Boron nitride is a material that can occur in either soft or hard compounds. However, the forms of boron nitride and how they react to changes in pressure and temperature are significantly less well understood than those of other materials like carbon.
Researchers at Rice University combined cubic boron nitride, which is second only to diamond in hardness, with hexagonal boron nitride, a soft variety also known as “white graphite,” and discovered that the resulting nanocomposite interacted with light and heat in unanticipated ways that could be helpful in future microchips, quantum devices, and other advanced technology applications.
Hexagonal boron nitride is widely used in a variety of products, such as coatings, lubricants and cosmetics. It is quite soft and it is a great lubricant, and very lightweight. It is also cheap and very stable at room temperature and under atmospheric pressure. Cubic boron nitride is also a very interesting material, with properties that make it very promising for use in electronics. Unlike hexagonal boron nitride, it is super hard—it is close to diamond in hardness, actually.
Abhijit Biswas, Study Lead Author and Research Scientist, Rice University
The combination of these two seemingly opposing materials exceeded its parent materials in several capabilities.
Biswas added, “We found the composite had low thermal conductivity, which means it could serve as a heat-insulating material in electronic devices, for instance. The thermal and optical properties of the mixed material are very different from an average of the two boron nitride varieties.”
Hanyu Zhu, one of the corresponding authors of the study, stated “The optical property we measure called second harmonic generation would be small for this type of disordered material. But it actually turns out to be quite large after heating, an order of magnitude more than both the individual material and the untreated mixture.”
According to him, the boron and nitrogen atoms in the composite were more regular and formed larger grains, where grain is the size of a collection of atoms aligned coherently in a lattice.
Zhu added, “We were surprised to find that the cubic boron nitride grains grow instead of diminish in this material from the small grains in the unmixed starting compounds.”
Based on theoretical predictions and experimental findings, there were conflicting statements concerning which of the two boron nitride variants was more stable.
“Some theorists say that, at ambient conditions, cubic boron nitride is more stable. Experimentally, people have seen that hexagonal boron nitride is very stable. So, if you ask someone which boron nitride phase is the most stable, they will likely say hexagonal boron nitride. What we are seeing experimentally is the opposite of what people are saying theory-wise, and it is still up for debate,” Biswas added.
The composite changed into hexagonal boron nitride when exposed to the quick, high-temperature process known as spark plasma sintering. This supported theoretical hypotheses, according to Biswas, and provided a clearer picture of “which varieties of boron nitrides appear at what conditions.”
Additionally, the hexagonal boron nitride that was produced following this process was of greater quality than the one that was first utilized for the combination.
Biswas noted, “What we will be looking at next is whether the spark plasma sintering technique improves the quality of hexagonal boron nitride all on its own, or whether you need the composite to get that effect.”
Pulickel Ajayan, a corresponding author of the study and chair of Rice’s Department of Materials Science and Nanoengineering, added, “What is fascinating about this study is that it opens up possibilities to tailor boron nitride materials with the right amounts of hexagonal and cubic structures, thus enabling a broad range of tailored mechanical, thermal, electrical and optical properties in this material.”
Ajayan is the Benjamin M. and Mary Greenwood Anderson Professor of Engineering and a professor of materials science and nanoengineering, chemistry, and chemical and biomolecular engineering.
Zhiting Tian, an associate professor at Cornell University’s Sibley School of Mechanical and Aerospace Engineering and the Eugene A. Leinroth Sesquicentennial Faculty Fellow, is another corresponding author.
The Office of Naval Research (N00014-22-1-2357), the National Science Foundation (2005096), the Army Research Office (W911NF-19-2-0269), and the Department of Energy (DE-SC0012311) provided funding for the study.
Biswas, A., et al. (2023) Phase Stability of Hexagonal/Cubic Boron Nitride Nanocomposites. Nano Letters doi:10.1021/acs.nanolett.3c01537