Aluminium oxide: Difference between revisions
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{{Short description|Chemical compound | {{Short description|Chemical compound}} | ||
{{About|aluminium(III) oxide, Al<sub>2</sub>O<sub>3</sub>|other uses|Aluminium oxides}} | {{About|aluminium(III) oxide, Al<sub>2</sub>O<sub>3</sub>|other uses|Aluminium oxides}} | ||
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| ImageFile2 = Aluminium oxide A.jpg | | ImageFile2 = Aluminium oxide A.jpg | ||
| OtherNames = Dialuminium trioxide | | OtherNames = {{ubl|Dialuminium trioxide|Aluminium sesquioxide}} | ||
| IUPACName = Aluminium oxide | | IUPACName = Aluminium oxide | ||
| SystematicName = Aluminium(III) oxide | | SystematicName = Aluminium(III) oxide | ||
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'''Aluminium oxide''' (or '''aluminium(III) oxide''') is a [[chemical compound]] of [[aluminium]] and [[oxygen]] with the [[chemical formula]] {{chem2|Al2O3}}. It is the most commonly occurring of several [[Aluminium oxide (compounds)|aluminium oxides]], and specifically identified as '''aluminium oxide'''. It is commonly called '''alumina''' and may also be called '''aloxide''', '''aloxite''', '''ALOX''' or '''alundum''' in various forms and applications and alumina is refined from [[bauxite]].<ref name="Myers">{{Cite book|last=Myers|first=Richard L.|url=https://books.google.com/books?id=0AnJU-hralEC|title=The 100 Most Important Chemical Compounds: A Reference Guide|date=2007|publisher=ABC-CLIO|isbn=978-0-313-33758-1|language=en|pages=24–26|access-date=21 November 2015|archive-date=17 June 2016|archive-url=https://web.archive.org/web/20160617093705/https://books.google.com/books?id=0AnJU-hralEC|url-status=live}}</ref> It occurs naturally in its crystalline [[Polymorphism (materials science)|polymorphic]] [[phase (matter)|phase]] α-Al<sub>2</sub>O<sub>3</sub> as the [[mineral]] [[corundum]], varieties of which form the precious [[gemstone]]s [[ruby]] and [[sapphire]],which have an alumina content approaching 100%.<ref name="Myers" /> Al<sub>2</sub>O<sub>3</sub> is used as feedstock to produce aluminium metal, as an [[abrasive]] owing to its [[hardness]], and as a [[refractory]] material owing 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|access-date = 2007-10-27|archive-url = https://web.archive.org/web/20071010063029/http://www.azom.com/details.asp?ArticleID=1389|archive-date = 10 October 2007|url-status = dead|df = dmy-all|date = 2002-05-03}}</ref> | '''Aluminium oxide''' (or '''aluminium(III) oxide''') is a [[chemical compound]] of [[aluminium]] and [[oxygen]] with the [[chemical formula]] {{chem2|Al2O3}}. It is the most commonly occurring of several [[Aluminium oxide (compounds)|aluminium oxides]], and specifically identified as '''aluminium oxide'''. It is commonly called '''alumina''' and may also be called '''aloxide''', '''aloxite''', '''ALOX''' or '''alundum''' in various forms and applications and alumina is refined from [[bauxite]].<ref name="Myers">{{Cite book|last=Myers|first=Richard L.|url=https://books.google.com/books?id=0AnJU-hralEC|title=The 100 Most Important Chemical Compounds: A Reference Guide|date=2007|publisher=ABC-CLIO|isbn=978-0-313-33758-1|language=en|pages=24–26|access-date=21 November 2015|archive-date=17 June 2016|archive-url=https://web.archive.org/web/20160617093705/https://books.google.com/books?id=0AnJU-hralEC|url-status=live}}</ref> It occurs naturally in its crystalline [[Polymorphism (materials science)|polymorphic]] [[phase (matter)|phase]] α-Al<sub>2</sub>O<sub>3</sub> as the [[mineral]] [[corundum]], varieties of which form the precious [[gemstone]]s [[ruby]] and [[sapphire]], which have an alumina content approaching 100%.<ref name="Myers" /> Al<sub>2</sub>O<sub>3</sub> is used as feedstock to produce aluminium metal, as an [[abrasive]] owing to its [[hardness]], and as a [[refractory]] material owing 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|access-date = 2007-10-27|archive-url = https://web.archive.org/web/20071010063029/http://www.azom.com/details.asp?ArticleID=1389|archive-date = 10 October 2007|url-status = dead|df = dmy-all|date = 2002-05-03}}</ref> | ||
==Natural occurrence== | ==Natural occurrence== | ||
[[Corundum]] is the most common naturally occurring [[crystallinity|crystalline]] form of aluminium oxide.<ref>{{Cite book|url=https://books.google.com/books?id=9WyXTae58DgC&q=Corundum+is+the+most+common+naturally+occurring+crystalline+form+of+aluminium+oxide.&pg=PA46|title=Atomic Layer Deposition Applications 6|last=Elam|first=J. W.|date=October 2010|publisher=The Electrochemical Society|isbn=9781566778213|language=en}}</ref> [[ruby|Rubies]] and [[sapphire]]s are gem-quality forms of corundum, which owe their characteristic colours to trace impurities. Rubies are given their characteristic deep red colour and their [[laser]] qualities by traces of [[chromium]]. Sapphires come in different colours given by various other impurities, such as iron and titanium. An extremely rare delta form occurs as the mineral deltalumite.<ref>{{cite web |title=Mindat.org - Deltalumite |url=https://www.mindat.org/min-47933.html |website=www.mindat.org |publisher=Hudson Institute of Mineralogy |access-date=15 May 2025}}</ref><ref>{{Cite web|url=https://www.ima-mineralogy.org/Minlist.htm|title=List of Minerals|date=21 March 2011}}</ref> Although | [[Corundum]] is the most common naturally occurring [[crystallinity|crystalline]] form of aluminium oxide.<ref>{{Cite book|url=https://books.google.com/books?id=9WyXTae58DgC&q=Corundum+is+the+most+common+naturally+occurring+crystalline+form+of+aluminium+oxide.&pg=PA46|title=Atomic Layer Deposition Applications 6|last=Elam|first=J. W.|date=October 2010|publisher=The Electrochemical Society|isbn=9781566778213|language=en}}</ref> [[ruby|Rubies]] and [[sapphire]]s are gem-quality forms of corundum, which owe their characteristic colours to trace impurities. Rubies are given their characteristic deep red colour and their [[laser]] qualities by traces of [[chromium]]. Sapphires come in different colours given by various other impurities, such as iron and titanium. An extremely rare delta form occurs as the mineral deltalumite.<ref>{{cite web |title=Mindat.org - Deltalumite |url=https://www.mindat.org/min-47933.html |website=www.mindat.org |publisher=Hudson Institute of Mineralogy |access-date=15 May 2025}}</ref><ref>{{Cite web|url=https://www.ima-mineralogy.org/Minlist.htm|title=List of Minerals|date=21 March 2011}}</ref> Although aluminium is the most abundant metal in the Earth's crust, it must be extracted from bauxite as alumina to produce aluminium metal.<ref name="Myers" /> | ||
==History== | ==History== | ||
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Al<sub>2</sub>O<sub>3</sub> is an [[Insulator (electricity)|electrical insulator]] but has a relatively high [[thermal conductivity]] ({{nowrap|30 Wm<sup>−1</sup>K<sup>−1</sup>}})<ref name="properties">[http://www.makeitfrom.com/data/?material=Alumina Material Properties Data: Alumina (Aluminum Oxide)] {{webarchive|url=https://web.archive.org/web/20100401131344/http://www.makeitfrom.com/data/?material=Alumina |date=2010-04-01 }}. Makeitfrom.com. Retrieved on 2013-04-17.</ref> for a ceramic material. Aluminium oxide is insoluble in water. 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/> | Al<sub>2</sub>O<sub>3</sub> is an [[Insulator (electricity)|electrical insulator]] but has a relatively high [[thermal conductivity]] ({{nowrap|30 Wm<sup>−1</sup>K<sup>−1</sup>}})<ref name="properties">[http://www.makeitfrom.com/data/?material=Alumina Material Properties Data: Alumina (Aluminum Oxide)] {{webarchive|url=https://web.archive.org/web/20100401131344/http://www.makeitfrom.com/data/?material=Alumina |date=2010-04-01 }}. Makeitfrom.com. Retrieved on 2013-04-17.</ref> for a ceramic material. Aluminium oxide is insoluble in water. 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 the resistance of metallic aluminium to [[weathering]]. Metallic aluminium is very reactive with atmospheric oxygen, and a thin [[Passivation (chemistry)|passivation layer]] of aluminium oxide ( | Aluminium oxide is responsible for the resistance of metallic aluminium to [[weathering]]. Metallic aluminium is very reactive with atmospheric oxygen, and a thin [[Passivation (chemistry)|passivation layer]] of aluminium oxide (~5 nm thickness) forms on any exposed aluminium surface almost instantly.<ref>{{cite book |last1=Hatch |first1=John E. |title=Aluminum: properties and physical metallurgy |date=1984 |publisher=American Society for Metals |location=Metals Park, Ohio |isbn=978-1-61503-169-6 |page=242}}</ref> 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 aluminium oxide generated by anodising is typically [[amorphous]], but discharge-assisted oxidation processes such as [[plasma electrolytic oxidation]] result in a significant proportion of crystalline aluminium oxide 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 the EPA's [[Toxics Release Inventory]] list if it is a fibrous form.<ref name=TRI>{{cite web|title=EPCRA Section 313 Chemical List For Reporting Year 2006 |url=http://www.epa.gov/tri/chemical/chemical%20lists/RY2006ChemicalList.pdf |archive-url=https://wayback.archive-it.org/all/20080522232533/http://www.epa.gov/tri/chemical/chemical%20lists/RY2006ChemicalList.pdf |url-status=dead |archive-date=2008-05-22 |publisher=US EPA |access-date=2008-09-30 }}</ref> | Aluminium oxide was taken off the [[United States Environmental Protection Agency]]'s chemicals lists in 1988. Aluminium oxide is on the EPA's [[Toxics Release Inventory]] list if it is a fibrous form.<ref name=TRI>{{cite web|title=EPCRA Section 313 Chemical List For Reporting Year 2006 |url=http://www.epa.gov/tri/chemical/chemical%20lists/RY2006ChemicalList.pdf |archive-url=https://wayback.archive-it.org/all/20080522232533/http://www.epa.gov/tri/chemical/chemical%20lists/RY2006ChemicalList.pdf |url-status=dead |archive-date=2008-05-22 |publisher=US EPA |access-date=2008-09-30 }}</ref> | ||
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|title=Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences |journal=Journal of the American Ceramic Society |volume=81 |issue=8 |pages=1995–2012 |doi=10.1111/j.1151-2916.1998.tb02581.x }}</ref> The oxygen ions form a nearly [[Close-packing of equal spheres|hexagonal close-packed]] structure with the aluminium ions filling two-thirds of the octahedral interstices. Each Al<sup>3+</sup> center is [[octahedral molecular geometry|octahedral]]. In terms of its [[crystallography]], corundum adopts a [[trigonal]] [[Bravais lattice]] with a [[space group]] of [[Hexagonal crystal family|R{{overline|3}}c]] (number 167 in the International Tables). The [[primitive cell]] contains two formula units of aluminium oxide. | |title=Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences |journal=Journal of the American Ceramic Society |volume=81 |issue=8 |pages=1995–2012 |doi=10.1111/j.1151-2916.1998.tb02581.x }}</ref> The oxygen ions form a nearly [[Close-packing of equal spheres|hexagonal close-packed]] structure with the aluminium ions filling two-thirds of the octahedral interstices. Each Al<sup>3+</sup> center is [[octahedral molecular geometry|octahedral]]. In terms of its [[crystallography]], corundum adopts a [[trigonal]] [[Bravais lattice]] with a [[space group]] of [[Hexagonal crystal family|R{{overline|3}}c]] (number 167 in the International Tables). The [[primitive cell]] contains two formula units of aluminium oxide. | ||
Aluminium oxide also exists in other metastable phases, including the cubic γ and η phases, the monoclinic θ phase, the hexagonal χ phase, the orthorhombic κ phase and the δ phase | Aluminium oxide also exists in other metastable phases, including the cubic γ and η phases, the monoclinic λ, θ, θ', and θ{{''}} phase, the hexagonal χ phase, the orthorhombic κ phase the orthorhombic δ phase and the δ' tetragonal phase.<ref name=Levin /><ref name=Paglia>{{cite news| author = Paglia, G. | title =Determination of the Structure of γ-Alumina using Empirical and First Principles Calculations Combined with Supporting Experiments| publisher = Curtin University of Technology, Perth| year = 2004| url = http://espace.library.curtin.edu.au/R?func=search-simple-go&ADJACENT=Y&REQUEST=adt-WCU20040621.123301|format=free download|access-date = 2009-05-05}}</ref> Each has a unique crystal structure and properties. Cubic γ-Al<sub>2</sub>O<sub>3</sub> has important technical applications. The so-called β-Al<sub>2</sub>O<sub>3</sub> proved to be NaAl<sub>11</sub>O<sub>17</sub>.<ref name = "Wiberg&Holleman">{{cite book|author1=Wiberg, E. |author2=Holleman, A. F. |year = 2001| title = Inorganic Chemistry| publisher = Elsevier| isbn = 978-0-12-352651-9}}</ref> | ||
Molten aluminium oxide near the melting temperature is roughly 2/3 [[tetrahedral]] (i.e. 2/3 of the Al are surrounded by 4 oxygen neighbors), and 1/3 5-coordinated, with very little (<5%) [[octahedral]] Al-O present.<ref name="Skinner2013">{{cite journal|title=Joint diffraction and modeling approach to the structure of liquid alumina|doi=10.1103/PhysRevB.87.024201|year=2013|author=Skinner, L.B.|journal=Phys. Rev. B|volume=87|issue=2| | Molten aluminium oxide near the melting temperature is roughly 2/3 [[tetrahedral]] (i.e. 2/3 of the Al are surrounded by 4 oxygen neighbors), and 1/3 5-coordinated, with very little (<5%) [[octahedral]] Al-O present.<ref name="Skinner2013">{{cite journal|title=Joint diffraction and modeling approach to the structure of liquid alumina|doi=10.1103/PhysRevB.87.024201|year=2013|author=Skinner, L.B.|journal=Phys. Rev. B|volume=87|issue=2|article-number=024201|bibcode=2013PhRvB..87b4201S|display-authors=etal|doi-access=free}}</ref> Around 80% of the oxygen atoms are shared among three or more Al-O polyhedra, and the majority of inter-polyhedral connections are corner-sharing, with the remaining 10–20% being edge-sharing.<ref name = "Skinner2013"/> The breakdown of octahedra upon melting is accompanied by a relatively large volume increase (~33%), the density of the liquid close to its melting point is 2.93 g/cm<sup>3</sup>.<ref>{{cite journal|title=Non-Contact Thermophysical Property Measurements of Liquid and Undercooled Alumina |doi=10.1143/JJAP.43.1496|year=2004|author=Paradis, P.-F.|journal= Jpn. J. Appl. Phys. |volume=43|issue=4| pages=1496–1500|bibcode = 2004JaJAP..43.1496P |s2cid=250779901 |display-authors=etal}}</ref> The structure of molten alumina is temperature dependent and the fraction of 5- and 6-fold aluminium increases during cooling (and supercooling), at the expense of tetrahedral AlO<sub>4</sub> units, approaching the local structural arrangements found in amorphous alumina.<ref>{{cite journal |last1=Shi |first1=C |last2=Alderman |first2=O L G |last3=Berman |first3=D |last4=Du |first4=J |last5=Neuefeind |first5=J |last6=Tamalonis |first6=A |last7=Weber |first7=R |last8=You |first8=J |last9=Benmore |first9=C J |title=The structure of amorphous and deeply supercooled liquid alumina |journal=Frontiers in Materials |date=2019 |volume=6 |issue=38 |pages=38 |doi=10.3389/fmats.2019.00038 |bibcode=2019FrMat...6...38S |doi-access=free }}</ref> | ||
==Production== | ==Production== | ||
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{{See also|List of countries by aluminium oxide production}} | {{See also|List of countries by aluminium oxide production}} | ||
Aluminium [[hydroxide]] minerals are the main component of [[bauxite]], the principal [[ore]] of [[aluminium]]. A mixture of the minerals comprise bauxite ore, including [[gibbsite]] (Al(OH)<sub>3</sub>), [[boehmite]] (γ-AlO(OH)), and [[diaspore]] (α-AlO(OH)), along with impurities of [[iron oxide]]s and hydroxides, quartz and [[clay minerals]].<ref>{{cite web| url = | Aluminium [[hydroxide]] minerals are the main component of [[bauxite]], the principal [[ore]] of [[aluminium]]. A mixture of the minerals comprise bauxite ore, including [[gibbsite]] (Al(OH)<sub>3</sub>), [[boehmite]] (γ-AlO(OH)), and [[diaspore]] (α-AlO(OH)), along with impurities of [[iron oxide]]s and hydroxides, quartz and [[clay minerals]].<ref>{{cite web| url = https://minerals.usgs.gov/minerals/pubs/commodity/bauxite/|publisher = USGS| access-date = 2009-05-05| title = Bauxite and Alumina Statistics and Information| archive-url= https://web.archive.org/web/20090506220703/http://minerals.usgs.gov/minerals/pubs/commodity/bauxite/| archive-date= 6 May 2009 | url-status= live}}</ref> Bauxites are found in [[laterite]]s. Bauxite is typically purified using the [[Bayer process]]: | ||
: Al<sub>2</sub>O<sub>3</sub> + H<sub>2</sub>O + NaOH → NaAl(OH)<sub>4</sub> | : Al<sub>2</sub>O<sub>3</sub> + H<sub>2</sub>O + NaOH → NaAl(OH)<sub>4</sub> | ||
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The solid Al(OH)<sub>3</sub> [[Gibbsite]] is then [[calcined]] (heated to over 1100 °C) to give aluminium oxide:<ref name = azom/> | The solid Al(OH)<sub>3</sub> [[Gibbsite]] is then [[calcined]] (heated to over 1100 °C) to give aluminium oxide:<ref name = azom/> | ||
: 2& | : 2 Al(OH)<sub>3</sub> → Al<sub>2</sub>O<sub>3</sub> + 3 H<sub>2</sub>O | ||
The product aluminium oxide tends to be multi-phase, i.e., consisting of several phases of aluminium oxide rather than solely [[corundum]].<ref name=Paglia/> The production process can therefore be optimized to produce a tailored product. The type of phases present affects, for example, the solubility and pore structure of the aluminium oxide product which, in turn, affects the cost of aluminium production and pollution control.<ref name=Paglia/> | The product aluminium oxide tends to be multi-phase, i.e., consisting of several phases of aluminium oxide rather than solely [[corundum]].<ref name=Paglia/> The production process can therefore be optimized to produce a tailored product. The type of phases present affects, for example, the solubility and pore structure of the aluminium oxide product which, in turn, affects the cost of aluminium production and pollution control.<ref name=Paglia/> | ||
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===Paint=== | ===Paint=== | ||
{{Main|Alumina effect pigment}} | {{Main|Alumina effect pigment}} | ||
Aluminium oxide flakes are used in paint for reflective decorative effects, such as in the automotive or cosmetic industries.{{ | Aluminium oxide flakes are used in paint for reflective decorative effects, such as in the automotive or cosmetic industries.<ref>{{Cite patent|number=US8052957B2|title=Method of manufacturing flake aluminum oxide using microwave|gdate=2011-11-08|invent1=Seo|invent2=Bak|invent3=Park|invent4=Lee|inventor1-first=Geum Seok|inventor2-first=Hui Chan|inventor3-first=Seong Soo|inventor4-first=Sang Geun|url=https://patents.google.com/patent/US8052957B2/en}}</ref> | ||
===Biomedical applications=== | ===Biomedical applications=== | ||
Aluminium oxide is a representative of bioinert ceramics.<ref>{{cite book |last1=Ishikawa|first1=K. |last2=Matsuya |first2= S. |date=2003 |title=Comprehensive Structural Integrity |url=https://www.sciencedirect.com/referencework/9780080437491/comprehensive-structural-integrity |publisher=Elsevier Science |volume=9 |pages=169–214 |isbn=978-0-08-043749-1 |access-date=May 27, 2024}}</ref> Due to its excellent biocompatibility, high strength, and wear resistance, alumina ceramics are used in medical applications to manufacture artificial bones and joints.<ref>{{cite web |url=https://www.preciseceramic.com/products/alumina-al2o3.html |title=Alumina ( | Aluminium oxide is a representative of bioinert ceramics.<ref>{{cite book |last1=Ishikawa|first1=K. |last2=Matsuya |first2= S. |date=2003 |title=Comprehensive Structural Integrity |url=https://www.sciencedirect.com/referencework/9780080437491/comprehensive-structural-integrity |publisher=Elsevier Science |volume=9 |pages=169–214 |isbn=978-0-08-043749-1 |access-date=May 27, 2024}}</ref> Due to its excellent biocompatibility, high strength, and wear resistance, alumina ceramics are used in medical applications to manufacture artificial bones and joints.<ref>{{cite web |url=https://www.preciseceramic.com/products/alumina-al2o3.html |title=Alumina (Al<sub>2</sub>O<sub>3</sub>), Aluminum Oxide |website=Precise Ceramic |access-date=May 27, 2024}}</ref> In this case, aluminium oxide is used to coat the surfaces of medical implants to give biocompatibility and corrosion resistance.<ref>[https://powder.samaterials.com/aluminum-oxide-game-changer-in-optical-coating-technology.html Aluminum Oxide: A Game Changer in Optical Coating Technology], Stanford Advanced Materials.</ref> It is also used for manufacturing dental implants, joint replacements, and other medical devices.<ref>{{cite web |url=https://www.usgs.gov/media/images/metals-and-minerals-medical-implants|title=Metals and Minerals in Medical Implants |website=USGS |date=15 March 2021 |access-date=May 27, 2024}}</ref> | ||
===Composite fiber=== | ===Composite fiber=== | ||
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===Abrasion protection=== | ===Abrasion protection=== | ||
An aluminium oxide layer can be grown as a protective coating on aluminium by [[anodizing]] or by [[plasma electrolytic oxidation]] (see the "Properties" above). Both the [[hardness]] and abrasion-resistant characteristics of the coating originate from the high strength of aluminium oxide, yet the porous coating layer produced with conventional direct current anodizing procedures is within a 60–70 Rockwell hardness C range<ref>{{cite web|first=Joseph H.|last=Osborn|title=understanding and specifying anodizing: what a manufacturer needs to know|url=http://www.omwcorp.com/understandingano/anoindex.html|website=OMW Corporation|year=2014|url-status=dead|archive-url=https://web.archive.org/web/20161120010024/http://www.omwcorp.com/understandingano/anoindex.html|archive-date=2016-11-20|access-date=2018-06-02}}</ref> which is comparable only to hardened carbon steel alloys, but considerably inferior to the hardness of natural and synthetic corundum. Instead, with [[plasma electrolytic oxidation]], the coating is porous only on the surface oxide layer while the lower oxide layers are much more compact than with standard DC anodizing procedures and present a higher crystallinity due to the oxide layers being remelted and densified to obtain α- | An aluminium oxide layer can be grown as a protective coating on aluminium by [[anodizing]] or by [[plasma electrolytic oxidation]] (see the "Properties" above). Both the [[hardness]] and abrasion-resistant characteristics of the coating originate from the high strength of aluminium oxide, yet the porous coating layer produced with conventional direct current anodizing procedures is within a 60–70 Rockwell hardness C range<ref>{{cite web|first=Joseph H.|last=Osborn|title=understanding and specifying anodizing: what a manufacturer needs to know|url=http://www.omwcorp.com/understandingano/anoindex.html|website=OMW Corporation|year=2014|url-status=dead|archive-url=https://web.archive.org/web/20161120010024/http://www.omwcorp.com/understandingano/anoindex.html|archive-date=2016-11-20|access-date=2018-06-02}}</ref> which is comparable only to hardened carbon steel alloys, but considerably inferior to the hardness of natural and synthetic corundum. Instead, with [[plasma electrolytic oxidation]], the coating is porous only on the surface oxide layer while the lower oxide layers are much more compact than with standard DC anodizing procedures and present a higher crystallinity due to the oxide layers being remelted and densified to obtain α-Al<sub>2</sub>O<sub>3</sub> clusters with much higher coating hardness values circa 2000 Vickers hardness.{{Citation needed|date=December 2019|reason=removed citation to content from predatory publisher}} | ||
Alumina is used to manufacture tiles which are attached inside pulverized fuel lines and flue gas ducting on coal fired power stations to protect high wear areas. They are not suitable for areas with high impact forces as these tiles are brittle and susceptible to breakage. | Alumina is used to manufacture tiles which are attached inside pulverized fuel lines and flue gas ducting on coal fired power stations to protect high wear areas. They are not suitable for areas with high impact forces as these tiles are brittle and susceptible to breakage. | ||
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[[File:Aluminum Oxide Ice Cream IMG20250316190022.jpg|thumb|Aluminium residue from a '50s vintage ice cream scoop.]] | [[File:Aluminum Oxide Ice Cream IMG20250316190022.jpg|thumb|Aluminium residue from a '50s vintage ice cream scoop.]] | ||
Before the advent of domestic [[plastics]], aluminium [[ice cream scoop]]s would, with wear and tear, leave aluminium residue.<ref>{{cite web | url=https://www.budget101.com/tips-n-tricks/4128-how-to-fix-oxidized-kitchen-utensils/ | title=How to Fix Oxidized Kitchen Utensils - by Budget101 | date=22 October 2020 }}</ref><ref>{{cite journal | pmc=5388732 | date=2017 | last1=Stahl | first1=T. | last2=Falk | first2=S. | last3=Rohrbeck | first3=A. | last4=Georgii | first4=S. | last5=Herzog | first5=C. | last6=Wiegand | first6=A. | last7=Hotz | first7=S. | last8=Boschek | first8=B. | last9=Zorn | first9=H. | last10=Brunn | first10=H. | title=Migration of aluminum from food contact materials to food—a health risk for consumers? Part I of III: Exposure to aluminum, release of aluminum, tolerable weekly intake (TWI), toxicological effects of aluminum, study design, and methods | journal=Environmental Sciences Europe | volume=29 | issue=1 | page=19 | doi=10.1186/s12302-017-0116-y | doi-access=free | pmid=28458989 }}</ref> | Before the advent of domestic [[plastics]], aluminium [[ice cream scoop]]s would, with wear and tear, leave aluminium residue.<ref>{{cite web | url=https://www.budget101.com/tips-n-tricks/4128-how-to-fix-oxidized-kitchen-utensils/ | title=How to Fix Oxidized Kitchen Utensils - by Budget101 | date=22 October 2020 }}</ref><ref>{{cite journal | pmc=5388732 | date=2017 | last1=Stahl | first1=T. | last2=Falk | first2=S. | last3=Rohrbeck | first3=A. | last4=Georgii | first4=S. | last5=Herzog | first5=C. | last6=Wiegand | first6=A. | last7=Hotz | first7=S. | last8=Boschek | first8=B. | last9=Zorn | first9=H. | last10=Brunn | first10=H. | title=Migration of aluminum from food contact materials to food—a health risk for consumers? Part I of III: Exposure to aluminum, release of aluminum, tolerable weekly intake (TWI), toxicological effects of aluminum, study design, and methods | journal=Environmental Sciences Europe | volume=29 | issue=1 | page=19 | doi=10.1186/s12302-017-0116-y | doi-access=free | pmid=28458989 }}</ref> | ||
In lighting, translucent aluminium oxide is used in some [[sodium vapor lamp]]s.<ref>{{cite web|url=http://www.ge.com/innovation/timeline/eras/science_and_research.html|title=GE Innovation Timeline 1957–1970|access-date=2009-01-12| archive-url= https://web.archive.org/web/20090216233917/http://www.ge.com/innovation/timeline/eras/science_and_research.html| archive-date= 16 February 2009 | url-status= live}}</ref> Aluminium oxide is also used in preparation of coating suspensions in [[compact fluorescent lamp]]s. | In lighting, translucent aluminium oxide is used in some [[sodium vapor lamp]]s.<ref>{{cite web|url=http://www.ge.com/innovation/timeline/eras/science_and_research.html|title=GE Innovation Timeline 1957–1970|access-date=2009-01-12| archive-url= https://web.archive.org/web/20090216233917/http://www.ge.com/innovation/timeline/eras/science_and_research.html| archive-date= 16 February 2009 | url-status= live}}</ref> Aluminium oxide is also used in preparation of coating suspensions in [[compact fluorescent lamp]]s. | ||
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In its finest powdered (white) form, called diamantine, aluminium oxide is used as a superior polishing abrasive in [[watchmaking]] and [[clockmaking]].<ref>{{Cite book | title=Practical Watch Repair|pages=164 | last=de Carle|first=Donald|publisher=N.A.G. Press Ltd.|isbn=0719800307|language=en|year=1969 }}</ref> | In its finest powdered (white) form, called diamantine, aluminium oxide is used as a superior polishing abrasive in [[watchmaking]] and [[clockmaking]].<ref>{{Cite book | title=Practical Watch Repair|pages=164 | last=de Carle|first=Donald|publisher=N.A.G. Press Ltd.|isbn=0719800307|language=en|year=1969 }}</ref> | ||
Aluminium oxide is also used in the coating of [[ | Aluminium oxide is also used in the coating of [[stanchion]]s in the motocross and mountain-bike industries. This coating is combined with [[molybdenum disulfide]] to provide long-term lubrication of the surface.<ref>{{Cite web|url=http://www.kashima-coat.com/global/service/kashima-coat/|title = Kashima Coat - Products / Services {{pipe}} Next-generation anodize boasting light weight, high lubrication, and superb wear resistance. The answer is Miyaki's Kashima Coat}}</ref> | ||
==See also== | ==See also== | ||
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==External links== | ==External links== | ||
{{Commons category | {{Commons category}} | ||
* [https://www.cdc.gov/niosh/npg/npgd0021.html CDC - NIOSH Pocket Guide to Chemical Hazards] | * [https://www.cdc.gov/niosh/npg/npgd0021.html CDC - NIOSH Pocket Guide to Chemical Hazards] | ||
Latest revision as of 07:22, 20 November 2025
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Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula Template:Chem2. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly called alumina and may also be called aloxide, aloxite, ALOX or alundum in various forms and applications and alumina is refined from bauxite.[1] It occurs naturally in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire, which have an alumina content approaching 100%.[1] Al2O3 is used as feedstock to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point.[2]
Natural occurrence
Corundum is the most common naturally occurring crystalline form of aluminium oxide.[3] Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colours to trace impurities. Rubies are given their characteristic deep red colour and their laser qualities by traces of chromium. Sapphires come in different colours given by various other impurities, such as iron and titanium. An extremely rare delta form occurs as the mineral deltalumite.[4][5] Although aluminium is the most abundant metal in the Earth's crust, it must be extracted from bauxite as alumina to produce aluminium metal.[1]
History
The field of aluminium oxide ceramics has a long history. Aluminium salts were widely used in ancient and medieval alchemy. Several vintage textbooks cover the history of the field.[6][7]
Properties
Al2O3 is an electrical insulator but has a relatively high thermal conductivity (30 Wm−1K−1)[8] for a ceramic material. Aluminium oxide is insoluble in water. 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.[2]
Aluminium oxide is responsible for the resistance of metallic aluminium to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of aluminium oxide (~5 nm thickness) forms on any exposed aluminium surface almost instantly.[9] 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 bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The aluminium oxide generated by anodising is typically amorphous, but discharge-assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline aluminium oxide 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 the EPA's Toxics Release Inventory list if it is a fibrous form.[10]
Amphoteric nature
Aluminium oxide is an amphoteric substance, meaning it can react with both acids and bases, such as hydrofluoric acid and sodium hydroxide, acting as an acid with a base and a base with an acid, neutralising the other and producing a salt.
- Al2O3 + 6 HF → 2 AlF3 + 3 H2O
- Al2O3 + 2 NaOH + 3 H2O → 2 NaAl(OH)4 (sodium aluminate)
Structure
The most common form of crystalline aluminium oxide is known as corundum, which is the thermodynamically stable form.[11] The oxygen ions form a nearly hexagonal close-packed structure with the aluminium ions filling two-thirds of the octahedral interstices. Each Al3+ center is octahedral. In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a space group of R3c (number 167 in the International Tables). The primitive cell contains two formula units of aluminium oxide.
Aluminium oxide also exists in other metastable phases, including the cubic γ and η phases, the monoclinic λ, θ, θ', and θTemplate:'' phase, the hexagonal χ phase, the orthorhombic κ phase the orthorhombic δ phase and the δ' tetragonal phase.[11][12] Each has a unique crystal structure and properties. Cubic γ-Al2O3 has important technical applications. The so-called β-Al2O3 proved to be NaAl11O17.[13]
Molten aluminium oxide near the melting temperature is roughly 2/3 tetrahedral (i.e. 2/3 of the Al are surrounded by 4 oxygen neighbors), and 1/3 5-coordinated, with very little (<5%) octahedral Al-O present.[14] Around 80% of the oxygen atoms are shared among three or more Al-O polyhedra, and the majority of inter-polyhedral connections are corner-sharing, with the remaining 10–20% being edge-sharing.[14] The breakdown of octahedra upon melting is accompanied by a relatively large volume increase (~33%), the density of the liquid close to its melting point is 2.93 g/cm3.[15] The structure of molten alumina is temperature dependent and the fraction of 5- and 6-fold aluminium increases during cooling (and supercooling), at the expense of tetrahedral AlO4 units, approaching the local structural arrangements found in amorphous alumina.[16]
Production
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Aluminium hydroxide minerals are the main component of bauxite, the principal ore of aluminium. A mixture of the minerals comprise bauxite ore, including gibbsite (Al(OH)3), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)), along with impurities of iron oxides and hydroxides, quartz and clay minerals.[17] Bauxites are found in laterites. Bauxite is typically purified using the Bayer process:
- Al2O3 + H2O + NaOH → NaAl(OH)4
- Al(OH)3 + NaOH → NaAl(OH)4
Except for SiO2, the other components of bauxite do not dissolve in base. Upon filtering the basic mixture, Fe2O3 is removed. When the Bayer liquor is cooled, Al(OH)3 precipitates, leaving the silicates in solution.
- NaAl(OH)4 → NaOH + Al(OH)3
The solid Al(OH)3 Gibbsite is then calcined (heated to over 1100 °C) to give aluminium oxide:[2]
- 2 Al(OH)3 → Al2O3 + 3 H2O
The product aluminium oxide tends to be multi-phase, i.e., consisting of several phases of aluminium oxide rather than solely corundum.[12] The production process can therefore be optimized to produce a tailored product. The type of phases present affects, for example, the solubility and pore structure of the aluminium oxide product which, in turn, affects the cost of aluminium production and pollution control.[12]
Sintering Process
The Sintering Process is a high-temperature method primarily used when the Bayer Process is not suitable, especially for ores with high silica content or when a more controlled product morphology is required.[18] Firstly, Bauxite is mixed with additives like limestone and soda ash, then heating the mixture at high temperatures (1200 °C to 1500 °C) to form sodium aluminate and calcium silicate.[19] After sintering, the material is leached with water to dissolve the sodium aluminate, leaving behind impurities. Sodium aluminate is then precipitated from the solution and calcined at around 1000 °C to produce alumina.[20] This method is useful for the production of complex shapes and can be used to create porous or dense materials.[21]
Applications
Known as alpha alumina in materials science, and as alundum (in fused form) or aloxite[22] in mining and ceramic communities, aluminium oxide finds wide use. Annual global production of aluminium oxide in 2015 was approximately 115 million tonnes, over 90% of which was used in the manufacture of aluminium metal.[2] The major uses of speciality aluminium oxides are in refractories, ceramics, polishing and abrasive applications. Large tonnages of aluminium hydroxide, from which alumina is derived, are used in the manufacture of zeolites, coating titania pigments, and as a fire retardant/smoke suppressant.
Over 90% of aluminium oxide, termed smelter grade alumina (SGA), is consumed for the production of aluminium, usually by the Hall–Héroult process. The remainder, termed specialty alumina, is used in a wide variety of applications which take advantage of its inertness, temperature resistance and electrical resistance.[23]
Fillers
Being fairly chemically inert and white, aluminium oxide is commonly used as a filler for plastics. Aluminium oxide is a common ingredient in sunscreen[24] and is often also present in cosmetics such as blush, lipstick, and nail polish.[25]
Glass
Many formulations of glass have aluminium oxide as an ingredient.[26] Aluminosilicate glass is a commonly used type of glass that often contains 5% to 10% alumina.
Catalysis
Aluminium oxide catalyses a variety of reactions that are useful industrially. In its largest scale application, aluminium oxide is the catalyst in the Claus process for converting hydrogen sulfide waste gases into elemental sulfur in refineries. It is also useful for dehydration of alcohols to alkenes.
Aluminium oxide serves as a catalyst support for many industrial catalysts, such as those used in hydrodesulfurization and some Ziegler–Natta polymerizations.
Gas purification
Aluminium oxide is widely used to remove water from gas streams.[27]
Abrasion
Aluminium oxide is used for its hardness and strength. Its naturally occurring form, corundum, is a 9 on the Mohs scale of mineral hardness (just below diamond). It is widely used as an 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 cutoff tools. As the powdery abrasive mineral aloxite, it is a major component, along with silica, of the cue tip "chalk" used in billiards. Aluminium oxide powder is used in some CD/DVD polishing and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste. It is also used in microdermabrasion, both in the machine process available through dermatologists and estheticians, and as a manual dermal abrasive used according to manufacturer directions.
Paint
Script error: No such module "Labelled list hatnote". Aluminium oxide flakes are used in paint for reflective decorative effects, such as in the automotive or cosmetic industries.[28]
Biomedical applications
Aluminium oxide is a representative of bioinert ceramics.[29] Due to its excellent biocompatibility, high strength, and wear resistance, alumina ceramics are used in medical applications to manufacture artificial bones and joints.[30] In this case, aluminium oxide is used to coat the surfaces of medical implants to give biocompatibility and corrosion resistance.[31] It is also used for manufacturing dental implants, joint replacements, and other medical devices.[32]
Composite fiber
Aluminium oxide has been used in a few experimental and commercial fiber materials for high-performance applications (e.g., Fiber FP, Nextel 610, Nextel 720).[33] Alumina nanofibers in particular have become a research field of interest.
Armor
Some body armors utilize alumina ceramic plates, usually in combination with aramid or UHMWPE backing to achieve effectiveness against most rifle threats. Alumina ceramic armor is readily available to most civilians in jurisdictions where it is legal, but is not considered military grade.[34]
Abrasion protection
An aluminium oxide layer can be grown as a protective coating on aluminium by anodizing or by plasma electrolytic oxidation (see the "Properties" above). Both the hardness and abrasion-resistant characteristics of the coating originate from the high strength of aluminium oxide, yet the porous coating layer produced with conventional direct current anodizing procedures is within a 60–70 Rockwell hardness C range[35] which is comparable only to hardened carbon steel alloys, but considerably inferior to the hardness of natural and synthetic corundum. Instead, with plasma electrolytic oxidation, the coating is porous only on the surface oxide layer while the lower oxide layers are much more compact than with standard DC anodizing procedures and present a higher crystallinity due to the oxide layers being remelted and densified to obtain α-Al2O3 clusters with much higher coating hardness values circa 2000 Vickers hardness.Script error: No such module "Unsubst".
Alumina is used to manufacture tiles which are attached inside pulverized fuel lines and flue gas ducting on coal fired power stations to protect high wear areas. They are not suitable for areas with high impact forces as these tiles are brittle and susceptible to breakage.
Electrical insulation
Aluminium oxide is an electrical insulator used as a substrate (silicon on sapphire) for integrated circuits,[36] but also as a tunnel barrier for the fabrication of superconducting devices such as single-electron transistors, superconducting quantum interference devices (SQUIDs) and superconducting qubits.[37][38]
For its application as an electrical insulator in integrated circuits, where the conformal growth of a thin film is a prerequisite and the preferred growth mode is atomic layer deposition, Al2O3 films can be prepared by the chemical exchange between trimethylaluminium (Al(CH3)3) and H2O:[39]
- 2 Al(CH3)3 + 3 H2O → Al2O3 + 6 CH4
H2O in the above reaction can be replaced by ozone (O3) as the active oxidant and the following reaction then takes place:[40][41]
- 2 Al(CH3)3 + O3 → Al2O3 + 3 C2H6
The Al2O3 films prepared using O3 show 10–100 times lower leakage current density compared with those prepared by H2O.
Aluminium oxide, being a dielectric with relatively large band gap, is used as an insulating barrier in capacitors.[42]
Other
Before the advent of domestic plastics, aluminium ice cream scoops would, with wear and tear, leave aluminium residue.[43][44]
In lighting, translucent aluminium oxide is used in some sodium vapor lamps.[45] Aluminium oxide is also used in preparation of coating suspensions in compact fluorescent lamps.
In chemistry laboratories, aluminium oxide is a medium for chromatography, available in basic (pH 9.5), acidic (pH 4.5 when in water), and neutral formulations. Additionally, small pieces of aluminium oxide are often used as boiling chips.
Health and medical applications include it as a material in hip replacements[2] and birth-control pills.[46]
It is used as a scintillator[47] and dosimeter for radiation protection and therapy applications for its optically stimulated luminescence properties.Script error: No such module "Unsubst".
Insulation for high-temperature furnaces is often manufactured from aluminium oxide. Sometimes the insulation contains a percentage of silica depending on the temperature rating of the material. The insulation can be made in blanket, board, brick, and loose fiber forms for various application requirements.
It is also used to make spark-plug insulators.[48]
Using a plasma-spray process and mixed with titania, it is coated onto the braking surface of some bicycle rims to provide abrasion and wear resistance.Script error: No such module "Unsubst".
Most ceramic eyes on fishing rods are circular rings made from aluminium oxide.Script error: No such module "Unsubst".
In its finest powdered (white) form, called diamantine, aluminium oxide is used as a superior polishing abrasive in watchmaking and clockmaking.[49]
Aluminium oxide is also used in the coating of stanchions in the motocross and mountain-bike industries. This coating is combined with molybdenum disulfide to provide long-term lubrication of the surface.[50]
See also
- Aluminium oxide nanoparticle
- Bauxite tailings
- Beta-alumina solid electrolyte, a fast ion conductor
- Charged Aerosol Release Experiment (CARE)
- List of alumina refineries
- Micro-pulling-down
- Transparent alumina
References
External links
Template:Aluminium compounds Template:Oxides Template:Oxygen compounds
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- ↑ Material Properties Data: Alumina (Aluminum Oxide) Template:Webarchive. Makeitfrom.com. Retrieved on 2013-04-17.
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- ↑ Hudson, L. Keith; Misra, Chanakya; Perrotta, Anthony J.; Wefers, Karl and Williams, F. S. (2002) "Aluminum Oxide" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. Script error: No such module "doi"..
- ↑ Template:Cite patent
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- ↑ Aluminum Oxide: A Game Changer in Optical Coating Technology, Stanford Advanced Materials.
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