CAREER: Pushing the Extremes of Heat Conduction via Multiscale Phonon Modeling from First-Principles

纳米材料和加工技术的最新进展使得大量的非均质纳米结构能够用于各种应用，包括热电发电、微电子冷却、热障材料、太阳能电池和能量存储。由纳米、中观和宏观层次结构组成的复杂系统的数量正在迅速增加。这些器件的热建模需要注意广泛的长度尺度和物理现象。这些分层材料中的小尺度输运很少被理解，主要是由于缺乏对这些多尺度结构中称为声子的振动的能量输运的适当描述。对多尺度声子输运的严格理解对于推动热传导的极端发展至关重要，这对于各种变革性应用的进步至关重要，例如需要超低导热系数的经济热电能量转换，以及需要超高导热系数的更有效的电子冷却。通过提高能源转换和散热效率，该项目可以从根本上为全球可持续能源解决方案做出贡献。CAREER项目的教育目标是促进学术多样性和平等的教育机会，并通过为公众提供创造性的博物馆展览，鼓励对热科学和工程的兴趣，通过新颖的国际合作课程参与研究活动，以及为幼儿园到大学的学生开展外展活动，为STEM领域培养受过高等教育的劳动力。并与工业伙伴分享最先进的研究成果。本CAREER项目的研究目标是从第一性原理出发，全面了解多尺度声子输运，从而推动热导率的上下边界。在过去的几十年里，尽管宏观尺度的热输运理论已经建立，纳米尺度的热输运也取得了重大进展，但对中尺度的热输运仍然知之甚少。本项目重点研究中尺度声子输运，以弥合纳米尺度和宏观尺度声子输运之间的知识差距。研究任务如下：(1)利用密度泛函理论，原子和纳米尺度第一性原理计算，生成中尺度模拟的关键输入参数，声子平均自由程和界面透射率。S函数法，从头算分子动力学模拟；(2)采用蒙特卡罗模拟方法求解中尺度输运的Boltzmann输运方程；(3)利用时域热反射测量对多尺度模拟结果进行验证；(4)为热工程师和传热研究人员开发一个紧凑的鲁棒分析模型。该项目的成果有望成为对多尺度声子输运的基本理解的重大飞跃，使具有前所未有的热输运特性的新材料能够用于包括热能转换和管理在内的许多应用。

Recent advances in nanomaterials and processing technologies have enabled the creation of a large number of heterogeneous nanostructures for a variety of applications including thermoelectric energy generation, microelectronics cooling, thermal barrier materials, solar cells, and energy storage. The number of complex systems consisting of hierarchical structures spanning the nano-, meso- and macro- scales is increasing rapidly. Thermal modeling of these devices requires attention to a broad range of length scales and physical phenomena. Small scale transport in these hierarchical materials is poorly understood mainly due to the lack of a proper description of energy transport by vibrations in the structures, called phonons, across these multiple scales. A rigorous understanding of multiscale phonon transport is crucial for pushing the extremes of heat conduction for the advancement of diverse, transformative applications such as economical thermoelectric energy conversion, which requires ultralow thermal conductivity, and more efficient electronics cooling, which demands ultrahigh thermal conductivity. By improving the efficiency of energy conversion and heat rejection, the project can essentially contribute to global sustainable energy solutions. The educational objective of this CAREER project is to promote academic diversity and equal educational opportunities and to prepare a highly educated workforce in the STEM fields by encouraging interest in thermal science and engineering via a creative museum exhibit for the general public, engaging in research activities via a novel international collaborative course and outreach activities for kindergarten-to-college students, and sharing of the state-of-the-art research findings with industrial partners. The research objective of this CAREER project is to obtain a comprehensive understanding of multiscale phonon transport from first-principles in order to push the upper and lower boundaries of thermal conductivity. Despite well-established theories at the macroscale and the significant progress made at the nanoscale over the past few decades, mesoscale thermal transport remains poorly understood. This project focuses on mesoscale phonon transport to bridge the knowledge gap between nanoscale and macroscale phonon transport. The research tasks are below: (1) Generate the key input parameters for mesoscale simulations, phonon mean free path and interface transmittance, from atomic- and nano-scale first-principles calculations using density functional theory, atomistic Green?s function method, and ab initio molecular dynamics simulations; (2) Solve the Boltzmann transport equation using Monte Carlo simulations for mesoscale transport; (3) Validate multiscale simulation results using time-domain thermoreflectance measurements; (4) Develop a compact robust analytical model for thermal engineers and heat transfer researchers. The outcome of this project is expected to be a major leap in the fundamental understanding of multiscale phonon transport, enabling the creation of novel materials with unprecedented thermal transport properties for numerous applications including thermal energy conversion and management.