Hafnium

Hafnium is a chemical element; it has symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869, though it was not identified until 1922, by Dirk Coster and George de Hevesy. Hafnium is named after Hafnia, the Latin name for Copenhagen, where it was discovered. The element is obtained only by separation from zirconium, with most of the world's hafnium production coming from processes that also produce zirconium. These processes make use of heavy mineral sands ore deposits, which include the minerals zircon, rutile, and ilmenite, among others.

Hafnium is most often used in alloys with nickel, and was used in larger quantities to produce the control rods used in nuclear reactors. Hafnium's large neutron capture cross section makes it a good material for neutron absorption in control rods in nuclear power plants, but at the same time requires that it be removed from the neutron-transparent corrosion-resistant zirconium alloys used in nuclear reactors. It is ductile, and is also used in filaments and electrodes. Some semiconductor fabrication processes use its oxide for integrated circuits at 45 nanometres (1.8×10⁻⁶ in) and smaller, and superalloys used for special applications can contain hafnium in combination with niobium, titanium, or tungsten.

Pure hafnium is not toxic, but is extremely flammable to the point of being pyrophoric—capable of spontaneous combustion in air. Several industrial processes involved in the production of hafnium have by-products that can be hazardous when released into the environment, and several hafnium compounds have hazards of their own. One nuclear isomer of hafnium, ^178m2Hf, was the source of a controversy for its potential use as a weapon, but it has never been successfully produced for practical use.