Role of interparticle potential in controlling the morphology of spray-dried powders from aqueous nanoparticle sols

Precursor nanopowders for producing advanced ceramics are often prepared by spray-drying concentrated sols, to produce aggregated particles for ease of handling during subsequent processing. In this study, the effect of ionic strength and solids content on the nanostructure and morphology of mesoporous titania gel microspheres obtained by spray-drying concentrated nanoparticle sols has been investigated using combined small-angle and ultra-small-angle X-ray scattering (SAXS/USAXS), small-angle neutron scattering, electron microscopy, and N2 adsorption. The USAXS and conventional SAXS of the as-prepared powders were measured over four decades in q, enabling their structure to be probed on length scales from 0.5 to >1000 nm. Absolute scaled measurements in this range show two exponential regimes and a transition region, which were modeled to obtain (i) the surface area of the nanosized pores, (ii) the total porosity of the microspheres, and (iii) the average size of the microspheres. The relationship between the interaction potential (ionic strength) in the sol and the porosity (microstructure) of the resulting spray-dried powders will be discussed. The macroscopic structure (morphology) of the microspheres was also found to be strongly influenced by the ionic strength. In the absence of added salt, fragmentation of the atomized droplets occurs during drying, leading to the formation of fine powder, in addition to gel particles with distorted shapes (e.g. donuts). In contrast, addition of appropriate salts results in the formation of well-defined spherical particles. It is shown that these effects can be attributed to the relative magnitudes of the Laplace and osmotic pressures immediately before the sol-to-gel transition. If the Laplace pressure exceeds the osmotic pressure, then well-defined spherical particles are obtained. Conversely, distorted particles are formed when the osmotic pressure exceeds the Laplace pressure.