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.
Characteristics
Physical characteristics
Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant and chemically similar to zirconium in that they have the same number of valence electrons and are in the same group. Also, their relativistic effects are similar: The expected expansion of atomic radii from period 5 to 6 is almost exactly canceled out by the lanthanide contraction. Hafnium changes from its alpha form, a hexagonal close-packed lattice, to its beta form, a body-centered cubic lattice, at 2,388 K (2,115 °C; 3,839 °F). The physical properties of hafnium metal samples are markedly affected by zirconium impurities, especially the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity. A notable physical difference between these metals is their density, with zirconium having about one-half the density of hafnium. The most notable nuclear properties of hafnium are its high thermal neutron capture cross section, roughly three orders of magnitude greater than that of zirconium, and that the nuclei of several different hafnium isotopes readily absorb two or more neutrons apiece. Because zirconium is practically transparent to thermal neutrons, it is commonly used for the metal components of nuclear reactors—especially the cladding of their nuclear fuel rods.
Chemical characteristics
Hafnium reacts in air to form a protective film of hafnium oxide in the monoclinic phase that inhibits further corrosion. Despite this, the metal is attacked by hydrofluoric acid and concentrated sulfuric acid, and can be oxidized with halogens or burnt in air. Like its sister metal zirconium, finely divided hafnium can ignite spontaneously in air. The metal is resistant to concentrated alkalis. As a consequence of lanthanide contraction, the chemistry of hafnium and zirconium is so similar that the two cannot be separated based on differing chemical reactions. The melting and boiling points of the compounds and the solubility in solvents are the major differences in the chemistry of these twin elements.
Isotopes
At least 40 isotopes of hafnium have been observed, ranging in mass number from 153 to 192. The five stable isotopes have mass numbers from 176 to 180 inclusive; the primordial 174Hf has a very long half-life of 3.8×1016 years. The extinct radionuclide 182Hf has a half-life of 8.90 million years, and is an important tracker isotope for the formation of planetary cores. No other radioisotope has a half-life over 1.87 years. The longest-lived nuclear isomer 178m2Hf (31 years) was at the center of a controversy for several years regarding its potential use as a weapon. Because of its high energy compared to the ground state 178Hf, the isomer was put under scrutiny as being capable of induced gamma emission, which could be weaponized to produce large amounts of gamma radiation all at once. Applications of the isomer have been frustrated due to the difficulty of producing it without the product being immediately destroyed as well as its extremely high cost.
Occurrence
Hafnium is estimated to make up about between 3.0 and 4.8 ppm of the Earth's upper crust by mass. It does not exist as a free element on Earth, but is found combined in solid solution with zirconium in natural zirconium compounds such as zircon, ZrSiO4, which usually has about 1–4 % of the Zr replaced by Hf. Rarely, the Hf/Zr ratio increases during crystallization to give the isostructural mineral hafnon (Hf,Zr)SiO4, with atomic Hf > Zr. An obsolete name for a variety of zircon containing unusually high Hf content is alvite. A major source of zircon (and hence hafnium) ores is heavy mineral sands ore deposits, pegmatites, particularly in Brazil and Malawi, and carbonatite intrusions, particularly the Crown Polymetallic Deposit at Mount Weld, Western Australia. A potential source of hafnium is trachyte tuffs containing rare zircon-hafnium silicates eudialyte or armstrongite, at Dubbo in New South Wales, Australia.
Production
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