14 Externe wissenschaftliche Einrichtungen
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/15
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Item Open Access Fatigue of Al thin films at ultra high frequencies(2005) Eberl, Christoph; Arzt, Eduard (Prof. Dr. phil.)Ultra high-cycle fatigue at frequencies in the GHz regime leads to a characteristic void and extrusion formation in patterned metal thin films. Resulting from the microstructural damage formation a significant degradation in form of a shift of the resonance frequency and failures by short circuits in Surface Acoustic Wave (SAW) test devices take place. To study fatigue at ultra high cycles, SAW test devices were used to test continuous and patterned Al thin films at ultra high frequencies. For stress amplitudes as low as 14 MPa lifetime measurements showed no fatigue limit for 400 nm Al thin films. The resulting damage sites appeared in regions of cyclic stress concentration as identified by Finite Element Analysis. In situ measurements revealed that the characteristic extrusion/void formation mechanism operates on a short time scale. The post-test analysis of microstructural changes reveals extrusion and void formation concentrated at grain boundaries. This finding and the observed grain growth indicates a high material flux at the grain boundaries induced by the cyclic load. Quantitative analysis also shows a correlation between extrusion density and electrical devices performance. This direct correlation shows a functional agreement with a common theory on the influence of crack density on intrinsic stresses in thin metal films. Advanced Finite Element (FEM) calculations simulate very well the sensitivity of the resonance frequency to damage structure in interconnects such as cracks, voids and extrusions. The experimentally observed linear correlation between damage density and frequency shift is reproduced by the FEM model. The estimation of the short circuit probability from the extrusion length distribution revealed an exponential dependency on the electrode distance. The observed damage formation is explained by the combined action of dislocation motion and stress-induced diffusion processes.Item Open Access Modelling of crystal plasticity effects in the fracture of a metal/ceramic interface - bridging the length scales(2006) Siddiq, Muhammad Amir; Schmauder, Siegfried (Prof. Dr. rer. nat.)Abstract: Metal/ceramic interfaces play a vital role in modern materials technology, as evident by their use in a variety of applications. High-strength materials, such as metal-matrix composites consist of internal interfaces between ceramic (e.g. SiC or Al2O3) particles or filaments within a metallic host. In microelectronics packaging, interfaces between metallic (Cu and/or Al) interconnects and SiO2, carbide/nitride (TiCN) or oxide (Al2O3) ceramics are commonplace, and impact the performance and longevity of solid state devices. Despite their widespread use, a basic understanding of these interfaces has been elusive. For example, given a particular metal/ceramic interface, it is not yet possible to accurately predict such fundamental properties as its fracture energy. In most of the cases, improvements in interface properties proceed via a costly and time consuming trial-and-error process in which numerous materials are evaluated until suitable performance is obtained. Computational methods provide a wide range of possibilities to study the fracture behaviour of such metal/ceramic interfaces. In the first part of the presented work, the deformation behaviour of niobium single crystals has been simulated using crystal plasticity theory. An automatic identification procedure has been proposed to identify the crystal plasticity parameters for each family of slip systems and simulation results of the mechanical behaviour of single crystal niobium are compared with the experiment. Good agreement between the experimental and simulation results was found. The second part presents effects of the different niobium single crystalline material orientations on crack initiation energies of the bicrystal niobium/sapphire four-point-bending-test specimens for a stationary crack tip. The trends of crack initiation energies are found to be similar to those observed during experiments. In the third part, crack propagation analyses of niobium/alumina bicrystal interface fracture have been performed using a cohesive modelling approach for three different orientations of single crystalline niobium. Parametric studies have been performed to study the effect of different cohesive law parameters, such as work of adhesion and cohesive strength, where work of adhesion is the area under the cohesive law curve while cohesive strength is the peak stress value of the cohesive law. The results show that cohesive strength has a stronger effect on the macroscopic fracture energy as compared to work of adhesion. Cohesive model parameters are identified for different combinations of cohesive strength and work of adhesion by applying a scale bridging procedure. In the last part, a correlation among the macroscopic fracture energy, cohesive strength, work of adhesion and yield stress of niobium single crystalline material will be derived.Item Open Access 3D-printed stimuli-responsive soft microrobots(2023) Lee, Yunwoo; Sitti, Metin (Prof. Dr.)Untethered microrobots, i.e., mobile microrobots, with overall sizes less than 1 mm are receiving significant attention due to their great potential to conduct targeted and minimally invasive therapeutic delivery and medical treatment of diseases in the local region of the biological environment. However, there are many technical barriers in the integration of conventional on-board sensors, actuators, and batteries into micro-scale systems. To overcome these limitations, stimuli-responsive active materials with both sensing and actuation properties are integrated to mobile microrobots. Stimuli-responsive materials can morph their shape and size via swelling and deswelling mechanisms in response to external chemical, physical and other stimuli, such as heat, pH, light, magnetic field, and acoustic field, with no aid from complex wires, sensors, or batteries. While conventional fabrication methods with passive materials have led to the production of static structures, 3D printing with stimuli-responsive materials opens a new direction for 3D objects possessing volumetric transformation behavior, material property change, and shape morphing ability. In this dissertation, I integrate different stimuli-responsive materials and 3D microprinting to provide more multifunctional, versatile and complex microrobot designs based on bioinspired design principles, with a wide range of stimuli-responsive sensing and actuation properties to use them in potential biomedical applications inside the human body. In the first part, I introduce magnetically steerable 3D-printed microroller and microscrew robot designs with stimuli-responsive materials to develop volume-controllable microrobots for spatial adaptation. These wirelessly controlled microrobots possess the ability to function in response to multiple stimuli, including magnetic fields, temperature, pH, and cations, which can enhance the adaptability of the microbots to various unstructured environments. Second, octopus-inspired architectures with temperature-responsive materials to control the adhesion properties for medical purposes is proposed. Introduction of pNIPAM material leads temperature-responsive volume morphing behavior enabled controllable tissue adhesion by using externally applied magnetic fields. Furthermore, I demonstrate the capability of implementing a wide range of medical tasks repeatedly via showing the repetition of attachment and detachment processes using an external magnetic field. Finally, I present a multifunctional pollen-grain-inspired hydrogel robot by 3D direct laser printing in order to enhance the functional diversity of microrobots in biological environments. I also demonstrated multi-responsive hydrogel structures to decouple the stimulus inputs of magnetically actuated locomotion, temperature-responsive controllable attachment, and pH-responsive on-demand cargo release, respectively. The temperature-responsive outer crust shells made of pNIPAM enabled controlling the attachment of the microrobot by shrinking up to 49%, revealing the robot’s spikes. In addition, the inner pollen-grain-inspired structure with spikes made of PETA with FePt nanoparticles demonstrated an improved attachment performance and magnetically guided locomotion along biological surfaces. The inner sphere made of pNIPAM-AAc successfully released drugs by pH-induced swelling.