Particle synthesis in jet flames generating multi-component aerosols

摘要

Of the many different methods used to produce nanophase powders, few are able to achieve high production rates while minimizing the agglomeration of particles. Furthermore, the powders produced, if non-oxide, must be handled using costly techniques due to their highly reactive nature. Recently, a flame synthesis process conceptually similar to commercial flame processes used to produce millions of tons of ceramic and carbon black powder annually, was extended to allow the production of unagglomerated, non-oxide nanophase powders. This technique employs an encapsulation step during the aerosol growth stage to control particle size, prevent agglomeration and protect the powders from oxidation/hydrolysis during post-synthesis handling. The encapsulation material (typically a salt) can be removed during post-flame processing. A Monte Carlo numerical simulation suggests that the encapsulation process occurs in two steps. Initially the flame-synthesized particles grow normally, but upon condensation of the encapsulate material (salt) the size distribution transitions into a bimodal size distribution, with the coarse mode consisting primarily of salt. The coarse mode then acts to scavenge the fine mode. To fully exploit the benefits of encapsulating the nanoparticles high encapsulation efficiencies must be attained. Experimental results will be presented in which the encapsulation efficiency is analyzed and found to asymptotically approach 100%. The final morphology of the core particles will depend on the scavenging process and the behavior of the core particles within the scavenging particles. Experimental results will be presented for three materials, each with different physical properties. Differences in morphology can be explained by considering differences in the coalescence mechanisms and solid-state diffusion coefficients. To demonstrate the scalability of the process the existing laminar jet flame facility has been converted to a turbulent jet facility. Experimental results will be presented for the synthesis of metal and ceramic powders in the turbulent jet facility showing the influence of various process parameters on encapsulation efficiency as well as particle morphology.