Influences of defects and defect formation on the wet grinding limit

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Top-down processing by wet comminution in stirred media mills is a rather simple, easily scalable yet versatile process towards large scale production of nanoparticles. Fundamental processes leading on the one hand to fracture of particles even in the nanometer size range and on the other hand to a limiting particle size are not yet understood.Within the first funding period of this project significant progress in the understanding of deformation and fracture of submicron spherical model particles as well as a microstructural interpretation of the grinding limit has been achieved: a strong structure-property relation between crosslinking of the underlying silica structure and the mechanical properties has been shown. The brittle-to-ductile transition for this material system was found at a particle size slightly below 1 µm; however, crack formation was still observed. For CaF2, SnO2, ZrO2, and Al2O3 a connection between the observed grinding limit and the particle size dependent capabilities to create and store defects within the particle volume could be shown. To this end, X-ray diffraction, electron microscopy, gas adsorption and molecular dynamic calculations were used. Additionally, a size effect was shown by molecular dynamic calculations for metal nanoparticles: below a limiting particle size of 30 nm a decrease of the elastic properties is observed. Scaling occurs in a linear fashion with the total number of atoms for a given particle size. The underlying reason is seen in the significantly lowered coordination number of the surface near atoms and their increasing influence with decreasing particle size. Further hints pointing towards the commonly underestimated role of the freshly broken surface and its interaction with the surrounding solvent was found for TiO2, SiO2 and a complex glass composition (45S55 Bioglass®): dependent on the used solvent fusion of individual particles, changes in the chemical composition and overall structure, as well as an increasing hydroxylation was found.In addition to breakage and structural changes during comminution, interactions of the solid with the surrounding solvent shall be investigated as an additional aspect during processing. Conditions of true breakage, as observable by in-situ electron microscopy techniques, shall be separated from mainly solvent influenced breakage conditions. By thorough analysis of the solid phase and the surrounding liquid with respect to dissolved species, a mechanistic understanding of the surface mediated interaction between the mechanically stressed solid and the surrounding liquid phase shall be derived. Furthermore, the size dependent true breakage of submicron particles shall be characterized, below the brittle-ductile transition by (multiple) stressing in-situ in a SEM and a TEM. To approach anisotropic fracture fragments, as expected during wet grinding, a stepwise variation of particle microstructure and particle morphology will be performed.