Saturday, 25 October 2014
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Carbon crusade

UK firms are pioneering the production techniques that could bring graphene to the masses.

It’s been proclaimed a ‘wonder material’ that will be the springboard for a new technological revolution. It has earned its discoverers Nobel prizes, formed the foundation for dedicated institutes and, if publicity is to be believed, is set to inveigle itself into every aspect of our lives. Name a superlative property and someone will claim it has it. Graphene has received more hype than a whole series of blockbuster superhero movies.

But graphene is real; and huge research budgets are focusing on it worldwide. For the uninitiated, graphene is a pure form of carbon consisting of a single flat sheet of carbon atoms arranged in hexagons: in effect, a single layer of graphite. It’s often referred to as a two-dimensional material, although, strictly speaking, it isn’t; it’s one atom thick and atoms are not dimensionless.

graphere

Perfect graphene structure such as this is, in practice, almost impossible to achieve

First isolated at Manchester University in 2003 by Russian-born materials scientists Andre Geim and Konstantin Novoselov, graphene had previously been studied theoretically but was thought to be thermodynamically unstable. Geim proved this was wrong when he took a thin piece of graphite that had been prepared by a student and ‘peeled’ off a sheet of graphene using a piece of adhesive tape.

‘We believe that the first commercial applications are likely to be conductive inks in the next five years

Paul Mason, Technology Strategy Board

Intensive investigation of the properties of small samples of this new material triggered excitement in the scientific community and the technology industry: although infinitesimally thin, it seemed to be immensely strong, with a breaking strain 100 times greater than a film of steel of the same thickness. It conducts heat as well as diamond and appears to enable ‘ballistic transport’ of electrons — zero-resistance electrical conductivity. It’s transparent, can convert visible light into electric current,  and is flexible and biocompatible.

The deluge of encouraging results triggered a rush to commercialise graphene — a rush that shows no signs of slowing down. Electronics companies, with South Korea’s Samsung in the lead, have thrown resources at the problem, hoping that graphene will be the key for anything from new forms of touchscreen display to innovative batteries. This has led to concerns that, with its relatively smaller budget available for research, the UK will once again see a discovery made on these shores exploited for profit elsewhere.

Geim

Andre Geim: The first scientist to isolate graphene, and incidentally the only person ever to win a Nobel Prize and an IgNobel Prize (for levitating frogs)

Despite this, there is a small but growing band of UK companies — alongside the National Graphene Institute, based at Manchester — exploring ways to manufacture graphene in bulk. There are still no products containing the material on the market — these companies sell graphene to companies developing applications. While for university materials science departments, isolated flakes of graphene are fine for experimentation, to a company developing products, a reliable supply of graphene in amounts large enough to make prototypes, with robust and repeatable properties, is vital. Once commercial applications are developed, dependable manufacturing techniques will be indispensible.

There are two ways to make graphene. Geim and Novoselov’s method, flaking sheets of graphene off a chunk of graphite, is known as ‘top-down’. In a manufacturing context, this isn’t done using sticky tape, no matter how appealing the image might be: the graphite can be vibrated with soundwaves (sonication); treated with acid and thermally shocked; or bombarded with ions in a plasma to break the weak bonds between the graphene layers that make up graphite and exfoliate sheets of the material from the bulk.

The other method doesn’t use graphite: rather, it seeks to force individual carbon atoms from some other source, generally a hydrocarbon, to combine into graphene’s hexagonal grid; this is known as a ‘bottom-up’ approach. This is what electronics companies such as Samsung are favouring, as they require larger areas of graphene. A process called chemical vapour deposition (CVD) is commonly used, where a carbon containing vapour is deposited onto a flat substrate — often copper — guided by a catalyst that can be a solid or in the vapour phase.

black powder

Powdered graphene made using a bottom-up process by Applied Graphene Materials

The UK graphene community contains proponents of both top-down and bottom-up processes. But there is a complicating factor: the properties of the materials produced are very strongly influenced by the manufacturing process and this, in turn, influences the applications for which they can be developed.

‘We believe that the first commercial applications are likely to be conductive inks in the next five years, and for that you need very small fragments of graphene, a few layers thick: what’s known as nanoplatelets,’ says Paul Mason, head of innovation at the Technology Strategy Board, which is funding development of graphene applications via a £2.5m competition for feasibility studies, alongside the Engineering and Physical Sciences Research Council (EPSRC). ‘And conductive inks, used alongside a jet printer and a 3D printing mindset, could be a powerful technology. Think of them in conjunction with the plastic electronics sector, in which the UK has a strong foothold. Following on from that, we’re expecting to see commercial applications in composites, using graphene’s physical properties and, again, the UK is very strong there. The electronics applications, such as displays and touchscreens, will come in after that and we think we’re looking at 10 to 15 years for those to hit the markets. It’s a long game.’

Ultimately, Mason says, the goal for the competition is for graphene to contribute to the UK economy. ‘But we aren’t sticklers for the exploitation itself to be in the UK,’ he adds. ‘If someone came to us with an idea for a technology that Samsung would license from them for £10m, we’d consider that a UK gain. And we aren’t concerned about what sector it’s in. Electronics using graphene are often seen as a South Korean strength, but we aren’t about to tell ARM not to work on a graphene transistor.’

The thought of perfect sheets of graphene forming a flexible, transparent touch-screen fall down because of the realities of manufacturing, says Ian Walters, who founded one of the UK’s first graphene makers, top-down producer Heydale, in Cardiff, and has since founded another, Perpetuus Carbon Technologies, near Swansea. ‘You can’t make a perfect graphene sheet over a wide area,’ he said. ‘You get islands of crystallinity — and it’s the crystallinity you’re after because otherwise you just have amorphous carbon, which is useless — and those islands grow together. But then you have grain boundaries, and those disrupt the conductivity of the sheet.’

That’s not the only problem, he adds. ‘You’ve got to get the sheet off the substrate and the material isn’t robust. If you manage to isolate it from the metal, then all that happens is that it rolls up and forms a nanotube.’

Perpetuus, which like so many high-tech startups in the UK is based on a nondescript industrial estate next to a pet-food warehouse, uses a top-down method to make graphene nanoplatelets. Its anonymous buildings house a reactor that can hold some 30kg of powdered graphite, which is subjected to treatment with a plasma of high-energy ions generated by UV light and a high voltage from a specific arrangement of electrodes. This, Walters says, gives it the ability to produce 100 tonnes of graphene products per year; plans are afoot to install a more advanced analysis lab at its HQ and to build a new production facility in nearby Port Talbot with two larger reactors.

Graphene dispersion

Dispersions of graphene in a liquid are important for making conductive inks and anti-fouling coatings

‘One of the problems with processing graphene is that it’s very inert stuff; it doesn’t like to mix with anything,’ he says. ‘So we can functionalise it; by changing the process gas in the reactor, we can make surface-functionalised graphene with oxygen groups, carbonyl groups; amine groups; or fluorine. Oxygen-functionalised with disperse in water, and can also form covalent bonds with epoxy, as can amine groups. There’s a fair bit of trial and error in making the desired product,’ he adds, ‘because on an atomic level, we don’t understand what’s happening in the reactor; we can tweak things such as the level of vacuum, the electrode voltages, the vapour composition and so on.’

The result is a graphene nanoplatelet powder that visually appears little different from the graphite that went in, but can be characterised using analytical techniques such as scanning electron microscopy (which reveals images of the wrinkled sheets of graphene) and X-ray diffraction (which shows the level of crystallinity); an increase in volume also indicates the transformation, as the layers of graphite are forced apart. ‘It’s important to remember that graphene is graphitic carbon, and as soon as you have stacked layers you essentially have graphite,’ Walters says. The fewer layers in a nanoplatelet and the more space between them, the more the graphene-like properties predominate over the graphitic ones.

Perpetuus can also produce graphene that is studded — or as Walters puts it, decorated — with micrometre-scale nuggets of other minerals. Decorating the edge of the graphene sheet with silver, for example, enhances the conductivity of the material in ink, and decorating with zinc oxide imparts piezoelectric properties. This raises possibilities such as using a graphene-ink printed stretchy membrane as a wound healing dressing, which could be used internally or externally, with movement generating an electric current and field to stimulate blood flow to damaged tissue; it could also be used for stress-strain actuators. Other techniques include intercalation, where other elements can be placed between graphene layers with some freedom of movement.

‘We can make a film of dispersed nanoplatelets that is more than 85 per cent transparent and approaches the properties of indium tin oxide for display screens,’ Walters says.

Walters, like others in the UK sector, isn’t overly concerned with the Asian electronics firms amassing patents in the field. ‘At the moment, it works much like the pharmaceutical industry,’ he says. ‘The larger companies are waiting for smaller, more agile research-oriented companies to make breakthroughs, then they dive in with support.’

Another graphene maker, Claudio Marinelli, business development director of Applied Graphene Materials, agrees. ‘South Korean IP isn’t as much of a problem as some people think it is; in the first place, many of those patents are somewhat speculative and in all probability won’t be defensible,’ he tells The Engineer. ‘You can’t equate patents to commercialisation in this kind of field. And the UK has the largest number of experts per capita working in this field than anywhere apart from South Korea; anyone is just as likely to come through with commercial applications, and it’s a fact that the cost of development of technology is only a tenth of the cost of going to market: that’s the main parallel with pharmaceuticals and why large companies are keen to support innovation from smaller ones.’

Claudio Marinelli

’Graphene will come into its own where you need a combination of two or more of its properties, and even then you’ll have to prove that it’s better than what’s available.

Claudio Marinelli, Applied Graphene Materials

Unlike Perpetuus, AGM makes graphene using a bottom-up process, developed by Karl Coleman of Durham University, . But the process isn’t CVD, Marinelli insists, because although it starts with carbon in the form of a vapour of simple alcohols, it isn’t deposited on anything. ‘We prefer to call it a continuous gas-phase synthesis,’ he says. The carbon-containing vapour is brought into contact with a catalyst that isn’t incorporated into the graphene, making free nanoplatelets 2–10 graphene layers thick, and about 2–3µm in size. ‘We think this is a superior product of high purity, because with exfoliation you always have some non-exfoliated graphite in the sample,’ adds Marinelli. ‘These platelets are dispersible with surfactants, and can be used in paints, inks, coatings and composites.’ Like Mason, Marinelli expects inks to lead the commercial market by some years.

UK manufacturers tend to shun the acid-heat top-down method. ‘It produces an extremely large amount of contaminated water,’ says Liam Britnell, technical director of Bluestone UK, affiliated to the National Graphene Institute and one of the few UK producers to use CVD to make graphene sheets, depositing it onto a variety of substrates. ‘That has to be treated, which pushes the cost up and adds a lot of complexity to your processing.’

Konstantin Novoselov

Konstantin (Kostya) Novoselov in his lab at Manchester University. Novoselov shared the Nobel Prize with Andre Geim

The challenge for graphene, as Claudio Marinelli sees it, is to transfer the properties that are predicted by theory or have been found in very small amounts of material to bulk materials or dispersions. ‘If you’re looking at exploiting any one particular property the chances are that there will be other materials available that can do it. Graphene will come into its own where you need a combination of two or more of its properties, and even then you’ll have to prove that it’s better than what’s available.’

Ian Walters puts it slightly differently. ‘Graphene is an inspiration’, he says. ‘In developing applications, you move towards the properties you want. Sometimes, it turns out you can use something else to give the same effect, but you wouldn’t have been looking at all if it weren’t for graphene.’

 

 


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