Engineers create nanolayered composites

Adapting a trick used for centuries by both metalsmiths and pastry makers, a team of researchers at MIT has found a way to efficiently create composite materials containing hundreds of layers that are atoms thick but span the full width of the material. The discovery could open up wide-ranging possibilities for designing new, easy-to-manufacture composites for optical devices, electronic systems, and high-tech materials.

Researchers have been searching for ways of using nanomaterials, such as graphene and carbon nanotubes, to add strength to composite materials, in 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 stirring them into a batch of liquid resin before it sets doesn’t work. 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 or puff pastry. A layer of material is spread out flat, then doubled over on itself, rolled out, and then doubled over and over again.

With each fold, the number of layers doubles, producing an exponential increase in the layering. Just 20 folds would produce more than a million perfectly aligned layers.

In this research, rather than folding the material, the MIT team cut the block —consisting of alternating layers of graphene and the composite material — into quarters, and then slid one on top of another, quadrupling the number of layers, and then repeating the process. The result was the same: a uniform stack of layers, quickly produced, and already embedded in the matrix material 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 less than 1/10 of a percent by weight, it led to a clear-cut improvement in overall strength.

One unexpected feature of the new layered composites, the team said, is that the graphene layers, which are electrically conductive, maintain their continuity all the way across their composite sample without any short-circuiting to the adjacent layers. This could ultimately lead to new kinds of complex multilayered electronics.

Tom Austin-Morgan

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