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