It's colourless, tasteless, odourless, inert, and very useful. For applications ranging from light bulbs to the growing of single silicon crystals for the semiconductor industry, argon is essential. Argon is also increasingly used in modified atmosphere packaging of foods and as an insulator in double-glazing. But it is for its metals applications that argon really shines. Argon is widely used in metals production, processing and fabrication where its role is nearly always to exclude atmospheric air from contact with metal alloys and metals - such as chromium, vanadium, silicon, titanium, aluminium and magnesium - that will either react with nitrogen contained in air, or take nitrogen into solution.
Argon is typically used in applications where a gas more inert than nitrogen is needed, or where a very low thermal conductivity is required. It plays an important role in welding, including metal inert gas (MIG) and tungsten inert gas (TIG) processes where, as a shielding gas, it is used to exclude atmospheric air from contact with those metals that can react with oxygen and nitrogen under the higher temperatures experienced.
Argon is also the shielding gas of choice for welding alloys such as carbon and stainless steels, where small amounts of dissolved nitrogen can reduce ductility. When combined with small amounts of oxygen or carbon dioxide, argon forms a shielding gas that helps to broaden and stabilise the arc and improve bead profile. Argon is also used to prevent oxidation during die casting of hot light metal alloys, such as aluminium and magnesium alloys, for products such as car wheels.
Aside from shielding, argon's most important metals processing applications are in steel making. Steel producers rely on the use of argon in a number of processes. For example, the wide range of ladle refining techniques used in the steel industry all involve bubbling an inert gas through the molten steel either to remove unwanted gases or to promote the carbon-oxygen reaction. Because of its inertness, argon is also used to stir the molten alloy during production to ensure that the temperature and composition are homogenous throughout the melt. Argon stirring is also used to introduce calcium or magnesium compounds into the melt to lower sulphur levels. In addition, bubbling argon through the melt is a useful way to remove unwanted gases such as nitrogen and hydrogen which affect the physical properties of steel. Argon is also used as a flotation gas for removing solid inclusions in order to produce 'clean' steels.
The single largest application for argon is in the production of stainless steel. Although stainless steel accounts for less than five percent of the global steel output, this strategic and important sector looks likely to grow, particularly in the Far East.
In stainless steel production argon plays a crucial role in controlled decarburisation, or carbon removal. Although, in theory, oxygen alone could be used, oxygen decarburisation would reduce the levels of readily oxidised elements, including chromium and nickel, which are responsible for key stainless steel characteristics, such as corrosion resistance. To avoid this problem argon-oxygen decarburisation (AOD) is used for the refining of most of the world's output of stainless steel.
During the first part of the AOD process, gas mixtures of predominantly oxygen are blown through the molten crude stainless steel. Then, as carbon levels are reduced and the thermodynamics for carbon removal become less favourable, an argon/oxygen gas mixture is substituted. The argon does not directly participate in the decarburisation reaction, but instead helps to maintain a thermodynamic driving force that continues to favour carbon removal over chromium oxidation. As more carbon is removed the ratio of argon to oxygen is increased to maintain the desired thermodynamic conditions.
Argon also plays an important role in powder metallurgy, where molten metals are atomised using high-pressure jets of argon, nitrogen or water to create powders. This technique is widely used in the automotive and white goods industries to fabricate metallic components near to final shape or to produce alloy materials that cannot be produced in the molten state. Argon is the gas of choice when the molten metal must be protected from oxidation and is the better choice where it might react with nitrogen.
It is also used for hot isostatic pressing (HIP), a process which involves mixing together metal powders, then compacting and sintering them at temperatures up to 2,300Â°C and pressures as high as 2,000 bar to produce high performance alloys. By using a gas, pressure can be applied equally in all directions. Argon is usually chosen because it is the lowest cost medium that won't react with either the molybdenum or graphite heating elements used.
Outside of the metals industries, argon plays an important role in the semiconductor industry, where it is serves as a protective atmosphere in the Czochralski process. This system accounts for the production of most of the monocrystalline silicon from which wafers used by semiconductor manufacturers are cut. The demand for high purity argon from the semiconductor industry is rising, particularly in Asia. This can be attributed to an increase in 300mm wafer fabs, a growth in production of the larger diameter 300mm wafers, and also the development of new applications. Also pushing demand is the increasing use of wafers as a substrate for growing photovoltaic cells. $quot;The demand for argon with increasingly stringent purity requirements is growing very fast, particularly in countries such as China, Singapore and Taiwan,$quot; notes Noel Leeson, President, Linde Electronics based in Hong Kong, $quot;and this shows no signs of decreasing.$quot;
But argon's use in semiconductor applications is relatively recent. In fact, argon has been lighting up our lives since tungsten filament bulbs came into use after 1913. The familiar incandescent light bulbs contain a tungsten filament which produces light when an electric current is passed through it. In an air-filled bulb, the filament would quickly oxidise. To extend filament life, the glass envelope, or capsule, of the bulb is generally filled with argon or an argon/nitrogen mixture. Argon is chosen because it does not react with the tungsten, even at very high temperatures. In addition, argon's low heat conductivity and relatively high molecular weight means that the tungsten in the filament evaporates more slowly - so the bulb lasts longer. Argon mixed with a little neon is used to fill luminous electric-discharge tubes, or neon-type bulbs, to create a blue light in neon-type bulbs.
On another front, argon is increasingly being used in the food industry in applications such as modified atmosphere packaging (MAP). Here its physical properties offer certain advantages over the nitrogen, oxygen or carbon dioxide atmospheres, or gas mixtures, typically used in MAP to prolong the quality shelf life or freshness of packaged foods. For example, because it is chemically inert, argon doesn't react with food components as oxygen or carbon dioxide might. It also inhibits the action of some oxidase enzymes which cause food spoilage. And since it is denser and more soluble in both water and oil it is more effective than nitrogen for displacing oxygen from the oils and fats in foods.
Because it prevents oxidation, the drinks industry also uses argon in a number of dispensing units and keeper cap systems. And argon's greater solubility also means that it is more effective than nitrogen for inerting oxygen-sensitive aqueous products such as wine - an application that is surely worth the extra expense!
How it is made
•In lighting mixtures
•In shielding gases for welding
•In plasma spraying for ceramic coatings
•For stirring in steelmaking
•In silicon crystal growing
•For cleaning silicon chips
•For laser cutting of materials such as titanium that would react with either oxygen or nitrogen
•In lasing gas mixtures for excimer lasers used for eye surgery
•In the manufacture of stainless steel
•As a gas for thermal insulation in energy efficient windows
•By museum conservators to protect old materials or documents which are prone to gradual oxidation in the presence of air.
Production and Supply
Argon makes up around 1 percent of the air we breathe, and is the most abundant of the rare gases.
It is generally produced in air separation units (ASUs) alongside oxygen and nitrogen, by means of a secondary distillation of the liquid oxygen, rather than from the primary distillation of air. It can also be recovered from ammonia plant purge gas streams, and a number of cryogenic process systems have been developed to recover a high percentage of the hydrogen and argon in the purge gas (see for example, www.uigi.com). In these, the hydrogen is recycled back into the ammonia plant, while the argon is recovered and purified for sale.
Because the boiling point of argon is between that of nitrogen and oxygen, an argon-rich mixture can be taken from a tray near the centre of the distillation column, then catalytically burned with hydrogen to remove oxygen, and finally processed in a pure argon distillation column to remove un-combined hydrogen and residual nitrogen to produce a product with purity in the range of 99.99 percent or greater. This process has been superseded, thanks to advances in packed-column distillation technology it is now possible to remove the oxygen cryogenically, using a distillation column without the need of hydrogen.
Argon for heat-treating applications is usually supplied as a cryogenic liquid, delivered to a customer's storage tank by cryogenic road tanker. Large steel works tend to receive their low purity argon via pipeline direct from an ASU. Smaller scale users can also receive argon in cylinders.