Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
33 results
Search Results
Item Open Access Synthesis and characterization of carbon nanotube reinforced copper thin films(2006) Otto, Cornelia; Arzt, Eduard (Prof. Dr.)Two model composites of copper and carbon nanotubes were fabricated by very different deposition methods. Copper electrodeposition in a plating bath containing nanotubes created a 3D matrix of randomly oriented CNTs within a thick, 20 micron Cu film. In contrast, sandwiching a layer of well-separated nanotubes between two sub-micron sputtered Cu layers produced a 2D-composite with nanotubes lying parallel to the substrate surface. These composites, which were mechanically tested using various techniques, proved to be well suited to explore the nature of the CNT/Cu matrix interface. Columns approximately 600 nm in diameter and 1.4 microns high were cut from the sputter-deposited composite and microcompression tested in a nanoindenter. No influence of the presence of nanotubes on the stress-strain-curves was observed, which was attributed to the low nanotube content. On the other hand, microscopic analysis showed an influence of the nanotube on the copper immediately surrounding it, resulting in funnel-like depressions on the column surface. In addition to deformation by slip, twinning was observed in some columns, which has never before been reported in the literature for micron-sized columns. Macroscopic tensile tests were performed on the electrodeposited films and the samples with the highest carbon content showed an increase in toughness of over 100% with respect to the CNT-free control samples produced by the same method. Finally, short copper electrodepositions into carbon nanotube carpets revealed large regions with conformally coated nanotubes. Until now, it was assumed that copper would not wet the nanotubes and that the interfacial strength between copper and CNTs would be low, since copper does not form a carbide. However, these experiments all revealed clear evidence of adhesion exceeding the copper shear strength. To our knowledge, this is the first time such strong adhesion was demonstrated between a nanotube and a metal matrix. We attribute this unexpected, but highly desirable adhesion and wetting behaviour to the defect structure in the nanotubes used. Most of the experiments were done with nitrogen-doped carbon nanotubes, which are known to be rich in defects. The nanotube carpets used in the last experiment were not doped but had a high defect density due to the synthesis method used. As a good adhesion between fiber and matrix is a prerequisite for the successful use of carbon nanotubes in metal matrix composites, these results are very encouraging. The composites and methods presented here provide a foundation for further studies needed to understand the nanotube-metal interaction in more detail and thus ultimately for successful metal-carbon nanotube composites.Item Open Access Novel X-ray lenses for direct and coherent imaging(2019) Sanli, Umut Tunca; Schütz, Gisela (Prof. Dr.)Item Open Access Interaction of carbon and nitrogen in iron(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Göhring, Holger; Mittemeijer, Eric Jan (Prof. Dr. Ir.)Item Open Access Grain growth and texture evolution in copper thin films(2010) Sonnweber-Ribic, Petra; Arzt, Eduard (Prof. Dr. phil)An improved basic understanding of mechanisms causing grain growth and texture evolution in Cu thin films contains the potential to improve performance and reliability of components and devices. In this work, the influence of film thickness, strain and temperature on grain growth and texture evolution in Cu thin films was investigated. By varying the parameters, information about the underlying mechanisms were revealed. The 0.5 to 10 micrometer thick Cu films were deposited on 125 micrometer thick polyimide substrates (Kapton®, DuPont) using a UHV magnetron sputtering system. For detailed observation of grain growth and texture evolution an EBSD-based in situ testing appliance was constructed. This system allowed the simultaneous observation of grain growth and texture evolution, giving new insight into growth kinetics and details of grain growth. In a first step, Cu thin films of thicknesses in between 0.5 and 10 micrometer were deposited on polymer substrates and annealed at 330°C for 30 min. Their resulting texture and microstructure were investigated by EBSD. A texture transition from (111) to (100) was observed at film thicknesses between 3 and 5 micrometer. The experimental findings were explained by the texture evolution model of Thompson and Carel. A significant observation which cannot be explained by a purely energetic argument is the broad texture transition. In order to get more information about the critical role of strain energy, uniaxial tensile tests were carried out on 3 micrometer thick films. In contrast to theoretical predictions, various tensile tests revealed no influence of strain on grain growth behaviour. Neither at room temperature nor at elevated temperatures, further (100) grain growth was observed. In a next step, the abnormal growth of individual (100) oriented grains was recorded for more than 24 hours at temperatures between 90 and 118°C. Annealing was carried out inside a Leo 1530-VP SEM equipped with a heating facility. Detailed analysis of grain growth and estimates of the possibly acting driving forces indicated that the reduction of dislocation density played an important role for abnormal grain growth. A further hint for the critical importance of defect density was given by the HWHM of the (100) texture fraction. Nevertheless, it was not clear why this driving force favours the growth of (100) oriented grains. A possible answer could be given by the strain energy release maximization (SERM) model developed by Lee. In addition, when analysing the activation energy for grain growth, they were found to possess a higher grain boundary mobility, supporting the preferred growth of (100) oriented grains. A new texture map, considering dislocation density as driving force, was constructed. Assuming dislocation density to play a significant role for grain growth and texture evolution in Cu thin films, the influence of deposition parameters is pointed out.Item Open Access Analytical and numerical investigations of form-finding methods for tensegrity structures(2007) Gomez Estrada, Giovani; Bungartz, Hans-Joachim (Prof. Dr.)The analysis of statically indeterminate structures requires the calculation of an initial equilibrium geometry. Tensegrity structures are one of such statically indeterminate structures, with the additional constraint of holding their equilibrium configuration with the action of internal forces and without any anchorage point or external forces. The only source of balance is the state of self-stress held among tensile and compression forces. Tensegrity structures are thus statically indeterminate structures in a stable state of self-stressed self-equilibrium. The basic problem with the modelling of statically indeterminate structures is that there is no unique solution for the forces or geometry that equilibrate a structure. This is where form-finding comes into play. The process of determining their three-dimensional equilibrium shape is commonly called form-finding. This dissertation presents two investigations, one analytical and one numerical on the form-finding of tensegrity structures. Both are in fact complementary. The main results from these investigations appear in [77, 78, 79, 80]. The analytical form-finding for a class of highly symmetric structures with cylindrical shape is first presented, while the numerical procedure for general structures is given in the second part. A thorough analysis of tensegrity cylinders, e.g., the triplex and the quadruplex, is presented in analytical form. Moreover, the numerical procedure here presented is able to reproduce the results obtained with other form-finding methods with great accuracy. The versatility of the novel numerical form-finding procedure is nonetheless demonstrated by solving not only cylindrical and spherical but also new tensegrity structures.Item Open Access Nitriding of iron-based alloys : residual stresses and internal strain fields(2007) Vives Díaz, Nicolás; Mittemeijer, Eric (Prof. Dr. Ir.)Different iron-chromium alloys (4, 8, 13 and 20 wt.% Cr) were nitrided in NH3/H2 gas mixtures at 580 ºC. The nitrided microstructure was investigated by X-ray diffraction, light microscopy, hardness measurements and scanning electron microscopy. Composition depth-profiles of the nitrided zone were determined by electron probe microanalysis. Various microstructures develop, depending on the nitriding conditions and the alloy composition (chromium content). The initial development of coherent, sub-microscopical CrN nitrides leads to a state of hydrostatic stress allowing the uptake of excess nitrogen dissolved in the ferrite matrix. It is shown that the outcome of the subsequent discontinuous coarsening process, which leads to a lamellar microstructure, has a close relation to the nitrogen supersaturation. As a result, the occurrence of a distinct gradient in hardness across the nitrided zone can be understood. Residual stress-depth profiles of the nitrided specimens were measured using the (X-ray) diffraction sin^2 (psi) method in combination with cumulative sublayer removals and correction for corresponding stress relaxations. Unusual, nonmonotonous changes of stress with depth could be related to the microstructure of the nitrided zone. A model description of the evolution of the residual stress as function of depth and nitriding time is given. Specimens of Fe-2.23 at.% V alloy were nitrided in a NH3/H2 gas mixture at 580 ºC. The nitrided microstructure was investigated by X-ray diffraction, and (conventional and high resolution) transmission electron microscopy. For specimens homogeneously nitrided during relatively short times no separate VN reflections developed but instead sidebands associated with ferrite reflections, most pronouncedly for the Fe-200 reflection, appeared. The diffractograms measured for the different specimens were interpreted as the result of coherent diffraction of the nitride platelets with the surrounding ferrite matrix, which is tetragonally distorted: the distorted ferrite matrix and the nitride platelets are represented by a single b.c.t. lattice, whereas the remaining part of the ferrite is described by a b.c.c. lattice. Analysis of the microstructure of the nitrided specimens using high resolution transmission electron microscopy investigations confirmed the existence of very tiny VN platelets, coherent with the surrounding matrix. Annealing at elevated temperatures (up to 750 ºC) after nitriding led to (moderate) coarsening of the nitride precipitates. The coarsening is associated with the occurrence of local disruptions/bending of lattice planes in the VN platelet. This effect causes that the VN platelets appear segmented in the diffraction contrast images. The specific changes in the X-ray diffractograms, as function of the stage of aging, could be consistently described as consequence of the transition from coherent to incoherent diffraction of the nitride platelets with reference to the surrounding ferrite matrix.Item Open Access Structural and mechanical investigations of biological materials using a Focussed Ion Beam microscope(2005) Orso, Steffen; Arzt, Eduard (Prof. Dr. phil.)Biological materials have been evolved over millions of years of evolution to fulfil the requirements posed by the organism and environment. A closer inspection of these materials reveals that they are composites with a highly hierarchical structure. A detailed understanding of the behaviour and function of these materials is possible only if the structure and the mechanical properties down to the smallest level of the hierarchy are known. This requires specimens of very small scale to be analysed. This thesis describes the development and application of a novel technique for the quantitative investigation of both the three-dimensional structure and the mechanical properties of biological materials. This technique allows the micromechanical testing in bending and tension of samples of a few tens of micrometers in length and a few micrometers or less in diameter. It uses a Focussed Ion Beam system (FIB) as an in situ laboratory for structural investigations, sample preparation and sample fixation. Mechanical tests are carried out in situ in a FIB and a scanning electron microscope (SEM). Advantages of this method are that samples from larger objects can be prepared site-specifically using the FIB, and that testing in tension is possible without end effects due to gripping, since the samples are affixed by metal ‘tapes’ deposited using the FIB. Forces are measured with a piezoresistive Atomic Force Microscope (AFM) tip attached to a micromanipulator for high precision positioning. The displacement is determined from micrographs taken during the test. The mechanical properties of three different polymeric and biological materials and structures were measured in bending in situ inside an SEM: polyimide (Kapton®), horse hair (keratin) and spruce wood cell wall material (cellulose-fibre composite). Four different biological materials were tested in tension in situ in a FIB: a single element (seta) of the hairy attachment system of a beetle Gastrophysa viridula, wind-receptor hairs from the filiform sensor of crickets (Acheta domesticus) (both chitin-fibre composites), natural spider silk from the garden cross spider (Araneus diadematus) and artificial spider silk (protein fibres). Some of the biological samples could be tested for the first time using the newly designed testing method. They showed exceptional high mechanical properties when compared to technical materials.Item Open Access Vortex-Kern-Korrelation in gekoppelten Systemen(2014) Jüllig, Patrick; Schütz, Gisela (Prof. Dr.)In der vorliegenden Arbeit wurden strukturierte ferromagnetische Dreischichtsysteme zum einen auf ihre statische in-plane- sowie out-of-plane-Magnetisierungsverteilung als auch auf deren dynamisches Verhalten hin untersucht. Die sowohl quadratischen als auch kreisförmigen Strukturen bestanden aus zwei ferromagnetischen Lagen mit einer Dicke von jeweils 50nm, welche durch eine nicht magnetische Cu-Zwischenschicht getrennt waren. Die Dicke dieser Zwischenschicht variierte schrittweise von t(Cu)=3nm bis 15nm. Als Magnetmaterialien kamen für die untere Schicht Kobalt (Co) und für die obere Schicht das magnetisch isotrope Permalloy (Ni80Fe20) zum Einsatz. Die lateralen Abmessungen sowie das Aspektverhältnis der beiden Einzelschichten wurden so gewählt, dass der Vortexzustand die stabile Domänenkonfiguration ist. Somit resultierten zwei vertikal übereinander angeordnete Vortexkonfigurationen, sodass deren Wechselwirkung sowohl im statischen als auch im dynamischen Fall untersucht werden konnte. Aufgrund der gewählten Cu-Schichtdicke von mindestens 3nm war gewährleistet, dass die Kopplung der in-plane-Schichtmagnetisierung hauptsächlich durch die elektrostatische Streufeldenergie beeinflusst wurde und somit der Beitrag der Oszillatorischen Zwischenschichtaustauschwechselwirkung vernachlässigt werden konnte. Im Falle zweier vertikal übereinander angeordneter Vortexstrukturen kann man bezüglich der Zirkulation C (beschreibt die Orientierung der in-plane-Magnetisierung) und der Polarisation P (beschreibt die Orientierung der out-of-plane-Komponente des Vortexkerns) unter Berücksichtigung der Symmetrie vier verschiedene Konfigurationen voneinander unterscheiden: Die beiden Fälle, bei denen C und P jeweils bzw. orientiert sind, sowie die beiden Fälle, bei denen lediglich C oder P parallel ausgerichtet ist. Der erste Schritt dieser Arbeit bestand in der Probenpräparation. Als Strukturierungsverfahren kamen zum einen das Ionenstrahlätzen und zum anderen die Elektronenstrahllithographie zum Einsatz. Anhand von Röntgenbeugungsexperimenten konnte herausgefunden werden, dass beide Schichtmaterialien, sowohl das Permalloy als auch das Kobalt, eine polykristalline, fasertexturierte Schichtstruktur mit einer fcc-Gitterstruktur aufwiesen. Diese Erkenntnisse waren vor allem für die korrekte Parameterwahl für die nachfolgend durchgeführten mikromagnetischen Simulationen von großer Bedeutung. Messungen der Oberflächenrauigkeiten mittels des AFM ließen darauf schließen, dass neben dem Beitrag der Streufeldenergie ebenso korrelierte bzw. unkorrelierte Zwischenschichtrauigkeiten zur gegenseitigen Ausrichtung der in-plane-Schichtmagnetisierungen beitrugen. Mit Hilfe von SQUID-Messungen bei T=40K an unstrukturierten Co/Cu/Py-Dreischichtsystemen konnte nachgewiesen werden, dass erst für Proben mit Cu-Schichtdicken ab t(Cu)=2,0nm beide ferromagnetische Materialien chemisch voneinander getrennt vorlagen und keine direkte ferromagnetische Kopplung aufgrund von sogenannten Pinholes auftrat. Somit konnte geschlussfolgert werden, dass erst ab einer Dicke von t(Cu) größer gleich 2,0nm eine vollständig geschlossene Cu-Schicht vorlag. Die ersten statischen in-plane-Messungen am STXM zeigten, dass Proben, welche im as-sputtered Zustand eine undefinierte metastabile Mehrdomänenkonfigurationen aufwiesen, durch einen Entmagnetisierungsprozess in den stabilen Vortexzustand überführt werden konnten. Neben antiparallel gekoppelten Systemen bezüglich der Zirkulation C wurden mit einer ähnlich hohen Wahrscheinlichkeit Proben mit einer parallelen Ausrichtung der in-plane-Magnetisierung gefunden. Dies zeigte, dass die Kopplung der Schichtmagnetisierungen nicht allein durch die Streufelder realisiert wurde, sondern ein weiterer Beitrag hinzukam, dessen Ursache mit hoher Wahrscheinlichkeit in den Zwischenschichtrauigkeiten zu finden war. Statische mikromagnetische Simulationen an quadratischen Co/Spalt/Py-Strukturelementen haben gezeigt, dass die in-plane-Magnetisierungsverteilung der Systeme mit C=parallel eine merklich verzerrte Landaustruktur aufwies. Zudem lag bei Konfigurationen mit P=antiparallel ein lateraler Shift bezüglich der Gleichgewichtspositionen der Kerne vor, was aufgrund der Interaktion der out-of-plane-Streufelder zu erwarten war. Dies spiegelte sich auch in der Energiebetrachtung wieder, wobei die beiden Systeme mit der Konfiguration C=parallel deutlich höhere Gesamtenergien aufwiesen als diejenigen mit C=antiparallel. Allgemein lagen im Falle von parallelen Kernpolarisationen die Energiewerte etwas niedriger als bei antiparallel ausgerichteten Kernen. Die dynamische Anregung der ferromagnetischen Schichtsysteme wurde experimentell mittels eines in-plane-Magnetfeldpulses realisiert, welcher durch die lineare Stripline generiert wurde. Die Pulsdauer betrug je nach Element 0,5 bis 1,6ns, und bezüglich der Pulsamplitude mussten Feldstärken von B(Puls)=3,1mT bis zu 6,0mT angelegt werden, um eine Gyrationsbewegung beobachten zu können.Item Open Access Deposition of metal oxide thin films from solutions containing organic additives(2007) Lipowsky, Peter; Aldinger, Fritz (Prof. Dr.)In bio-inspired materials synthesis the principles of biomineralization are employed for the fabrication of materials with favourable functional properties at near-ambient temperature and with little expenditure: Organic templates direct the formation of inorganic matter. In aqueous solution, zinc compounds with manifold morphologies are produced by thermal hydrolysis of zinc nitrate in the presence of biomolecules like amino acids and dipeptides. In methanol, ZnO films are deposited by hydrolysis of zinc acetate in the presence of polymers like polyvinylpyrrolidone (PVP) and polyethylene glycol. With PVP, particularly smooth, uniform and stable films are fabricated. Their thickness is determined by the deposition time and the polymer concentration. Various microscopic and spectroscopic measurements prove that the films consist of textured nanocrystalline zinc oxide. Selected properties of the films, such as their photoluminescence, are investigated. Film deposition is possible on substrates with organic coatings bearing certain functional groups. Patterned films can be deposited after local decomposition of the organic coating by UV light. The mechanism of film formation is treated in detail. Like in biomineralization, an amorphous transient state of matter occurs before crystallization. This state succumbs to ZnO nanocrystals, which either aggregate in solution or adsorb to the substrate. It is demonstrated in what way the additive controls the reaction. Sulfonate-modified polystyrene beads are coated with zinc oxide and used as sacrificial templates for the fabrication of zinc oxide hollow spheres. Laminates of alternating layers of zinc oxide and poly(amino acids) are deposited and exhibit an improved mechanical performance compared to the monolithic zinc oxide.Item Open Access Microstructural changes and intermetallic compound formation in metallic bilayers(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2016) Rossi, Paul J.; Mittemeijer, Eric Jan (Prof. Dr. Ir.)This thesis investigates interdiffusion and intermetallic compound (IMC) formation, as well as their effects on the microstructure, in metallic thin-film bilayers. The investigation focuses on bilayers based on the Ag-Sn and Ag-In binary systems, which are technologically important as basis for lead free solders. Due to the enhanced diffusional mechanisms in these systems, diffusion occurs readily even at room and low temperatures. The proceeding interdiffusion eventually leads to IMC formation in the bilayers, allowing for the investigation of the kinetics of IMC formation and the associated microstructural changes at room and low temperatures. The combination of the properties special to thin films with the diffusional mechanisms in the binary Ag-Sn and Ag-In systems leads to interesting effects, such as the dependence of IMC formation on the stacking sequence in the bilayers. The obtained experimental results for both systems could be explained using thermodynamic and kinetic models. Experimental characterization of the bilayers mainly relied on X-ray diffraction (XRD) and electron microscopy. In order to investigate the effect of the deposition process on IMC formation and the microstructure of the bilayers, different physical vapor deposition (PVD) techniques, especially thermal evaporation and magnetron sputtering, were used for the preparation of the bilayers. During investigation of the Ag-Sn system it was found that ambiguity exists among the published crystal structures of the Ag3Sn IMC. Therefore, the crystal structure of Ag3Sn has been reinvestigated using high-resolution XRD in connection with Rietveld refinements.