Nanoparticles can have different form but predominantly are obtained as solid grains. The properties of many materials based on these nanopowders are relatively well known. Using ultrasonic spray pyrolysis (2,6 MHz) nanopowders composed of hollow ball-shaped grains were obtained. The diameter of grains vary in the range 2 - 400 nm. The obtained nanopowder materials were metallic (Ru, Ag, Pd, Ag-Pd, Ag-Rh), ceramic (Al₂O₃, Al₂O₃-Cr₂O₃, CeO₂, CeO₂-ZrO₂) as well as composite (cermetals: NiO-Au, CeO₂-ZrO₂-Pt and CeO₂-ZrO₂-Au).
As a precursors aqueous solutions of adequate nitrates(V), chlorides, acetates or alkoxides of appropriate metal were used. The total concentration of initial salts solution was in the range 0.1-0.03 mol.dm^-3. The concentration of particular solution corresponded to the assumed nanopowder product composition. Aerosols based on such a solution were obtained by ultrasonic nebulization in the stream of adequate carrier gas (i.e. air, Ar+H₂) and introduced into horizontal reactor heated to 500-800^oC, depending of the type of solution, carrier gas and its flow rate, were the decomposition of salts occurred. Products of decomposition were caught in electrofilter.
Microstructure of the powders obtained was examined using different methods like TEM, SEM and EDAX.
The hollow nanoparticles enable formation of porous ceramic composite membranes of higher permeability than those made of solid particles, which is important for catalytic applications. On the other hand, the dense ceramic membranes can be applied as cathodes in ion transport. The basic advantage of hollow nanoparticles is their low specific weight and presence of metal admixture atoms (Ag, Au, Pt, Ru, Rh) on the surface of nanoparticles. This can give the significant economic effect for many applications e.g. in electronic engineering for conductive path fabrication by screen- or ink-printing techniques.

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