Aluminium oxide (data page)

Aluminium oxide, also known as alumina, is a chemical compound with the formula Al₂O₃. It is an amphoteric oxide of aluminium, meaning it can react with both acids and bases. Aluminium oxide exists in several crystalline forms, the most common of which is corundum (α-Al₂O₃).

Properties:

• Appearance: Typically a white powder, though crystalline forms can be transparent or colored due to impurities.
• Melting Point: Very high, around 2072 °C (3762 °F; 2345 K).
• Boiling Point: Approximately 2977 °C (5391 °F; 3250 K).
• Density: Varies depending on the crystalline form, but typically around 3.95 - 4.0 g/cm³.
• Hardness: Corundum is extremely hard, with a Mohs hardness of 9, making it suitable for abrasive applications.
• Solubility: Insoluble in water. It can dissolve in strong acids and bases.
• Electrical Properties: Generally an electrical insulator, but its conductivity can be modified by doping.
• Chemical Inertness: Highly chemically inert and resistant to corrosion in many environments.

Occurrence and Production:

Aluminium oxide is found naturally as corundum (ruby and sapphire are gem varieties), as well as in bauxite, the principal ore of aluminium. It is commercially produced via the Bayer process, which involves dissolving bauxite in hot sodium hydroxide solution, followed by precipitation of aluminium hydroxide and subsequent calcination to form aluminium oxide.

Applications:

Aluminium oxide has a wide range of applications, including:

• Abrasives: Due to its hardness, it is used in grinding wheels, polishing compounds, and sandpaper.
• Refractory materials: Its high melting point and chemical inertness make it suitable for use in furnace linings and other high-temperature applications.
• Ceramics: Used as a component in various ceramic materials, providing strength and chemical resistance.
• Catalysis: It serves as a catalyst or catalyst support in various chemical processes.
• Adsorbent: Used as a desiccant and adsorbent in chromatography and other separation techniques.
• Electronics: Used as an insulator in integrated circuits and other electronic devices.
• Gemstones: Corundum is a valuable gemstone when found in transparent and colored varieties (ruby and sapphire).
• Medical Implants: Due to its biocompatibility and inertness, it's used in some medical implants.

Safety:

Aluminium oxide is generally considered to be of low toxicity. However, inhalation of fine aluminium oxide dust can cause respiratory irritation. Appropriate safety measures, such as wearing dust masks, should be taken when handling powdered forms of the material.