Zirconium

For criticism see Criticism of Zirconium

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silvery white
colspan=2 style="text-align: center; background-color: #FFC0C0; color: #000000 " | General properties
Name, symbol, number	zirconium, Zr, 40
Element category	transition metal
Group, period, block	4, 5, d
Standard atomic weight	91.224
Electron configuration	[Kr] 5s2 4d2
Electrons per shell	2, 8, 18, 10, 2 (Image)
colspan=2 style="text-align: center; background-color: #FFC0C0; color: #000000 " | Physical properties
Phase	solid
Density (near r.t.)	6.52 g·cm−3
Liquid density at m.p.	5.8 g·cm−3
Melting point	2128 K, 1855 °C, 3371 °F
Boiling point	4682 K, 4409 °C, 7968 °F
Heat of fusion	14 kJ·mol−1
Heat of vaporization	573 kJ·mol−1
Molar heat capacity	25.36 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2639 2891 3197 3575 4053 4678
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Oxidation states	4, 3, 2, 1,[1] (amphoteric oxide)
Electronegativity	1.33 (Pauling scale)
Ionization energies	1st: 640.1 kJ·mol−1
	2nd: 1270 kJ·mol−1
	3rd: 2218 kJ·mol−1
Atomic radius	160 pm
Covalent radius	175±7 pm
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Crystal structure	hexagonal close-packed
Magnetic ordering	paramagnetic[2]
Electrical resistivity	(20 °C) 421 nΩ·m
Thermal conductivity	22.6 W·m−1·K−1
Thermal expansion	(25 °C) 5.7 µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 3800 m·s−1
Young's modulus	88 GPa
Shear modulus	33 GPa
Bulk modulus	91.1 GPa
Poisson ratio	0.34
Mohs hardness	5.0
Vickers hardness	903 MPa
Brinell hardness	650 MPa
CAS registry number	7440-67-7
colspan=2 style="text-align: center; background-color: #FFC0C0; color: #000000 " | Most stable isotopes
Main article: Isotopes of zirconium
iso NA half-life DM DE (MeV) DP 88Zr syn 83.4 d ε - 88Y γ 0.392D - 89Zr syn 78.4 h ε - 89Y β+ 0.902 89Y γ 0.909D - 90Zr 51.45% 90Zr is stable with 50 neutrons 91Zr 11.22% 91Zr is stable with 51 neutrons 92Zr 17.15% 92Zr is stable with 52 neutrons 93Zr trace 1.53×106 y β− 0.060 93Nb 94Zr 17.38% 1.1×1017 y β−β− - 94Mo 96Zr 2.8% 2.0×1019 y[3] β−β− 3.348 96Mo
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Zirconium (pronounced /zərˈkoʊniəm/ zər-koh-ni-əm) is a chemical element with the symbol Zr and atomic number 40. It is a lustrous, grey-white, strong transition metal that resembles titanium. Zirconium is used as an alloying agent due to its high resistance to corrosion. It is never found as a native metal; it is obtained mainly from the mineral zircon, which can be purified by chlorine. Zirconium was first isolated in an impure form in 1824 by Jöns Jakob Berzelius.

Zirconium has no known biological role. Zirconium forms both inorganic and organometallic compounds such as zirconium dioxide and zirconocene dichloride, respectively. There are five naturally-occurring isotopes, three of which are stable. Short-term exposure to zirconium powder causes minor irritation, and inhalation of zirconium compounds can cause skin and lung granulomas.

Characteristics

Zirconium is a lustrous, grayish-white, soft, ductile, and malleable metal which is solid at room temperature, though it becomes hard and brittle at lower purities.^[4][5] In powder form, zirconium is highly flammable, but the solid form is far less prone to igniting. Zirconium is highly resistant to corrosion by alkalis, acids, salt water, and other agents.^[6] However, it will dissolve in hydrochloric and sulfuric acid, especially when fluorine is present.^[7] Alloys with zinc become magnetic below 35 K.^[6]

The melting point of zirconium is at 1855°C, and the boiling point is at 4409°C.^[6] Zirconium has an electronegativity of 1.33 on the Pauling scale. Of the elements within d-block, zirconium has the fourth lowest electronegativity after yttrium, lutetium, and hafnium.^[8]

Applications

Because of zirconium's excellent resistance to corrosion, it is often used as an alloying agent in materials that are exposed to corrosive agents, such as surgical appliances, explosive primers, vacuum tube getters and filaments. Zirconium dioxide (ZrO₂) is used in laboratory crucibles, metallurgical furnaces, as a refractory material,^[6] and it can be sintered into a ceramic knife. Zircon (ZrSiO₄) is cut into gemstones for use in jewelry. Zirconium carbonate (3ZrO₂·CO₂·H₂O) was used in lotions to treat poison ivy, but this was discontinued as it caused bad skin reactions in some cases.^[4]

Ninety percent of all zirconium produced is used in nuclear reactors (in the form of zircaloys) because of its low neutron-capture cross-section and resistance to corrosion.^[5][6] Zirconium alloys are used in space vehicle parts for their resistance to heat, an important quality given the extreme heat associated with atmospheric reentry.^[9] Zirconium is also a component in some abrasives, such as grinding wheels and sandpaper.^[10] Zirconium is used in weapons such as the BLU-97/B Combined Effects Bomb for incendiary effect.

High temperature parts such as combustors, blades and vanes in modern jet engines and stationary gas turbines are to an ever increasing extent being protected by thin ceramic layers which reduce the metal temperatures below and keep them from undergoing (too) extensive deformation which could possibly result in early failure. They are absolutely necessary for the most modern gas turbines which are driven to ever higher firing temperatures to produce more electricity at less CO₂. These ceramic layers are usually composed by a mixture of zirconium and yttrium oxide.^[11]

Refining

Upon being collected from coastal waters, the solid mineral zircon is purified by spiral concentrators to remove excess sand and gravel and by magnetic separators to remove ilmenite and rutile. The byproducts can then be dumped back into the water safely, as they are all natural components of beach sand. The refined zircon is then purified into pure zirconium by chlorine or other agents, then sintered until sufficiently ductile for metalworking.^[5] Zirconium and hafnium are both contained in zircon and they are quite difficult to separate due to their similar chemical properties.^[9]

History

Zirconium crystal bar, 99.97%, made by the crystal bar process

The zirconium-containing mineral zircon, or its variations (jargoon, hyacinth, jacinth, ligure), were mentioned in biblical writings.^[6][9] The mineral was not known to contain a new element until 1789,^[10] when Klaproth analyzed a jargoon from the island of Sri Lanka in the Indian Ocean. He named the new element Zirkonerde (zirconia).^[6] Humphry Davy attempted to isolate this new element in 1808 through electrolysis, but failed.^[4] Zirconium (from Syriac ܙܐܪܓܥܢܥ zargono,^[12] Arabic zarkûn ئشقنعى from Persian zargûn زرگون meaning "gold like")^[9] was first isolated in an impure form in 1824 by Berzelius by heating a mixture of potassium and potassium-zirconium fluoride in a small decomposition process conducted in an iron tube.^[6]

The crystal bar process (or 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 pure metallic zirconium. The process involved thermally decomposing zirconium tetraiodide. It was superseded in 1945 by the much cheaper Kroll process developed by William Justin Kroll, in which zirconium tetrachloride is broken down by magnesium.^[5][13]

Occurrence

Geological

Zirconium output in 2005

World production trend of zirconium mineral concentrates

Zirconium has a concentration of about 130 mg/kg within the earth's crust and about 0.026 Î¼g/L in sea water,^[14] though it is never found in nature as a native metal. The principal commercial source of zirconium is the zirconium silicate mineral, zircon (ZrSiO₄),^[4] which is found primarily in Australia, Brazil, India, Russia, South Africa, and the United States, as well as in smaller deposits around the world.^[5] 80% of zircon mining occurs in Australia and South Africa.^[4] Zircon resources exceed 60 million metric tons worldwide^[15] and annual worldwide zirconium production is approximately 900,000 metric tons.^[14]

Zircon is a by-product of the mining and processing of the titanium minerals ilmenite and rutile, as well as tin mining.^[16] From 2003 to 2007, zircon prices have steadily increased from $360 to $840 per metric ton.^[15] Zirconium also occurs in more than 140 other recognized mineral species including baddeleyite and kosnarite.^[17] This metal is commercially produced mostly by the reduction of the zirconium(IV) chloride with magnesium metal in the Kroll process.^[6] Commercial-quality zirconium for most uses still has a content of 1% to 3% hafnium.^[4]

This element is relatively-abundant in S-type stars, and it has been detected in the sun and in meteorites. Lunar rock samples brought back from several Apollo program missions to the moon have a quite high zirconium oxide content relative to terrestrial rocks.^[6]

Biological

Zirconium has no known biological role, though zirconium salts are of low toxicity. The human body contains, on average, only 1 milligram of zirconium, and daily intake is approximately 50 Î¼g per day. Zirconium content in human blood is as low as 10 parts per billion. Aquatic plants readily take up soluble zirconium, but it is rare in land plants. 70% of plants have no zirconium content at all, and those that do have as little as 5 parts per billion.^[4]

Compounds

As a transition metal, zirconium forms various inorganic compounds, such as zirconium dioxide (ZrO₂). This compound, also referred to as zirconia, has exceptional fracture toughness and chemical resistance, especially in its cubic form.^[18] These properties make zirconia useful as a thermal barrier coating,^[19] though it is also a common diamond substitute.^[18] Zirconium tungstate is an unusual substance in that it shrinks in all directions when heated, whereas most other substances expand when heated.^[6] ZrZn₂ is one of only two substances to exhibit superconductivity and ferromagnetism simultaneously, with the other being UGe₂.^[20] Other inorganic zirconium compounds include zirconium(II) hydride, zirconium nitride, and zirconium tetrachloride (ZrCl₄), which is used in the Friedel-Crafts reaction.^[21]

Organozirconium chemistry is the study of compounds containing a carbon-zirconium bond. These organozirconium compounds are often employed as polymerization catalysts. The first such compound was zirconocene dibromide, prepared in 1952 by John M. Birmingham at Harvard University.^[22] Schwartz's reagent, prepared in 1970 by P. C. Wailes and H. Weigold,^[23] is a metallocene used in organic synthesis for transformations of alkenes and alkynes.^[24]

Isotopes

A zirconium rod

Naturally-occurring zirconium is composed of five isotopes. ⁹⁰Zr, ⁹¹Zr, and ⁹²Zr are stable. ⁹⁴Zr has a half-life of 1.10×10¹⁷ years. ⁹⁶Zr has a half-life of 2.4×10¹⁹ years, making it 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.^[25]

28 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 also the shortest-lived, with an estimated half-life of only 30 milliseconds. Radioactive isotopes at or above mass number 93 decay by β^−, whereas those at or below 89 decay by β⁺. The only exception is ⁸⁸Zr, which decays by ε.^[25]

Zirconium also has six 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.^[25]

Toxicity

Short-term exposure to zirconium powder can cause irritation, but only contact with the eyes requires medical attention.^[26] Inhalation of zirconium compounds can cause skin and lung granulomas. Zirconium aerosols can cause pulmonary granulomas. Persistent exposure to zirconium tetrachloride resulted in increased mortality in rats and guinea pigs and a decrease of blood hemoglobin and red blood cells in dogs. OSHA recommends a 5 mg/m³ time weighted average limit and a 10 mg/m³ short-term exposure limit.^[27]

See also

• Zirconium compounds
• Zirconium minerals

Notes

External links

Wikimedia Commons has media related to: Zirconium

Look up [//en.wiktionary.org/wiki/zirconium zirconium] in Wiktionary, the free dictionary.

• WebElements.com: Zirconium
• Chemistry in its element podcast (MP3) from the Royal Society of Chemistry's Chemistry World: Zirconium

Template:Zirconium compounds