(10c) Dispersion of Flame-Synthesized Silica Nanoparticles in an Aqueous Solution

Flame synthesized nanoparticles are applied as dispersed phase to nanostructured materials improving optical, mechanical, thermal or handling properties of coatings or bulk materials. The processing often requires a re-dispersion of the dry particles in a liquid phase, however flame made particles often tend to be agglomerated. The scope of this study is the investigation of the mechanical dispersion focusing on the break-up of agglomerates using high pressure dispersion. Silica nanoparticles were produced by oxidation of hexamethyldisiloxane (HMDSO) in a co-flow diffusion flame reactor. The primary particle diameter of the product powders was measured by nitrogen adsorption and the particle morphology was investigated by transmission electron microscopy. Primary particle size and degree of agglomeration could be controlled by varying the oxygen and HMDSO flow rates. Spherical, non-agglomerated silica particles were produced at low O2 flow rates, while smaller and highly agglomerated particles were made at high O2 flow rates at a silica production rate of 5 g/h. Increasing the silica production rate up to 14 g/h keeping the O2 flow rate constant, the primary particle diameter increases while the degree of agglomeration is virtually unchanged. The influence of primary particle diameter and degree of agglomeration on the dispersion behaviour of the flame made silica particles in aqueous solutions has been studied in terms of the break-up of hard and soft agglomerates. The high pressure dispersion system allows imposing a pressure difference of up to 1500 bars with a nozzle of 125 µm inner diameter. Hydrodynamic stresses result in a complete break-up of the soft agglomerates and yield hard agglomerate sizes in the range of 100 to 180 nm characterized by dynamic light scattering.