Rice University scientists find 3-D boron nitride structures
will excel at thermal management for electronics
(July 15, 2015) 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 heat flow 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 boron nitride nanotubes 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 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.