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72. HafniumName: Hafnium
Most zirconium minerals contain 1 to 3% hafnium. Hafnium is a ductile metal with a brilliant silver lustre. Its properties are influenced considerably by the impurities of zirconium present. Of all the elements, zirconium and hafnium are two of the most difficult to separate. Hafnium is a Group 4 transition element. Because hafnium has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used for nuclear reactor control rods. Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory metal nitride (m.p. 3310°C). Hafnium (Latin Hafnia for "Copenhagen") was discovered by Dirk Coster and Georg von Hevesy in 1923 in Copenhagen, Denmark. Soon after, the new element was predicted to be associated with zirconium by using the Bohr theory and was finally found in zircon through X-ray spectroscope analysis in Norway. It was separated from zirconium through repeated recrystallization of double ammonium or potassium fluorides by Jantzen and von Hevesey. Metallic hafnium was first prepared by Anton Eduard van Arkel and Jan Hendrik deBoer by passing tetraiodide vapor over a heated tungsten filament. Care needs to be taken when machining hafnium because when it is divided into fine particles, it is pyrophoric and can ignite spontaneously in air. Compounds that contain this metal are rarely encountered by most people and the pure metal is not normally toxic but all its compounds should be handled as if they are toxic (although there appears to be limited danger to exposed individuals). Quick links
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