Assembly of Magnetic Microcomposites From Low pH Precursors Using a Novel Micro-Fluidic-Jet-Spray-Dryer

Spray drying is a convenient method to produce particles in high yields within a short period of time. The benefits include efficient energy and solvent usages, allowing for scale-up. However, a major challenge in most conventional spray drying is the polydispersity of the produced particles. A novel spray dryer called micro-fluidic-jet-spray-drier (MFJSD) coupled with micro-fluidic-aerosol-nozzle (MFAN) could generate droplets in a single trajectory pattern, to produce monodisperse particles. The drying temperature ranges from 90-300^oC, so that it can be used to accommodate heat sensitive materials if necessary. Here we investigated the use of MFJSD to generate uniform magnetic microcomposites, specifically focusing on the effects of precursor composition, droplet size, and secondary heat treatment on the final properties of the particles.  The presence of silica nanoparticles in the precursor was demonstrated to directly affect the morphology of the particles. Precursor containing silica nanoparticles generated particles with bowl-like shapes due to slower redistribution of solutes to support the particle skeleton during drying. In the absence of silica nanoparticles, the particles were almost perfectly spherical albeit with dimpled surfaces. After being subjected to calcination after drying, iron oxide crystals were found on the particle surfaces accounting for the overall magnetic property of the microcomposites, with lower magnetisation observed for particles containing non-magnetic silica.

SEM pictures showing (a) spray-dried particles from precursors containing silica sol and iron chloride; (b) the particles after calcination at 450^oC in N₂ environment; (c) iron oxide nanocrystals (magnetite and hematite) formed on the surface after calcination