When one thinks of the word “nanotube,” the other word that’s likely to come to mind is “carbon.” Even a Google search reveals this bias.
It’s easy to understand why. Since 1991, when carbon nanotubes emerged most prominently on the scientific scene, they have captured our collective imagination. These nano-sized cylinders of strictly organized carbon atoms are, gram for gram, more than 100 times stronger than steel and 30 times stronger than the Kevlar used in bulletproof vests. For this reason, carbon nanotubes have been hailed as the wonder material that can give us everything from better energy storage to super-strong tethers for space elevators.
But carbon isn’t the only nanotube game in town and, depending on the applications, not necessarily the best. Boron nitride nanotubes are beginning to step out of the carbon shadow, thanks in part to a new production process developed and demonstrated by scientists at the National Research Council of Canada (NRC). “Canada now has the largest boron nitride nanotube production capacity in the world,” the NRC disclosed in August, pointing out that it can make the fibrous, cotton candy-like material 100 times faster than earlier approaches. “Applications and product development previously limited by low production volumes can now be expected to grow rapidly.”
The National Research Council of Canada team that conducted the first pilot scale demonstration of boron nitride nanotubes. (L-R): Keun Su Kim, Mark Plunkett, Benoit Simard, Christopher Kingston, Jingwen Guan and Michael Jakubinek. Photo Credit: National Research Council of Canada
To understand why this matters, it’s important to compare and contrast the properties of boron nitride nanotubes (BNNTs) with those of carbon nanotubes (CNTs). The two share similar molecular structures and this gives them comparable thermal conductivity, strength and weight — all considered ideal for making super strong but exceptionally light parts for cars, airplanes and spaceships. Lower weight and increased strength leads to enhanced fuel efficiency and safety.
The tubular structure at the atomic level is what gives nanotubes, whether made of carbon or boron nitride, their incredible strength. “If you apply force outside of a two-dimensional sheet it has no strength,” says Chris Kingston, a materials scientist at the NRC. “But it gains stiffness when it is reinforced as a tube and you gain these amazing mechanical properties as a result of this shape,” Kingston says.
BNNTs can excel where CNTs can’t. For example, BNNTs are much better than CNTs at withstanding heat. “We’ve had the material in open flames as high as 2,000 C, while at that temperature carbon nanotubes would just burn away,” says Kingston. This feature could lead to a new generation of fire-retardant materials for building construction or product packaging. It’s also important for making aluminum, titanium and other metal composites, as well as lightweight ceramic composites, which require process temperatures well above 400 C, at which point carbon nanotubes begin to oxidize. Such strength-enhanced composites will be particularly ideal for making products — such as parts for jet engines — that can tolerate high heat exposure. “Wherever you want to have a fibrous composite at high temperatures, you go with boron nitrides,” says Roy Whitney, president and chief executive officer of BNNT, LLC, based in Newport News, Va. Like the NRC, Whitney’s company is working on boosting the speed at which BNNTs can be produced.
This past September, BNNT, LLC announced it had licensed a production method from the United States Department of Energy’s Lawrence Berkeley National Laboratory, an approach that will complement techniques the company jointly developed with the DOE’s Jefferson Lab, NASA Langley Research Center and the National Institute of Aerospace.
Whitney says that the high thermal stability of BNNTs holds promise for additive manufacturing — more popularly known as 3D printing — by which objects are printed layer by layer from the bottom up using instantly hardening “ink” consisting of laser-melted metal powders. BNNTs, as part of metal composites, would be able to withstand melting processes that CNTs could never survive. “To my knowledge no one has done this. People still need to figure this out, of course,” Whitney says. “I’m talking about things that are far down the road.”
The unique characteristics of BNNTs don’t end there. Kingston says that carbon nanotubes are highly conductive of electricity, whereas BNNTs are excellent insulators. This makes them ideal, for example, as an exceptionally lightweight insulator for wiring, including CNT wiring, which could dramatically reduce the weight of equipment used in aerospace applications.
Boron in BNNTs also makes the material highly effective at neutron and ultraviolet shielding. Researchers envision using BNNTs as coatings that protect solar cells from damaging ultraviolent light and as paper-like linings that shield airplane cockpits, satellites and space vehicles from radiation exposure. It’s this unique combination of properties that make BNNTs so sought after by certain industries, particularly aerospace and security: ultra light, super strong, resistant to high heat, thermally conductive, electrically insulating and impermeable to radiation. Many materials offer one or a few of these characteristics but they don’t offer them all.
Keun Su Kim of the National Research Council of Canada pulls boron nitride nanotubes (BNNT) from the plasma reactor. Photo Credit: National Research Council of Canada
Perhaps what most sets BNNTs apart from CNTs is the ability to make them clear. Unlike carbon, “they don’t absorb light in the visible spectrum,” says Kingston. “If you do the chemistry properly, you can envision having transparent materials.”
Transparent armour for vehicles and soldiers — for example, windows in a tank that can withstand bomb blasts, radiation, and fire — are among the first products the NRC expects industry to produce, based on interest from clients. But this could eventually be extended to civilian vehicles, building materials and medical products. And we’ve all heard talk of commercial airplanes with partly transparent fuselage. You can’t do that with carbon nanotubes but the potential is there with BNNTs.
The potential is also there to come up with highly efficient energy harvesting devices, as BNNTs have promising piezoelectric properties, meaning they can generate an electrical current when subjected to mechanical stress, either on their own or in a plastic composite. This could lead to a new class of self-powered sensors, motors and energy-generation devices that can operate in unconventional environments.
“Boron nitride nanotubes provide a versatile platform for additional novel physics phenomena and applications,” wrote Marvin Cohen and Alex Zettl in their 2010 overview of BNNTs that appeared in Physics Today. Both men are materials scientists at the Lawrence Berkeley National Laboratory. Cohen is credited for first theorizing in 1994 that BNNTs could be made, speculating that boron and nitrogen — carbon’s closest periodic table cousins — formed the same strong bonds with each other that exist with carbon nanotubes. Zettl, a year later, was first to synthesize it in a lab. “The material’s high surface area and ability to bond to hydrogen and other molecules could put it into the service of clean and efficient fuel storage,” the pair wrote. “Their ability to bond with other nanocrystals also suggests they may be used as harsh-environment catalysts, water purification systems, plasmonic devices, and photovoltaics.”
Australian researchers reported in 2009 that BNNTs were highly effective at desalting water when compared to existing membrane-based desalination systems or even carbon nanotubes. “Using boron nitride nanotubes, and the same operating pressure as current desalination methods, we can achieve 100 percent salt rejection for concentrations twice that of seawater with water flowing four times faster, which means a much faster and more efficient desalination process,” says Tamsyn Hilder, a computational biophysics scientist at the Australian National University.
This same property of BNNTs is also being considered for large-scale energy generation. It has long been known that the osmotic flow of water from a lower to higher salinity gradient can be harnessed to produce an electric current. In nature, this situation is presented at the mouths of rivers, where fresh water meets ocean water. A team led by physicists at the Institut Lumière Matière in Lyon and Institut Néel reported in a 2013 paper in Nature that osmotic flow through BNNTs produces electric currents with 1,000 times the efficiency of any previous system.
As if BNNTs weren’t impressive enough, they also appear helpful in the fight against all sorts of soft tissue cancers. Essentially, the nanotubes turbo-boost a treatment option called “irreversible electroporation,” which involves using short pulses of electricity to put holes in the walls of tumour cells to promote cell suicide, or apoptosis.
Life science researchers at Italy’s Sant’Anna School of Advanced Studies, working with NASA Langley Research Center, found that when they applied tiny strands of BNNTs to tumour cells there was a 2.2-fold reduction in cell survival during the electroporation process compared to cells not exposed to BNNTs.
So if BNNTs are such a great material, and if they were first developed only a few years after CNTs, why have they not developed at the same pace? Benoit Simard, group leader in the security and disruptive technologies division of the NRC, says that one of the same properties that makes BNNTs attractive — high heat resistance — also makes it difficult to synthesize. “In order to make boron nitride nanotubes, you need to have very high temperature and pressure, which is completely different from carbon nanotubes,” Simard says. “Because of this, it has taken a longer time to develop methods that can produce BNNTs in sufficient quantities.”
For example, Zettl first synthesized BNNTs using an arc-discharge and arc-jet plasma method. Since then, production methods for making high-quality BNNTs have measured output in the milligrams — hardly enough to drive a materials revolution. “For 15 years people could only make small amounts and could not get it to where it was viable or commercial,” says Whitney at BNNT, LLC, which since 2009 has been working on a new synthesis technique that can scale up production.
The method relies on a high-temperature vaporization and condensation process that takes place in a pressurized, nitrogen-filled chamber. Boron powder placed inside the chamber is hit with a laser to create boron gas, which is cooled to form droplets. Those droplets react with the nitrogen and magically self-assemble into boron nitride nanotubes that are long enough to spin into a yarn. Again, the white cotton-candy image comes to mind.
The NRC, however, contends that its own vaporization process takes production a big step forward. “We know BNNT, LLC very well,” says Simard. “Our production capability is much, much higher than theirs.”
It starts with hexagonal boron nitride powder, which looks microscopically like two-dimensional sheets of hexagonal-shaped molecular bonds similar to graphene powder, which is why it’s sometimes referred to as “white graphene.” Hexagonal boron nitride powder is available in industrial quantities and used most frequently as a dry lubricant. The powder is vaporized in a reactor that is equipped with a superhot plasma torch, supplied by Tekna Plasma Systems of Sherbrooke, Que. Hydrogen is used as a catalyst to boost the chemical reactions. “You put the powder in, let the reactor do its thing and the sheets naturally assemble into tubes as they follow their path through the reactor,” says Kingston, who works closely with Simard. The result: 20 grams of BNNT produced per hour. “The yields demonstrated by this plasma synthesis process mean that kilogram quantities of high purity, highly crystalline BNNT are now accessible for the first time,” Kingston, Simard and NRC colleagues wrote in a 2014 paper published in ACS Nano. BNNT fibres, fabrics, thin films and “buckypapers” can now be made in large quantities and tested in real-world applications, they wrote, calling the speedy throughput of their process a “seminal milestone” for the material.
Asked about the NRC’s rival accomplishment, Whitney was gracious in his reply. “It’s a great group over there. We were very excited,” he says. “The rising tide raises all ships and we think it’s a good thing at this time.” Whitney adds that he expects, 10 years from now, BNNTs will be as common as CNTs in a variety of consumer products, from computing devices to sports equipment. And if a space elevator is ever constructed, “it will be built with BNNTs and some carbon,” he says. “We think the materials will be quite complementary.”