Metal Profile: Gallium
The Minor Metal That Helps LED Lights Shine Bright
Gallium is a corrosive, silver-colored minor metal that melts near room temperature and is most often used in the production of semiconductor compounds.
- Atomic Symbol: Ga
- Atomic Number: 31
- Element Category: Post-transition metal
- Density: 5.91 g/cm³ (at 73°F / 23°C)
- Melting Point: 85.58°F (29.76°C)
- Boiling Point: 3999°F (2204°C)
- Moh's Hardness: 1.5
Pure gallium is silvery-white and melts at temperatures under 85°F (29.4°C). The metal remains in a melted state up to nearly 4000°F (2204°C), giving it the largest liquid range of all metal elements.
Gallium is one of only a few metals that expands as it cools, increasing in volume by just over 3%.
Although gallium easily alloys with other metals, it is corrosive, diffusing into the lattice of, and weakening most metals. Its low melting point, however, makes it useful in certain low melt alloys.
As opposed to mercury, which is also liquid at room temperatures, gallium wets both skin and glass, making it more difficult to handle. Gallium is not nearly as toxic as mercury.
Discovered in 1875 by Paul-Emile Lecoq de Boisbaudran while examining sphalerite ores, gallium was not used in any commercial applications until the latter part of the 20th century.
Gallium is of little use as a structural metal, but its value in many modern electronic devices cannot be understated.
Commercial uses of gallium developed from the initial research on light-emitting diodes (LEDs) and III-V radio frequency (RF) semiconductor technology, which began in the early 1950s.
In 1962, IBM physicist J.B. Gunn's research on gallium arsenide (GaAs) led to the discovery of high-frequency oscillation of the electrical current flowing through certain semiconducting solids - now known as the 'Gunn Effect.' This breakthrough paved the way for early military detectors to be constructed using Gunn diodes (also known as transfer electron devices) that have since been used in various automated devices, from car radar detectors and signal controllers to moisture content detectors and burglar alarms.
The first LEDs and lasers based on GaAs were produced in the early 1960s by researchers at RCA, GE, and IBM.
Initially, LEDs were only able to produce invisible infrared lightwaves, limiting the lights to sensors, and photo-electronic applications. But their potential as energy efficient compact light sources was evident.
By the early 1960s, Texas Instruments began offering LEDs commercially. By the 1970s, early digital display systems, used in watches and calculator displays, were soon developed using LED backlighting systems.
Further research in the 1970s and 1980s resulted in more efficient deposition techniques, making LED technology more reliable and cost-effective. The development of gallium-aluminium-arsenic (GaAlAs) semiconductor compounds resulted in LEDs that were ten-times brighter than previous, while the color spectrum available to LEDs also advanced based on new, gallium-containing semi-conductive substrates, such as indium-gallium-nitride (InGaN), gallium-arsenide-phosphide (GaAsP), and gallium-phosphide (GaP).
By the late 1960s, GaAs conductive properties were also being researched as part of solar power sources for space exploration. In 1970, a Soviet research team created the first GaAs heterostructure solar cells.
Critical to the manufacture of optoelectronic devices and integrated circuits (ICs), demand for GaAs wafers soared in the late 1990s and beginning of the 21st century in correlation with the development of mobile communication and alternative energy technologies.
Not surprisingly, in response to this growing demand, between 2000 and 2011 global primary gallium production more than double from approximately 100 metric tons (MT) per year to over 300MT.
The average gallium content in the earth's crust is estimated to be about 15 parts per million, roughly similar to lithium and more common than lead. The metal, however, is widely dispersed and present in few economically extractable ore bodies.
As much as 90% of all primary gallium produced is currently extracted from bauxite during the refining of alumina (Al2O3), a precursor to aluminum. A small amount of gallium is produced as a by-product of zinc extraction during refining of sphalerite ore.
During the Bayer Process of refining aluminum ore to alumina, crushed ore is washed with a hot solution of sodium hydroxide (NaOH). This converts alumina to sodium aluminate, which settles in tanks while the sodium hydroxide liquor that now contains gallium is collected for re-use.
Because this liquor is recycled, the gallium content increases after each cycle until it reaches a level of about 100-125ppm. The mixture can then be taken and concentrated as gallate via solvent extraction using organic chelating agents.
In an electrolytic bath at temperatures of 104-140°F (40-60°C), sodium gallate is converted to impure gallium. After washing in acid, this can then be filtered through porous ceramic or glass plates to create 99.9-99.99% gallium metal.
99.99% is the standard precursor grade for GaAs applications, but new uses require higher purities that can be achieved by heating the metal under vacuum to remove volatile elements or electrochemical purification and fractional crystallization methods.
Over the past decade, much of the world's primary gallium production has moved to China who now supplies about 70% of the world's gallium. Other primary producing nations include the Ukraine and Kazakhstan.
About 30% of annual gallium production is extracted from scrap and recyclable materials such as GaAs-containing IC wafers. Most gallium recycling occurs in Japan, North America, and Europe.
The US Geological Survey estimates that 310MT of refined gallium was produced in 2011.
The world's largest producers include Zhuhai Fangyuan, Beijing Jiya Semiconductor Materials, and Recapture Metals Ltd.
When alloyed gallium tends to corrode or make metals like steel brittle. This trait, along with its extremely low melting temperature, means that gallium is of little use in structural applications.
In its metallic form, gallium is used in solders and low melt alloys, such as Galinstan®, but it is most often found in semiconductor materials.
Gallium's main applications can be categorized into five groups:
1. Semiconductors: Accounting for about 70% of annual gallium consumption, GaAs wafers are the backbone of many modern electronic devices, such as smartphones and other wireless communication devices that rely on the power saving and amplification ability of GaAs ICs.
2. Light Emitting Diodes (LEDs): Since 2010, global demand for gallium from the LED sector has reportedly doubled, owing to the use of high brightness LEDs in mobile and flat screen display screens. The global move towards greater energy efficiency has also led to government support for the use of LED lighting over incandescent and compact fluorescent lighting.
3. Solar energy: Gallium's use in solar energy applications is focused on two technologies:
- GaAs concentrator solar cells
- Cadmium-indium-gallium-selenide (CIGS) thin film solar cells
As highly efficient photovoltaic cells, both technologies have had success in specialized applications, particularly related to aerospace and military but still face barriers to large-scale commercial use.
4. Magnetic materials: High strength, permanent magnets are a key component of computers, hybrid automobiles, wind turbines and various other electronic and automated equipment. Small additions of gallium are used in some permanent magnets, including neodymium-iron-boron (NdFeB) magnets.
5. Other applications:
Softpedia. History of LEDs (Light Emitting Diodes).
Anthony John Downs, (1993), "Chemistry of Aluminium, Gallium, Indium, and Thallium." Springer, ISBN 978-0-7514-0103-5
Barratt, Curtis A. "III-V Semiconductors, a History in RF Applications." ECS Trans. 2009, Volume 19, Issue 3, Pages 79-84.
Schubert, E. Fred. Light-Emitting Diodes. Rensselaer Polytechnic Institute, New York. May 2003.
USGS. Mineral Commodity Summaries: Gallium.
SM Report. By-Product Metals: The Aluminum-Gallium Relationship.