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Zirconium, ₄₀Zr

Zirconium
Pronunciation	/zɜːrˈkoʊniəm/ ​(zur-KOH-nee-əm)
Appearance	silvery white
Standard atomic weight Ar°(Zr)
	91.224±0.002[1] 91.224±0.002 (abridged)[2]
Zirconium 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 Ti ↑ Zr ↓ Hf yttrium ← zirconium → niobium
Atomic number (Z)	40
Group	group 4
Period	period 5
Block	d-block
Electron configuration	[Kr] 4d2 5s2
Electrons per shell	2, 8, 18, 10, 2
Physical properties
Phase at STP	solid
Melting point	2125 K ​(1852 °C, ​3365 °F)
Boiling point	4650 K ​(4377 °C, ​7911 °F)
Density (at 20° C)	6.505 g/cm3[3]
when liquid (at m.p.)	5.8 g/cm3
Heat of fusion	14 kJ/mol
Heat of vaporization	591 kJ/mol
Molar heat capacity	25.36 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2639 2891 3197 3575 4053 4678
Atomic properties
Oxidation states	−2, 0, +1,[4][citation needed] +2, +3, +4 (an amphoteric oxide)
Electronegativity	Pauling scale: 1.33
Ionization energies	1st: 640.1 kJ/mol 2nd: 1270 kJ/mol 3rd: 2218 kJ/mol
Atomic radius	empirical: 160 pm
Covalent radius	175±7 pm
Spectral lines of zirconium
Other properties
Natural occurrence	primordial
Crystal structure	​hexagonal close-packed (hcp) (hP2)
Lattice constants	a = 323.22 pm c = 514.79 pm (at 20 °C)[3]
Thermal expansion	5.69×10−6/K (at 20 °C)[3][a]
Thermal conductivity	22.6 W/(m⋅K)
Electrical resistivity	421 nΩ⋅m (at 20 °C)
Magnetic ordering	paramagnetic[5]
Young's modulus	88 GPa
Shear modulus	33 GPa
Bulk modulus	91.1 GPa
Speed of sound thin rod	3800 m/s (at 20 °C)
Poisson ratio	0.34
Mohs hardness	5.0
Vickers hardness	820–1800 MPa
Brinell hardness	638–1880 MPa
CAS Number	7440-67-7
History
Naming	after zircon, zargun زرگون meaning "gold-colored".
Discovery	Martin Heinrich Klaproth (1789)
First isolation	Jöns Jakob Berzelius (1824)
Isotopes of zirconium v e
Main isotopes[6] Decay abun­dance half-life (t1/2) mode pro­duct 88Zr synth 83.4 d ε 88Y γ – 89Zr synth 78.4 h ε 89Y β+ 89Y γ – 90Zr 51.5% stable 91Zr 11.2% stable 92Zr 17.1% stable 93Zr trace 1.53×106 y β− 93Nb 94Zr 17.4% stable 96Zr 2.80% 2.0×1019 y[7] β−β− 96Mo
Category: Zirconium view talk edit | references

Zirconium is a chemical element; it has symbol Zr and atomic number 40. The name zirconium is derived from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian zargun (zircon; zar-gun, "gold-like" or "as gold").^[8] It is a lustrous, grey-white, strong transition metal that closely resembles hafnium and, to a lesser extent, titanium.

Zirconium forms a variety of inorganic and organometallic compounds such as zirconium dioxide and zirconocene dichloride, respectively. Five isotopes occur naturally, four of which are stable.

Zirconium is mainly used as a refractory and opacifier, although small amounts are used as an alloying agent for its strong resistance to corrosion. Zirconium compounds have no known biological role.

Characteristics

Zirconium is a lustrous, greyish-white, soft, ductile, malleable metal that is solid at room temperature, though it is hard and brittle at lesser purities.^[9] In powder form, zirconium is highly flammable, but the solid form is much less prone to ignition. Zirconium is highly resistant to corrosion by alkalis, acids, salt water and other agents.^[10] However, it will dissolve in hydrochloric and sulfuric acid, especially when fluorine is present.^[11] Alloys with zinc are magnetic at less than 35 K.^[10]

The melting point of zirconium is 1855 °C (3371 °F), and the boiling point is 4409 °C (7968 °F).^[10] Zirconium has an electronegativity of 1.33 on the Pauling scale. Of the elements within the d-block with known electronegativities, zirconium has the fifth lowest electronegativity after hafnium, yttrium, lanthanum, and actinium.^[12]

At room temperature zirconium exhibits a hexagonally close-packed crystal structure, α-Zr, which changes to β-Zr, a body-centered cubic crystal structure, at 863 °C. Zirconium exists in the β-phase until the melting point.^[13]

Isotopes

Naturally occurring zirconium is composed of five isotopes. ⁹⁰Zr, ⁹¹Zr, ⁹²Zr and ⁹⁴Zr are stable, although ⁹⁴Zr is predicted to undergo double beta decay (not observed experimentally) with a half-life of more than 1.10×10¹⁷ years. ⁹⁶Zr has a half-life of 2.4×10¹⁹ years, and is the longest-lived radioisotope of zirconium. Of these natural isotopes, ⁹⁰Zr is the most common, making up 51.45% of all zirconium. ⁹⁶Zr is the least common, comprising only 2.80% of zirconium.^[14]

Twenty-eight artificial isotopes of zirconium have been synthesized, ranging in atomic mass from 78 to 110. ⁹³Zr is the longest-lived artificial isotope, with a half-life of 1.53×10⁶ years. ¹¹⁰Zr, the heaviest isotope of zirconium, is the most radioactive, with an estimated half-life of 30 milliseconds. Radioactive isotopes at or above mass number 93 decay by electron emission, whereas those at or below 89 decay by positron emission. The only exception is ⁸⁸Zr, which decays by electron capture.^[14]

Five isotopes of zirconium also exist as metastable isomers: ^83mZr, ^85mZr, ^89mZr, ^90m1Zr, ^90m2Zr and ^91mZr. Of these, ^90m2Zr has the shortest half-life at 131 nanoseconds. ^89mZr is the longest lived with a half-life of 4.161 minutes.^[14]

Occurrence

Zirconium has a concentration of about 130 mg/kg within the Earth's crust and about 0.026 μg/L in sea water. It is the 18th most abundant element in the crust.^[15] It is not found in nature as a native metal, reflecting its intrinsic instability with respect to water. The principal commercial source of zirconium is zircon (ZrSiO₄), a silicate mineral,^[9] which is found primarily in Australia, Brazil, India, Russia, South Africa and the United States, as well as in smaller deposits around the world.^[16] As of 2013, two-thirds of zircon mining occurs in Australia and South Africa.^[17] Zircon resources exceed 60 million tonnes worldwide^[18] and annual worldwide zirconium production is approximately 900,000 tonnes.^[15] Zirconium also occurs in more than 140 other minerals, including the commercially useful ores baddeleyite and eudialyte.^[19]

Zirconium is relatively abundant in S-type stars, and has been detected in the sun and in meteorites. Lunar rock samples brought back from several Apollo missions to the moon have a high zirconium oxide content relative to terrestrial rocks.^[10]

EPR spectroscopy has been used in investigations of the unusual 3+ valence state of zirconium. The EPR spectrum of Zr³⁺, which has been initially observed as a parasitic signal in Fe‐doped single crystals of ScPO₄, was definitively identified by preparing single crystals of ScPO₄ doped with isotopically enriched (94.6%)⁹¹Zr. Single crystals of LuPO₄ and YPO₄ doped with both naturally abundant and isotopically enriched Zr have also been grown and investigated.^[20]

Production

Occurrence

Zirconium is a by-product formed after mining and processing of the titanium minerals ilmenite and rutile, as well as tin mining.^[21] From 2003 to 2007, while prices for the mineral zircon steadily increased from $360 to $840 per tonne, the price for unwrought zirconium metal decreased from $39,900 to $22,700 per ton. Zirconium metal is much more expensive than zircon because the reduction processes are costly.^[18]

Collected from coastal waters, zircon-bearing sand is purified by spiral concentrators to separate lighter materials, which are then returned to the water because they are natural components of beach sand. Using magnetic separation, the titanium ores ilmenite and rutile are removed.

Most zircon is used directly in commercial applications, but a small percentage is converted to the metal. Most Zr metal is produced by the reduction of the zirconium(IV) chloride with magnesium metal in the Kroll process.^[10] The resulting metal is sintered until sufficiently ductile for metalworking.^[16]

Separation of zirconium and hafnium

Commercial zirconium metal typically contains 1–3% of hafnium,^[22] which is usually not problematic because the chemical properties of hafnium and zirconium are very similar. Their neutron-absorbing properties differ strongly, however, necessitating the separation of hafnium from zirconium for nuclear reactors.^[23] Several separation schemes are in use.^[22] The liquid-liquid extraction of the thiocyanate-oxide derivatives exploits the fact that the hafnium derivative is slightly more soluble in methyl isobutyl ketone than in water. This method is used mainly in United States. In India, TBP-Nitrate solvent extraction process is used for the separation.

Zr and Hf can also be separated by fractional crystallization of potassium hexafluorozirconate (K₂ZrF₆), which is less soluble in water than the analogous hafnium derivative.

Fractional distillation of the tetrachlorides, also called extractive distillation, is used primarily in Europe.

The product of a quadruple VAM (vacuum arc melting) process, combined with hot extruding and different rolling applications is cured using high-pressure, high-temperature gas autoclaving. This produces reactor-grade zirconium that is about 10 times more expensive than the hafnium-contaminated commercial grade.

Hafnium must be removed from zirconium for nuclear applications because hafnium has a neutron absorption cross-section 600 times greater than zirconium.^[24] The separated hafnium can be used for reactor control rods.^[25]

Compounds

Like other transition metals, zirconium forms a wide range of inorganic compounds and coordination complexes.^[26] In general, these compounds are colourless diamagnetic solids wherein zirconium has the oxidation state +4. Far fewer Zr(III) compounds are known, and Zr(II) is very rare.

Oxides, nitrides, and carbides

The most common oxide is zirconium dioxide, ZrO₂, also known as zirconia. This clear to white-coloured solid has exceptional fracture toughness (for a ceramic) and chemical resistance, especially in its cubic form.^[27] These properties make zirconia useful as a thermal barrier coating,^[28] although it is also a common diamond substitute.^[27] Zirconium monoxide, ZrO, is also known and S-type stars are recognised by detection of its emission lines.^[29]

Zirconium tungstate has the unusual property of shrinking in all dimensions when heated, whereas most other substances expand when heated.^[10] Zirconyl chloride is a rare water-soluble zirconium complex with the relatively complicated formula Cl₈.

Zirconium carbide and zirconium nitride are refractory solids. The carbide is used for drilling tools and cutting edges. Zirconium hydride phases are also known.

Lead zirconate titanate (PZT) is the most commonly used piezoelectric material, with applications such as ultrasonic transducers, hydrophones, common rail injectors, piezoelectric transformers and micro-actuators.

Halides and pseudohalides

All four common halides are known, ZrF₄, ZrCl₄, ZrBr₄, and ZrI₄. All have polymeric structures and are far less volatile than the corresponding monomeric titanium tetrahalides. All tend to hydrolyse to give the so-called oxyhalides and dioxides.

The corresponding tetraalkoxides are also known. Unlike the halides, the alkoxides dissolve in nonpolar solvents. Dihydrogen hexafluorozirconate is used in the metal finishing industry as an etching agent to promote paint adhesion.^[30]

Organic derivatives

Organozirconium chemistry is key to Ziegler–Natta catalysts, used to produce polypropylene. This application exploits the ability of zirconium to reversibly form bonds to carbon. Zirconocene dibromide ((C₅H₅)₂ZrBr₂), reported in 1952 by Birmingham and Wilkinson, was the first organozirconium compound.^[31] Schwartz's reagent, prepared in 1970 by P. C. Wailes and H. Weigold,^[32] is a metallocene used in organic synthesis for transformations of alkenes and alkynes.^[33]

Most complexes of Zr(II) are derivatives of zirconocene, one example being (C₅Me₅)₂Zr(CO)₂.

History

The zirconium-containing mineral zircon and related minerals (jargoon, jacinth, or hyacinth, ligure) were mentioned in biblical writings.^[10][23] The mineral was not known to contain a new element until 1789,^[34] when Klaproth analyzed a jargoon from the island of Ceylon (now Sri Lanka). He named the new element Zirkonerde (zirconia).^[10] Humphry Davy attempted to isolate this new element in 1808 through electrolysis, but failed.^[9] Zirconium metal was first obtained in an impure form in 1824 by Berzelius by heating a mixture of potassium and potassium zirconium fluoride in an iron tube.^[10]

The crystal bar process (also known as the Iodide Process), discovered by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925, was the first industrial process for the commercial production of metallic zirconium. It involves the formation and subsequent thermal decomposition of zirconium tetraiodide (ZrI₄), and was superseded in 1945 by the much cheaper Kroll process developed by William Justin Kroll, in which zirconium tetrachloride (ZrCl₄) is reduced by magnesium:^[16][35]

Applications

Approximately 900,000 tonnes of zirconium ores were mined in 1995, mostly as zircon.^[22]

Most zircon is used directly in high-temperature applications. Because it is refractory, hard, and resistant to chemical attack, zircon finds many applications. Its main use is as an opacifier, conferring a white, opaque appearance to ceramic materials. Because of its chemical resistance, zircon is also used in aggressive environments, such as moulds for molten metals.

Zirconium dioxide (ZrO₂) is used in laboratory crucibles, in metallurgical furnaces, and as a refractory material^[10] Because it is mechanically strong and flexible, it can be sintered into ceramic knives and other blades.^[36] Zircon (ZrSiO₄) and cubic zirconia (ZrO₂) are cut into gemstones for use in jewelry. Zircon is also used in dating of rocks.

Zirconium dioxide is a component in some abrasives, such as grinding wheels and sandpaper.^[34]

A small fraction of the zircon is converted to the metal, which finds various niche applications. Because of zirconium's excellent resistance to corrosion, it is often used as an alloying agent in materials that are exposed to aggressive environments, such as surgical appliances, light filaments, and watch cases. The high reactivity of zirconium with oxygen at high temperatures is exploited in some specialised applications such as explosive primers and as getters in vacuum tubes. The same property is (probably) the purpose of including Zr nanoparticles as pyrophoric material in explosive weapons such as the BLU-97/B Combined Effects Bomb. Burning zirconium was used as a light source in some photographic flashbulbs. Zirconium powder with a mesh size from 10 to 80 is occasionally used in pyrotechnic compositions to generate sparks. The high reactivity of zirconium leads to bright white sparks.^[37]

Nuclear applications

Cladding for nuclear reactor fuels consumes about 1% of the zirconium supply,^[22] mainly in the form of zircaloys. The desired properties of these alloys are a low neutron-capture cross-section and resistance to corrosion under normal service conditions.^[16][10] Efficient methods for removing the hafnium impurities were developed to serve this purpose.

One disadvantage of zirconium alloys is the reactivity with water, producing hydrogen, leading to degradation of the fuel rod cladding:

Hydrolysis is very slow below 100 °C, but rapid at temperature above 900 °C. Most metals undergo similar reactions. The redox reaction is relevant to the instability of fuel assemblies at high temperatures.^[38] This reaction occurred in the reactors 1, 2 and 3 of the Fukushima I Nuclear Power Plant (Japan) after the reactor cooling was interrupted by the earthquake and tsunami disaster of March 11, 2011, leading to the Fukushima I nuclear accidents. After venting the hydrogen in the maintenance hall of those three reactors, the mixture of hydrogen with atmospheric oxygen exploded, severely damaging the installations and at least one of the containment buildings.

Zirconium is a constituent of the uranium zirconium hydride (UZrH) nuclear fuel used in TRIGA reactors.

Space and aeronautic industries

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