Heat-Resistant, Stable Boron Nitride Nanotube Fibers Developed

Heat-Resistant, Stable Boron Nitride Nanotube Fibers A team from Rice University led by Professors Matteo Pasquali and Angel Martí has simplified the process of making nanotubes—which are highly valuable for large-scale applications including aerospace, electronics and energy-efficient materials—more practical. Researchers reported in Nature Communications that boron nitride nanotubes, also known as BNNTs, self-assemble into liquid crystals under the right conditions, specifically at concentrations exceeding 170 parts per million by weight in chlorosulfonic acid. These liquid crystals consist of aligned BNNTs, which are much easier to process than mixed nanotubes typically formed in solution. The laboratory began producing fibers and films from the liquid crystal solutions. Martí, who designed the solutions and helped characterize fibers produced in Pasquali's laboratory, said "BNNT fibers are attractive for manufacturing various products with applications ranging from wearable devices to aerospace vehicles." Boron nitride nanotubes resemble carbon nanotubes, but their hexagonal lattices contain alternating boron and nitrogen atoms instead of carbon. Both types of nanotubes are strong, but unlike electrically conductive carbon nanotubes, BNNTs are good electrical insulators and remain thermally and chemically stable in air up to 900 degrees Celsius (1,652 degrees Fahrenheit). To form liquid crystals, researchers had to ensure their nanotubes contained no impurities. Unfortunately, these impurities were mostly boron nitride particles that threatened to clog the process. "Early BNNT samples contained numerous non-nanotube boron nitride structures," said Cedric Ginestra, a graduate student and lead author, adding "They were either chemically bonded to the BNNTs or simply stuck physically in a way that prevented the BNNTs from dispersing in the acid and aligning at higher concentrations. Separating these boron nitride allotropes from BNNTs and measuring their concentrations was difficult. All the different boron nitride types essentially looked the same with basically every quantitative technique we tried." Cedric Ginestra said that working with suppliers to optimize the BNNT purification process and using a purification process developed in the Pasquali laboratory helped obtain better BNNT batches for the formation of liquid crystal solutions. After obtaining suitable material, the Pasquali group was prepared to rapidly adapt wet-spinning techniques used for carbon nanotube fibers to produce the first boron nitride filaments through the process. "There are reports of others taking solid puffs of BNNTs and stretching and twisting them to make a thread, but that's quite different from our process," Ginestra said. "Our goal was to make a fiber with a very high degree of alignment because properties are better along the length of the nanotubes." Explaining that "liquid crystals are ideal precursors for fibers because the nanotubes within them are already aligned," Ginestra said BNNT alignment in the liquid crystals was characterized microscopically through birefringence, a phenomenon where the crystals split light in a prism-like manner even though they appear clear. Ginestra said the films also demonstrated how BNNT solution processing can adopt methods developed for carbon nanotubes. Such transparent thin films could be useful in next-generation electronics. "BNNT film and fiber properties will evolve as our understanding of the material and liquid crystal solution develops," he said. Martí noted that BNNT films could be useful as filters for ultraviolet light, toxic dyes and corrosion protection.   Source: More information: Cedric J. Simonsen Ginestra et al, Liquid crystals of neat boron nitride nanotubes and their assembly into ordered macroscopic materials, Nature Communications (2022). DOI:10.1038/s41467-022-30378-5 / Journal information: Nature Communications / Provided by Rice University / https://phys.org/news/2022-06-heat-tolerant-stable-boron-nitride-nanotube.html