by In chemistry, everything has a structure. Compounds with the same chemical formula can have different properties depending on the arrangement of the molecules they are made of. And compounds with different chemical formula but similar molecular arrangement can have similar properties.
The latter group contains graphene and a form of boron nitride known as hexagonal boron nitride. Graphene is made up of carbon atoms. Boron nitride, BN, is composed of boron and nitrogen atoms. Although their chemical formulas differ, they have a similar structure – so similar that many chemists refer to hexagonal boron nitride as “white graphene”.
Carbon-based graphene has many useful properties. It is thin but strong, and conducts heat and electricity very well, making it suitable for use in electronics.
Similarly, hexagonal boron nitride has many properties similar to graphene that could improve biomedical imaging and drug delivery, as well as computers, smartphones and LEDs. Researchers have studied this type of boron nitride for many years.
But, hexagonal boron nitride is not the only useful form this compound comes in.
As materials engineers, our research team is investigating another type of boron nitride called cubic boron nitride. We want to know if combining the properties of hexagonal boron nitride with cubic boron nitride could open the door to more useful applications.
Hexagonal versus cubic
Hexagonal boron nitride is, as you might guess, boron nitride molecules arranged in the shape of a flat hexagon. It looks honeycomb-shaped, like graphene. Cubic boron nitride has a three-dimensional lattice structure and resembles diamond at the molecular level.
H-BN is thin, soft and used in cosmetics to give them a silky texture. It does not melt or degrade even under extreme heat, making it useful in electronics and other applications. Some scientists predict that it could be used to build a radiation shield for spacecraft.
C-BN is hard and resistant. It is used in manufacturing to make cutting tools and drills, and can maintain its sharp edge even at high temperatures. It can also help dissipate heat in electronics.
Although h-BN and c-BN may look different, when combined, our research found that they have greater potential than either alone.
Both types of boron nitride conduct heat and can provide electrical insulation, but one, h-BN, is soft, while the other, c-BN, is hard. So, we wanted to see if they could be used together to create materials with interesting properties.
For example, a coated material could be effective for high temperature structural applications and combine their different behaviours. C-BN could provide strong adhesion to a surface, and the lubricating properties of h-BN could resist wear and tear. Both would keep the material from overheating.
Making boron nitride
This class of material does not occur naturally, so scientists must make it in the laboratory. In general, it has been difficult to synthesize high-quality c-BN, but it is relatively easy to make h-BN as high-quality films, using methods known as vapor phase deposition methods.
In vapor phase deposition, we burn up boron and nitrogen materials until they evaporate. The evaporated molecules are then deposited on a surface, cool down, bond together and form a thin BN film.
Our research team has worked on combining h-BN and c-BN using similar processes to vapor phase deposition, but we can also mix powders of both together. The idea is to build a material with the right mix of h-BN and c-BN for thermal, mechanical and electronic properties that we can fine-tune.
Our team discovered that the composite substance made from combining the two forms of BN together has a variety of potential applications. When you point a laser beam at the substance, it glows brightly. Researchers could use this property to create display screens and improve radiation therapies in the medical field.
We also discovered that we can tailor how thermally conductive the composite material is. This means that engineers could use this BN composite in machines that manage heat. The next step is to try to manufacture large plates made of h-BN and c-BN composite. If done precisely, we can tailor the mechanical, thermal and optical properties to specific applications.
In electronics, h-BN could act as a dielectric – or insulator – alongside graphene in certain low-power electronics. As a dielectric, h-BN would help electronics work efficiently and maintain their charge.
C-BN could work alongside diamond to create ultrawide band gap materials that allow electronic devices to operate at much higher power. Diamond and c-BN conduct heat well, and together they could help cool these high-powered devices, which generate a lot of extra heat.
H-BN and c-BN separately can lead to electronics that perform very well in a variety of contexts – together, they also have a multitude of potential applications.
Our BN composite could improve heat spreaders and insulators, and could work in energy storage machines like supercapacitors, which are fast-charging energy storage devices, and rechargeable batteries.
We will continue to study the properties of BN, and how we can use it in lubricants, coatings and wear-resistant surfaces. Developing ways to increase production further will be essential to explore its applications, from materials science to electronics and even environmental science.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was written by: Pulickel Ajayan, Rice University and Abhijit Biswas, Rice University.
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Pulickel Ajayan receives funding from the Army Research Laboratory and the Army Research Office.
Abhijit Biswas does not work for, consult with, share in, or be funded by any company or organization that would benefit from this article, and has disclosed no material affiliations beyond their academic appointment.
