Chem: Silicon Explosives now

Date: Thu Aug 02 2001 - 06:43:13 MDT

Superpowerful explosive arrives with a bang
19:00 01 August 01
Justin Mullins
An accidental explosion in a German physics lab has led to the identification
of a superpowerful explosive. The substance - an exotic form of silicon -
releases seven times as much energy as TNT, and explodes a million times

"This might be the strongest explosive ever discovered," 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.

"We realised immediately that oxygen was condensing on the sample and that
the oxidation was happening explosively," says Kovalev.

Chain reaction

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

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.

"Most explosive molecules contain too few oxygen atoms for a complete burn,"
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. "It requires a rather special set of conditions to
explode, so in normal use it is quite safe," he says. His results will appear
in Physical Review Letters.

Satellite thrusters

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 "why use conventional explosives when the silicon itself could provide
the thrust," asks Canham.

However, Peter Haskins, a theoretical chemist specialising in explosives,
questions whether porous silicon will be ever be useful as an explosive:
"There's certainly a lot of energy in there but whether it'd be practical to
use at this temperature, I'm not so sure."
19:00 01 August 01


This archive was generated by hypermail 2b30 : Fri Oct 12 2001 - 14:40:01 MDT