Aluminium oxide (oraluminium(III) oxide) is achemical compound ofaluminium andoxygen with thechemical formulaAl2O3. It is the most commonly occurring of severalaluminium oxides, and specifically identified asaluminium oxide. It is commonly calledalumina and may also be calledaloxide,aloxite,ALOX oralundum in various forms and applications and alumina is refined frombauxite.[7] It occurs naturally in its crystallinepolymorphicphase α-Al2O3 as themineralcorundum, varieties of which form the preciousgemstonesruby andsapphire, which have an alumina content approaching 100%.[7] Al2O3 is used as feedstock to produce aluminium metal, as anabrasive owing to itshardness, and as arefractory material owing to its high melting point.[8]
Corundum is the most common naturally occurringcrystalline form of aluminium oxide.[9]Rubies andsapphires are gem-quality forms of corundum, which owe their characteristic colours to trace impurities. Rubies are given their characteristic deep red colour and theirlaser qualities by traces ofchromium. 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.[10][11] Although aluminium is the most abundant metal in the Earth's crust, it must be extracted from bauxite as alumina to produce aluminium metal.[7]
The field of aluminium oxide ceramics has a long history. Aluminium salts were widely used in ancient and medievalalchemy. Several vintage textbooks cover the history of the field.[12][13]
Al2O3 is anelectrical insulator but has a relatively highthermal conductivity (30 Wm−1K−1)[2] for a ceramic material. Aluminium oxide is insoluble in water. In its most commonly occurring crystalline form, calledcorundum or α-aluminium oxide, its hardness makes it suitable for use as anabrasive and as a component incutting tools.[8]
Aluminium oxide is responsible for the resistance of metallic aluminium toweathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thinpassivation layer of aluminium oxide (~5 nm thickness) forms on any exposed aluminium surface almost instantly.[14] This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process calledanodising. A number ofalloys, such asaluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The aluminium oxide generated by anodising is typicallyamorphous, but discharge-assisted oxidation processes such asplasma electrolytic oxidation result in a significant proportion of crystalline aluminium oxide in the coating, enhancing itshardness.
Aluminium oxide is anamphoteric substance, meaning it can react with bothacids andbases, such ashydrofluoric acid andsodium hydroxide, acting as an acid with a base and a base with an acid, neutralising the other and producing a salt.
The most common form of crystalline aluminium oxide is known ascorundum, which is the thermodynamically stable form.[16] The oxygen ions form a nearlyhexagonal close-packed structure with the aluminium ions filling two-thirds of the octahedral interstices. Each Al3+ center isoctahedral. In terms of itscrystallography, corundum adopts atrigonalBravais lattice with aspace group ofR3c (number 167 in the International Tables). Theprimitive 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 θ″ phase, the hexagonal χ phase, the orthorhombic κ phase the orthorhombic δ phase and the δ' tetragonal phase.[16][17] Each has a unique crystal structure and properties. Cubic γ-Al2O3 has important technical applications. The so-called β-Al2O3 proved to be NaAl11O17.[18]
Molten aluminium oxide near the melting temperature is roughly 2/3tetrahedral (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.[19] 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.[19] 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.[20] 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.[21]
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)3Gibbsite is thencalcined (heated to over 1100 °C) to give aluminium oxide:[8]
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 solelycorundum.[17] 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.[17]
The Sintering Process is a high-temperature method primarily used when the Bayer Process is not suitable, especially forores with highsilica content or when a more controlled product morphology is required.[23] Firstly,Bauxite is mixed with additives likelimestone and soda ash, then heating the mixture at high temperatures (1200 °C to 1500 °C) to formsodium aluminate andcalcium silicate.[24] After sintering, the material is leached with water to dissolve thesodium aluminate, leaving behind impurities. Sodium aluminate is then precipitated from the solution and calcined at around 1000 °C to produce alumina.[25] This method is useful for the production of complex shapes and can be used to create porous or dense materials.[26]
Known asalpha alumina inmaterials science, and asalundum (in fused form) oraloxite[27] inmining andceramic communities, aluminium oxide finds wide use. Annual global production of aluminium oxide in 2015 was approximately 115 milliontonnes, over 90% of which was used in the manufacture of aluminium metal.[8] 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 ofzeolites, coatingtitania pigments, and as a fire retardant/smoke suppressant.
Over 90% of aluminium oxide, termedsmelter grade alumina (SGA), is consumed for the production of aluminium, usually by theHall–Héroult process. The remainder, termedspecialty alumina, is used in a wide variety of applications which take advantage of its inertness, temperature resistance and electrical resistance.[28]
Being fairly chemically inert and white, aluminium oxide is commonly used as a filler for plastics. Aluminium oxide is a common ingredient insunscreen[29] and is often also present in cosmetics such as blush, lipstick, and nail polish.[30]
Many formulations ofglass have aluminium oxide as an ingredient.[31] Aluminosilicate glass is a commonly used type of glass that often contains 5% to 10% alumina.
Aluminium oxide catalyses a variety of reactions that are useful industrially. In its largest scale application, aluminium oxide is the catalyst in theClaus process for converting hydrogen sulfide waste gases into elemental sulfur in refineries. It is also useful fordehydration ofalcohols toalkenes.
Aluminium oxide is used for its hardness and strength. Its naturally occurring form,corundum, is a 9 on theMohs scale of mineral hardness (just below diamond). It is widely used as anabrasive, including as a much less expensive substitute forindustrial diamond. Many types ofsandpaper use aluminium oxide crystals. In addition, its low heat retention and lowspecific heat make it widely used in grinding operations, particularlycutoff tools. As the powdery abrasive mineralaloxite, it is a major component, along withsilica, of thecue tip "chalk" used inbilliards. Aluminium oxide powder is used in someCD/DVDpolishing and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste. It is also used inmicrodermabrasion, both in the machine process available through dermatologists and estheticians, and as a manual dermal abrasive used according to manufacturer directions.
Aluminium oxide is a representative of bioinert ceramics.[34] Due to its excellent biocompatibility, high strength, and wear resistance, alumina ceramics are used in medical applications to manufacture artificial bones and joints.[35] In this case, aluminium oxide is used to coat the surfaces of medical implants to give biocompatibility and corrosion resistance.[36] It is also used for manufacturing dental implants, joint replacements, and other medical devices.[37]
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).[38] Aluminananofibers in particular have become a research field of interest.
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.[39]
An aluminium oxide layer can be grown as a protective coating on aluminium byanodizing or byplasma electrolytic oxidation (see the "Properties" above). Both thehardness 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[40] which is comparable only to hardened carbon steel alloys, but considerably inferior to the hardness of natural and synthetic corundum. Instead, withplasma 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.[citation needed]
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.
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 isatomic layer deposition, Al2O3 films can be prepared by the chemical exchange betweentrimethylaluminium (Al(CH3)3) and H2O:[44]
2 Al(CH3)3 + 3 H2O → Al2O3 + 6 CH4
H2O in the above reaction can be replaced byozone (O3) as the active oxidant and the following reaction then takes place:[45][46]
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 largeband gap, is used as an insulating barrier incapacitors.[47]
In chemistry laboratories, aluminium oxide is a medium forchromatography, available inbasic (pH 9.5),acidic (pH 4.5 when in water), and neutral formulations. Additionally, small pieces of aluminium oxide are often used asboiling chips.
Insulation for high-temperature furnaces is often manufactured from aluminium oxide. Sometimes the insulation contains a percentage ofsilica 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.
Using aplasma-spray process and mixed withtitania, it is coated onto the braking surface of somebicycle rims to provide abrasion and wear resistance.[citation needed]
Most ceramic eyes on fishing rods are circular rings made from aluminium oxide.[citation needed]
In its finest powdered (white) form, called diamantine, aluminium oxide is used as a superior polishing abrasive inwatchmaking andclockmaking.[54]
Aluminium oxide is also used in the coating ofstanchions in the motocross and mountain-bike industries. This coating is combined withmolybdenum disulfide to provide long-term lubrication of the surface.[55]
^Raymond C. Rowe; Paul J. Sheskey; Marian E. Quinn (2009). "Adipic acid".Handbook of Pharmaceutical Excipients. Pharmaceutical Press. pp. 11–12.ISBN978-0-85369-792-3.
^abZumdahl, Steven S. (2009).Chemical Principles 6th Ed. Houghton Mifflin Company.ISBN978-0-618-94690-7.
^abI. Levin; D. Brandon (1999). "Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences".Journal of the American Ceramic Society.81 (8):1995–2012.doi:10.1111/j.1151-2916.1998.tb02581.x.
^Alton T. Tabereaux, Ray D. Peterson (2014). "Chapter 2.5 - Aluminum Production". In Seshadri Seetharaman (ed.).Treatise on Process Metallurgy. Elsevier. pp. 839–917.ISBN9780080969886.
^Bordboland, Reza; Azizi, Asghar; Khani, Mohammad (2024). "Extracting Alumina from a Low-grade (Shale) Bauxite Ore using a Sintering Process with Lime-soda followed by Alkali Leaching".Journal of Mining and Environment.15 (3):1131–1148.doi:10.22044/jme.2024.13905.2588.
^"Aloxite". ChemIndustry.com database.Archived from the original on 25 June 2007. Retrieved24 February 2007.
^Evans, K. A. (1993). "Properties and uses of aluminium oxides and aluminium hydroxides". In Downs, A. J. (ed.).The Chemistry of Aluminium, Indium and Gallium. Blackie Academic.ISBN978-0751401035.
^"Alumina".INCI Decoder. Archived fromthe original on 5 February 2023. Retrieved20 June 2023.
^Hudson, L. Keith; Misra, Chanakya; Perrotta, Anthony J.; Wefers, Karl and Williams, F. S. (2002) "Aluminum Oxide" inUllmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim.doi:10.1002/14356007.a01_557.
^US8052957B2, Seo, Geum Seok; Bak, Hui Chan & Park, Seong Soo et al., "Method of manufacturing flake aluminum oxide using microwave", issued 2011-11-08
^Higashi GS, Fleming (1989). "Sequential surface chemical reaction limited growth of high quality Al2O3 dielectrics".Appl. Phys. Lett.55 (19):1963–65.Bibcode:1989ApPhL..55.1963H.doi:10.1063/1.102337.
^Kim JB; Kwon DR; Chakrabarti K; Lee Chongmu; Oh KY; Lee JH (2002). "Improvement in Al2O3 dielectric behavior by using ozone as an oxidant for the atomic layer deposition technique".J. Appl. Phys.92 (11):6739–42.Bibcode:2002JAP....92.6739K.doi:10.1063/1.1515951.
^Kim, Jaebum; Chakrabarti, Kuntal; Lee, Jinho; Oh, Ki-Young; Lee, Chongmu (2003). "Effects of ozone as an oxygen source on the properties of the Al2O3 thin films prepared by atomic layer deposition".Mater Chem Phys.78 (3):733–38.doi:10.1016/S0254-0584(02)00375-9.
