Spray-drying is a process by which a ¯uid feed material is transformed into a dry powder by spraying the feed into a hot medium. The feed materials are either water-based suspensions with air as drying gas or organic solvent-based suspensions (usually ethanol) with nitrogen as drying gas. In general, there are ®ve factors to in¯uence the characteristics of spray-dried powders: type of atomiser, suspension composition, feeding rate, drying temperature and atomising pressure. The spray-drying theory for ceramics has been discussed by Lukasiewicz and Masters. The spray process is a result of many interwoven complex mechanisms and no comprehensive mathematical treatment of atomisation has been developed. For external-mixing nozzles (as the type used in this work).
The compaction of spray-dried powders has received considerable attention in the literature. Due to its high ¯owability and hence the achieved high green density, the spray-dried powder can provide sintered bodies with high densities and highly precise dimensions. The particle size, the size distribution and the particle strength, which determines the behaviour during cold compaction, are the most important parameters for a spray-dried powder with high sinterability. A new application of spray-dried powders in TBCs has been developed. Spray-dried powders oer an advantage in the application of TBCs where a high porosity (10±20%) is necessary. On the other hand, particles of the spray-dried powders are usually spherical and free ¯owing, the powder can be easily fed by the plasma-spraying gun. As reported by Wigren,14 the coating whose starting powders were prepared by spraydrying/sintering had longer thermal shock lives than those coatings whose powders were prepared by precipitation, sintering/crushing and hollow-sphericalpowder (HOSP).
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