Is boron nitride the new carbon in nanotube science?

Engineers from MIT and the University of Tokyo have fabricated centimeter-sized structures, large enough to see by the eye, filled with hundreds of billions of aligned hexagonal boron nitride hollow fibers, or nanotubes.

Hexagonal boron nitride, or hBN, is a one-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 also withstand higher temperatures than graphene and is electrically insulating rather than conductive. When hBN is rolled into nanotubes or nanotubes, its extraordinary properties are greatly enhanced.

The team’s findings, published in the journal today ACS nano, provide a route to mass fabrication of aligned boron nitride nanotubes (A-BNNTs). The researchers plan to use the technique to create large arrays of these nanotubes, which can then be combined with other materials to make stronger, more heat-resistant composites, for example to shield space structures and supersonic aircraft.

Because hBN is transparent and electrically insulating, the team also plans to build the BNNTs into transparent windows and use them to electrically isolate sensors in electronic devices. The team is also studying ways to weave the nanofibers into membranes for water filtration and for ‘blue energy’ – a renewable energy concept in which electricity is generated from the ion filtration of salt water into fresh water.

Brian Wardle, a professor of aerospace engineering at MIT, compares the team’s findings to decades of continuous striving by scientists to produce carbon nanotubes on a mass scale.

“In 1991, a single carbon nanotube was identified as an interesting thing, but it’s been 30 years since we’ve achieved mass-aligned carbon nanotubes, and the world isn’t even there yet,” says Wardle. “With the work we’re doing, we just short-circuited about 20 years to get mass versions of aligned boron nitride nanotubes.”

Wardle is the senior author of the new study, which includes lead author and MIT research scientist Luiz Acauan, former MIT postdoc Haozhe Wang, and collaborators at the University of Tokyo.

A vision, aligned

Like graphene, hexagonal boron nitride has a molecular structure resembling chicken wire. In graphene, this wire mesh configuration consists entirely of carbon atoms arranged in a repeating pattern of hexagons. In hBN, the hexagons consist of alternating boron and nitrogen atoms. In recent years, researchers have discovered that two-dimensional hBN foils exhibit exceptional strength, stiffness, and elasticity properties at high temperatures. When sheets of hBN are rolled into nanotube form, these properties are further enhanced, especially when the nanotubes are oriented like small trees in a densely packed forest.

However, finding ways to synthesize stable, high-quality BNNTs has proven difficult. A handful of attempts to do this have produced inferior, misaligned 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,” says Wardle.

In 2020, Rong Xiang and colleagues from the University of Tokyo found they could fabricate high-quality boron nitride nanotubes by first using a conventional chemical vapor deposition approach to grow a forest of short, a few microns long, carbon nanotubes. They then coated the carbon-based forest with “precursors” of boron and nitrogen gas, which crystallized on the carbon nanotubes when baked in an oven at high temperatures to form high-quality hexagonal boron nitride nanotubes with carbon nanotubes inside.

Burning scaffolding

In the new study, Wardle and Acauan have expanded and scaled Xiang’s approach, essentially removing the underlying carbon nanotubes and leaving the long boron nitride nanotubes to their own devices. The team drew on the expertise of Wardle’s group, which has focused on fabricating high-quality aligned carbon nanotube arrays for years. In their current work, researchers sought ways to optimize 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 complete ugly junk,” Wardle recalls. “The tubes curled up into a ball and didn’t work.”

Eventually, the team encountered a combination of temperatures, pressures, and forerunners that did the trick. Using this combination of processes, the researchers first reproduced the steps Xiang took to synthesize the boron nitride-coated carbon nanotubes. Because hBN is resistant to higher temperatures than graphene, the team then turned up the heat to burn away the underlying framework of black carbon nanotubes, while leaving the transparent, free-standing boron nitride nanotubes intact.

In microscopic images, the team observed clear crystalline structures – proof that the boron nitride nanotubes are of high quality. The structures were dense, too: Within a square centimeter, the researchers were able to synthesize a forest of more than 100 billion aligned boron nitride nanotubes that was about a millimeter high – large enough to be visible to the naked eye. By standards of nanotube engineering, these dimensions are considered “masses”.

“We are now able to produce these nanofibers on a large scale, which has never been shown before,” Acauan says.

To demonstrate the flexibility of their technique, the team synthesized larger carbon-based structures, including a web of carbon fibers, a mat of “flaky” carbon nanotubes, and sheets of randomly oriented carbon nanotubes known as “buckypaper.” They coated each carbon-based sample with boron and nitrogen precursors, then ran their process to burn away the underlying carbon. At each demonstration, they were left with a boron nitride replica of the original black carbon framework.

They were also able to “rip down” the BNNT forests by producing horizontally oriented fibrous films, which are a preferred configuration for incorporation into composite materials.

“We are now working on 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,” says Wardle. “You could make transparent materials reinforced 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 the MIT Nano-Engineered Composite Aerospace Structures (NECST) Consortium.

Written by Jennifer Chu, MIT News Office

Additional background

Paper: “Micro- and Macro-structures of Aligned Boron Nitride Nanotube Arrays”

https://pubs.acs.org/doi/10.1021/acsnano.2c05229