(202a) Theoretical and Experimental Investigations on Sintering Kinetics of Silica Nanoparticles

High temperature aerosol processing is a major route for the generation of nanopowders. In general the size and structure of these nanoparticles ? and consequently the product properties and product value ? are determined by extremely fast fundamental processes acting simultaneously, i.e. chemical reaction, nucleation, growth, coagulation and sintering. In order to understand product formation the kinetics, i.e. functional dependencies including the respective kinetic parameters, of all these fundamental processes have to be known precisely. This presentation will show detailed experimental as well as theoretical investigations on sintering kinetics as one important fundamental mechanism. As exemplary material so far silica has been chosen. The experimental investigations aim at studying the sintering kinetics of airborne nanoparticles from neck formation to complete coalescence, separated from all other fundamental processes. For this purpose, nanoparticle doublets consisting of unsintered, spherical primary particles are produced and directly fed into a high-temperature short-time sintering reactor allowing for precise control of all relevant parameters, i.e. temperature, residence time and gas composition (Kirchhof et al. 2004). The reactor can be operated at temperatures up to 1600 °C and sintering residence times in the range 8-1000 ms. This parameter range well reflects the operation conditions in industrial production facilities. In order to get a well-defined temperature and residence time history the nanoparticles - in this study silica - are rapidly heated and cooled by water-cooled inlet and outlet probes, respectively. During passage of the sintering reactor an extremely flat temperature profile has been achieved. Changes of particle contacts are recorded by transmission electron microscopy (TEM) analysis after passage through the reactor and related to the respective sintering temperature and sintering time. This way for the first time the growth kinetics of the sintering necks as depending on temperature and primary particle size is determined. It will be demonstrated that experimentally observed sintering kinetics is severely faster than predicted by macroscopic models, e. g. described by Pokluda et al. 1997, and Koch & Friedlander 1990. Furthermore, activation energy of sintering by Arrhenius-plots is investigated and its dependency on primary particle size and temperature is shown. For glasseous materials such as silica the predominant sintering mechanism is supposed to be viscous flow. Therefore, the sintering process of silica nanoparticles is investigated theoretically by simulations using a volume of fluid method implemented in the commercial computational fluid dynamics code CFX. Additionally, van der Waals interactions are considered determining an additional momentum source during the viscous flow process. It will be shown that van der Waals interactions accelerate sintering kinetics considerably and that a much better agreement with experimental observations can be achieved. Finally, using the volume of fluid simulations the influence of aggregate structure is investigated by simulation of the sintering process of different aggregate structures consisting of up to seven primary particles. It is shown that there is a significant influence of the aggregate structure on sintering kinetics.