Some researchers are using filters to remove the graphene from biochar, a residue of biofuel production. Others are experimenting with a detonation method, introducing a hydrocarbon gas and oxygen into a chamber and detonating it using a spark plug. The gases burn off, leaving behind a graphene-containing carbon residue. Barkan says there are many other methods of graphene manufacture that are currently being explored and/or moved into commercial-scale production.
Graphene comes in a variety of forms, including sheets, nanoplatelets, flake powder and dissolved in solutions. Each form has different properties and characteristics, including lateral size, that make it appropriate for specific applications.
Graphene can also be functionalized with other materials to impart certain properties. For example, graphene functionalized with boron nitride makes a better insulator than a molecular conductor.
“The main one that people use is graphene oxide; you can make that in a batch process, putting graphene in with specific chemicals and providing some agitation. You end up with few-layer graphene oxide flakes,” says Mark Dickie, application manager for composites at GEIC. Because graphene oxide disperses more easily than some other forms of graphene and is compatible with many organic systems and polymers, it is being used in many applications.
The goal is to tune the properties of the graphene to the matrix of the materials to achieve the desired results, Dickie adds.
Opportunities and Obstacles
One of the biggest hurdles manufacturers face when adding graphene to composites is overcoming its tendency to agglomerate.
“Where people fail is in not using the correct equipment to disperse it properly,” says Dickie. “They think they can just whisk it in there, but since it sticks to itself you need to use something like a high shear mixer so that it separates the agglomerates.” The rate of addition is also important; putting the graphene in slowly and mixing it well works best.
Another approach is using graphene incorporated into liquid resins. “In many cases it makes a lot of sense to disperse your graphene material in a monomer and then introduce that into a polymer,” says Barkan. “You can do high shear mixing. You can do melt mixing. You just want to make sure the process is controlled enough that you get adequate dispersion.”
Functionalizing graphene to alter its surface chemistry and surface energy may assist in dispersion, but there are drawbacks. “The problem is that if you don’t think it through very carefully, in the process of altering [graphene] to get good dispersion you may actually degrade or destroy the very properties that you are putting it in for,” says Steven Rodgers, technology consultant and principal, EmergenTek, and a board member of the National Graphene Association. If you’re incorporating graphene for its electrical properties, for example, introducing certain chemicals could tie up a graphene sheet’s receptors, reducing the ability of electrons to transfer between sheets.