Glossary

Aluminum oxide (chemical formula Al₂O₃) is a white, crystalline oxide of aluminum that occurs naturally as the mineral corundum. It is one of the most important industrial compounds, known for its exceptional hardness, high melting point, chemical stability, and electrical insulation properties.

In its pure crystalline form, aluminum oxide appears as colorless sapphire or red ruby, depending on trace impurities such as chromium or iron. Artificially produced aluminum oxide is typically a fine white powder, widely used in metallurgy, ceramics, abrasives, and electronics.

Aluminum oxide forms naturally when aluminum metal reacts with oxygen. This reaction creates a thin, adherent oxide film that protects aluminum from further corrosion or oxidation—this is why aluminum is so corrosion-resistant. The oxide layer is often thickened intentionally through anodizing, a process that enhances wear and corrosion resistance while allowing for decorative coloring.

In industrial applications, aluminum oxide is produced from bauxite ore through the Bayer process, which extracts and purifies the oxide as calcined alumina. This refined Al₂O₃ serves as the feedstock for aluminum metal production via the Hall–Héroult electrolytic process, where it is dissolved in molten cryolite and reduced to pure aluminum.

Aluminum oxide’s hardness (9 on the Mohs scale) makes it ideal for abrasives, such as sandpaper, grinding wheels, and cutting tools. It’s also an excellent thermal and electrical insulator, used in spark plugs, electronic substrates, resistors, and high-temperature crucibles. In ceramics and refractories, Al₂O₃ provides strength and heat resistance up to temperatures exceeding 2,000°C (3,632°F).

Additionally, aluminum oxide plays a role in chemical and medical fields—it’s used as a catalyst support, chromatography medium, and even as an ingredient in certain dental and orthopedic implants due to its biocompatibility and inertness.

AKA: Alumina