North Carolina State University (NCSU) has discovered a new phase of boron nitride (Q-BN) that has potential applications for both manufacturing tools and electronic displays. The researchers have also developed a new technique for creating cubic boron nitride (c-BN, with a cubic crystalline structure, analogous to diamond) at ambient temperatures and air pressure, which has applications including the development of advanced power grid technologies.
"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 boron nitride," says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering and lead author of the paper (Jagdish Narayan and Anagh Bhaumik, 'Direct conversion of h-BN into pure c-BN at ambient temperatures and pressures in air', APL Materials, 4, 020701 (2016)).
"We have also developed a faster, less expensive way to create c-BN, making the material more viable for applications such as high-power electronics, transistors and solid-state devices," Narayan says. "c-BN nanoneedles and microneedles, which can be made using our technique, also have potential for use in biomedical devices."
The 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.
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 2800K. The material is then quenched, using a substrate that quickly absorbs the heat. The whole process takes about 0.2 microseconds 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 3500K and applying 95,000 atmospheres of pressure.
c-BN has similar properties to diamond, but has several advantages over diamond: it has a higher bandgap (attractive for use in high-power devices); it can be doped to give it positively and negatively charged layers (so 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 that it could be used to make both 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 work was supported by the National Science Foundation under grant DMR-1304607.
