Influences of defects and defect formation on the wet grinding limit

在搅拌介质磨机中采用自顶向下的湿法粉碎是一种相当简单、易于扩展且通用性强的纳米颗粒大规模生产工艺。在这个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对研磨极限的微观结构解释方面取得了重大进展：表明了潜在二氧化硅结构的交联性和机械性能之间的强烈结构-性能关系。在颗粒尺寸略小于1微米的情况下，该材料体系发生了脆-韧转变，但仍观察到裂纹的形成。对于CaF2、SnO2、ZrO2和Al_2O_3，可以显示观察到的研磨极限和颗粒体积内产生和存储缺陷的颗粒尺寸相关能力之间的联系。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，分子动力学计算还发现了金属纳米颗粒的尺寸效应：当颗粒尺寸小于30 nm时，弹性性能下降。比例以线性方式发生，对于给定的颗粒大小，原子总数为。其根本原因在于表面原子附近的配位数显著降低，并且随着颗粒尺寸的减小，配位数的影响越来越大。对于二氧化钛、二氧化硅和一种复杂的玻璃成分(45S55 Bioglass®)，进一步的迹象表明，新破碎的表面及其与周围溶剂的相互作用通常被低估了：根据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，以及越来越多的羟化作用。除了粉碎过程中的破碎和结构变化外，应将固体与周围溶剂的相互作用作为加工过程中的一个额外方面进行研究。通过原位电子显微镜技术观察到的真实破损条件应与主要受溶剂影响的破损条件分开。通过对固相和周围液体中溶解物质的透彻分析，可以从机理上理解受机械应力作用的固体和周围液体之间的表面相互作用。此外，亚微米颗粒的大小相关的真实断裂应通过在扫描电子显微镜和透射电子显微镜中原位施加(多重)应力来表征脆性-延性转变下方的尺寸。为了接近各向异性的断裂碎片，正如在湿法磨削过程中所预期的那样，将执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.