ALUMINUM OXIDE (ALUMINA – Al₂O₃)

Introduction:
Aluminum Oxide (Al₂O₃), also known as alumina, is the most common oxide of aluminum. It naturally occurs in the crystalline corundum (α-Al₂O₃) form, which has extremely high hardness—second only to diamond on the Mohs scale (Mohs 9)—and a very high melting point (~2050 °C). Al₂O₃ is an amphoteric oxide, chemically and thermally stable, and is a fundamental raw material for advanced ceramics and abrasive materials.

1. Classification and Composition

• Chemical formula: Al₂O₃
• Molar mass: 101.96 g/mol
• Main chemical constituents: Aluminum and oxygen
– Source: Primarily refined from bauxite ore using the Bayer process.
• Classification:
– Industrial-grade alumina: Typically used for refractory materials, with Al₂O₃ content ≥ 90%.
– Technical / ceramic-grade alumina: High-purity alumina (≥ 99%) used for alumina ceramic products.

2. Technical Specifications

Parameter	Value / Characteristics
Appearance	White, crystalline solid powder
Hardness (Mohs)	9 (second only to diamond)
Melting point	Extremely high (~2050 °C)
Chemical properties	Amphoteric oxide; highly inert and corrosion-resistant
Electrical / thermal properties	Excellent electrical insulator with good thermal conductivity

3. Key Applications in Industry and Ceramics

Ceramics & Advanced Ceramic Industry (Alumina Ceramics):
– Increased Hardness and Wear Resistance: Al₂O₃ is the core component of alumina ceramics, widely used for cutting tools, grinding media, bearings, and wear-resistant machine parts.
– Refractory Materials: Withstands extremely high temperatures; used for kiln linings, high-temperature ceramic substrates, and heat-resistant components (e.g., jet engine parts and electrical insulators).
Applications in Glazes and Ceramic Bodies:
– Structural Stabilization: Alumina is added to glazes to control viscosity, prevent excessive glaze flow, and improve chemical durability.
– Mullite Formation: In ceramic bodies, Al₂O₃ reacts with SiO₂ to form mullite, enhancing mechanical strength and thermal stability.
Other Industries:
– Abrasives: Due to its Mohs hardness of 9, Al₂O₃ is used in abrasives, grinding wheels, and sandpaper.
– Chemical Industry: Used as a catalyst and as a desiccant (activated alumina).
– Jewelry: Crystalline corundum forms gemstones such as ruby (red) and sapphire (various colors).