'White graphene' structures can take the heat

July 15, 2015 by Mike Williams
A 3-D structure of hexagonal boron nitride sheets and boron nitride nanotubes could be a tunable material to control heat in electronics, according to researchers at Rice University. Credit: the Shahsavari Group

Three-dimensional structures of boron nitride might be the right stuff to keep small electronics cool, according to scientists at Rice University.

Rice researchers Rouzbeh Shahsavari and Navid Sakhavand have completed the first theoretical analysis of how 3-D boron nitride might be used as a tunable material to control in such devices.

Their work appears this month in the American Chemical Society journal Applied Materials and Interfaces.

In its two-dimensional form, hexagonal boron nitride (h-BN), aka white graphene, looks just like the atom-thick form of carbon known as graphene. One well-studied difference is that h-BN is a natural insulator, where perfect graphene presents no barrier to electricity.

But like graphene, h-BN is a good conductor of heat, which can be quantified in the form of phonons. (Technically, a phonon is one part—a "quasiparticle" – in a collective excitation of atoms.) Using boron nitride to control heat flow seemed worthy of a closer look, Shahsavari said.

"Typically in all electronics, it is highly desired to get heat out of the system as quickly and efficiently as possible," he said. "One of the drawbacks in electronics, especially when you have layered materials on a substrate, is that heat moves very quickly in one direction, along a conductive plane, but not so good from layer to layer. Multiple stacked graphene layers is a good example of this."

Heat moves ballistically across flat planes of boron nitride, too, but the Rice simulations showed that 3-D structures of h-BN planes connected by would be able to move phonons in all directions, whether in-plane or across planes, Shahsavari said.

The researchers calculated how phonons would flow across four such structures with nanotubes of various lengths and densities. They found the junctions of pillars and planes acted like yellow traffic lights, not stopping but significantly slowing the flow of from layer to layer, Shahsavari said. Both the length and density of the pillars had an effect on the heat flow: more and/or shorter pillars slowed conduction, while longer pillars presented fewer barriers and thus sped things along.

While researchers have already made graphene/carbon nanotube junctions, Shahsavari believed such junctions for boron nitride materials could be just as promising. "Given the insulating properties of , they can enable and complement the creation of 3-D, graphene-based nanoelectronics.

"This type of 3-D thermal-management system can open up opportunities for thermal switches, or thermal rectifiers, where the heat flowing in one direction can be different than the reverse direction," Shahsavari said. "This can be done by changing the shape of the material, or changing its mass – say one side is heavier than the other – to create a switch. The heat would always prefer to go one way, but in the reverse direction it would be slower."

Explore further: 3-D nanostructure could benefit nanoelectronics, gas storage

More information: "Dimensional Crossover of Thermal Transport in Hybrid Boron Nitride Nanostructures." ACS Appl. Mater. Interfaces, Just Accepted Manuscript DOI: 10.1021/acsami.5b03967

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Sigh
not rated yet Jul 15, 2015
The heat would always prefer to go one way, but in the reverse direction it would be slower
So if I put that material between two compartments that experience uncorrelated temperature fluctuations, one of them would heat up? And if that works at small enough scales, wouldn't this be Maxwell's demon? I never learned a lot of thermodynamics, so what does this mean for the second law?
Captain Stumpy
not rated yet Jul 15, 2015
h-BN is a good conductor of heat
@sigh
IMHO -my guess is that it would be used as part of the substrate to facilitate heat dispersal ...
then there is the last paragraph
This type of 3-D thermal-management system can open up opportunities for thermal switches, or thermal rectifiers, where the heat flowing in one direction can be different than the reverse direction," Shahsavari said. "This can be done by changing the shape of the material, or changing its mass – say one side is heavier than the other – to create a switch. The heat would always prefer to go one way, but in the reverse direction it would be slower
not maxwell's demon so much as a new switch/rectifier

i am sure Thermodynamics or Antialias_physorg could expound upon this with more clarity
docile
Jul 15, 2015
This comment has been removed by a moderator.
antialias_physorg
5 / 5 (3) Jul 15, 2015
So if I put that material between two compartments that experience uncorrelated temperature fluctuations, one of them would heat up?

Not infinitely so. Transport will still be along the gradient from hotter to colder. It just means that when area A is hotter transport towards area B will be slower compared to heat transfer towards area A when area B is hotter.
With truly random heat fluctuations on both sides you would end up with a mean heat gradient. It will saturate because the chance of getting a "hotter than current" temperature from your temperature fluctuations will diminish as that end gets warmer (i.e. the chance the ambient temperature cooling it down will increase. Conveserly for the 'cold' end the chance of ambient temperature being higher will increase)

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