Why ‘white graphene’ structures are cool

July 16, 2015

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: Shahsavari Group/Rice University)

Three-dimensional structures of boron nitride are a viable candidate as a tunable material to keep electronics cool, according to scientists at Rice University researchers Rouzbeh Shahsavari and Navid Sakhavand.

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 difference: h-BN is a natural insulator, where perfect graphene presents no barrier to electricity (is a natural electrical conductor).

But like graphene, h-BN is a also a good conductor of heat, which can be quantified in the form of phonons. (Technically, a phonon is a “quasiparticle” in a collective excitation of atoms.)

“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 boron nitride nanotubes would be able to move phonons in all directions, whether in-plane or across planes, Shahsavari said.

Phonon flows

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 significantly slowed down the flow of phonons 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.

Researchers have already made graphene/carbon nanotube junctions, but 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 3-D, graphene-based nanoelectronics,” Shahsavari said.

“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. 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.”

Abstract of Dimensional Crossover of Thermal Transport in Hybrid Boron Nitride Nanostructures

Although Boron Nitride nanotubes (BNNT) and hexagonal-BN (h-BN) are superb one-dimensional (1D) and 2D thermal conductors respectively, bringing this quality into 3D remain elusive. Here, we focus on Pillared Boron Nitride (PBN) as a class of 3D BN allotropes and demonstrate how the junctions, pillar length and pillar distance control phonon scattering in PBN and impart tailorable thermal conductivity in 3D. Using reverse non equilibrium molecular dynamics simulations, our results indicate that while a clear phonon scattering at the junctions accounts for the lower thermal conductivity of PBN compared to its parent BNNT and h-BN allotropes, it acts as an effective design tool and provides 3D thermo-mutable features that are absent in the parent structures. Propelled by the junction spacing, while one geometrical parameter, e.g., pillar length, controls the thermal transport along the out-of-plane direction of PBN, the other parameter, e.g., pillar distance, dictates the gross cross-sectional area, which is key for design of 3D thermal management systems. Furthermore, the junctions have a more pronounced effect in creating a Kapitza effect in the out-of-plane direction, due to the change in dimensionality of the phonon transport. This work is the first report on thermo-mutable properties of hybrid BN allotropes and can potentially impact thermal management of other hybrid 3D BN architectures.