Indium

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Indium (Template:PronEng) is a chemical element with chemical symbol In and atomic number 49. This rare, soft, malleable and easily fusible poor metal is chemically similar to aluminium or gallium but more closely resembles zinc (zinc ores are also the primary source of this metal). Its current primary application is to form transparent electrodes from indium tin oxide in liquid crystal displays. 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's also used for making particularly low melting point alloys, and is a component in some lead-free solders.

Notable characteristics

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.

One unusual property of indium is that its most common isotope is slightly radioactive; it very slowly decays by beta emission to tin. This radioactivity is not considered hazardous, mainly because its half-life is 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. Unlike its period 5 neighbor cadmium, indium is not a notorious cumulative poison.

Applications

The first large-scale application for indium was as a coating for bearings in high-performance aircraft engines during World War II. Afterwards, 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. Other uses:

• For manufacture of low-melting-temperature alloys. An alloy consisting of 24% indium and 76% gallium is liquid at room temperature.
• Some indium compounds such as indium antimonide, indium phosphide, and indium nitride are semiconductors with useful properties.
• Component required for 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 (LDs) 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.
• Can also be plated onto metals and evaporated onto glass which forms a mirror which is as good as those made with silver but has higher corrosion resistance.
• Indium oxide (In₂O₃) is used as a transparent conductive glass substrate in the making of electroluminescent panels.
• Used as a light filter in low pressure sodium vapor lamps.
• Indium's freezing 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.
• ¹¹¹In emits gamma radiation and is used in scintigraphy, a technique of medical imaging. Scintigraphy has many applications, including early phase drug development, and monitoring the activity of white blood cells. A blood test is taken from the patient, white cells removed and labelled with the radioactive ¹¹¹In, then re-injected back into the patient. Gamma imaging will reveal any areas of high white cell activity such as an abscess.
• Very small amounts used in aluminium alloy sacrificial anodes (for salt water applications) to prevent passivation of the aluminium.
• In the form of a wire it is used as a vacuum seal in cryogenics applications.
• Used as a calibration material for thermogravimetric analysis devices.

History

Indium (named after the country of India) was discovered by the Indian Ferdinand Reich and Hieronymous Theodor Richter in 1863 while they were testing Lead ores with a pick in search of gold. Richter went on to isolate the metal in 1867.

Occurrence and consumption

Ductile Indium wire

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 . Up until 1924, there was only about a gram of isolated indium on the planet. 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, BC, is the largest single source, with production of 32,500 kg in 2005, 41,800 kg in 2004 and 36,100 kg in 2003.

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 televisions, 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 is likely to continue to increase with large-scale manufacture of CIGS-based thin film solar technology starting by several companies in 2008, including Nanosolar and Miasole.

Based on content of indium in zinc ore stocks, there is a world-wide 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, such estimates are often regarded as alarmist and scaremongering . 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, a less abundant element, is currently mined at approximately 18,300 tonnes per annum , which is 40 times greater than current indium mining rates.

Precautions

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.

This may not be the case with indium compounds: there is some unconfirmed evidence that suggests that indium has a low level of toxicity. For example, indium trichloride anhydrous (InCl₃) is quite toxic, while indium phosphide (InP) is both toxic and a suspected carcinogen.

See also

• Indium compounds

References

• Los Alamos National Laboratory – Indium

External links

• WebElements.com – Indium
• The Indium Corporation
• Reducing Agents > Indium low valent

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