Exploring new ideas for making large-scale graphene sheets
Imagine you are a company like SpaceX. A material like graphene that is 200 times stronger than steel will allow you to make stronger lightweight rockets. That means cheaper flights with increased payloads. You contact suppliers to source this amazing material and quickly find it is only available in tiny pieces, but you need metre scale sheets of the stuff. If you want to know how to make large sheets of graphene this is how it could be done, read on…
In part one we looked at a new way of making graphene by bubbling methane through molten metal. In this part we anticipate some of the problems inherent in this approach and think about the principles for building a machine that might be capable of making graphene in large-scale sheets.
Passing methane gas through the molten metal breaks it down into hydrogen gas and carbon; both rise to the surface. The hydrogen escapes as gas and the carbon floats as a solid. If the surface of the melt was smooth then graphene could form at the surface. However the hydrogen gas will form bubbles that will disrupt the formation of any layer. A basic reactor will only produce hydrogen gas and amorphous carbon (soot). To make graphene a smooth metal surface is needed. A side chamber acting as a flotation tank solves this problem.
This arrangement should circulate molten metal with enough dissolved carbon to produce a layer of graphene at the metal surface. I say ‘should’ because as far as I can tell this is a new idea and no one has published experimental work to prove this, yet.
Working on the assumption that we can produce sheet graphene by this flotation method, we now have another problem to solve. The snowflake growth problem. This is where the graphene layer starts to form from multiple points on a surface. These points grow in to domains that can look like hexagons, flowers and snowflakes. These domains eventually meet and this causes defects in the graphene layer. When producing sheet graphene these defects will disrupt the electrical, thermal and mechanical properties of the large-scale graphene product.
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The obvious way of preventing these defects is to constrain the starting point of the graphene sheet. There are several possible ways this could work; the most straightforward is a sliding mask shaped to reveal a point area first.
The mask is in contact with and completely covers the molten metal surface in the flotation tank. As the mask begins to slide over the surface a small area of melt is revealed on which graphene can form. Pulling the mask further will reveal more metal surface for the graphene layer to grow. As the graphene layer started from a small point this should produce a defect free sheet of graphene.
At this point, dear InvestorIntel reader, you may be thinking how can we retrieve the graphene layer from the flotation tank and how can we get graphene out of the machine as a continuous sheet? Well, it should be possible to solve both problems with another idea that we can call a freezewall.
One side of the graphene flotation tank has a removable double wall. The inner wall is slightly lower than the outer one. Once the graphene has formed on the surface the outer wall is cooled slightly to freeze the molten metal nearby. This has two consequences, firstly the graphene layer is stuck to the surface of the frozen metal and secondly the frozen metal will stick to the side of the freezewall, especially if the wall has a dovetail groove shaped into the melt side of the wall.
When the melt freezes into the groove it is locked in place and part of the graphene sheet is frozen onto the surface of the melt. The melt level can be adjusted to the level of the lower inside wall. Then pulling the freezewall away from the flotation tank will pull the graphene layer as a sheet from the surface of the melt.
At the other side of the flotation tank, the graphene layer will be pulled away from the wall exposing fresh melt surface. This will prompt more dissolved carbon to migrate to the melt surface and attach to the edge of the graphene layer. This should spontaneously form a continuous sheet as the whole graphene layer is pulled from the flotation tank.
The width of the sheet is limited only by the width of the flotation tank, so this method is capable of producing sheet of graphene with a hammock index of zero (and above) namely a one square metre sheet.
Dear reader, you will appreciate that there are some significant engineering challenges to be overcome in making large-scale sheets of graphene using these principles. The purpose of this column is to put a set of ideas into the public domain so they are free to use by anyone. Companies such as SpaceX are used to achieving the impossible. This column moves graphene manufacture into the realm of the possible. Time will tell if smart, motivated and well-resourced people pick up these ideas and use them to help build their vision of the future.
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>