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During many decades product engineers and technologists showed no interest in tantalum. As a matter of fact, there existed no tantalum as such: this metal was first obtained in its pure dense form only in XX century. Werner von Bolton, a German chemist, was the first to do that in 1903. Many scientists tried to isolate pure tantalum even earlier, Henri Moissan among them. But the metal powder obtained by Moissan, who reduced tantalum pentoxide Ta2O5 with carbon in an electric furnace, was no pure tantalum – the powder contained 0,5% of carbon.
Tantalum is a heavy metal with its density 16,6 g/cm^3 (worth noting is that to transport a cubic meter of tantalum, six three-ton trucks would be needed). High lasting quality and hardness are combined with perfect plastic properties. Pure tantalum can be well mechanically processed, easily forged, can be reprocessed in thinnest sheets (about 0,04 mm thin) and wire. High thermal conductivity makes its characteristic feature. But the most important physical property of tantalum is rather its refractory quality: it dissolves at about 3000°C (rather at 2996°C), letting by only tungsten and rhenium.
As it became known that tantalum is high refractory, scientists thought to use it as a material for bulb strings. But only several years later tantalum gave way to more refractory and not so expensive tungsten.
Tantalum was not used for practical purposes for another several years. Only in 1922 it started to be used in alternating-current rectifiers (covered with an oxide layer, tantalum lets the current flow only in one direction), and in a year – in radio valves. At the same time industrial ways of tantalum production were being worked out. The first industrially produced tantalum, obtained by an American company in 1922, was a size of a match head. Twenty years later the same company put into operation a specialised factory for tantalum production.
Lately of great importance were extraction ways of tantalum isolation, based upon differentiating solvability of tantalum and niobium salts in some organic solvents. Methyl-isobutyl-ketone and cyclohexanone were experimentally shown to have best extraction properties.
Nowadays, electrolysis of cast fluor-tantalate in graphite, cast-iron and nickeliferous crucibles, serving at the same time as cathodes, makes a basic technique to get metal tantalum – tantalum powder precipitates on the walls of the crucible. Withdrawn from the crucible, this powder is first moulding in planes of the rectangular cross-section (if stock material is meant for rolling in sheets) or in fillets of the square section (for wire dragging), and then al is subject to sintering treatment.
Powdered tantalum, the end product of this reaction, is then sintered. In the late two decades other powder processing ways appeared – arc or induction melting in vacuum and electron-beam melting.
Tantalum unique property is its biological compatibility, т.е. its capability to survive in the body without irritation of surrounding tissues. Due to this tantalum is widely applied in the medical science, mostly in the recovery surgery – to repair a human body. Plates out of this metal are used, for example, at skull injuries – they cover fractures of a brainpan. There was a case, as an artificial ear was made out of a tantalum plane. And the transplanted skin established itself so good, that you would not be able to differentiate a tantalum ear from a real one. Tantalum yarn is sometimes used to compensate for losses of sinewy tissues. With the help of thin tantalum planes surgeons strengthen abdomen walls after an operation. Tantalum staples, similar to those used to make copy-books, connect safely blood vessels. Tantalum meshwork is used in artificial eyes. Fibres out of this metal can replace tendons and are even used to stitch up nerve filaments.
But medical needs consume only 5% of the whole produced tantalum; about 20% is taken up by the chemical industry. The major tantalum part – more than 45% – serves the metallurgy. Tantalum is more and more often used in the last couple of years as an alloying agent in alloy steels – ultra strong, rust-resisting, heat-resisting. Tantalum influences steels similar to niobium. Addition of these elements to typical chromium steels increases their resistance and reduces frailty after hardening and bake-out.
Another very important area of tantalum application is the production of heat-resisting alloys, more and more needed in missile and space technology. 90% tantalum-10% tungsten alloy possesses extraordinary qualities. In the shape of sheets is such an alloy operational at temperatures up to 2500°C, and bulkier parts bear more than 3300°C! This alloy is considered safe to be used in production of blowpipes, tailpipes, system knots of gas check-up and of many responsible units of spaceships. In cases if rocket nozzles are chilled down with liquid corrosion enabling metal (lithium or natrium) you cannot do without a tantalum-tungsten alloy.
Parts out of a tantalum-tungsten alloy grow even more heat-resisting, if they are covered with a tantalum carbide layer (this coating melting point is more than 4000°C). At rockets pilot launches such nozzles were capable of enduring enormous temperatures, at which the alloy itself rusts and disintegrates very quickly.
Another tantalum carbide advantage is its hardness, similar to that of a diamond, thus introducing it into production of carbide tooling for rapid metal cutting.
About one fourth of a tantalum world production goes into electro-technical and electro-vacuum industries. Thanks to the high chemical inactivity of tantalum and its oxide layer, electrodeposited tantalum condensers are very stable in operation, reliable and long-living: their lifetime adds up to12 years and more. Miniature tantalum condensers are used in radio-station transmitters, radar mountings and other electronic systems. It is interesting to know, that these condensers can repair themselves: suppose, a high voltage spark destroyed insulating material – at the same time an insulating oxide layer appears at the breakdown spot, and a condenser goes on working, as if nothing has happened.
Tantalum oxide possesses another property, highly valuable for electro-technical industry: if we let changeable current pass through a solution with submersed tantalum, covered with a thinnest (only a few microns!) oxide layer, it will flow in only one direction – from solution to metal. Tantalum rectifiers are based on this feature and are used in, for example, railroad signal services, telephone switchboards, fire-fighting signal systems.
Tantalum serves as a material for various parts in electro-vacuum appliances. As niobium, it makes a perfect getter, i.е. gas absorber. For example, at 800°C tantalum is capable of absorbing a gas amount, which is 740 times more than his own size. Hot bulb attachments – anodes, grids, indirectly heated cathodes and other warming parts – are also made out of tantalum. Tantalum is very useful for bulbs, which should preserve neat characterization for a long period of time irrespective high temperatures and voltages. Tantalum wire is used in cryotrons, which are superconducting elements needed in, for example, computer engineering. back |
