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Characteristics

Aluminium is a soft, durable, lightweight, malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. Aluminium is nontoxic, nonmagnetic, and nonsparking. It is also insoluble in alcohol, though it can be soluble in water in certain forms. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.^[2] Aluminium has about one-third the density and stiffness of steel. It is ductile, and easily machined, cast, and extruded.

Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper.^[2] This corrosion resistance is also often greatly reduced when many aqueous salts are present however, particularly in the presence of dissimilar metals.

Aluminium atoms are arranged in a face-centered cubic (FCC) structure. Aluminium has a high stacking-fault energy of approximately 200 mJ/m².^[3]

Aluminium is one of the few metals that retain full silvery reflectance in finely powdered form, making it an important component of silver paints. Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3000–10000 nm (far IR) regions, while in the 400–700 nm visible range it is slightly outdone by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper.^{[citation needed]}

Aluminium is a good thermal and electrical conductor, by weight better than copper. Aluminium is capable of being a superconductor, with a superconducting critical temperature of 1.2 kelvins and a critical magnetic field of about 100 gauss.^[4]

Isotopes

Main article: isotopes of aluminium

Aluminium has nine isotopes, whose mass numbers range from 23 to 30. Only ²⁷Al (stable isotope) and ²⁶Al (radioactive isotope, t_1/2 = 7.2 × 10⁵ y) occur naturally; however, ²⁷Al has a natural abundance of 99.9+ %. ²⁶Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of ²⁶Al to ¹⁰Be has been used to study the role of transport, deposition, sediment storage, burial times, and erosion on 10⁵ to 10⁶ year time scales.^[5] Cosmogenic ²⁶Al was first applied in studies of the Moon and meteorites. Meteoroid fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial ²⁶Al production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further ²⁶Al production, and its decay can then be used to determine the meteorite's terrestrial age. Meteorite research has also shown that ²⁶Al was relatively abundant at the time of formation of our planetary system. Most meteoriticists believe that the energy released by the decay of ²⁶Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.^[6]

Natural occurrence

In the Earth's crust, aluminium is the most abundant (8.13%) metallic element, and the third most abundant of all elements (after oxygen and silicon). However, because of its strong affinity to oxygen, it is almost never found in the elemental state; instead it is found in oxides or silicates. Feldspars, the most common group of minerals in the earth's crust, are aluminosilicates. Native aluminium metal can be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes.^[7]

Although aluminium is an extremely common and widespread element, the common aluminium minerals are not economic sources of the metal. Almost all metallic aluminium is produced from the ore bauxite. Bauxite occurs as a weathering product of low iron and silica bedrock in tropical climatic conditions.^[8]

Production and refinement

Although aluminium is the most abundant metallic element in the Earth's crust (believed to be 7.5 to 8.1 percent), it is rare in its free form, occurring in oxygen-deficient environments such as volcanic mud, and it was once considered a precious metal more valuable than gold. Napoleon III, emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the other guests had to make do with gold.^[9][10] The Washington Monument was completed, with the 100 ounce (2.8 kg) aluminium capstone being put in place on December 6, 1884, in an elaborate dedication ceremony. It was the largest single piece of aluminium cast at the time. At that time, aluminium was more expensive than silver, gold, or platinum. Aluminium has been produced in commercial quantities for just over 100 years.

Aluminium is a strongly reactive metal that forms a high-energy chemical bond with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the energy required to reduce aluminium oxide (Al₂O₃). For example, direct reduction with carbon, as is used to produce iron, is not chemically possible, since aluminium is a stronger reducing agent than carbon. Aluminium oxide has a melting point of about 2,000 °C. Therefore, it must be extracted by electrolysis. In this process, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. The operational temperature of the reduction cells is around 950 to 980 °C. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a chemical compound of aluminium, sodium, and calcium fluorides: (Na₃AlF₆). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process of Karl Bayer. (Previously, the Deville process was the predominant refining technology.)

The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the ore is in the molten state, its ions are free to move around. The reaction at the cathode (negative electrode) is

Al³⁺ + 3 e⁻ → Al

Here the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off.

At the anode (positive electrode), oxygen is formed:

2 O²⁻ → O₂ + 4 e⁻

This carbon anode is then oxidized by the oxygen, releasing carbon dioxide.

O₂ + C → CO₂

The anodes in a reduction cell must therefore be replaced regularly, since they are consumed in the process.

Unlike the anodes, the cathodes are not oxidized because there is no oxygen present, as the carbon cathodes are protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.

World production trend of aluminium

Aluminium electrolysis with the Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The worldwide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced (52 to 56 MJ/kg). The most modern smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg). (Compare this to the heat of reaction, 31 MJ/kg, and the Gibbs free energy of reaction, 29 MJ/kg.) Reduction line currents for older technologies are typically 100 to 200 kA; state-of-the-art smelters ^[11] operate at about 350 kA. Trials have been reported with 500 kA cells.

Recovery of the metal via recycling has become an important facet of the aluminium industry. Recycling involves melting the scrap, a process that requires only five percent of the energy used to produce aluminium from ore. However, a significant part (up to 15% of input material) is lost as dross (ash-like oxide).^[12] Recycling was a low-profile activity until the late 1960s, when the growing use of aluminium beverage cans brought it to the public consciousness.

Electric power represents about 20% to 40% of the cost of producing aluminium, depending on the location of the smelter. Smelters tend to be situated where electric power is both plentiful and inexpensive, such as South Africa, the South Island of New Zealand, Australia, the People's Republic of China, the Middle East, Russia, Quebec and British Columbia in Canada, and Iceland.

Aluminium output in 2005

In 2005, the People's Republic of China was the top producer of aluminium with almost one-fifth world share, followed by Russia, Canada, and USA, reports the British Geological Survey.

Over the last 50 years, Australia has become a major producer of bauxite ore and a major producer and exporter of alumina.^[13] Australia produced 62 million tonnes of bauxite in 2005. The Australian deposits have some refining problems, some being high in silica but have the advantage of being shallow and relatively easy to mine.^[14]

See also: Category:Aluminium minerals

Chemistry

 Oxidation state one

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• AlH is produced when aluminium is heated in an atmosphere of hydrogen.
• Al₂O is made by heating the normal oxide, Al₂O₃, with silicon at 1800 °C in a vacuum.
• Al₂S can be made by heating Al₂S₃ with aluminium shavings at 1300 °C in a vacuum. It quickly disproportionates to the starting materials. The selenide is made in a parallel manner.
• AlF, AlCl and AlBr exist in the gaseous phase when the tri-halide is heated with aluminium.

Aluminium halides usually exist in the form AlX₃. e.g. AlF₃, AlCl₃, AlBr₃, AlI₃ etc.

Oxidation state two

• Aluminium monoxide, AlO, is present when aluminium powder burns in oxygen.

Oxidation state three

• Fajans' rules show that the simple trivalent cation Al³⁺ is not expected to be found in anhydrous salts or binary compounds such as Al₂O₃. The hydroxide is a weak base and aluminium salts of weak acids, such as carbonate, can't be prepared. The salts of strong acids, such as nitrate, are stable and soluble in water, forming hydrates with at least six molecules of water of crystallization.
• Aluminium hydride, (AlH₃)_n, can be produced from trimethylaluminium and an excess of hydrogen. It burns explosively in air. It can also be prepared by the action of aluminium chloride on lithium hydride in ether solution, but cannot be isolated free from the solvent.
• Aluminium carbide, Al₄C₃ is made by heating a mixture of the elements above 1000 °C. The pale yellow crystals have a complex lattice structure, and react with water or dilute acids to give methane. The acetylide, Al₂(C₂)₃, is made by passing acetylene over heated aluminium.
• Aluminium nitride, AlN, can be made from the elements at 800 °C. It is hydrolysed by water to form ammonia and aluminium hydroxide.
• Aluminium phosphide, AlP, is made similarly, and hydrolyses to give phosphine.
• Aluminium oxide, Al₂O₃, occurs naturally as corundum, and can be made by burning aluminium in oxygen or by heating the hydroxide, nitrate or sulfate. As a gemstone, its hardness is only exceeded by diamond, boron nitride, and carborundum. It is almost insoluble in water.
• Aluminium hydroxide may be prepared as a gelatinous precipitate by adding ammonia to an aqueous solution of an aluminium salt. It is amphoteric, being both a very weak acid, and forming aluminates with alkalis. It exists in various crystalline forms.
• Aluminium sulfide, Al₂S₃, may be prepared by passing hydrogen sulfide over aluminium powder. It is polymorphic.
• Aluminium iodide, (AlI₃)₂, is a dimer with applications in organic synthesis.
• Aluminium fluoride, AlF₃, is made by treating the hydroxide with HF, or can be made from the elements. It consists of a giant molecule which sublimes without melting at 1291 °C. It is very inert. The other trihalides are dimeric, having a bridge-like structure.
• Aluminium fluoride/water complexes: When aluminium and fluoride are together in aqueous solution, they readily form complex ions such as AlF(H₂O)₅⁺², AlF₃(H₂O)₃⁰, AlF₆^-3. Of these, AlF₆^-3 is the most stable. This is explained by the fact that aluminium and fluoride, which are both very compact ions, fit together just right to form the octahedral aluminium hexafluoride complex. When aluminium and fluoride are together in water in a 1:6 molar ratio, AlF₆^-3 is the most common form, even in rather low concentrations.
• Organo-metallic compounds of empirical formula AlR₃ exist and, if not also giant molecules, are at least dimers or trimers. They have some uses in organic synthesis, for instance trimethylaluminium.
• Alumino-hydrides of the most electropositive elements are known, the most useful being lithium aluminium hydride, Li[AlH₄]. It decomposes into lithium hydride, aluminium and hydrogen when heated, and is hydrolysed by water. It has many uses in organic chemistry, particularly as a reducing agent. The aluminohalides have a similar structure.

 Clusters

In the journal Science of 14 January 2005 it was reported that clusters of 13 aluminium atoms (Al₁₃) had been made to behave like an iodine atom; and, 14 aluminium atoms (Al₁₄) behaved like an alkaline earth atom. The researchers also bound 12 iodine atoms to an Al₁₃ cluster to form a new class of polyiodide. This discovery is reported to give rise to the possibility of a new characterisation of the periodic table: superatoms. The research teams were led by Shiv N. Khanna (Virginia Commonwealth University) and A. Welford Castleman Jr (Penn State University).^[15]

 Applications

 General use

A piece of aluminium metal.

Aluminium is the most widely used non-ferrous metal.^[16] Global production of aluminium in 2005 was 31.9 million tonnes. It exceeded that of any other metal except iron (837.5 million tonnes).^[17] Relatively pure aluminium is encountered only when corrosion resistance and/or workability is more important than strength or hardness. A thin layer of aluminium can be deposited onto a flat surface by physical vapor deposition or (very infrequently) chemical vapor deposition or other chemical means to form optical coatings and mirrors. When so deposited, a fresh, pure aluminium film serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared.

Pure aluminium has a low tensile strength, but when combined with thermo-mechanical processing, aluminium alloys display a marked improvement in mechanical properties, especially when tempered. Aluminium alloys form vital components of aircraft and rockets as a result of their high strength-to-weight ratio. Aluminium readily forms alloys with many elements such as copper, zinc, magnesium, manganese and silicon (e.g., duralumin). Today, almost all bulk metal materials that are referred to loosely as "aluminium," are actually alloys. For example, the common aluminium foils are alloys of 92% to 99% aluminium.^[18]

Some of the many uses for aluminium metal are in:

household aluminium foil

• Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles etc.)
• Packaging (cans, foil, etc.)
• Water treatment
• Treatment against fish parasites such as Gyrodactylus salaris.
• Construction (windows, doors, siding, building wire, etc.)
• Cooking utensils
• Electrical transmission lines for power distribution
• MKM steel and Alnico magnets
• Super purity aluminium (SPA, 99.980% to 99.999% Al), used in electronics and CDs.
• Heat sinks for electronic appliances such as transistors and CPUs.
• Substrate material of metal-core copper clad laminates used in high brightness LED lighting.
• Powdered aluminium is used in paint, and in pyrotechnics such as solid rocket fuels and thermite.
• In the blades of prop swords and knives used in stage combat.
• Aluminium is widely used in watch production as it provides durability and resists tarnishing and corrosion.^[19]

 Aluminium compounds

• Aluminium ammonium sulfate ([Al(NH₄)](SO₄)₂), ammonium alum is used as a mordant, in water purification and sewage treatment, in paper production, as a food additive, and in leather tanning.

• Aluminium acetate is a salt used in solution as an astringent.

• Aluminium borate (Al₂O₃ B₂O₃) is used in the production of glass and ceramic.

• Aluminium borohydride (Al(BH₄)₃) is used as an additive to jet fuel.
• Aluminium bronze (CuAl₅)
• Aluminium chloride (AlCl₃) is used: in paint manufacturing, in antiperspirants, in petroleum refining and in the production of synthetic rubber.

• Aluminium chlorohydride is used as an antiperspirant and in the treatment of hyperhidrosis.

• Aluminium fluorosilicate (Al₂(SiF₆)₃) is used in the production of synthetic gemstones, glass and ceramic.

• Aluminium hydroxide (Al(OH)₃) is used: as an antacid, as a mordant, in water purification, in the manufacture of glass and ceramic and in the waterproofing of fabrics.

• Aluminium oxide (Al₂O₃), alumina, is found naturally as corundum (rubies and sapphires), emery, and is used in glass making. Synthetic ruby and sapphire are used in lasers for the production of coherent light.

• Aluminium phosphate (AlPO₄) is used in the manufacture: of glass and ceramic, pulp and paper products, cosmetics, paints and varnishes and in making dental cement.

• Aluminium sulfate (Al₂(SO₄)₃) is used: in the manufacture of paper, as a mordant, in a fire extinguisher, in water purification and sewage treatment, as a food additive, in fireproofing, and in leather tanning.

• In many vaccines, certain aluminium salts serve as an immune adjuvant (immune response booster) to allow the protein in the vaccine to achieve sufficient potency as an immune stimulant.

Aluminium alloys in structural applications

Aluminium foam

Main article: Aluminium alloy

Aluminium alloys with a wide range of properties are used in engineering structures. Alloy systems are classified by a number system (ANSI) or by names indicating their main alloying constituents (DIN and ISO).

The strength and durability of aluminium alloys vary widely, not only as a result of the components of the specific alloy, but also as a result of heat treatments and manufacturing processes. A lack of knowledge of these aspects has from time to time led to improperly designed structures and gained aluminium a bad reputation. (See main article)

One important structural limitation of aluminium alloys is their