A tangle of unprocessed boron nitride nanotubes seen through a scanning electron microscope. Image: Pasquali Research Group/Rice University.
A tangle of unprocessed boron nitride nanotubes seen through a scanning electron microscope. Image: Pasquali Research Group/Rice University.

According to researchers at Rice University, boron nitride nanotubes used to be hard to process. But not anymore.

A Rice team led by professors Matteo Pasquali and Angel Martí has simplified handling of the highly valuable nanotubes to make them more suitable for large-scale applications, including aerospace, electronics and energy-efficient materials.

In a paper in Nature Communications, the researchers report that boron nitride nanotubes (BNNTs) can assemble themselves into liquid crystals under the right conditions, primarily concentrations above 170 parts per million by weight in chlorosulfonic acid. These liquid crystals consist of aligned BNNTs that are far easier to process than the tangled nanotubes that usually form in solution. The lab proceeded to form fibers and films from the liquid crystalline solutions.

“BNNT fibers are attractive for the manufacture of a variety of products, with applications that range from wearables to aerospace vehicles,” said Martí, whose lab designed solutions and helped to characterize the fibers produced in Pasquali’s lab.

BNNTs are like carbon nanotubes, but their hexagonal lattices are made up of alternating boron and nitrogen atoms rather than carbon atoms. Both types of nanotubes are strong, but unlike electrically conductive carbon nanotubes, BNNTs are good electrical insulators, and are thermally and chemically stable in air up to 900°C (1652°F).

To form liquid crystals, the researchers needed to be sure their nanotubes were free of contaminants. Unfortunately, those contaminants were mostly bits of boron nitride that threatened to gum up the works.

“Early BNNT samples contained lots of non-nanotube boron nitride structures,” said graduate student and lead author Cedric Ginestra. “They were either chemically bound to the BNNTs or just physically adhered in a way that prevented BNNTs from dispersing in acid and aligning at higher concentrations.

“It is difficult to separate these boron nitride allotropes from BNNTs, and hard to even measure their concentration. All the different types of boron nitride appear identical by basically every quantitative technique that we’ve tried so far.”

To obtain better batches of BNNTs, the researchers took advantage of a purification process developed in the Pasquali lab and worked with their supplier to optimize this BNNT purification process for the formation of liquid crystalline solutions. Once suitable material was produced, the Pasquali group was primed to quickly adapt the wet-spinning techniques it had developed for carbon nanotube fibers to make the first boron nitride threads.

“There are reports of others taking solid puffs of BNNTs and stretching and twisting them to make a yarn, but that’s very different from our process,” Ginestra said. “Our goal was to make a very highly aligned fiber because the properties are better along the length of the nanotubes.”

Liquid crystals are the ideal precursor for fibers because the nanotubes within are already aligned. The researchers microscopically identified BNNT alignment in the liquid crystals by their birefringence, a phenomenon by which crystals split light, prism-like, even if they appear to be clear.

The films also demonstrate how BNNT solution processing can adopt methods developed for carbon nanotubes. Such transparent thin films could be useful in next-generation electronics. “The BNNT film and fiber properties will improve as the material and our understanding of the liquid crystalline solution improves,” Ginestra said.

Martí noted that BNNT films would be useful as filters for ultraviolet light, antifouling coatings and for corrosion protection.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.