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nonirritating

Tantalum

[tan-tl-uhm]
Tantalum (formerly tantalium /tænˈtæliəm/) is a chemical element with the symbol Ta and atomic number 73. A rare, hard, blue-grey, lustrous, transition metal, tantalum is highly corrosion-resistant and occurs naturally in the mineral tantalite.

Characteristics

Tantalum is dark, dense, ductile, very hard, easily fabricated, and highly conductive of heat and electricity. The metal is renowned for its resistance to corrosion by acids; in fact, at temperatures below 150 °C tantalum is almost completely immune to attack by the normally aggressive aqua regia. It can be dissolved with hydrofluoric acid or acidic solutions containing the fluoride ion and sulfur trioxide, as well as with a solution of potassium hydroxide. Tantalum's high melting point of 3017 °C (boiling point 5458 °C) is exceeded only by tungsten and rhenium for metals, and carbon.

Applications

The major use for tantalum, as the metal powder, is in the production of electronic components, mainly capacitors and some high-power resistors. Tantalum electrolytic capacitors exploit the tendency of tantalum to form a protective oxide surface layer, using tantalum powder, pressed into a pellet shape, as one "plate" of the capacitor, the oxide as the dielectric, and an electrolytic solution or conductive solid as the other "plate". Because the dielectric layer can be very thin (thinner than the similar layer in, for instance, an aluminium electrolytic capacitor), a high capacitance can be achieved in a small volume. Because of the size and weight advantages, tantalum capacitors are attractive for portable telephones, pagers, personal computers, and automotive electronics.

Tantalum is also used to produce a variety of alloys that have high melting points, are strong and have good ductility. Alloyed with other metals, it is also used in making carbide tools for metalworking equipment and in the production of superalloys for jet engine components, chemical process equipment, nuclear reactors, and missile parts. Because of its ductility, tantalum can be drawn into fine wires or filaments, which are used for evaporating metals such as aluminium.

Due to the fact that it resists attack by body fluids and is nonirritating, tantalum is widely used in making surgical instruments and implants. For example, porous tantalum coatings are used in the construction of orthopedic implants due to tantalum's ability to form a direct bond to hard tissue. The oxide is used to make special high refractive index glass for camera lenses. The metal is also used to make vacuum furnace parts.

Shaped charge and explosively formed penetrator liners have been constructed from tantalum.

History

Tantalum was discovered in Sweden in 1802 by Anders Ekeberg and isolated in 1820 by Jöns Berzelius. Many contemporary chemists believed niobium and tantalum were the same elements until 1844 and later 1866 when researchers showed that niobic and tantalic acids were different compounds. Early investigators were only able to isolate impure metal and the first relatively pure ductile metal was produced by Werner von Bolton in 1903. Wires made with tantalum metal were used for light bulbs until tungsten replaced it.

Its name is derived from the character Tantalus, father of Niobe in Greek mythology, who was punished after death by being condemned to stand knee-deep in water with perfect fruit growing above his head, both of which eternally tantalized him - if he bent to drink the water, it drained below the level he could reach, and if he reached for the fruit, the branches moved out of his grasp. This was considered similar to tantalum's general non-reactivity—it sits among reagents and is unaffected by them. The English word tantalize was named after Tantalus, and tantalum was named after the tantalizing problems posed by the inertness of the element and its compounds.

For many years, the commercial technology for separating tantalum from niobium involved the fractional crystallization of potassium heptafluorotantalate away from potassium oxypentafluoroniobate monohydrate, that had been discovered by Marignac in the 1860s. The method has been supplanted by solvent extraction from fluoride-containing solutions.

Occurrence

There are many species of tantalum minerals, only some of which are so far being used by industry as raw materials: tantalite, microlite, wodginite, euxenite, polycrase. Tantalite [(Fe,Mn) Ta2O6] is the most important mineral for tantalum extraction.

Other minerals include samarskite and fergusonite.

The main production of tantalum occurs in Australia, where the largest producer, Talison Minerals (formerly part of the Sons of Gwalia company), operates the Wodgina mine. Tantalum minerals are also mined in Brazil, Canada, China, Ethiopia and Mozambique. Tantalum is also produced in Thailand and Malaysia as a by-product of tin mining and smelting.

Future large sources of supply, in order of magnitude, are being explored in Saudi Arabia, Egypt, Greenland, China, Mozambique, Canada, Australia, U.S.A., Finland and Brazil.

A comprehensive, 2002 review of non-Australian mines is available.

Tantalite has the same mineral structure as columbite [(Fe,Mn) (Ta,Nb)2O6]; when there is more Ta than Nb it is called tantalite and when there is more Nb than Ta is it called columbite (or niobite). In central Africa the colloquial term coltan is used to refer to the two minerals equally, an example being the Democratic Republic of the Congo which the United States Geological Survey reports in its 2006 yearbook as having produced a little less than 1% of the world's tantalum for the past four years.

Ethical questions have been raised about responsible corporate behaviour, human rights and endangered wildlife, due to the exploitation of resources such as coltan in the conflict regions of the Congo. According to United Nations report smuggling and exportation of coltan helped fuel the war in the Congo, a crisis that has resulted in approximately 5.4 million deaths since 1998 – making it the world’s deadliest documented conflict since WW II.

Several complicated steps are involved in the extraction of tantalum from tantalite, the first being crushing of the mineral and physical concentration by gravity separation which is generally carried out near the mine site. Further processing by chemical separation is generally accomplished by treating the ores with a mixture of hydrofluoric acid and sulfuric acids at over 90°C. This causes the tantalum and niobium to dissolve as complex fluorides and be separated from the impurities. The resulting potassium fluorotantalate salt is generally treated by reduction with molten sodium to produce a coarse tantalum powder.

Compounds

Los Alamos National Laboratory scientists have developed a tantalum carbide-graphite composite material that is one of the hardest materials ever synthesized. Korean researchers have developed an amorphous tantalum-tungsten-copper alloy which is more flexible and two to three times stronger than traditional steel alloys.

There are two tantalum aluminides, TaAl3 and Ta3Al; they are stable, refractory and reflective, and have been proposed as mirror coatings for use in the IR. Tantalum carbide, like the more commonly used tungsten carbide, is a very hard ceramic used in cutting tools. Tantalum (III) nitride is used as a thin film insulator in some microelectronic fabrication processes.

Tantalum prefers the +5 oxidation state; the only oxide is tantalum pentoxide Ta2O5, the normal chloride is the (unstable to hydrolysis) tantalum(V) chloride, and similarly for the other halides. A tantalum-tellurium alloy forms quasicrystals

See also Tantalum compounds.

Isotopes

Natural tantalum consists of two isotopes: 180mTa (0.012%) and 181Ta (99.988%). 181Ta is a stable isotope. 180mTa (m denotes a metastable state) is predicted to decay in three ways: isomeric transition to the ground state of 180Ta, beta decay to 180W, or electron capture to 180Hf. However, any radioactivity of this nuclear isomer was never observed. Only a lower limit on its half life of over 1015 years has been set. The ground state of 180Ta has a half life of only 8 hours.

180mTa is the only naturally occurring nuclear isomer (excluding radiogenic and cosmogenic short-living nuclides). It is also the rarest isotope in the Universe, taking into account the elemental abundance of tantalum and isotopic abundance of 180mTa in the natural mixture of isotopes.

Tantalum has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of 181Ta, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope 182Ta with a half-life of 114.43 days and produce approximately 1.12 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several months. Such a weapon is not known to have ever been built, tested, or used.

Precautions

Compounds containing tantalum are rarely encountered in the laboratory. The metal is highly biocompatible and is used for body implants and coatings, therefore attention may be focused on other elements or the physical nature of the chemical compound.

The only concern in the laboratory with tantalum is with the powder form: as is common with finely divided metal powders this may catch fire.

A single study from 1956 (Oppenheimer, B.S., Oppenheimer, E.T., Danishefsky, I. & Stout, A.P. (1956) Carcinogenic effects of metals in rodent. Cancer Res., 16, 439-441) is the only reference in literature ever linking tantalum to local sarcomas. It is possible the result was due to other factors not considered in the study. The study was quoted in IARC Monograph vol. 74 which includes the following "Note to the reader": "Inclusion of an agent in the Monographs does not imply that it is a carcinogen, only that the published data have been examined.

References

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