The secret to the Sword of Damascus

Carbon nanotubes are rolled up tubes of graphene, which you may have heard of. These are far stronger than steel yet far lighter than steel making them a very interesting material. Although carbon nanotubes sound like a new fad, they actually date back to a very long time ago.  Legend tells of a sword with mystical properties. Blacksmith Magnus Sigurdson tells us the story of the sword of Damascus, with Michael de Volder, materials engineer from the University of Cambridge, on hand to explain to Sally Le Page and Katie King the science behind the myth...

Magnus - There are many stories from the third crusades of King Richard meeting Saladin. They decide to compare sword blades. King Richard the Lionheart produces his sword of Frankish steel and cuts through an inch thick iron bar, having no damage to his blade whatsoever. Saladin then produces his lighter blade, which shines with a blue gray effect. Looking closely at the blade, it looks like a starry night being viewed over a rippling pond. With this he lays out a silken scarf and cuts through it with one swipe. King Richard swears he has never seen such a blade so sharp.

Sally - Okay. Not gonna lie. That was a great story. But what does any of that have to do with carbon nanotubes?

Katie - There is a lot of speculation, but a few years ago, this mythical mystical magical material, Damascus steel was discovered to contain these carbon nanotubes and they reckon that it might be these nanotubes is what made the sword so strong.

Sally - So you're telling me that you've been bigging up nanotechnology at the start of the show as something new and "cutting edge" when actually we've been using it for a thousand years.

Katie - It's true that these structures have existed for a while, but it's only recently that we've been able to control how we make these structures and therefore control their properties.

Sally - Someone who knows all about that is Michael de Volder, materials engineer from the University of Cambridge, who specializes in carbon nanotubes. Michael, what makes Damascus steel and therefore Saladin sword so strong?

Michael - I'm not sure anyone really knows but as you mentioned what people have discovered in 2006, is by actually dissolving a little bit of the Damascus sword in acid, is that there is carbon tubes inside. Now that brings about two mysteries. How did the carbon tubes get in there in the first place? And what do they do to the steel to make it better?

Sally - Well exactly. I think carbon nanotubes are something modern and fancy you need a lab for. How did the nanotubes get in there that many years ago?

Michael - For that we need to understand how carbon nanotubes are made. Although they sound very fancy, actually their synthesis is very much like gardening; something you've all done. For gardening you need some seeds, you need some energy from the sun and you need soil and fertilizer to make the plants grow. In the case of carbon nanotubes, those seeds are little metal clusters, often times iron. The food you feed them instead of soil would be hydrocarbon. Then you need some energy from heat. If you think about how a sword is made, there's lots of iron in the steel which connects as a seed. You heat them up to high temperature, which is also what's needed for carbon nanotubes. Then the question is where would the carbon source come from to make the nanotubes, and turns out that in the recipe of the Damascus sword, people did add various leaves and wood to the steel as they were making it and that could very much have been the carbon source or one of the carbon sources to make the nanotubes.

Sally - So they knew that they knew that they were adding these leaves and they ended up with stronger steel, but obviously they wouldn't have known that they were building these carbon nanotubes, but they did it anyway.

Michael - Exactly. A lot of times discoveries are made by serendipity and people do things without necessarily knowing how exactly they are made.

Sally - What gives carbon nanotubes their strength? Why does having them in your steel make your sword so strong?

Michael - The bond between the carbon atoms in a material like graphine, which is essentially the same as the surface of a carbon nanotube, is a very special bond that is mechanically very strong. It also allows the carbon atoms to share electrons and makes material electrically conductive. It's a very, very special bond that we have in between the carbon atoms in carbon nanotubes.

Sally - How long are these tubes? I mean, we call them nanotubes, so obviously they're small in diameter, but how long can they get?

Michael - That's a very interesting question. The pieces that were found in the Damascus sword, which I've seen images of, were fairly short, but the current world record in the length of a single nanotube is about half a meter long.

Sally - That's not nano, but it's only a few atoms across and half a meter long?

Michael - That's correct. The diameter is tiny. The ratio between the diameter of planet earth and the tennis ball, is about the same as the tennis ball and that nanotube. Although it's very long, it's diameter is still at a nano scale.

Sally - Are we still using nanotubes? Are they still useful or are they just a marketing fad?

Michael - When carbon nano tubes were discovered and were looking for an application, they were often advertised for sporting goods and so on as a marketing tool. But then people start finding it quite easy to use them as additives in for instance, polymers and then making those electrically conductive. That turned out to be a big commercial success in automotive applications and a couple of other applications as well. Making a plastic conductive tends to be very interesting if you want to paint cars or use certain plastics in fuel tanks where by law, the material needs to be conductive.