Controlling the size distribution of alumina hinges on regulating the precursors, reaction conditions, additives, and heat treatment processes during preparation. This allows for precise control over particle size and its distribution range, meeting the performance requirements of various applications.
Precursors and Preparation Methods Determine Initial Particle Size Distribution
Raw Material Particle Size Control: The initial particle size of aluminum hydroxide or boehmite directly affects the final alumina particle size distribution. For example, fine precursor grains, after calcination, form nano-alumina with numerous micropores and a large specific surface area (D50 can be as low as 5–20 nm); while large-particle-size precursors contribute to obtaining activated alumina with larger pore sizes and wider distributions.
Sol-Gel Method: By adjusting the pH value and hydrolysis rate, nano-alumina (5–50 nm) with a narrow particle size distribution (PDI≤0.15) can be achieved, suitable for high-precision catalysis and coating applications.
High-energy ball milling:
This method grinds micron-sized alumina to 50–80 nm, but it easily introduces impurities and is suitable for abrasive applications where high purity is not required.
Additives and pore-forming agents for regulating pore structure and particle aggregation:
Organic pore-forming agents (e.g., starch, PEG, cellulose): Added during the molding process, they burn off after calcination to form controllable pores, significantly increasing pore size and improving porosity. Higher addition amounts result in larger pore sizes and higher pore volumes, and can be used to prepare porous alumina with pore sizes of 100–250 nm.
Surfactants (e.g., CTAB, PVA): Inhibit growth by adsorbing onto specific crystal planes, regulating particle morphology and size distribution, and improving uniformity.
Inorganic pore expanders (e.g., TiO₂, SiO₂): Inhibit grain coarsening during high-temperature sintering, stabilize the macroporous structure, and improve thermal stability.
Optimization of Grain Growth and Distribution through Heat Treatment
Calcination temperature is a key parameter for controlling particle size distribution:
400–600℃: Generates γ-Al₂O₃, retaining a high specific surface area (200–600 m²/g), small pore size (2–10 nm), and concentrated distribution, suitable for desiccants and catalyst supports.
>1200℃: Gradually transforms into α-Al₂O₃, resulting in grain growth, a wider particle size distribution, and a D50 exceeding 66.9 μm, suitable for electrolytic aluminum and refractory materials.
Heating rate and holding time: Slow heating promotes uniform grain growth and reduces size dispersion; long holding times promote grain agglomeration, leading to increased particle size and distribution shift.
