Vibrational Structure and Thermal Transport in Statically and Dynamically Disordered Crystals

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。在从节能建筑到冷却电子设备到将废热转化为电能的应用中，热传输是一个关键的考虑因素。导热性是描述热量如何通过材料的一种特性。预测固体热导率的标准方法是基于一个假设，即它的原子要么是完全有序的，要么是完全无序的。然而，许多固体的原子结构既有有序的成分，也有无序的成分。例如，一类被称为金属卤化物钙钛矿的材料，它可以将阳光转化为电能，它包含有规律有序位置的分子，但分子会旋转，这会造成混乱。它们的热导率必须知道，以确定设备的工作温度，不能用标准方法预测。该项目首先寻求建立执行此类计算的理论工具和计算框架。然后将应用该框架来了解金属卤化物钙钛矿和超氧化物钠共存的有序和无序如何影响导热性，这是钠-空气电池的重要组成部分。该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。参与的研究生和本科生将开展跨越材料科学、物理学和机械工程的前沿跨学科研究。一个模块化的、开放获取的分子模拟课程将被开发并分发给科学界。一项探索秩序紊乱谱系的拓展活动将被开发出来，并呈现给匹兹堡地区的中学生和高中生。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的热输运的基本理解。这种材料具有明确的晶格和基，在较高的温度下出现无序元素。在具有静态无序的晶体中，多阱势能表面导致了单元胞之间的差异（例如，超氧化钠中氧二聚体的取向）。在具有动态无序的晶体中，一些原子不会围绕平衡位置振动（例如，卤化物钙钛矿中旋转的甲基铵离子）。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（离域、非传播振动模式）和缺陷的综合处理来描述。将建立一个集成有限温度力常数、晶格动力学、玻尔兹曼输运方程、艾伦-费尔德曼理论和微扰理论的计算框架，以预测模式相关的性质和导热系数。新的贡献将包括首次实现用于导热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制定，以及使用不稳定平衡结构来包括动态无序散射。然后，该框架将被应用于了解超氧化物钠和卤化物钙钛矿的静态和动态无序如何影响导热性。参与的研究生和本科生将开展跨越材料科学、物理学和机械工程的前沿跨学科研究。一个模块化的、开放获取的分子模拟课程将通过nanoHUB开发和分发。一项探索秩序紊乱谱系的拓展活动将被开发出来，并呈现给匹兹堡地区的中学生和高中生。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.