Morphology of alumina granules obtained by spray freeze drying with twin‐fluid atomization

Abstract

This study presents the results of the morphological dependence of alumina granules obtained by spray freeze drying (SFD) with twin‐fluid atomization. It is shown that the introduction of 1‐, 2‐ and 4‐wt.% polyvinyl alcohol (PVA) as a binder in an alumina suspension leads to the formation of irregular granules. Depending on atomization conditions, spherical, “tadpole‐like,” granule‐satellite and granule aggregates may form during SFD. Increasing the flight time of droplets before the fixing of their shape when they come in contact with liquid nitrogen effectively contributes to the spheroidization of alumina granules. Compacting PVA‐free SFD granules leads to the formation of diagonal flaws in a green‐body structure. Adding PVA prevents the abovementioned defects by increasing the strength of alumina granules. Increasing the amount of PVA leads to an increase in “tadpole‐like” granule content and, consequently, a decrease in a green‐body density.

Selecting an appropriate binder to prepare alumina granules via spray freeze granulation drying

“…Reducing the amount of binder [poly vinyl alcohol (PVA) in this case], decreasing the spray gas pressure, or increasing the distance between the nozzle and liquid nitrogen (refrigerant to freeze droplets) was effective for obtaining spherical granules. 19) Reducing the molecular mass of binder [polyethylene glycol (PEC) in this case] was also effective for obtaining spherical granules. 20) As the formation of non-spherical granules affects the formability and sinterability of ceramics, leading to a reduced density and strength, the reduction of non-spherical granules is essential to improve the density and strength of the sintered ceramics.…”

Solubility parameters for ensuring a castable design in the Al₂O₃-based gel-casting system

“…The residual bubble content in the slurry increases by stabilizing the bubbles and hindering their further increase. 29),30) Kuzmin et al 31) estimated an increase in the viscosity and surface tension of Al 2 O 3 slurry with the addition of PVA. This effect causes defect-volume and porosity in the sintered body [Figs.…”