Los Angeles [US], January 21 : Rice University researchers have developed a new method for producing high-purity boron nitride nanotubes. These hollow cylindrical structures can withstand temperatures up to 900 degrees Celsius (~1652 Fahrenheit) and are stronger than steel by weight.
Strong, lightweight materials capable of withstanding extremely high temperatures might open the way for next-generation spacecraft, improve existing electronics, and enable the development of unique biomedical imaging or hydrogen storage applications, among other things.
According to a report published in Chemistry of Materials, Rice researchers, led by Angel Marti, discovered how to use phosphoric acid to remove tough impurities from boron nitride nanotubes and fine-tune the procedure.
"The challenge is that during the synthesis of the material, in addition to tubes, we end up with a lot of extra stuff," said Kevin Shumard, a chemistry doctoral student and lead author of the study. "As scientists, we want to work with the purest material we can so that we can limit variables as we experiment. This work gets us one step closer to making materials with the potential to revamp whole industries when used as additives to metals or ceramic composites to make those even stronger."
The "extra stuff" that usually mars the quality and usefulness of the nanotubes are boron nitride cages, hollow sphere-shaped structures that encapsulate boron particles. A paper that showed phosphoric acid acted as a boron nitride wetting agent inspired the researchers to explore whether they could use the acid to remove the cages.
"We didn't expect a reaction," said Marti, professor of chemistry, bioengineering, materials science and nanoengineering, chair of chemistry and faculty director of the Rice Emerging Scholars Programme.
And, indeed, at room temperature, nothing happened. But when they heated things up, the researchers got a surprise.
"When we looked through the microscope, we saw no tubes and no cages," Marti said. "Instead, there were pyramids."
The researchers learned that the high temperatures and acid concentrations were destructive for the boron nitride, so they revised their hypothesis and instead aimed to tune the reaction to destroy only unwanted structures in the material.
"Through a lot of experimentation, we developed a completely new direction for the purification of nanotubes," Shumard said.
"I have spent a lot of time in front of an electron microscope and have read a lot of papers with images of boron nitride nanotubes. The material that we can make is by far the purest tube that I have seen when compared to others."
The researchers plan to continue their efforts to improve reaction yields so as to produce enough nanotubes to make fibres, which could be a suitable and more sustainable alternative to steel.
"Nitrogen makes up 70 per cent of our atmosphere, and boron is highly abundant in rocks," Shumard said. "This work could be a stepping stone to much better building materials, both in terms of strength and sustainability."
The structure of boron nitride nanotubes is very similar to that of carbon nanotubes, and so are some of their properties, such as tensile strength and thermal conductivity. However, boron nitride nanotubes are more resilient, and some of their properties are complementary to their carbon counterparts.
"For example, carbon nanotubes can be electrical conductors or semiconductors, while boron nitride nanotubes are insulators," Marti said.
"The science of boron nitride nanotubes is not as well developed as the science of carbon nanotubes, a gap we were hoping to address in our research because we think the ability to produce pure boron nitride nanotubes efficiently and reliably could be important for a wide range of industries."
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