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Aluminum oxide: structure, properties, uses, nomenclature

It has great hardness, corrosion resistance and high electrical resistance. These properties, among others, have allowed aluminum oxide to have numerous applications, such as: electrical insulation, abrasive material, bone and tooth implants, etc.

Bauxite is the main source for obtaining aluminum oxide. Corundum, on the other hand, is an aluminum oxide mineral that is found in the free state in the form of α-Al 2 O 3 . Corundum has a hardness of 9.0, and due to some impurities, it produces precious stones, such as sapphire and ruby.

Structure

Crystal structure of the α-Al2O3 or corundum phase

Corundum or the α-Al 2 O 3 phase , being the most thermodynamically stable polymorph for aluminum oxide, its structure (top image) is considered the main one at the time of its characterization. This structure is trigonal, with Al 3+ ions surrounded by six O 2- , and therefore, establishing AlO 6 octahedra with Al-O or Al-O interactions.

In addition to corundum, Al 2 O 3 also exists as other metastable polymorphs, including: γ-Al 2 O 3 and η-Al 2 O 3 (cubic), θ-Al 2 O 3 (monoclinic), Χ-Al 2 O 3 (hexagonal), κ-Al 2 O 3 (orthorhombic) and δ-Al 2 O 3 (tetragonal or orthorhombic).

As Al 2 O 3 heats up , the octahedra “break up” into AlO 4 tetrahedra or AlO 5 pentahedra . Once melted and cooled again, a vitreous, that is, amorphous, aluminum oxide can be obtained with mixtures of AlO 5 and AlO 6 polyhedra .

Properties of aluminum oxide

Aluminum oxide coating molten pieces of metallic aluminum

Physical appearance

White crystalline solid or powder

Molar mass

101.96 g / mol

Odor

Lavatory

Density

3,798 g / cm 3

Melting point

2072 ºC

Boiling point

2977 ºC

Solubility

Insoluble in all solvents, including water.

Thermal conductivity

30 W · m -1 · K -1 . Despite its use in ceramics, it has a relatively high thermal conductivity.

Refractive index

1,768-1,760

Hardness

It is a compound that has a very high hardness, with a value of 9.0 on the Mohs hardness scale in the form of α-aluminum oxide (corundum).

Electrical resistivity

At 300 ° C: 1.2 x 10 13 ohms · cm. Aluminum oxide or alumina is therefore an electrical insulator.

Although Al 2 O 3 reacts weakly with alkalis, and to a greater degree with acids, it is considered a chemical amphoteric. However, when it is heated to more than 800 ºC, it acquires an almost inert state to the action of acids.

This oxide is responsible for protecting aluminum against environmental corrosive agents, as it firmly covers the metal surface without crumbling or cracking.

Nomenclature

The nomenclature of Al 2 O 3 is simple. It is systematically known as dialuminium trioxide. As this oxide only exists for aluminum, its stock nomenclature does not specify the valence of the metal (III). Also, traditionally its name is summarized as aluminum oxide, as it has only one valence.

Uses / applications

Glass

Aluminum oxide is involved in the production of aluminum-silicate glass. This refractory glass, resistant to thermal shock, can be used at higher temperatures than borosilicate glass; although it has less chemical resistance.

The addition of a small amount of magnesia transforms opaque alumina glass into translucent, allowing it to be used as a gas container in high pressure sodium vapor light bulbs. It is also used in the preparation of coating suspensions for certain types of fluorescent lamps.

Catalyst

Activated alumina has a granular and porous characteristic that can have catalytic activity, or serve as a support for the activity of other catalysts. Among other processes, alumina is involved in the catalysis of the ethylene oxide polymerization.

Microbicidal activity

The aluminum oxide nanoparticles adhere to the surface of the bacteria, causing them to die. Likewise, a similar effect has been shown on the fungus of the genus Candida.

Electrical insulator

Aluminum oxide has a high electrical resistivity, which gives it the ability to act as an electrical insulator. Therefore it is used in integrated circuit packages. It is also used in superconducting devices, and as a capacitor or capacitor dielectric. In addition, it is used as an insulator in electric spark plugs.

Abrasive

Corundum, the hardest, crystalline form of aluminum oxide, is used to build abrasive tools. Source: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

The high hardness of the mineral corundum allows it to be used as an abrasive, lower in cost than diamonds, in industry and in oil drilling.

Corundum takes the form of a precious stone, like red ruby, due to contamination with chromium; or like blue sapphire, due to contamination with iron and titanium. Many types of sandpaper used on metal and wood have aluminum oxide crystals.

The toughness or ability to prevent breakage of aluminum oxide is increased with the addition of zirconia or silicon carbide, enabling it to be used in industrial cutting tools. Also, aluminum oxide is used in toothpastes for polishing teeth.

Also some presentations of aluminum oxide are used as abrasives by dermatologists in the treatment of certain skin conditions.

Gas dehumidifier

Aluminum oxide is a compound that is avid for water, so it can be used to extract it from gaseous streams.

Flame retardant

The nanoparticles of aluminum oxide hydroxide (boehmite) act as a heat sink, releasing water and retarding the appearance of flames.

Hard tissue implants

Aluminum oxide is a highly hard compound, chemically inert and resistant to corrosion, as well as having a white color. Due to these qualities, it is used as a substitute for hard tissues such as bones and teeth. Likewise, the material built with aluminum oxide has been used in the replacement of joints.

Obtaining

Bauxite sample. Source: Max.kit, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Bauxite is the main mineral for obtaining aluminum oxide, being formed by a mixture of three minerals: gibbsite, laboehmite and diaspora.

Aluminum oxide is extracted from its minerals by the Bayer procedure. This consists of the hot reaction of bauxite with sodium hydroxide, a reaction that produces sodium aluminate [NaAl (OH) 4 ].

When the above solution is cooled, the precipitation of aluminum hydroxide in solid form occurs. Then this hydroxide is heated to 1100 ºC, producing aluminum oxide.

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