Thermal Transport in Large Unit Cell Crystals

非技术总结该奖项支持理论和计算研究和教育，旨在实现对热能如何在具有大晶胞的晶体中传输的基本了解。在晶体材料中，原子的排列可以用一个晶胞来描述，该晶胞在各个方向上周期性地重复，从而组成大块晶体。在一些晶体中，例如金和硅，单位晶胞只包含几个原子，而在另一些晶体中，单位晶胞可能包含数百个以上的原子。大单胞晶体的例子包括“沸石”和“富勒烯”，前者在催化、分子分离和气体储存方面有应用，后者在分子电子学和太阳能转换方面有应用。了解这些材料中的热传输对于预测它们将如何响应周围环境的温度波动以及如何分散设备运行过程中产生的多余热量至关重要。热在晶体材料中流动的方式取决于它的晶胞。对热输运的传统理解是基于为小晶胞晶体发展的理论。然而，实验证据表明，这些理论不适合于模拟大晶胞晶体中的热输运。该项目的目标是使用原子水平的计算工具来开发一个框架来预测大单元晶体的导热系数，并将其应用于沸石和富勒烯。这一结果将对在日常应用中使用这些材料的科学家和工程师具有直接的重要性。开发的计算工具将适用于其他大型单元晶体的建模。通过这一项目，研究团队将通过系列讲座将纳米科学整合到卡内基梅隆大学的本科工程课程中，并通过国家科学基金会资助的NanHUB分发材料。将开展与大单胞晶体有关的推广活动，并向匹兹堡的初中生和高中生介绍技术总结该奖项支持理论和计算研究和教育，旨在实现对大单胞晶体中热能传输的基本理解。具有大单位电池的晶体材料在催化、分子分离、气体储存、热电能量转换和太阳能转换等领域面临着广泛的与能源相关的挑战和机遇。在其中许多领域，热运输发挥着关键作用，但受到的关注很少。这个项目的中心假设是，在一个大的晶胞晶体中，并不是所有的振动模都会传播，并且在小于晶格常数的长度尺度上的能量传输机制是重要的。总体目标是揭示潜在的物理机制，并提出设计具有定制热性能的材料的策略。将使用一套原子级别的计算工具来考虑两种不同的材料系统，包括分子动力学模拟和晶格动力学计算。首先，我们将研究沸石分子筛，在沸石分子筛中，由于共价和静电相互作用，所有的原子都是强键的。非谐和谐和对热传输的影响将被严格模拟，以量化不同类型的振动模式对热传导性的贡献。骨架铝、非骨架阳离子和吸附物种对导热系数的影响将被量化。其次，将研究富勒烯分子晶体，其中分子内相互作用较强，但由于范德华作用力分子间相互作用较弱。在较低温度下，将建立降阶模型，而在较高温度下，将探索基于分子间热导的网络模型。建模预测将通过与实验研究小组的合作进行验证。开发的方法和工具将转化为对其他大单胞晶体中热输运的研究，如笼合物、方钛矿、锌化合物、气体水合物和金属有机骨架。通过这个项目，研究小组将通过一系列讲座将纳米科学整合到卡内基梅隆大学的本科工程课程中，并通过国家科学基金会资助的纳米HUB分发材料。将开展与大单位细胞晶体有关的推广活动，并向匹兹堡的初中生和高中生介绍。

NON-TECHNICAL SUMMARYThis award supports theoretical and computational research and education aimed at achieving a fundamental understanding of how thermal energy is transported in crystals with large unit cells. In a crystalline material, the arrangement of atoms can be described in terms of a unit cell which periodically repeats in all directions to make up the bulk crystal. In some crystals such as gold and silicon, the unit cell contains only a few atoms, while in others, the unit cell may contain upwards of hundreds of atoms. Examples of large unit cell crystals include "zeolites", which have application in catalysis, molecular separation, and gas storage, and "fullerenes", which have applications in molecular electronics and solar energy conversion. Understanding thermal transport in these materials is critical for predicting how they will respond to temperature fluctuations in their surroundings and how they can dissipate excess heat generated during device operation. The way that heat flows through a crystalline material depends on its unit cell. Conventional understanding of thermal transport is based on theory developed for small unit cell crystals. Experimental evidence, however, suggests that these theories are not suitable for modeling thermal transport in large unit cell crystals. The objective of this project is to use atomic-level computational tools to develop a framework for predicting the thermal conductivity of large unit crystals and apply it to zeolites and fullerenes. The results will be of direct importance to scientists and engineers using these materials in everyday applications. The computational tools developed will be suitable for modeling other large unit cell crystals.Through this project, the research team will integrate nanoscience into the undergraduate engineering curriculum at Carnegie Mellon University through a lecture series and distribute the materials through the National Science Foundation-supported nanoHUB. Outreach activities related to large unit cell crystals will be developed and presented to middle school and high school students in Pittsburgh.TECHNICAL SUMMARYThis award supports theoretical and computational research and education aimed at achieving a fundamental understanding of how thermal energy is transported in crystals with large unit cells. Crystalline materials with large unit cells are relevant in a wide range of energy-related challenges and opportunities, such as in catalysis, molecular separation, gas storage, thermoelectric energy conversion, and solar energy conversion. In many of these areas, thermal transport plays a critical role, but has received minimal attention. The central hypothesis of this project is that not all vibrational modes in a large unit cell crystal propagate and that energy transport mechanisms at length scales smaller than the lattice constant are important. The overarching objective is to uncover the underlying physical mechanisms and to suggest strategies for the design of materials with tailored thermal properties.Two distinct materials systems will be considered using a suite of atomic-level computational tools including molecular dynamics simulations and lattice dynamics calculations. First, zeolites, where all atoms are strongly bonded due to covalent and electrostatic interactions will be investigated. Anharmonic and harmonic effects on thermal transport will be rigorously modeled to quantify the contributions of different types of vibrational modes to thermal conductivity. The effects of framework aluminum, non-framework cations, and adsorbed species on thermal conductivity will be quantified. Second, fullerene-based molecular crystals, where the intramolecular interactions are strong but the intermolecular interactions are weak due to van der Waals forces, will be investigated. At low temperatures, a reduced-order model will be developed, while at higher temperatures a network model based on the thermal conductance between molecules will be explored. The modeling predictions will be validated through collaboration with experimental research groups. The methods and tools developed will translate to studies of thermal transport in other large unit cell crystals, such as clathrates, skutterudites, Zintl compounds, gas hydrates, and metal-organic frameworks.Through this project, the research team will integrate nanoscience into the undergraduate engineering curriculum at Carnegie Mellon University through a lecture series and distribute the materials through the National Science Foundation-supported nanoHUB. Outreach activities related to large unit cell crystals will be developed and presented to middle school and high school students in Pittsburgh.