Phonon Heat Conduction in Nanostructures: 3D to 1D Transition

众所周知，在大多数纳米结构材料中，例如薄膜、超晶格和纳米线，由于声子的表面和界面散射，热导率降低。另一方面，理论预测，真正的一维材料，如原子链，具有发散（即无限）的热导率。随着3D块体材料的尺寸减小，人们会期望从由增强的边界效应引起的低热导率过渡到高热导率，这是由于难以满足1D结构中的散射作用并因此降低散射率。 这种从三维声子热传导到一维行为的转变具有重要的理论意义和实际意义。一个理论和实验计划，旨在探索这种重要的转变，在实际材料系统的建议。 感兴趣的材料系统是聚合物，对于聚合物，预期从单个聚合物股线的高热导率转变为添加链以形成纤维的低热导率。 将进行分子动力学模拟。 声子弛豫时间将通过分析轨迹来提取，并且可以获得关于这种3D到1D过渡的见解。 实验上，聚合物纤维束的热导率的测量将使用双层悬臂梁进行。 结合实验和理论的工作将导致新的洞察三维到一维的声子热传导过渡制度。 PI将把这些研究成果整合到课程和书籍中，同时通过在网上发布讲座笔记和讲座录音，向世界其他地区推广。 本科生，高中生，和代表性不足的少数民族学生将被招募和支持参加研究。智力优点。 虽然从3D到1D声子输运的过渡可以预期从现有的工作中得出的见解的基础上，主题本身还没有收到任何系统的研究。这项工作将对纳米结构和块状材料中的声子热传导机制产生新的见解，并可能激发新的方法来设计具有高热导率的材料。 更广泛的影响。 从分子动力学模拟中分析声子动力学和测量单个纳米结构热导率的技术也将引起更广泛的科学界的极大兴趣。 开发的分子动力学模拟代码将被纳入桑迪亚国家实验室现有的开源代码，以覆盖更广泛的科学界。拟议的教育工作将在纳米技术领域对世界其他地区产生真实的影响，并有助于增加研究和高等教育中代表性不足的群体的存在。

0755825ChenIt is well-known that in most nanostructured materials, such as thin films, superlattices and nanowires, thermal conductivity is reduced due to surface and interface scattering of phonons. On the other hand, theory has predicted that truly 1D materials, such as an atomic chain, have a divergent (i.e. infinite) thermal conductivity. As 3D bulk materials are reduced in size, one would expect a transition from low thermal conductivity, resulting from enhanced boundary effects, to high thermal conductivity, due to difficulties in satisfying scattering roles in 1D structures and hence reduced scattering rates. This type of transition from 3D phonon heat conduction to 1D behavior is of both fundamental and practical interests. A theoretical and experimental program aimed at probing such an important transition in practical materials systems proposed. The material system of interests is polymers, for which a transition from high thermal conductivity for a single polymer strand to low thermal conductivity as chains are added to form a fiber is expected. Molecular dynamics simulation will be carried out. Phonon relaxation times will be extracted by analyzing the trajectories and insight can be gained with respect to this 3D to 1D transition. Experimentally, measurements of the thermal conductivity of polymer fiber strands will be carried out using a bilayer cantilever. The combined experimental and theoretical work will lead to new insight on the 3D to 1D transition regime in phonon heat conduction. The PI will integrate these research results into courses and books, while also reaching out to the rest of the world by posting lecture notes and audio recordings of lectures on web. Undergraduate students, high school students, and underrepresented minority students will be recruited and supported to participate in research.Intellectual Merits. Although a transition from 3D to 1D phonon transport can be expected based on insight drawn from existing work, the topic itself has not received any systematic study. The proposed work will generate new insights on phonon heat conduction mechanisms in nanostructures as well as in bulk materials, and could potentially inspire new ways to engineer materials with high thermal conductivity. Broader Impacts. Techniques developed for analyzing phonon dynamics from molecular dynamics simulation and for measuring thermal conductivity of individual nanostructures will also be of great interest to the broader scientific community. The molecular dynamics simulation codes developed will be incorporated into existing open-source code in Sandia National Laboratory to reach broader scientific community. The proposed educational effort will make real impacts for the rest of world in the area of nanotechnology and contributes to increasing the presence of underrepresented groups in research and higher education.