Modeling of spray pyrolysis—why are the synthesized Y2O3 microparticles hollow?

There are some major advantages to be gained in processing micronic europium doped yttrium oxide Y2O3 particles for phosphor applications using spray pyrolysis. To better understand the influence of the operating conditions and also why hollow particles are formed, a complete one-dimensional modeling of the pure evaporation then evaporation/precipitation steps of micronic water/Y(NO3)3 droplets has been performed. Among the main results, it appears that no concentration gradient exists inside droplets during the pure evaporation stage and that the evaporation/precipitation stage, occurring in less than 10−3 s, leads to compact particles theoretically formed of a coherent porous medium of Y(NO3)3·5H2O. But the analysis of phenomena occurring during the thermolysis step between 95 and 300°C has revealed that a partial liquefaction of the hydrated yttrium nitrate occurs with concomitant release of gas. As a consequence, a crust of lightly hydrated yttrium nitrate probably appears, followed by particle inflation and leading to the formation of broken hollow particles. © 2007 American Institute of Chemical Engineers AIChE J, 2008