Indium is a member of the same group, or family of elements as aluminium, but whereas the latter is the most abundant metal in the earth’s crust, indium is the 61st most plentiful.
Discovery and Extraction of Indium
The name has nothing to do with India, but refers to the manner of indium's discovery. In 1863, German chemists, Ferdinand Reich and Hieronymus Richter isolated a yellow powder they were able to extract from a sample of the zinc ore, sphalerite. On testing this with a flame and analysing the light produced, they found a distinctive indigo coloured line in the spectrum, which could not be related to any previously known element. They named the metal after this spectral line.
An ore of indium is known, but it is extremely rare and of no commercial value. The metal is obtained as a by-product of the extraction of zinc and lead from their ores, of which indium may comprise up to 1%. The impure metal is then purified by electrolysis.
Properties of Indium
Indium is almost unique among the elements in that, although it does have stable isotopes; its most abundant isotope, that with an atomic mass number of 115, is very weakly radioactive, though its half-life is thousands of times longer than the present age of the universe. Another unusual property of indium is that, when bent, it emits an audible squeak.
Uses of Indium
Indium was used during World War II to form a thin, lubricating film on the bearings of aircraft engines. Its vapour, when condensed onto a glass surface, makes a mirror of high quality. It can be used in low melting point alloys, in sprinkler systems and to make lead-free solder. An alloy of tin and indium with gallium, another member of the aluminium family of elements, is liquid down to a temperature of -19°C, and is employed as a non-toxic alternative to mercury in thermometers. The same alloy can also substitute for mercury in liquid mirror telescopes.
Modern semiconductor technology, particularly in the field of communications, has discovered new uses for indium. Light emitting diodes (LEDs) make use of alloys of indium and gallium with either nitrogen or phosphorus. CIGS semiconductors (copper-indium-gallium-selenium) are used in flexible thin-film solar cells.
Indium Tin Oxide (ITO)
The most widely applied compound, however, is indium tin oxide (ITO), which is made of 90% indium oxide and 10% tin oxide, and is transparent, yet able to conduct electricity. This almost unique combination of properties has led to its extensive use as the electrodes in liquid crystal displays (LCDs) and flat screen televisions. In these devices, each pixel consists of a light absorbing material sandwiched between the ITO electrodes, which are able to convert the light energy into electricity.
Touchscreens in the newer generations of mobile phones and e-readers also use ITO electrodes separated by a tiny gap, the capacitance of which alters when touched by a finger. ITO is brittle, but the manufacturers of mobile phones operate on the expectation of the lifetimes of their devices stretching to little more than one-and-a-half years. Items such as e-readers, however, which would be expected to last longer, might not have sufficiently durable touch screens.
Supply and Demand Problems
Questions have been raised as to problems with the supply of Indium. Around 1200 tonnes are used each year, about 40% from mining and the rest from re-cycling. Indeed, the touch screen market alone is at present worth almost $1.5 billion per year, and rising rapidly. Known reserves are estimated at 16 000 tonnes, over 60% of which are found in China, which has recently begun to restrict its export of metals, among them indium.
Possible alternatives to ITO are generally less transparent, less conducting, more brittle, toxic or a combination of these. However, The Indium Corporation, which controls much of the supply of the rare metal, has invested in the discovery of new deposits and more efficient extraction and recovery methods. Even as the demand soars for the modern devices we are becoming ever more dependent upon, the corporation remains confident that supplies can meet the demand.
Sources:
Nature's Building Blocks (John Emsley, Oxford University Press)
New Scientist, Vol 208 No. 2783 (James Mitchell Crow)
Anthony Toole - I was born and brought up in the English Lake District, where I have walked and climbed for most of my life. I was educated as a scientist ...
Join the Conversation