Spray drying and sintering of zirconia based hollow powders

Abstract

A study has been performed on both the spray drying of ceramic slurries to determine how the characteristics of the spray dried powder can be affected by the process parameters and the sintering of these agglomerated powders. Two materials have been investigated: yttria partially stabilized ziconia (PSZ) and zirconium silicate. Indeed spray drying enables the fabrication of powder by aggregation of different small particles using an appropriate organic binder. The rapid heat and mass transfer which occurs during drying combined with the presence of various slurry compounds result in dried granules having a large variety of shapes, from uniform solid elongated, pancake, donut-shaped, needlelike or hollow granules. First experiments have been focused on the slurry parameters: dispersant amount, binder type and pH. The colloidal stability of aqueous ceramic suspensions as a function of poly(acrylic) acid content, pH and binder nature has been investigated. The addition of a binder may modify the state of dispersion. Thanks to sedimentation tests, stable (dispersed) and unstable (flocculated) suspensions were defined. Then, dried granules have been produced by means of a single droplet drying method. Individual slurry droplets were suspended using a hypodermic syringe in a stream of heated air. The aim was to assess the effect of the state of dispersion and sedimentation stability of the slurry on the drying of a single droplet. It has been shown that granules prepared from a dispersed slurry (low Ratio Sediment Height—RSH < 50% for PSZ) result in hollow shapes that contain a single large open pore. Changing the nature of the binder from latex to poly(vinyl) alcohol (PVA) has an effect on the wall thickness of the hollow granules. Conversely, granules obtained from a flocculated slurry (high RSH > 60% for PSZ) result in solid granules. The sedimentation volume, which represents the state of dispersion of the suspension, is the major factor controlling the droplet drying mechanism. According to the former results, powders have been elaborated by the spray drying process using several slurries formulations. Image analyses have been performed on the granules to determine their morphologies. The shape of the agglomerated powder was mostly spherical and hollow. These results confirmed the morphologies predictions of the drying test. Finally, the evolution of the morphology of the PSZ spray dried hollow powder after sintering in an oxyacetylene flame or an atmospheric furnace was investigated. When the powder is sintered in a furnace, low shrinkage and approximately no shape variations were noticed, whereas using an oxyacetylene flame tends to densify the powder.