WEDNESDAY, AUGUST 9, 2023
Researchers from Rice University are reportedly analyzing a material produced from forms of boron nitride, as well as its response to changing temperatures and pressures.
Hexagonal boron nitride is found in products such as coatings, lubricants and cosmetics, but when combined with cubic boron nitride could be useful in other technology applications.
About the Research
According to the press release, university scientists mixed hexagonal boron nitride, specifically “white graphite,” with cubic boron nitride, which is second to diamonds in its hardness. They reportedly found that the nanocomposite that was created interacted with light and heat in unexpected ways.
This new material reportedly has the potential to be useful in next-generation 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,” said Abhijit Biswas, a research scientist who is the lead author of a study about the research published in Nano Letters. “It’s quite soft and it is a great lubricant, and very lightweight. It’s 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’s super hard—it’s close to diamond in hardness, actually.”
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| Jeff Fitlow / Rice University |
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Researchers from Rice University is reportedly analyzing a material produced from forms of boron nitride, as well as its response to changing temperatures and pressures. |
According to the university, the composite of the two materials reportedly outperformed its parent materials in different functionalities.
“We found the composite had low thermal conductivity, which means it could serve as a heat-insulating material in electronic devices, for instance,” Biswas said. “The thermal and optical properties of the mixed material are very different from an average of the two boron nitride varieties.”
Hanyu Zhu, a corresponding author on the study, said he expected that “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,” Zhu said.
He said the boron and nitrogen atoms in the composite displayed greater regularity and formed larger grains, where a grain designates the size of a group of atoms aligned coherently in a lattice.
“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,” said Zhu.
“Some theorists say that, at ambient conditions, cubic boron nitride is more stable,” Biswas said. “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’ll likely say hexagonal boron nitride. What we’re seeing experimentally is the opposite of what people are saying theory-wise, and it’s still up for debate.”
Rice reports that when the composite is subjected to a rapid, high-temperature technique known as spark plasma sintering, it transformed into hexagonal boron nitride. Biswas said this confirmed theoretical predictions and helped paint a fuller picture of “which varieties of boron nitrides appear at what conditions.”
Additionally, the hexagonal boron nitride produced after this method was of higher quality than the one initially used for the mixture.
“What we’ll 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,” Biswas said.
“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,” said Pulickel Ajayan, a corresponding author on the study and chair of Rice’s Department of Materials Science and Nanoengineering.
Zhiting Tian, Eugene A. Leinroth Sesquicentennial Faculty Fellow and an associate professor in Cornell University’s Sibley School of Mechanical and Aerospace Engineering, is also a corresponding author.
The research was supported by the Army Research Office, the National Science Foundation, the Office of Naval Research and the Department of Energy.
Hexagonal Boron Nitride Coatings Use
Last year, researchers at Purdue University developed a new thinner, lighter formulation for its “whitest white” paint, making it ideal for cooling vehicles or aircraft. Using nanoparticles of barium sulfate to reflect 98.1% of the sunlight, the white cooling paint can cool outdoor surfaces, but the research team knew certain structures would need a lighter formulation.
The research team initially created the ultra-white paint in October 2020, led by Xiulin Ruan, a Purdue professor of mechanical engineering, but the coating was more recently updated to be formulated to be thinner and lighter.
“To achieve this level of radiative cooling below the ambient temperature, we had to apply a layer of paint at least 400 microns thick,” Ruan said. “That’s fine if you’re painting a robust stationary structure, like the roof of a building. But in applications that have precise size and weight requirements, the paint needs to be thinner and lighter.”
To resolve this, Ruan’s team began experiment with other materials to push the limit of its capability to scatter sunlight. According to Purdue’s release, the latest formulation is nanoporous paint incorporating hexagonal boron nitride as the pigment, a substance mostly used in lubricants.
As a result, the new paint achieves nearly the same level of solar reflectance at 97.9% with a single 150-micron layer of paint.
“Hexagonal boron nitride has a high refractive index, which leads to strong scattering of sunlight,” said Andrea Felicelli, a Purdue Ph.D. student in mechanical engineering who worked on the project. “The particles of this material also have a unique morphology, which we call nanoplatelets.”
Purdue reports that the paint also incorporates voids of air, which make it highly porous on a nanoscale. This, in combination with the thinner application, makes the new paint weigh 80% less than barium sulfate paint while achieving nearly identical solar reflectance.
Ruan added that they are in discussions right now to commercialize the paint, while there are “still a few issues that need to be addressed, but progress is being made.”
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