Researchers develop New Phase of Boron Nitride
Scanning electron micrograph of c-BN nanoneedles and microneedles up to three microns in length. Image credit: Anagh Bhaumik
Researchers at North Carolina State University have discovered a new phase of the material boron nitride (Q-BN), which has potential applications for electronics.
The researchers have also developed a faster, less expensive technique for creating cubic boron nitride (c-BN) at ambient temperatures and air pressure. C-BN has many uses including the development of advanced power grid technologies.
Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and lead author of a paper describing the research said: "We have bypassed what were thought to be the limits of boron nitride's thermodynamics with the help of kinetics and time control to create this new phase of BN."
Q-BN has a low work function and negative electron affinity, which effectively means that it glows in the dark when exposed to very low levels of electrical fields. These characteristics make it a promising material for energy-efficient display technologies, according to the research team.
To make Q-BN, researchers begin with a layer of thermodynamically stable hexagonal boron nitride (h-BN), which can be up to 500-1000nm thick. The material is placed on a substrate and researchers then use high-power laser pulses to rapidly heat the h-BN to 2,800k. The material is then quenched, using a substrate that quickly absorbs the heat. The whole process takes approximately one-fifth of a microsecond and is done at ambient air pressure.
By manipulating the seeding substrate beneath the material and the time it takes to cool the material, researchers can control whether the h-BN is converted to Q-BN or c-BN. These same variables can be used to determine whether the c-BN forms into microneedles, nanoneedles, nanodots, microcrystals or a film.
"Using this technique, we are able to create up to a 100- to 200-square-inch film of Q-BN or c-BN in one second," Narayan says.
By comparison, previous techniques for creating c-BN required heating hexagonal boron nitride to 3,500K and applying 95,000 atmospheres of pressure.
C-BN has similar properties to diamond, but with several advantages: c-BN has a higher bandgap, which is attractive for use in high-power devices; c-BN can be doped to give it positively- and negatively-charged layers, which means it could be used to make transistors; and it forms a stable oxide layer on its surface when exposed to oxygen, making it stable at high temperatures. This last characteristic means it could be used to make solid state devices and protective coatings for high-speed machining tools used in oxygen-ambient environments.
"We're optimistic that our discovery will be used to develop c-BN-based transistors and high-powered devices to replace bulky transformers and help create the next generation of the power grid," Narayan says.
The paper, 'Direct conversion of h-BN into pure c-BN at ambient temperatures and pressures in air' was published online Feb. 3 in the open-access journal APL Materials. The paper was co-authored by NC State PhD student Anagh Bhaumik. The work was supported by the National Science Foundation under grant DMR-1304607.
