Aluminium oxide
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{{redirect|Alumina|the company|Alumina Limited}}
{{Chembox new
| Name = Aluminium oxide
| ImageFile = Corundum-3D-balls.png
| ImageSize = 250
| ImageFile2 = Aluminium_oxide2.png
| Section1 = {{Chembox Identifiers
| CASNo = 1344-28-1
}}
| Section2 = {{Chembox Properties
| MolarMass = 101.96 g/mol
| Density = 3.97 g cm<sup>−3</sup>, solid
| Solubility ='''insoluble'''
| MeltingPt = 2054 °C
| BoilingPt = 2980 °C <ref>{{cite web|last=Mallinckrodt Baker| first=MSDS | title = Aluminum Oxide: Material Safety Data Sheet (A28440) | url=http://www.jtbaker.com/msds/englishhtml/a2844.htm}} </ref>
}}
| Section3 = {{Chembox Structure
| Coordination = [[octahedral]]
| Structure = [[trigonal]]
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = −1675.7 kJ mol<sup>−1</sup>
| Entropy = 50.92 J mol<sup>−1</sup> K<sup>−1</sup>
}}
| Section7 = {{Chembox Hazards
| EUClass = Not listed.
| FlashPt = non-flammable
}}
| Section8 = {{Chembox Related
| OtherAnions = [[aluminium hydroxide]]
| OtherCations = [[boron trioxide]]<br />[[gallium oxide]]<br />[[indium oxide]]<br />[[thallium oxide]]}}
}}
'''Aluminium oxide''' is an [[amphoteric oxide]] of [[aluminium]] with the [[chemical formula]] {{aluminium}}<sub>2</sub>{{oxygen}}<sub>3</sub>. It is also commonly referred to as '''alumina''' or '''aloxite'''<ref name="CI14835">[http://www.chemindustry.com/chemicals/14835.html "Aloxite"], ''ChemIndustry.com'' database, retrieved [[24 February]] [[2007]]</ref> in the [[mining]], [[ceramic]] and [[materials science]] communities. It is produced by the [[Bayer process]] from [[bauxite]]. Its most significant use is in the production of [[aluminium]] metal, although it is also used as an abrasive due to its hardness and as a [[refractory]] material due to its high melting point.<ref name = azom>{{cite web | title = Alumina (Aluminium Oxide) – The Different Types of Commercially Available Grades | url = http://www.azom.com/details.asp?ArticleID=1389 | publisher = The A to Z of Materials | accessdate = 2007-10-27}}</ref>
==Natural occurrence==
[[Corundum]] is the most common naturally occurring [[crystalline]] form of aluminium oxide. [[ruby|Rubies]] and [[sapphire]]s are gem-quality forms of corundum with their characteristic colors due to trace impurities in the corundum structure.
==Properties==
Aluminium oxide is an electrical [[Electrical insulation|insulator]] but has a relatively high [[thermal conductivity]] (40 W/m K). In its most commonly occurring crystalline form, called [[corundum]] or α-aluminium oxide, its hardness makes it suitable for use as an [[abrasive]] and as a component in [[cutting tools]].<ref name = azom/>
Aluminium oxide is responsible for metallic aluminium's resistance to [[weathering]]. Metallic aluminium is very reactive with atmospheric [[oxygen]], and a thin [[passivation layer]] of alumina quickly forms on any exposed aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called [[anodising]]. A number of [[alloys]], such as [[aluminium bronze]]s, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The alumina generated by anodising is typically [[amorphous]], but discharge assisted oxidation processes such as [[plasma electrolytic oxidation]] result in a significant proportion of crystalline alumina in the coating, enhancing its [[hardness]].
Aluminium oxide was taken off the [[United States Environmental Protection Agency]]'s chemicals lists in [[1988]]. Aluminium oxide is on EPA's TRI list if it is a fibrous form.{{Fact|date=October 2007}}
==Crystal structure==
The most common form of crystalline alumina, α-aluminium oxide, is known as [[corundum]]. Corundum has a [[trigonal]] Bravais lattice with a space group of R-3c (number 167 in the International Tables). The primitive cell contains two formula units of aluminium oxide. The oxygen ions nearly form a hexagonal close-packed structure with aluminium ions filling two-thirds of the octahedral interstices. Alumina also exists in other phases, namley beta, eta, chi, gamma, delta and theta aluminas. Each has a unique crystal structure and properties.
==Production==
Aluminium [[oxide]], also known as alumina, is the main component of [[bauxite]], the principal [[ore]] of [[aluminium]]. The largest [[manufacturing|manufacturers]] in the world of alumina are [[Alcoa]], [[Alcan]] and [[Rusal]].{{Fact|date=October 2007}} Companies which specialise in the production of speciality aluminium oxides and aluminium hydroxides include Alcan and [[Almatis, Inc.|Almatis]]. The bauxite ore is made up of impure Al<sub>2</sub>O<sub>3</sub>, Fe<sub>2</sub>O<sub>3</sub>, and SiO<sub>2</sub>. Bauxite is purified by the [[Bayer process]]:
: Al<sub>2</sub>O<sub>3</sub> + 3 H<sub>2</sub>O + 2 NaOH → 2NaAl(OH)<sub>4</sub>
The Fe<sub>2</sub>O<sub>3</sub> does not dissolve in the base. The SiO<sub>2</sub> dissolves as silicate Si(OH)<sub>6</sub><sup>2-</sup>. Upon filtering, Fe<sub>2</sub>O<sub>3</sub> is removed. When the Bayer liquor is cooled, Al(OH)<sub>3</sub> precipitates, leaving the silicates in solution. The mixture is then [[calcined]] (heated strongly) to give aluminium oxide:<ref name = azom/>
2Al(OH)<sub>3</sub> + heat → Al<sub>2</sub>O<sub>3</sub> + 3H<sub>2</sub>O
The formed Al<sub>2</sub>O<sub>3</sub> is alumina. The alumina formed tends to be multi-phase, i.e. constituting several of the alumina phases rather than solely corundum.{{Fact|date=July 2008}} The production process can therefore be optimized to produce a taylored product. The type of phases present affects, for example, the solubility and pore structure of the alumina product which, in turn, affects the cost of aluminium production and pollution control.{{Fact|date=July 2008}}
==Uses==
[[Image:2005alumina.PNG|thumb|right|Alumina output in 2005]]
Annual world production of alumina is approximately 45 million [[tonne]]s, over 90% of which is used in the manufacture of aluminium metal.<ref name="azom" />. The major uses of speciality aluminium oxides are in refractories, ceramics, polishing and abrasive applications. Large tonnages are also used in the manufacture of [[zeolites]], coating titania pigments and as a fire retardant/smoke suppressant.
In lighting photogrpahy, and synthetic chemistry, alumina is a medium for [[chromatography]], available in [[Base (chemistry)|basic]] ([[pH]] 9.5), [[acid]]ic (pH 4.5 when in water) and neutral formulations. In 1961, GE developed "[[Lucalox]]", a transparent alumina used in [[sodium vapor lamp]]s.{{Fact|date=October 2007}} Aluminium oxide is also used in preparation of coating suspensions in [[compact fluorescent lamp]]s.
Health and medical applications include it as a material in [[hip replacement]]s,<ref name="azom" />
in water filters (derived water treatment chemicals such as [[aluminium sulfate]], [[aluminium chlorohydrate]] and [[sodium aluminate]], are one of the few methods available to filter water-soluble fluorides out of water), and even in toothpaste formulations.
Aluminium oxide is also used for its strength. Most pre-[[Wood finishing|finish]]ed wood [[flooring]] now uses aluminium oxide as a hard protective [[coating]]. In [[2004]], [[3M]] developed a technique for making a [[ceramic]] composed of aluminium oxide and [[rare earth element]]s to produce a strong [[glass]] called [[transparent alumina]]. Alumina can be grown as a coating on aluminium by [[anodising]] or by [[plasma electrolytic oxidation]] ''(see the "Properties" section, above)''. Both its strength and abrasive characteristics are due to aluminium oxide's great [[hardness]] (position 9 on the [[Mohs scale of mineral hardness]]).
It is widely used as a coarse or fine [[abrasive]], including as a much less expensive substitute for [[industrial diamond]]. Many types of [[sandpaper]] use aluminium oxide crystals. In addition, its low heat retention and low [[specific heat]] make it widely used in grinding operations, particularly [[metalworking|cutoff]] tools. As the powdery abrasive mineral aloxite, it is a major component, along with [[silica]], of the [[Cue stick|cue tip]] "chalk" used in [[Cue sports|billiards]]. ''(See [[William A. Spinks]], cue chalk co-inventor, for more information.)'' Aluminium oxide powder is used in some [[Compact disc|CD]]/[[DVD]] [[polishing]] and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste.
Aluminium oxide is widely used in the fabrication of [[superconducting]] devices, particularly [[single electron transistor]]s and superconducting quantum interference devices ([[SQUID]]), where it is used to form highly resistive [[quantum tunnelling]] barriers.
Aluminium oxide is considered a Welsbach material. It has been suggested that this chemical could be sprayed into the upper atmosphere to reflex sunlight and thus lower the global temperature.{{cite web|title= |url=http://www.freepatentsonline.com/5003186.html}}
==See also==
* [[Bayer process]]
* [[List of Alumina Refineries]]
* [[Transparent aluminium]]
==References==
{{Refimprove|date=November 2007}}
<references/>
==External links==
*[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc03/icsc0351.htm International Chemical Safety Card 0351]
*[http://physicsweb.org/article/news/8/8/9 PhysicsWeb article on Transparent alumina]
[[Category:Aluminium compounds]]
[[Category:Oxides]]
[[Category:Refractory materials]]
[[Category:Inorganic compounds]]
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