The transfer of heat at the nanoscale has been a prominent issue for a multitude of nanotechnology applications. Currently, there are two main problems. The first is the administration and conduction of heat developed inside nanotechnology devices to preserve the performance and reliability of their components. The second is actually using the nanotechnology to control the flow of heat as well as its conversion to energy. These issues arise in areas such as thermophotovoltaics, integrated circuits, and semiconductor lasers.
In a recent study published in Nature Nanotechnology, several teams of researchers from Stanford, Cornell, and Columbia Engineering have proven that heat transmission can be produced nearly one hundred times stronger than once thought. This has been demonstrated by bringing two conducting surfaces very close; at nanoscale distances apart. Lead researchers Shanhui Fan from Stanford University’s school of engineering, and electrical engineering Professor Michal Lipson from Columbia Engineering have spearheaded a research project aimed at creating larger and more efficient heat transfer.
Heat transfer significantly increases at the nanoscale. Image courtesy of Raphael St-Gelais of the Lipson Nanophotonics Group
The transfer of heat at nanoscale distances was believed to be much different than that of the micro- and macroscales. As device length approaches the nanoscale, it also approaches the wavelength and mean free path distance of heat carriers such as photons, electrons, and molecules. When a structure or device length approaches these nanoscale distances, our classical laws become invalid; new techniques and calculations must be made to anticipate the heat transfer of such devices. Just as Ohm’s law is ironclad for electrical conductors, Fourier’s law can be seen as the empirical rule of heat transfer in solids. Fourier’s law states that the thermal conductivity is independent of the sample length, and tends to be violated when reaching one dimensional and nanoscale distance.
Lipson states “At separations as small as 40 nanometres, we achieved almost a 100-fold enhancement of heat transfer compared to classical predictions.” This discovery is significantly more ground-breaking than once expected to be as our conventional thermal radiation laws predicted the results to be far less efficient. Many teams of researchers have delved into demonstrating the interaction and heat transfer of nanoscale systems before, but none has produced results that could be used for energy applications such as converting heat directly into electricity.
Heat exchange using light is considered to be a very weak form of energy transfer, as we commonly use conduction or convection to produce much more efficient and larger heat transfer results. The primary issue with radiative heat transfer at these distances is that it is extremely challenging to maintain uniform thermal gradients, as well as avoiding conduction and convection.
Video of a MEMS device taken under with microscope to display heat transfer rising as the beams get closer. Also of the Lipson Nanophotonics Group
Lipson’s nanophotonics team placed objects with different temperatures within 100nm or one billionth of a meter next to one another. They demonstrated enhanced near-field radiative heat transfer between parallel SiC nanobeams in the deep subwavelength regime. A microelectronic system was used to control the gap length between the nanobeams. This allowed them to exploit the stability of the nanobeams to reduce buckling at high temperatures, as well as control the separation and uniformity even at large thermal gradients. Lipson’s team was able to reproduce this experiment with temperatures differences as much as 500 degrees F, which looks very promising for energy conversion applications.
“An important implication of our work is that thermal radiation can now be used as a dominant heat transfer mechanism between objects at different temperatures,” -Raphael St-Gelais.
Similar mechanisms and techniques that are used for manipulating light can now be used to control the transfer of heat to electricity. The research is pointing toward applications in energy conversion, biotechnology, nanomaterial synthesis and nanofabrication as well as a wide range of contemporary technologies that can exploit the unusual heat transfer physics in nanostructures.