(119c) The Engineering of Hollow Nanocrystalline Iron Oxide and Barium Titanate Particles Produced by Aerosol Pyrolysis

Aerosol pyrolysis is a well-known approach to produce near-spherical particles for a wide variety of crystalline inorganic materials. Thick-walled hollow particles are commonly produced because of the concentration gradients that evolve during solvent evaporation. While hollow particles are often undesired, for some applications hollow particles are desirable. For example, grain growth of iron oxide can be frustrated in ultrathin-walled hollow particles, resulting in highly friable nanocrystalline hollow particles. Precursor chemistry is a very powerful way to influence the morphology of particles produced by aerosol pyrolysis. We have explored the use of gelling precursors and polymeric or gel-forming additives to intentionally produce hollow iron oxide and barium titanate particles using aerosol pyrolysis. For multicomponent oxides, gel formation with multidentate cation complexation plays a role in preventing undesired secondary phase formation. Precursors or additives that promote surface gelation can lead to ultrathin-walled balloon-like particles, while other types of additive and precursor combinations promote thicker walled particles that may lead to spherical, dimpled or collapsed morphology. In gelling precursor systems, experimental evidence shows that the thin wall forms at relatively low temperature and is retained through calcinations to the final hollow oxide particle. In addition to experimental studies, droplet evaporation modeling has been conducted, indicating that normal multicomponent solution evaporation (with constant diffusion coefficients) cannot account for the steep concentration gradients that are likely needed to produce ultrathin-walled hollow particles; however concentration-dependent diffusion can predict very steep surface gradients that are consistent with such particles. A summary of our experimental and modeling results will be presented.