Indium ( /ˈɪndiəm/ IN-dee-əm) is a chemical element with chemical symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to aluminium. Indium was discovered in 1863 and named for the indigo blue line in its spectrum that was the first indication of its existence in zinc ores, as a new and unknown element. The metal was first isolated in the following year. Zinc ores continue to be the primary source of indium, where it is found in compound form. Very rarely the element can be found as grains of native (free) metal, but these are not of commercial importance.
Indium's current primary application is to form transparent electrodes from indium tin oxide in liquid crystal displays and touchscreens, and this use largely determines its global mining production. It is widely used in thin-films to form lubricated layers (during World War II it was widely used to coat bearings in high-performance aircraft). It is also used for making particularly low melting point alloys, and is a component in some lead-free solders.
Indium is not known to be used by any organism. In a similar way to aluminum salts, indium (III) ions can be toxic to the kidney when given by injection, but oral indium compounds do not have the chronic toxicity of salts of heavy metals, probably due to poor absorption in basic conditions. Radioactive indium-111 (in very small amounts on a chemical basis) is used in nuclear medicine tests, as a radiotracer to follow the movement of labeled proteins and white blood cells in the body.
- 1 Characteristics
- 2 Isotopes
- 3 Creation
- 4 History
- 5 Occurrence and consumption
- 5.1 Resources
- 5.2 Production
- 6 Applications
- 6.1 Electronics
- 6.2 Metal and alloys
- 6.3 Other uses
- 7 Precautions
- 8 See also
- 9 References
- 10 External links
Indium wetting the glass surface of a test tube
Indium is a very soft, silvery-white, relatively rare true metal with a bright luster. As a pure metal, indium emits a high-pitched "cry", when it is bent. Both gallium and indium are able to wet glass. A number of standard electrode potentials, depending on the reaction under study, is reported for indium:
- 0.40 In2+ + e− ↔ In+
- 0.49 In3+ + e− ↔ In2+
- 0.40 In2+ + 3 e− ↔ In+
- 0.338 In3+ + 3 e− ↔ Ino
Unlike its period 5 neighbor cadmium, indium is not a cumulative poison.
Indium demonstrates the inert pair effect to a limited extent, forming some In (I) compounds such as Indium(I) bromide. The most stable oxidation state for indium, however, is still (III). Unlike the corresponding thallium compounds, indium (I) compounds are not stable in water.
Main article: Isotopes of indium
Indium in nature consists of two primordial nuclides. One unusual property of indium (shared only with rhenium) is that although it possesses a stable isotope, its most common (abundant) isotope (95.7%) is slightly and measurably radioactive. This isotope, indium-115 very slowly decays by beta emission to tin. This decay has a half-life of 4.41×1014 years, four orders of magnitude larger than the age of the universe and nearly 50,000 times longer than that of natural thorium.
Indium is created via the long S-process in low-medium mass stars (.6 -> 10 solar masses). This takes thousands of years to do. It requires a cadmium atom to capture sufficient neutrons and then undergo Beta decay.
In 1863 the German chemists Ferdinand Reich and Hieronymous Theodor Richter were testing ores from the mines around Freiberg, Saxony. They dissolved the minerals pyrite, arsenopyrite, galena and sphalerite in hydrochloric acid and distilled the raw zinc chloride. As it was known that ores from that region sometimes contain thallium they searched for the green emission lines with spectroscopic methods. The green lines were absent but a blue line was present in the spectrum. As no element was known with a bright blue emission they concluded that a new element was present in the minerals. They named the element with the blue spectral line indium, from the indigo color seen in its spectrum. Richter went on to isolate the metal in 1864. At the World Fair 1867 an ingot of 0.5 kg (1.1 lb) was presented.
 Occurrence and consumption
Indium ranks 61st in abundance in the Earth's crust at approximately 0.25 ppm, which means it is more than three times as abundant as silver, which occurs at 0.075 ppm. Fewer than 10 indium minerals are known, none occurring in significant deposits. Examples are the dzhalindite (In(OH)3) and indite (FeIn2S4).
Based on content of indium in zinc ore stocks, there is a worldwide reserve base of approximately 6,000 tonnes of economically viable indium. This figure has led to estimates suggesting that, at current consumption rates, there is only 13 years' supply of indium left. However, the Indium Corporation, the largest processor of indium, claims that, on the basis of increasing recovery yields during extraction, recovery from a wider range of base metals (including tin, copper and other polymetallic deposits) and new mining investments, the long-term supply of indium is sustainable, reliable and sufficient to meet increasing future demands.
This conclusion also seems reasonable in light of the fact that silver, three times less abundant than indium in the earths crust, is currently mined at approximately 18,300 tonnes per annum, which is 40 times greater than current indium mining rates.
The lack of indium mineral deposits and the fact that indium is enriched in sulfidic lead, tin, copper, iron and predominately in zinc deposits, makes zinc production the main source for indium. The indium is leached from slag and dust of zinc production. Further purification is done by electrolysis.
Indium is produced mainly from residues generated during zinc ore processing but is also found in iron, lead, and copper ores. Canada is a leading producer of indium. The Teck Cominco refinery in Trail, British Columbia, is the largest single source indium producer, with production of 32,500 kg in 2005, 41,800 kg in 2004 and 36,100 kg in 2003. South American Silver's Malku Khota property in Bolivia is the largest resource of indium with an indicated resource of 845,000 kg and inferred resource of 968,000 kg.Adex Mining Inc.’s Mount Pleasant Mine in New Brunswick, Canada, holds about 15 to 20% of the world’s total known indium resources.
The amount of indium consumed is largely a function of worldwide LCD production. Worldwide production is currently 476 tonnes per year from mining and a further 650 tonnes per year from recycling. Demand has risen rapidly in recent years with the popularity of LCD computer monitors and television sets, which now account for 50% of indium consumption. Increased manufacturing efficiency and recycling (especially in Japan) maintain a balance between demand and supply. Demand increased as the metal is used in LCDs and televisions, and supply decreased when a number of Chinese mining concerns stopped extracting indium from their zinc tailings. In 2002, the price was US$94 per kilogram. The recent changes in demand and supply have resulted in high and fluctuating prices of indium, which from 2005 to 2007 ranged from US$700/kg to US$1,000/kg. Demand for indium may increase with large-scale manufacture of CIGS-based thin film solar technology starting by several companies in 2008, including Nanosolar and Miasole.
A magnified image of an LCD screen showing RGB pixels. Individual transistors are seen as white dots in the bottom part.
The first large-scale application for indium was as a coating for bearings in high-performance aircraft engines during World War II. Afterward, production gradually increased as new uses were found in fusible alloys, solders, and electronics. In the 1950s, tiny beads of it were used for the emitters and collectors of PNP alloy junction transistors. In the middle and late 1980s, the development of indium phosphide semiconductors and indium tin oxide thin films for liquid crystal displays (LCD) aroused much interest. By 1992, the thin-film application had become the largest end use.
- Indium oxide (In2O3) and indium tin oxide (ITO) are used as a transparent conductive coating applied to glass substrates in the making of electroluminescent panels.
- Some indium compounds such as indium antimonide, indium phosphide, and indium nitride are semiconductors with useful properties.
- Indium is used in the synthesis of the semiconductor copper indium gallium selenide (CIGS), which is used for the manufacture of thin film solar cells.
- Used in light-emitting diodes (LEDs) and laser diodes based on compound semiconductors such as InGaN, InGaP that are fabricated by Metalorganic Vapor Phase Epitaxy (MOVPE) technology.
- The ultrapure metalorganics of indium, specifically high purity trimethylindium (TMI) is used as a precursor in III-V compound semiconductors, while it is also used as the semiconductor dopant in II-VI compound semiconductors.
- As one of many substitutes for mercury in alkaline batteries to prevent the zinc from corroding and releasing hydrogen gas (e.g., US Pat US5188869)
 Metal and alloys
Ductile indium wire
- Very small amounts used in aluminium alloy sacrificial anodes (for salt water applications) to prevent passivation of the aluminium.
- To bond gold electrical test leads to superconductors, indium is used as a conducting glue and applied under a microscope with precision tweezers.
- In the form of a wire it is used as a vacuum seal and a thermal conductor in cryogenics and ultra-high vacuum applications. For example, in manufacturing gaskets which deform to fill gaps.
- Used as a calibration material for Differential scanning calorimetry.
- It is an ingredient in the alloy Galinstan, which is liquid at room temperature while not being toxic like mercury.
 Other uses
- Indium tin oxide is used as a light filter in low pressure sodium vapor lamps. The infrared radiation is reflected back into the lamp, which increases the temperature within the tube and therefore improves the performance of the lamp.
- Indium's melting point of 429.7485 K (156.5985 °C) is a defining fixed point on the international temperature scale ITS-90.
- Indium's high neutron capture cross section for thermal neutrons makes it suitable for use in control rods for nuclear reactors, typically in an alloy containing 80% silver, 15% indium, and 5% cadmium.
- In nuclear engineering, the (n,n') reactions of 113In and 115In are used to determine magnitudes of neutron fluxes.
- Indium is also used as a thermal interface material by personal computer enthusiasts in the form of pre-shaped foil sheets fitted between the heat-transfer surface of a microprocessor and its heat sink. The application of heat partially melts the foil and allows the indium metal to fill in any microscopic gaps and pits between the two surfaces, removing any insulating air pockets that would otherwise compromise heat transfer efficiency.
- 111In emits gamma radiation and is used in indium leukocyte imaging, or indium scintigraphy, a technique of medical imaging. Indium leukocyte scintigraphy has many applications, including early phase drug development, and the monitoring of activity of white blood cells. For the test, blood is taken from the patient, white cells removed, labeled with the radioactive 111In, then re-injected back into the patient. Gamma imaging will then reveal any areas of on-going white cell localization such as new and developing areas of infection.
Pure indium in metal form is considered non-toxic by most sources. In the welding and semiconductor industries, where indium exposure is relatively high, there have been no reports of any toxic side-effects.
Indium compounds, like aluminum compounds, complex with hydroxyls to form insoluble salts in basic conditions, and are thus not well-absorbed from food, giving them fairly low oral toxicty. Soluble indium (III) is toxic when delivered parenterally, however, causing damage primarily to the kidney (both inner and outer parts). Other indium compounds are toxic when administered outside the GI tract: for example, anhydrous indium trichloride (InCl3) and indium phosphide (InP) are quite toxic when delivered into the lungs (the latter is a suspected carcinogen).[
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