|Name, Symbol, Number||indium, In, 49|
|Chemical series||poor metals|
|Group, Period, Block||13, 5, p|
|Appearance||silvery lustrous gray
|Atomic mass||114.818(3) g/mol|
|Electron configuration||[Kr] 4d10 5s2 5p1|
|Electrons per shell||2, 8, 18, 18, 3|
|Density (near r.t.)||7.31 g/cm³|
|Liquid density at m.p.||7.02 g/cm³|
|Melting point||429.75 K
(156.60 °C, 313.88 °F)
|Boiling point||2345 K
(2072 °C, 3762 °F)
|Heat of fusion||3.281 kJ/mol|
|Heat of vaporization||231.8 kJ/mol|
|Heat capacity||(25 °C) 26.74 J/(mol·K)|
|Electronegativity||1.78 (Pauling scale)|
|1st: 558.3 kJ/mol|
|2nd: 1820.7 kJ/mol|
|3rd: 2704 kJ/mol|
|Atomic radius||155 pm|
|Atomic radius (calc.)||156 pm|
|Covalent radius||144 pm|
|Van der Waals radius||193 pm|
|Magnetic ordering||no data|
|Electrical resistivity||(20 °C) 83.7 nΩ·m|
|Thermal conductivity||(300 K) 81.8 W/(m·K)|
|Thermal expansion||(25 °C) 32.1 µm/(m·K)|
|Speed of sound (thin rod)||(20 °C) 1215 m/s|
|Speed of sound (thin rod)||(r.t.) 11 m/s|
|Brinell hardness||8.83 MPa|
|CAS registry number||7440-74-6|
Indium (chemical symbol In, atomic number 49) is a rare, soft, malleable and easily fusible metal. It is chemically similar to aluminum and gallium, but it looks more like zinc. Zinc ores are also the primary source of this metal.
Currently, the primary application of indium is to make thin, transparent electrodes from indium tin oxide for liquid crystal displays (LCDs). It is also used as a semiconductor dopant, as a plating on metals and glass (for mirrors), as a light filter in sodium vapor lamps, and as a component in low-melting-temperature alloys. Several indium compounds are useful as semiconductors, and the oxide is good for making electroluminescent panels. In addition, indium, antimonide, and arsenide are used in infrared detectors.
Indium is produced mainly from residues generated during zinc ore processing, but it is also found in iron, lead, and copper ores. The amount of indium consumed is largely a function of worldwide LCD production. 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.
Up until 1924, there was only about one gram of isolated indium on the planet. The Earth is estimated to contain about 0.1 parts per million (ppm) of indium. This means it is about as abundant as silver, but indium is nearly three times more expensive by weight. Canada is a leading producer of indium. Worldwide production is typically over 300 metric tons per year, but demand has risen rapidly with the increased popularity of LCD computer monitors and television sets.
Indium was discovered by Ferdinand Reich and Hieronymous Theodor Richter in 1863, when they were testing zinc ores with a spectrograph in search of thallium. The element was named after the indigo line in its atomic spectrum. It is interesting to note that most elements were discovered while searching for other elements. Richter went on to isolate the metal in 1867.
In the periodic table, indium lies in group 13 (former group 3A), between gallium and thallium, and in the same group as aluminum. Consequently, its properties resemble those of these three elements. In addition, it is situated in period 5, between cadmium and tin. It is also said to be one of the "poor metals"—elements located between the transition metals and metalloids in the periodic table.
One unusual property of indium is that its most common isotope, 115In, is slightly radioactive—it decays very slowly by beta emission to tin. The estimated abundance of 115In is about 95.7%, while that of the stable isotope, 113In, is 4.3%.
The radioactivity of 115In is not considered hazardous, mainly because its decay rate is nearly 50,000 times slower than that of natural thorium, with a half-life of 4×1014 years. Also, indium is not a notorious cumulative poison, like its neighbor cadmium, and is relatively rare.
Numerous other radioactive isotopes of indium are known, but most of them are extremely short-lived.
- Indium antimonide (InSb): This crystalline compound is a narrow-gap semiconductor material. It is used in infrared detectors, including thermal-imaging cameras, infrared homing missile guidance systems, and instruments for infrared astronomy.
- Indium arsenide or indium monoarsenide (InAs): It is a semiconductor composed of indium and arsenic. It has the appearance of gray cubic crystals, with a melting point 942 °C. It is used for the construction of infrared detectors (wavelength range of 1-3.8 µm) and diode lasers. Alloyed with gallium arsenide, it forms indium gallium arsenide, which is also used in the semiconductor industry.
- Indium nitride (InN): This is a small bandgap semiconductor material that has potential applications in solar cells and high speed electronics. Currently there is research into developing solar cells using nitride-based semiconductors.
- Indium phosphide (InP): It is a binary semiconductor, composed of indium and phosphorus. It is used in high-power and high-frequency electronics because of its superior electron velocity, compared with the more common semiconductors silicon and gallium arsenide. It has a direct bandgap, making it useful for optoelectronics devices such as laser diodes.
The first large-scale application for indium was as a coating for bearings in high-performance aircraft engines during World War II. Later, production gradually increased as new uses were found in fusible alloys, solders, and electronics. In the middle and late 1980s, the development of indium phosphide semiconductors and indium-tin oxide thin films for liquid crystal displays (LCDs) aroused much interest. By 1992, the thin-film application had become the largest end use.
Other uses are as follows:
- Indium is used in the manufacture of low-melting-temperature alloys. An alloy consisting of 24 percet indium and 76 percent gallium is liquid at room temperature.
- It is also used as a semiconductor dopant.
- It can be plated onto metals and evaporated onto glass to form a mirror that is as good as those made with silver, but with higher corrosion resistance.
- Several indium compounds—such as indium antimonide, indium arsenide, indium phosphide, and indium nitride—are semiconductors with useful properties.
- Indium antimonide and arsenide are used in infrared detectors.
- Its oxide is used in making electroluminescent panels.
- It is used as a light filter in low-pressure sodium vapor lamps.
- Its freezing point of 429.7485 K (156.5985 °C) is a defining fixed point on the international temperature scale.
- It is occasionally used as a component of nuclear reactor control rods.
- Very small amounts of indium are used in aluminum alloy sacrificial anodes (for salt water applications), to prevent passivation of the aluminum.
Pure indium in metallic form is considered nontoxic by most sources. In the welding and semiconductor industries, where exposure to indium and its compounds is relatively high, there have been no reports of any toxic side-effects. Yet, some sources maintain that indium has a low level of toxicity, and its compounds are highly toxic.
- ↑ WebElements states that "All indium compounds should be regarded as highly toxic. Indium compounds damage the heart, kidney, and liver, and may be teratogenic." For example, indium trichloride anhydrous (InCl3) is quite toxic, while indium phosphide (InP) is both toxic and a suspected carcinogen.
- Cotton, F. Albert; and Geoffrey Wilkinson. 1980. Advanced Inorganic Chemistry 4th ed. New York: Wiley. ISBN 0471027758.
- Chang, Raymond. 2006. Chemistry 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031.
- Greenwood, N.N. and A. Earnshaw. 1998. Chemistry of the Elements 2nd ed. Oxford, U.K.; Burlington, Massachusetts: Butterworth-Heinemann, Elsevier Science. ISBN 0750633654. Online version available at . Retrieved on October 29, 2006.
- Indium Los Alamos National Laboratory. Accessed on October 29, 2006.
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