Indium (In) is rare metal of Boron group of periodic table with atomic number 49. It is silver-white with a slight bluish tint. Indium is the softest metal except for alkali metals that can even be scratched with nails. Indium has strong plasticity, ductility and can be pressed into sheets. Indium is a face-centered tetragonal crystal structure at 27℃ with a=0.4600 and c=0.4946. The tetragonality of unit (=c/a) at 27℃ is 1.076.^[1]

                                                                                                                     Figure 1 Indium sheets ^[2]

Although it is hardly researched or used for a hundred year since its discovery, it has been attached more and more importance with the rapid development of world industry.^[3] Indium was discovered in 1863 by Ferdinand Reich and Hilnham Theodore Richter by spectroscopic methods. After thallium was discovered, Reich hoped to obtain some thallium to research the properties. So, in 1863 he began searching for the metal in zinc mines produced in Schimansfort, Freiburg. Although the experiment took a lot of time, he didn't extract the element he wanted. However, he obtained a straw-yellow precipitate of unknown composition and decided to analyse it by spectroscopic method. Because he was an achromate, he turned to his assistant Hilnham Theodore Richter. He found a bright indigo line in the spectroscope, which did not match the two bright blue lines of caesium, and named it indium after the Greek word for "indikon". In 1867, a 0.5 kg ingot was unveiled at the Expo.^[4]

                                                                                                                 Figure 2 Ferdinand Reich (1799 – 1882) ^[5]

From the discovery of indium to the 1950s, the research and utilization of indium are as rare as its amount, the attention of scientists to indium is synchronized with the industrial development of the world. Due to the special physical properties of indium, its application is rapidly expanding, especially in the past 10 years. Research on indium has since become more extensive.

Much of the research on indium began in the 1950s. The researchers in this period were mainly western scholars, focusing on the geochemical properties of indium ^[6], indium intrusive rocks and minerals ^[7], and trace elements including indium sulfides ^[8] are representative of this period. These studies roughly determined the distribution of indium in various rocks and laid the foundation for later research on indium.

The Soviet scholars had pushed the research on indium to the climax during the 1960s to 1970s. They studied the distribution of indium in different rock and ore deposits within the territory of the former Soviet and found a group of rich indium deposits. Studies on indium in rocks and deposits of different ages show that indium content increases from old to new. During this period, other scholars also made research work in the west, such as the content of indium in meteorites and different rock.

During the 1980s and 1990s, Soviet scholars continued to strengthen their research on indium. Greta ^[9] found that coal contained a high amount of indium. Indium deposits or indium orebodies have been reported in Sweden, France, Canada, and the United States. During this period, Japanese scholars made great progress in the research on indium enrichment and mineralization. Indium orebodies and deposits were discovered in Miami, Kagoshima, Fengyu, Toyota, Nakakoshi, and other places, making Japan's indium resources leap to the front in the world. ^[10]

At the same time, the research on indium mineralization has been significantly strengthened by foreign scholars, who have successively carried out studies on indium presence form ^[11] experimental studies on indium mineral synthesis ^[12] studies on the relationship between indium and tin in basalt, sulfide, and mantle.

Indium does not have its own mineral and often extracted from smelting plant of lead and zinc as by-product. The most common purification method is solvent extraction in the world. This method uses acidic organic reagents including simple acidic-type extractants as well as acidic chelating-type extractants to form neutral complex species. In addition, it takes advantages of solvating organic reagents and basic organic reagents to form solvation and ion pair species.^[13] There are several extraction technologies which utilize liquid membranes, chelating resins, impregnated resins and microcapsules as separation materials. These new technologies provide new options for indium extraction and recovery.

Indium has a wide range of applications. The production of ITO targets (for the production of liquid crystal displays and flat panel screens) is the main consumption area of indium ingots. ^[14] A considerable amount of indium is used in the production of semiconductor materials. ^[15] Indium is also used in gaskets or linings for high-altitude instruments and aerospace equipment. In the atomic energy industry, indium is used as control rods for nuclear reactors.^[16] In addition, indium has a good application prospect in the field of stomatology and can be used as an alloy of denture.

The manufacture processes of indium are similar to those of metals. For metals, the fabrication methods are roughly classified into forming operations, casting and miscellaneous according to the operation sequences. The specific methods suitable for indium were chosen depend on several factors. The properties of indium are the most important. The size and shape of the finished piece are also significant.

Indium is rare metal obtained by purification which has many excellent properties. It has a promising future in aerospace, radio and electronic industries, medical, national defense, high-tech, energy and other fields.

[1] Oshima, Y., Nangou, T., Hirayama, H. and Takayanagi, K. (2001). Face centered cubic indium nano-particles studied by UHV-transmission electron microscopy. Surface Science, 476(1-2), pp.107-114.

[2] aliexpress.com. 2020. US $49.02 5% OFF|Indium Sheet Indium Foil Size: 100Mm*200Mm*0.1Mm, Laser Cooling Coating Sealing Material|Air Conditioner Parts| - Aliexpress. [online] Available at: <https://www.aliexpress.com/item/32871611153.html> [Accessed 12 June 2020].

[3] Encyclopedia Britannica. (2020). Indium | chemical element. [online] Available at: https://www.britannica.com/science/indium [Accessed 5 Mar. 2020].

[4] Reich, F. and Richter, T. (1863). Ueber das Indium. Journal fr Praktische Chemie, 90(1), pp.172-176.

[5] Helmenstine, T., 2020. Today In Science History - February 19 - Ferdinand Reich And The Discovery Of Indium. [online] Science Notes and Projects. Available at: <https://sciencenotes.org/today-science-history-february-19-ferdinand-reich/> [Accessed 12 June 2020].

[7]Shaw D. The geochemistry of indium. Geochim Cosmochim Acta. 1952;2(3):185-206. doi:10.1016/0016-7037(52)90006-9
[8]Wager L, Smit J, Irving H. Indium content of rocks and minerals from the Skaergaard intrusion, East Greenland. Geochim Cosmochim Acta. 1958;13(2-3):81-86. doi:10.1016/0016-7037(58)90041-3
[9]Frenkel R, Sherby O, Dorn J. Activation energies for creep of cadmium, indium, and tin. Acta Metallurgica. 1955;3(5):470-472. doi:10.1016/0001-6160(55)90137-1
[10] Eskenazy G. On the geochemistry of indium in coal-forming process. Geochim Cosmochim Acta. 1980;44(7):1023-1027. doi:10.1016/0016-7037(80)90290-2
[11]MATSUSAKI K, SATA T. Removal of sulfate interference in the determination of indium and gallium by graphite furnace AAS. Bunseki kagaku. 1994;43(8):641-643. doi:10.2116/bunsekikagaku.43.641
[12] Rodriguez J. Acetylacetonate of indium: Structure and reactivity with 1,3-dienes. Reaction Kinetics and Catalysis Letters. 1988;36(1):91-96. doi:10.1007/bf02071146
[13] Rousina R, Shivakumar G. Electrical contacts to semiconducting indium telluride films. J Mater Sci Lett. 1988;7(5):463-465. doi:10.1007/bf01730689 [13] Paiva, A. (2001). RECOVERY OF INDIUM FROM AQUEOUS SOLUTIONS BY SOLVENT EXTRACTION. Separation Science and Technology, 36(7), pp.1395-1419.

[14] The Electroluminescent Light Sabre. Nanotechnology News Archive. Azonano. June 2, 2005.

[15] Bachmann, K. (1981). Properties, Preparation, and Device Applications of Indium Phosphide. Annual Review of Materials Science, 11(1), pp.441-484.

[16] Scoullos, M. (2001). Mercury, cadmium, lead. Dordrecht: Kluwer Academic Publishers, p.222.