Pure hafnium is nottoxic, but is extremelyflammable to the point of beingpyrophoric—capable ofspontaneous combustion in air. Several industrial processes involved in the production of hafnium haveby-products that can be hazardous when released into the environment, and severalhafnium compounds have hazards of their own. Onenuclear isomer of hafnium,178m2Hf, was the source ofa controversy for its potential use as a weapon, but it has never been successfully produced for practical use.
Hafnium is a shiny, silvery,ductilemetal[13] that iscorrosion-resistant and chemically similar to zirconium[14] in that they have the same number ofvalence electrons and are in the same group. Also, theirrelativistic effects are similar: The expected expansion of atomic radii from period 5 to 6 is almost exactly canceled out by thelanthanide 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).[15] 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.[14]
A notable physical difference between these metals is theirdensity, with zirconium having about one-half the density of hafnium. The most notablenuclear properties of hafnium are its highthermalneutron capture cross section, roughly threeorders of magnitude greater than that of zirconium,[13] and that the nuclei of several different hafnium isotopes readily absorb two or moreneutrons apiece.[14] Because zirconium is practically transparent to thermal neutrons, it is commonly used for the metal components of nuclear reactors—especially the cladding of theirnuclear fuel rods.[14]
Hafnium reacts in air to form aprotective film ofhafnium oxide in themonoclinic phase that inhibits furthercorrosion.[16] Despite this, the metal is attacked by hydrofluoric acid and concentrated sulfuric acid, and can be oxidized withhalogens[17] or burnt in air. Like its sister metal zirconium, finely divided hafnium can ignite spontaneously in air.[13] The metal is resistant to concentratedalkalis.[17]
As a consequence oflanthanide 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 thesolubility in solvents are the major differences in the chemistry of these twin elements.[18]
At least 40 isotopes of hafnium have been observed, ranging inmass number from 153 to 192.[19] The five stable isotopes have mass numbers from 176 to 180 inclusive; theprimordial174Hf has a very long half-life of3.8×1016 years.[11]
The longest-livednuclear isomer178m2Hf (31 years) was at thecenter of a controversy for several years regarding its potential use as a weapon. Because of its high energy compared to the ground state178Hf, the isomer was put under scrutiny as being capable ofinduced gamma emission, which could be weaponized to produce large amounts ofgamma radiation all at once.[22] Applications of the isomer have been frustrated due to the difficulty of producing it without the product being immediately destroyed[23] as well as its extremely high cost.[24]
Hafnium is estimated to make up about between 3.0 and 4.8ppm of theEarth's uppercrust by mass.[25]: 5 [26] It does not exist as a free element on Earth, but is found combined insolid solution with zirconium in naturalzirconium compounds such aszircon, 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 mineralhafnon(Hf,Zr)SiO4, with atomic Hf > Zr.[27] An obsolete name for a variety of zircon containing unusually high Hf content isalvite.[28]
Melted tip of a hafnium consumable electrode used in anelectron beamremelting furnace, a 1 cm cube, and an oxidized hafnium electron beam-remelted ingot (left to right)
The heavy mineral sands ore deposits of thetitanium oresilmenite andrutile yield most of the mined zirconium, and therefore also most of the hafnium.[30] 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 of hafnium.[14]
The chemical properties of hafnium and zirconium are nearly identical, which makes the two difficult to separate.[31] The methods first used—fractional crystallization of ammonium fluoride salts[32] or the fractional distillation of the chloride[33]—did not prove suitable for an industrial-scale production. After zirconium was chosen as a material for nuclear reactor programs 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 producing hafnium.[34] Other methods to purify hafnium from zirconium includemolten salt extraction and crystallization offluorozirconates.[35] About half of all hafnium metal manufactured is produced as a by-product of zirconium refinement. The end product of the separation ishafnium(IV) chloride.[36] The purified hafnium(IV) chloride is converted to the metal by reduction withmagnesium orsodium, as in theKroll process.[37]
Further purification is effected by achemical transport reaction developed byArkel and de Boer: In a closed vessel, hafnium reacts withiodine at temperatures of 500 °C (900 °F), forminghafnium(IV) iodide; at a tungsten filament of 1,700 °C (3,100 °F) the reverse reaction happens preferentially, and the chemically bound iodine and hafnium dissociate into the native elements. The hafnium forms a solid coating at the tungsten filament, and the iodine can react with additional hafnium, resulting in a steady iodine turnover and ensuring thechemical equilibrium remains in favor of hafnium production.[18][38]
Due to thelanthanide contraction, theionic radius of hafnium(IV) (0.78 ångström) is almost the same as that ofzirconium(IV) (0.79 angstroms).[39] Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties.[39] Hafnium and zirconium tend to occur together in nature and the similarity of their ionic radii makes their chemical separation rather difficult. Hafnium tends to forminorganic compounds in the oxidation state of +4.Halogens react with it to form hafnium tetrahalides.[39] At higher temperatures, hafnium reacts withoxygen,nitrogen,carbon,boron,sulfur, andsilicon.[39] Some hafnium compounds in lower oxidation states are known.[40]
The whitehafnium oxide (HfO2), with a melting point of 2,812 °C (3,085 K; 5,094 °F) and a boiling point of roughly 5,100 °C (5,400 K; 9,200 °F), is very similar tozirconia, but slightly more basic.[18]Hafnium carbide is the mostrefractorybinary compound known, with a melting point over 3,890 °C (4,163 K; 7,034 °F), and hafnium nitride is the most refractory of all known metal nitrides, with a melting point of 3,310 °C (3,583 K; 5,990 °F).[39]Hafnium carbonitride has the highest known melting point for any material, which is confirmed to be above 4,000 °C (4,270 K; 7,230 °F) by experiment,[43] while calculations predict its melting point to be 4,110 °C (4,380 K; 7,430 °F).[44]
Photographic recording of the characteristicX-ray emission lines of some elements
Hafnium's existence waspredicted by Dmitri Mendeleev in 1869.In his report onThe Periodic Law of the Chemical Elements, in 1869,Dmitri Mendeleev had implicitlypredicted 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 theiratomic masses and placedlanthanum (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.[45]
TheX-ray spectroscopy done byHenry Moseley in 1914 showed a direct dependency betweenspectral line andeffective nuclear charge. This led to the nuclear charge, oratomic number of an element, being used to ascertain its place within the periodic table. With this method, Moseley determined the number oflanthanides and showed the gaps in the atomic number sequence at numbers 43, 61, 72, and 75.[46]
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.[47]Georges Urbain asserted that he found element 72 in therare earth elements in 1907 and published his results onceltium in 1911.[48] Neither the spectra nor the chemical behavior he claimed matched with the element found later, and therefore his claim was turned down after a long-standing controversy.[49] The controversy was partly because the chemists favored the chemical techniques which led to the discovery ofceltium, 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.[49] In 1921,Charles R. Bury[50][51] suggested that element 72 should resemble zirconium and therefore was not part of the rare earth elements group. By early 1923,Niels Bohr and others agreed with Bury.[52][53] These suggestions were based on Bohr's theories of the atom which were identical to chemist Charles Bury,[50] the X-ray spectroscopy of Moseley, and the chemical arguments ofFriedrich Paneth.[54][55]
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 andGeorg von Hevesy were motivated to search for the new element in zirconium ores.[56] Hafnium was discovered by the two in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev.[57][58][59] It was ultimately found inzircon in Norway through X-ray spectroscopy analysis.[60] The place where the discovery took place led to the element being named for the Latin name for "Copenhagen",Hafnia, the home town ofNiels Bohr.[61][62][63] Today, theFaculty of Science of theUniversity of Copenhagen uses in itsseal a stylized image of the hafnium atom.[64]
Hafnium was separated from zirconium through repeated recrystallization of the doubleammonium orpotassium fluorides byValdemar Thal Jantzen and von Hevesey.[32]Anton Eduard van Arkel andJan Hendrik de Boer were the first to prepare metallic hafnium by passing hafnium tetraiodide vapor over a heatedtungsten filament in 1924.[33][38] This process for differential purification of zirconium and hafnium is still in use today.[14]
In 1923, six predicted elements were still missing from the periodic table: 43 (technetium), 61 (promethium), 85 (astatine), and 87 (francium) are radioactive elements and are only present in trace amounts in the environment,[65] thus making elements 75 (rhenium) and 72 (hafnium) the last twostable elements to be discovered. The elementrhenium was found in 1908 byMasataka Ogawa, though its atomic number was misidentified at the time, and it was not generally recognised by the scientific community until its rediscovery byWalter Noddack,Ida Noddack, andOtto Berg in 1925. This makes it somewhat difficult to say if hafnium or rhenium was discovered last.[66]
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.[67] Hafnium products, such as tubes and sheets of the metal, could be purchased at€250/kg($170/lb) in 2009.[13]
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)[68] is about 600 times that of zirconium (other elements that are good neutron-absorbers for control rods arecadmium andboron). Excellent mechanical properties and exceptional corrosion-resistance properties allow its use in the harsh environment ofpressurized water reactors.[34] The German research reactorFRM II uses hafnium as a neutron absorber.[69] It is also common in military reactors, particularly in US naval submarine reactors, to slow reactor rates that are too high.[70][71] It is seldom found in civilian reactors, the first core of theShippingport Atomic Power Station (a conversion of a naval reactor) being a notable exception.[72]
Small additions of hafnium increase the adherence of protective oxide scales on nickel-based alloys. It thereby improves thecorrosion resistance, especially under cyclic temperature conditions that tend to break oxide scales, by inducing thermal stresses between the bulk material and the oxide layer.[74][75][76] An alloy that includes as little as 1% hafnium can withstand temperatures that are 50 °C (122 °F) higher than the same alloy without hafnium.[13]
Hafnium-based compounds are employed ingates of transistors as insulators in the 45 nm (and below) generation ofintegrated circuits fromIntel,IBM and others.[77][78] Hafnium oxide-based compounds are practicalhigh-k dielectrics, allowing reduction of the gate leakage current which improves performance at such scales.[79][80][81]
In most geologic materials,zircon is the dominant host of hafnium (>10,000 ppm) and is often the focus of hafnium studies ingeology.[86] Hafnium is readily substituted into the zirconcrystal lattice, and is therefore very resistant to hafnium mobility and contamination. Zircon also has an extremely low Lu/Hf ratio, making any correction for initial lutetium minimal. Although the Lu/Hf system can be used to calculate a "model age", i.e. the time at which it was derived from a given isotopic reservoir such as thedepleted mantle, these "ages" do not carry the same geologic significance as do other geochronological techniques as the results often yield isotopic mixtures and thus provide an average age of the material from which it was derived.[87][88]
Garnet is another mineral that contains appreciable amounts of hafnium to act as a geochronometer. The high and variable Lu/Hf ratios found in garnet make it useful for datingmetamorphic events.[89]Mass spectrometry also makes use of these ratios to date garnet formed throughigneous events.[90]
Due to its heat resistance and its affinity to oxygen and nitrogen, hafnium is a good scavenger for oxygen and nitrogen in gas-filled andincandescent lamps. Hafnium is also used as the electrode inplasma cutting because of its ability to shed electrons into the air.[91] Hafniummetallocene compounds can be prepared fromhafnium tetrachloride and variouscyclopentadiene-typeligand species. Perhaps the simplest hafnium metallocene is hafnocene dichloride. Hafnium metallocenes are part of a large collection of Group 4transition metal metallocene catalysts that are used worldwide in the production ofpolyolefin resins likepolyethylene andpolypropylene.[92] A pyridyl-amidohafnium catalyst can be used for the controlled iso-selective polymerization of propylene, which can then be combined with polyethylene to make a tougher recycled plastic.[93]
The high energy content of178m2Hf was the concern of aDARPA-funded program in the US. This program eventually concluded that using the178m2Hfnuclear isomer of hafnium to construct high-yield weapons with X-ray triggering mechanisms—an application ofinduced gamma emission—was infeasible because of its expense and difficulty to manufacture.[23] Seehafnium controversy.[24]
Hafnium is apyrophoric 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.[97]Machining hafnium is particularly hazardous because of the potential for fine particles of the metal to be produced and immediately introduced tofrictional force. Compounds that contain this metal are rarely encountered by most people.[98] The pure metal is not considered toxic, though it has been observed to accumulate in theliver when injected into rats.[13] 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.[98]Hafnium tetrachloride andhafnium 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 andhydrobromic acid, respectively). Additionally, hafnium tetrachloride has been observed as causing liver damage at high exposure levels.[13]
Because the mineral zircon is often associated with traces of the radioactive elementsuranium andthorium, the chemically destructive processes used to separate zirconium from hafnium have potential to release these radioactive elements and theirdecay products into the environment along with other reaction wastes. Additionally, synthesis pathways that involve liquid-liquid extraction introduceammonium chloride andsulfate into reaction mixtures, which aseffluent can reduce available oxygen in water sources or producecyanides if it comes into contact withthiocyanate-containing compounds.[13]
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^US 6013553, Wallace, Robert M.; Stoltz, Richard A. & Wilk, Glen D., "Zirconium and/or hafnium oxynitride gate dielectric", published 2000-01-11, assigned toTexas Instruments Inc.
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^Söderlund, Ulf; Patchett, P. Jonathan; Vervoort, Jeffrey D.; Isachsen, Clark E. (March 2004). "The 176Lu decay constant determined by Lu–Hf and U–Pb isotope systematics of Precambrian mafic intrusions".Earth and Planetary Science Letters.219 (3–4):311–324.Bibcode:2004E&PSL.219..311S.doi:10.1016/S0012-821X(04)00012-3.
^Albarède, F.; Duchêne, S.; Blichert-Toft, J.; Luais, B.; Télouk, P.; Lardeaux, J.-M. (5 June 1997). "The Lu–Hf dating of garnets and the ages of the Alpine high-pressure metamorphism".Nature.387 (6633):586–589.Bibcode:1997Natur.387..586D.doi:10.1038/42446.S2CID4260388.
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