Spray freeze drying of YSZ nanopowder
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Spray freeze drying of yttria stabilised zirconia nanopowders with a primary particle size of ~16 nm has been undertaken using different solids content starting suspensions, with the effect of the latter on the flowability and crushability of the granules being investigated. The flowability and fill density of the granules increased with an increase in the solid content of the starting suspension, whilst the crushability decreased. The powder flowability, measured using a Hall flowmeter and model shoe-die filling tests, showed that the flowability of otherwise poorly flowable nanopowders can be improved to match that of the commercial spray dried submicron powder. The 5.5 vol.% solid content based suspension yielded soft agglomerates whilst a 28 vol.% solid content suspension formed hard agglomerates on spray freeze drying; the granule relics were visible in the fracture surface of the die pressed green compact in the latter case. The increase in granule strength is explained by the reduction in inter-particle distance based on the theories developed by Rumpf and Kendall. The flaw sizes computed using the Kendall model are comparable with those seen in the micrographs of the granule. With an optimum solid content, it is possible to have a granulated nanopowder with reasonable flowability and compactability resulting in homogeneous green bodies with ~54 % of theoretical density.
KeywordsYSZ Spray freeze drying Agglomerate strength Powder flowability Die pressing Nitrogen adsorption isotherms Nanostructured ceramics
The authors would like to thank EPSRC/PowdermatriX for financial support, Prof. Alan Cocks, Oxford University, UK, and Dr. Farhad Motazedian, Leicester University, UK, for the model shoe-die filing experiments and Mr. Nikolaos Vlachos and Prof. Issac Chang, Birmingham University, UK, for use of their Hall flowmeter. Single granule strength tests were done by Ms. Susannah Eckhard and Ms. Jing (Sherry) Liu at the Fraunhofer Institut Keramische Technologien und Systeme in Dresden, Germany, and their time and access to the equipment is gratefully acknowledged.
Supplementary material 1 (WMV 3951 kb)
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