A team of researchers at Rice University has completed the first analysis of how 3D boron nitride might be used as a tunable material to control heat flow in small electronics.

In its 2D form, hexagonal boron nitride (otherwise known as white graphene) looks just like the atom-thick form of carbon known as graphene. One well-studied difference is that the hexagonal boron nitride is a natural insulator, where perfect graphene presents no barrier to electricity. But like graphene, the hexagonal boron nitride is a good conductor of heat, which can be quantified in the form of phonons.

“Typically in all electronics, it is highly desired to get heat out of the system as quickly and efficiently as possible,” said Dr Rouzbeh Shahsavari, lead author of a paper published online in the journal Applied Materials and Interfaces.

“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 simulations showed that 3D structures of hexagonal boron nitride planes connected by boron nitride nanotubes would be able to move phonons in all directions, whether in-plane or across planes.

Dr Rouzbeh Shahsavari and his co-author, Dr Navid Sakhavand, 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.

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.

“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,” Dr 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.”

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Rouzbeh Shahsavari & Navid Sakhavand. Dimensional Crossover of Thermal Transport in Hybrid Boron Nitride Nanostructures. ACS Appl. Mater. Interfaces, published online July 9, 2015; doi: 10.1021/acsami.5b03967