Thermal Transport in Nanostructured Materials

A detailed understanding of thermal transport at the nanoscale is becoming increasingly needed. This need is largely being driven by two distinct technological challenges. First, continuous miniaturization of electronic devices and the ever-increasing density of the components on integrated circuits (ICs) place ever-increasing thermal loads on systems. Indeed, it is now necessary to make thermal design an integral part of the development of many electronic devices, not just at the packaging level, but also at the device and individual junction level. Moreover, as in the case of the electric conductivity, as the characteristic feature sizes ICs rapidly approach the nanoscale, the thermal transport behavior demonstrates unexpected features such as hotspots,1 which need to be clearly understood and incorporated into complete thermal–electrical–mechanical designs. Second, ever-increasing use of the energy resources of our planet requires a realistic set of solutions that includes additional, and more efficient, ways of energy generation, energy usage, and energy distribution. Consequently, efficient energy technologies require significantly improved thermal management. In some cases, such as ICs, the primary objective is to make the thermal transport as efficient as possible. In other cases, such as thermal barrier coatings2,3 and thermoelectric devices,4,5 the primary objective is to minimize thermal transport. In yet other cases, such as in the case of UO2 and other nuclear fuels, whose chemistry and microstructure evolve during burnup, the chief challenge is to understand and control the thermal environment throughout its lifetime. The thermal conductivity of electrical insulators, from which many thermal management systems are engineered, is mediated by lattice vibrations, phonons in the language of quantum mechanics. Lattice vibrations couple to each other and can strongly couple with any structural defects—surfaces, grain boundaries, dislocations, precipitates, or point defects. As such, the thermal conductivity can be very 19