A way for mass production of aligned boron nitride nanotubes

A technology for synthesizing many "white graphene" nanotubes at one time paves the way for manufacturing stronger and heat-resistant composite materials and renewable energy films

The engineers synthesized a well-arranged forest of nanofibers made of boron nitride or "white graphene". They hope to use this technology to make large arrays of these nanotubes, and then combine with other materials to make stronger and more heat-resistant composite materials, such as protecting space structures and hypersonic aircraft.

Hexagonal boron nitride (hBN) is a kind of monoatomic thin material, which is called "white graphene" because of its transparent appearance and its molecular structure and strength similar to carbon cornerstone graphene. It can also withstand higher temperatures than graphene, and is insulated, not conductive. When hBN is rolled into nanotubes or nanotubes, its special properties are significantly enhanced.

The team's research results, published today in ACS Nano magazine, provide a way for mass production of aligned boron nitride nanotubes (a - bnnts). Researchers plan to use this technology to produce large arrays of these nanotubes, which can then be combined with other materials to make stronger and more heat-resistant composite materials, such as shielding space structures and hypersonic aircraft.

Since hBNs are transparent and electrically insulated, the team also envisages incorporating bnnts into the transparent window and using them to electrically insulate sensors in electronic devices. The team is also studying how to weave nanofibers into membranes for water filtration and "blue energy" - a concept of renewable energy, which generates electricity by filtering salt water into fresh water ions.

Brian Wardle, a professor of aeronautics and astronautics at the Massachusetts Institute of Technology, compared the team's research results to scientists' continuous pursuit of manufacturing large-scale carbon nanotubes for decades.

"In 1991, a single carbon nanotube was considered to be an interesting thing, but it has taken 30 years to achieve the mass arrangement of carbon nanotubes, and the world has not even fully achieved this goal." "Through the work we are doing, we have shortened the time of about 20 years to achieve a large-scale version of boron nitride nanotubes."

Wardell is the senior author of the new study, the main author of the study, Luiz Acauan, a research scientist at the Massachusetts Institute of Technology, Haozhe Wang, a former post-doctoral fellow at the Massachusetts Institute of Technology, and collaborators at the University of Tokyo.

Vision, consistency

Like graphene, the molecular structure of hexagonal boron nitride is similar to that of iron wire mesh. In graphene, the chicken silk structure is completely composed of carbon atoms and arranged into a repeated hexagon. For hBN, the hexagon is composed of boron atoms and nitrogen atoms alternately. In recent years, researchers have found that the two-dimensional sheets of hBN exhibit excellent strength, stiffness and resilience at high temperatures. When the hBN sheet is rolled into the form of nanotubes, these characteristics are further enhanced, especially when the nanotubes are arranged in order, just like the small trees in the dense forest.

However, it is challenging to find a method to synthesize stable and high-quality bnnt. Some of these efforts have resulted in low quality discontinuous fibers.

"If you can align them, you will have a better chance to use the characteristics of BNNTs on a large scale to manufacture actual physical devices, composites and membranes," Wardell said

In 2020, Rong Xiang and colleagues at the University of Tokyo found that they could plant a short and several micron-long carbon nanotube forest by first using the traditional method of chemical vapor deposition to produce high-quality boron nitride nanotubes. Then, they coated the "precursors" of boron and nitrogen on the carbon-based forest. When baking in the oven at high temperature, these precursors crystallized on the carbon nanotubes to form high-quality hexagonal boron nitride nanotubes, which contained carbon nanotubes.

Combustion support

In the new study, Wardle and Acauan extended and expanded Xiang's method, essentially removing the carbon nanotubes at the bottom layer and making the long boron nitride nanotubes exist independently. The team made use of the expertise of the Waddell team, which has been focusing on manufacturing high-quality carbon nanotube arrays for many years. In their current work, researchers have found a way to adjust the temperature and pressure of the chemical vapor deposition process to remove carbon nanotubes while retaining the integrity of boron nitride nanotubes.

"In the first few times, what we did was totally ugly garbage," Waddell recalled. "The pipe curled up in a ball, which didn't work."

Finally, the research team found a combination of temperature, pressure and precursor. Through this process combination, researchers first reproduced the steps of synthesizing boron nitride coated carbon nanotubes by Xiang. Since hBN is more resistant to high temperature than graphene, the research team then raised the temperature and burned the black carbon nanotube support at the bottom, while retaining transparent and independent boron nitride nanotubes.

In the microscope image, the research team observed clear crystal structure - boron nitride nanotubes have high-quality evidence. These structures are also very dense: in the range of one square centimeter, researchers can synthesize a forest composed of more than 100 billion aligned boron nitride nanotubes, with a height of about one millimeter, which is visible to the naked eye. According to the nanotube engineering standards, these dimensions are considered "bulk" in scale.

"We are now able to produce these nanofibers on a large scale, which is unprecedented before," Acauan said

In order to prove the flexibility of their technology, the team synthesized larger carbon-based structures, including carbon fiber braiding, "fuzzy" carbon nanotube pads, and randomly oriented carbon nanotube segments called "Barker paper". They coated each carbon-based sample with boron and nitrogen precursors, and then burned the underlying carbon through their process. In each demonstration, they left a boron nitride replica of the original black carbon support.

They can also "destroy" the bnnt forest and produce a horizontal arrangement of fiber membranes, which is the preferred configuration for incorporating composite materials.

"We are now committed to using fibers to strengthen ceramic matrix composites, for hypersonic and high-temperature space applications, and for equipment windows that require optical transparency," Wardell said. "You can use these very strong nanotubes to strengthen transparent materials.

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