AN EXOTIC type of nitrogen could form the next generation of safe, energy-dense rocket fuels or explosives, provided someone can figure out how to make it.
In its most common state, nitrogen is an inert gas whose molecules contain two atoms held together by a triple covalent bond. But nitrogen is also a component in many explosives, such as TNT.
Twenty years ago physicists predicted that nitrogen could form another molecule consisting of four atoms connected to one another with single covalent bonds. This 鈥減olymeric鈥� nitrogen would release a tremendous amount of energy as it transformed to its inert form. Estimates suggest it would be more powerful than TNT or HMX, a high explosive used in military applications. Also, the only by-product would be inert nitrogen, making it attractive as a fuel.
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鈥淧olymeric nitrogen would release a tremendous amount of energy and the only by-product would be inert nitrogen鈥�
In 2004, researchers at the Max Planck Institute for Chemistry in Mainz, Germany, reported they had synthesised this molecule at extremely high pressures and temperatures: 110 gigapascals and 2000 kelvin. However, when they reduced the pressure the molecule collapsed to inert nitrogen.
Although other forms of single-bonded nitrogen have been proposed, no one has managed to synthesise one that remains stable at ambient pressures and temperatures.
Now Hakima Abou-Rachid of Defence Research and Development Canada in Valcartier, Quebec, and colleagues suggest another approach. Using computer simulations, they calculate that an eight-atom nitrogen chain could be stable at ambient temperatures and pressures if it could be formed inside a carbon nanotube (Physical Review Letters, ).
Although the nitrogen chain would be unstable by itself, calculations show that the carbon nanotube would stabilise it. The nanotube and the nitrogen do not form a covalent bond. Instead, the attraction between a net positive charge on the inner wall of the nanotube and a net negative charge on the nitrogen chain stabilises the polymer.
Abou-Rachid says that she and her colleagues have experimentally tried to deposit nitrogen on carbon nanotubes, but this has not yet produced the polynitrogen.
, who researches advanced propellants at the NASA Glenn Research Center in Cleveland, Ohio, says the material is theoretically interesting, but faces a number of hurdles. First, someone has to make it and show it can be stored and handled safely. It would also have to be made cost-effectively in large quantities.
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