Modeling of Precipitate Formation in Solution Precursor Droplets in a Microwave Plasma

摘要

A comprehensive computational model based on finite difference method was developed to study the heat and mass transport within solution precursor droplets injected into a laminar microwave air plasma flow field. Plasma flow field was simulated as hot gas flowing in a quartz tube generated by volumetric heat addition in the microwave coupling region. The resulting air plasma had a maximum temperature of 6000 K. Droplets containing zirconium acetate precursor of different diameters and solute concentrations were injected into the axisymmetric laminar plasma flow along the centerline of the plasma. Variation of transport properties of the plasma surrounding spherical droplets and absorption of microwave radiation within these droplets were considered in the model. Model predictions suggest that solution droplets are not affected by the microwave radiation in the presence of high convective heat flux from microwave plasma. Smaller droplets and high solute concentrations result in formation of thicker precipitate shells around them based on the homogeneous precipitation hypothesis. Mass transport is found to be slower than heat transfer in the droplets. Microwave plasma allowing the possibility of injecting droplets axially into the high temperature plasma environment present the opportunity to produce more consistent precipitate states as compared to DC arc plasmas into which droplets are typically injected transversely in thermal spray applications.