Study of agglomeration of alumina nanoparticles by atomic force microscopy (AFM) and photon correlation spectroscopy (PCS)

Alumina (Al2O3) nanopowders dispersed by ultrasonic treatment in solvents of different hydrophobicity are simultaneously investigated using photon correlation spectroscopy (PCS) and, after sedimentation to a thin layer, by atomic force microscopy (AFM). This simultaneous information is crucial for the optimisation of a wet process yielding a certain dry product. Largely varying structures of agglomerates are observed both in dispersions and in the thin sedimented layers as a function of the liquid used for the dispersions. Surprisingly, at 20% alumina, an excellent dispersion was obtained in water and in heptan with 3% high molecular weight dispersant Solsperse 17,000 (S17000) but not in liquids of intermediate hydrophobicity, such as benzyl alcohol. The size distribution and structure of the agglomerates indicates that in hydrophilic solvents the formation of a relatively tightly bound solvent shell due to H-bonding with alumina nanoparticles, but in solvents with the investigated dispersants the steric hindrance by the copolymers are major forces that prevent particles from hard agglomeration. Conclusions for optimal dispersion procedures are drawn.