Compaction-induced restructuring of aggregated nanoparticle films using the discrete element method

Aggregated nanoparticle films find application in many fields such as gas sensing, solar cells and batteries. These films are characterized by size distributions of polydisperse primary particles and sintered particle aggregates that dictate both the response to external load and functional properties such as heat or charge transport. Mechanical compaction of the films strongly affects these properties in a way that can be quantified by the change of porosity and pore size distributions on the applied compacting pressure. The exact restructuring mechanisms of the aggregate architecture, however, remain unknown. Here, we apply Discrete Element Method (DEM) simulations to gain access to such restructuring mechanisms in TiO2 nanoparticle films synthesized by flame-spray pyrolysis. The ability of the sintered TiO2 aggregates to rearrange via mutual detachment, rolling or sliding events dictated by non-covalent, humidity-dependent interactions are known to be crucial to predict the correct response to compaction. In this work, the importance of elastic deformation of aggregates according to a novel sinter bridge model is elucidated. The best match between DEM simulations and experiments is obtained for bridges with a tensile and bending strengths substantially larger than bulk TiO2, consistently with a low probability of critical fracture initiation in nanometer-scale structures.