Transition metal
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. …
Hafnium's existence was predicted by Dmitri Mendeleev in 1869.
In his report on The Periodic Law of the Chemical Elements, in 1869, Dmitri Mendeleev had implicitly predicted the existence of a heavier analog of titanium and zirconium. At the time of his formulation in 1871, Mendeleev believed that the elements were ordered by their atomic masses and placed lanthanum (element 57) in the spot below zirconium. The exact placement of the elements and the location of missing elements was done by determining the specific weight of the elements and comparing the chemical and physical properties.
The X-ray spectroscopy done by Henry Moseley in 1914 showed a direct dependency between spectral line and effective nuclear charge. This led to the nuclear charge, or atomic number of an element, being used to ascertain its place within the periodic table. With this method, Moseley determined the number of lanthanides and showed the gaps in the atomic number sequence at numbers 43, 61, 72, and 75.
The discovery of the gaps led to an extensive search for the missing elements. In 1914, several people claimed the discovery after Henry Moseley predicted the gap in the periodic table for the then-undiscovered element 72. Georges Urbain asserted that he found element 72 in the rare earth elements in 1907 and published his results on celtium in 1911. …
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.
Hafnium reacts in air to form a protective film of hafnium oxide in the monoclinic phase that inhibits further corrosion. …
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.
Much of the hafnium produced is used in the manufacture of control rods for nuclear reactors and as an additive in nickel alloys to increase their heat resistance.
Hafnium has limited technical applications due to a few factors. It is very similar to zirconium, a more abundant element that can be used in most cases, and pure hafnium wasn't widely available until the late 1950s, when it became a byproduct of the nuclear industry's need for hafnium-free zirconium. Additionally, hafnium is rare and difficult to separate from other elements, making it expensive. After the Fukushima disaster reduced the demand for hafnium-free zirconium, the price of hafnium increased significantly from around $500–$600/kg ($227-$272/lb) in 2014 to around $1000/kg ($454/lb) in 2015. Hafnium products, such as tubes and sheets of the metal, could be purchased at €250/kg ($170/lb) in 2009.
The nuclei of several hafnium isotopes can each absorb multiple neutrons. This makes hafnium a good material for nuclear reactors' control rods. Its neutron capture cross section (Capture Resonance Integral Io ≈ 2000 barns) is about 600 times that of zirconium (other elements that are good neutron-absorbers for control rods are cadmium and boron). Excellent mechanical properties and exceptional corrosion-resistance properties allow its use in the harsh environment of pressurized water reactors. The German research reactor FRM II uses hafnium as a neutron absorber. …
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.
Hafnium is a pyrophoric material, and as such fine particles can spontaneously combust upon exposure to air. Hafnium powder is often wetted with at least 25% water by weight to be considered safe - the metal is insoluble in water. Machining hafnium is particularly hazardous because of the potential for fine particles of the metal to be produced and immediately introduced to frictional force. Compounds that contain this metal are rarely encountered by most people. The pure metal is not considered toxic, though it has been observed to accumulate in the liver when injected into rats. Hafnium compounds should be handled as if they were toxic because the ionic forms of metals are normally at greatest risk for toxicity, and limited animal testing has been done for hafnium compounds. Hafnium tetrachloride and hafnium tetrabromide, which are often part of industrial processes that use the element, are of particular note, with both compounds releasing acidic fumes on contact with water (hydrochloric and hydrobromic acid, respectively). Additionally, hafnium tetrachloride has been observed as causing liver damage at high exposure levels.
People can be exposed to hafnium in the workplace by breathing, swallowing, skin, and eye contact. In the United States, the Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for exposure to hafnium and hafnium compounds in the workplace as TWA 0.5 mg/m3 over an 8-hour workday. …