CHEMICAL engineers at MIT, US have created a composite material containing hundreds of graphene layers that are a few atoms thick but span the full width of the material using a method which stacks the layers efficiently.
MIT say the discovery could “open up wide-ranging possibilities” for designing new, easy-to-manufacture composites for optical devices, electronic systems, and protective materials.
Graphene is a two-dimensional form of pure carbon with strong bonds within its structure. Traditional production methods lack cohesion or uniform structuring due to relatively weaker interactions between layers stacked on top of one another. Carbon nanotubes – the cylindrical form – suffer from the same problem when stacked as multiple layers. Both the sheet and tube forms have a tendency to clump together, so just stirring them into a batch of liquid resin before it sets does not work.
The research team has adapted a technique used by metalsmiths to make sword blades, and pastry chefs to make puff pastry. Material is spread out flat, and then the material is doubled over on itself. It is then pounded or rolled out to shape, then doubled over again, and the technique is repeated again and again.
With each fold, the number of layers doubles, producing an exponential increase in the layering. The team say 20 folds would produce more than a million perfectly-aligned layers.
“Folding” on the nanoscale does not mean the same as the traditional sense. The team cut a block of material – containing alternating layers of graphene and the composite (polycarbonate) material – into quarters, and then slid one quarter on top of another, quadrupling the number of layers, and then repeating the process. The team say the result was the same – achieving a uniform stack of graphene layers that was produced quickly and already embedded in the matrix material.
In the proof-of-concept tests the team produced composites with up to 320 layers of graphene embedded in them. Even though the total amount of graphene added to the material was less than 0.14% by weight — it led to a storage modulus increase of 0.32—0.38GPa.
The team also found a method to make structured fibres from graphene, potentially enabling the creation of yarns and fabrics with embedded electronic functions, and another type of composites. The method uses a shearing mechanism, like a cheese slicer, to peel off layers of graphene in a way that causes them to roll up into a scroll-like shape.
That could overcome the slippery drawback of graphene and nanotubes, in terms of their ability to be woven into long fibres. Because they are perfectly smooth, strands slip past each other instead of sticking together in a bundle. The team says the new scrolled strands not only overcome that problem, but they are also stretchy, unlike other strong materials such as Kevlar. That means they might lend themselves to being woven into protective materials that could ‘give’ without breaking.
Michael Strano, professor of chemical engineering at MIT, said one unexpected feature of the new layered composites, is that the graphene layers, which are very electrically conductive, maintain their continuity all the way across the composite sample without any short-circuiting to the adjacent layers. 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.
Strano says that this could ultimately lead to new kinds of complex multilayered electronics, and that the research will continue to explore the incorporation of the composite materials into commercially-viable products.
Science, DOI: 10.1126/science.aaf4362
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