Hafnium

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Hafnium, 72Hf
Hf-crystal bar.jpg
Hafnium
Pronunciation /ˈhæfniəm/ (HAF-nee-əm)
Appearancesteel gray
Standard atomic weight Ar, std(Hf)178.486(6) [1]
Hafnium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Zr

Hf

Rf
lutetiumhafniumtantalum
Atomic number (Z)72
Group group 4
Period period 6
Block d-block
Element category   Transition metal
Electron configuration [ Xe ] 4f14 5d2 6s2
Electrons per shell2, 8, 18, 32, 10, 2
Physical properties
Phase at  STP solid
Melting point 2506  K (2233 °C,4051 °F)
Boiling point 4876 K(4603 °C,8317 °F)
Density (near r.t.)13.31 g/cm3
when liquid (at m.p.)12 g/cm3
Heat of fusion 27.2  kJ/mol
Heat of vaporization 648 kJ/mol
Molar heat capacity 25.73 J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)268929543277367941944876
Atomic properties
Oxidation states −2, +1, +2, +3, +4 (an  amphoteric oxide)
Electronegativity Pauling scale: 1.3
Ionization energies
  • 1st: 658.5 kJ/mol
  • 2nd: 1440 kJ/mol
  • 3rd: 2250 kJ/mol
Atomic radius empirical:159  pm
Covalent radius 175±10 pm
Color lines in a spectral range Hafnium spectrum visible.png
Color lines in a spectral range
Spectral lines of hafnium
Other properties
Natural occurrence primordial
Crystal structure hexagonal close-packed (hcp)
Hexagonal close packed.svg
Speed of sound thin rod3010 m/s(at 20 °C)
Thermal expansion 5.9 µm/(m·K)(at 25 °C)
Thermal conductivity 23.0 W/(m·K)
Electrical resistivity 331 nΩ·m(at 20 °C)
Magnetic ordering paramagnetic [2]
Magnetic susceptibility +75.0·10−6 cm3/mol(at 298 K) [3]
Young's modulus 78 GPa
Shear modulus 30 GPa
Bulk modulus 110 GPa
Poisson ratio 0.37
Mohs hardness 5.5
Vickers hardness 1520–2060 MPa
Brinell hardness 1450–2100 MPa
CAS Number 7440-58-6
History
Namingafter Hafnia . Latin for: Copenhagen, where it was discovered
Prediction Dmitri Mendeleev (1869)
Discovery and first isolation Dirk Coster and George de Hevesy (1922)
Main isotopes of hafnium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
172Hf syn 1.87 y ε 172Lu
174Hf0.16%2×1015 y α 170Yb
176Hf5.26% stable
177Hf18.60%stable
178Hf27.28%stable
178m2 Hfsyn31 y IT 178Hf
179Hf13.62%stable
180Hf35.08%stable
182Hfsyn8.9×106 y β 182Ta
Folder Hexagonal Icon.svg  Category: Hafnium
| references

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 many zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869, though it was not identified until 1923, by Coster and Hevesy, making it the last stable element to be discovered. Hafnium is named after Hafnia, the Latin name for Copenhagen, where it was discovered. [4] [5]

Contents

Hafnium is used in filaments and electrodes. Some semiconductor fabrication processes use its oxide for integrated circuits at 45 nm and smaller feature lengths. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten.

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.

Characteristics

Physical characteristics

Pieces of hafnium Hafnium bits.jpg
Pieces of hafnium

Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant and chemically similar to zirconium [6] (due to its having the same number of valence electrons, being in the same group, but also to relativistic effects; the expected expansion of atomic radii from period 5 to 6 is almost exactly cancelled 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 2388 K. [7] 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. [6]

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 and that the nuclei of several different hafnium isotopes readily absorb two or more neutrons apiece. [6] In contrast with this, zirconium is practically transparent to thermal neutrons, and it is commonly used for the metal components of nuclear reactors – especially the cladding of their nuclear fuel rods.

Chemical characteristics

Hafnium dioxide Hafnium(IV) oxide.jpg
Hafnium dioxide

Hafnium reacts in air to form a protective film that inhibits further corrosion. The metal is not readily attacked by acids but can be oxidized with halogens or it can be burnt in air. Like its sister metal zirconium, finely divided hafnium can ignite spontaneously in air. The metal is resistant to concentrated alkalis.

The chemistry of hafnium and zirconium is so similar that the two cannot be separated on the basis of differing chemical reactions. 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. [8]

Isotopes

At least 34 isotopes of hafnium have been observed, ranging in mass number from 153 to 186. [9] [10] The five stable isotopes are in the range of 176 to 180. The radioactive isotopes' half-lives range from only 400  ms for 153Hf, [10] to 2.0 petayears (1015 years) for the most stable one, 174Hf. [9]

The nuclear isomer 178m2Hf was at the center of a controversy for several years regarding its potential use as a weapon.

Occurrence

Zircon crystal (2x2 cm) from Tocantins, Brazil Zircao.jpeg
Zircon crystal (2×2 cm) from Tocantins, Brazil

Hafnium is estimated to make up about 5.8 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. [11] An obsolete name for a variety of zircon containing unusually high Hf content is alvite. [12]

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. [13]

Hafnium reserves have been infamously estimated to last under 10 years by one source if the world population increases and demand grows. [14] In reality, since hafnium occurs with zirconium, hafnium can always be a byproduct of zirconium extraction to the extent that the low demand requires.[ citation needed ]

Production

Melted tip of a hafnium consumable electrode used in an electron beam remelting furnace, a 1 cm cube, and an oxidized hafnium electron beam-remelted ingot (left to right) Hafnium ebeam remelted.jpg
Melted tip of a hafnium consumable electrode used in an electron beam remelting furnace, a 1 cm cube, and an oxidized hafnium electron beam-remelted ingot (left to right)

The heavy mineral sands ore deposits of the titanium ores ilmenite and rutile yield most of the mined zirconium, and therefore also most of the hafnium. [15]

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. [6]

Hafnium oxidized ingots which exhibit thin film optical effects. Hafnium pellets with a thin oxide layer.jpg
Hafnium oxidized ingots which exhibit thin film optical effects.

The chemical properties of hafnium and zirconium are nearly identical, which makes the two difficult to separate. [16] The methods first used — fractional crystallization of ammonium fluoride salts [17] or the fractional distillation of the chloride [18] — have not proven suitable for an industrial-scale production. After zirconium was chosen as 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 the production of hafnium. [19] 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. [20] The purified hafnium(IV) chloride is converted to the metal by reduction with magnesium or sodium, as in the Kroll process. [21]

HfCl4 + 2 Mg (1100 °C) → 2 MgCl2 + Hf

Further purification is effected 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. [8] [22]

Hf + 2 I2 (500 °C) → HfI4
HfI4 (1700 °C) → Hf + 2 I2

Chemical compounds

Due to the lanthanide contraction, the ionic radius of hafnium(IV) (0.78 ångström) is almost the same as that of zirconium(IV) (0.79  angstroms). [23] Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties. [23] 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 form inorganic compounds in the oxidation state of +4. Halogens react with it to form hafnium tetrahalides. [23] At higher temperatures, hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. [23] Some compounds of hafnium in lower oxidation states are known. [24]

Hafnium(IV) chloride and hafnium(IV) iodide have some applications in the production and purification of hafnium metal. They are volatile solids with polymeric structures. [8] These tetrachlorides are precursors to various organohafnium compounds such as hafnocene dichloride and tetrabenzylhafnium.

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 more basic. [8] 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. [23] 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 (Ta
4
HfC
5
) possesses the highest melting point of any currently known compound, 4215 K (3942 °C, 7128 °F). [25] Recent supercomputer simulations suggest a hafnium alloy with a melting point of 4400 K. [26]

History

Photographic recording of the characteristic X-ray emission lines of some elements Moseley step ladder.jpg
Photographic recording of the characteristic X-ray emission lines of some elements

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. [27]

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. [28]

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. [29] Georges Urbain asserted that he found element 72 in the rare earth elements in 1907 and published his results on celtium in 1911. [30] 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. [31] The controversy was partly because 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. [31] By early 1923, several physicists and chemists such as Niels Bohr [32] and Charles R. Bury [33] 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 Moseley, and the chemical arguments of Friedrich Paneth. [34] [35]

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. [36] Hafnium was discovered by the two in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev. [37] [38] It was ultimately found in zircon in Norway through X-ray spectroscopy analysis. [39] 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. [40] Today, the Faculty of Science of the University of Copenhagen uses in its seal a stylized image of the hafnium atom. [41]

Hafnium was separated from zirconium through repeated recrystallization of the double ammonium or potassium fluorides by Valdemar Thal Jantzen and von Hevesey. [17] Anton Eduard van Arkel and Jan Hendrik de Boer were the first to prepare metallic hafnium by passing hafnium tetraiodide vapor over a heated tungsten filament in 1924. [18] [22] This process for differential purification of zirconium and hafnium is still in use today. [6]

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, [42] thus making elements 75 (rhenium) and 72 (hafnium) the last two unknown non-radioactive elements. Since rhenium was discovered in 1908, hafnium was the last element with stable isotopes to be discovered.

Applications

Most of the hafnium produced is used in the manufacture of control rods for nuclear reactors. [19]

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. [6] When the demand for zirconium dropped following the Fukushima disaster, the price of hafnium increased sharply from around $500–600/kg in 2014 to around $1000/kg in 2015. [43]

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 (Capture Resonance Integral Io ≈ 2000 barns) [44] 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. [19] The German research reactor FRM II uses hafnium as a neutron absorber. [45] It is also common in military reactors, particularly in US naval reactors, [46] but seldom found in civilian ones, the first core of the Shippingport Atomic Power Station (a conversion of a naval reactor) being a notable exception. [47]

Alloys

Hafnium-containing rocket nozzle of the Apollo Lunar Module in the lower right corner Apollo AS11-40-5866.jpg
Hafnium-containing rocket nozzle of the Apollo Lunar Module in the lower right corner

Hafnium is used in alloys with iron, titanium, niobium, tantalum, and other metals. 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. [48]

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. [49] [50] [51]

Microprocessors

Hafnium-based compounds are employed in gate insulators in the 45 nm generation of integrated circuits from Intel, IBM and others. [52] [53] Hafnium oxide-based compounds are practical high-k dielectrics, allowing reduction of the gate leakage current which improves performance at such scales. [54] [55]

Isotope geochemistry

Isotopes of hafnium and lutetium (along with ytterbium) are also used in isotope geochemistry and geochronological applications, in lutetium-hafnium dating. It is often used as a tracer of isotopic evolution of Earth's mantle through time. [56] This is because 176Lu decays to 176Hf with a half-life of approximately 37 billion years. [57] [58] [59]

In most geologic materials, zircon is the dominant host of hafnium (>10,000 ppm) and is often the focus of hafnium studies in geology. [60] Hafnium is readily substituted into the zircon crystal 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 the depleted 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.

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 dating metamorphic events. [61]

Other uses

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. [62]

The high energy content of 178m2Hf was the concern of a DARPA-funded program in the US. This program determined that 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—was infeasible because of its expense. See hafnium controversy .

Hafnium metallocene compounds can be prepared from hafnium tetrachloride and various cyclopentadiene-type ligand species. Perhaps the simplest hafnium metallocene is halfnocene dichloride. Hafnium metallocenes are part of a large collection of Group 4 transition metal metallocene catalysts [63] that are used worldwide in the production of polyolefin resins like polyethylene and polypropylene.

Precautions

Care needs to be taken when machining hafnium because it is pyrophoric—fine particles can spontaneously combust when exposed to air. 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. [64]

People can be exposed to hafnium in the workplace by breathing it in, swallowing it, skin contact, and eye contact. 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. The National Institute for Occupational Safety and Health (NIOSH) has set the same recommended exposure limit (REL). At levels of 50 mg/m3, hafnium is immediately dangerous to life and health. [65]

See also

Related Research Articles

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Lutetium Chemical element with atomic number 71

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Neptunium Chemical element with atomic number 93

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Periodic table Tabular arrangement of the chemical elements ordered by atomic number

The periodic table, also known as the periodic table of elements, is a tabular display of the chemical elements, which are arranged by atomic number, electron configuration, and recurring chemical properties. The structure of the table shows periodic trends. The seven rows of the table, called periods, generally have metals on the left and nonmetals on the right. The columns, called groups, contain elements with similar chemical behaviours. Six groups have accepted names as well as assigned numbers: for example, group 17 elements are the halogens; and group 18 are the noble gases. Also displayed are four simple rectangular areas or blocks associated with the filling of different atomic orbitals.

Rutherfordium Chemical element with atomic number 104

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Thorium Chemical element with atomic number 90

Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately hard, malleable, and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided.

Zirconium Chemical element with atomic number 40

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A period 2 element is one of the chemical elements in the second row of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate/display recurring (periodic) trends in the chemical behavior of the elements as their atomic number increases; a new row is started when chemical behavior begins to repeat, creating columns of elements with similar properties.

Group 4 element group of chemical elements

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Control rod Nuclear reactor component

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Thorium dioxide Chemical compound

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Neutron capture Atomic nuclear process

Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, which are repelled electrostatically.

Hafnium tetrachloride chemical compound

Hafnium(IV) chloride is the inorganic compound with the formula HfCl4. This colourless solid is the precursor to most hafnium organometallic compounds. It has a variety of highly specialized applications, mainly in materials science and as a catalyst.

Zirconium hydride

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Zirconium carbide chemical compound

Zirconium carbide (ZrC) is an extremely hard refractory ceramic material, commercially used in tool bits for cutting tools. It is usually processed by sintering.

Yttrium Chemical element with atomic number 39

Yttrium is a chemical element with the symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost always found in combination with lanthanide elements in rare-earth minerals, and is never found in nature as a free element. 89Y is the only stable isotope, and the only isotope found in the Earth's crust.

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