Known as
alpha alumina in
materials science, and as
alundum (in fused form) or
aloxite in
mining and
ceramic communities, aluminium oxide finds wide use. Annual global production of aluminium oxide in 2015 was approximately 115 million
tonnes, over 90% of which was used in the manufacture of aluminium metal.
Fillers Being fairly chemically inert and white, aluminium oxide is commonly used as a filler for plastics. Aluminium oxide is a common ingredient in
sunscreen and is often also present in cosmetics such as blush, lipstick, and nail polish.
Glass Many formulations of
glass have aluminium oxide as an ingredient. Aluminosilicate glass is a commonly used type of glass that often contains 5% to 10% alumina.
Catalysis Aluminium oxide catalyses a variety of reactions that are useful industrially. In its largest scale application, aluminium oxide is the catalyst in the
Claus process for converting hydrogen sulfide waste gases into elemental sulfur in refineries. It is also useful for
dehydration of
alcohols to
alkenes. Aluminium oxide serves as a
catalyst support for many industrial catalysts, such as those used in
hydrodesulfurization and some
Ziegler–Natta polymerizations.
Gas purification Aluminium oxide is widely used to remove water from gas streams.
Abrasion Aluminium oxide is used for its hardness and strength. Its naturally occurring form,
corundum, is a 9 on the
Mohs scale of mineral hardness (just below diamond). It is widely used as an
abrasive, including as a much less expensive substitute for
industrial diamond. Many types of
sandpaper use aluminium oxide crystals. In addition, its low heat retention and low
specific heat make it widely used in grinding operations, particularly
cutoff tools. As the powdery abrasive mineral
aloxite, it is a major component, along with
silica, of the
cue tip "chalk" used in
billiards. Aluminium oxide powder is used in some
CD/
DVD polishing and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste. It is also used in
microdermabrasion, both in the machine process available through dermatologists and estheticians, and as a manual dermal abrasive used according to manufacturer directions.
Paint Aluminium oxide flakes are used in paint for reflective decorative effects, such as in the automotive or cosmetic industries.
Biomedical applications Aluminium oxide is a representative of bioinert ceramics. Due to its excellent biocompatibility, high strength, and wear resistance, alumina ceramics are used in medical applications to manufacture artificial bones and joints. In this case, aluminium oxide is used to coat the surfaces of medical implants to give biocompatibility and corrosion resistance. It is also used for manufacturing dental implants, joint replacements, and other medical devices.
Composite fiber 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). Alumina
nanofibers in particular have become a research field of interest.
Armor Some body armor utilizes 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 it is not considered military grade.
Abrasion protection An aluminium oxide layer can be grown as a protective coating on aluminium by
anodizing or by
plasma electrolytic oxidation (see the "Properties" above). Both the
hardness 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 which is comparable only to hardened carbon steel alloys, but considerably inferior to the hardness of natural and synthetic corundum. Instead, with
plasma 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. 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.
Electrical insulation Aluminium oxide is an electrical
insulator used as a substrate (
silicon on sapphire) for
integrated circuits, but also as a
tunnel barrier for the fabrication of
superconducting devices such as
single-electron transistors, superconducting quantum interference devices (
SQUIDs) and
superconducting qubits. 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 is
atomic layer deposition, Al2O3 films can be prepared by the chemical exchange between
trimethylaluminium (Al(CH3)3) and H2O: :2 Al(CH3)3 + 3 H2O → Al2O3 + 6 CH4 H2O in the above reaction can be replaced by
ozone (O3) as the active oxidant and the following reaction then takes place:
Other Before the advent of domestic
plastics, aluminium
ice cream scoops would, with wear and tear, leave aluminium residue. In lighting, translucent aluminium oxide is used in some
sodium vapor lamps. Aluminium oxide is also used in preparation of coating suspensions in
compact fluorescent lamps. In chemistry laboratories, aluminium oxide is a medium for
chromatography, available in
basic (pH 9.5),
acidic (pH 4.5 when in water), and neutral formulations. Additionally, small pieces of aluminium oxide are often used as
boiling chips. Health and medical applications include it as a material in
hip replacements It is used as a
scintillator and
dosimeter for radiation protection and therapy applications for its
optically stimulated luminescence properties. Insulation for high-temperature furnaces is often manufactured from aluminium oxide. Sometimes the insulation contains a percentage 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. It is also used to make
spark-plug insulators. Using a
plasma-spray process and mixed with
titania, it is coated onto the braking surface of some
bicycle rims to provide abrasion and wear resistance. Most ceramic eyes on fishing rods are circular rings made from aluminium oxide. In its finest powdered (white) form, called diamantine, aluminium oxide is used as a superior polishing abrasive in
watchmaking and
clockmaking. Aluminium oxide is also used in the coating of
stanchions in the motocross and mountain-bike industries. This coating is combined with
molybdenum disulfide to provide long-term lubrication of the surface. ==See also==