Additionally, an important advantage of the USP method can be found in the recycling by synthesis of nanoparticles from the initial scrap.

The influence of different USP parameters, such as solution concentrations, ultrasonic frequencies, temperature, as well as the synthesis mechanisms of metal, oxide, composite nano-quantities, and submicron particles are mainly described in the studies of Stopić et al. In the next stage, dry droplets enter into the reaction zone, where chemical decomposition takes place at high temperatures in the presence of reaction gas. The choice of the appropriate precursor as well as the solute concentration are very important factors because the chemical composition of the desired nanoparticles depends on them, as well as on the technological transport by carrier gas into the evaporation zone, where water evaporation takes place. The first step in USP synthesis is the formation of an aerosol generated by ultrasound from a precursor that contains metal ions in drops of solute (usually water). One of the potential methods is the production of nanoparticles by ultrasonic spray pyrolysis (USP), which is a very fast, continuous, and relatively flexible technique. A mechanism for the formation of CCNPs is proposed, based on the thermochemical analysis of element reduction with the help of H 2 and based on the mixing enthalpy of binary systems. Research has shown that CCNPs belong to the group with medium entropy characteristics. A microstructural examination of the synthesised CCNPs showed that round and mostly spherical (not 100%) nanoparticles were formed, with an average diameter of 200 nm. As a consequence of the incomplete dissolution of the cast alloy and the formation of Pt and Ni complexes, it was found that the complete recycling of the alloy is not possible. Hydrometallurgical treatment was used to dissolve the cast alloy, from which the USP precursor was prepared. The development of new multicomponent nanoparticles is gaining increasing importance due to their specific functional properties, i.e., synthesised new complex concentrated nanoparticles (CCNPs) in the form of powder using ultrasonic spray pyrolysis (USP) and lyophilisation from the initial cast Ag 20Pd 20Pt 20Cu 20Ni 20 alloy, which was in the function of the material after its catalytic abilities had been exhausted.

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