h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The new "material of the future" of the day is graphene. As far as I can tell, this is large 2-dimensional sheets of carbon, like that which graphite is made from. Only in graphite, each 2-dimensional layer is connected to another 2-dimensional layer through weak bonding. One of it's properties is that it's meant to be stronger than steel. I understand that steel at that thickness wouldn't be very strong to begin with. However graphite could then be compared to many steel plates on top of each other, help together with glue. Ripping them apart would be easy. However wouldn't you then expect that from certain angles, it would be damn near unbreakable as you would be not ripping the plates away from each other but actually needing to break the plates? So why can I so easily snap a piece of graphite in half?

Graphite is a strongly planar material. It consists of hexagonal layers of carbon atoms, very strongly bonded together. But the bonds between layers are comparatively weak

http://www.nano-enhanced-wholesale-technologies.com/images/structure-graphite.gif

When you break graphite, you break the bonds between layers.

Graphene, diamond and buckyballs are all made of pure carbon atoms, same as graphite, but the bonding forms a three-dimensional lattice which is strong in all directions.

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As an addendum, remember that microstructure of materials is not the same as macrostructure. Your steel-plate analogy falls apart because an actual, tangible bit of graphite (say, the middle of a pencil) is made up of lots of microscopic graphite grains, each with a different orientation. If by some miracle of chaos and randomness, each grain had aligned up perfectly, then your graphite would indeed be unbreakable along two of its three axes. However, real life doesn't work like that - when you apply stress to a piece of graphite, it's the planes closest to perpendicular to the stress direction that will fracture first. This will then increase the stress on other grains that are oriented in 'breakable' way, and eventually the whole lot gives up.

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That's what I meant, that breaking the bonds between layers should be easy. Surely from some angles, you would need to actually break the layers themselves.

If you take a piece of wood and try to hack it apart with an old rusty saw, it will end up splitting along the grain (not the same sense of grain that I used in post 3). Same thing - just because you are applying stress in one direction, it does not mean that fracture will happen along one continuous line in this direction.

If you take a piece of pencil 'lead' and snap it in half and examine the broken surface, you will rarely see a clean break along one plane - it will nearly always be jagged.

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In passing, I've been lurking around this poxy forum for six years waiting for someone to post a materials question. I'm so pleased when it happens!

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I'm glad I could make you feel useful.

I saw something the other day which said that when you draw with a graphite pencil you are actually laying down layers of graphene..... Am I right, or have a misunderstood something?

No, that sounds about right.

Hmmm, hmmm, sort of.

It goes back to the difference between nanoscopic, microscopic and nanoscopic. 'Graphene' is very much a term describing a material and all the macroscopic properties (strength, hardness etc) that are associated with it.

You might as well describe a single carbon atom as a diamond.

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