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

众所周知，在大多数纳米结构材料中，如薄膜、超晶格和纳米线，由于声子的表面和界面散射，热导率会降低。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)的热传导性。随着3D块体材料尺寸的减小，人们预计会从边界效应增强导致的低导热系数过渡到高导热系数，这是因为难以满足一维结构中的散射作用，从而降低了散射率。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用价值。提出了一个理论和实验方案，旨在探索实际材料系统中的这种重要转变。感兴趣的材料系统是聚合物，对于聚合物，预计将从单一聚合物链的高导热系数过渡到随着链的添加而形成纤维的低导热系数。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转变的洞察。在实验上，将使用双层悬臂来测量聚合物纤维束的导热系数。实验和理论相结合的工作将对声子热传导中的三维到一维转变机制有新的认识。PI将把这些研究成果整合到课程和书籍中，同时还通过在网络上张贴讲座笔记和讲座的录音来接触世界其他地区。招收和支持本科生、高中生、少数民族学生参加研究。尽管基于现有工作的洞察力，从3D声子传输到1D声子传输的过渡是可以预期的，但这个主题本身还没有得到任何系统的研究。这项拟议的工作将对纳米结构和块体材料中的声子热传导机制产生新的见解，并可能启发设计具有高导热系数的材料的新方法。更广泛的影响。为通过分子动力学模拟分析声子动力学和测量单个纳米结构的导热系数而开发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整合到桑迪亚国家实验室现有的开放源代码中，以推广到更广泛的科学界。拟议的教育努力将在纳米技术领域对世界其他地区产生实际影响，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.