Conditioning and Transformation of Aerosol Particles

Conditioning of the aerosol produced by a particular generator is frequently required for a specific application to prevent coagulation or loss of the particles. Particle concentration and size distribution are commonly modified. Aerosol particles generated are frequently charged and are usually neutralized with a bipolar ion source. Aerosol particle concentrations can easily be diluted after generation with the addition of clean dilution gas. The size distribution of the original aerosol can be modified with the use of cyclones, impactors, or electrical mobility classifiers. The intent is the elimination of undesired agglomerates or large particles. The morphology of particles generated from the evaporation of solution droplets can be controlled, as described previously, by controlling the conditions of evaporation.

An aerosol may be conditioned by heating to modify the physical, chemical, and morphological properties of the particles. Such a technique is often used to produce insoluble particles from generation techniques that require solutions or liquids. Kanapilly et al. [73] generated metal oxide particles by nebulizing solutions of metal chelates and thermally decomposing the particles to solid oxide particles. Hollow particles and shells were obtained under some conditions of drying of the solution droplets and heating of the particles. Jenkins et al. [74] generated monodisperse cerium oxide particles from colloidal suspensions using a spinning-top generator.

Ramamurthi and Leong [75] applied a similar technique to generate monodisperse aerosols of metallic, metal oxide, and carbon particles. The primary particles were generated using the vibrating-orifice aerosol generator. Metal oxide particles were obtained by thermal decomposition of the primary particles. The conversion of the metal oxide to metal was achieved by the use of a reducing gas (hydrogen). Particles obtained were generally spheroidal, with wrinkled or dimpled surfaces. Hollow copper oxide particles were produced when copper sulfate particles were thermally decomposed. The monodisperse hollow spheres fragmented to polydisperse copper particles when reduced, indicating the need for relatively solid particles for monodispersity to be maintained. Monodisperse carbon particles were generated by pyrolysis of polystyrene spheres.

The technique of using a gaseous reactant to modify an aerosol was studied by Robbins and Cadle [76] for a sulfuric acid aerosol and ammonia gas system. A sulfuric acid aerosol was produced by an evaporation-condensation technique and was reacted with ammonia gas. The uptake of ammonia by the acid particles was examined. Movilliat [77] generated silica aerosols by reacting a water aerosol with silicon chloride gas. Further treatment of the aerosol with heat yielded silica gel particles. At higher temperatures, silica particles were obtained. This technique of modifying particle properties was used by Durand-Keklikian and Parteli [16] to generate oil droplets coated with a ceramic. Water vapor reacted with the alkoxides (titanium or aluminum) in solution to form a coating of oxide. Hence, the use of a vapor-phase reactant can be exploited for the generation and transformation of the physical and chemical properties of an aerosol.

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