By Will Soutter
Graphene's unique properties have made it the focus of a great deal of research and media coverage in the last few years. A huge number of potential commercial applications have been proposed, for graphene itself and for various derivative materials like graphene oxides.
The majority of these applications, however, rely on the production of synthetic graphene in extremely high quality on an industrial scale. Some more advanced applications in nanoelectronics also require construction of intricate nanoscale structures like graphene nanoribbons.
Whilst graphene production of this quality is possible, it has yet to be scaled up to the throughput necessary to meet the demands of mainstream applications, and it will continue to be an expensive option for the near future.
Graphene has superb materials properties across the board - its mechanical strength, thermal conductivity and electron transport properties are unprecedented. Many in the materials industry are keen to harness the potential of this "wonder material", without waiting for synthetic graphenes to become sufficiently affordable and scalable.
Using graphene in composite materials allows this - the presence of graphene enhances the conductivity and strength of bulk materials, and can make use of graphene produced by less expensive methods like graphite exfoliation. These non-synthetic graphenes are of insufficient quality to be used in most electronics applications, but they are perfect for use in nanocomposites.
Researchers have now developed tough, lightweight materials by adding a small amount of graphene to metals, polymers and ceramics. The composites typically conduct electricity and withstand heat far better than than the bulk materials alone.
Graphene nanoplatelets are much easier to create than flawless single-layer graphene sheets, and they can be combined with other materials to make composites which inherit some of the properties of graphene. Image credit: CMSC, Michigan State University.
Graphene Composites in Battery Electrodes
In 2011, researchers at Northwestern University, in partnership with Argonne National Laboratory developed a silicon-graphene nanocomposite material for lithium battery anodes. Graphite is the current standard material for this application, as it has a very high theoretical charge capacity. However, this large capacity is not practically accessible, as the lithium ions are physically prevented from penetrating deep enough into the graphitic sheets.
The Northwestern researchers added nanometer-sized defects into the graphene, to allow charge to flow more easily between layers, and added silicon nanoparticles in the 20-50 nm range between the sheets graphene - this resulted in up to a threefold increase in the energy capacity of the battery..
This technology was the subject of a licensing agreement between Argonne and CalBattery, a Los Angeles based startup, which aims to rapidly develop and commercialize the technology.
In 2012, German researchers assessed the performance of solution styrene butadiene rubber composites containing graphene nanoplatelets, and compared them with composites with carbon nanotubes and expanded graphite.
The graphene nanoplatelets were effective at improving the electrical properties of the rubber - the resistivity of the material began to decrease at 15% loading. However, multi-walled carbon nanotubes (MWCNTs) began to affect the resistivity at just 5% loading.
Similar trends were observed for mechanical properties testing. Whilst MWCNTs were more effective than graphene nanoplatelets at improving tensile strength due to their ability to form a network at a lower concentration, the nanoplatelets still performed well.
Whilst MWCNTs may perform better in rubber composites, it is likely that graphene nanoplatelets will be more cost effective to manufacture in high quantities.
Titanium is a major structural material for commercial, military and industrial applications, due to its corrosion resistance, high strength and light weight. However, its applications are restricted in many fields due to its low thermal conductivity.
Studies suggest that adding graphene to titanium would significantly improve its thermal properties. In November 2012, XG Sciences and Oak Ridge National Lab announced a joint development program for advanced titanium-graphene composites - combining XG Sciences' capabilities in mass production of graphene nanoplatelets with the low-temperature metal processing technologies developed at Oak Ridge.
In a more hi-tech field, researchers from the Yunnan University published work on the adsorption of hydrogen molecules on titanium-decorated graphene in 2010. Their research showed that the presence of titanium made the hydrogen molecules bind to the graphene much more easily - suggesting potential applications for this type of material as a high-capacity hydrogen storage medium.
In recent years, scientists have developed a number of composite materials using graphene with enhanced physical and thermal properties, very few with good electrical conductivity.
However, a new graphene-based polymer composite developed at the Chinese Academy of Sciences in Shenyang was found to be an excellent conductor of electricity. The material consists of a flexible, interconnected network of graphene embedded in a poly(dimethylsiloxane) matrix.
The graphene foam was produced by depositing carbon onto a nickel foam template. Poly(methylmethacrylate) was then deposited on top of the thin carbon layer to prevent deformation of the structure. Treatment with a strong acid removed the nickel substrate, and the graphene foam was impregnated with poly(dimethylsiloxane) to create a light, flexible and highly conductive graphene-polymer composite.
In 2012, Basheer from the King Fahd University of Petroleum and Minerals developed a titanium dioxide-graphene (TiO2-G) composite for use in the photodegradation of dangerous organic compounds in wastewater samples. He prepared TiO2-G composites using calcination and sonochemical methods.
It was found that the synthesized TiO2-G composites had enhanced photocatalytic efficiencies when compared to pristine TiO2. Graphene provides a large surface area support for the TiO2 photocatalyst and stabilizes charge separation by capturing electrons transferred from the TiO2, thus hindering charge transfer and enhancing photocatalytic efficiency.
During the last few decades, a number of novel composite materials have been produced using graphene, which is stronger than any known material. Most of the properties of these graphene composites were better than the base polymer matrix, and other carbon filler-based composites.
These improved properties of the graphene composites were obtained at very low graphene contents, which make them much easier to commercialize than some other, more demanding applications.
The potential applications of graphene composites include sports equipment, medical implants, and engineering materials in fields like aerospace and renewables. The discovery of graphene has opened a new dimension for developing light weight, inexpensive and high-performance composite materials for a wide range of applications.