Graphene: what is it good for?
Image credit: Vollebak
Once regarded as a wonder material for the future, graphene is now finding numerous commercial applications in the hands of innovative companies. We take a whistle-stop tour of everyday items with a twist of graphene.
Graphene was originally identified for use as a transistor in computing, but as researchers catalogued its properties, it became clear that it could be used for many other applications.
Graphene is strong, light, thin and flexible. It is the thinnest substance capable of conducting electricity, is an efficient thermal conductor and is optically transparent.
In a functionalised form (i.e. when compatible chemical groups are added to the material surface to disperse the graphene) it is used with carbon fibre and composite products to strengthen and reduce the weight of sports equipment. In protective helmets, graphene-enhanced composites increase the strength to weight ratio to improve impact resistance.
In the 2018 Winter Olympics in PyeongChang, South Korea, British bronze medallist Dominic Parson used an X22 skeleton sled that used Nanene, a layered, graphene nano-platelet powder produced by Versarien. Dispersing Nanene in resin removes the carbon structure to make products lightweight, says Stephen Hodge, head of research at Versarien.
Adding graphene to plastic composites can improve the tensile strength and stiffness of packaging. “Graphene won’t make the material indestructible,” says Hodge, “but it may be possible to reduce packaging size while maintaining the same properties.” This has obvious advantages for transporting fragile goods and may also contribute to recycling. Today, recycling plastics degrades the quality of the plastic – it can be recycled an average of three times; but adding graphene to recycled plastics can improved its strength so that it can be recycled many times more.
Graphene can also be used in printing ink to re-engineer touchpoints and replace displays in familiar objects. Printing ink onto a substrate embedded into a car dashboard, for example, connects objects to the digital world without a computer screen, explains Chris Jones, technical manager at Novalia, a partner in the EU’s Graphene Flagship. “Our mission statement is to make technology disappear into everyday items,” he says.
At Mobile World Congress in Barcelona this year, the company presented a board game that has sensors printed on the board using graphene ink. The ink is supplied by the University of Cambridge and produced by micro fluidisation. This process uses a mix of water and graphite, which is passed, under high pressure, through an intensifier pump, which cleaves the graphite into graphene.
“Because they are printed, [the capacitive touch sensors] can be any size or shape and printed in volume,” says Jones. “They can be tremendously inexpensive compared to traditional sensor designs and switches, which can fail over time; the printed plane cannot fail.” The graphene-ink-printed buttons are located behind a graphic on the board game. Touching the graphic activates a spoken instruction, such as ‘Move forward three spaces’. Using capacitive touch technology means that the presence of a finger is enough to activate the sensor.
The next step is to print sensors on a sheet of paper, which can be overlaid on to a vehicle dashboard for convenient operation but also to reduce weight in the vehicle. At the moment, says Jones, there is limited traction for peelable electronic sensors in vehicles, as it is relatively inexpensive to use an iPad to control navigation.
An ink based on graphene is also yielding interesting results in reducing the cost of manufacturing perovskite solar cells (PSCs). Researchers at the Instituto Italiano di Technologia, University of Rome Tor Vergata and BeDimensional – all Graphene Flagship partners – developed an ink using molybdenum disulphide quantum dots and graphene to stabilise PSCs. The ink can be applied to practically any surface to produce electricity, say the research teams, meaning that solar cells could be painted onto the roofs of buildings.
What is graphene?
In 2004, two researchers at the University of Manchester, Professor Andre Geim and Professor Konstantin Novoselov, removed flakes from a lump of graphite using sticky tape. They repeated the process on the fragments until the flakes were one atom thick and the 2D graphene was isolated.
The structure of graphene is a hexagonal lattice of carbon atoms. It is approximately 200 times stronger than steel and harder than a diamond of the same dimensions. It is lightweight, weighing 0.77mg per square metre, and is stretchable with a high tensile strength; graphene can stretch by up to 20 per cent of its original size without breaking.
Graphene has very high electrical conductivity,far better than copper or silicon, because carbon atoms have four ‘outer shell’ electrons, three of which form bonds in the lattice, leaving the fourth free for conduction.
It also has a higher thermal conductivity than carbon nanotubes, graphite and diamond and acts as an isotropic conductor, conducting heat in all directions. Moreover, its conductivity increases with size. This challenges conventional thermal conduction law and means that graphene could in theory absorb an unlimited amount of heat.
Graphene is also more resistant to tearing than steel and is almost impermeable. In addition, it is thin, around 0.34nm, and absorbs approximately 2.3 per cent of white light, which makes it transparent to the human eye so it can be used as a transparent conductor.
Although perfectly flat sheets are inert, structural or chemical irregularities enable graphene to be modified to create, for example, graphene oxide or fluorinated graphene..
Every atom in graphene is exposed to its environment, so sensors detect changes in temperature, atmospheric conditions and the presence of harmful gases.
Today, graphene is available as a powder, mixed with polymers, oil or water in micro-fluidisation processes, for use in composites, paints, inks and coatings.
In 2013 the European Union launched a huge 10-year Graphene Flagship programme working with academia and industry to understand what graphene can do and to find products and commercial applications.
Researchers at the University of Manchester have been engineering graphene flakes to develop graphene-based yarns that can be knitted into a garment for use as a flexible sensor to send temperature or pressure data to a device via Bluetooth or radio frequency identification (RFID).
Dr Nazmul Karim, Knowledge Exchange Fellow (Graphene) at the National Graphene Institute, University of Manchester, explains that the team used graphene oxide that had been functionalised to coat textile yarn. “Pure graphene, derived from mechanical exfoliation, is not scalable, so we used a liquid phase exfoliation to produce reduced graphene oxide [rGO],” he says. Using a very simple dyeing machine, the team produced electro-conductive textile yarns that can be knitted into garments.
The coated yarn has “excellent temperature sensitivity” reports the team, responding to even small changes in temperature. The yarn was exposed to mechanical agitation (using 10 steel balls) and washing powder to simulate a typical laundry cycle and was still conductive after 10 washes.
“We are focusing on using existing machinery, processes and systems,” says Karim. “Scalable production will reduce the cost for high-performance, functional clothing, which can be anti-bacterial, anti-static and fire retardant.”
Brothers Nick and Steve Tidball founded outdoor clothing company Vollebak in 2015 and introduced a prototype reversible graphene jacket in 2018. It was created by blending graphene nano-platelets with polyurethane to create a thin membrane that could be coated onto one material. The other side of the jacket is a black, nylon material with 15 per cent elastane for stretch.
The jacket is priced at £525 and is expected to be available this summer. It is particularly suited to outdoor pursuits as it can equalise body temperature in cold weather. Graphene is impermeable to gases and most liquids, except water molecules, so sweat can evaporate through it. It is also bacteriostatic, while the jacket’s polyurethane membrane makes it waterproof to 10,000mm.
Another sporting use for graphene is inov-8’s MudClaw running shoe. Working with the National Graphene Institute, the company infused rubber with graphene for the rubber soles and 8mm grips. The shoes grip better than rubber on rocky and wet terrain and do not wear down as quickly. The material also reduces the weight of the shoes.
The thermal conductivity of graphene means that, when mixed with paint, less energy is required for heating and air conditioning. Spanish paint producer Graphenstone mixes a small amount (less than 2 per cent) with its lime-based paint. “The lime-based, natural mineral paints are alkaline, so resistant to mildew and mould growth,” says Ben Sturges, head of business development at The Graphene Company, which supplies the paint in the UK. “Lime is a natural disinfectant, and graphene improves the bond strength and flexibility.”
Natural minerals can be brittle when exposed to heat or cold or applied to hairline cracks in buildings or timber, but the flexibility of graphene means that the paint can move with the building. The porous paint allows walls to breathe, improves air quality and reduces room humidity. Rooms can be occupied within a few hours of application.
In concrete, graphene acts as a mesh to increase the tensile strength of a building and to increase resistance to compression and wear. It can protect buildings in earthquake zones by being flexible and delaying cracks and fissures. Spanish company Graphenano Smart Materials adds graphene concrete additives to its products for projects in Spain, Morocco, the US and Mexico. It reports that concretes with graphene additives can increase service life by up to 50 years compared to those using conventional materials.
For interiors, graphene added to ceramic tiles provides underfloor heating. US company Radiant Panel Technologies applies a graphene heat film between layers of its Easy Elements stone or tile floor panels. The panels can be cut to size and can bear up to 40lb per square inch (28,000kgf/m²).
Graphene can also be added to shower tray resin for waterproof, bacteriostatic shower trays. Core Graphene says that its SolidGraphen natural slate texture shower trays can withstand distributed loads of over 600kg without breaking or bending. Graphene also helps to prevent mould and mildew growth in the humid environment.
Although graphite is readily available, producing graphene in sufficiently large quantities can be an expensive and specialist area. There are also no industry standards for graphene, and many commentators see this as an obstacle to future development. According to Novalia’s Jones: “It can be mixed to order for a particular project, but there are no formal standards to define it, which is essential.” At the moment, developers specify a particle size or property to create their own standards, he says.
Another hurdle is that the large surface area of graphene means that it can be difficult to deliver. The difference in scale is immediately visible, says Jones. “For 1kg of [ink] pigment, I use a one-litre container but for 1kg of graphene, I need a 25-litre drum, due to the surface area,” he explains.
The graphene dress: tech meets couture
For the 2017 Innovation Festival at the Manchester Trafford Centre, shopping centre management company Intu approached technology-fashion label CuteCircuit to collaborate with the National Graphene Institute at the University of Manchester to create an exhibit.
A dress was designed to illustrate the material’s strength, transparency and conductivity. The shape and decoration of the dress represent the design of a graphene crystal. “We examined graphene under a microscope to see the hexagonal structure and enlarged it to help people understand graphene’s molecular structure,” says Francesca Rosella, co-founder of CuteCircuit.
Lightweight, transparent graphene was used to illuminate elements on the bodice of the dress, which changed colour according to the depth of the model’s breathing.
“We used a rubber, stretchable sensor with graphene to create a capacitive sensor,” explains Rosella. “This was connected to a microcontroller in the dress which was connected to light-emitting diodes (LEDs). When the model took a breath, the LEDs were illuminated. The effect was ‘magical’.”
The rubber sensor had to be embedded with nano granules to compensate for the changes in conductivity caused by stretching. Nano particles of graphene were deposited onto squares of material using chemical vapour deposition.
Explaining the next step, Rosella says: “Designers and scientists worked together but the scientists only envisage the material and cannot see the design angle.” To create a bodice for the dress, the squares had to be cut into triangles. The material was laser edged into quadrants and a ‘foot’ of each LED was placed in each section.
Producing the material is a long process, but Rosella believes when it becomes more readily available on a commercial scale, graphene’s properties will enable indestructible garments that will not rip or tear, with lightweight sensors for illumination, communication and medical purposes.
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