Freeze granulation: Powder processing for transparent alumina applications

Use of freeze granulated powders is successfully used as an industrially viable alternative to loose powder sintering for transparent polycrystalline alumina (PCA). Freeze granulation with narrow granule size distribution was realized after suspension condition optimization, with very good flowability and regular spherical shapes. The key factors are low viscosity slurries linked to the complex interactions between the organic processing additives and their interaction with dopant ions in solution. Real in-line transmittances (RITs) of 52% were achieved by pulsed electric current sintering (PECS) of dry pressed green bodies from doped granulated powders. This is the first example of a high RIT (>50%) alumina produced from simple dry pressing of a granulated powder. The results indicate that higher granule solid loads and lower organic additive concentrations give better RITs after PECS. Aging of the powder slurry before freeze granulation proves a crucial step for the optical performance of the final transparent PCA.