Experimental study of first-stage short-time sintering kinetics of airborne silica nanoparticles

摘要

Aerosol processing is of major interest for generation of nanopowders with well-defined product properties which strongly depend on particle characteristics such as primary particle size,specific surface area and aggregate structure.Industrial production of nanoparticles in the gas phase is conducted in flame,hot wall tubular,laser and plasma reactors.These processes had been established several decades ago,but the fundamental mechanisms are not yet well understood.One of the reasons is that chemical reaction and particle growth are extremely fast and consequently representative sample collection is very difficult (Pratsinis 1998).Two important mechanisms for particle synthesis in the gas phase are coagulation and sintering of clusters originating from either chemical reactions or evaporation.Hence,the sintering process is an essential mechanism influencing size and structure of the created solid particles.Several sintering mechanisms,e.g.viscous flow,evaporation and recondensation,surface and solid state diffusion are known.Model constants like characteristic sintering times differ in literature over more than an order of magnitude.The reason is that the process parameters were not varied completely separated from each other and also not all influencing parameters of the sintering process have been considered to former measurements.In addition,common sintering models are not valid for the overall process.Therefore a short-time high-temperature flow reactor was realized that enables sintering of nanoparticles by separate variation of the sintering parameters,i.e.temperature,residence time,particle size and material,and gas composition.