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A breakthrough in carbon nanotube composite materials could lead to superior body armor.  (Source: Agile Nano)

[Click to enlarge] The addition of polymer, cross linked by heat treatment, resists lateral motion, strengthening the carbon nanotube yarn.  (Source: ACS Nano/Northwestern University/Horacio Espinosa)

[Click to enlarge] The new material stacks up favorably to kevlar and has plenty of room for even more improvement.  (Source: ACS Nano/Northwestern University/Horacio Espinosa)
Mixture could lead to futuristic body armor, parachutes, and more

The chief goal of industrial chemistry is to produce compounds with useful characteristics at as low a price as possible.  Researchers from Northwestern University and various other institutions, along with corporate partners, have certainly fulfilled the first objective.  They have created an exotic blend of polymer and the ubiquitous nanomaterial, carbon nanotubes, that is stronger than Kevlar.

The new material has extremely high specific strength and energy-to-failure ratios.  That means that it can absorb a lot of impact without tearing, which in turn means that the force of the impact will spread out rather than be concentrated in a single destructive point.  The yarn was produced from double-walled carbon nanotubes and cross-linkable organic polymers like polyvinylalcohol (PVA).

Interestingly, the yarn fibers themselves had strength characteristics slightly inferior to kevlar.  For the engineers out there, the maximal reported values for ductility was ~20%, ~100Jg
-1 for the energy to failure ratios, and ~1.4GPa for the specific strength.  But when the fiber bundles were woven together, forming a macro-fiber with specific strength of ~6 GPa and energy to failure ratios of ~500Jg-1.

These materials of the woven cloth compared favorably to the 3 GPa strength of kevlar and its approximately 30 Jg
-1 energy to failure ratio.  In other words, that's good enough to substantiate the University's claim that the material "could be tougher than Kevlar", at least to some extent.

Heat treatment is one key to increasing the energy to failure ratio, likely because it activates the polymer cross-linking.

Engineering professor Horacio Espinosa led the study.  He states, "We want to create new-generation fibers that exhibit both superior strength and toughness.  A big issue in engineering fibers is that they are either strong or ductile — we want a fiber that is both. The fibers we fabricated show very high ductility and a very high toughness. They can absorb and dissipate large amounts of energy before failure. We also observed that the strength of the material stays very, very high, which has not been shown before. These fibers can be used for a wide variety of defense and aerospace applications."

The research was significant as past strength tests focused on pure-nanotube mixtures, which had the tendency for the tubes to slip laterally when stressed, weakening the resulting material.  The new composite material is much stronger as the polymers fix the nanotubes in place.

The research was funded by a $7.5M USD grant from the Army Research Office to investigate new materials for next generation bulletproof vests, parachutes, or composite materials used in vehicles, airplanes and satellites.  The project is part of the Department of Defense’s Multidisciplinary University Research Initiative (MURI) program, which promotes defense-minded projects that require a variety of engineering fields to collaborate.

The study on the intriguing new material is titled "A Multiscale Study of High Performance Double-Walled Nanotube−Polymer Fibers" and is published in the journal 
ACS Nano.

While future work will focus in part on refining the material strength even further, the crucial next step will be refining methods to mass produce the fibers.  A great deal of work has already been put into mass producing carbon nanotubes, so commercial super-strong carbon-polymer-based body armor may not be as far off as one might think.

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Old folks
By RugMuch on 12/6/2010 11:59:37 AM , Rating: 0
My favorite conversation with experts is the carbon nano tube one...

When talking robotics I ask is anyone using CNTube and I always get the same disgruntled reply. Then I hear a side conversation of (everyone thinks thats the solution to everything)

Well, when it cheaper lighter and out preforms in just about every manner, i have to just grrr at these oldies.

In 0.5 secs choose a car frame of steel or CNT. Isn't that what a car commercial did a few years ago?

I know it's starting to mass produce but giddy up

RE: Old folks
By MozeeToby on 12/6/2010 1:13:09 PM , Rating: 2
Well, when it cheaper lighter and out preforms in just about every manner
In theory maybe. In practice, the factories need to be retooled, workers need to be retrained, supply lines need to be completely reworked and all that assumes that the technology even exists to make the nanotubes of the right length and quantity for your application. Look at how many problems Boeing had on the 787, and they were just moving to the relatively old technology of carbon fiber construction, let alone trying to mass produce nanotubes at an industrial level.

RE: Old folks
By MrTeal on 12/6/2010 1:50:16 PM , Rating: 2
In 0.5 secs choose a car frame of steel or CNT. Isn't that what a car commercial did a few years ago?

And you're going to love replacing the car when the frame shatters once some dumb kid texting on their cell phone rear ends you.

RE: Old folks
By Solandri on 12/6/2010 1:53:23 PM , Rating: 5
From a structural engineering standpoint, the main drawback of fiber composites like carbon nanotubes or graphite are its low Young's Modulus. That is, under the same stress, they will deform a lot more than other materials like metal. So replacing the metal in your designs with fiber composites frequently turns your design from being strength-limited to being deflection-limited. You then have to (1) redesign your parts to also stay within the deflection tolerances rather than only to withstand the expected stresses, and (2) you have to add more material to accomplish (1). Granted, with carbon fiber you frequently end up having less weight of material for the same strength and deflection, but frequently the weight savings ends up being a lot less than you'd expect looking at just strength-to-weight ratios. Frequently, it's just not worth the trouble of the extra work and cost. Planes and and performance race cars are highly weight-sensitive so it's worth it there. Skis and sports racquets are easy to design so it's not much extra work there (flat sheets and rolled tubes are the easiest designs). But in general applications, from a design-to-production standpoint, it's not always a given that carbon fibers are a better material.

Also, the fibers are highly isotropic (stronger in one direction than another). The math for dealing with them is not that difficult, but involves a ton of matrix math so almost always has to be done on a computer. With an anisotropic material like metal, the math is straightforward and can be done by hand on the back of an envelope. For most people, that gives them a better "feel" for the characteristics of the anisotropic materials, freeing them to be more creative with their designs.

RE: Old folks
By drank12quartsstrohsbeer on 12/6/2010 3:31:15 PM , Rating: 2
Joining individual composite parts is much more difficult than metal. You cannot just drill a hole or cut threads into a composite part.

So you either have to make the entire assembly as one part, or do a secondary bonding of the individual parts, or bond on metal connections. All of which are difficult and expensive.

RE: Old folks
By JediJeb on 12/6/2010 3:40:46 PM , Rating: 3
Then if it also costs 5x as much or more try putting that into a common consumer automobile and trying to sell it. Anyone want to pay $100k for a Nissan Sentra? These materials have their place, but we can't just go replacing everything with them yet, and for some time to come I imagine.

RE: Old folks
By Howard on 12/6/2010 7:52:49 PM , Rating: 1
I've never seen the word "frequently" used so much in one paragraph.

RE: Old folks
By Fenixgoon on 12/7/2010 10:15:39 PM , Rating: 2
You have isotropic and anisotropic backwards (isotropic = same in all directions), but otherwise spot on. Tons of applications are deflection (modulus) limited rather than strength limited.

Also correct about it being easier to design with isotropic materials since it only takes a handful of values to define operational parameters, but highly anisotropic materials can be tricky. If only tungsten weren't so dense, we could use it for everything since it's almost perfectly isotropic ;)

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