AN ACCIDENTAL explosion in a German physics lab has led to the identification
of a superpowerful explosive. The substance鈥攁n exotic form of
silicon鈥攔eleases seven times as much energy as TNT, and explodes a million
times faster.
鈥淭his might be the strongest explosive ever discovered,鈥� says Dmitri Kovalev,
the physicist who runs the laboratory at the Technical University of Munich in
Garching.
Kovalev and his team were studying the optical properties of porous silicon,
a sponge-like material. The group had cooled the silicon in a vacuum to the
temperature of liquid nitrogen, when suddenly a leak in their equipment allowed
air into the device. The silicon exploded.
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鈥淲e realised immediately that oxygen was condensing on the sample and that
the oxidation was happening explosively,鈥� says Kovalev. Porous silicon has a
layer of hydrogen just one atom thick covering its surface. This creates a
barrier between oxygen atoms and the silicon atoms beneath. But when a single
hydrogen bond breaks, an oxygen atom can bind to the silicon, starting a chain
reaction that rips through the structure like wildfire.
The explosion is so violent because oxidising silicon releases a huge amount
of energy compared with conventional explosives. And using liquid oxygen rather
than gas means there are lots of oxygen atoms at the silicon surface. 鈥淢ost
explosive molecules contain too few oxygen atoms for a complete burn,鈥� says
Kovalev. Because the silicon is sponge-like, it has a very high
surface-area-to-volume ratio and this creates a very efficient burn.
So far nobody has been hurt by exploding silicon and Kovalev believes that it
can be handled safely. 鈥淚t requires a rather special set of conditions to
explode, so in normal use it is quite safe,鈥� he says. His results will appear in
Physical Review Letters.
Leigh Canham is a physicist who specialises in porous silicon and has set up
a company called pSiMedica in Malvern to exploit the material. He thinks that
porous silicon could one day provide thrust for small satellites.
Scientists are currently building thrusters comprising a silicon chip with an
array of holes packed with conventional explosives
(New Scientist, 10 April 1999, p 38).
Each hole can be detonated separately. But 鈥渨hy use
conventional explosives when the silicon itself could provide the thrust,鈥� asks
Canham.
However, Peter Haskins, a theoretical chemist specialising in explosives,
questions whether porous silicon will be ever be useful as an explosive:
鈥淭here鈥檚 certainly a lot of energy in there but whether it鈥檇 be practical to use
at this temperature, I鈥檓 not so sure.鈥�
