In its purest form, graphene is a one-atom-thick sheet of carbon atoms arranged in a dense, hexagonal lattice pattern. Although scientists theoretically knew about graphene for several years, it was difficult to produce. The University of Manchester researchers used adhesive tape to lift flakes from a piece of graphite, then continued to split those flakes with more tape until they reached a single-atom sheet.
Although manufacturing methods have progressed far beyond the adhesive tape level, producing monolayer graphene remains difficult and expensive. Researchers have found, however, that graphene doesn’t have to be in this pure form to be used effectively.
“Few-layer graphene (FLG), typically less than 10 layers, still displays many of the unique properties of graphene. It is a more cost-effective solution for many applications, including composites,” says Lisa Scullion, application manager at the University of Manchester’s Graphene Engineering Innovation Centre (GEIC).
“The common assumption is that graphene material that has fewer layers is superior quality, but it really does depend on what the particular application is,” says Barkan. “You could have a 20-carbon-layer-count material that still imparts the mechanical or thermal conductivity benefits you might want, but it also might be quite a bit less expensive than trying to find a single or two-layer material.” Composites manufacturers often use multi-layer graphene (MLG), which consists of 11 to 20 carbon atom layers.
Graphene today is produced in many ways. Chemical vapor deposition (CVD) yields the purest form, which consists of one to two layers of carbon atoms. A 25-micron, thin foil copper sheet in a vacuum vessel, heated up to 1000 C, is bathed with a mixture of argon, helium and methane. Graphene sheets collect on the copper’s surface, and the copper is then digested by chemicals like hydrochloric acid, leaving the graphene.
CVD is a very expensive approach, producing graphene costing up to $500,000 a gram, and even continuous CVD lines can’t produce the necessary output for large-scale manufacturing. So manufacturers have developed bulk production methods to obtain FLG and MLG, using physical, mechanical, chemical and thermal forces to exfoliate the graphite feedstock.
In one technique, rock crushers and a ball mill break the graphite down into a fine powder, which is then put into a solvent like alcohol or methyl pyrrolidone and hit with ultrasonic vibrations to further break down the graphene layers. The longer the processing time, the fewer the layers of graphene. In powder or solution form, this graphene can sell from $100 to $500 a gram for pure graphene or FLG and for as little as $50 a gram for MLG.