Engineers from MIT and the University of Tokyo have produced centimeter-scale structures, large enough for the eye to see, that contain hundreds of billions of aligned hollow fibers, or nanotubes, made from hexagonal boron nitride.
Hexagonal boron nitride, or hBN, is a thin, single-atom material that was coined “white graphene” for its transparent appearance and similarity to carbon-based graphene in terms of molecular structure and strength. It can also withstand higher temperatures than graphene and is electrically insulating rather than conductive. When hBN is coiled into nanoscale tubes, or nanotubes, its exceptional properties are dramatically enhanced.
The team’s findings, published today in the journal ACS Nano, provide a route to the fabrication of aligned boron nitride nanotubes (A-BNNTs) in bulk. The researchers plan to exploit the technique to make large-scale arrays of these nanotubes, which can then be combined with other materials to make stronger, more heat-resistant composites, for example to protect space structures and hypersonic aircraft.
Because hBN is transparent and electrically insulating, the team also plans to embed BNNTs in transparent windows and use them to electrically isolate sensors from electronic devices. The team is also investigating ways to weave the nanofibers into membranes for water filtration and for “blue energy” – a concept of renewable energy in which electricity is generated from the ion filtering of light. salt water into fresh water.
Brian Wardle, a professor of aeronautics and astronautics at MIT, compares the team’s results to the continued pursuit of scientists for decades to manufacture large-scale carbon nanotubes.
“In 1991, a single carbon nanotube was identified as an interesting thing, but it’s been 30 years since mass-aligned carbon nanotubes were developed, and the world isn’t even quite there yet,” says Wardle. “With the work we’re doing, we’ve just short-circuited about 20 years to get to large-scale versions of aligned boron nitride nanotubes.”
Wardle is lead author on the new study, which includes lead author and MIT researcher Luiz Acauan, former MIT postdoctoral fellow Haozhe Wang, and collaborators from the University of Tokyo.
An aligned vision
Like graphene, hexagonal boron nitride has a chicken wire-like molecular structure. In graphene, this chicken-wire configuration is made entirely of carbon atoms, arranged in a repeating pattern of hexagons. For hBN, the hexagons are composed of alternating boron and nitrogen atoms. In recent years, researchers have discovered that two-dimensional sheets of hBN exhibit exceptional properties of strength, stiffness and resilience at elevated temperatures. When sheets of hBN are rolled up into nanotubes, these properties are further enhanced, especially when the nanotubes are aligned, like tiny trees in a densely populated forest.
But finding ways to synthesize stable, high-quality BNNTs has proven difficult. A handful of efforts to achieve this have produced poor quality unaligned fibers.
“If you can align them, you have a much better chance of exploiting the properties of BNNTs on a large scale to fabricate real physical devices, composites and membranes,” says Wardle.
In 2020, Rong Xiang and his colleagues at the University of Tokyo discovered that they could produce high-quality boron nitride nanotubes by first using a conventional chemical vapor deposition approach to grow a forest of short carbon nanotubes a few microns long. They then coated the carbon-based forest with “precursors” of boron and nitrogen gas, which, when baked in a high-temperature oven, crystallized on the carbon nanotubes to form high-quality nanotubes of hexagonal boron nitride with carbon nanotubes inside.
Burning scaffolds
In the new study, Wardle and Acauan extended and scaled Xiang’s approach, essentially removing the underlying carbon nanotubes and leaving the long boron nitride nanotubes to stand on their own. The team drew on the expertise of Wardle’s group, which for years focused on fabricating high-quality aligned arrays of carbon nanotubes. With their current work, the researchers looked for ways to adjust the temperatures and pressures of the chemical vapor deposition process to remove the carbon nanotubes while leaving the boron nitride nanotubes intact.
“The first few times we did it, it was absolutely ugly garbage,” recalls Wardle. “The tubes curled up into a ball and they didn’t work.”
Eventually, the team found a combination of temperatures, pressures and precursors that did the trick. With this combination of processes, the researchers first replicated the steps Xiang followed to synthesize the boron nitride-coated carbon nanotubes. Because hBN withstands higher temperatures than graphene, the team then turned up the heat to burn off the underlying black carbon nanotube scaffold, while leaving the transparent, self-contained boron nitride nanotubes intact.
In microscopic images, the team observed clear crystal structures, proof that the boron nitride nanotubes are of high quality. The structures were also dense: in a square centimeter, the researchers were able to synthesize a forest of more than 100 billion aligned boron nitride nanotubes, measuring about a millimeter in height, large enough to be visible to the naked eye. According to nanotube engineering standards, these dimensions are considered large scale.
“We are now able to manufacture these nanoscale fibers on a large scale, which has never been demonstrated before,” says Acauan.
To demonstrate the flexibility of their technique, the team synthesized larger carbon-based structures, including a weave of carbon fibers, a mat of “fuzzy” carbon nanotubes, and sheets of randomly oriented carbon nanotubes. called “buckypaper”. They coated each carbon-based sample with boron and nitrogen precursors, then went through their process to burn off the underlying carbon. In each demonstration, they ended up with a boron nitride replica of the original black carbon scaffold.
They were also able to “cut down” BNNT forests, producing horizontally aligned fiber films that are a preferred configuration for incorporation into composite materials.
“We are now working on fibers to reinforce ceramic matrix composites, for hypersonic and space applications where temperatures are very high, and for device windows that need to be optically transparent,” says Wardle. “You could make reinforced transparent materials with these very strong nanotubes.”
This research was supported, in part, by Airbus, ANSYS, Boeing, Embraer, Lockheed Martin, Saab AB, and Teijin Carbon America through MIT’s Nano-Engineered Composite aerospace STructures (NECST) consortium.
Written by Jennifer Chu, MIT News Office
Additional context
Article: “Micro- and macro-structures of arrays of aligned boron nitride nanotubes”
https://pubs.acs.org/doi/10.1021/acsnano.2c05229
The title of the article
“Micro- and macro-structures of arrays of aligned boron nitride nanotubes”
#nanotube #science #boron #nitride #carbon