tantalum

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tantalum

(tăn`tələm) [from TantalusTantalus
, in Greek mythology, king of Sipylos, son of Zeus and father of Pelops and Niobe. He was admitted to the society of the gods, but his abominable behavior aroused their anger, and Zeus condemned him to suffer eternally at Tartarus.
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], metallic chemical element; symbol Ta; at. no. 73; at. wt. 180.94788; m.p. 2,996°C;; b.p. 5,400±100°C;; sp. gr. 16.65 at 20°C;; valence +2, +3, +4, or +5. Tantalum is a rare, hard, blue-gray metal with a body-centered cubic crystalline structure. Its chemical characteristics resemble those of niobium, the element above it in Group 5 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. Pure tantalum is extremely ductile and can be drawn into a very thin wire. It is malleable and highly resistant to common acids and to corrosion at temperatures below about 150°C;. Tantalum is obtained chiefly from the mineral tantalite, although it also occurs in euxenite, samarskite, and some other rare minerals. The major sources of tantalum ore are Australia, Brazil, and Canada. Tantalum is almost always found in association with niobium; separation of the two metals is difficult. Major uses of tantalum include electrolytic capacitors, chemical equipment, and parts for vacuum furnaces, aircraft, and missiles. Tantalum was used in the filaments of electric light bulbs and electronic tubes but has been largely replaced by tungsten for these uses. It is often alloyed with other metals; it imparts strength, ductility, corrosion resistance, and a high melting point. Because it is unaffected by body fluids and causes no adverse tissue reactions, it is used in dental and surgical instruments and prostheses. Useful tantalum compounds include the carbide TaC2, an abrasive that is almost as hard as diamond; and the oxide Ta2O5, used in making special highly refractive glass. Tantalum was discovered in 1802 by A. G. Ekeberg but for some time was confused with niobium.

Tantalum

 

Ta, a chemical element of group V of Mendeleev’s periodic system. Atomic number, 73; atomic weight, 180.948. A gray metal with a slight leaden tint. Tantalum occurs in nature in the form of two isotopes: stable 181Ta (99.99 percent) and radioactive 180Ta (0.012 percent, half-life of 1012 years). Of the artificially produced isotopes, radioactive 182Ta (half-life of 115.1 days) is used as a radioactive indicator.

The element was discovered in 1802 by the Swedish chemist A. G. Ekeberg and, owing to the difficulties encountered in obtaining the element in pure form, was named after Tantalus, a figure in Greek mythology. Ductile tantalum metal was first produced in 1903 by the German chemist W. von Bolton.

Occurrence. The average abundance of tantalum in the earth’s crust (clarke) is 2.5 × 10–4 percent by weight. It is a characteristic element of granitic and sedimentary layers (average abundance reaching 3.5 × 10–4 percent); deeper in the crust, however, and especially in the upper mantle, little tantalum is found (1.8 × 10–6 percent in ultrabasic rocks). Tantalum is dispersed in the biosphere and in most magmatic rocks; its content in the hydrosphere and in organisms has not been established. There are 17 known tantalum minerals and more than 60 tantalum-bearing minerals, with the origin of all these minerals linked to magmatic activity (tantalite, columbite, loparite, pyrochlore). In minerals, tantalum occurs in association with niobium because of the similar physical and chemical properties of the two elements. Tantalum ores are found in the pegmatites of granitic and alkali rocks, in carbonatites, and in hydrothermal veins. Placer deposits of the ores have the greatest practical value.

Physical and chemical properties. Tantalum has a body-centered cubic lattice, with a = 3.296 angstroms (Å). The atomic radius is 1.46 Å, and the ionic radius of Ta2+ is 0.88 Å and of Ta5+ is 0.66 Å. The element has a density of 16.6 g/cm3 at 20°C, a melting point of 2996°C, a boiling point of 5300°C, a specific heat in the temperature range 0°–100°C of 0.142 kilojoule/kg–°K (0.034 calorie/g–°C), a thermal conductivity in the range 20°–100°C of 54.47 watts/m·°K (0.13 calorie/cm·sec·°C), and a coefficient of linear thermal expansion of 8.0 × 10–6(20°–1500°C). The electrical resistivity of 0°C is 13.2 × 10–8 ohm·m and at 2000°C is 87 × 10–8 ohm·m. At 4.38°K the element becomes a superconductor. Tantalum is paramagnetic, with a magnetic susceptibility of 0.849 × 10–6 (18°C). Pure tantalum is a ductile metal that can be easily cold-pressed. It can be cogged by 99 percent without intermediate annealing. No transition from a ductile to a brittle state is observed upon cooling tantalum to – 196°C. The elastic modulus of the element is 190 giganewtons/m2 (190 × 102 kilograms-force/mm2) at 25°C, and the tensile strength of high-purity annealed tantalum is 206 meganewtons/m2 (20.6 kilograms-force/mm2) at 27°C and 190 meganewtons/m2 (19 kilograms-force/mm2) at 490°C; elongation is 36 percent (27°C) and 20 percent (490°C). The Brinell hardness of pure recrystallized tantalum is 500 meganewtons/m2 (50 kilograms-force/mm2). The properties of tantalum depend largely on the degree of purity: admixtures of hydrogen, nitrogen, oxygen, and carbon make the metal brittle.

The electronic configuration of the outer subshells of the Ta atom is 5d36s2. The most characteristic oxidation state for tantalum is +5. Compounds with a lower oxidation state, for example, TaCl4, TaCl3, and TaCl2, are known, but the formation of such compounds is less characteristic of tantalum than of niobium.

Under normal conditions, tantalum, like niobium, is only slightly reactive. Pure, consolidated tantalum is stable upon exposure to air, with oxidation commencing at 280°C. The element’s only stable oxide, the pentoxide Ta2O5, exists in two modifications: the white α-form below 1320°C and the gray β-form above 1320°C; the oxide is acidic in nature. At approximately 250°C, tantalum forms a solid solution with hydrogen containing up to 20 atomic percent of hydrogen at 20°C; here, tantalum becomes brittle. At 800°–1200°C in a high vacuum, hydrogen separates from the metal and ductility is restored. At approximately 300°C, tantalum reacts with nitrogen to form a solid solution and the nitrides Ta2N and TaN; in a high vacuum at temperatures above 2200°C, the absorbed nitrogen separates from the metal. The existence of three phases has been established in the Ta═C system at temperatures up to 2800°C: a solid solution of carbon in tantalum, a phase comprising the lower carbide Ta2C, and a phase of the higher carbide TaC. Tantalum reacts with halogens at temperatures above 250°C (with fluorine at room temperature) to form halides, mainly of the type TaX5 (where X = F, Cl, Br, I). Upon heating, tantalum reacts with C, B, Si, P, Se, Te, H2O, CO, CO2, NO, HCl, H2S.

Pure tantalum is exceptionally stable to the action of many liquid metals, including Li, Pb, Na, and K, alloys containing Na or K, and U═Mg and Pu═Mg alloys. Tantalum is characterized by an extremely high resistance to the corrosive action of most inorganic and organic acids, including nitric, hydrochloric, sulfuric, and perchloric acids and aqua regia, and of many other aggressive media. However, tantalum will react with fluorine, hydrogen fluoride, hydrofluoric acid, mixtures of hydrofluoric and nitric acid, and solutions and melts of alkalies. The known salts of tantalic acids include tantalates of the general formula xMe2O·yTa2O5·H2O, metatantalates MeTaO3, orthotantalates Me3TaO4, and salts of the type Me5TaO5, where Me is an alkali metal; in addition, pertantalates are formed in the presence of hydrogen peroxide. The most important tantalates of alkali metals are KTaO3 and NaTaO3; these salts are ferroelectrics.

Preparation. Tantalum-bearing ores are rare, complex, and deficient in tantalum. Dressing processes are carried out on ores containing up to hundredths of a percent of (Ta, Nb)2O5 and on slags from the reduction smelting of tin concentrates. The main starting materials in the preparation of tantalum and tantalum alloys and compounds are tantalite and loparite concentrates, which contain approximately 8 percent Ta2O5 and 60 percent or more Nb2O5. The three stages involved in the treatment of the concentrates are the decomposition of the ore, the separation of Ta and Nb and preparation of their pure compounds, and the reduction and purification of Ta. Tantalite concentrates are decomposed by acids or alkalies, whereas loparite concentrates are chlorinated. The separation of Ta and Nb with the derivation of pure compounds of the two elements is effected through extraction, for example, with tributyl phosphate from hydrofluoric acid solutions, or through the fractionation of chlorides.

The processes available for obtaining metallic tantalum include the reduction of the element from Ta2O5 by carbon black in one or two stages (with preliminary production of TaC from a mixture of Ta2O5 and carbon black in a CO or H2 atmosphere at 1800°–2000°C), the reduction by electrolysis from melts containing K2TaF7 and Ta205, and the reduction by sodium of K2TaF 7upon heating. There are also processes involving the thermal dissociation of tantalum chloride or the reduction of tantalum from the chloride using hydrogen. The metal is consolidated either by vacuum arc melting, electron-beam melting, or plasma-arc melting or by the techniques of powder metallurgy. The ingots and sintered bars obtained are then cold-pressed. Single crystals of exceptionally pure tantalum are obtained by electron-beam zone refining.

Use. Tantalum possesses many valuable properties, including good ductility, strength, weldability, corrosion resistance at moderate temperatures, refractoriness, low vapor pressure, high heat transfer coefficient, low electron work function, and ability to form an anode film (Ta2O5) with specific dielectric characteristics. In addition, tantalum is inert to body fluids and tissues. Thus, the element finds use in electronics, machine building (chemical equipment), nuclear engineering, metallurgy (refractory alloys, stainless steel), and medicine; tantalum in the form of TaC is used in the production of hard alloys.

Pure tantalum serves in the manufacture of electric capacitors for semiconductor devices, vacuum tube components, corrosion-resistant equipment for the chemical industry, and spinnerets for man-made filaments. In addition, pure tantalum is used in making laboratory ware, crucibles for the melting of metals (rare-earth) and alloys, heaters for high-temperature furnaces, and heat exchangers for nuclear power systems. In surgery, plates and gauze of tantalum are used in repairing bones and nerves, and the element can also be used in making suture wire. Alloys and compounds of tantalum have a variety of uses.

REFERENCE

Zelikman, A. N., and G. A. Meerson. Metallurgiia redkikh metallov. Moscow, 1973.

O. P. KOLCHIN

tantalum

[′tant·əl·əm]
(chemistry)
A metallic transition element, symbol Ta, atomic number 73, atomic weight 180.9479; black powder or steel-blue solid soluble in fused alkalies, insoluble in acids (except hydrofluoric and fuming sulfuric); melts about 3000°C.
(metallurgy)
A lustrous, platinum-gray ductile metal used in making dental and surgical tools, pen points, and electronic equipment.

tantalum

a hard greyish-white metallic element that occurs with niobium in tantalite and columbite: used in electrical capacitors in most circuit boards and in alloys to increase hardness and chemical resistance, esp in surgical instruments. Symbol: Ta; atomic no.: 73; atomic wt.: 180.9479; valency: 2, 3, 4, or 5; relative density: 16.654; melting pt.: 3020°C; boiling pt.: 5458±100°C