Modeling of Thermophysical Processes in Liquid Ceramic Precursor Droplets Heated by Monochromatic Irradiation

A transient heat and mass transfer model is formulated to describe radiative heating of ceramic precursor droplets in a nonconvective environment. Heating causes vaporization of solvent from the droplet and concentration of the solute within the droplet leading to precipitation of the solute. It is found that the temperatures within the droplets are fairly uniform, but show different spatial profiles depending on the characteristics of solute absorptivity and duration of radiative heating. Incident laser irradiance and wavelength were found to play a significant role in the temperature profiles within droplets due to the absorption characteristics of the solute and the solvent. Lower levels of incident laser irradiation allows longer times for mass diffusion within a droplet leading to a gradual increase of the solute concentration from its center to its surface. Based on an equilibrium homogeneous precipitation hypothesis, it is found that the droplets heated with low laser irradiance tend to form thick precipitate shells as compared to those exposed to higher irradiances and consequently faster rates of vaporization. Large droplets form thin shells through surface precipitation, while small droplets may precipitate into shells of varying thickness depending on the magnitude of irradiance. Comparisons with convective heating in a high temperature plasma indicate that, with proper tuning of the laser irradiance, similar internal temperatures and solute concentration distributions are achievable. These modeling results suggest that different particle morphologies can be obtained from processing of liquid ceramic precursor containing droplets by proper tailoring of radiation parameters (wavelength and irradiance level).