To meet the requirements of different industries, industrial alumina or aluminum hydroxide can be subjected to special processing such as calcination/sintering, hydraulic separation, and grinding. These processes yield alumina products with particle size, shape, and specific surface area significantly different from metallurgical-grade alumina. There are two main preparation methods for alumina: - **Physical Methods**: Techniques like ball milling and other mechanical comminution methods can only refine particle size to a certain extent. These methods alter only the physical properties rather than the chemical properties of the product, and they tend to be energy-intensive. - **Chemical Methods**: Including sol-gel, hydrothermal synthesis, and other approaches, these methods can meet strict requirements for purity, particle size, and specific surface area. Although they involve more complex operational procedures, chemical methods have become the predominant choice for modern alumina production.Also known as high-temperature alumina or calcined alumina, α-Al₂O₃ crystallizes in a rhombohedral system with a relative molecular mass of 101.96. This white powdery material exhibits exceptional physical and chemical stability, featuring a melting point of 2050°C, boiling point of 2980°C, linear expansion coefficient of 8.6×10⁻⁸ K⁻¹, and thermal conductivity of 0.2888 W/(cm•K). Notable properties include:
- Small specific surface area with uniform particle size distribution
- Easy dispersibility, high hardness (Mohs hardness 9.0), low water absorption (≤2.5%)
- Excellent electrical insulation, mechanical strength, wear resistance, and thermal shock resistance
- Insolubility in water, slight solubility in acids/alkalis, good sinterability, and corrosion resistance
These characteristics make α-alumina ideal for high-temperature and high-mechanical-stress applications.
Naβ-alumina is a compound with the formula Na₂O•11Al₂O₃, consisting of 5% (mass fraction) Na₂O and 95% Al₂O₃. It features uniformly distributed small crystal grains, a melting point of approximately 2000°C, refractive index (ε) of 1.635–1.650, bulk density of 3.25 g/cm³, low porosity (sintering degree >97%), high mechanical strength, and superior thermal shock resistance. Key structural parameters include:
- Axial expansion coefficients: ~5.7×10⁻⁶ (a-axis), ~7.7×10⁻⁶ (c-axis)
- High ionic conductivity (300°C resistivity: 35 Ω•cm) due to low grain boundary resistance
Its primary applications include:
-Separator material in sodium-sulfur batteries, serving as both an ionic conductor and physical barrier between sodium and sulfur electrodes
- Raw material for room-temperature batteries, sodium thermal sensors, glass, refractories, and ceramics
Predominantly in γ and ρ crystalline forms, activated alumina is a highly dispersed, porous material characterized by large specific surface area, high pore volume, and strong adsorption capacity. With acidic surface properties and excellent thermal stability, it acts as a versatile material in:
- Adsorbents and dehydrants in petroleum, chemical, pharmaceutical, and environmental industries
- Catalysts and catalyst supports for chemical reactions requiring high surface activity
Its porous structure and surface reactivity make it indispensable for separation, purification, and catalytic processes.
Defied as powders with purity >99.99% and uniform particle size, high-purity alumina includes three main types:
- Single-crystal powder alumina
- Ultrafine alumina (particle size ≤1 μm)
- Nanometer alumina (particle size ≤0.1 μm)
These materials exhibit outstanding physical, chemical, thermal, optical, and mechanical properties:
- High density, hardness, and corrosion resistance
- Excellent high-temperature stability and processability
Critical applications span advanced technologies:
- Integrated circuits, high-pressure sodium lamp tubes, ceramic sensors, and microchips
- Insulating switches, ceramic substrates, catalyst carrier coatings
- Precision instruments, aviation optical components, and capacitors
Their ultra-purity and fine particle control enable use in high-reliability electronic and optoelectronic devices.
