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Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable isotope element to be discovered (rhenium was identified two years later). Hafnium was found by Dirk Coster and Georg von Hevesy in 1923 in Copenhagen, Denmark, and named Hafnia after the Latin name for "Copenhagen".
Hafnium is used in filaments, electrodes, and some semiconductor fabrication processes for integrated circuits at 45 nm and smaller feature lengths. Its large neutron capture cross-section makes hafnium a good material for neutron absorption in control rods in nuclear power plants. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten.
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. Neither the spectra nor the chemical behavior matched with the element found later, and therefore his claim was turned down after a long-standing controversy. The controversy was partly due to the fact that the chemists favored the chemical techniques which led to the discovery of celtium, while the physicists relied on the use of the new X-ray spectroscopy method that proved that the substances discovered by Urbain did not contain element 72. By early 1923, several physicists and chemists such as Niels Bohr and Charles R. Bury suggested that element 72 should resemble zirconium and therefore was not part of the rare earth elements group. These suggestions were based on Bohr's theories of the atom, the X-ray spectroscopy of Mosley, and the chemical arguments of Friedrich Paneth. 
Encouraged by these suggestions and by the reappearance in 1922 of Urbain's claims that element 72 was a rare earth element discovered in 1911, Dirk Coster and Georg von Hevesy were motivated to search for the new element in zirconium ores. Hafnium was discovered by the two in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev. It was ultimately found in zircon in Norway through X-ray spectroscopy analysis. The place where the discovery took place led to the element being named for the Latin name for "Copenhagen", Hafnia, the home town of Niels Bohr. Today, the Faculty of Science of the University of Copenhagen uses in its seal a stylized image of the hafnium atom.
Hafnium was separated from zirconium through repeated recrystallization of the double ammonium or potassium fluorides by Valdemar Thal Jantzen and von Hevesey. Anton Eduard van Arkel and Jan Hendrik de Boer were the first to prepare metallic hafnium by passing hafnium tetra-iodide vapor over a heated tungsten filament in 1924. This process for differential purification of zirconium and hafnium is still in use today.
In 1923, four predicted elements were still missing from the periodic table: 43 (technetium) and 61 (promethium) are radioactive elements and are only present in trace amounts in the environment, thus making elements 75 (rhenium) and 72 (hafnium) the last two unknown non-radioactive elements. Since rhenium was discovered in 1925, hafnium was the next to last element with stable isotopes to be discovered.
Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant and chemically similar to zirconium. The physical properties of hafnium metal samples are markedly affected by zirconium impurities, as these two elements are among the most difficult ones to separate because of their chemical similarity. A notable physical difference between them is their density (zirconium being about half as dense as hafnium). The most notable physical property of hafnium is its high thermal neutron-capture cross-section, and the nuclei of several hafnium isotopes can each absorb multiple neutrons. Hafnium does react in air to form a protective film that prevents any further reaction.
At least 34 isotopes of hafnium have been observed, ranging in mass number from 153 to 186. The five stable isotopes are in the range of 176 to 180. The radioactive isotopes' half-lives range from only 400 ms for 153Hf, to 2.0 petayears (1015 years) for the most stable one, 174Hf.
The nuclear isomer 178m2Hf is also a source of cascades of gamma rays whose energies total 2.45 MeV per decay. It is notable because it has the highest excitation energy of any comparably long-lived isomer of any element. One gram of this pure isotope could release approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT. Possible applications requiring such highly concentrated energy storage are of interest. For example, it has been studied as a possible power source for gamma ray lasers.
As a tetravalent transition metal, hafnium forms various inorganic compounds, generally in the oxidation state of +4. The metal is resistant to concentrated alkalis, but halogens react with it to form hafnium tetrahalides. At higher temperatures, hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Due to the lanthanide contraction of the elements in the sixth period, zirconium and hafnium have nearly identical ionic radii. The ionic radius of Zr4+ is 0.79 Ångström and that of Hf4+ is 0.78 Ångström.
This similarity results in nearly identical chemical behavior and in the formation of similar chemical compounds. The chemistry of hafnium is so similar to that of zirconium that a separation on chemical reactions was not possible, only the physical properties of the compounds differ. The melting points and boiling points of the compounds and the solubility in solvents are the major differences in the chemistry of these twin elements.
Like zirconium, hafnium reacts with halogens forming the tetrahalogen compound with the oxidation state of +4 for hafnium. Hafnium(IV) chloride and hafnium(IV) iodide have some applications in the production and purification of hafnium. The white hafnium oxide (HfO2), with a melting point of 2812 °C and a boiling point of roughly 5100 °C, is very similar to zirconia, but slightly basic. Hafnium carbide is the most refractory binary compound known, with a melting point over 3890 °C, and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of 3310 °C. This has led to proposals that hafnium or its carbides might be useful as construction materials that are subjected to very high temperatures. The mixed carbide tantalum hafnium carbide (Ta4HfC5) possesses the highest melting point of any currently known compound, 4215 °C.
Hafnium is estimated to make up about 5.8 ppm of the Earth's upper crust by weight. It does not exist as a free element in nature, but is found combined in solid solution for zirconium in natural zirconium compounds such as zircon, ZrSiO4, which usually has a 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 old (obsolete) name for a variety of zircon containing unusually high Hf content is alvite.
A major source of zircon (and hence hafnium) ores are 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.
The heavy mineral sands ore deposits of the titanium ores ilmenite and rutile yield most of the mined zirconium, and therefore also most the hafnium.
Zirconium is a good nuclear fuel-rod cladding metal, with the desirable properties of a very low neutron capture cross-section and good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear-reactor applications. Thus, a nearly complete separation of zirconium and hafnium is necessary for their use in nuclear power. The production of hafnium-free zirconium is the main source for hafnium.
Several details contribute to the fact that there are only a few technical uses for hafnium: First, the close similarity between hafnium and zirconium makes it possible to use zirconium for most of the applications; second, hafnium was first available as pure metal after the use in the nuclear industry for hafnium-free zirconium in the late 1950s. Furthermore, the low abundance and difficult separation techniques necessary make it a scarce commodity.
Hafnium and zirconium have nearly identical chemistry, which makes the two difficult to separate. The methods first used — fractionated crystallization of ammonium fluoride salts or the fractionated distillation of the chloride — were not suitable for an industrial-scale production. After zirconium was chosen as material for the nuclear reactor program in the 1940s, a separation method had to be developed. Liquid-liquid extraction processes with a wide variety of solvents were developed, and are still used for the production of hafnium. About half of all hafnium metal manufactured is produced as a by-product of zirconium refinement. The end product of the separation is hafnium(IV) chloride. The conversion to the metal is done through reducing hafnium(IV) chloride with magnesium or sodium in the Kroll process.
HfCl4 + 2 Mg (1100 °C) → 2 MgCl2 + Hf
Further purification is done by a chemical transport reaction developed by Arkel and de Boer: In a closed vessel, hafnium reacts with iodine at temperatures of 500 °C, forming hafnium(IV) iodide; at a tungsten filament of 1700 °C the reverse reaction happens, and the iodine and hafnium are set free. The hafnium forms a solid coating at the tungsten filament, and the iodine can react with additional hafnium, resulting in a steady turn over.
Hf + 2 I2 (500 °C) → HfI4
Most of the hafnium produced is used in the production of control rod for nuclear reactors.
The nuclei of several hafnium isotopes can each absorb multiple neutrons. This makes hafnium a good material for use in the control rods for nuclear reactors. Its neutron-capture cross-section 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 a pressurized water reactors. The German research reactor FRM II uses hafnium as a neutron absorber.
Hafnium is used in iron, titanium, niobium, tantalum, and other metal alloys. An alloy used for liquid rocket thruster nozzles, for example the main engine of the Apollo Lunar Modules is C103, which consists of 89% niobium, 10% hafnium and 1% titanium.
Small additions of hafnium increase the adherence of protective oxide scales on nickel based alloys. It improves thereby the corrosion resistance especially under cyclic temperature conditions that tend to break oxide scales by inducing thermal stresses between the bulk material and the oxide layer.
The electronics industry discovered that hafnium-based compound can be employed in gate insulators in the 45 nm generation of integrated circuits from Intel, IBM and others. Hafnium oxide-based compounds are practical high-k dielectrics, allowing reduction of the gate leakage current which improves performance at such scales.
Due to its heat resistance and its affinity to oxygen and nitrogen, hafnium is a good scavenger for oxygen and nitrogen in gas-filled and incandescent lamps. Hafnium is also used as the electrode in plasma cutting because of its ability to shed electrons into air,
The high energy content of 178m2Hf is the concern of a DARPA funded program in the US. This program should determine the possibility of using a nuclear isomer of hafnium (the above mentioned 178m2Hf) to construct high yield weapons with X-ray triggering mechanisms—an application of induced gamma emission. That work follows over two decades of basic research by an international community into the means for releasing the stored energy upon demand. There is considerable opposition to this program because uninvolved countries might perceive an "isomer weapon gap" that would justify their further development and stockpiling of nuclear weapons. A related proposal is to use the same isomer to power Unmanned Aerial Vehicles, which could remain airborne for months at a time.
Care needs to be taken when machining hafnium because, like its sister metal zirconium, when hafnium is divided into fine particles, it is pyrophoric and can ignite spontaneously in air—similar to that obtained in Dragon's Breath. Compounds that contain this metal are rarely encountered by most people. The pure metal is not considered toxic, but 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.
* Nuclear isomer
1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.