Exploring new ideas for making large scale graphene sheets – Part One
This lightest and strongest material in the world was first isolated in 2004. It can be made in small pieces, and scaling up its manufacture seems to be a slow process. This short series of articles looks at the current state of the art and explores potentially new ideas for making the material at a large scale. Dear InvestorIntel reader please read on to find out more…
At the present time, graphene manufacturing is focussed on making powders and suspensions of few layer graphene nano-platelets that are used as additives in other products.
Sheet graphene can be made at a few centimetres scale, but these sheets are deposited on copper foil and are not defect free. To realise the full promise of this material it needs to be made on a larger scale, creating defect-free sheets metres wide. Building on current science, here is a set of ideas about how defect free sheet graphene could be made. If these thoughts are not patented already then this column creates ‘prior art’ that means any novel ideas expressed in this column are free for anyone to use.
There are no reports of sheet graphene occurring naturally any larger than at the nanoscale. So we can assume for the time being that if we need large sheets of the material these must be manufactured.
Sheet graphene manufacturing methods
One possibility is to persuade these nanoplatelets to weld together and form larger sheets but there is no published evidence that this is possible. We are left with the other possibility: to make graphene sheets atom by atom, the bottom up approach, and here there is much published work we can draw on.
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Making sheets of graphene by the bottom up approach at any useful scale needs to overcome a series of problems:
- Separating carbon atoms from their natural state
- Reassembling the carbon atoms so they form a hexagonal layer of graphene on a substrate.
- Controlling the growth of the hexagonal layer to prevent defects forming on the substrate
- Separating the sheet graphene from the substrate, making it ready to use.
The problems faced by current manufacturing methods
The Chemical Vapour Deposition (CVD) method is the current state of the art for making sheet graphene from methane and copper foil. Methane is a gas where four hydrogen atoms are bonded to one carbon atom. The methane gas is heated to high temperatures to separate hydrogen from the carbon. The carbon atoms then form a graphene monolayer on the copper foil. Unfortunately the copper foil is not smooth at the nanoscale and contains grain boundaries that create defects and stop the formation of a continuous hexagonal layer of graphene.
Research teams have found that graphene will grow on a molten metal surface and this will eliminate the grain boundary problem. However a further problem called snowflake deposition needs to be overcome. This is where graphene grows on the surface from multiple points and defects are created where the growing layers collide.
Ideas for producing graphene
Encouraging graphene to grow on a molten metal surface is a good starting point. Let’s try and avoid the obvious…
Rather than the current CVD method, methane gas can be bubbled directly in to the hot melt. Hydrogen will dissociate from carbon in methane if a metal with a high enough melting point is used. Researchers working on hydrogen generation have proved this, bubbling methane gas into the bottom of a reactor of liquid metal, and generating hydrogen bubbles out from the top of the reactor with carbon as a waste product. Our point of view is exactly the reverse, we want the carbon, and hydrogen is an incidental nuisance.
Can carbon dissolved in metal form graphene? In 2010 a team at the University of California–Riverside designed an experiment to find out. They found that molten metal could dissolve solid graphite and form graphene at the surface.
The team also found that nickel was better than copper at dissolving solid graphite. However they also found that the process was difficult to control and produced single layer and multiple layer graphene. They did not make any mention of the control factors. The primary issue was probably that they had used solid graphite as the carbon source. This means that the release of carbon in to the melt could not be controlled. It is also possible that the graphite was exfoliating into the melt rather than dissolving. Running the experiment again with another source of carbon would have eliminated this possibility. Their attention seems to have been on the graphene outputs rather than the raw material inputs and this probably explains why they appear not to have considered other sources of carbon.
By now you’ll be ahead of me. If we combine these research findings we can create the outline of a new controllable process by bubbling methane through molten metal and producing a layer of graphene at the surface.
Now you understand the principles and know they are planted on solid foundations. We will need a few more ideas before we can make sheet graphene. To do this we will need to think about the principles for a machine that can produce graphene sheet on a large scale by a continuous process. These ideas will be explored in part two…
Adrian Nixon began his career as a scientist and is a Chartered Chemist and Member of the Royal Society of Chemistry. As a scientist and ... <Read more about Adrian Nixon>