Indium is a lustrous, silver metal that is predominantly used in the production of flat panel display screens.
- Atomic Symbol: In
- Atomic Number: 49
- Element Category: Post-transition metal
- Density: 7.31 g/cm3
- Melting Point: 313.87 °F (156.6 °C)
- Boiling Point: 3762 °F (2072 °C)
- Mohs hardness: 1.2
Indium metal melts at 313.87 °F (156.6 °C), but alloys of indium and indium solders often have melt temperatures under 212 °F (100 °C).
In small quantities, indium can also strengthen or harden certain alloys. Indium has a number of unique characteristics, one of which is its ability to cold weld (join together without being heated). Another is its ability to remain soft at very low temperatures.
Molten, oxide-free indium metal can wet (cling to) clean glass and other surfaces. Also, because indium is considered non-toxic it has become a favored replacement for lead, particularly in solders.
Discovered in 1863 by Ferdinand Reich and Theodor Richter while examining zinc ores, indium was named for the indigo lines that it produced during spectroscopy analysis.
A 500g (1.1lb) ingot of indium was displayed at the 1867 world fair in Paris, but it was not for another 70 years that indium began to be used in commercial applications; first in dental alloys, and then along with bismuth, lead, tin and cadmium in low-melt alloys.
In 1934, the Indium Corporation was established in Utica, New York to study the potential uses of this unique metal.
During World War II, indium was used to coat bearings in aircraft engines.
Semiconductors containing indium were first developed in the 1950s, and some of the first transistor radios were made containing indium.
The growth of nuclear energy - and weapons - during the 1970s resulted in indium's use in nuclear control rods.
More recently, in the 1990s, the development of indium tin oxide (ITO) coatings for flat panel display screens significantly reshaped global demand for the metal. This industry is now the largest consumer of indium.
Global indium production was an estimated 1800 metric tons (mt) in 2009. Of this, about 35% (640mt) was attributable to primary production. The remaining 1160mt was recovered and recycled - predominantly from ITO scrap.
As a minor metal, indium is produced as a by-product of other metals. Primary indium is most often extracted from zinc dust and waste produced during zinc refining, but it is also found in copper, tin and lead residues.
To remove indium concentrate from zinc waste material, the material must first be leached with sulfuric acid and hydrochloric acid to remove zinc and lead. Soda is then added to neutralize and precipitate most of the indium, resulting in a filtrate that contains 10-50mg/L of indium.
Further leaching produces a 20% indium hydroxide. Finally, the remaining copper and arsenic are cemented using iron, tin and lead using indium and, ultimately, indium using aluminum.
Indium recovered from ITO processing has now become a major source of the metal. Part of the reason for this is because the ITO sputtering targets that are used to deposit indium in thin layers on flat panel display screens is quite inefficient, depositing only about 30% of the indium from the target and leaving 70% behind.
Most of the material not deposited, however, can be recovered (about 85%). Most large end-users now have closed-loop systems whereby this material is sent for reprocessing and returned to the end-users as ITO targets.
According to the US Geological Survey, in 2011 the largest primary indium producing countries were China (53%), Korea (16%) and Japan (11%). The largest indium producers include Teck Resources (primary production), Dowa Metals & Mining (primary and secondary), Huludao Zinc (primary) and Asahi Pretec (secondary).
The development of flat panel displays in the 1990s and significant growth in demand from this market after 2000 led to a major shift in the demand structure for indium. In 2000, 240mts of indium were produced, whereas this figure had 640 by 2011.
ITO coatings, which are used on all types of flat panel displays, including liquid crystal displays (LCDs), plasma display panels (PDPs) and organic light emitting diode (OLEDs) screens, now account for upwards of 80% of all demand.
Lead free solders, which had previously been the major end-use for indium now account for only about 5% of demand.
Other uses for indium include:
- As a lubricant on bearings
- In low-melt alloys (e.g. galinstan)
- In cathodic protection systems
- As a glass coating for aircraft windshields
- In the synthesis of CIGS thin film solar panels
- In the manufacture of III-V semiconductors (trimethylindium)
- In the production of light emitting diodes (LED)
- As a light filter in sodium vapour lamps
- As a substitute for mercury in alkaline batteries
- In electronic seal assemblies
- As cryogenic seals
- Along with silver and cadmium for neutron capture in nuclear control rods
- In body scanner technology