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Physical Properties Tantalum

Tantalum is a white metal with a greyish tinge and is very similar to platinum in colour and general appearance. When it is heated to 1600° C. in vacuo it assumes a crystalline form. Examination of the powdered metal by X-ray analysis has shown that the arrangement of the atoms is on the plan of a body-centred cube of side 3.272 Å, obtained by dividing the space of a crystal into equal closely packed cubes and placing an atom at each cube corner and each cube centre; the distance between the nearest atoms is 2.833 Å. The specific gravity of the fused metal is 16.6; a sample drawn into wire 0.05 mm. diameter had a density of 16.5; the density calculated from X-ray data is 17.09.

Pure tantalum possesses valuable mechanical properties, in which it closely resembles molybdenum and tungsten. It can be worked in the cold state to a remarkable extent without being previously annealed, although it is subject to strain-hardening, much like copper and silver. Photomicrographs of cold-worked and annealed tantalum are given in the reference cited. The metal can be rolled into sheet 0.1 mm. thick or even less, and can be drawn into filament wire 0.03 mm. diameter. The ultimate tensile strength of the hard-drawn wire (0.08 mm. diameter) is 93 kilograms per square millimetre or 57 tons per square inch; this figure increases as the diameter of the wire diminishes, reaching 150 to 160 kilograms per square millimetre for wire 0.05 mm. thick, which is considerably higher than the value for hard-drawn copper, nickel, or platinum; the tensile strength of a carbon steel containing 0.89 per cent, of carbon is about 52 tons per square inch. Young's modulus of elasticity for tantalum wire (0.08 mm. diameter) in kilograms per square millimetre is 19,000. The Brinell hardness number is 46, compared with 290 for tungsten, 147 for molybdenum, and from 100 to 300 for steels; the hardness is increased by the addition of small quantities of silicon, carbon, boron, aluminium, tin or titanium, and by traces of oxygen. When heated to redness and hammered, the metal becomes so hard through the formation of an oxide film that a diamond will not scratch it, and at the same time it retains its toughness. When heated in a poor vacuum, tantalum becomes very brittle and is easily powdered; its ductility and toughness are restored by heating in a good vacuum to a white heat. The compressibility under pressure has been determined; the average fractional change in volume caused by 1 megabar pressure between applications of 100 and 500 megabars at 20° C. is 0.54×10-6. The linear coefficient of expansion per ° C. from 0° to 400° C. is approximately 6.46×10-6, as determined with a Fizeau dilatometer; this figure is less than that given by platinum and enables tantalum to be fused into glass. An earlier determination over the range 0° to 50° C. gave a linear expansion of 7.9×10-6. The cubical coefficient of expansion is 24×10-6; the atomic volume is 10.9, and the calculated internal pressure of the atom, using the last two figures, is 455,000 megabars.

The melting-point of tantalum lies between 2850° and 3000° C.; the most recent determinations are: 2798° C., 2850° C., 2910° C., 3000° C., 3030° C. The specific heat in calories per gram per degree between 16° and 100° C. is 0.0365, and increases at higher temperatures; the coefficient of thermal conductivity is 0.130 calories per cc. at 17° C. and 0.129 calories at 100° C. Tantalum displays weak paramagnetism; the magnetic susceptibility decreases with the temperature. The electrical behaviour of tantalum has received considerable attention because of its application in the manufacture of electric lamps. The electrical resistance of 1 cc. of a sample which had been well annealed by heating for a period of between 100 and 200 hours in vacuo at 1900° C. was 14.6 microhms at 20° C.; the corresponding figure for copper is 1.87. The electrical resistance increases with increase in temperature; the variation over the range from -180° to 2000° C. has been measured by Pirani.1 At 2000° C. the resistance is more than six times that at room temperatures; at -180° C. it is reduced to approximately one-third. The influence on the electrical conductivity of pressures varying from 700 to 2000 atmospheres has also been measured. For an investigation of the Hall effect and allied phenomena, see the reference cited. Tantalum has been employed as one of the elements in thermocouples, with copper or tungsten as the other element; a tantalum/tungsten thermocouple is stated to be four times as sensitive as platinum/platinum-rhodium.

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