The effect of the slurry formulation upon the morphology of spray-dried yttria stabilised zirconia particles

A study has been performed to determine how the characteristics of spray-dried granules prepared from aqueous yttria doped zirconia slurries can be affected by the spray-drying process parameters: dispersant amount, pH and binder type. First, the colloidal stability of aqueous zirconia suspensions as a function of polyacrylic acid content, pH and binder nature has been investigated in order to establish a stability map. The concentration of dispersant required to stabilise the zirconia suspensions decreases with increasing pH (for instance Γmax=0.93 mg m−2 at pH 4 and 0.1 mg m−2 at pH 10). The addition of a binder may modify the state of dispersion. The stability map makes it possible to define regions of stable (dispersed) and unstable (flocculated) suspensions. Then, dried granules have been produced by means of a single droplet drying method. Granules prepared from a dispersed slurry (low Ratio Sediment Height—RSH <53%) result in hollow shapes that contain a single large open pore. Changing the nature of the binder from latex to an hydrosoluble compound (polyvinyl alcohol) has an effect on the wall thickness of the hollow granules. Conversely, granules obtained from a flocculated slurry (high RSH>62%) result in full granules. The sedimentation volume, which represents the state of dispersion of the suspension, is the major factor controlling the droplet drying mechanism.