In the thesis "Modeling of Particle Contacts in Aggregated Nanoparticles" mechanical interactions between aggregated nanoparticles from the gas phase as well as their effects on processing and their characterization are shown. Various methods are developed or adapted to describe structural properties of the particles and their films. With the help of these models, predictions about film properties such as porosity, pore size distribution or electrical resistance can be made and more precise statements about the particle-particle contacts in heterogeneous particles (consisting of several components) can be made on the basis of image analysis.

Aggregated nanoparticles and their films consist of seemingly randomly arranged primary particles with diameters of a few nanometers. This special arrangement results in a structure that is characterized by very high porosity and continuous pores and thus enables fluid molecules, for example, to reach the entire surface of the particles. This makes these particle structures particularly attractive for catalysis. Particles structured in films are also characterized by a high number of percolation paths (continuous chain of connected primary particles) along which, for example, charges such as electrons can be transported. This structure is essentially determined by two types of contact between primary particles: (1) non-bonded particle contacts by weak physical forces (e.g. capillary forces or van der Waals forces) and (2) sinter bridges consisting of strong covalent bonds. In heterogeneous particles (consisting of at least two different components), both types of contact can form both within the same component and between different components.

In this thesis the Discrete Element Method (DEM) is used to represent the interactions based on contacts in aggregated nanoparticles and to calculate the compaction of films of such particles in simulations. The resulting film models correspond to experimentally obtained films with very high accuracy in terms of porosity and pore structure at a given compaction pressure. In addition, these film models allow the analysis of the exact percolation paths. Thus, the electrical resistance of nanoparticle films can be calculated and compared with experimental values. The electrical resistance as a function of the compaction pressure is very similar to the experiments.

The dissertation also shows how the contacts can be described in heterogeneous aggregates. The number of heterogeneous contacts between the different components can be adjusted by the number of primary particles per cluster of a component during aggregate generation. The relationship between heterogeneous coordination number (heterogeneous contacts per primary particle) and primary particles per cluster exactly follows the analytical prediction. Based on this model and the resulting findings, 2D projections of heterogeneous aggregates are created. The direct correlation of the 2D parameters and the exact 3D parameters (coordination number, primary particles per cluster) enables the determination of the 3D heterogeneous coordination number on the basis of experimental 2D image analyses. Thus, the analytical methods for the characterization of e.g. transport processes (mass, charge) are extended by a very detailed possibility to resolve the exact contacts.

The shown models and methods give manufacturers of battery electrodes, gas sensors or catalysts access to a new possibility to investigate the contacts between particles, the porous structure and the percolation paths in aggregated nanoparticles and thus make it easier to optimize these particle structures for their application.

Schlagwörter: Nanoparticles; Diskrete Element Method; Modellierung; Verfahrenstechnik

Forschungsberichte aus dem Leibniz-Institut für Werkstofforientierte Technologien