Chemists report a new process for making carbon fibers from graphene oxide that promises to be a scalable, organic-solvent-free route to new kinds of strong, lightweight materials (ACS Nano 2014, DOI: 10.1021/nn501098d). Unlike conventional carbon fibers, these graphene oxide fibers can be knotted and knitted, with potential applications in energy-storing textiles, novel optical materials, and wearable electronics.
Conventional carbon fiber is made by spinning a polymer solution, typically polyacrylonitrile, into thin fibers. The fibers are oxidized, then heated to high temperatures in the absence of oxygen to carbonize the materials. Manufacturers can control the resulting fibers’ strength, ductility, and thermal conductivity based on the polymer precursor and the carbonization method used. The resulting fibers often end up in lightweight, strong, structural composites in which the fibers are held together by polymer glue, in a similar fashion to paper-mâché.
Some materials scientists want to make carbon fibers out of nanomaterials, such as graphene and carbon nanotubes, to take advantage of the materials’ strength, as well as their high electrical and thermal conductivity. Unlike conventional carbon fibers, these nanomaterial fibers are flexible enough to knot without breaking. It might be possible to fashion them into knotted or knitted textiles, without the need for the gluey polymers used to make carbon-fiber composites, says Mauricio Terrones of Pennsylvania State University.
Terrones and his team set out to make fibers from graphene oxide because it can be used on a larger scale than graphene. First, they coat a large surface with an aqueous solution of graphene oxide and let the water evaporate, leaving a dried sheet of the nanomaterial. Then by taping down one end of the sheet and attaching an electric screwdriver to the other end, they can spin the sheet into a yarn. This method, Terrones admits, is a primitive proof of concept and could be improved upon. He adds that the graphene oxide fibers can be reduced to graphene after spinning.
Graphene oxide yarns made with this dry-spinning method are tough, withstanding forces as strong as 17 J/g, which are in the range of what conventional carbon fiber can handle. They’re also stretchy, elongating 76% before fracturing. Those with the best properties were about 300 μm in diameter and made from graphene oxide films about 5 μm thick. The yarns can be coiled and knotted. The researchers could produce composite fibers by coating the films with another material, such as carbon nanotubes to improve strength or silver nanowires for electrical conductivity, before spinning it into a yarn.
The properties of these fibers are useful but not exceptional—probably because the production process hasn’t yet been optimized, says Ray H. Baughman, a materials scientist at the University of Texas, Dallas, who is working on carbon nanotube fibers. “But it’s clearly a scalable process, and that’s exciting,” he says. “I’m confident they can improve the mechanical properties.”