Reviewed by Mila PereraNov 1 2022
A team of engineers from the Massachusetts Institute of Technology (MIT) and the University of Tokyo has created centimeter-scale structures that are composed of hexagonal boron nitride (hBN) and loaded with hundreds of billions of hollow aligned fibers (nanotubes). These centimeter-scale structures are large enough for the naked eye to see.
MIT engineers fabricate a forest of “white graphene” nanotubes (shown here patterned as MIT) by burning away a scaffold of black carbon. Image Credit: Courtesy of the researchers
hBN is a single-atom-thin material, which has been called “white graphene” due to its transparent look and resemblance to carbon-based graphene in molecular structure and strength. It can endure higher temperatures than graphene and is electrically insulating instead of conductive. When hBN is rolled into nanoscale tubes (or nanotubes), its remarkable properties are considerably improved.
The team’s findings, published recently in ACS Nano, offer a path toward making aligned boron nitride nanotubes (A-BNNTs) in bulk. The scientists aim to utilize the method to produce large volumes of these nanotubes, which can then be integrated with other materials to create sturdier, more heat-resilient composites, for instance, to protect hypersonic aircraft and space structures.
As hBN is electrically insulating and transparent, the researchers also intend to integrate the BNNTs into transparent windows and employ them to insulate sensors inside electronic gadgets electrically.
The researchers are also examining methods to knit the nanofibers into membranes for water filtration and “blue energy,” a novel idea for renewable energy wherein electricity is generated from the ionic sifting of salt water into fresh water.
Brian Wardle, professor of aeronautics and astronautics at MIT, compares the team’s outcomes to researchers’ decades-long, ongoing quest of manufacturing mass carbon nanotubes.
In 1991, a single carbon nanotube was identified as an interesting thing, but it’s been 30 years getting to bulk-aligned carbon nanotubes, and the world’s not even fully there yet. With the work we’re doing, we’ve just short-circuited about 20 years in getting to bulk-scale versions of aligned boron nitride nanotubes.
Brian Wardle, Study Senior Author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
Wardle is the study’s senior author. The study also comprises lead author and MIT research scientist Luiz Acauan, former MIT postdoctoral researcher Haozhe Wang, and co-workers at the University of Tokyo.
Similar to graphene, hBN has a molecular structure akin to chicken wire. In graphene, this chicken wire formation is composed of carbon atoms organized in a repeating pattern of hexagons.
For hBN, the hexagons are made up of alternating atoms of nitrogen and boron. In the last few years, scientists have learned that two-dimensional (2D) sheets of hBN display excellent stiffness, strength, and resilience properties at elevated temperatures.
When sheets of hBN are rolled into nanotube structures, these properties are additionally improved, mainly when the nanotubes are aligned, like miniature trees in a tightly packed forest.
But discovering ways to manufacture stable, superior-quality BNNTs has been difficult, and some efforts have only created low-quality, nonaligned fibers.
If you can align them, you have much better chance of harnessing BNNTs properties at the bulk scale to make actual physical devices, composites, and membranes.
Brian Wardle, Study Senior Author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
In 2020, Rong Xiang and colleagues at the University of Tokyo discovered that they could make superior-quality boron nitride nanotubes by first using a traditional chemical vapor deposition method to develop a forest of minute, few-micron-long carbon nanotubes.
They then coated the carbon-based forest with “precursors” of nitrogen and boron gas. When baked in a high-temperature oven, this crystallized onto the carbon nanotubes to develop superior-quality nanotubes of hBN containing carbon nanotubes inside.
In the new research, Wardle and Acauan have extended and scaled Xiang’s method, eliminating the underlying carbon nanotubes and allowing the long boron nitride nanotubes to remain. The researchers drew insight from Wardle’s group, which has concentrated for years on constructing superior-quality aligned arrays of carbon nanotubes.
The team searched for methods to fine-tune the pressures and temperatures of the chemical vapor deposition process to eliminate the carbon nanotubes while allowing the boron nitride nanotubes to be complete.
The first few times we did it, it was completely ugly garbage. The tubes curled up into a ball, and they didn’t work.
Brian Wardle, Study Senior Author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
The team discovered a combination of pressures, temperatures, and precursors that resolved the issues. Using this combination of processes, the scientists first replicated Xiang's steps to manufacture the boron-nitride-coated carbon nanotubes.
As hBN is impervious to higher temperatures than graphene, thus, the researchers increased the heat to burn away the primary black carbon nanotube scaffold while allowing the transparent, freestanding boron nitride nanotubes to remain intact.
In microscopic pictures, the researchers noticed clear crystalline structures, proof that the boron nitride nanotubes are high quality. The structures were also dense, and within a square centimeter, the team was able to manufacture a forest of over 100 billion aligned boron nitride nanotubes that measured around a millimeter in height, large enough to be seen by the naked eye. By nanotube engineering principles, these dimensions are “bulk” in scale.
“We are now able to make these nanoscale fibers at bulk scale, which has never been shown before,” Acauan says.
To show the versatility of their method, the researchers created larger carbon-based structures, including a mat of “fuzzy” carbon nanotubes, a weave of carbon fibers, and sheets of randomly arranged carbon nanotubes called “buckypaper.”
They coated each carbon-based sample with nitrogen and boron precursors and then performed their process of burning away the underlying carbon. Each experiment was left with a boron-nitride reproduction of the original black carbon scaffold.
They could also “knock down” the forests of BNNTs, creating horizontally aligned fiber films that are a favored configuration for integrating into composite materials.
We are now working toward fibers to reinforce ceramic matrix composites, for hypersonic and space applications where there are very high temperatures, and for windows for devices that need to be optically transparent. You could make transparent materials that are reinforced with these very strong nanotubes.
Brian Wardle, Study Senior Author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
This study was partly assisted by Saab AB, Airbus, Boeing, ANSYS, Lockheed Martin, Embraer, and Teijin Carbon America through MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium.
Journal Reference
Acauan, L. H., et al. (2022) Micro- and Macrostructures of Aligned Boron Nitride Nanotube Arrays. ACS Nano. doi.org/ 10.1021/acsnano.2c05229.