by
A team of engineers from the Massachusetts Institute of Technology (MIT) and the University of Tokyo have created centimeter-scale structures made of hexagonal boron nitride (hBN) and loaded with hundreds of billions of hollow aligned fibers (nanotubes). These centimeter-scale structures are large enough to be seen by the naked eye.
MIT engineers make a forest of “white graphene” nanotubes (shown here in an MIT pattern) by burning a carbon black scaffold. Image credit: Courtesy of the researchers
hBN is a thin, single-atom material that has been called “white graphene” because of its transparent appearance and its 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 improved.
The team’s findings, recently published in ACS Nano, offer a route to the fabrication of aligned boron nitride nanotubes (A-BNNTs) in bulk. The scientists aim to use the method to produce large volumes of these nanotubes, which can then be combined with other materials to create stronger, more heat-resistant composites, for example, to protect hypersonic aircraft and space structures .
As hBN is electrically insulating and transparent, the researchers also plan to incorporate BNNTs into transparent windows and use them to electrically insulate sensors inside electronic gadgets.
The researchers are also looking at methods of weaving the nanofibers into membranes for water filtration and “blue energy,” a new renewable energy concept where electricity is generated by ionic sieving of saltwater into fresh water.
Brian Wardle, a professor of aeronautics and astronautics at MIT, compares the team’s results to researchers’ decades-long, ongoing quest to build mass carbon nanotubes.
In 1991, a single carbon nanotube was recognized as an interesting thing, but it’s been 30 years since we got to bulk aligned carbon nanotubes, and the world isn’t even fully there yet. With the work we’re doing, we’ve just short-circuited about 20 years to get to mass-scale versions of aligned boron nitride nanotubes.
Brian Wardle, Senior Study Author and Professor of Aeronautics and Astronautics, Massachusetts Institute of Technology
Wardle is the senior author of the study. The study also includes lead author and MIT researcher Luiz Acauan, former MIT postdoctoral researcher Haozhe Wang, and colleagues at the University of Tokyo.
Similar to graphene, hBN has a molecular structure similar to chicken wire. In graphene, this chicken wire formation consists of carbon atoms organized 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) hBN sheets exhibit excellent properties of stiffness, strength, and elasticity at high temperatures.
When hBN sheets are wrapped into nanotube structures, these properties are further enhanced, especially when the nanotubes are aligned, like tiny trees in a densely packed forest.
But finding ways to make stable, higher-quality BNNTs has been difficult, and some efforts have produced only low-quality, misaligned fibers.
If you can align them, you have a much better chance of harnessing the properties of BNNT on a massive scale to create real physical devices, composites and films.
Brian Wardle, Senior Study 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 grow a forest of tiny carbon nanotubes a few microns long.
They then coated the carbon-based forest with nitrogen “precursors” and boron gas. When baked in a high-temperature oven, this crystallized into the carbon nanotubes to create superior 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 on insights from Wardle’s team, which has focused for years on making superior-quality aligned arrays of carbon nanotubes.
The team sought methods to adjust the pressures and temperatures of the chemical vapor deposition process to eliminate the carbon nanotubes while allowing the boron nitride nanotubes to be completed.
The first few times we did it, it was complete ugly rubbish. The tubes curled up into a ball and didn’t work.
Brian Wardle, Senior 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 repeated Xiang’s steps to make the boron nitride-coated carbon nanotubes.
As hBN is impermeable at higher temperatures than graphene, the researchers turned up the heat to burn off the pristine black carbon nanotube scaffold while allowing the transparent, free-standing boron nitride nanotubes to remain intact.
In microscopic images, the researchers observed clear crystal structures, proof that the boron nitride nanotubes are of high quality. The structures were also dense, and within one square centimeter, the team was able to construct a forest of over 100 billion aligned boron nitride nanotubes that were about a millimeter tall, large enough to be seen with the naked eye. According to the principles of nanotube engineering, these dimensions are “bulky” in scale.
“We are now able to manufacture these nanoscale fibers on a massive scale, which has never been seen before.” says Acauan.
To demonstrate the versatility of their method, the researchers created larger carbon-based structures, including a layer of “fuzzy” carbon nanotubes, a mesh 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 carried out the process of burning the underlying carbon. Each experiment was left with a boron nitride reproduction of the original carbon black scaffold.
They could also “collapse” forests of BNNTs, creating horizontally aligned fiber films that are a preferred configuration for incorporation into composites.
We are now working on fibers to reinforce ceramic matrix composites, for ultrasonic 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, Senior Study Author and Professor of Aeronautics and Astronautics, 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.
Journal report
Acauan, LH, et al. (2022) Micro- and macrostructures of aligned boron nitride nanotube arrays. ACS Nano. doi.org/ 10.1021/acsnano.2c05229.
Source: https://mit.edu
#Simultaneous #synthesis #White #Graphene #nanotubes
