Influences of defects and defect formation on the wet grinding limit

在搅拌介质粉碎机中湿式粉碎自上而下的处理是一种相当简单，易于扩展且通用的大规模生产纳米颗粒的过程。导致颗粒断裂的基本过程，一方面是在纳米尺寸范围内，另一方面是一个有限的颗粒尺寸，目前还不清楚。在该项目的第一个资助期内，在理解亚微米球形模型颗粒的变形和断裂以及磨削极限的微观结构解释方面取得了重大进展：显示了底层二氧化硅结构交联与机械性能之间的强结构-性能关系。该材料体系在粒径略低于1µm时发生脆性向延性转变；然而，仍然观察到裂纹的形成。对于CaF2， SnO2， ZrO2和Al2O3，观察到的磨削极限与颗粒尺寸相关的能力之间的联系可以显示在颗粒体积内产生和存储缺陷。为此，使用了x射线衍射、电子显微镜、气体吸附和分子动力学计算。此外，金属纳米颗粒的分子动力学计算显示了尺寸效应：在极限粒径30 nm以下，观察到弹性性能下降。在给定的粒子尺寸下，标度与原子总数成线性关系。其根本原因在于原子附近表面的配位数显著降低，并且随着粒径的减小，配位数的影响越来越大。对于TiO2、SiO2和一种复杂的玻璃成分（45S55 Bioglass®），发现了通常被低估的新破裂表面及其与周围溶剂相互作用的进一步线索：根据所使用的溶剂对单个颗粒的融合，发现了化学成分和整体结构的变化，以及羟基化作用的增加。在粉碎过程中，除了破碎和结构变化之外，固体与周围溶剂的相互作用应作为加工过程中的一个附加方面进行调查。通过原位电子显微镜技术观察到的真正破损情况，应与主要受溶剂影响的破损情况分开。通过对固相和周围液体中溶解物质的深入分析，可以从力学角度理解机械应力下的固体和周围液体之间的表面相互作用。此外，亚微米颗粒的真实破裂与尺寸有关，在脆性-延性转变之下，应在SEM和TEM中通过原位（多重）应力来表征。为了接近各向异性断裂碎片，在湿磨削过程中，将进行颗粒微观结构和颗粒形态的逐步变化。

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.