Properties of Aluminum Oxide (Alumina)

Properties of Aluminum Oxide (Alumina)

Compound of aluminum and oxygen with the chemical formula Al2O3. Commonly called alumina, and may also be called aloxide, aloxite, or alundum depending on particular forms or applications. Al2O3 is significant in its use to produce aluminum metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point. Aluminum hydroxide minerals are the main component of bauxite, the principal ore of aluminum.

Over 90% of the aluminum oxide, normally termed Smelter Grade Alumina (SGA) produced is consumed by the production of aluminum. The remainder, normally called specialty alumina is used in a wide variety of applications which reflect its inertness, temperature resistance and electrical resistance.

Aluminum oxide is fairly inert and white, and is a favored filler for plastics. It is a common ingredient in sunscreen and is sometimes present in cosmetics such as blush, lipstick, and nail polish. Many formulations of glass have aluminum oxide as an ingredient.

In its largest scale application, it 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. Aluminum oxide serves as a catalyst support for many industrial catalysts, such as those used in hydrodesulfurization and some Ziegler-Natta polymerizations. Aluminum oxide is widely used to remove water from gas streams.

Due to its hardness and strength it is widely used as an abrasive, including as a much less expensive substitute for industrial diamond. Many types of sandpaper use aluminum 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. Aluminum oxide powder is used in some CD/DVD polishing and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste.

Flakes are used in paint for reflective decorative effects, such as in the automotive or cosmetic industries.

Used in a few experimental and commercial fiber materials for high-performance applications (e.g., Fiber FP, Nextel 610, Nextel 720).

Can be grown as a coating on aluminum by anodizing or by plasma electrolytic oxidation. Both the hardness and abrasion-resistant characteristics of the coating originate from the high strength of aluminum 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.

In lighting, transparent aluminum oxide is used in some sodium vapor lamps. Aluminum oxide is also used in preparation of coating suspensions in compact fluorescent lamps. In chemistry laboratories, aluminum oxide is a medium for chromatography, available in basic (pH 9.5), acidic (pH 4.5 when in water) and neutral formulations.

Health and medical applications include it as a material in hip replacements. It is used as a dosimeter for radiation protection and therapy applications for its optically stimulated luminescence properties.

Aluminum 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 and superconducting quantum interference devices (SQUIDs).

Insulation for high-temperature furnaces is often manufactured from aluminum 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 aluminum oxide are often used as boiling chips in chemistry.

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 aluminum oxide.