Engineers from MIT and the University of Tokyo have created a (Is boron nitride the new carbon?) centimeter-scale structure, large enough for the eye to see, that consists of hundreds of billions of open fibers, or nanotubes, made of hexagonal boron nitride.
Hexagonal boron nitride, or hBN, is a one-atom thin material coined “white graphene” for its optical properties and similarities to carbon-based graphene in terms of structure and strength. It can also withstand higher temperatures than graphene and insulates electricity instead of conducting. When hBN is injected into nanoscale tubes, or nanotubes, its unique properties are greatly increased.
The team’s research, published in the journal ACS Nano, paves the way for the production of more boron nitride nanotubes (A-BNNTs). The researchers plan to use the process to make different types of these nanotubes, which will be combined with other materials to make stronger, heat-resistant materials. heat, for example to protect space in hypersonic aircraft.
Because hBN is transparent and electrically insulating, the team plans to attach BNNTs to transparent windows and use them to isolate electrical sensors and electronic devices. The group is also investigating ways to weave nanofibers into membranes for water filtration and for “blue energy” – the concept of renewable energy generated by electricity from ionizing filters.
Brian Wardle, a professor of aerospace engineering at MIT, compared the team’s results to the decades-long pursuit of scientists to produce large carbon nanotubes.
“In 1991, a carbon nanotube was discovered as an interesting substance, but carbon nanotubes have been attached to large objects for 30 years, and the world is not there yet,” says Wardle. “With our work, we’ve gone about 20 years to get a large version of boron nitride nanotubes.”
Wardle is the lead author of the new study, which includes author and MIT researcher Luiz Acauan, former MIT postdoctoral fellow Haozhe Wang, and colleagues at the University of Tokyo.
Appropriate vision
Like graphene, hexagonal boron nitride has a chicken wire like structure. In graphene, the chicken-wire structure is made entirely of carbon atoms, arranged in a repeating hexagonal pattern. For hBN, the hexagon is made up of alternating boron and nitrogen atoms. In recent years, researchers have discovered that two sheets of hBN exhibit unique characteristics of strength, stiffness and resilience at high temperatures. When hBN sheets are converted into nanotubes, these properties are increased, especially when the nanotubes are aligned, like small trees in a densely populated forest.
But finding ways to create stable, high-quality BNNTs has proven difficult. A few attempts to achieve this have yielded a poor quality string. “If you can organize them, you have a better chance of using the properties of BNNTs on a large scale to create real physical devices, composites and skins,” Wardle says.
In 2020, Rong Xiang and his colleagues at the University of Tokyo discovered that they could produce high-quality boron nitride nanotubes by using a chemical vapor deposition method to grow a forest of small carbon nanotubes a few microns long. They cover a forest that contains carbon and “precursors” of boron and nitrogen gas, which, when cooked in a hot oven, glow in the carbon nanotubes to form high-density hexagonal boron nitride nanotubes and carbon nanotubes inside.
Skin burns
In the new study, Wardle and Acauan extended and expanded Xiang’s method, removing the underlying carbon nanotubes and leaving the long boron nitride nanotubes to stand on their own. The team relies on the expertise of Wardle’s team, which has focused for years on designing highly flexible carbon nanotubes. Through their current work, the researchers are looking for a way to change the temperature and pressure of the liquid injection process to remove the carbon nanotubes while leaving the boron nitride nanotubes.
“The first few times we did it, it was a horrible mess,” recalls Wardle. “The days rolled into a ball and they didn’t work.”
Eventually, the team found a combination of heat, pressure and catalyst that did the trick. By combining these techniques, the researchers first replicated Xiang’s process to produce boron nitride-coated carbon nanotubes. Because hBN is more heat resistant than graphene, the team then heated it to burn the carbon nanotubes underneath, leaving the boron nitride nanotubes free-standing.
In the images, the team saw a clear crystal structure, indicating that the boron nitride nanotubes are very fine. The structures are also very large: in one square centimeter, the researchers were able to create a forest of more than 100 billion boron nitride nanotubes, measuring about a millimeter in height, large enough to be seen with the naked eye. straight. According to nanotube engineering principles, these dimensions are considered large. “Now we can produce these nanoscale fibers in large quantities, which has never been demonstrated before,” says Acauan.
To demonstrate the flexibility of their process, the team has produced large-scale carbon structures, including layers of carbon fibers, mats of “fuzzy” carbon nanotubes and random sheets of carbon nanotubes called “buckypaper”. They coated each carbon-based sample with boron and nitrogen precursors, then went through their process to burn the underlying carbon. In each presentation, they closed with a boron nitride copy of the original black carbon scaffold.
They can also “cut” forests of BNNTs, creating vertically oriented fiber films that are the ideal structure for incorporation into composites.
“We are currently working on fibers to strengthen ceramic matrix composites, for hypersonic applications in high-temperature environments, and for transparent device windows,” says Wardle. saying. “You can use these strong nanotubes to make light.”
