Comparison of spray freeze dried nanozirconia granules using ultrasonication and twin-fluid atomisation

Granulation of nanostructured 3 mol% yttria stabilised zirconia using spray freeze drying was investigated to achieve flowable and crushable granules for subsequent die pressing. Commercial nanosuspension consisting of ∼16 nm particles was concentrated to ∼55 wt% solids content via a patented technique, followed by spraying into liquid nitrogen using either a vibrating ultrasonic probe or a twin-fluid atomizer and freeze dried to yield spherical granules. Control of the granule size fractions was investigated by changing the amplitude and the feeding rate of the nanosuspension during ultrasonication, whilst the flow rate of compressed air used for spraying was varied during twin-fluid atomisation. Granules retaining good crushability for pressing were in a size range of 125–250 μm, which were achieved with ∼60 wt% yields using the atomisation route, whilst a maximum of 35 wt% of granules in this size range were produced in previous research using ultrasonication.