Y2O3:Eu micronic particles synthesised by spray pyrolysis: Global modelling and optimisation of the evaporation stage

There are a number of some major advantages to be gained in processing micronic europium-doped yttrium oxide Y2O3 particles for phosphor applications using spray pyrolysis. In order to maximise production rates, it is tempting to use relatively dense sprays, but then coalescence occurs increasing final particle diameters, which must be prevented. Moreover, the influence of the operating conditions on the process behaviour is poorly understood. A complete one-dimensional model of the evaporation stage of micronic water/Y(NO3)3 droplets considering only the evaporation process and then both evaporation and gravity-induced coalescence phenomena has been established. Calculations of pure evaporation have shown that the amounts of evaporated water and droplet compositions depend only on the local temperature and not on the thermal history of the spray. Coupled calculations have shown that, in comparison with evaporation, coalescence plays a minor role on droplet diameter, but non-negligible as the increase of the final mean droplet diameter due to coalescence reaches up to 10% at low flow rates in the operating conditions tested. Injecting a preheated air flow directly into the nebuliser is a promising method to minimise coalescence effects: optimal operating conditions for which coalescence is completely insignificant were obtained by simulation.