Aluminium oxide

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Aluminium oxide

Aluminium oxide is an amphoteric oxide of aluminium

Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
with the chemical formula

Chemical formula

A chemical formula is a way of expressing information about the atoms that constitute a particular chemical compound, and how the relationship between those atoms changes in chemical reactions....
₂₃. It is also commonly referred to as alumina or aloxite in the mining

Mining

Mining is the extraction of value minerals or other geology materials from the earth, usually from an ore body, vein or seam. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, Sodium chloride and potash....
, ceramic

Ceramic

File:Bridge from dental porcelain.jpgFile:Qing vase p1070256.jpgA ceramic is an inorganic, nonmetal solid prepared by the action of heat and subsequent cooling....
and materials science

Materials science

Materials science or materials engineering is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering....
communities. It is produced by the Bayer process

Bayer process

The Bayer process is the principal industrial means of refining bauxite to produce alumina.Bauxite, the most important ore of aluminium, contains only 30-54% alumina, Al2O3, the rest being a mixture of silica, various iron oxides, and titanium dioxide....
from bauxite

Bauxite

Bauxite is the most important aluminium ore. It consists largely of the minerals gibbsite Al3, boehmite ?-AlO, and diaspore a-AlO, together with the iron oxides goethite and hematite, the clay mineral kaolinite and small amounts of anatase TiO2....
. Its most significant use is in the production of aluminium

Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
metal, although it is also used as an abrasive due to its hardness and as a refractory

Refractory

A refractory material is one that retains its strength at high temperatures. ASTM International C71 defines refractories as "non-metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to environments above 1000 ?F "....
material due to its high melting point.

lass="link1" onMouseover='showByLink("m611308",this)' onMouseout='hide("m611308")'href="http://www.absoluteastronomy.com/topics/Corundum">Corundum

Corundum

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Natural occurrence

Corundum

Ruby

A ruby is a pink to blood-red gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium....
and sapphire

Sapphire

Sapphire refers to gem varieties of the mineral corundum, an aluminium oxide , when it is a color other than red, in which case the gem would instead be a ruby....
s are gem-quality forms of corundum with their characteristic colors due to trace impurities in the corundum structure. Rubies are given their characteristic deep red color and their laser

Laser

A laser is a device that emits light through a process called stimulated emission. The term laser is an acronym for light amplification by stimulated emission of radiation....
qualities by traces of the metallic element chromium

Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
. Sapphires come in different colors given by various other impurities, such as iron and titanium.

Properties

Aluminium oxide is an electrical insulator

Electrical insulation

An insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons....
but has a relatively high thermal conductivity

Thermal conductivity

In physics, thermal conductivity, , is the List of materials properties of a material that indicates its ability to conduct heat. It appears primarily in Heat conduction#Fourier's law for heat conduction....
(40 W/mK). In its most commonly occurring crystalline form, called corundum

Corundum

Abrasive

An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away....
and as a component in cutting tools.

Aluminium oxide is responsible for metallic aluminium's resistance to weathering

Weathering

Weathering is the decomposition of earth Rock , soils and their minerals through direct contact with the planet's atmosphere. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity....
. Metallic aluminium is very reactive with atmospheric oxygen

Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
, 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

Anodising

Anodizing, or anodising, is an electrolyte passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts....
. A number of alloys, such as aluminium bronze

Aluminium bronze

Aluminium bronze is a type of bronze in which aluminium is the main alloying metal added to copper. A variety of aluminium bronzes of differing compositions have found industrial use, with most ranging from 5% to 11% aluminium by weight, the remaining mass being copper; other alloying agents such as iron, nickel, manganese, and silicon are...
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

Plasma electrolytic oxidation

Plasma electrolytic oxidation , also known as microarc oxidation , is an Electrochemistry surface treatment process for generating oxide coatings on metals....
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

United States Environmental Protection Agency

The U.S. Environmental Protection Agency is an List of United States federal agencies of the federal government of the United States charged to Regulation of chemicals and protect human health by safeguarding the natural environment: air, water, and land....
's chemicals lists in 1988. Aluminium oxide is on EPA's TRI list if it is a fibrous form.

Crystal structure

The most common form of crystalline alumina, a-aluminium oxide, is known as corundum

Corundum

Corundum is a crystalline form of aluminium oxide and is one of the rock -forming minerals. It is naturally clear, but can have different colors when impurities are present....
. 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, namely eta, chi, gamma, delta and theta aluminas. Each has a unique crystal structure and properties. The so-called ß-alumina proved to be NaAl₁₁O₁₇.

Production

Aluminium hydroxide

Hydroxide

In chemistry, hydroxide is the name for the Diatomic molecule anion OH-, consisting of oxygen and hydrogen atoms, usually derived from the Dissociation of a base ....
minerals are the main component of bauxite

Bauxite

Ore

An ore is a type of Rock that contains minerals such as gemstones and metals that can be extracted through mining and refined for use. Samples of ore in the form of exceptionally beautiful crystals, exotic layering visible when sectioned or polished or metallic presentations such as large nuggets or crystalline formations of metals suc...
of aluminium

Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
. The bauxite ore is made up of a mixture of the minerals gibbsite

Gibbsite

Gibbsite, Al3, is one of the mineral forms of aluminium hydroxide. It is often designated as ?-Al3 . It is also sometimes called hydrargillite ....
(Al(OH)₃), boehmite

Boehmite

Boehmite or B?hmite is an aluminium oxide hydroxide mineral, a component of the aluminium ore bauxite. It is dimorphous with diaspore. It crystallizes in the orthorhombic dipyramidal system and is typically massive in habit....
(?-AlO(OH)), and diaspore

Diaspore

Diaspore is a native aluminium oxide hydroxide, a-AlO, crystallizing in the orthorhombic system and isomorphous with goethite and manganite....
(a-AlO(OH)) along with iron oxide

Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. Altogether, there are sixteen known iron oxides and oxyhydroxides....
s and hydroxides, quartz and clay minerals

Clay minerals

Clay minerals are hydrous aluminium Silicate_minerals#Phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations....
.

Bauxite is purified by the Bayer process

Bayer process

Al₂O₃ + 3 H₂O + 2 NaOH ? 2NaAl(OH)₄

The Fe₂O₃ does not dissolve in the base. The SiO₂ dissolves as silicate Si(OH)₆^2-. Upon filtering, Fe₂O₃ is removed. When the Bayer liquor is cooled, Al(OH)₃ precipitates, leaving the silicates in solution. The mixture is then calcined (heated strongly) to give aluminium oxide:

2Al(OH)₃ + heat ? Al₂O₃ + 3H₂O

The formed Al₂O₃ is alumina. The alumina formed tends to be multi-phase; i.e., constituting several of the alumina phases rather than solely corundum. 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 alumina product which, in turn, affects the cost of aluminium production and pollution control.

Uses

Annual world production of alumina is approximately 45 million tonnes, over 90% of which is used in the manufacture of aluminium metal.. The major uses of speciality aluminium oxides are in refractories, ceramics, and polishing and abrasive applications. Large tonnages are also used in the manufacture of zeolites, coating titania pigments, and as a fire retardant/smoke suppressant.

Alumina is a medium for chemical chromatography

Chromatography

Chromatography is the collective term for a family of laboratory techniques for the separation of mixtures. It involves passing a mixture dissolved in a "mobile phase" through a stationary phase, which separates the analyte to be measured from other molecules in the mixture and allows it to be isolated....
, available in basic

Base (chemistry)

In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
(pH 9.5), acid

Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
ic (pH 4.5 when in water) and neutral formulations.

In lighting, GE developed "Lucalox" in 1961, a transparent alumina used in sodium vapor lamp

Sodium vapor lamp

A sodium vapor lamp is a gas discharge lamp which uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure....
s. Aluminium oxide is also used in preparation of coating suspensions in compact fluorescent lamp

Compact fluorescent lamp

File:Energiesparlampe 01 retouched.jpgA compact fluorescent lamp , also known as a compact fluorescent light or energy saving light , is a type of fluorescent lamp....
s.

Health and medical applications include it as a material in hip replacement

Hip replacement

Hip replacement, also hip arthroplasty, is a surgery procedure in which the hip joint is replaced by a prosthetic implant . Such joint replacement orthopaedic surgery generally is conducted to relieve arthritis pain or fix severe physical joint damage as part of the hip fracture treatment....
s. It is used in water filters (derived water treatment chemicals such as aluminium sulfate

Aluminium sulfate

Aluminium sulfate, written as Aluminium23 or Aluminium2OxygenSulfur3 Aluminium sulfate is an industrial chemical used as a Flocculation in the purification of drinking water and waste water treatment plants, and also in paper manufacturing....
, aluminium chlorohydrate

Aluminium chlorohydrate

Aluminium chlorohydrate is a group of salts having the general formula AluminiumnChloridem. It is used in deodorants and antiperspirants and as a flocculation in water purification....
and sodium aluminate

Sodium aluminate

Sodium aluminate is an important commercial inorganic chemical. It works as an effective source of aluminium hydroxide for many industrial and technical applications....
, are one of the few methods available to filter water-soluble fluoride

Fluoride

Fluoride is the Redox form of fluorine. Both organic compounds and inorganic compounds containing the chemical element fluorine are considered fluorides....
s out of water). It is also used in toothpaste formulations.

Aluminium oxide is used for its hardness and strength. Most pre-finish

Wood finishing

Wood finishing refers to the process of embellishing and/or protecting the surface of a wooden material. The process starts with surface preparation, either by sandpaper by hand , Card scraper, or Plane ....
ed wood flooring now uses aluminium oxide as a hard protective coating

Coating

Coating is a covering that is applied to an object. The aim of applying coatings is to improve surface properties of a bulk material usually referred to as a Substrate ....
. In 2004, 3M

3M Company , formerly Minnesota Mining and Manufacturing Company until 2002, is an United States multinational corporation Conglomerate corporation with a worldwide presence....
developed a technique for making a ceramic

Ceramic

File:Bridge from dental porcelain.jpgFile:Qing vase p1070256.jpgA ceramic is an inorganic, nonmetal solid prepared by the action of heat and subsequent cooling....
composed of aluminium oxide and rare earth element

Rare earth element

According to IUPAC, rare earth elements or rare earth metals are a collection of seventeen chemical elements in the periodic table, namely scandium, yttrium, and the fifteen lanthanoids....
s to produce a strong glass

Glass

Glass generally refers to a Hardness, brittle, transparency amorphous solid, such as that used for windows, many Glass Bottles, or eyewear, including, but not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovite , or aluminium oxynitride....
called transparent alumina

Transparent alumina

Transparent alumina is a transparent form of aluminium oxide, Al2O3. In bulk, solid form, alumina is a colorless, transparent solid....
. Alumina can be grown as a coating on aluminium by anodising

Anodising

Anodizing, or anodising, is an electrolyte passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts....
or by plasma electrolytic oxidation

Plasma electrolytic oxidation

Plasma electrolytic oxidation , also known as microarc oxidation , is an Electrochemistry surface treatment process for generating oxide coatings on metals....
(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

Mohs scale of mineral hardness

Not to be confused with Siemens_#Mho, a unit of electric conductance.The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material....
).

It is widely used as a coarse or fine abrasive

Abrasive

An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away....
, including as a much less expensive substitute for industrial diamond. Many types of sandpaper

Sandpaper

Sandpaper is a form of paper where an abrasive material has been fixed to its surface.Sandpaper is part of the "coated abrasives" family of abrasive products....
use aluminium oxide crystals. In addition, its low heat retention and low specific heat make it widely used in grinding operations, particularly cutoff

Metalworking

Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships, bridges and oil refineries to delicate jewellery....
tools. As the powdery abrasive mineral aloxite, it is a major component, along with silica, of the cue tip

Cue stick

A cue stick , is an item of sporting equipment essential to the games of Pocket billiards, snooker and carom billiards. It is used to strike a Billiard ball, usually the ....
"chalk" used in billiards. Aluminium oxide powder is used in some CD

Compact Disc

A Compact Disc is an optical disc used to store Data , originally developed for storing digital audio. The CD, available on the market since October 1982, remains the standard physical medium for sale of commercial Sound recording and reproduction to the present day....
/DVD

DVD

DVD, also known as "Digital Versatile Disc" or "Digital Video Disc,"is a popular optical disc data storage device media format. Its main uses are video and data storage....
polishing

Polishing

Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with significant specular reflection and minimal diffuse reflection....
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 transistors and superconducting quantum interference devices (SQUID

Squid

Squid are marine cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, Symmetry #Bilateral_symmetry, a mantle , and cephalopod arms....
), where it is used to form highly resistive quantum tunnelling

Quantum tunnelling

In quantum mechanics, wave-mechanical tunneling is an evanescent wave that occurs because the behaviour of particles is governed by Schroedinger equation....
barriers.

Aluminium oxide

Natural occurrence

Properties

Crystal structure

Production

Uses

See also

External links