The work is described this week in a
paper in Science by Michael Strano, the Carbon P. Dubbs Professor in
Chemical Engineering; postdoc Pingwei Liu; and 11 other MIT students, postdocs,
and professors.
Materials such as graphene, a
two-dimensional form of pure carbon, and carbon nanotubes, tiny cylinders that
are essentially rolled-up graphene, are "some of the strongest, hardest materials
we have available," says Strano, because their atoms are held together
entirely by carbon-carbon bonds, which are "the strongest nature gives
us" for chemical bonds to work with. So, researchers have been searching
for ways of using these nanomaterials to add great strength to composite
materials, much the way steel bars are used to reinforce concrete.
The biggest obstacle has been finding
ways to embed these materials within a matrix of another material in an orderly
way. These tiny sheets and tubes have a strong tendency to clump together, so
just stirring them into a batch of liquid resin before it sets doesn't work at
all. The MIT team's insight was in finding a way to create large numbers of
layers, stacked in a perfectly orderly way, without having to stack each layer
individually.
Although the process is more complex
than it sounds, at the heart of it is a technique similar to that used to make
ultrastrong steel sword blades, as well as the puff pastry that's in baklava
and napoleons. A layer of material—be it steel, dough, or graphene—is spread
out flat. Then, the material is 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.
Now, it doesn't work out exactly that
way on the nanoscale. 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 their 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 1/10 of a percent 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, somewhat like a cheese
slicer, 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 precise 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.
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