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Quasi-ballistic thermal transport from nanoscale interfaces observed using ultrafast coherent soft X-ray beams

2009; Nature Portfolio; Volume: 9; Issue: 1 Linguagem: Inglês

10.1038/nmat2568

1476-4660

Mark E. Siemens, Qing Li, Ronggui Yang, Keith A. Nelson, Erik H. Anderson, Margaret M. Murnane, Henry C. Kapteyn,

Heat Transfer and Optimization

Fourier theory of thermal transport considers heat transport as a diffusive process where energy flow is driven by a temperature gradient. However, this is not valid at length scales smaller than the mean free path for the energy carriers in a material, which can be hundreds of nanometres in crystalline materials at room temperature. In this case, heat flow will become 'ballistic'--driven by direct point-to-point transport of energy quanta. Past experiments have demonstrated size-dependent ballistic thermal transport through nanostructures such as thin films, superlattices, nanowires and carbon nanotubes. The Fourier law should also break down in the case of heat dissipation from a nanoscale heat source into the bulk. However, despite considerable theoretical discussion and direct application to thermal management in nanoelectronics, nano-enabled energy systems and nanomedicine, this non-Fourier heat dissipation has not been experimentally observed so far. Here, we report the first observation and quantitative measurements of this transition from diffusive to ballistic thermal transport from a nanoscale hotspot, finding a significant (as much as three times) decrease in energy transport away from the nanoscale heat source compared with Fourier-law predictions.