A team of engineers from Massachusetts Institute of Technology (MIT) and the University of Tokyo have created centimeter-sized structures made of hexagonal boron nitride (hBN) loaded with hundreds of billions of aligned hollow fibers (nanotubes). These centimeter-sized structures are large enough to be seen with the naked eye.
MIT engineers fabricate a forest of “white graphene” nanotubes (shown here patterned as MIT) by burning away a black carbon scaffold. Photo credit: Courtesy of the researchers
hBN is an atom thin material that has been dubbed “white graphene” due to its transparent appearance and similarity to carbon-based graphene in molecular structure and strength. It can withstand higher temperatures than graphene and is electrically insulating rather than conductive. When hBN is rolled into nanoscale tubes (or nanotubes), its remarkable properties are greatly enhanced.
The team’s findings, recently published in ACS nano, provide a way to fabricate aligned boron nitride nanotubes (A-BNNTs) in large quantities. The scientists plan to use the process to produce large quantities of these nanotubes, which can then be integrated with other materials to create stronger, more heat-resistant composites, for example to protect hypersonic aircraft and space structures.
Since hBN is electrically insulating and transparent, the researchers intend to integrate the BNNTs into transparent windows as well, thereby electrically isolating sensors in electronic devices.
The researchers are also investigating methods to knit the nanofibers into membranes for water filtration and “blue energy,” a novel renewable energy idea that generates electricity from the ionic sieving of saltwater into freshwater.
Brian Wardle, a professor of aerospace engineering at MIT, compares the team’s findings to the researchers’ decades-long quest to mass-produce carbon nanotubes.
In 1991, a single carbon nanotube was identified as an interesting thing, but it’s been 30 years since large-scale aligned carbon nanotubes have come about, and the world isn’t even there yet. With the work we are doing, we have just shortened about 20 years to achieve mass versions of aligned boron nitride nanotubes.
Brian Wardle, senior study author and aerospace professor, 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 postdoc Haozhe Wang, and collaborators from the University of Tokyo.
Similar to graphene, hBN has a molecular structure that resembles chicken wire. In graphene, this chain-link formation consists of carbon atoms organized in a repeating pattern of hexagons.
In hBN, the hexagons are made up of alternating nitrogen and boron atoms. In recent years, scientists have learned that two-dimensional (2D) hBN foils exhibit excellent stiffness, strength, and elasticity properties at elevated temperatures.
When sheets of hBN are rolled into nanotube structures, these properties are further enhanced, mainly when the nanotubes are aligned, like miniature trees in a densely packed forest.
However, finding ways to produce stable, top-quality BNNTs has been difficult, and some efforts have produced only inferior, unaligned fibers.
If you can align them, you have a much better chance of exploiting the properties of BNNTs on a large scale to make actual physical devices, composites, and membranes.
Brian Wardle, senior study author and aerospace professor, Massachusetts Institute of Technology
In 2020, Rong Xiang and colleagues from the University of Tokyo discovered they could produce the highest quality boron nitride nanotubes by first using a traditional chemical vapor deposition process to develop a forest of tiny carbon nanotubes a few microns long.
Then they coated the carbon-based forest with “precursors” of nitrogen and boron gas. When baked in a high-temperature oven, this crystallized on the carbon nanotubes to develop the highest quality hBN nanotubes containing carbon nanotubes inside.
In the new research, Wardle and Acauan extended and scaled Xiang’s method by eliminating the underlying carbon nanotubes and allowing the long boron nitride nanotubes to remain. The researchers drew lessons from Wardle’s group, which for years has focused on constructing high-quality aligned arrays of carbon nanotubes.
The team looked for methods to fine-tune the pressures and temperatures of the chemical vapor deposition process to eliminate the carbon nanotubes while making the boron nitride nanotubes complete.
The first few times we did it, it was complete ugly junk. The tubes curled up into a ball and they didn’t work.
Brian Wardle, senior study author and aerospace professor, Massachusetts Institute of Technology
The team discovered a combination of pressures, temperatures, and precursors that solved the problems. Using this combination of processes, scientists first replicated Xiang’s steps to fabricate the boron nitride-coated carbon nanotubes.
Because hBN is insensitive to higher temperatures than graphene, the researchers increased the heat to burn away the primary framework of black carbon nanotubes while leaving the transparent, free-standing boron nitride nanotubes intact.
In microscopic images, the researchers noticed clear crystalline structures, evidence that the boron nitride nanotubes are of high quality. The structures were also dense, and within a square centimeter the team was able to fabricate a forest of over 100 billion aligned boron nitride nanotubes, about a millimeter high and large enough to be seen with the naked eye. According to the principles of nanotube technology, these dimensions are “large” in scale.
“We are now able to produce these nanofibers on a large scale, which has never been shown before,” Akauan says.
To demonstrate the versatility of their method, the researchers created larger carbon-based structures, including a mat of “fuzzy” carbon nanotubes, a web 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 ran their process of burning away the underlying carbon. Each experiment was left with a boron nitride reproduction of the original black carbon framework.
They could also “knock down” the BNNT forests and create horizontally aligned fibrous films, which are a preferred configuration for incorporation into composite materials.
We are currently working on fibers for the reinforcement of ceramic matrix composites, for hypersonic and space applications where very high temperatures are involved, and for windows for devices that need to be optically transparent. They could make transparent materials reinforced with these very powerful nanotubes.
Brian Wardle, senior study author and aerospace professor, Massachusetts Institute of Technology
This study was supported in part by Saab AB, Airbus, Boeing, ANSYS, Lockheed Martin, Embraer, and Teijin Carbon America through MIT’s Nano-Engineered Composite Aerospace Structures (NECST) Consortium.
magazine reference
Akauan, LH, et al. (2022) Micro and macro structures of aligned boron nitride nanotube arrays. ACS nano. doi.org/10.1021/acsnano.2c05229.
Source: https://mit.edu
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