Diamonds are not forever

Scientists are busy looking for a compound which they say is going to replace diamond as the hardest substance

Published: Friday 31 May 1996

CARBON is a remarkable element; not only is it the basis of life as we know it, it also has an amazing diversity in its inorganic form. From coal to graphite to diamond and carbon-60, the variety in its structure and properties is unmatched. Diamond is the hardest substance known to us and finds many uses in industry because of this unique property. Scientists have been searching for another carbon compound which they believe will be even harder than diamond.

In the late '70s and early '80s, several theoretical studies indicated that there could exist a compound of carbon and nitrogen, carbon nitride, which would be harder than diamond. Since then, many experimental groups have tried to search for this compound without any success. Nevertheless, the search for this family of compounds still continues.

Last year, a team of Chinese scientists reported success in creating the compound by graphite ablation in atomic nitrogen. Ablation is a process of removal of material from the surface by decomposition or vaporisation. Another group in Australia has also reported making a film using sputtering (in which carbon and nitrogen atoms are vaporised from solid targets using ions). The structure of these films does not indicate that they have the desired hardness.

Recently, Charles Lieber and his group have reported some success in pr paring the desired family of compounds. Using the ablation method where a pulsed laser beam vaporises the cafbon from a rotating graphite target and a radio frequency gun breaks down nitrogen molecules to an atomic nitrogen beam, they claim to have made a -thin film which has properties like those addicted. Lieber and co-workers used annealing to get the desired purity. The annealed films have electrical properties like diamond films, but their hardness is much less. (Nature, March 14, 1996).

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