3D White Graphene Could Keep Electronics Cool

Three-dimensional structures of boron nitride might be the right stuff to keep small electronics cool.

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

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 says.

“Typically in all electronics, it is highly desired to get heat out of the system as quickly and efficiently as possible,” he explains. “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.”

Like Yellow Traffic Lights

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

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 phonons from layer to layer, Shahsavari says.

3D structure of hexagonal boron nitride sheets and boron nitride nanotubes

Credit: Shahsavari Group/Rice University

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 believes such junctions for boron nitride materials could be just as promising.

“Given the insulating properties of boron nitride, they can enable and complement the creation of 3D, graphene-based nanoelectronics.

This type of 3D 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 says.

“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.”

Navid Sakhavand and Rouzbeh Shahsavari
Dimensional Crossover of Thermal Transport in Hybrid Boron Nitride
Nanostructures ACS Appl. Mater. Interfaces, 2015, 7 (33), pp 18312–18319 DOI: 10.1021/acsami.5b03967

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