Freeze Granulation for Dry pressing of transparent alumina
In recent years, many advances have been made in production of transparent ceramics by various processing and sintering strategies [1]. A key for the industrialization is the use of simple low cost fabrication methods such as Dry pressing. When well controlled, self-flowing granulated powders allow automated production of highly dense and defect-free green bodies by relatively quick and simple uniaxial or isostatic pressing methods. We have investigated the use of freeze granulation, an alternative to spray drying, solving many of its limitations. Freezing being a fast process, diffusion dynamics are insufficient to induce binder migration, thereby better preserving the granule homogeneity sometimes difficult to achieve in spray drying.
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Schematic diagram of freeze granulation set-up |
Freeze dried granules of alumina used to produce Transparent alumina |
Freeze granulated powders were successfully used to produce transparent polycrystalline alumina (PCA). Real in-line transmittances (RIT) of 52 % were achieved by Spark Plasma Sintering (SPS) of dry pressed green bodies from doped granulated powders (450 ppm Mg, La, Y)[2]. Freeze granulation with narrow granule size distributions was achieved, after colloidal stability (link to colloidal stability in atomistic simulation) and suspension 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 [3] 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 SPS..
More information and example:
1. M. Stuer, Z.Zhao, U. Aschauer, P. Bowen, ”Transparent Polycrystalline Alumina using Spark Plasma Sintering: effect of Mg, Y and La doping” J Eur Ceram Soc. 30 (2010) 1335-
2. M. Stuer , Z. Zhe and P. Bowen, “ Freeze granulation: Powder processing for transparent alumina applications Alumina freeze drying” J.Eur.Ceram.Soc (2012)
3. M. Stuer and P. Bowen “Colloidal processing and yield stress modelling towards dry pressed green bodies for transparent polycrystalline alumina” Advanced Engineering materials ,16(6),774-784 (2014)