Collaborative Research: Hydrodynamic Thermal Transport in Graphitic Materials

石墨材料表现出固体中最高的导热系数值。这一特性导致了这些材料在传递热量和保持操作设备冷却方面的现有和新兴应用。然而，其高导热性背后的实际机制尚不清楚。最近的理论研究表明，石墨材料中一组特殊的高居子之间的频繁散射既不会直接引起电阻，也不会限制它们的导热性贡献。这种不寻常的声子输运行为被称为流体动力声子输运。在与技术应用相关的足够高的温度下，从未在任何材料中实验观察到流体动力声子输运。流体动力声子输运在石墨材料的设计和热建模中具有实际意义。因此，本研究进行了一组理论指导的先进实验，以验证石墨材料中异常热输运行为的存在。并将研究成果和方法整合到三个本科和研究生课程以及两个公众示范模块中。本研究的目的是阐明流体动力声子输运对石墨材料热输运性质的影响。这将通过理论和实验的努力来完成，包括基于第一性原理的声子输运理论研究，纳米尺度的固有热导的热输运测量，以及同位素纯化的纳米管、石墨烯和薄石墨样品中第二声的超快光热测量。这些理论和实验的努力可以进一步导致更好地理解晶格振动的能量传输的量子理论。如果成功，这项研究将为模拟声子输运提供一种新的方法。此外，通过克服探测纳米结构固有热输运性质的关键挑战，可以推进纳米尺度热输运测量的前沿。此外，它可以产生一种尖端的超快热输运方法来探测声子输运。

Graphitic materials exhibit some of the highest thermal conductivity values found in solids. This feature has resulted in existing and emerging applications of these materials for transferring heat and keeping operating devices cool. However, the actual mechanisms behind their high thermal conductivity are not well understood. Recent theoretical studies have suggested that frequent scattering among a peculiar group of high-population phonons in graphitic materials does not either cause resistance directly or limit their thermal conductivity contribution. This unusual phonon transport behavior has been referred as hydrodynamic phonon transport. Observation of hydrodynamic phonon transport has never been experimentally observed in any materials at a sufficiently high temperature that is relevant to technological applications. Hydrodynamic phonon transport can have practical implications in the design and thermal modeling of graphitic materials. Thus, a set of theoretically guided advanced experiments are conducted in this research to verify the existence of the unusual thermal transport behavior in graphitic materials. Also, the research findings and methods are integrated into three undergraduate and graduate courses and two demonstration modules for general public. The objective of this research is to elucidate the influence of hydrodynamic phonon transport on the thermal transport properties of graphitic materials. This will be accomplished through theoretical and experimental efforts, including first principles based theoretical studies of phonon transport, nanoscale thermal transport measurements of the intrinsic thermal conductance, and ultrafast opto-thermal measurements of second sound in isotopically purified nanotube, graphene, and thin graphite samples. These theoretical and experimental efforts can further lead to a better understanding of the quantum theory of energy transport by lattice vibrations. If successful, the research can lead to a new approach to simulate phonon transport. In addition, it can advance the frontier of nanoscale thermal transport measurements by overcoming a critical challenge in probing the intrinsic thermal transport properties of nanostructures. Moreover, it can result in a cutting edge ultrafast thermal transport method for probing phonon transport.