Iridium


Iridium

Iridium (pronEng|ɪˈrɪdiəm) is a chemical element that has the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can even be flammable. The most important iridium compounds in terms of use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in catalysis and in research. 191Ir and 193Ir are the only two naturally-occurring isotopes of iridium as well as the only stable isotopes; the latter is the more abundant of the two.

Iridium was discovered in 1803 by Smithson Tennant among insoluble impurities in natural platinum from South America. It is one of the rarest elements in the Earth's crust, with annual production and consumption of only three tonnes. However, iridium does find a number of specialized industrial and scientific applications. Iridium is employed when high corrosion resistance and high temperatures are needed, as in spark plugs, crucibles for recrystallization of semiconductors at high temperatures, electrodes for the production of chlorine in the chloralkali process, and radioisotope thermoelectric generators used in unmanned spacecrafts. Iridium compounds also find applications as catalysts for the production of acetic acid.

Iridium has been linked with the extinction of the dinosaurs and many other species 65 million years ago. The unusually high abundance of iridium in the clays of the K–T geologic boundary was a crucial clue that led to the theory that the extinction was caused by the impact of a massive extraterrestrial object with Earth—the so-called Alvarez hypothesis. Iridium is found in meteorites with an abundance much higher than its average abundance in the Earth's crust. It is thought that due to the high density and siderophilic character of iridium, most of the iridium on Earth is found in the inner core of the planet.

Characteristics

A platinum group metal, iridium is white, resembling platinum, but with a slight yellowish cast. One of the lesser-known members of the platinum group, iridium possesses quite remarkable chemical and physical properties. Due to its hardness, brittleness, and very high melting point (the tenth highest of all elements), solid iridium is difficult to machine, form, or work, and thus powder metallurgy is commonly employed instead. It is also the only metal to maintain good mechanical properties in air at temperatures above 1600 °C. Iridium also has a very high boiling point (11th among all elements) and becomes a superconductor under 0.14 K. [cite book | last = Kittel | first = C. | coauthors = | title=Introduction to Solid state Physics, 7th Edition| publisher=Wiley-India | year=2004 | isbn=8126510455| oclc = ]

Iridium is the most corrosion-resistant metal known: it is not attacked by any acid, by aqua regia, by any molten metals, or by silicates at high temperatures. It can, however, be attacked by some molten salts, such as sodium cyanide and potassium cyanide, as well as oxygen and the halogens (particularly fluorine)cite book |title = Handbook of Inorganic Compounds |author=Perry, D. L. |pages = 203–204 | date = 1995 | isbn = 0-8492-8671-3 | publisher = CRC Press |ISBN status = May be invalid - please double check |unused_data = |ISBN status = May be invalid - please double check] at higher temperatures.cite book |title = Chemistry Foundations and Applications | volume = 2 |editor = Lagowski, J. J. | pages=250–251 | date = 2004 | isbn = 0-02-865732-3 | publisher = Thomson Gale |ISBN status = May be invalid - please double check |unused_data = |ISBN status = May be invalid - please double check]

Iridium's modulus of elasticity is the second highest among the metals, only being surpassed by osmium. This, together with a high modulus of rigidity and a very low figure for Poisson's ratio (the relationship of longitudinal to lateral strain), indicate the high degree of stiffness and resistance to deformation that have rendered its fabrication into useful components a matter of great difficulty over the long period since its discovery. Despite these limitations and iridium's high cost, a number of applications have developed in more recent years where mechanical strength is an essential factor in some of the extremely severe conditions encountered in modern technology.

The measured density of iridium is only slightly lower (by about 0.1%) than that of osmium, the densest element known. [cite journal |title=Osmium, the Densest Metal Known |author=Arblaster, J. W. |journal=Platinum Metals Review |volume=39 |issue=4 |year=1995 |pages=164 |url=http://www.platinummetalsreview.com/dynamic/article/view/pmr-v39-i4-164-164] There has been some ambiguity regarding which element is the densest due to the small size of the difference in density and the difficulty in measuring it accurately,cite book | author=Lide, D. R. | title=CRC Handbook of Chemistry and Physics (70th Edn.) | publisher=Boca Raton (FL):CRC Press | year=1990] but the best available calculations from X-ray crystallographic data give densities of 22.56 g/cm3 for iridium and 22.59 g/cm3 for osmium. [cite journal |url=http://www.platinummetalsreview.com/pdf/pmr-v33-i1-014-016.pdf |title=Densities of osmium and iridium: recalculations based upon a review of the latest crystallographic data |author=Arblaster, J. W. |journal=Platinum Metals Review |volume=33 |issue=1 |year=1989 |pages=14–16]

Isotopes

Iridium has two naturally occurring, stable isotopes, 191Ir and 193Ir, with natural abundances of 37.3% and 62.7%, respectively. At least 34 radioisotopes have also been synthesized, ranging in mass number from 164 to 199. Twenty-seven of these are lighter than the stable isotopes, whereas only six are heavier than the stable isotopes. 192Ir, which falls between the two stable isotopes, is the most stable radioisotope, with a half-life of 73.827 days, and finds application in brachytherapy. Three other isotopes have half-lives of at least a day—188Ir, 189Ir, 190Ir—while the rest usually have a half-life of at least 1 ms. One of the least stable isotopes is 165Ir with a half-life of 1 µs. Isotopes with masses below 191 decay by some combination of β+ decay, α decay, and proton emission, with the exceptions of 189Ir, which decays by electron capture, and 190Ir, which decays by positron emission. Synthetic isotopes heavier than 191 decay by β decay, although 192Ir also has a minor electron capture decay path.cite journal| last = Audi| first = G. | title = The NUBASE Evaluation of Nuclear and Decay Properties| journal = Nuclear Physics A| volume = 729| pages = 3–128| publisher = Atomic Mass Data Center| date = 2003| doi=10.1016/j.nuclphysa.2003.11.001] All known isotopes of iridium were discovered between 1934 and 2001; the most recent is 171Ir. [cite journal |title=The discoverers of the iridium isotopes: the thirty-six known iridium isotopes found between 1934 and 2001 |author=Arblaster, J. W. |journal=Platinum Metals Review |volume=47 |issue=4 |year=2003 |pages=167–174 |url=http://www.platinummetalsreview.com/dynamic/article/view/47-4-167-174]

At least 32 metastable isomers have been characterized, ranging in mass number from 164 to 197. The most stable of these is 192m2Ir, which decays by isomeric transition with a half-life of 241 years, making it more stable than any of iridium's synthetic isotopes in their ground states. The least stable isomer is 190m3Ir with a half-life of only 2 µs. The isotope 191Ir was the first one of any element to be shown to present a Mössbauer effect. This renders it useful for Mössbauer spectroscopy for research in physics, chemistry, biochemistry, metallurgy, and mineralogy.

Compounds

Iridium forms compounds in the oxidation states of −3 and all in the range from −1 to +6; the most common oxidation states are +3 and +4. Well-characterized examples of the highest oxidation state are rare, but include chem|IrF|6 and two mixed oxides chem|Sr|2|MgIrO|6 and chem|Sr|2|CaIrO|6.cite book | last=Greenwood |first = N. N. |coauthors = Earnshaw, A. | title=Chemistry of the Elements |edition = 2nd Edition |publisher=Oxford:Butterworth-Heinemann | year=1997 | isbn=0-7506-3365-4 |pages=1113–1143,1294 | oclc=213025882 37499934 41901113] [cite journal| last = Jung | first = D. | title = High Oxygen Pressure and the Preparation of New Iridium (VI) Oxides with Perovskite Structure: Sr2MIrO6 (M = Ca, Mg) | journal = Journal of Solid State Chemistry | volume = 115 | issue = 2 | year = 1995 |pages = 447–455| publisher = | date = | doi= 10.1006/jssc.1995.1158]

Iridium dioxide, chem|IrO|2, a brown powder, is the only well-characterized oxide of iridium. A sesquioxide, chem|Ir|2|O|3, has also been described as a blue-black powder which is oxidized to chem|IrO|2 by chem|HNO|3. The corresponding disulfides, diselenides, sesquisulfides and sesquiselenides are known and chem|IrS|3 has also been reported. Iridium also forms iridates with oxidation states +4 and +5, such as K2IrO3 and KIrO3, which can be prepared from the reaction of potassium oxide or potassium superoxide with iridium at high temperatures. [cite journal |title=The chemistry of ruthenium, osmium, rhodium, iridium, palladium and platinum in the higher oxidation states |journal=Coordination Chemistry Reviews |volume=46 |year=1982 |pages=1–127 |author=Gulliver, D. J; Levason, W. |doi=10.1016/0010-8545(82)85001-7]

While no binary hydrides of iridum, chem|Ir|x|H|y are known, complexes are known that contain chem|IrH|54− and chem|IrH|63−, where iridium has the +1 and +3 oxidation states, respectively. [cite book | last = Holleman | first = A. F. | coauthors = Wiberg, E.; Wiberg, N. | title=Inorganic Chemistry, 1st Edition | publisher=Academic Press | year=2001 | isbn=0123526515| oclc = ] The ternary hydride chem|Mg|6|Ir|2|H|11 is believed to contain both the chem|IrH|54− and the 18-electron chem|IrH|45− anion. [cite journal| last = Černý | first = R. |coauthors = Joubert, J.-M.; Kohlmann, H.; Yvon, K. | title = chem|Mg|6|Ir|2|H|11, a new metal hydride containing saddle-like chem|IrH|45− and square-pyramidal chem|IrH|54− hydrido complexes| journal = Journal of Alloys and Compounds| volume = 340| issue = 1-2 | year = 2002 |pages = 180-188 | publisher = | date = | doi=10.1016/S0925-8388(02)00050-6 ]

No monohalides or dihalides are known, whereas trihalides, IrX3, are known for all of the halogens. For oxidation states +4 and above, only the tetrafluoride, pentafluoride and hexafluoride are known. Iridium hexafluoride, IrF6, is a volatile and highly reactive yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to IrF4, a crystalline solid, by iridium black. Iridium pentafluoride has similar properties but it is actually a tetramer, chem|Ir|4|F|20, formed by four corner-sharing octahedra.

Hexachloroiridic(IV) acid, chem|H|2|IrCl|6, and its ammonium salt are the most important iridium compounds from an industrial perspective. They are involved in the purification of iridium and used as precursors for most other iridium compounds, as well as in the preparation of anode coatings. The [IrCl6] 2− ion has an intense dark brown color, and can be readily reduced to the lighter-colored [IrCl6] 3− and vice versa. Iridium trichloride, IrCl3, which can be obtained in anhydrous form from direct oxidation of iridium powder by chlorine at 650 °C, or in hydrated form by dissolving Ir2O3 in hydrochloric acid, is often used as a starting material for the synthesis of other Ir(III) compounds. Another compound used as a starting material is ammonium hexachloroiridate(III), (chem|NH|4|)|3|IrCl|6. Iridium(III) complexes are diamagnetic (low-spin) and generally have an octahedral molecular geometry.

Organoiridium compounds contain iridium–carbon bonds where the metal is usually in lower oxidation states. For example, oxidation state zero is found in tetrairidium dodecacarbonyl, chem|Ir|4|(CO)|12, which is the most common and stable binary carbonyl of iridium. In this compound, each of the iridium atoms is bonded to the other three, forming a tetrahedral cluster. Some organometallic Ir(I) compounds are notable enough to be named after their discoverers. One is Vaska's complex, chem|IrCl(CO) [P(C|6|H|5|)|3|] |2, which has the unusual property of binding to the dioxygen molecule, O2. [cite journal| first =L. | last= Vaska | coauthors = DiLuzio, J.W. | title = Carbonyl and Hydrido-Carbonyl Complexes of Iridium by Reaction with Alcohols. Hydrido Complexes by Reaction with Acid |journal=Journal of the American Chemical Society |date = 1961 |volume = 83 |pages = 2784–2785 | doi = 10.1021/ja01473a054| authorlink=Lauri Vaska] Another one is Crabtree's catalyst (s, which accounts for their reactivity. [cite book | title=The Organometallic Chemistry of the Transition Metals| url=http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471662569.html| author=Crabtree, R. H.| date=2005| publisher=Wiley| isbn=978-0-471-66256-3 | oclc=224478241 61520528 85820471 |authorlink=Robert H. Crabtree]

Occurrence

Iridium is one of the rarest elements in the Earth's crust; with an average abundance of 0.001 ppm in crustal rock, it is 4 times less abundant than gold, 10 times less abundant than platinum, and 80 times less abundant than silver and mercury. Tellurium is about as abundant as iridium, and only three naturally-occurring elements are less abundant: rhenium, ruthenium, and rhodium, the last two being 10 times less abundant than iridium. In contrast to its low abundance in crustal rock, iridium is relatively common in meteorites, with concentrations of 0.5 ppm or more. It is thought that the overall concentration of iridium on Earth is much higher than what is observed in crustal rocks, but because of the density and siderophilic nature of iridium, it descended below the crust and into the Earth's core at a time when the planet was young and still molten.cite book |author=Renner, H.; Schlamp, G.; Kleinwächter, I.; Drost, E.; Lüschow, H. M.; Tews, P.; Panster, P.; Diehl, M.; Lang, J.; Kreuzer, T.; Knödler, A.; Starz, K. A.; Dermann, K.; Rothaut, J.; Drieselman, R. |chapter=Platinum group metals and compounds |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley |year=2002 |doi=10.1002/14356007.a21_075]

Iridium is found in nature as an uncombined element or in natural alloys; especially the iridium–osmium alloys, osmiridium (osmium rich), and iridiosmium (iridium rich).cite book | title = Nature's Building Blocks: An A-Z Guide to the Elements | last = Emsley | first=J. | publisher = Oxford University Press | year = 2003 | location = Oxford, England, UK | isbn = 0198503407 | chapter = Iridium | pages=201–204] In the nickel and copper deposits the platinum group metals occur as sulfides (i.e. (Pt,Pd)S)), tellurides (i.e. PtBiTe), antimonides (PdSb), and arsenides (i.e. PtAs2), in all of these compounds platinum is exchanged by a small amount of iridium and osmium. All the platinum group metals end up as alloys with raw nickel or raw copper. [cite journal | doi = 10.1016/j.mineng.2004.04.001 | journal = Minerals Engineering | volume = 17 | year = 2004 | pages = 961–979 | title =Characterizing and recovering the platinum group minerals—a review | first = Z. | last = Xiao | coauthors= Laplante, A. R.] Clarifyme|How does the previous sentence relate to iridium?|date=September 2008

Within the Earth's crust, iridium is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa, the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin, Canada with its large ore deposits are the two other large deposits. Smaller reserves can be found in the United States. Iridium is also found in secondary deposits, combined with platinum and other platinum group metals in alluvial deposits. The alluvial deposits used by pre-Columbian people in the Chocó Department, Colombia are still a source for platinum group metals. Total world reserve amounts have not been estimated.

K–T boundary presence

The K–T boundary (, are still releasing iridium. [cite book |title=The Cretaceous-Tertiary Event and Other Catastrophes in Earth History |author=Ryder, G.; Fastovsky, D. E.; Gartner, S. |publisher=Geological Society of America |year=1996 |isbn=0813723078 |pages=47] [cite journal |author=Toutain, J.-P.; Meyer, G. |year=1989 |title=Iridium-Bearing Sublimates at a Hot-Spot Volcano (Piton De La Fournaise, Indian Ocean) |journal=Geophysical Research Letters |volume=16 |issue=12 |pages=1391–1394]

History

The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. Native platinum used by ancient Ethiopianscite journal | title = The So-Called 'Platinum' Inclusions in Egyptian Goldwork | first = J. M. | last = Ogden | journal = The Journal of Egyptian Archaeology | volume = 62 | year = 1976 | pages = 138–144 | url = http://www.jstor.org/stable/3856354 | doi = 10.2307/3856354] and by South American culturescite journal | journal = Platinum Metals Rev.| year = 1980| volume = 24 | issue = 21 | pages = 70–79 | title =The Powder Metallurgy of Platinum| first = J. C. | last =Chaston] always contained a small amount of the other platinum group metals, including iridium. Platinum reached Europe as "platina" ("small silver"), found in the 17th century by the Spanish conquerors in a region today known as Department of Chocó, in Colombia. [cite journal |title=The Platinum of New Granada: Mining and Metallurgy in the Spanish Colonial Empire |author=McDonald, M. |journal=Platinum Metals Review |volume=3 |issue=4 |year=959 |pages=140–145 |url=http://www.platinummetalsreview.com/dynamic/article/view/pmr-v3-i4-140-145] The discovery that this metal was not an alloy of known elements, but instead a distinct new element, did not occur until 1748. [cite book|author=Juan, J.; de Ulloa, A.|year=1748|title=Relación histórica del viage a la América Meridional|volume=1|pages=606 |language=Spanish]

Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue. Joseph Louis Proust thought that the residue was graphite. The French chemists Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments.

In 1803, British scientist Smithson Tennant analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternatively with alkali and acids and obtained a volatile new oxide, which he believed to be of this new metal—which he named "ptene", from the Greek word _el. πτηνος (ptènos) for winged. [cite book |title=A System of Chemistry of Inorganic Bodies |author=Thomson, T. |publisher=Baldwin & Cradock, London; and William Blackwood, Edinburgh |year=1831 |pages=693] cite journal |doi=10.1595/147106704X4844 |title=Bicentenary of Four Platinum Group Metals. Part II: Osmium and iridium – events surrounding their discoveries |author=Griffith, W. P. |journal=Platinum Metals Review |volume=48 |issue=4 |year=2004 |pages=182–189] However, Tennant, who had the advantage of a much greater amount of residue, continued his research and identified the two previously undiscovered elements in the black residue, iridium and osmium. He obtained dark red crystals (probably of chem|Na|2| [IrCl|6] ·"n"chem|H|2|O) by a sequence of reactions with sodium hydroxide and hydrochloric acid. He named iridium after Iris ( _el. Ιρις), the Greek winged goddess of the rainbow and the messenger of the Olympian gods, because many of the salts he obtained were strongly colored."Iridium" literally means "of rainbows".] cite book| title = Discovery of the Elements | pages = 414–418 | author = Weeks, M. E. | year= 1968 | edition = 7 | publisher = Journal of Chemical Education| isbn = 0848685792| oclc = 23991202] Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.cite journal | title= A History of Iridium | first =L. B. | last =Hunt | journal = Platinum Metals Review| volume =31 | issue = 1 | year = 1987 | url = http://www.platinummetalsreview.com/dynamic/article/view/pmr-v31-i1-032-041 | pages= 32–41] [cite journal | title= On Two Metals, Found in the Black Powder Remaining after the Solution of Platina | first = S. | last = Tennant | journal = Philosophical Transactions of the Royal Society of London | volume = 94 | year =1804| pages= 411–418 | url = http://www.jstor.org/pss/107152 | doi= 10.1098/rstl.1804.0018]

John George Children was the first to manage to melt a sample of iridium in 1813 with the aid of "the greatest galvanic battery that has ever been constructed" (at that time). The first to obtain iridium in a high purity was Robert Hare in 1842. He found that it had a density of around 21.8 g/cm3 and noted that the metal is nearly unmalleable and very hard. The first melting in appreciable quantity was done by Henri Sainte-Claire Deville and Jules Henri Debray in 1860. They required burning more than 300 L of pure chem|O|2 and chem|H|2 for each kilogram of iridium.

These extreme difficulties in melting the metal limited the possibilities for handling iridium. John Isaac Hawkins was looking to obtain a fine and hard point for fountain pen nibs and in 1834 managed to create an iridium pointed gold pen. In 1880 John Holland and William Lofland Dudley were able to melt iridium by adding phosphorus and patented the process in the United States; British company Johnson Matthey later stated that they were using a similar process since 1837 and had already presented fused iridium at a number of World Fairs. The first use of an alloy of iridium with ruthenium in thermocouples was made by Otto Feussner in 1933. These allowed for the measurement of high temperatures in air, up to 2000 °C.

In 1957, Rudolf Mössbauer, in what has been called one of the "landmark experiments in twentieth century physics", [cite book |pages=179–190 |title=Landmark Experiments in Twentieth Century Physics |author=Trigg, G. L. |publisher=Courier Dover Publications |isbn=048628526X] discovered the resonant and recoil-free emission and absorption of gamma rays by atoms in a solid metal sample containing only 191Ir. [cite journal | first=R. L. | last = Mössbauer | authorlink = Rudolf Mössbauer | title = Gammastrahlung in Ir191 | journal = Zeitschrift für Physik A Hadrons and Nuclei | volume = 151 | pages = 124–143 | year = 1958 | language = German | doi = 10.1007/BF01344210] This phenomenon, known as the Mössbauer effect, has since been observed for other nuclei, such as 57Fe, and, developed as Mössbauer spectroscopy, has made important contributions to research in physics, chemistry, biochemistry, metallurgy, and mineralogy.cite book |title=Handbook of Ceramics and Composites |author=Chereminisoff, N. P. |publisher=CRC Press |year=1990 |isbn=082478006X |pages=424] Mössbauer received the Nobel Prize in Physics in 1961, just three years after he published his discovery. [cite book |title=Nobel Lectures, Physics 1942-1962 |publisher=Elsevier |year=1964 |chapter=The Nobel Prize in Physics 1961: presentation speech| first =I. | last =Waller | url =http://nobelprize.org/nobel_prizes/physics/laureates/1961/press.html]

Production

Iridium is obtained commercially as a by-product from nickel and copper mining and processing. During electrorefining of copper and nickel, noble metals such as silver, gold and the platinum group metals as well as selenium and tellurium settle to the bottom of the cell as "anode mud", which forms the starting point for the extraction of the platinum group metals.cite book | url = http://minerals.usgs.gov/minerals/pubs/commodity/platinum/myb1-2006-plati.pdf |publisher = United States Geological Survey USGS | title = 2006 Minerals Yearbook: Platinum-Group Metals| first = M. W. | last = George]

After ruthenium and osmium have been removed, iridium is separated by precipitating (NH4)2IrCl6 or by extracting [IrCl6] 2− with organic amines. The first method is similar to the procedure Tennant and Wollastone used for their separation. The second method can be planned as continuous liquid–liquid extraction and is therefore more suitable for industrial scale production. In either case, the product is reduced using hydrogen, yielding the metal as a powder or sponge that can be treated using powder metallurgy techniques. [cite journal | title =Processing of Iridium and Iridium Alloys | first = E. K. | last = Ohriner | journal = Platinum Metals Review | volume = 52 | issue = 3 | year = 2008 | pages = 186–197 | doi =10.1595/147106708X333827] [cite journal | first = L. B. | last = Hunt | coauthors = Lever, F. M. | journal = Platinum Metals Review | volume = 13 | issue = 4 | year = 1969 | pages = 126–138 | title = Platinum Metals: A Survey of Productive Resources to industrial Uses | url = http://www.platinummetalsreview.com/pdf/pmr-v13-i4-126-138.pdf | accessdate = ]

Annual production of iridium circa 2000 was around 3 tonnes or about 100,000 troy ounces (ozt).Like other precious metals, iridium is customarily traded in troy ounces, which are equivalent to about 31.1 grams.] The price of iridium as of 2007 was $440 USD/ozt, but the price fluctuates considerably, as shown in the table. The high volatility of the prices of the platinum group metals has been attributed to supply, demand, speculation, and hoarding, amplified by the small size of the market and instability in the producing countries. [cite journal |author=Hagelüken, C. |journal=Metall |volume=60 |issue=1–2 |year=2006 |pages=31–42 |title=Markets for the catalysts metals platinum, palladium, and rhodium |url=http://www.preciousmetals.umicore.com/publications/articles_by_umicore/general/show_Metal_PGMmarkets_200602.pdf ]

Applications

The global demand for iridium in 2007 was 119,000 troy ounces (3,700 kg), out of which 25,000 ozt (780 kg) were used for electrical applications such as spark plugs; 34,000 ozt (1,100 kg) for electrochemical applications such as electrodes for the chloralkali process; 24,000 ozt (750 kg) for catalysis; and 36,000 ozt (1,100 kg) for other uses.cite book | first = D. | last = Jollie | title=Platinum 2008 |publisher=Johnson Matthey |date=May 2008 | ISSN = 0268-7305 |url=http://www.platinum.matthey.com/uploaded_files/Pt2008/08_complete_publication.pdf| format = pdf]

Industrial

The high melting point, hardness and corrosion resistance of iridium and its alloys determine most of its applications. Iridium and especially iridium–platinum alloys or osmium–iridium alloys have a low wear and are used for example for multi-pored spinnerets, through which a plastic polymer melt is extruded to form fibers, such as rayon. [cite journal |title= Spinnerets for viscose rayon cord yarn | journal = Fibre Chemistry | volume =10 | issue = 4 | year = 1979 | doi = 10.1007/BF00543390 | pages = 377–378 | first = R. V. | last = Egorova | coauthors = Korotkov, B. V.; Yaroshchuk, E. G.; Mirkus, K. A.; Dorofeev N. A.; Serkov, A. T.] Osmium–iridium is used for compass bearings and for balances. [cite web | url = http://www.rsc.org/chemsoc/visualelements//PAGES/pdf/iridium.PDF | title = Iridium |publisher = Royal Society of Chemistry | work = Visual Elements Periodic Table | format = PDF | author = Emsley, J. | date = 2005-01-18 | accessdate = 2008-09-17]

Corrosion and heat resistance makes iridium an important alloying agent. Certain long-life aircraft engine parts are made of an iridium alloy and an iridium–titanium alloy is used for deep-water pipes because of its corrosion resistance. Iridium is also used as a hardening agent in platinum alloys. The Vickers hardness of pure platinum is 56 HV while platinum with 50% of iridium can reach over 500 HV. [cite journal | url = http://www.platinummetalsreview.com/pdf/pmr-v4-i1-018-026.pdf | journal = Platinum Metals Review | title = Iridium Platinum Alloys | first = A. S. |last = Darling | year = 1960 | volume =4| issue =l | pages = 18–26] [cite journal |doi = 10.1595/147106705X24409 | title = The Hardening of Platinum Alloys for Potential Jewellery Application| first = T. |last = Biggs| coauthors= Taylor, S. S.; van der Lingen, E.| journal = Platinum Metals Review | year = 2005 | volume = 49 | issue = 1 | pages = 2–15]

Devices that must withstand extremely high temperatures are often made from iridium. For example, high-temperature crucibles made of iridium are used in the Czochralski process to produce oxide single-crystals (such as sapphires) for use in computer memory devices and in solid state lasers. [cite journal |title= On the Use of Iridium Crucibles in Chemical Operations | first = W. | last = Crookes |authorlink=William Crookes| journal = Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character| volume = 80 | issue = 541 | year = 1908 | pages = 535–536 | url = http://www.jstor.org/pss/93031 |doi= 10.1098/rspa.1908.0046] cite journal |title= Increasing Applications for Iridium | first = J. R. | last = Handley | journal = Platinum Metals Review | volume = 30 | issue = 1 | year = 1986 | pages = 12–13 | url = http://www.platinummetalsreview.com/dynamic/article/view/pmr-v30-i1-012-013] The crystals, such as gadolinium gallium garnet and yttrium gallium garnet, are grown by melting pre-sintered charges of mixed oxides under oxidizing conditions at temperatures up to 2100 oC. Its resistance to arc erosion makes iridium alloys ideal for electrical contacts for spark plugs). [cite book | url = http://books.google.de/books?hl=de&lr=&id=I03qepnj2IwC | title = Euromat 99 | author = Stallforth, H.; Revell, P. A. | publisher= Wiley-VCH | year = 2000]

Iridium compounds are used as catalysts in the Cativa process for carbonylation of methanol to produce acetic acid.cite book |first=H. |last= Cheung |coauthors = Tanke, R. S.; Torrence, G. P.|chapter=Acetic acid |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley |year=2000 |doi=10.1002/14356007.a01_045] Iridium itself is used as a catalyst in a type of automobile engine introduced in 1996 called the direct-ignition engine.

Iridium is commonly used in complexes like Ir(mppy)3 and other complexes in organic light emitting diode technology to increase the efficiency from 25% to almost 100% due to triplet harvesting. [cite journal |title = Electrophosphorescence from substituted poly(thiophene) doped with iridium or platinum complex | doi = 10.1016/j.tsf.2004.05.095| year = 2004 | journal = Thin Solid Films | volume = 468 | issue = 1–2| pages = 226–233 | first = X. | last = Wang| coauthors = Andersson, M. R.; Thompson, M. E.; Inganäsa, O. ] One of the major uses for these family of complexes have been the flat panel displays that are found in televisions or monitors. [cite web|url=http://sa.rochester.edu/jur/issues/fall2002/tonzetich.pdf|title=Organic Light Emitting Diodes|publisher=Rochester University] [cite journal| title=New Trends in the Use of Transition Metal-Ligand Complexes for Applications in Electroluminescent Devices | author = Holder, E. |coauthors = Langefeld, B. M. W.; Schubert, U. S. | publisher = | journal = Advanced Materials | volume = 17 | issue = 9 | pages = 1109–1121 | date = 2005-04-25 | accessdate = |doi=10.1002/adma.200400284]

cientific and medical

An alloy of 90% platinum and 10% iridium was used in 1889 to construct the International Prototype Meter and kilogram mass, kept by the International Bureau of Weights and Measures near Paris. The meter bar was replaced as the definition of the fundamental unit of length in 1960 by a line in the atomic spectrum of krypton,The definition of the meter was changed again in 1983. The meter is currently defined as the distance traveled by light in a vacuum during a time interval of frac|299,792,458 of a second.] but the kilogram prototype is still the international standard of mass.cite web | url=http://www.mel.nist.gov/div821/museum/timeline.htm | publisher = NIST |first =W. B. | last = Penzes |title=Time Line for the Definition of the Meter |year=2001 |accessdate=2008-09-16]

Iridium has been used in the radioisotope thermoelectric generators of unmanned spacecraft such as the "Voyager", "Viking", "Pioneer", "Cassini", "Galileo", and "New Horizons". Iridium was chosen to encapsulate the plutonium-238 fuel in the generator because it can withstand the operating temperatures of up to 2000 °C and for its great strength.

Another use in astronomy concerns X-ray telescopes. The mirrors of the Chandra X-ray Observatory are coated with a layer of iridium 60 nm thick. Iridium proved to be the best choice for reflecting X-rays after nickel, gold, and platinum were tested. The iridium layer, which had to be smooth to within a few atoms, was applied by depositing iridium vapor under high vacuum on a base layer of chromium. [cite web |title=Face-to-Face with Jerry Johnston, CXC Program Manager & Bob Hahn, Chief Engineer at Optical Coating Laboratories, Inc., Santa Rosa, CA |publisher=Harvard-Smithsonian Center for Astrophysics; Chandra X-ray Center |url=http://chandra.harvard.edu/press/bios/johnston.html |accessdate=2008-09-24 |year=1995]

Iridium is used in particle physics for the production of antiprotons, a form of antimatter. Antiprotons are made by shooting a high-intensity proton beam at a "conversion target", which needs to be made from a very high density material. Although tungsten may also be used, iridium has the advantage of better stability under the shock waves induced by the temperature rise due to the incident beam. [cite journal |title = Production of low-energy antiprotons | journal =Zeitschrift Hyperfine Interactions | volume =109| year = 1997 |doi = 10.1023/A:1012680728257 | pages = 33–41 | first = D. | last =Möhl]

Carbon–hydrogen bond activation (C–H activation) is an active area of research that investigates reactions that cleave carbon–hydrogen bonds, a type of bond traditionally regarded as unreactive. The first reported successes at activating C–H bonds in saturated hydrocarbons, published in 1982, [cite journal |title=Carbon-hydrogen activation in completely saturated hydrocarbons: direct observation of M + R-H -> M(R)(H) |author=Janowicz, A. H.; Bergman, R. G. |journal=Journal of the American Chemical Society |year=1982 |volume=104 |issue=1 |pages=352–354 |doi=10.1021/ja00365a091] used organometallic iridium complexes that undergo an oxidative addition with the hydrocarbon.

Iridium complexes are also being investigated as catalysts for asymmetric hydrogenation. These catalysts have been used in the synthesis of natural products and able to hydrogenate certain difficult substrates, such as unfunctionalized alkenes, enantioselectively. [cite journal |doi=10.1002/chem.200500755 |year=2006 |author=Källström, K; Munslow, I; Andersson, P. G. |title=Ir-catalysed asymmetric hydrogenation: Ligands, substrates and mechanism |volume=12 |issue=12 |pages=3194–3200 |pmid=16304642 |journal=Chemistry – A European Journal] [cite journal |doi=10.1021/ar700113g |year=2007 |author=Roseblade, S. J.; Pfaltz, A. |title=Iridium-catalyzed asymmetric hydrogenation of olefins |volume=40 |issue=12 |pages=1402–1411 |pmid=17672517 |journal=Accounts of Chemical Research]

The radioisotope iridium-192 is used as a radiography source for non-destructive testing of materials. [cite journal |title= The use and scope of Iridium 192 for the radiography of steel | first = R. | last = Halmshaw | year = 1954 | journal = Br. J. Appl. Phys. | volume = 5 | pages = 238–243 | doi = 10.1088/0508-3443/5/7/302] Additionally, 192Ir is used as a source of gamma-ray radioactivity for the treatment of cancer using brachytherapy, a form of radiotherapy where a sealed radioactive source is placed inside or next to the area requiring treatment. Specific treatments include high dose rate prostate brachytherapy, bilary duct brachytherapy, and intracavitary cervix brachytherapy.

Historical

Iridium–osmium alloys were used to tip fountain pen nibs. The first major use of iridium was in 1834 in nibs mounted on gold. Since 1944, the famous Parker 51 fountain pen was fitted with a nib tipped a ruthenium and iridium alloy (with 3.8% iridium). The tip material in modern fountain pens is still conventionally called "iridium," although there is seldom any iridium in it; other metals such as tungsten have taken its place. [cite journal |url=http://www.pencollectors.com/nib-corner.html |journal=The PENnant |volume=XIII |issue=2 |year=1999 |title=Notes from the Nib Works—Where's the Iridium? |author=Mottishaw, J.]

An iridium–platinum alloy was used for the touch holes or vent pieces of cannons. According to a report of the Paris Exhibition of 1867, one of the pieces being exhibited by Johnson and Matthey "has been used in a Withworth gun for more than 3000 rounds, and scarcely shows signs of wear yet. Those who know the constant trouble and expense which are occasioned by the wearing of the vent-pieces of cannon when in active service, will appreciate this important adaptation". [cite journal |editor=Crookes, W. |volume=XV |year=1867 |journal=The Chemical News and Journal of Physical Science |title=The Paris Exhibition |pages=182 | doi= ]

The pigment "iridium black", which consists of very finely divided iridium, is used for painting porcelain an intense black; it was said that "all other porcelain black colors appear grey by the side of it". [cite book |title=The Playbook of Metals: Including Personal Narratives of Visits to Coal, Lead, Copper, and Tin Mines, with a Large Number of Interesting Experiments Relating to Alchemy and the Chemistry of the Fifty Metallic Elements |author=Pepper, J. H. |publisher=Routledge, Warne, and Routledge |year=1861 |pages=455]

Precautions

Iridium in bulk metallic form is not biologically important or hazardous to health due to its lack of reactivity with tissues; there are only about 20 parts per trillion of iridium in human tissue. However, finely divided iridium powder can be hazardous to handle, as it is an irritant and may ignite in air.cite book |title=Kirk Othmer Encyclopedia of Chemical Technology |first =R. J. | last = Seymour | coauthors = O'Farrelly, J. I. |chapter=Platinum-group metals |doi=10.1002/0471238961.1612012019052513.a01.pub2 |year=2001 |publisher=Wiley] Very little is known about the toxicity of iridium compounds because they are used in very small amounts, but soluble salts, such as the iridium halides, could be hazardous due to elements other than iridium or due to iridium itself. However, most iridium compounds are insoluble, which makes absorption into the body difficult.

A radioisotope of iridium, 192Ir, is dangerous like other radioactive isotopes. The only reported injuries related to iridium concern accidental exposure to radiation from 192Ir used in brachytherapy.cite book |title=Encyclopaedia of Occupational Health and Safety |first =J. | last = Mager Stellman | coauthors = |chapter=Iridium |isbn=9789221098164 |year=1998|publisher=International Labour Organization| pages =63.19 | url = http://books.google.de/books?id=nDhpLa1rl44C |oclc=35279504 45066560] High-energy gamma radiation from 192Ir can increase the risk of cancer. External exposure can cause burns, radiation poisoning, and death. Ingestion of 192Ir can burn the linings of the stomach and the intestines. [cite web| title = Radioisotope Brief: Iridium-192 (Ir-192)| work = Radiation Emergencies| publisher = Center for Disease Control and Prevention| date = 2004-08-18| url = http://emergency.cdc.gov/radiation/isotopes/pdf/iridium.pdf| format = PDF| accessdate = 2008-09-20] 192Ir, 192mIr, and 194mIr tend to deposit in the liver, and can pose health hazards from both gamma and beta radiation.cite web| title = Iridium| work = Human Health Fact Sheet| publisher = Argonne National Laboratory| date = March 2005| url = http://www.ead.anl.gov/pub/doc/Iridium.pdf| format = PDF| accessdate = 2008-09-20]

Notes

References

External links


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