Mechanical behavior of bulk supercrystalline ceramic-organic nanocomposites

这项工作旨在阐明块体超晶陶瓷-有机纳米复合材料的力学行为。这些材料由无机的，通常是结晶的纳米颗粒组成，用有机配体表面功能化，并组装成周期性结构，类似于原子晶格。它们代表了材料科学和纳米工程的一个新兴领域，在光电子学、等离子体、磁性材料、电池电极、催化剂和传感器中得到了应用。然而，它们在结构上的应用在很大程度上还没有被探索，即使在块状陶瓷基超晶材料中已经开始出现良好的力学性能。通过在固定在陶瓷纳米颗粒表面的有机链之间诱导交联来提高其力学性能的有效方法已经被发现。这限制了无机构建块彼此之间的移动性，不仅导致强度、硬度和刚性的高值，而且还导致材料在载荷下的行为发生显著变化。初步工作指出材料压实、类塑性变形和外在增韧机制的共同作用。还发现了让人想起晶体材料中常见缺陷的缺陷，如间隙、堆叠断层、位错和剪切带。力学行为和局部纳米结构变形之间的内在联系尚不清楚，这也是本提案旨在解决的问题。对于有机配体具有不同程度交联度的材料，需要推导出应力-应变关系、蠕变和压缩行为。同时，还将描述由材料加工和机械加载引起的缺陷和变形。选择的方法包括带有不同尖端的纳米压痕，用于推导材料的本构响应，以及原子力和电子显微镜，用于可视化缺陷，从点缺陷到位错和超晶界。结合它们的经验评估，将通过应用和采用文献中关于滑移系统激活、线缺陷周围的应变场和应力场以及位错滑移的经典理论来模拟超晶变形和整体力学行为之间的关系。然后将推断材料加工、超晶结构和机械性能之间的关系。

The proposed work aims at elucidating the mechanical behavior of bulk supercrystalline ceramic-organic nanocomposites. These are materials that consist of inorganic, usually crystalline, nanoparticles, surface-functionalized with organic ligands, and assembled into periodic structures, analogous to atomic lattices. They represent a rising field of materials science and nanoengineering, which has been finding applications in optoelectronics, plasmonics, magnetic materials, battery electrodes, catalysts and sensors. Their use for structural purposes is however largely unexplored, even if promising mechanical properties have started to emerge in bulk ceramics-based supercrystalline materials.An effective way to enhance their mechanical properties has been found in inducing crosslinking among the organic chains that are anchored to the ceramic nanoparticles’ surfaces. This limits the mobility of the inorganic building blocks with respect to each other, leading not only to high values of strength, hardness and stiffness, but also to significant changes in the materials’ behavior under loading. Preliminary work points towards the concurrent effects of material compaction, plastic-like deformation and extrinsic toughening mechanisms. Defects remindful of the ones typically observed in crystalline materials – such as interstitials, stacking faults, dislocations and shear bands – have also been detected.The interconnections among mechanical behavior and the local nanostructure deformations are yet to be clarified, and this is what the present proposal aims at tackling. For materials with various degrees of crosslinking of the organic ligands, the stress-strain relationships, creep and compressibility behavior are to be derived. In parallel, the defects and deformations induced by both material processing and mechanical loading will be characterized. The methods of choice involve nanoindentation with various tips, for the derivation of the materials’ constitutive response, and atomic force and electron microscopy for the visualization of imperfections, from point-defects to dislocations and inter-supercrystalline boundaries. Together with their empirical evaluation, the connections between supercrystalline deformations and overall mechanical behavior will be modeled by applying and adapting the classic theories from the literature on slip systems activation, strain and stress fields around line defects, and dislocation glide. The correlations between material processing, supercrystalline structure and mechanical properties will then be inferred.