Header image credit: Walter Myers
Recent Nanotech Discovery may lead to Building a Better Spaceship
Carbon nanotubes are legendary in their strength ...
By some estimates, when carbon nanotubes are mixed with lightweight plastics and epoxies -- lightweight polymers, in other words -- they are more than 30 times stronger that Kevlar.
In a way, the nanotubes act like steel rebar in reinforced concrete. The nanotubes strenghten and harden the polymers, reinforcing them, and making them incredibly strong.
But now a team of scientists at Binghamton University's Thomas J. Watson School of Engineering have found nanotubes that are even stronger than carbon. They are called ...
Boron Nitride Nanotubes (BNNTs).
Bind thousands of these tubes together and they are still thinner than a single strand of human hair, but they may lead to stronger cars and planes, golf clubs and tennis rackets.
They could even lead to building a better spaceship.
As Changhong Ke, lead researcher and an associate professor in the mechanical engineering department at the State University of New York at Binghamton, said:
We think that this could be the first step in a process that changes the way we design and make materials that affect the future of travel on this planet and exploration of other worlds beyond our own. Those materials may be way off still, but someone needed to take the first step, and we have.
The Advantages of Boron Nitride Nanotubes
Boron nitride, like carbon, can form single-atom-thick sheets that are rolled into cylinders to create nanotubes. By themselves boron nitride nanotubes are almost as strong as carbon nanotubes, but their real advantage in a composite material comes from the way they stick strongly to the polymer.
As Ke noted:
The weakest link in these nanocomposites is the interface between the polymer and the nanotubes,
If you break the composite, the nanotubes left sticking out have clean surfaces, as opposed to having chunks of polymer still stuck to them. The clean break indicates that the connection between the tubes and the polymer fails, according to Ke.
Ke and his colleagues devised a novel way to test the strength of the nanotube-polymer link.
They sandwiched boron nitride nanotubes between two thin layers of polymer, with some of the nanotubes left sticking out. Selecting only those tubes that were sticking straight out of the polymer, they then welded the nanotube to the tip of a tiny cantilever beam.
The team then applied a force on the beam, gradually increasing it, until the nanotube broke free of the polymer.
At first, as they increased the nanotube length, the researchers found that the force required to pluck out a nanotube also increased -- but then it reached a peak and plateaued. According to Ke, that behavior is a sign that the connection between the nanotube and the polymer is failing through a crack that forms and then spreads.
In their tests, the researchers only tested two forms of polymer: epoxy and poly(methyl methacrylate), or PMMA, which is the same material used for Plexiglas, and found that the epoxy polymers outperformed the PMMA polymers.
But, more importantly, they also found that both polymer-boron nitride nanotube binding strengths were higher than those reported for carbon nanotubes -- 35 percent higher for the PMMA interface and approximately 20 percent higher for the epoxy interface.
And a stronger interface means that a larger load can be transferred from the polymer to nanotubes, a critical characteristic for building a better spaceship.
As Ke explained, Boron nitride nanotubes likely bind more strongly to polymers because of the way the electrons are arranged in the molecules. I
n carbon nanotubes, all carbon atoms have equal charges in their nucleus, so the atoms share electrons equally.
In boron nitride, the nitrogen atom has more protons than the boron atom, so it hogs more of the electrons in the bond. That unequal charge distribution leads to a stronger attraction between the boron nitride and the polymer molecules, as verified by molecular dynamics simulations performed by Ke’s colleagues in Dr. Xianqiao Wang’s group at the University of Georgia.
Boron nitride nanotubes also have additional advantages over carbon nanotubes, according to Ke.
- They are more stable at high temperatures,
- They can better absorb neutron radiation (both advantageous properties in the extreme environment of outer space), and
- They are piezoelectric (which means they can generate an electric charge when stretched which means they can generate energy as well as act as sensors)
The main drawback to boron nitride nanotubes, however, is the cost.
Currently they sell for about $1,000 per gram, compared to the $10-20 per gram for carbon nanotubes, Ke said. He is optimistic that the price will come down, though, noting that carbon nanotubes were similarly expensive when they were first developed.
As Ke noted:
I think boron nitride nanotubes are the future for making polymer composites for the aerospace industry.
The work was funded by the US Air Force Office of Scientific Research - Low Density Materials program, with materials provided by NASA. Co-authors Xianqiao Wang and graduate student Liuyang Zhang from the University of Georgia provided verification and explanation data through computational simulations after the experiments were conducted in Binghamton.
Catharine Fay from the NASA Langley Research Center and Cheol Park of the Center and the University of Virginia are co-authors on the paper.
The paper, "Mechanical Strength of Boron Nitride Nanotube-Polymer Interfaces," was published in the latest issue of Applied Physics Letters.