A team of engineers from the Massachusetts Institute of Technology (MIT) and the University of Tokyo have created centimeter-scale structures composed of hexagonal boron nitride (hBN) and loaded with hundreds of billions of aligned hollow fibers (nanotubes ). These centimeter-scale structures are large enough to be seen with the naked eye.
MIT engineers make a forest of “white graphene” nanotubes (shown here as MIT) by burning a scaffold of black carbon. Image credit: Courtesy of the researchers
hBN is a thin single-atom material, called “white graphene” because of its transparent appearance and its similarity to carbon-based graphene in terms of molecular structure and strength. It can withstand higher temperatures than graphene and is electrically insulating instead of conductive. When hBN is rolled into nanometer tubes (or nanotubes), its remarkable properties are dramatically enhanced.
The team’s findings, recently published in ACS Nano, offer a route to the bulk fabrication of aligned boron nitride nanotubes (A-BNNTs). The scientists aim to use the method to produce large volumes of these nanotubes, which can then be integrated with other materials to create stronger and more heat-resistant composites, for example, to protect hypersonic aircraft and structures. spatial.
Since hBN is electrically insulating and transparent, the researchers also plan to embed BNNTs in transparent windows and use them to electrically isolate sensors inside electronic gadgets.
Researchers are also looking at methods for knitting the nanofibers into membranes for water filtration and “blue energy,” a new renewable energy idea in which electricity is generated from the ion sieving of carbon dioxide. salt water into fresh water.
Brian Wardle, professor of aeronautics and astronautics at MIT, compares the team’s results to the ongoing quest of researchers for decades to make mass-produced carbon nanotubes.
In 1991, a single carbon nanotube was identified as an interesting thing, but it’s been 30 years since carbon nanotubes have lined up en masse, and the world isn’t even quite there yet. 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.
Brian Wardle, lead study author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
Wardle is the lead author of the study. The study also includes lead author and MIT researcher Luiz Acauan, former MIT postdoctoral researcher Haozhe Wang, and colleagues from the University of Tokyo.
Similar to graphene, hBN has a molecular structure similar to chicken wire. In graphene, this chicken wire formation is made up of carbon atoms arranged in a repeating pattern of hexagons.
For hBN, the hexagons consist of alternating nitrogen and boron atoms. In recent years, scientists have learned that two-dimensional (2D) sheets of hBN exhibit excellent stiffness, strength, and resilience properties at elevated temperatures.
When hBN sheets are rolled into nanotube structures, these properties are further enhanced, primarily when the nanotubes are aligned, like miniature trees in a dense forest.
But discovering ways to make stable, high-quality BNNTs has been difficult, and some efforts have only created low-quality, unaligned fibers.
If you can align them, you have a much better chance of exploiting the properties of BNNTs at scale to fabricate real physical devices, composites and membranes.
Brian Wardle, lead study author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
In 2020, Rong Xiang and his colleagues at the University of Tokyo discovered that they could make superior boron nitride nanotubes by first using a traditional chemical vapor deposition method to grow a forest of nanotubes. of tiny carbon a few microns long.
They then coated the carbon-based forest with “precursors” of nitrogen and boron gas. When fired in a high temperature oven, it crystallizes on the carbon nanotubes to grow superior quality hBN nanotubes containing carbon nanotubes inside.
In the new research, Wardle and Acauan extended and scaled up Xiang’s method, eliminating the underlying carbon nanotubes and allowing the long boron nitride nanotubes to remain. The researchers drew inspiration from Wardle’s group, which for years focused on building high-quality aligned arrays of carbon nanotubes.
The team researched methods to fine-tune the pressures and temperatures of the chemical vapor deposition process to remove the carbon nanotubes while allowing the boron nitride nanotubes to be complete.
The first few times we did it was completely ugly garbage. The tubes curled up into a ball and they didn’t work.
Brian Wardle, lead study author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
The team discovered a combination of pressures, temperatures and precursors that solved the problems. Using this combination of processes, the scientists first replicated Xiang’s steps to fabricate the boron nitride-coated carbon nanotubes.
Since hBN is impervious to higher temperatures than graphene, researchers increased the heat to burn off the primary scaffold of black carbon nanotubes while allowing the transparent, self-contained boron nitride nanotubes to remain intact.
In microscopic images, the researchers noticed clear crystal structures, proof that the boron nitride nanotubes are of high quality. The structures were also dense, and within a square centimeter the team was able to craft a forest of more than 100 billion aligned boron nitride nanotubes measuring about a millimeter in height, large enough to be seen with the naked eye. According to the principles of nanotube engineering, these dimensions are large scale.
“We are now able to manufacture these nanoscale fibers on a large scale, which has never been demonstrated before”, Acauan said.
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 randomly arranged sheets of carbon nanotubes called “buckypaper”.
They coated each carbon-based sample with nitrogen and boron precursors, then performed their process of burning the underlying carbon. Each experiment was left with a boron nitride reproduction of the original black carbon scaffold.
They could also “cut down” BNNT forests, creating horizontally aligned fiber films that are a preferred configuration for embedding in composite materials.
We are now working towards fibers for reinforcing ceramic matrix composites, for hypersonic and space applications where temperatures are very high, and for windows for devices that need to be optically transparent. You could make reinforced transparent materials with these very strong nanotubes.
Brian Wardle, lead study author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
This study was assisted in part by Saab AB, Airbus, Boeing, ANSYS, Lockheed Martin, Embraer and Teijin Carbon America through the Nano-Engineered Composite aerospace STtructures (NECST) consortium at MIT.
Journal reference
Acauan, LH, et al. (2022) Microstructures and macrostructures of arrays of aligned boron nitride nanotubes. ACS Nano. doi.org/10.1021/acsnano.2c05229.
Source: https://mit.edu
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