Aluminium oxide
Overview
 
Aluminium oxide is an amphoteric oxide with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 23. It is commonly referred to as alumina, or corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry. Its most significant use is in the production of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 metal, although it is also used as an abrasive owing to its hardness and as a refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...

 material owing to its high melting point.
Corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

 is the most common naturally occurring crystalline
Crystallinity
Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. In a gas, the relative positions of the atoms or...

 form of aluminium oxide.
Encyclopedia
Aluminium oxide is an amphoteric oxide with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 23. It is commonly referred to as alumina, or corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry. Its most significant use is in the production of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 metal, although it is also used as an abrasive owing to its hardness and as a refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...

 material owing to its high melting point.

Natural occurrence

Corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

 is the most common naturally occurring crystalline
Crystallinity
Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. In a gas, the relative positions of the atoms or...

 form of aluminium oxide. Rubies
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...

 and sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...

s are gem-quality forms of corundum, which owe their characteristic colors to trace impurities. Rubies are given their characteristic deep red color and their laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 qualities by traces of chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

. Sapphires come in different colors given by various other impurities, such as iron and titanium.

Properties

Aluminium oxide is an electrical insulator
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...

 but has a relatively high thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

 (30 Wm−1K−1) for a ceramic material. In its most commonly occurring crystalline form, called corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

 or α-aluminium oxide, its hardness makes it suitable for use as an 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...

 and as a component in cutting tools.

Aluminium oxide is responsible for resistance of metallic aluminium to weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...

. Metallic aluminium is very reactive with atmospheric oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, and a thin passivation layer of alumina (4 nm thickness) forms in about 100 picoseconds 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 in British English, is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called "anodizing" because the part to be treated forms the anode electrode of an electrical circuit...

. 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, in contrast to standard bronze or brass...

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 electrochemical surface treatment process for generating oxide coatings on metals. It is similar to anodizing, but it employs higher potentials, so that discharges occur and the resulting plasma modifies the structure of the...

 result in a significant proportion of crystalline alumina in the coating, enhancing its hardness.

Aluminium oxide is completely insoluble in water. However it is an amphoteric substance, meaning it can react with both acids and bases, such as hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

 and sodium hydroxide.
Al2O3 + 6 HCl → 2 AlCl3 + 3 H2O
Al2O3 + 6 NaOH + 3 H2O → 2 Na3Al(OH)6


Aluminium oxide was taken off the United States Environmental Protection Agency
United States Environmental Protection Agency
The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

's chemicals lists in 1988. Aluminium oxide is on EPA's TRI list if it is a fibrous form.

Structure

The most common form of crystalline alumina is known as corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

. The oxygen ions nearly form a hexagonal close-packed structure with aluminium ions filling two-thirds of the octahedral interstices. Each Al3+ center is octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...

. In terms of its crystallography
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...

, corundum adopts 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.

Alumina also exists in other phases, namely γ-, δ-, η-, θ-, and χ-aluminas. Each has a unique crystal structure and properties. The so-called β-alumina proved to be NaAl11O17.


Production

Aluminium hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...

 minerals are the main component of bauxite
Bauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...

, the principal ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....

 of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

. A mixture of the minerals comprise bauxite ore, including gibbsite
Gibbsite
Gibbsite, Al3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al3 . It is also sometimes called hydrargillite ....

 (Al(OH)3), boehmite
Boehmite
Boehmite or Böhmite 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. It is white with tints of yellow, green, brown or red due to impurities...

 (γ-AlO(OH)), and diaspore
Diaspore
Diaspore is a native aluminium oxide hydroxide, α-AlO, crystallizing in the orthorhombic system and isomorphous with goethite. It occurs sometimes as flattened crystals, but usually as lamellar or scaly masses, the flattened surface being a direction of perfect cleavage on which the lustre is...

 (α-AlO(OH)), along with impurities of iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...

s and hydroxides, quartz and clay minerals
Clay minerals
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...

. Bauxites are found in laterite
Laterite
Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock...

s. Bauxite is purified 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. The alumina must be purified before it can...

:
Al2O3 + 3 H2O → 2 Al(OH)3


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. The solid is then calcined (heated strongly) to give aluminium oxide:
2 Al(OH)3 → Al2O3 + 3 H2O


The product alumina tends to be multi-phase, i.e., consisting of several phases of alumina rather than solely corundum
Corundum
Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

. 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.

Known as alundum (in fused form) or aloxite in the mining
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

, ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

, and materials science
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...

 communities, alumina finds wide use. Annual world production of alumina is approximately 45 million tonne
Tonne
The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...

s, over 90% of which is used in the manufacture of aluminium metal. The major uses of specialty 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.

Applications

The great majority of alumina is consumed for the production of aluminium, usually by the Hall process.

As a filler

Being fairly chemically inert and white, alumina is a favored filler for plastics. Alumina is a common ingredient in sunscreen
Sunscreen
Sunblock is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet radiation on the skin exposed to sunlight and thus helps protect against sunburn...

 and is sometimes present in cosmetics such as blush, lipstick, and nail polish.

As a catalyst and catalyst support

Alumina catalyses a variety of reactions that are useful industrially. In its largest scale application, alumina is the catalyst in the Claus process
Claus process
The Claus process is the most significant gas desulfurizing process, recovering elemental sulfur from gaseous hydrogen sulfide. First patented in 1883 by the scientist Carl Friedrich Claus, the Claus process has become the industry standard....

 for converting hydrogen sulfide waste gases into elemental sulfur in refineries. It is also useful for dehydration of alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

s to alkenes.

Alumina serves as a catalyst support
Catalyst support
In chemistry, a catalyst support is the material, usually a solid with a high surface area, to which a catalyst is affixed. The reactivity of heterogeneous catalysts occurs at the surface atoms. Consequently great effort is made to maximize the surface area of a catalyst by distributing it over...

 for many industrial catalysts, such as those used in hydrodesulfurization
Hydrodesulfurization
Hydrodesulfurization is a catalytic chemical process widely used to remove sulfur from natural gas and from refined petroleum products such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils...

 and some Ziegler-Natta polymerizations. Zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...

s are produced from alumina.

Gas purification and related absorption applications

Alumina is widely used to remove water from gas streams. Other major applications are described below.

As an abrasive

Aluminium oxide is used for its hardness and strength. 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, also known as glasspaper, is a heavy paper with abrasive material attached to its surface.Sandpaper is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother , to remove a layer of material...

 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 and bridges to precise engine parts and delicate jewelry. It therefore includes a correspondingly wide range of skills,...

 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 pool, snooker and carom billiards. It is used to strike a ball, usually the...

 "chalk" used in billiards. Aluminium oxide powder is used in some CD
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...

/DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

 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 a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...

 and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste. Alumina can be grown as a coating on aluminium by anodising
Anodising
Anodizing, or anodising in British English, is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called "anodizing" because the part to be treated forms the anode electrode of an electrical circuit...

 or by plasma electrolytic oxidation
Plasma electrolytic oxidation
Plasma electrolytic oxidation , also known as microarc oxidation , is an electrochemical surface treatment process for generating oxide coatings on metals. It is similar to anodizing, but it employs higher potentials, so that discharges occur and the resulting plasma modifies the structure of the...

 (see the "Properties" above). Both its strength and abrasive characteristics originate from the high hardness (9 on the Mohs scale of mineral hardness
Mohs scale of mineral hardness
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 was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...

) of aluminium oxide. 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 sanding by hand , scraping, or planing. Imperfections or nail holes on the surface may be filled using wood putty or pores may be filled...

ed wood flooring
Flooring
Flooring is the general term for a permanent covering of a floor, or for the work of installing such a floor covering. Floor covering is a term to generically describe any finish material applied over a floor structure to provide a walking surface...

 now uses aluminium oxide as a hard protective coating
Coating
Coating is a covering that is applied to the surface of an object, usually referred to as the substrate. In many cases coatings are applied to improve surface properties of the substrate, such as appearance, adhesion, wetability, corrosion resistance, wear resistance, and scratch resistance...

.

In dentistry, it is used as a polishing agent to remove stains. It is an alternative to sodium bicarbonate, for patients that have high blood pressure.

As an effect pigment

Aluminium oxide flakes are base material for effect pigments. These pigments are widely used for decorative applications e.g. in the automotive or cosmetic industry. See main article Alumina effect pigment
Alumina effect pigment
Alumina effect pigment are pearlescent pigments based on alumina .The most important class of special effect pigments today exists of natural mica platelets coated with thin films of transparent metal oxides like titanium oxide and iron oxide....

.

Niche applications and research themes

In lighting, transparent alumina is used in some sodium vapor lamp
Sodium vapor lamp
A sodium vapor lamp is a gas discharge lamp that 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
A compact fluorescent lamp , also called compact fluorescent light, energy-saving light, and compact fluorescent tube, is a fluorescent lamp designed to replace an incandescent lamp; some types fit into light fixtures formerly used for incandescent lamps...

s.

In chemistry laboratories, alumina is a medium for chromatography
Chromatography
Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures....

, available in basic
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

 (pH 9.5), acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic (pH 4.5 when in water) and neutral formulations.

Health and medical applications include it as a material in hip replacement
Hip replacement
Hip replacement is a surgical procedure in which the hip joint is replaced by a prosthetic implant. Hip replacement surgery can be performed as a total replacement or a hemi replacement. Such joint replacement orthopaedic surgery generally is conducted to relieve arthritis pain or fix severe...

s.

As well, it is used as a dosimeter
Dosimeter
Dosimeters measure an individual's or an object'sexposure to something in the environment — particularly to a hazard inflicting cumulative impact over long periods of time, or over a lifetime...

 for radiation protection and therapy applications for its optically stimulated luminescence
Optically stimulated luminescence
In physics, optically stimulated luminescence is a method for measuring doses from ionizing radiation.The method makes use of electrons trapped between the valence and conduction bands in the crystalline structure of certain types of matter . The trapping sites are imperfections of the lattice -...

 properties.

Aluminium oxide is widely used in the fabrication of superconducting devices, particularly single electron transistors and superconducting quantum interference devices (SQUID
SQUID
A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions....

), where it is used to form highly resistive quantum tunneling barriers.

Insulation for high temperature furnaces is often manufactured from aluminium oxide. Sometimes the insulation has varying percentages 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.

Small pieces of alumina are often used as boiling chips in chemistry.

It is also used to make spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...

 insulators.

Using a plasma spray process and mixed with titania, it is coated onto the braking surface of some aluminum bicycle
Bicycle
A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....

 rims to provide abrasion and wear resistance.

See also

  • Charged Aerosol Release Experiment
    Charged Aerosol Release Experiment
    The Charged Aerosol Release Experiment also known as CARE, is a project run by NASA which will use a rocket to release of dust in the upper atmosphere to form a dusty plasma in space. NASA plans to trigger cloud formation around the rocket's exhaust particles...

     (CARE)
  • Corundum
    Corundum
    Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

  • List of alumina refineries
  • Micro-Pulling-Down
    Micro-pulling-down
    -Basics:The micro-pulling-down method is a crystal growth technique based on continuous transport of the melted substance through micro-channel made in a crucible bottom. Continuous solidification of the melt is progressed on a liquid/solid interface positioned under the crucible...

  • Ruby
    Ruby
    A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...

  • Sapphire
    Sapphire
    Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...

  • Transparent alumina
    Transparent alumina
    Many ceramic materials, both glassy and crystalline, have found use as optically transparent materials in various forms from bulk solid-state components to high surface area forms such as thin films, coatings and fibers....

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