7 junio, 2024

Aluminum oxide: structure, properties, uses, nomenclature

He aluminum oxide, also called alumina, aluminum oxide, corundum or aluminum trioxide, is an inorganic compound whose chemical formula is Al2O3. It is an amphoteric oxide, although it can become, due to treatment, an almost inert compound.

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 found in the free state in the form of α-Al2O3. Corundum has a hardness of 9.0, and due to some impurities, it produces precious stones, such as sapphire and ruby.

The electrolysis of aluminum oxide formed the basis for the isolation of aluminum by Humphry Davy in 1808. This same method, with some adaptations, is used today in the production of metallic aluminum.

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Structure

Corundum or the α-Al2O3 phase, being the most thermodynamically stable polymorph for aluminum oxide, its structure (upper image) is considered the main one at the time of its characterization. This structure is trigonal, with the Al3+ ions surrounded by six O2-, thus establishing AlO6 octahedrons with Al—O or Al-O interactions.

In addition to corundum, Al2O3 also exists as other metastable polymorphs, including: γ-Al2O3 and η-Al2O3 (cubic), θ-Al2O3 (monoclinic), Χ-Al2O3 (hexagonal), κ-Al2O3 (orthorhombic), and δ- Al2O3 (tetragonal or orthorhombic).

As the Al2O3 is heated, the octahedrons “break” into AlO4 tetrahedrons or AlO5 pentahedra. Once melted and cooled again, a glassy aluminum oxide can be obtained, that is, amorphous, with mixtures of AlO5 and AlO6 polyhedra.

Aluminum Oxide Properties

Physical appearance

White crystalline solid or powder

molar mass

101.96 g/mol

Smell

Lavatory

Density

3.798 g/cm3

Melting point

2072ºC

Boiling point

2977ºC

Solubility

Insoluble in all solvents, including water.

Thermal conductivity

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

Refractive index

1768-1760

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 ºC: 1.2 x 1013 ohms cm. Aluminum oxide or alumina is therefore an electrical insulator.

Reactivity

Al2O3, despite reacting weakly with alkalis, and to a greater degree with acids, is considered a chemical amphoteric. However, when it is heated to more than 800 ºC, it becomes almost inert to the action of acids.

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

Nomenclature

The nomenclature of Al2O3 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 summed up to aluminum oxide, for having a single 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 lower chemical resistance.

The addition of a small amount of magnesia makes opaque alumina glass translucent, making it suitable for use as a gas container in high-pressure sodium vapor 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 polymerization of ethylene oxide.

microbicidal activity

Aluminum oxide nanoparticles adhere to the surface of bacteria, causing their death. 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 dielectric for capacitors or capacitors. In addition, it is used as an insulator in electric spark plugs.

Abrasive

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

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

The toughness or ability to prevent aluminum oxide from breaking increases with the addition of zirconia or silicon carbide, which makes it suitable for use in industrial cutting tools. Additionally, aluminum oxide is used in toothpastes to polish 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 with avidity for water, so it can be used to extract it from gaseous streams.

flame retardant

The nano-particles 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 very 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. Also, the material constructed with aluminum oxide has been used in joint replacement.

Obtaining

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

Aluminum oxide is extracted from its ores by the Bayer process. 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.

References

Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
Wikipedia. (2020). Aluminum oxide. Retrieved from: en.wikipedia.org
National Center for Biotechnology Information. (2020). Aluminum oxide. PubChem Compound Summary for CID 9989226. Retrieved from: pubchem.ncbi.nlm.nih.gov
Kushagri S. et al. (2019). Antiviral and Antimicrobial Potentiality of Nano Drugs: Aluminum Oxide Nanoparticles. Retrieved from: sciencedirect.com
The Editors of Encyclopaedia Britannica. (2020). Alumina. Recovered from: britannica.com
Lohninger Hans. (August 15, 2020). Aluminum Oxide. Chemistry LibreTexts. Retrieved from: chem.libretexts.org

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