Plasma blasts way to new materials

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

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.

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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