The Massachusetts Institute of Technology (MIT, Cambridge, Massachusetts) reports that it has found a way to create composite materials containing hundreds of layers of carbon nanotubes that are just atoms thick but span the full width of the material.
The work is described in a paper in Science ("Layered and scrolled nanocomposites with aligned semi-infinite graphene inclusions at the platelet limit"), by Michael Strano, the Carbon P. Dubbs Professor in Chemical Engineering; postdoc Pingwei Liu; and 11 other MIT students, postdocs, and professors.
Nanomaterials such as graphene and carbon nanotubes are held together entirely by carbon-carbon bonds, which are the strongest in nature. The biggest obstacle in the application of nanomaterials has been finding ways to embed these materials within a resin matrix without the agglomeration that normally occurs. The MIT team's insight was in finding a way to create large numbers of layers of nanomaterials, stacked in an orderly way, without having to stack each layer individually.
At the heart of it is a technique similar to that used to make steel sword blades: A layer of material is spread out flat, then doubled over on itself, pounded or rolled out, and then doubled over again, and again, and again. With each fold, the number of layers doubles, thus producing an exponential increase in the layering. Just 20 simple folds would produce more than a million perfectly aligned layers.
In this research, rather than folding the material, the team cut the whole block — itself consisting of alternating layers of graphene and the composite material — into quarters, and then slid one quarter on top of another, quadrupling the number of layers, and then repeating the process. But the result was the same: a uniform stack of layers, quickly produced and already embedded in the matrix material, in this case polycarbonate, to form a composite.
In proof-of-concept tests, the MIT team produced composites with up to 320 layers of graphene embedded in them. They were able to demonstrate that even though the total amount of the graphene added to the material was minuscule — less than 0.1%1/10 by weight — it led to a clear-cut improvement in overall strength.
"The graphene has an effectively infinite aspect ratio," Strano says, since it is infinitesimally thin yet can span sizes large enough to be seen and handled. "It can span two dimensions of the material," even though it is only nanometers thick. Graphene and a handful of other known 2-D materials are "the only known materials that can do that," he says.
The team also found a way to make structured fibers from graphene, potentially enabling the creation of yarns and fabrics with embedded electronic functions, as well as yet another class of composites. The method uses a shearing mechanism to peel off layers of graphene in a way that causes them to roll up into a scroll-like shape, technically known as an Archimedean spiral.
That could overcome one of the biggest drawbacks of graphene and nanotubes, in terms of their ability to be woven into long fibers: their extreme slipperiness. Because they are so perfectly smooth, strands slip past each other instead of sticking together in a bundle. And the new scrolled strands not only overcome that problem, they are also extremely stretchy, unlike other super-strong materials such as Kevlar. That means they might lend themselves to being woven into protective materials that could "give" without breaking.
One unexpected feature of the new layered composites, Strano says, is that the graphene layers, which are extremely electrically conductive, maintain their continuity all the way across their composite sample without any short-circuiting to the adjacent layers. So, for example, simply inserting an electrical probe into the stack to a certain depth would make it possible to uniquely "address" any one of the hundreds of layers. This could ultimately lead to new kinds of complex multilayered electronics, he says.
The research was supported by the U.S. Army Research Office through the Institute for Soldier Nanotechnologies at MIT.
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