Spinning Boron Nitride Nanotubes into Yarn
What do a clown’s makeup and a spinning wheel have in common? Boron nitride is the white material found in clown makeup and face powder. Old meets new as the ancient art of spinning is used to produce a very modern material: a boron nitride nanotube (BNNT) yarn. Researchers have long been able to make nanotubes out of carbon — super-tough carbon nanotube fibers are suitable for weaving into electronic cloth, are four times tougher than spider silk, and 17 times tougher than the Kevlar used in bullet-proof vests. Here’s a video that shows the spinning process for carbon nanotubes:
Creating such fibers from boron nitride has proved elusive. Carbon and boron nitride are about the same strength, but BNNTs can survive temperatures that are twice as high as carbon nanotubes can survive –- 800°C and higher. Up until now, researchers have only been able to create high-quality BNNTs a micron long. Larger versions have been perforated with defects in the crystalline structure.
These problems now appear to be largely resolved. In a recent paper published in Nanotechnology, A team of materials scientists at the NASA Langley Creativity and Innovation Program, the NASA Subsonic Fixed Wing program, DOE’s Jefferson Lab and the Commonwealth of Virginia, describe the ability to create high-quality, uniformly crystalline BNNTs in large quantities. "Other labs can make really good nanotubes that are short or really crummy ones that are long. We’ve developed a technique that makes really good ones that are really long," said Mike Smith, a staff scientist at NASA’s Langley Research Center.*
A cotton-like mass of nanotubes is finger-twisted into a yarn about one millimeter wide. "They’re big and fluffy, textile-like," said Kevin Jordan, a staff electrical engineer at Jefferson Lab. "This means that you can use commercial textile manufacturing and handling techniques to blend them into things like body armor and solar cells and other applications."
The “spinning” process involves a laser aimed at a cake of boron inside a chamber filled with nitrogen. This forms a plume of boron gas that shoots upward. A cooled metal wire is then inserted into the gas, causing the gas to cool and form liquid droplets. The droplets combine with the nitrogen to self-assemble into BNNTs. "It’s like fuel-air-spark in an engine," says NASA aerospace scientist Michael Smith. "The reaction advances violently, creating the superlong tubes in just milliseconds."
Why boron nitride rather than carbon? Building large amounts of inexpensive boron nitride nanotubes opens the door for lighter, faster car frames; affordable space vehicles and ultralightweight armor. Because of their excellent thermal and chemical stability, boron nitride ceramics are traditionally used as parts of high-temperature equipment. Boron nitride has great potential for nanotechnology applications –- BNNTs are more thermally and chemically stable than carbon nanotubes. And BNNTs can be produced with a structure similar to that of carbon nanotubes. However, their properties are very different –- carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius, whereas BNNT is an electrical insulator with a wide band gap of ~5.5 eV (the same as diamond). Chemical resistance is better for BNNTs, which are able to survive in air up to much higher temperatures. According to ScienceNOW, BNNTs also offer the potential for “pinpoint precision to attack cancer cells by sticking to tumors, absorbing neutrons from a targeted beam, and generating localized alpha radiation to kill the cancer.”
Building large amounts of inexpensive boron nitride nanotubes opens the door for lighter, faster car frames; affordable space vehicles and ultralightweight armor.
"This is the start of a revolution in materials," says Dennis Bushnell, a NASA engineer who has hopes of using BNNTs for space vehicles. "Just about everything can be made lighter, and hopefully, cheaper. You’re talking about energy savings all over the place."