Effective notebook cooling: Near-field radiation therapy

Posted by Wolfgang Gruener

Chicago (IL) – Researchers from Lehigh University, IBM’s T.J. Watson Research Center and at the Ioffe Institute in St. Petersburg, Russia said they have found a way to cool notebook chips much more efficiently than it is the case today.
 
A method described as 'near-field' radiation therapy promises to dissipate heat by overcoming low rate of thermal coupling between carbon nanotubes and substrate: Carbon nanotube electronics are cooled by utilizing nonconventional radiation in a "near-field zone" just above the substrate, or surface, on which the nanotubes rest. The method channels excess heat from the nanotubes into the substrate which, can be more effectively cooled by the vents that push cool air through laptops, the researchers said.
 
Typically, nanotubes and substrate are made of heterogeneous materials and their rate of heat release is relatively low, similar to that of dry wood. This makes it difficult to dissipate heat from the nanotubes to the substrate through classical thermal conduction. The new approach requires the nanotubes' substrate be composed of a polar material such as silicon-dioxide (SiO2).

"Other methods of heat dissipation do not succeed at discharging heat from within the channel of the nanotube or nanowire," said Slava Rotkin, one of the project leaders. "Our method enables the heat to leave the channel and move to the substrate, while also scattering the hot electrons. This constitutes a novel cooling mechanism without any moving parts or cooling agents."

Rotkin and his colleagues utilize what they call surface phonon-polariton (SPP) thermal coupling by exploiting the high level of electron scattering that occurs in non-suspended carbon nanotube transistors.

A wave called a surface polariton is caused by this electron scattering, according to the scientist. This polariton is particularly strong in the near field zone just above the substrate on which the carbon nanotubes rest. "If you put a graphene monolayer, or layer of carbon nanotubes, in a near field zone," Rotkin said, "this enables the hot electrons to be scattered by the surface polariton and to give out energy to the substrate. Heat is dissipated into the substrate as radiation tunnels from the nanotube through the near field zone to the substrate.”

"If you move the nanotube away from the substrate, the near field tunneling ceases and the mechanism is absent. We achieve all of our coupling through surface polariton scattering (SPP) because of a large enhancement of the electrical field of the polariton in the near field zone. Most semiconductor devices fabricated now have the nanotube or nanowire placed directly on a silica substrate, which is polar. With this mechanism, if the substrate is polar and if there's a small van der Waals gap, our new near-field channel totally dominates thermal coupling," Rotkin said.

"We have shown that SPP thermal coupling increases the effective thermal conductance over the interface between nanotube and [polar substrate] by an order of magnitude."