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能够产生大量异质纳米结构用于各种应用，包括热电能产生、微电子冷却、热障材料、太阳能电池和能量存储。由跨越纳米、介观和宏观尺度的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。这些分级材料中的小尺度输运知之甚少，主要是由于缺乏对结构中振动（称为声子）跨这些多尺度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化以促进多样化的变革性应用至关重要，例如经济的热电能量转换，这需要超低的热导率，以及更有效的电子冷却，这需要更高的热导率。通过提高能源转换和散热效率，该项目可以为全球可持续能源解决方案做出贡献。这个职业生涯项目的教育目标是促进学术多样性和平等的教育机会，并通过为公众举办创意博物馆展览，鼓励对热科学和工程的兴趣，为STEM领域培养受过高等教育的劳动力，通过新颖的国际合作课程和针对大学生的外联活动参与研究活动，并与工业伙伴分享最先进的研究成果。这个CAREER项目的研究目标是从第一性原理中全面了解多尺度声子输运，以推动热导率的上限和下限。尽管在宏观尺度上建立了完善的理论，在纳米尺度上取得了重大进展，在过去的几十年里，中尺度热传输仍然知之甚少。该项目的重点是中尺度声子输运，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、原子绿色？s函数方法和从头算分子动力学模拟;（2）使用Monte Carlo模拟求解Boltzmann输运方程进行中尺度输运;（3）使用时域热反射测量对多尺度模拟结果进行分析;（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。该项目的成果预计将是对多尺度声子输运基本理解的重大飞跃，从而能够创造出具有前所未有的热输运特性的新型材料，用于包括热能转换和管理在内的众多应用。

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.