Michael,
Funny you should mention that "But he lives in a world where the lifting body has not yet been invented. Where propellant development has ceased. Where bright ideas have stopped being thought of." I had read this yesterday in the London Times and thought you might be interested the potential of this discovery for propellant uses.
The creation of N*, an atomic freak of nature, has stunned the world of chemistry. Nicholas Booth reports
The next big bang: explosive the size of salt grains
There are two sorts of explosion in chemistry - predictable ones and those that are entirely unexpected. To hear Karl Christe describe the events of one day last November, the explosion that destroyed part of his apparatus came as no surprise. "We knew what we were looking for," he says with a studied nonchalance. "We knew it would be very unstable and spectacular."
The "it" in question has the normally staid world of inorganic chemistry agog. For Dr Christe's research team, working for the US Air Force, has formed an atomic freak of nature and one of the most virulently explosive substances ever created.
Known as "Nitrogen 5" (N*), many chemists doubted this form of polynitrogen could ever exist, let alone be created in a laboratory. And yet when it was synthesised in the form of a few grains of salt, the effects were quite spectacular.
"I was quite relieved when it blew up," Dr Christe says. "If you expect something to be that energetic, you're going to have to deliver." Although he is at pains to point out that his work is purely fundamental research, it does promise hitherto unexpected advances in rocket propellants and explosives.
Dr Christe was the leader of a team of 15 chemists, who created this man-made form of nitrogen. Normally regarded as one of the more staid and boring of elements, nitrogen is the invisible gas that forms four fifths of our atmosphere. Gaseous nitrogen comprises two atoms joined together as N*, which is stable and unreactive. It was first isolated in 1772 by, among others, the British scientist Henry Cavendish. Yet its stability makes it useful as a potential explosive: when some of its electrons are stripped, the positively charged fragments (ions) will go to any lengths, even violent ones, to regain stability. A second form of nitrogen was found in the 1890s in the form of azides or N*, which temporarily binds three nitrogen atoms together.
Azides are so unstable that they usually have to be kept in a crystalline form. The lattice structure imprisons each azide so that it cannot come into contact with its neighbour - an explosion results if contact occurs.
A common form is the sodium azide that is found inside the airbags of cars. It is used to generate nitrogen rapidly. When a car undergoes a severe impact, the nitrogen ions come into contact and release the gas remarkably quickly.
The third form created by Dr Christe is more unstable still. N* consists of five nitrogen atoms bonded in a V-shape. Essentially, his team have pulled a rabbit out of a hat: binding more than three nitrogen atoms together was believed to be impossible. When Dr Christe presented his findings to the American Chemical Society last month, the audience were stunned and not because of the explosion.
Dr Christe's work is part of an initiative by the US Air Force to look into highly energetic materials, which could be used for making more efficient rocket fuel. His work is carried out at the Edwards Air Force Base, a vast dry lake in California's Mojave Desert, most familiar as the landing site for the space shuttle and famous as the home of the "right stuff" test pilots.
Although spectacular, all rocket launches are frustrating to their designers. Even the most powerful propellants have a performance ceiling that limits their efficiency. For every ton of equipment hoisted aloft - be it scientific equipment into orbit or a warhead beyond enemy lines - five tons of fuel are needed. This ratio is immutable with conventional rocket chemistry and Dr Christe's team in the Edwards Propulsion Directorate may have found a way around it.
It took four months to synthesise a stable form of the molecular fragment by combining gaseous nitrogen with a negatively charged mixture of arsenic and fluorine. The result was a few grains of a solid compound that, says Dr Christe, looks like table salt. "Except that if you put it in a salt cellar you'd soon know about it," he adds.
Its explosiveness comes from the way in which the positively charged molecular fragment latches on to its nearby brethren. Natural forms of nitrogen have attained the chemical equivalent of Zen, the lowest energy state, where it remains unreactive and stable. What Dr Christe has done is to break a barrier in energy terms. He uses the analogy of a river. "Water doesn't run uphill," he says. "You can make it go up a hill, but you have to put some energy in. Chemically speaking, we have kicked this form of nitrogen up the hill."
Dr Christe is particularly proud that they got it right first time. The research chemistry of today no longer uses just test tubes and blind faith, but rather expensive and complex equipment that takes up whole laboratories. Dr Christe's work has to be carried out in a vacuum, with tubes fashioned from stainless steel and Teflon, and complicated spectrometers which look for the tell-tale signs of unusual molecules on an infinitesimal scale.
Very little was left to chance. The use of supercomputers means that the innumerable permutations of chemical combinations can be predicted onscreen. "We can predict whether the material is stable and if it exists, minimise the processes needed to create it," he says.
Yet actually to create new molecular fragments is more an art than an exact science. The results still lie with "intuition and instinct", for others have failed to create any new forms of nitrogen. Dr Christe has scored some notable successes. In 1986 he succeeded in separating pure fluorine from a compound by chemical means rather than using vast amounts of energy. "I've had a pretty good batting average," Dr Christe adds.
Another surprise is that they have been able to create N* on a microscopic scale and not just as a handful of molecules. In November they produced about 100 milligrams, but now they could create half a gram. Because of its instability, they have to keep it cold and pack it within dry ice at a temperature of -80C. Even so, they are taking no chances: the ampoules, which contain the new form of nitrogen, are made of Teflon.
In the annals of inorganic chemistry, Dr Christe has produced a wonder stuff that some believe may be too unstable to use. But if it could be kept stable and manufactured it would be an ideal fuel for the upper stages of rockets and missiles.
He refuses to be drawn, merely saying he has no idea what it might lead to. "Scientifically, it is very spectacular," he says. "But if you want sure bets, go to Las Vegas. I can't predict what will come out of this work."
You don't have to be a rocket scientist to realise that the field of polynitrogen chemistry may surprise us yet.
I can't imagine what the cost to develop this N* salt is, but it looks to be the future of propellant. Give it 5 - 10 years before they've perfected the creation of the salt.
Interesting times ahead though.
Cheers, Rory. |
