Indium (symbol In) is a silvery-white metal with a bluish hue, whiter than tin. It has a specific gravity of 7.31, tensile strength of 103 MPa, and elongation 22%. It is very ductile and does not work-harden, because its re-crystallization point is below normal room temperature, and it softens during rolling. The metal is not easily oxidized, but above its melting point, 157°C, it oxidizes and burns with a violet flame.
Indium is now obtained as a by-product from a variety of ores. Because of its bright color, light reflectance, and corrosion resistance, it is valued as a plating metal, especially for reflectors. It is softer than lead, but a hard surface is obtained by heating the plated part to diffuse the indium into the base metal. It has high adhesion to other metals. When added to chromium plating baths it reduces brittleness of the chromium.
In spite of its softness, small amounts of indium will harden copper, tin, or lead alloys and increase the strength. About 1% in lead will double the hardness of the lead. In solders and fusible alloys it improves wetting and lowers the melting point. In lead-base alloys a small amount of indium helps to retain oil film and increases the resistance to corrosion from the oil acids. Small amounts may be used in gold and silver dental alloys to increase the hardness, strength, and smoothness. Small amounts are also used in silver-lead and silver-copper aircraft-engine bearing alloys. Lead-indium alloys are highly resistant to corrosion, and are used for chemical-processing equipment parts. Gold-indium alloys have high fluidity, a smooth lustrous color, and good bonding strength. An alloy of 77.5% gold and 22.5 indium, with a working temperature of about 500°C, is used for brazing metal objects with glass inserts. Silver-indium alloys have high hardness and a fine silvery color. A silver-indium alloy used for nuclear control rods contains 80% silver, 15% indium, and 5% cadmium. The melting point is 746°C, tensile strength 289 MPa, elongation 67%, and it retains a strength of 120 MPa at 316°C. It is stable to irradiation, and is corrosion-resistant in high-pressure water up to 360°C. The thermal expansion is about six times that of steel.
Mechanical Properties
Typical properties of annealed indium are as follows: tensile strength, 262 MPa; hardness, 0.9 Bhn; compressive strength, 215 MPa.
Because indium does not work-harden and almost all of the deformation occurring in the tensile test is localized, deformation is very low for such a low-strength material.
Reactivity
Indium is stable in dry air at room temperature. The metal boils at 2000°C but sublimes when heated in hydrogen or in vacuum. Because a thin, tenacious oxide film forms on its surface, indium resists oxidation up to, and a little beyond, its melting point. The film, however, dissolves in dilute hydrochloric acid.
Indium can be slowly dissolved in dilute mineral acids and more readily in hot dilute acids. It unites with the halogens directly when warm. Concentrated mineral acids react vigorously with indium but there is no attack by solutions of strong alkalies. The metal dissolves in oxalic acid, but not in acetic acid.
There is no evidence that the metal is toxic and it has no action as a skin irritant.
Applications
The three largest uses of indium are in semiconductor devices, bearings, and low melting-point alloys.
Semiconductors
Indium is used to form p-n junctions in germanium. Two characteristics — the fact that indium readily wets the germanium and dissolves it at 500 to 550°C, as well as the fact that after alloying, the indium does not set up contraction stresses in the germanium — make it suitable for this application.
If a piece of n-type germanium is dissolved in indium, germanium containing excess indium recrystallizes after subsequent cooling.
The excess indium changes the germanium from n-type to p-type.
Other compounds include InSb where interest is twofold. Its small energy gap (0.18 eV) makes it valuable as a photodetector in wavelengths from the infrared up to ~8 mm. InSb also has a very high electron mobility — up to 80,000 cm2/Vs at room temperature and up to 800,000 cm2/Vs at liquid nitrogen temperature. Consequently, the material can be used in devices based on magnetoresistance or the Hall effect (gyrators, switching elements, magnetometers, analog computers, etc.).
InAs is of interest as a semiconductor: energy gap ~0.47 eV; electron mobility ~50,000 cm2/Vs (room temperature). The energy gap is too small to make InAs practical as a transistor material. It is, however, a candidate for infrared photoconductor applications. The relatively high mobility and small dependence of electrical properties on temperature make InAs of particular interest for Hall effect and magneto-resistance devices.
InO3 is an n-type semiconductor finding use as a resistive element in integrated circuits, and InP is a semiconductor that is useful for rectifiers and transistors and is a promising material for electronic devices that operate at intermediate temperatures.
Bearings
Impregnating the surface of steel-backed lead-silver bearings increases the strength and hardness, improves resistance to corrosion by acids in the lubricants, and permits better retention of the bearing oil film. Indium-coated bearings can be used for high-duty service such as found in aircraft engines and diesel engines.
Alloying
A common glass sealing alloy contains approximately 50% tin-50% indium. A solder alloy containing 37.5% lead, 37.5% tin and 25% indium has greater resistance to alkalies than the 50% lead-50% tin solder.
Adding indium to gold for use in dental alloys increases the tensile strength and ductility of gold, improves resistance to discoloration, and improves bonding characteristics.
