed and presented to middle-school and high-school students in the Pittsburgh area.TECHNICAL SUMMARYThis award supports research and educational activities with an aim to advance our fundamental understanding of thermal transport in statically and dynamically disordered crystals. Such materials have a well-defined lattice and basis, with elements of disorder emerging at higher temperatures. In a crystal with static disorder, a multi-well potential energy surface leads to differences between unit cells (e.g., the oxygen dimer orientations in sodium superoxide). In a crystal with dynamic disorder, some atoms do not vibrate around an equilibrium position (e.g., the rotating methylammonium ion in a halide perovskite).The central hypothesis is that thermal transport in statically and dynamically disordered crystals can be described through an integrated treatment of phonons, diffusons (delocalized, non-propagating vibrational modes), and defects. A computational framework will be built that integrates finite-temperature force constants, lattice dynamics, the Boltzmann transport equation, Allen-Feldman theory, and perturbation theory to predict mode-dependent properties and thermal conductivities. Novel contributions will include the first-ever implementation of rotational lattice dynamics for thermal conductivity prediction, the formulation of a virtual crystal approximation approach for modeling scattering by static disorder, and the use of unstable equilibrium structures to include scattering by dynamic disorder. The framework will then be applied to understand how thermal conductivity is impacted by static and dynamic disorder in sodium superoxide and halide perovskites.The participating graduate and undergraduate students will carry out cutting-edge, interdisciplinary research that spans materials science, physics, and mechanical engineering. A modular, open-access molecular simulation course will be developed and distributed through nanoHUB. An outreach activity that explores the order-disorder spectrum will be developed and presented to middle-school and high-school students in the Pittsburgh area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.