CAREER: Thermal Transport Studies of Individual Grain Boundaries within Nanostructured Materials

多晶材料中单个晶界的热输运研究在纳米尺度上，界面可以通过散射热载体来强烈地限制热传递，热载体主要是非金属材料中的声子。这种界面声子散射及其产生的界面热阻对许多应用都很重要，从纳米电子和光学器件的热管理，到有效的隔热材料，再到热电能量转换。然而，经过几十年的研究，对于声子如何与界面相互作用的基本理解仍然有限，特别是当考虑到真实界面的复杂性时。特别是，还没有直接测量多晶块体材料或薄膜的单晶界(GB)的热阻。现有的热学研究只能通过拟合整个材料随温度变化的导热系数来提取平均GB热阻。为了解决这一关键问题，该项目结合了热测量和原子模拟来揭示声子在单个银河系中的详细输运。对于一般界面，本项目所获得的知识将为通过改变界面原子和纳米结构来定制界面声子输运提供重要的指导。这项综合教育计划旨在让本科生和高中生，特别是那些来自代表性不足群体的学生，参与尖端能源研究。创新的外展活动还包括在数学、工程、科学成就项目中为中学生开发一个挑战，并通过博物馆展览向公众展示纳米技术研究的重要性。拟议的研究目的是更好地了解多晶块状材料和薄膜中声子在单个GB中的传输。主要研究了两种广泛应用的材料合成技术，即化学气相沉积(CVD)法和热压法制备纳米块体材料。热阻测量是在单个GB上进行的，使用纳米级的热传感器来测量在给定热流下该GB上的稳态温度跳跃。这将为声子在这些多晶材料中的单能级输运提供前所未有的实验数据。作为热压块体材料中真实GB的一个可比较的例子，通过热压的平面薄膜-晶片界面也被测量了不同晶体在界面上的取向错误。所有的热测量都可以直接与基于原子学格林-S函数的预测相比较，该模拟使用了不同显微技术揭示的准确的界面原子结构(例如，位错、晶体取向、粗糙度、纳米级应变作为原子位移)。单个GB测量和AGF模拟的集成超出了以前的AGF研究，这些研究通常使用猜测的界面原子结构，很少被实验验证。从根本上讲，这项研究将阐明合成条件、界面原子结构和相应的GB热输运之间的关系。该项目的成功将极大地促进许多重要应用的热研究，如使用CVD薄膜和薄膜-晶片键合的纳米电子器件、多晶薄膜太阳能电池、热压结构和光学块体材料、热电材料和热障涂层。

Thermal Transport Studies of Individual Grain Boundaries within Polycrystalline MaterialsAt the nanoscale, interfaces can strongly restrict heat transfer by scattering the heat carriers, which are mainly phonons in nonmetallic materials. Such interfacial phonon scattering and its resulting interfacial thermal resistance are important to many applications, ranging from the thermal management of nanoelectronic and optical devices, to effective thermal insulation materials, to thermoelectric energy conversion. However, the fundamental understanding of how phonons interact with an interface is still limited after decades of research, especially when the complexities of a real interface are considered. In particular, the thermal resistance of a single grain boundary (GB) has not been directly measured for a polycrystalline bulk material or thin film. Existing thermal studies can only extract an averaged GB thermal resistance by fitting the temperature-dependent thermal conductivity of the whole material. To address this critical issue, this project combines thermal measurements and atomistic simulations to reveal the detailed phonon transport across individual GBs. For general interfaces, the knowledge gained from this project will provide important guidance for tailoring the interfacial phonon transport by varying the interfacial atomic and nanoscale structures. The integrated educational plan aims to involve undergraduate and high-school students, especially those from underrepresented groups, in cutting-edge energy research. Innovative outreach activities also include developing a challenge for middle-school students in the Mathematics, Engineering, Science Achievement program, and demonstrating the importance of nanotechnology research to the general public through museum exhibitions.The objective of the proposed research is to better understand the phonon transport across an individual GB within polycrystalline bulk materials and thin films. The investigations focus on GBs formed by two widely used techniques for materials synthesis, i.e., chemical vapor deposition (CVD) for thin films and hot press for nanostructured bulk materials. Thermal resistance measurements are carried out on a single GB, using nanofabricated thermal sensors to measure the steady-state temperature jump across this GB under a given heat flow. This will provide unprecedented experimental data for phonon transport across single GBs within these polycrystalline materials. As a comparable case of a real GB within hot-pressed bulk materials, planar film-wafer interfaces by hot press are also measured for varied crystal misorientations across the interface. All thermal measurements can be directly compared to predictions based on atomistic Green?s function (AGF) simulations that employ the exact interfacial atomic structure (e.g., dislocations, crystal orientation, roughness, nanoscale strain as atomic displacement) revealed by different microscopy techniques. The integration of individual GB measurements and AGF simulations reaches beyond previous AGF studies that often use guessed interfacial atomic structures and are seldom validated by experiments. Fundamentally, the proposed study will elucidate the relationship between the synthesis condition, interfacial atomic structure, and the corresponding GB thermal transport. The success of this project will significantly advance the thermal studies for many important applications, such as nanoelectronic devices using CVD films and film-wafer bonding, polycrystalline thin-film solar cells, structural and optical bulk materials by hot press, thermoelectric materials, and thermal barrier coatings.