Alumina (Al₂O₃) and white fused alumina (WFA) share visual similarities, both appearing as white solids, which often leads to confusion. However, these materials differ significantly in composition, structure, and application. White fused alumina is synthesized through high-temperature electric arc furnace processing of alumina as the primary raw material. This document describes its distinctions in physical properties, structural characteristics, and industrial applications.
Crystalline Structure and Composition
Alumina
Chemical Composition: Alumina is a naturally occurring inorganic compound with high hardness. It exists in multiple polymorphic forms, primarily γ-Al₂O₃, β-Al₂O₃, and α-Al₂O₃.
Key Phase: The α-Al₂O₃ phase corresponds to the mineral corundum (commonly referred to as “alumina” in industrial contexts). Natural corundum often contains impurities, while most commercially available alumina is synthetically produced.
Color Variation: Alumina typically appears as a white powder in its γ-phase but turns deep blue after high-temperature calcination.
White Fused Alumina
Synthesis: Derived from high-purity alumina subjected to melting and controlled crystallization in electric arc furnaces (above 2000°C).
Crystalline System: Trigonal crystal system, exhibiting a dense, granular microstructure.
Purity: Typically contains over 99% Al₂O₃, with trace impurities such as SiO₂ or Fe₂O₃.
Comparative Physical Properties
White Fused Alumina
- Density: 3.90 g/cm³
- Hardness:
- Knoop Hardness: 2000–2200 kg/mm²
- Mohs Hardness: 9.0
3. Thermal Properties:
- Melting Point: 2250°C
- Maximum Service Temperature: 1900°C
- Specific Heat Capacity: 0.26 cal/g·°C (20–90°C)
- Thermal Conductivity: 900 cal/cm³·sec·°C at room temperature
- Linear Expansion Coefficient: (7–9) × 10⁻⁶/K (0–1600°C)
4. Optical Properties: Refractive index of e=1.760 and ω=1.768 (measured at sodium line wavelength).
Alumina
- Morphology: White nanopowder (γ-phase) with an average particle size of 20 ± 5 nm.
- Purity: Exceeds 99.9% Al₂O₃ content.
- Thermal Properties:
- Melting Point: 2010–2050°C
- Boiling Point: 2980°C
4. Density: 3.97–4.0 g/cm³ (water = 1).
Industrial Applications
White Fused Alumina
Thermal and Chemical Stability: Its exceptional resistance to thermal shock and corrosive environments makes it ideal for refractory linings, abrasive tools, and precision grinding applications.
Gas Filtration: Sintered white fused alumina cylinders are widely adopted as filter media in gas sampling systems due to their controllable micropore structure, chemical inertness, and high-temperature endurance. Key sectors include chemical processing, marine exploration, and environmental monitoring.
Alumina
Catalysis and Adsorption: The γ-phase alumina, with its high surface area and porosity, serves as a catalyst support in petroleum refining and automotive exhaust systems.
Advanced Ceramics: Used in producing wear-resistant components, electrical insulators, and biomedical implants.
Optoelectronic Materials: High-purity alumina is critical in manufacturing sapphire glass for LEDs and optical sensors.
Conclusion
While alumina and white fused alumina are chemically related, their divergent synthesis methods and microstructures result in distinct physical properties and industrial roles. Alumina excels in high-purity nanomaterial applications, whereas white fused alumina dominates high-temperature and abrasive environments. Understanding these differences ensures optimal material selection for engineering and technological applications.
