Dr. Çetiner’s Blogs (Prof. Dr. Gültekin Çetiner)

A new album titled Nanotechnology Pictures was added to Photo Gallery. If you want to share interesting pictures of Nanotechnology materials, products, carbon nanotubes, or pictures regarding nano processes, you can share in the album. The link for the album is http://www.drcetiner.org/photo_gallery/main.php?g2_itemId=19631

Carbon nanotubes (CNTs) are an allotrope of carbon. They take the form of cylindrical carbon molecules and have novel properties that make them potentially useful in a wide variety of applications in nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.

Nanotubes are members of the fullerene structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 50,000 times smaller than the width of a human hair), while they can be up to several millimeters in length. There are two main types of nanotubes: single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).

The nature of the bonding of a nanotube is described by applied quantum chemistry, specifically, orbital hybridization. Nanotubes are composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, which is stronger than the sp3 bonds found in diamond, provides the molecules with their unique strength. Nanotubes naturally align themselves into “ropes” held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp2 bonds for sp3 bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking.

Carbon nanotubes are no longer the proud boast of 21^st century materials scientists. It appears their discovery was unwittingly pre-empted by mediaeval Muslim sword-smiths whose tough Damascus blades taught the Crusaders the true meaning of cold steel when they fought over the Holy Land. 

Peter Paufler and colleagues at Dresden’s Technical University discovered carbon nanotubes in the microstructure of a 17^th century Damascus sabre. Intriguingly, the nanotubes could have encapsulated iron-carbide nanowires that might give clues to the mechanical strength and sharpness of these swords.

To Europeans, Damascus steel blades seemed magical. Not only could they cut a piece of silk in half as it fell to the floor, they could cleave rocks and their own swords without losing sharpness. The problem facing sword smiths was how to produce steel that was both hard and malleable. Too much carbon and the steel is hard and brittle; too little and it is too soft and malleable to hold an edge when sharpened. Damascus steel blades were forged out of small pure cakes of steel containing around 1.6–1.7 per cent carbon, called wootz. Produced in India, wootz cakes were shipped to Damascus where expert sword smiths fashioned them into blades.

Steel that contains this amount of carbon forms plates of cementite (Fe₃C) which, on its own, makes the steel brittle. However, during the forging process at around 800^oC, small amounts of ‘impurities’ were added containing many first-row transition elements (such as V, Cr, Mn, Co, and Ni), tungsten, and some rare-earths. which together had the effect of forming the cementite into bands. This gave the blades great strength, malleability, and a distinctive wavy-band pattern known as a damask. The skill had been lost by the 18^th century,   when supplies of these ores and impurities ran out.

Micro-structural examination of the bands had previously shown they contained nanowires of Fe₃C. Now, Paufler’s team has uncovered the presence of carbon nanotubes by exposing a small piece of a blade to corrosion by hydrofluoric acid, and examining the effects under a high resolution scanning electron microscope. In some remnants the researchers saw evidence of incompletely dissolved Fe₃C nanowires, suggesting the nanotubes could have encapsulated the nanowires. This would not only have given the blades their renowned strength and sharpness, but also their characteristic banding pattern. ‘The nanotubes probably came from the addition of mandatory organic ingredients we know were added during wootz production, such as wood from the tree Cassia auriculata and leaves from Coltropis gigantean,’ said Paufler.   ‘So, by empirically optimising their blade-treatment procedures, these craftsmen made nanotubes more than 400 years ago.’

Lionel Milgrom

References

•  http://www.rsc.org/chemistryworld/News/2006/November/15110602.asp
• M Reibold et al, Nature, 2006, 444, 286