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

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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.