Generation of ultrafine particles by spark discharging

Ultrafine carbon, metal, and metal oxide particles were generated with a commercially available spark generator designed for the production of carbon particles. Aerosols with number concentrations up to 10(7) cm(-3) were produced at flow rates up to 150 lpm. Lognormal size distributions with modal diameters in the range of 18-150 nm and geometric standard deviations of about 1.5 were obtained. The chemical composition, size, number concentration, morphology, and surface area of the particles were varied, and the generation of particles with fixed characteristics could be maintained over many hours. The particle characteristics, however, could not be varied independently. For a certain chemical composition only size and number concentration were variable; morphology and surface area were fixed regardless of particle size. The particles grow by coagulation of primary particles formed by nucleation. The coagulated particles can either stick together and maintain their identity or fuse together and lose their identity. Each material used for the generation of ultrafine particles is thus associated with a certain morphology and surface area: silver with a low mass-related BET surface area (20 m(2) g(-1)), metal oxides and iridium with a low-to-intermediate BET surface area (50 m(2) g(-1) for cadmium oxide, 120 m(2) g(-1) for iridium, and 300 m(2) g(-1) for ferric oxide), and carbon with a large BET surface area (750 m(2) g(-1)). Iridium, on the other hand, has a huge volume-related BET surface area (2800 m(2) cm(-3)). It was not possible to generate ultrafine carbon particles without contaminations with the generator. However, these contaminations could be decreased in this study from 25% to 6% by replacing organic components of the generator by pure inorganic components.