A model flow reactor design for the study of nanoparticle structure formation under well-defined conditions

Structure formation models describe the change of the particle structure, e.g., by sintering or coating, as a function of the residence time and temperature. For the validation of these models, precise experimental data are required. The precise determination of the required data is difficult due to simultaneously acting mechanisms leading to particle structure formation as well as their dependency on various particle properties and process conditions in the reactor. In this work, a model flow reactor (MFR) is designed and optimized, supported by a validated computational fluid dynamic simulation, to determine the structure formation of nanoparticles under well-defined conditions. Online instrumentation is used to measure the particle mass and different equivalent diameter to detect changes of the particle shape and to calculate the particle structure, defined by the primary particle size, the number of primary particles per agglomerate, coating thickness, effective density, and fractal dimension, by means of structural models. High precision is achieved by examining size-selected particles in a low number concentration and a laminar flow field. Coagulation can be neglected due to the low particle number concentration. Structure formation is restricted to a defined region by direct particle trajectories from the water-cooled aerosol inlet to the water-cooled outlet. A preheated sheath gas is used to concentrate the aerosol on the centerline. The simulated particle trajectories exhibit a well-defined and narrow temperature residence time distribution. Residence times of at least 1 s in the temperature range from 500 K to 1400 K are achieved. The operation of the MFR is demonstrated by the sintering of size-selected Fe_xO_y agglomerates with measurements of the particle size and mass distribution as a function of the temperature. An increase of the effective density, resulting from the decreasing particle size at constant particle mass, is observed.