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传输的基本理解。热传输是从节能建筑到冷却电子产品再到将废热转化为电能的应用中的关键考虑因素。热导率是描述热量如何通过材料移动的属性。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可以将阳光转化为电能，它包含规则有序位置的分子，但分子旋转，这会造成混乱。它们的热导率，这必须是已知的，以确定设备的工作温度，不能使用标准的方法预测。该项目首先寻求建立理论工具和计算框架来执行此类计算。然后，该框架将被应用于了解热导率如何受到金属卤化物钙钛矿和超氧化钠中共存的有序和无序的影响，超氧化钠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包含有序和无序元素的任何固体中的热传输。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。将开发一个模块化的开放式分子模拟课程，并分发给科学界。一个探索有序无序光谱的推广活动将被开发并呈现给匹兹堡地区的初中和高中学生。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础，在较高的温度下出现无序元素。在具有静态无序的晶体中，多阱势能表面导致晶胞之间的差异（例如，超氧化钠中的氧二聚体取向）。在具有动态无序的晶体中，一些原子不围绕平衡位置振动（例如，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、非传播振动模式）和缺陷的综合处理来描述。将建立一个计算框架，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来预测模式相关的属性和热导率。新的贡献将包括有史以来第一次实施的旋转晶格动力学的热导率预测，制定一个虚拟的晶体近似方法建模散射静态无序，以及使用不稳定的平衡结构，包括散射动态无序。该框架将用于了解超氧化钠和卤化物钙钛矿中静态和动态无序对热导率的影响。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。将通过nanoHUB开发和分发一个模块化的开放式分子模拟课程。一个探索有序-无序光谱的外展活动将被开发并呈现给匹兹堡地区的初中和高中学生。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.