|Niobium is variously described as a dull grey, steel grey, or white metal with a yellowish tinge, giving a silver-white fracture. The metallic lustre is remarkably permanent, and is not removed even by prolonged boiling with aqua-regia. After being fused in vacuo, rhombic crystals several millimetres long are formed. The crystal structure of the metal has been studied. The density of the fused metal is 12.7 or 12.75 after being rolled into thin foil. Other figures which have been obtained for less pure samples are much lower, e.g. 8.4, 7.8, 7.06, 7.37. The hardness of the pure metal is about the same as that of wrought iron, and it will not scratch glass or quartz, but the presence of small quantities of carbon, aluminium, or oxygen increases the hardness considerably; a specimen containing about 3 per cent, of carbon scratched quartz easily. It is not very brittle. It can be hammered into foil 0.05 mm. thick, and it is possible, although difficult, to draw it into wire. It can be welded by hammering at a red heat. |
The specific heat between 21° and 100° C. is 0.071, giving 6.61 for the atomic heat, which figure is in conformity with the law of Dulong and Petit. von Bolton's sample melted at 1950° C. in vacuo, but a more recent determination gave the melting-point as 1700° C. Niobium displays weak paramagnetism. The electrical resistance of pure niobium wire, 1 metre long and 1 sq. mm. cross-section, is 0.187 ohm; this figure increases with rising temperature. The metal volatilises and scatters comparatively easily when made to glow in a vacuum, and is therefore unsuitable for use as the filament in electric lamps.