In 2004, working at the University of Manchester in the United Kingdom, researchers Andre Geim and Kostya Novoselov produced graphene, the world’s first two-dimensional, man-made material. In the following years, graphene was hailed as a wonder material because of its many remarkable properties. Despite being extremely lightweight and a million times thinner than a human hair, graphene is the world’s strongest material, with 200 times the tensile strength of steel. It has high electrical conductivity and thermal conductivity, is practically transparent, is impermeable and extremely flexible and stretchable. What makes graphene truly unique, however, is its ability to provide all these properties at once.
“With most materials, if you want to get one beneficial property you have to introduce some negative aspects, but with graphene you can impart multiple properties simultaneously and without the typical tradeoffs in many, many cases,” says Terrance Barkan, executive director of The Graphene Council.
Like many other high-tech discoveries, graphene didn’t initially live up to the hype. Early attempts to capitalize on its properties were unsuccessful, and many companies became dubious about its ability to deliver what was promised. There were several reasons for these failures.
“In many cases, the companies that thought they were working with graphene were actually working with material that may have had carbon in it, but wasn’t really graphene,” says Barkan. Manufacturers also found it very difficult to disperse graphene throughout a matrix to get the desired properties.
Early adopters were also confused by graphene’s unusual behavior. Composite material manufacturers, for example, are accustomed to adding more of a substance to a composite when they want to increase the properties it imparts to a material. But graphene has the reverse effect; reducing the quantity used generally improves the graphene-enhanced composites’ performance. Companies that initially added 1% graphene by weight to their composite materials achieved better results when they decreased the amount to 0.5% by weight.
“The magic of this material is that an incredibly small amount of it can dramatically improve the performance of other materials,” Barkan says.
With continued research and experimentation, companies have gained a better understanding of graphene’s behavior and are finally realizing its benefits. The nanomaterial is proving to be a valuable asset for the composites industry since it can be used for thermoset and thermoplastic composites, incorporated into glass, carbon and basalt fibers, and used with a variety of resins.
“The Graphene Council has identified more than 45 different vertical markets for graphene; composites are clearly the leading application market, and coatings is not far behind,” Barkan says.
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.
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.”