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Ultrathin coating of fluidized particles by means of aerosol
Deutsche Forschungsgemeinschaft (DFG) ;
Coated particles for various applications are usually produced by spraying solid-containing liquid on mechanically agitated or fluidized cores. Every spray droplet which is deposited on the surface of a core particle leaves behind a solid remnant after evaporation of the solvent or suspension liquid (preferably water). Each such deposit is a building block (BB) of the coating layer. However, spray droplets are quite large (typically 40 µm with two-fluid nozzles) in present technology, so that BBs are also large, resulting in coarse and thick coating. Radically thinner and finer resolved coating layers (down to the nanoscale) could be produced on fluidized particles by using aerosol (with droplet diameters around 1 µm or less) instead of common spray. Feasibility of the respective aerosol fluidized bed (AFB) coating process has recently been shown by a proof-of-principle experiment. On this basis, the present project aims at a thorough scientific investigation of the novel AFB process. This includes batch coating experiments with variation of operating parameters, materials, as well as aerosol generation and entrance conditions. The quality of coated particles is characterized thoroughly by scanning electron microscopy and various image analysis techniques in regard of intra-particle coating thickness distribution, inter-particle coating thickness distribution, average porosity, porosity distribution, and pore size distribution. Supported by such unique data, a stochastic (Monte Carlo) model is developed and parameterized to accurately simulate the buildup of coating layers on single particles and in the population of particles; Moreover, in the surface coverage period (possibly with island growth) and later on (in the coating layer growth period). Finally, measurements are conducted and a model is developed to predict solids yield of the process, which is equivalent to the efficiency of the fluidized bed in filtering aerosol droplets out of the gas flow.

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