Researchers at MIT have found a way to make composite materials using large area graphene films, in which large numbers of layers are stacked in an orderly manner, without having to stack each layer individually. This could enable creating composite materials containing hundreds of layers and open the door to various possibilities for designing new, easy-to-manufacture composites for optical devices, electronic systems, and more.
A major obstacle in creating graphene-based composites has been that graphene sheets and particles have a strong tendency to adhere together, so just stirring them into a batch of liquid resin before it sets is inefficient. The new technique could go a long way in solving this - while the process is more complex than it sounds, at the heart of it is a technique similar to that used to make puff pastry common in many desserts. A layer of material â dough, or graphene, in this case â 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.
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 â less than 1/10 of a percent by weight â it led to a clear-cut improvement in overall strength.
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 extremely smooth, strands slip past each other instead of sticking together in a bundle. 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, according to the team, 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.
