Graphene: What does the future hold outside of the lab?

The 2010 Nobel Prize catapulted Graphene onto the world stage, and it's since captured the attention of the materials world. No doubt there are extraordinary opportunities.

Mechanically it's 200 times stronger than steel, its in-plane electrical and thermal conductivity is higher than copper, and it has a staggering surface area in excess of 2,500m² per gram. It's no surprise, the 'wonder' material has been the subject of intense research.

The range of potential applications is vast. For example, a single or double layer of carbon shows a range of quantum effects that may be exploited for novel sensors, energy harvesting, and could even be the basis for the next generation of computer chips, replacing silicon.

Commercial graphene
So is all this really achievable? Many question the commercial reality of the material, and its relevance to real, commercial products and applications.

In many cases, taking an idea from the laboratory to the industrial world is fraught with difficulty and fantastic material properties on a laboratory scale do not always translate into cost effective applications on a large scale.

However, there is long term effort being made to produce single film 'perfect' graphene sheets on an industrial scale. But, by capitalising on less perfect but equally effective graphene nanoplatelets (GNP), a route to short term commercialisation may have opened up.

A complicated commercial picture
There are many target markets for graphene that actually make commercialisation more challenging. Each target market requires different performance levels and cost targets. There are also many types of graphene, each with a different set of properties.

There is a real need to standardise the growing number of graphene variants by recognising the cost benefits of each family and establishing the applications for which they are most suitable.

Realising the potential
Graphene, and graphene oxide, show promise as reinforcements in high-performance nano-composites and could yield outstanding mechanical properties. The strong interface between the reinforcement and the polymer matrix yields excellent mechanical and conductive properties.

However, the homogenous dispersion of nanomaterials is a fundamental requirement in realising their full potential. Nanomaterials tend to agglomerate, which can hinder good dispersions in something like a polymer used to make a composite. In order to prevent this, it's necessary to employ a surface functionalisation treatment.

Chemical treatment of nanoparticles using acids can effectively functionalise the surface of graphene, but it is at the expense of safe and proper disposal costs, which can be considerable. And acid treatments can damage graphene, seriously degrading the properties of the final product.

These deleterious effects can be avoided with processes including plasma functionalisation. With the correct chemical functionalisation, there is a far greater possibility of achieving chemical bonding (such as covalent bonding) within the matrix to ensure homogenous dispersion.

Functionalisation via plasma
As graphene is inherently inert, the challenge is to integrate graphene into target materials. However, the customised modification of a material's surface chemistry is key and is crucial to achieving adequate dispersion. This is seen as the key barrier to the material's commercialisation.

A low temperature plasma, under 100°C, process has been developed that overcomes this. This can 'functionalise' with a range chemical groups, such as O2, COOH, NH3, and F, and the degree of functionalisation can be tailored to specific requirements. Good dispersion optimises the properties and performance to ensure the required results are consistently achieved.

The plasma functionalisation process is not aggressive and may also remove impurities and repair some defects.

The environmental credentials of the plasma process are also further underlined by low energy consumption and the avoidance of a waste stream.

Graphene in composites
The mechanical performance of graphene is of particular interest as a filler for composites. Exciting results demonstrating the potential of nano-reinforcement resin have been reported where an epoxy resin reinforced by GNPs showed a 200% increase in tensile strength/modulus and a 125% increase in toughness.

These results point to surface functionalisation as the key parameter influencing nano-reinforcement due to dispersion promotion and chemical bonding with the epoxy matrix.

The flake graphene (or GNPs) form can be produced top-down from mined graphite, or bottom-up by chemical processes, which represent a wide range of different materials under the 'graphene' label that vary in purity, number of layers, flake size and morphology. These materials can be incorporated into inks, polymers, composites and coatings, to either exploit electrical or thermal conductivity, or add enhanced mechanical performance.

Realising the potential
The key to realising the performance in a matrix or ink is homogeneous dispersion, and chemical bonding within the matrix. These properties can be achieved by effective surface functionalisation of the graphene that can be achieved most effectively by plasma treatment compared to acid-based surface functionalisation.

Plasma is a clean, low energy process that avoids a toxic waste stream and can therefore realise the full effects of graphene in a cost and environmentally-effective way.

As with all new 'wonder' materials, commercialisation depends on many factors. With graphene the market demand is certainly widespread and the sources of raw material numerous. Now that the key role of functionalisation has been identified and a commercial route established, the scene is set for a successful future.

Justin Cunnigham

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