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November 23, 2006

Carbon Nanotubes and Ancient Sabres

Up until the 1980's the chemical element carbon was known to chemists in only two modifications: graphite and diamond. Those two could not be more different: graphite is a good conductor of electricity, diamond of heat. Graphite is black and opaque, a diamond usually transparent and so forth. In the 1980's another modification of carbon was detected: the fullerenes, spherical molecules of carbon atoms that are arranged in five- and sixmembered rings. They always consist of 12 pentagons and (n/2 - 10) hexagons where n is the number of carbon atoms. The most famous is C60 which looks exactly like the European soccer balls we had before the last Soccer World Cup. If you take one and count the edges, pentagons and hexagons you'll know what I mean.

The fullerenes where first artificially generated in laboratory experiments and then found to be combustion products in hydrocarbon/oxygen flames, although in very low concentrations. A few years later the nanotubes were detected. They consist only of 'sheets' of hexagons (no pentagons) which are rolled up to form molecular tubes. They have surprising properties and are still very much under research.

Now scientists have found nanotubes from the 17th century! In the blade of an ancient Damascus sabre. Damascus sabres of that time had extraordinary mechanical properties which were not known in their European couterparts and very sharp cutting edges. The process of making Damascus steel was apparently a very complex one and sadly the secret has been lost over by the end of the 18th centuries. The basic ingredients are believed to be small cakes of steel, named 'wootz', which were produced in ancient India.

Other necessary additions to the steel cakes are small quantities of the elements vanadium, chromium, manganese, cobalt, and nickel and carbon to improve its mechanical properties. It is still unknown how exactly the medieval blacksmiths overcame the inherent brittleness of plates of cementite (iron carbide) that forms in steel with a carbon content of 1-2 wt%. With the help of a high resolution transmission microscope German scientists have found that the Damascus sabre they examined contained cementite nanowires and carbon nanotubes. Fancy, blacksmiths creating nanotubes more than four hundred years ago!

The hypothesis is that the thermal cycling and cyclic forging process eventually caused the catalytic elements present to segregate and thereby enable the carbon nanotubes to form. The nanotubes in turn might have initiated the growth of the cementite nanowires. Cementite nanowires are very probably the cause of the characteristic surface pattern of Damascene sabres and knifes. What role exactly nanowires and nanotubes play regarding the mechanical properties and sharpness of the sabres still awaits further research.

The things Nature thinks up - never ending fascination, isn't it?

Posted by Mausi at November 23, 2006 09:06 PM

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