Direct current arc plasmas are contributing to the
production of new, very high performance materials in industrial
quantities. Tom Shelley reports
Particles fired in a plasma at supersonic speeds are producing
metal matrix composites of superior properties at surprisingly
modest cost.
The source for this claim is Sergey Alexeev, who has spent his
entire working life inventing and studying processes based on DC
arc plasmas. As chief executive of Flagman, a small
high-technology company spun out of what was then Leningrad
Polytechnic, Alexeev heads a team that has found ways of
injecting solid particles into plasma jets and accelerating them
to 200, 300 and even 500m/s. He says that it is not too difficult
to accelerate small particles - about 20 to 30 microns across -
at such speeds but his team is achieving similar speeds with
particles 40 to 100 microns across - which are up to ten times
heavier.
Now the company is targeting the tricky task of scaling up
production of Metal Matrix Composites (MMCs) to around 1/4
million tonnes. At the current scale of production a 100kW torch
can produce a 50g batch of composite in 30 minutes, with a 10%
solids content. It is possible to make composites with higher
solids content but doing so takes proportionately longer.
Examples of MMC components shown by Flagman at the recent Russian
Venture Fair included what were unquestionably missile fins, but
the company is now focussed on reducing costs to those more
appropriate to automotive production.
The author interviewing Sergey Alexeev at the Russian Venture Fair in St Petersburg in October 2001. The fair was organised by the Russian Venture Capital Association www.rvca.ru
According to Alexeev, one Korean car company customer wants to
include two 5kg MMC components in new models which it is hoped
will have production runs of up to 250,000 vehicles. So Flagman's
attendance at the Russian Venture Fair was primarily to attract
investment to scale up MMC production. However, Eureka was told
that a number of the machines already developed are capable of
producing the necessary quantities at an acceptable price. But
scaling up would undoubtedly result in significant cost
reductions and a much greater market potential.
Plasma gases can be argon, helium, nitrogen, hydrogen or their
mixtures in order to eliminate oxidation of the material being
sprayed. In such cases, the cost of the plasma gases is about 20
to 40% of overall costs. Flagman is the only company known to Eureka
that is also developing plasma spraying processes based on
compressed air. Alexeev says compressed air plasma spraying
technology can be used not only for spraying oxides but also to
spray chemically active metals such as magnesium, aluminium,
titanium and other's without oxidation in an open atmosphere.
Reactive gases, such as nitrogen, can be used to generate
nanometre-sized particles of aluminium nitride and to undertake
other plasma chemical reactions.
Ideally, metal oxide particles for making MMCs should be 5 to 160
microns across. The advantage of using plasma injection is that
the reinforcing particles are widely distributed in the composite
and tend to stay in the right places. Alternative technologies
based on infiltrating ceramic fibrous mats leave regions of
non-reinforced metal, especially in complex fabrications such as
diesel engine pistons.
Buckyballs and diamond
As well as injecting metal oxides and nitrides, it may be
possible to inject particles of much more exotic materials,
particularly fullerenes (also known as 'buckyballs'),
diamond-like carbon and diamond. Because fullerenes - clusters of
carbon atoms held in a hollow spherical structure - do not
decompose below 1,000 deg C, Alexeev believes it may be possible
to use them in MMCs.
He says Flagman has already developed a special power source
capable of making 100g/hr of fullerene containing soot. The
plasma gun involved in the process is rated at about 100kW, yet
is only the size of a packet of cigarettes. The plan is to
increase production capability ten-fold in the near future.
Similarly, it may be possible to make composites reinforced with
diamond or diamond-like particles. Russian scientists invented
the low pressure, radio frequency processes by which diamond and
diamond-like carbon coatings are laid down. It should, therefore,
come as no surprise that they have now cracked the problem of
increasing production quantities.
Other applications
New lubricants can be created which contain vast numbers of
fullerenes functioning as nanometre-sized ball bearings. And when
a fullerene is doped by inserting the right amount of potassium
or caesium into empty spaces within the crystal, it becomes not
just a superconductor but the best organic superconductor known.
Other possible applications include optical devices, chemical
sensors and chemical separation devices.
Experiments also suggest that fullerenes incorporating alkali
metals possess catalytic properties resembling those of platinum.
The C60 molecule can also absorb almost one hydrogen atom for
each carbon atom, without disrupting the structure. This suggests
that fullerenes may be a better storage medium for hydrogen than
metal hydrides, hence possibly a key factor in the development of
non-polluting cars powered by hydrogen fuel cells. Hydrogen
loaded fullerene powder could be used to fuel cars in a similar
manner to fuelling with petrol, with the spent powder
subsequently recovered to be re-loaded.
Fullerenes may also be used as a sorbent for radioactive waste.
And a thin layer of the C70 fullerene, when deposited on a
silicon chip, seems to provide a vastly improved template for
growing thin films of diamond. It's also worth noting that Xerox
owns patents for using 'buckyballs' to improve the resolution of
photocopies. They are 1,000 times smaller than the particles
currently used in conventional photocopier toner.
Plasmas blast bacteria or steel
As well as being able to inject particles into molten metal, the
combinations of very high temperatures and extreme speeds allow
the plasma torches to cut through steel up to 180mm thick. They
can also be used to deposit coatings and do fairly fearsome
things to surfaces.
Plasma torches can be used to sterilise water. Apparently five
seconds from a 40kW plasma jet kills everything present in 40
litres of water. The torch uses an oxygen plasma and is inserted
in the water. Its main function is to produce ozone in situ,
which immediately dissolves in the water, killing the bacteria
and other unwanted organisms. At the present time, the equipment
is only suitable for treating closed volumes of water, such as in
swimming pools or the drinking water tanks of boats. On a larger
scale, the technology might be attractive for water utilities.
Concerns about transporting chlorine about the countryside are
growing, in case some of it might be hijacked by terrorists.
There are also worries about generating ozone gas and then using
it, since it is a known carcinogen.
( More information at www.npf-flagman.inc.ru
)
Design Pointers
Metal Matrix Composites of superior quality can be made by firing
ceramic particles at sonic or supersonic speeds into molten metal
from a radio frequency plasma gun
Other DC arc plasma guns have been developed which can
mass-produce fullerenes ('buckyballs') and powders of diamond and
diamond-like carbon
The same technology can also be used to cut steel up to 180mm
thick or sterilise water in a manner less hazardous than by using
chlorine or ozone gas
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