Hafnium
Hafnium | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Pronunciation | /ˈhæfniəm/ | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
Appearance | steel gray | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
Standard atomic weight Ar°(Hf) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Hafnium in the periodic table | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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kJ/mol | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Heat of vaporization | 648 kJ/mol | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
Molar heat capacity | 25.73 J/(mol·K) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
Vapor pressure
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Atomic properties | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Discovery and first isolation | Dirk Coster and George de Hevesy (1922) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||
Isotopes of hafnium | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Hafnium is a
Hafnium is used in filaments and electrodes. Some semiconductor fabrication processes use its oxide for integrated circuits at 45 nanometers and smaller feature lengths. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten.
Hafnium's large
Characteristics
Physical characteristics
Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant and chemically similar to zirconium[12] 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 2388 K.[13] 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.[12]
A notable physical difference between these metals is their
Chemical characteristics
Hafnium reacts in air to form a protective film 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.[14]
Isotopes
At least 40 isotopes of hafnium have been observed, ranging in
The extinct radionuclide 182Hf has a half-life of 8.9±0.1 million years, and is an important tracker isotope for the formation of planetary cores.[18] The nuclear isomer 178m2Hf was at the center of a controversy for several years regarding its potential use as a weapon.
Occurrence
Hafnium is estimated to make up about between 3.0 and 4.8
A major source of zircon (and hence hafnium) ores is
Production
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.[23]
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.[12]
The chemical properties of hafnium and zirconium are nearly identical, which makes the two difficult to separate.[24] The methods first used—fractional crystallization of ammonium fluoride salts[25] or the fractional distillation of the chloride[26]—have not proven 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.[27] 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.[28] The purified hafnium(IV) chloride is converted to the metal by reduction with magnesium or sodium, as in the Kroll process.[29]
Further purification is effected by a
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).[31] Consequently, compounds of hafnium(IV) and zirconium(IV) have very similar chemical and physical properties.[31] 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.[31] At higher temperatures, hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon.[31] Some hafnium compounds in lower oxidation states are known.[32]
The white
4HfC
5) possesses the highest melting point of any currently known compound, 4,263 K (3,990 °C; 7,214 °F).[33] Recent supercomputer simulations suggest a hafnium alloy with a melting point of 4,400 K.[34]
History
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.[35]
The
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.
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,
Hafnium was separated from zirconium through repeated recrystallization of the double ammonium or potassium fluorides by Valdemar Thal Jantzen and von Hevesey.[25] 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.[26][30] This process for differential purification of zirconium and hafnium is still in use today.[12]
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,[51] thus making elements 75 (rhenium) and 72 (hafnium) the last two unknown non-radioactive elements.
Applications
Most of the hafnium produced is used in the manufacture of control rods for nuclear reactors.[27]
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 the more abundant zirconium for most 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.
Nuclear reactors
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)
Alloys
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.[58]
Small additions of hafnium increase the adherence of protective oxide scales on nickel-based alloys. It thereby improves 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.[59][60][61]
Microprocessors
Hafnium-based compounds are employed in
Isotope geochemistry
Isotopes of hafnium and
In most geologic materials,
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.[72]
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
The high energy content of 178m2Hf was the concern of a DARPA-funded program in the US. This program eventually concluded that using the above-mentioned 178m2Hf nuclear isomer of hafnium 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 hafnocene dichloride. Hafnium metallocenes are part of a large collection of Group 4 transition metal metallocene catalysts [74] that are used worldwide in the production of polyolefin resins like polyethylene and polypropylene.
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 much tougher recycled plastic.[75]
Hafnium diselenide is studied in spintronics thanks to its charge density wave and superconductivity.[76]
Precautions
Care needs to be taken when
People can be exposed to hafnium in the workplace by breathing, swallowing, skin, and eye contact. The
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{{cite journal}}
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Literature
- ISBN 978-0-19-539131-2.
External links
- Hafnium at Los Alamos National Laboratory's periodic table of the elements
- Hafnium at The Periodic Table of Videos(University of Nottingham)
- Hafnium Technical & Safety Data
- NLM Hazardous Substances Databank – Hafnium, elemental
- Don Clark: Intel Shifts from Silicon to Lift Chip Performance – WSJ, 2007
- Hafnium-based Intel 45nm Process Technology
- CDC – NIOSH Pocket Guide to Chemical Hazards
- https://colnect.com/en/coins/list/composition/168-Hafnium