Boron nitride is a chemical compound with chemical formula BN, consisting of equal numbers of boron and nitrogen atoms. BN is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms. The hexagonal form corresponding to graphite is the most stable and softest among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic (sphalerite structure) variety analogous to diamond is called c-BN. Its hardness is inferior only to diamond, but its thermal and chemical stability is superior. The rare wurtzite BN modification is similar to lonsdaleite and may even be harder than the cubic form.
Because of excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of high-temperature equipment. Boron nitride has potential use in nanotechnology. Nanotubes of BN can be produced that have a structure similar to that of carbon nanotubes, i.e. graphene (or BN) sheets rolled on themselves, but the properties are very different.
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.