In the chemical industry, silica sol solution, as an important inorganic material precursor, is widely used in catalyst supports, coating materials, and thermal insulation materials. However, during high-temperature calcination, silica sol particles are prone to agglomeration due to reduced surface energy, leading to a decrease in specific surface area and collapse of the pore structure, thus severely affecting the final performance of the material. Therefore, effectively suppressing particle agglomeration during high-temperature calcination has become a key technical challenge for improving the quality of silica sol materials.
The agglomeration of silica sol particles mainly originates from the abundant silanol (Si-OH) groups on their surface. Under high-temperature conditions, these hydroxyl groups form silicon-oxygen-silicon bonds (Si-O-Si) through dehydration condensation reactions, resulting in chemical bonding between particles. Simultaneously, solvent evaporation during calcination reduces the interparticle spacing, further exacerbating agglomeration caused by physical contact. Furthermore, if impurities or additives are present in the sol that have not been completely removed, their decomposition products may act as "binders," promoting particle aggregation. Therefore, suppressing agglomeration requires a synergistic approach involving blocking chemical bonding, controlling interparticle spacing, and reducing impurity interference.
Surface modification is one of the core methods for suppressing agglomeration. By introducing organosilane coupling agents (such as hexamethyldisilazane and trimethylchlorosilane), a hydrophobic organic coating layer can be formed on the surface of silica particles. This coating layer not only physically isolates the particles, preventing direct contact, but also hinders the dehydration reaction of hydroxyl groups through steric hindrance. The surface energy of the modified particles is significantly reduced, and the driving force for agglomeration is greatly weakened. In addition, some modifiers (such as polydimethylsiloxane) can decompose at high temperatures to generate low surface energy substances, further enhancing the anti-agglomeration effect.
Optimization of the calcination process is crucial for controlling agglomeration. Traditional rapid heating calcination easily leads to uneven reaction rates on the particle surface, and local overheating triggers violent agglomeration. Adopting a staged heating strategy, first slowly removing solvent and adsorbed water in the low-temperature zone, and then completing the crystal transformation in the high-temperature zone, can effectively reduce thermal stress concentration. At the same time, introducing an inert gas (such as nitrogen or argon) protective atmosphere can suppress surface defects caused by oxidation reactions and reduce particle activity. Furthermore, supercritical drying of the sol before calcination can prevent capillary forces from causing pore structure collapse, preserving more dispersion space for subsequent high-temperature processing.
The appropriate use of additives is an effective way to assist in preventing agglomeration. Introducing a small amount of inorganic salts (such as sodium chloride and aluminum nitrate) during the sol preparation stage can stabilize the particle dispersion through double-layer repulsion. These ions can form compounds with high melting points during calcination, covering the particle surface and forming a physical barrier. It is worth noting that the selection of additives must take into account their thermal decomposition characteristics to avoid residues becoming new sources of agglomeration. For example, using volatile organic amines as pH adjusters can completely remove them before calcination, avoiding inorganic salt residues.
Precise control of the sol-gel process is a fundamental measure to prevent agglomeration. By controlling the hydrolysis-condensation reaction rate, an initial sol with uniform particle size and good dispersibility can be prepared. For example, using acidic catalytic conditions to promote linear polycondensation can reduce particle entanglement caused by branched structures; adjusting solvent polarity (such as alcohol-water mixed solvents) can optimize the rheological properties of the sol and avoid pre-agglomeration caused by excessively high local concentrations. Furthermore, introducing template agents (such as surfactants and block copolymers) can directionally construct porous structures, providing structural support for high-temperature processing.
Post-calcination treatment techniques can further repair potential agglomerations. For materials that have already undergone slight agglomeration, secondary dispersion can be achieved using mechanical grinding or air jet milling, but energy input must be strictly controlled to avoid damaging the porous structure. A gentler method is chemical etching, which uses hydrofluoric acid or alkaline silica sol solutions to selectively dissolve the surface contact points of agglomerates, restoring particle independence. In recent years, plasma treatment technology, due to its non-thermal equilibrium characteristics, has been used for surface activation and dispersion of materials after high-temperature calcination, showing promising application prospects.
