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In this study, the effects of cycle dependent wear on the consecutive corrosion behaviour of AISI 420 in 0.15 M NaCl solution were investigated to determine the influence of the tribological microstructure alteration on the electrochemical behaviour. The results revealed adhesion dominated wear at low cycle counts that led to the formation of a lamellar microstructure with delamination and enlarged Cr-depleted areas, resulting in a significant decline in pitting corrosion resistance. As the number of cycles increased, a transition of the wear mechanisms towards abrasion was observed. The progressive deformation allowed strong grain refinement and homogenization of the microstructure, which led to a significant increase in the pitting corrosion potential.

In microelectromechanical system manufacturing and especially in the photovoltaic industry, wet-chemical baths are used for surface structuring, conditioning, and cleaning. Ozone-based wet-chemical cleaning processes show, in addition to the cleaning of the silicon wafers, the ancillary effect of intended material etch back. Previous studies observed inhomogeneities over the wafer surface occurring during the ozone-based wet chemical process. Since a detailed observation during the process in the basin is not possible, simulations are carried out to understand what causes these inhomogeneities. Therefore, an existing microscopic two-dimensional (2D) model for reaction modelling is extended stepwise from a length of several microns to the full length of 157 mm of an industrial solar cell. Subsequently, the 2D model is transferred to a three-dimensional (3D) model. In addition, an initial water layer, originating from the previous rinsing step, and the movement of the handling system were taken into account for modeling. This approach allows the estimation of the etching process over an entire wafer surface for the first time and enables better understanding of the reasons for the inhomogeneities. The water layer in connection with the fast movement of the handling system and the masking, originating from carrier rods, leads to stripes on the wafer surface, shown by differences in reflection. As the water layer is not removed homogeneously, the occurring chemical reaction in the masked areas is delayed and results in an uneven etching of the wafer surface. The removal over the entire wafer surface in the simulation (8.51 ± 0.37 nm) is in accordance with the experimental data (9.02 ± 1.10 nm). This presented model can serve as a base for future work to estimate the effects of parameter changes, e.g., plant design or the composition of the etching solution on the homogeneity of the wet chemical cleaning and etch back process, thus enabling a cost efficient process optimization.

This paper presents a novel Laser-Assisted Micro-Milling (LAMM) process of austenitic stainless steel X5CrNi18-10. The LAMM process is compared with the conventional micro-milling process. Ultra-short pulsed laser radiation is utilized for the structuring of the workpiece surface prior to the micro-milling process. Different laser structures are produced on the workpiece surface at a constant laser scanning speed with various laser powers and laser line spans. The high performance of the developed process is shown by experimental investigations. The effect of laser structuring on the micro-milling forces and temperature indicated the superior performance of the new LAMM process. Cutting forces and temperature could be reduced by up to 70% and 50%, respectively. The results of conventional micro-milling showed that increasing the cutting speed, at a constant undeformed chip thickness, reduced the micro-milling forces. Increasing the cutting speed from 50 to 250 m/min halved both the trust and normal forces, while it slightly improved the surface roughness. On the other hand, increasing the feed per tooth degraded the surface roughness and increased the cutting forces. Furthermore, in conventional milling the workpiece was subjected to high plastic deformation during the cutting process, while side flow, smeared material, metal debris, and cavities were observed on the workpiece surface.

Electrodeposition of metals directly on non-metal substrates without any seed layer reduces process steps; however, adhesion of the electrodeposited layer is a challenge. In this work, the influence of current density during seedless electroplating on the adhesion of Cu layers on Si substrates was investigated. Cu was electrodeposited on phosphorus-doped Si samples having different orientations in two stages. During the first stage, higher current densities were used for a few minutes to reach instantaneous nucleation and maximize the number of Cu islands on the substrate. The second stage had the goal of growing the nuclei previously formed. Therefore, lower current densities were used to create progressive nucleation. Adhesion values of Cu layers on Si were derived from scratch tests and tensile measurements. Higher adhesion was obtained either at higher current densities or at lower cathodic potentials at the beginning of potentiostatic or galvanostatic processes. To study the influence of hydrogen evolution on the adhesion of seedless electrodeposited Cu on Si, the transition time which indicates the moment when transferred charges are used to create hydrogen at the cathode, was determined experimentally. The transition time shows a strong dependence on current density. Therefore, control and optimization of adhesion by an appropriate choice of current densities within the two-stage process is possible.

Structured surfaces are used to reduce reflection and enhance light-trapping in silicon solar cells. In this simulation study, we investigated the relationship between the refractive index of front-side coupling structures on top of planar wafer-based crystalline silicon solar cells and the light-trapping performance of the structures. A crossed diffraction grating with a period of 1 mm and random pyramid structures with varying refractive indices were considered. Simulations were carried out both at the cell level and at the complete module stack level. It is shown that the single pass light path enhancement factor (LPEF) only provides a rough estimate of the light-trapping properties. The light-trapping behavior can only be reliably assessed in the complete system level and these results deviate from the estimated single pass LPEF. It can also be shown that the refractive index of the structure strongly influences the light-trapping behavior.

Structured interfaces to enhance light trapping are standard concepts for silicon solar cells. Within this simulation study we investigated the influence of the refractive index of front side coupling structures on top of a crystalline silicon solar cell on the light trapping performance. Simulations were carried out both at cell level and for the complete module stack. The light trapping behavior can only be reliably assessed if the complete system is investigated. It could be shown that the refractive index of the light trapping structure strongly influences the light trapping behavior.

Measurement of residual stress in plasma-sprayed coating is of key importance to optimize their microstructure and mechanical properties. In this study, the x-ray diffraction analysis was carried out using the sin2ψ method to evaluate the residual stress distribution of hydroxyapatite (HAp) coatings produced on titanium substrate by atmospheric plasma spraying (APS) and vacuum plasma spraying (VPS). The sin2ψ method measured strains at different tilt ψ and rotating φ angles around the specimen surface normal. A non-uniform and inhomogeneous stress distribution was present in the both coatings. The measured strain eψφ is plotted versus sin2ψ, showing a nonlinear (elliptical) behavior, which indicates the presence of inhomogeneous triaxial stress distributions within coating, due to the crystalline anisotropy, inhomogeneous cooling rate or solidification of the molten particles. The normal stress values within both HAp coatings produced were found to be tensile in nature, but the values of tensile stresses are significantly higher in APS coating than those values obtained for VPS coating.

Halar®, an ethylene chlorotrifluoroethylene (ECTFE) thermoplastic polymer, can be considered as an ideal protection coating material for medical application, because of its high purity and unique combination of the mechanical and chemical properties. In this study, Halar® polymer powder particles are deposited onto titanium substrate by the means of electrophoretic deposition (EPD). Differential scanning calorimetry was used to measure thermal characteristics of the raw Halar® powders including glass transition temperature (Tg), melting temperature (Tm) and melting enthalpy (∆Hm). A high crystallinity value was determined for the raw polymer powders by differential scanning calorimetry analysis. Additionally, the effect of heat treatment on Tg, Tm and crystallinity of the polymer are evaluated at different heating temperatures. It was found that heat-treatment of the Halar® powder at the temperature of 260 °C (above its melting temperature) results in increasing of Tg, Tm and crystallinity of this thermoplastic polymer. Thus, after deposition thermal treatment temperature was performed at 260 °C for 15 min which may improve the adhesion of the coating to the substrate and its mechanical properties. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to assess the quality and surface morphology of Ti coated with Halar® polymer. The deposited coating showed a high quality, smooth and homogenous microstructure without any cracks or holes. Nanomechanical properties (hardness and elastic modulus) of the deposited polymer coating were also determined using the nanoindentation technique. Moreover, a comparison is drawn between the fatigue behavior of uncoated and polymer coated Ti specimens by a three-point bend test. The results indicated a blunt crack tip and a higher fatigue strength of Halar® polymer coated Ti.

We report on a new type of polymer electrolyte fuel cell based on a hydroxide ion conductive polymer combined with a non-noble chromium–nickel (Cr–Ni) catalyst for the oxygen reduction reaction (ORR). We study variable fractions of Cr in Ni by density functional theory simulating the thermodynamic potentials characterizing the ORR. We found increased ORR catalytic activity employing the rotating disk electrode technique. The polarization curve and power densities measured for the constructed fuel cell indicate considerable performance improvement with the Cr–Ni catalyst. Thus we expect that this kind of fuel cell may open up alternative routes in fuel cell research using non-noble catalysts.

Effective catalysts for electrochemical oxygen reduction reaction should have both high activity and stability under relevant electrochemical conditions of fuel cells. For sustainable and scalable applications it is important to identify catalyst material candidates that are free of precious metals or rare elements. We combine modeling based on density functional theory and experimental characterization to study a nickel based catalyst modified with Cr. To describe the catalytic activity within the model, we computed the O and OH adsorbate binding energies, the effective reversible potential and the critical potential for the oxygen reduction reaction. The stability of different overlayer structures has been analyzed by means of surface Pourbaix diagrams which show that the decorating Cr atoms considerably stabilize the catalyst surface against dissolution for large ranges of potentials and pH. Both the measured voltammograms and the analysis of the simulated structures indicate increased catalytic activity of the Cr modified catalyst.

The influence of light intensity on photo-electrochemical cells (PECs) consisting of an n-type strontium titanate (SrTiO₃) photoanode and nickel cathode in potassium hydroxide electrolyte is studied. The band levels of an electrolyte-metal-semiconductor-electrolyte system are presented and the effect of different light intensities on the energy levels is investigated. Photocurrent density, quantum efficiency, and open circuit potential measurements are performed on the processed PECs under different light intensities (375 nm). It is demonstrated that a threshold value of the light intensity has to be reached in order to obtain positive photo activity and that beyond this value the performance remains nearly constant.

Within this work we investigate the influence of the refractive index of front side struc-tures on top of a planar silicon solar cell on the light trapping performance.

Simulating the fluid flow during a wet chemical process bath, allows for a deeper understanding of the flow conditions in the reaction basin. To validate the simulation, dye visualization as well as laser doppler velocimetry (LDV) and particle image velocimetry (PIV) were carried out for the first time in a transparent replica. Dye experiments led to a qualitative validation of the simulation model, as vortices and direction of rotation were detected in the basin. In summary, the repeated experiments show a good agreement with the simulation that there are areas in the basin where high relative standard deviations occur, mostly in areas with a very low velocity. The LDV velocity is higher compared to the theoretical velocity, while the simulation and PIV measurements match the theoretical value. The PIV measurements are in better agreement with the simulation, since the mean velocity in the ${z}$ -direction is 0.0039 m/s and 0.0033 m/s for the PIV and the simulation, respectively. The PIV results show a change in the velocity direction with time, which can also be seen in the simulation results. The PIV measurements therefore confirm the simulation results of a time-dependent change in the flow.

Silicon solar cells are typically textured by means of wet chemical etching in order to enhance absorption. Within this work, we apply an optically functional layer onto a planar silicon surface. This layer is made of a high refractive index sol-gel material and can be patterned by nanoimprint lithography (NIL). In first experiments, we investigated various sol-gel based TiO2 precursors and evaluated their refractive index as well as the possibility to apply them in NIL. The refractive index was determined to be up to 2.25 using ellipsometry. This result was achieved with a solution composed of amorphous TiO2 precursors mixed with ethanol and 1,5-pentanediol. The topography of the patterned TiO2 layers were investigated using an atomic force microscope (AFM) and a scanning electron microscope (SEM) revealing a period of 1 μm and a pattern depth of 60 nm after sintering. Furthermore, optical modeling was used to optimize the structure parameters in order to minimize the weighted reflectance of an encapsulated silicon solar cell.

Silicon carbide (SiC) is a high performance ceramic material which is increasingly used in applications which demand for high temperature stable materials. The microstructuring of this material in its sintered state is a challenge for conventional machining methods due to its extraordinary mechanical properties. To overcome these limitations the process of electrical discharge machining (EDM) was modified with a so called assisting electrode (AE) to enable the structuring of non-conductive ceramics such as SiC. This paper presents a parametric analysis of micro electrical discharge milling process of non-conductive SiC for two different tool materials, tungsten carbide (WC) and copper (Cu). A full factorial design with four factors was conducted. The significant factors are shown with main effects plots. Two sets of optimized parameters for maximum material removal rate (MRR) and minimum tool wear rate (TWR) for both tool materials are presented. Hardness and surface roughness are measured and compared to the non-machined material.

Biofilm-associated infections of medical devices are a global problem. For the prevention of such infections, biomaterial surfaces are chemically or topographically modified to slow down the initial stages of biofilm formation. In the bifunctional material here presented, chemical and topographical cues are combined, so that protein and bacterial adhesion as well as bacterial proliferation are effectively inhibited. Upon changes in the surface topography parameters and investigation of the effect of these changes on bioactivity, structure-property relationships are obtained. The target material is obtained by microcontact printing (μCP), a soft lithography method. The antimicrobial component, poly(oxanorbornene)-based synthetic mimics of an antimicrobial peptide (SMAMP), was printed onto a protein-repellent polysulfobetaine hydrogel, so that bifunctional 3D structured polymer surfaces with 1, 2, and 8.5 μm spacing are obtained. These surfaces are characterized with fluorescence microscopy, surface plasmon resonance spectroscopy, atomic force microscopy, and contact angle measurements. Biological studies show that the bifunctional surfaces with 1 and 2 μm spacing are 100% antimicrobially active against Escherichia coli and Staphylococcus aureus, 100% fibrinogen-repellent, and nontoxic to human gingival mucosal keratinocytes. At 8.5 μm spacing, the broad-band antimicrobial activity and the protein repellency are compromised, which indicates that this spacing is above the upper limit for effective simultaneous antimicrobial activity and protein repellency of polyzwitterionic-polycationic materials.

The lateral dimensions of micro- and nanostructures obtained by microcontact printing (μCP) can be easily varied by selecting stamps with the desired spacing and pattern. However, the height of these structures cannot be tuned as easily, and in most cases only 2D structures are obtained. Here, we show how the chemical cross-linking properties of polymer inks designed for μCP can be used to obtain 3D structures with heights ranging from 3 to 750 nm using the same μCP stamps. This is technologically relevant because the ink concentration affects the quality and resolution of the printed image, and therefore can only be varied in a certain range. By exploiting the cross-linking efficiency to tune the height, an additional parameter is available to reach the desired structure height without compromising the image quality. The inks were made from copolymers containing a low percentage of different UV cross-linkable repeat units: nitrobenzoxadiazole (NBD), coumarin (COU), and/or benzophenone (BP). The base polymer of the here presented model system was an antimicrobially active poly(oxanorbornene) (SMAMP), however the concept should be transferable to many other polymer backbones. We describe the fabrication and characterization of the printed micro- and nanostructures made from pure SMAMP, NBD-SMAMP, coumarin-SMAMP, BP-SMAMP, BP-NBD-SMAMP and BP-coumarin-SMAMP polymer inks. The photo-dimerization of COU during UV irradiation at λ = 254 nm was confirmed by UV-Vis spectroscopy. Since NBD and COU are fluorescent, the polymer could be visualized by fluorescence microscopy. Additionally, their height profiles were measured by atomic force microscopy (AFM). The heights of the 3D surface-attached polymer networks obtained from the here presented polymer inks correlated with the gel-content of the corresponding unstructured polymer layers, and thus with the cross-linking efficiency of the NBD, COU and BP cross-linkers. Due to being covalently cross-linked, these 3D-surface attached polymer structures were solvent-stable and stable in aqueous surroundings.

The influence of silicon doping on the adhesion of copper layers electroplated directly on (100) silicon without a seed layer was investigated in this work. The adhesion of Cu layers on Si(100) was derived from scratch tests where the critical loads and the types of failures of these layers on phosphorous- and boron-doped silicon were obtained. The maximum loads supported until complete layer removals were about twice as large for Cu layers electrodeposited on p-Si(100) than those deposited on n-Si(100). The Cu layers were also visually inspected using images taken with scanning electron microscopes, their topography was obtained by atomic force microscope measurements and crystal orientations by X-ray diffraction. Secondary neutral mass spectroscopy measurements and X-ray photoelectron spectroscopy were used to quantitatively determine the atomic Cu concentration at the surface of each sample. Both measurements have detected larger Cu concentrations at the p-Si(100) surface than at n-Si(100). These measurements were used to support a model previously presented by this group to explain the adhesion difference of Cu electroplated directly on p- or n-doped Si.

The scalable storage of renewable energy by means of converting water to hydrogen fuels electrochemically hinges on fundamental improvements in catalytic materials. However, many applications exist where an extended lifetime is virtually crucial for their functionality and success, e.g. in case of limited accessibility such as tire pressure sensors or biomedical implants. For these kinds of applications, the ultimate power supply should be a self-renewing energy source. This strategy is pursued by the concept of Micro Energy Harvesting (MEH). Within a MEH system a micro generator converts ambient energy to electrical energy for driving an application. Unfortunately, it is not ensured that the ambient energy level will maintain always high enough to provide sufficient power to the system as harvested energy usually manifests itself in rather irregular, random and low-energy bursts. One appealing form of integrated energy storage is the use of H2/air, a so called fuel cell type (FC) battery. Such devices promise very high volumetric energy densities of more than 2000 Wh/l. Consequently, this type of battery has recently attracted more and more attention and primary as well as secondary cells have been realized. Alkaline polymer electrolyte fuel cells have been recognized as the most promising solution in order to overcome the dependency on noble metal catalysts. Nevertheless, further improvements for these kinds of fuel cells have to be reached with respect to high power. Therefore, one promising approach is to increase the skin surface of porous chromium decorated nickel electrodes for enhancement of exchange current density by forming three-dimensional (3D) microstructures directly into the electrode. Therefore, a novel laser structuring process was applied using ultrashort laser pulses. Ultrashort laser processing of complex multimaterial systems for energy storage allow for precise material removal without changing the material properties. By applying this novel laser-based structuring technique, 3D microstructures could be formed permitting shortened diffusion lengths between the electrolyte and the electrode surface being necessary for increased exchange current densities.

In this paper single band gap photo electrochemical cells (PECs) are presented, which consist of strontium titanate (SrTiO3) photo anodes on nickel cathodes in potassium hydroxide electrolyte. SrTiO3 powders are deposited on nickel substrates by electrophoresis before sintering with varying temperatures, times, cooling rates, gas types, and gas flow rates. The external quantum efficiency (EQE) of such PECs mainly depends on the morphology and the amount of oxygen vacancies in SrTiO3 lattice. At first, the morphology is investigated, which can be adjusted by the particle size as well as the sinter temperature and time. Nanopowder-based PECs sintered above the starting sinter temperature indicate the best charge carrier transport and hence allow high EQEs. The sinter time influences the specific surface area, but not the EQE in this investigation. Secondly, the generation of oxygen vacancies is investigated, which depends on the oxygen partial pressure and the equilibration temperature. Low oxygen partial pressures and high equilibration temperatures increase the amount of oxygen vacancies, which can be set by the gas type and its flow rate or the cooling rate and an additional heating step, respectively. It can be shown that PECs have to possess a low amount of oxygen vacancies to reach high EQE values, but not too low to allow for sufficient conductivity. This point is shown through our finding that the samples with lower and higher concentrations exhibit very low photo activity. Oxygen vacancies can be considered as intrinsic donors and hence increase electrical conductivity which is necessary but also act as recombination centers. For SrTiO3 nanopowder-based samples, which have been sintered at 1200 °C for 20 min with a cooling rate of 10 K/s in reducing gas (with 5 vol% H2) and a low flow rate of 1.7 l/h, very high external quantum efficiencies of 64.2 % under 365 nm illumination can be achieved.

Electric discharge machining (EDM) is an electro-thermal non-traditional machining process used to machine conductive materials. EDM process can be used to machine non-conductive ceramics by applying an assisting electrode (AE) on top of the insulating ceramics. The initial discharges occur between the tool electrode and the assisting electrode. During the spark erosion process an intrinsic conductive layer is continuously produced on the insulating ceramic. This transitional conductive layer ensures that the electric contact is sustained. However, result of parametric analysis of the EDM of non-conductive ceramics is not available. This paper presents a parametric study to evaluate the influence of five major EDM process parameters (peak current, open-circuit voltage, servo, pulse width and gap voltage) on two response variables (material removal rate and the tool ware rate) when structuring non-conductive Si3N4ceramics using the die-sinking μ-EDM process with the help of the AE. A set of experimental trials planned with fractional factorial design with a resolution of V was performed to evaluate the primary, two-factor and quadratic effects.

Characterized by excellent material properties such has high mechanical, thermal and chemical stability technical ceramics such as ZrO2, SiC, Si3N4and AlN are increasingly being used for various applications. Traditional means of machining sintered ceramics are expensive and limited by geometry. Electrical discharge machining (EDM) is an electro-thermal machining process used to structure conductive materials. By applying a conductive layer (denoted as assisting electrode) on top of the non-conductive material, the EDM process can also be used to structure insulating ceramics. This paper presents a comparative study on the major machining parameters affecting the µEDM process of non-conductive SiC, ZrO2, Si3N4and AlN ceramics. The influence of five major machining parameters (current, open-circuit voltage, gap voltage, duty-cycle and servo) over two responses (material removal rate (MRR) and tool wear rate) is investigated for each ceramics material. The underlying reason for the variation in the MRR among the different ceramics is examined by comparing the material properties. Melting point of the ceramics material has an effect on the MRR for the µEDM of different ceramics. The bulk resistance value of the ceramic material does not have an influence on the MRR for the µEDM of different ceramics. Scanning electron microscope (SEM) images of the cross section of the unprocessed and µEDM processed surface of these ceramics have been analyzed. The SEM micrographs show that the µEDM process does not affect the ceramics bulk. It also confirmed spalling as one of the dominant material removal mechanism for ZrO2ceramics.

Electrochemical machining (ECM) is an appropriate technology for structuring a wide range of metals and alloys independent of their mechanical properties. Further advantages the occurence of no tool wear and almost no change in the material properties of the work piece material due to the machining. As different studies showed it is possible to manufacture structures in the µm-range and also in the sub µm-range. The resulting geometry and feature size depend on the electrolyte, the working gap, the electric field, the flow conditions within the working gap and the material structure. The working gap is the most important parameter regarding the critical dimensions of ECM. Actual high resolution processes are limited by the process time due low current densities, while faster processes are limited by the resolution due to the dimensions of the double layer and the need for purging.\\Regarding these limits an enhanced understanding of the electrochemical processes within the working gap is mandatory for the optimization of the ECM. Therefore a measurement device is realised utilising electrochemical impedance spectroscopy (EIS), which is one technique to obtain a wide set of thermodynamic and kinetic parameters related to the electrochemical processes like reactions, adsorption, desorption and diffusion in the gap. There the impedance, which is the complex ratio between the voltage and the current in an AC circuit, is measured as a function of the frequency and it is compared to an equivalent circuit, composed out of basic elements representing the chemical sub processes. The spectrometer is realized by combining a commercial potentiostat with computer based signal processing realized in MATLAB\copyright. The spectrometer implements time-domain measurement techniques (Fourier transformation) for a strong reduction of measurement time. This time reduction allows in situ measurements.\\The presented spectrometer enables characterization of standard material systems like cooper, as well as the identification of the time-critical chemical processes and steps in more complex systems.

Silicon Carbide is a ceramic material with extraordinary properties. Not only does it excel in mechanical properties, such as very high hardness and flexural strength, but also shows excellent thermal properties with a low thermal expansion and operating temperatures above 1000°C. These properties predestine silicon carbide for applications in harsh environments. Structuring silicon carbide in the sintered state with conventional methods is not feasible due to its hardness. A non-conventional process to structure materials independent of their mechanical properties is electrical discharge machining (EDM). A certain conductivity is however required for this process. To fulfill this requirement, the method of an assisting electrode (AE) is used. During the process, an intrinsic AE is generated from the cracked dielectric oil and deposited on the ceramic surface. The process can therefore continue even after the applied AE has been penetrated. For a deeper understanding of the present removal mechanism EDM of non-conducting ceramics, especially in the area of micro EDM, an investigation of the influence of conductivity is necessary. Therefore three silicon carbide ceramics with different electrical conductivity (S-SiC: 1 10-7S/cm; LR-SiC: 10 S/cm; HO-SiC: 5 10-9S/cm) have been microstructured and analysed. It is found that the conductivity of the silicon carbide materials has no influence on the machinability, all samples can be microstructured. The microsections of the machined samples show that the near-surface structure of the SiC materials is not negatively influenced by the EDM process. The analysis of the surface revealed indications that for S-SiC and for HO-SiC, thermal spalling is the present removal mechanism. The LR-SiC surface shows melting structures. The material removal rate of LR-SiC is 8 × 10-3mm3/min, whereas the material removal rate of the S-SiC and the HO-SiC ranges at 3 × 10-3mm3/min. The high MRR of the LR-SiC indicates a removal mechanism analog to Silicon infiltrated Silicon carbide (SiSiC), with removal of a conductive phase.

In this paper a new concept of a bidirectional air electrode for alkaline rechargeable batteries is introduced. The functionality of this concept, which bases on high surface nickel-foam, is shown by a demonstrator. In order to enable the charging-reaction, the concept integrates the catalytic properties of the nickel-foam based current collector. For the discharging-direction well established catalysts like carbon black or MnO2 can be utilized. Compared to common air electrodes this provides several advantages, like the avoidance of expensive catalysts and a very simple design.

A novel analyzing method is presented, which allows precise characterization of cell-free layers (CFLs) of blood flowing through microchannels. The CFL occurs due to axial migration of the erythrocytes (RBCs). A confocal laser scanning microscope (CLSM) is used to detect the reflected light of channel walls and cells within the blood flow. Since the presented method does not depend on emitted fluorescence signals, there is no necessity for a complex sample preparation as fluorescence marking of cells. Furthermore, it allows the characterization of the thickness of the CFL in whole blood. Due to the high vertical resolution of the used CLSM, the developed characterization method enables measurements along the optical axis of the microscope. It is exemplarily used to analyze the thickness of the CFL in human blood flowing through microchannels as a function of the hematocrit and blood flow velocity. The microchannels are made of silicone rubber with a height of 100 µm. The microchannels are intended for a gas exchange application.

The fatigue strength of polycrystalline cp-Ti depends on a variety of different factors especially its plastic deformation mechanism. Deformation twinning is activated as a major deformation mechanism in cp-Ti during cyclic deformation. The fatigue-induced twin transformation developing on the surface of the cp-Ti specimens is accompanied with the microstructure irregularities, which affect the cohesive properties of grain boundaries and thus its fatigue cracking. The twinned grains serve as preferential sites for the localized shear and act as preferred path for crack propagation. The texture evolution was observed during cyclic deformation in cp-Ti which can be explained by the high twinning activity. The residual stress relaxation takes place significantly by imposing a mechanical loading.

Reflectance, reduction, and light trapping enhancement are essential to maximize the absorption of silicon solar cells. The industrial state of the art method to improve the solar cell optics is wet chemical texturization of the front surface in combination with the deposition of antireflection coatings. This work analyzes an alternative route, namely a TiO2 pillar structure on the front side of a planar silicon solar cell encapsulated in ethylene vinyl acetate (EVA) and glass. It focuses on parameter variations of the structured TiO2 layer while taking the module encapsulation into account. It is shown that internal reflections at the front interface of the module play a crucial role for the structure design. This leads to optimized structures working in a different optical regime. While state of the art structures optimized for a half infinite encapsulation act as effective media, structures optimized for the full module show an improved performance by making use of diffractive effects. It could be shown that weighted reflectance of 4.7% can be reached for a solar module with TiO2 pillar structure on top of the silicon surface compared to 5.5% for a two-layer ARC with a TiO2 bottom layer and 2.3% for an isotexture, which is the state of the art structure for multicrystalline silicon cells.

Lamination processes are a wide spread method to generate polymer multilayer foils where foils with different functions are stacked on top of each other. However, the process is limited to compatible materials because sufficient adhesion between the different layers relies on the interdiffusion of polymer chains from one material to the other. In this paper, we report on a new reactive lamination process which leads to a stable connection between sheets of poly(methyl methacrylate) (PMMA) and a cyclic olefin copolymer (COC). This connection is mediated by using a compatibilizing layer consisting of a photo- or thermoreactive PMMA-copolymer. These polymers generate chemical bonds to both foils, leading to a strong adhesion between them. Initial tests show that the adhesion between the two foils is typically stronger than the cohesion of the weaker of the two materials.

The electrical discharge machining process is an established process for machining materials regardless of their mechanical properties. Thus this process is especially attractive for materials which are hard to machine with conventional machining methods. The only requirement a material has to fulfil is having a certain electrical conductivity. Ceramic materials, (e.g. zirconia, silicon nitride or silicon carbide) exhibit excellent mechanical properties but are mostly electrically non-conductive. This can be compensated by an applied, electrically conductive assisting electrode. With this modification, the electrical discharge machining of non-conductive ceramic material is enabled. In this study the micro electrical discharge machining of non-conductive sintered silicon carbide is investigated. The drilling process shows instabilities due to the excessive generation of carbon products. A stabilisation of the process up to the maximum depth of 420 μm is realized by two approaches: adapting process parameters and adapting the tool electrode geometry. An analysis of the amount of infeed used in a milling process shows that an infeed of 15 μm has the best material removal rate to tool wear rate ratio. A maximum material removal rate of 3.58 × 10?3 mm3/min is achieved. Detached microstructures with an aspect ratio of 30 are machined. A conducted surface analysis indicates that the present removal mechanism is thermally induced spalling. Furthermore no heat affected zone is present in the machined near-surface area.

A fatigue testing setup based on electromagnetic excitation was built to apply cyclic stresses to the specimens near their resonance frequency. A test near the resonance frequency has the advantage that higher stresses can be applied to test specimens at a reduced input power. Stress amplitude up to 1000 MPa can be applied to the test specimens and up to four specimens can be tested simultaneously. The setup can test specimens at high fatigue cycle regime i.e. 100 million stress cycles can be achieved in 48 hours.The setup has been used to study the effects of internal and external microstructure on the fatigue strength of materials. Specimens especially stainless steel-304 was prepared by different techniques i.e. electric discharge machining EDM, etching and laser cutting. Specimens prepared by these techniques were tested and their fatigue strengths were compared. To probe the material endurance limit, tests were also performed on the above mentioned steel specimens in very high stress cycle regime i.e. > 109cycles. In order to investigate the effect of internal microstructure on fatigue strength of material, CuZn37 fabricated by etching was tested and the effect of different grain size on fatigue strength was compared. SN curves have been plotted for materials with no prior fatigue strength data. Stainless steel-1.4404 specimens prepared by Rapid Prototyping (RP) has been tested for fatigue analysis. The test results showed higher degree of scattering when compared to the traditionally manufactured steel. Fractography revealed the existence of inherent material flaws which was the main reason of higher degree of test point scattering. In addition to these data, the SN curve was plotted for Innolot which is an important soldering alloy and prompts to fatigue failure in electronic assemblies.

Micro- and nanostructures can be used for reflectance reduction or light guidance in applications like photovoltaic solar cells, LEDs or display technology. The combination of interference lithography and nanoimprint lithography enables the fabrication and replication of high resolution structures on large areas. The origination of master structures, seamlessly patterned on areas as large as 1.2 × 1.2 m2 was shown using interference lithography. Within this work we demonstrate our current results on the up-scaling of the replication process chain based on nanoimprint lithography with in-line capable tools. Application examples in the fields of photovoltaics are demonstrated, e.g. the micron-scale patterning of multicrystalline silicon substrates to increase the solar cell efficiency. Furthermore, the lifetime of soft PDMS stamps is investigated. AFM force-distance measurements are introduced as suitable method to quantify the PDMS hardness as a parameter indicating stamp degradation. This technique is subsequently applied to evaluate two different resist materials. Applying the epoxy material (SU-8) with its more complex molecular structure results in a strongly increased stamp lifetime compared to the acrylate resist (Laromer LR 8996). This is a highly valuable result for further developments towards an up-scaled realization of nanoimprint lithography.

Surface textures can significantly improve anti-reflective and light trapping properties of silicon solar cells. Combining standard pyramidal front side textures with scattering or diffractive rear side textures has the potential to further increase the light path length inside the silicon and thereby increase the solar cell efficiency. In this work we introduce the OPTOS (Optical Properties of Textured Optical Sheets) simulation formalism and apply it to the modelling of silicon solar cells with different surface textures at front and rear side. OPTOS is a matrix-based method that allows for the computationally-efficient calculation of non-coherent light propagation within textured solar cells, featuring multiple textures that may operate in different optical regimes. After calculating redistribution matrices for each individual surface texture with the most appropriate technique, optical properties like angle dependent reflectance, transmittance or absorptance can be determined via matrix multiplications. Using OPTOS, we demonstrate for example that the integration of a diffractive grating at the rear side of solar cells with random pyramids at the front results in an absorptance gain that corresponds to a photocurrent density enhancement of 0.73 mA/cm 2 for a 250 μm thick cell. The re-usability of matrices enables the investigation of different solar cell thicknesses within minutes. For thicknesses down to 50 μm the simulated gain increases up to 1.22 mA/cm 2 . The OPTOS formalism is furthermore not restricted with respect to the number of textured interfaces. By combining two or more textured sheets to effective interfaces, it is possible to optically model a complete photovoltaic module including EVA and potentially textured glass layers with one calculation tool.

In microsystems technology quality control of micro structured surfaces with different surface properties is playing an ever more important role. The process of quality control incorporates three-dimensional (3D) reconstruction of specularand diffusive reflecting technical surfaces. Due to the demand on high measurement accuracy and data acquisition rates, structured light optical microscopy has become a valuable solution to solve this problem providing high vertical and lateral resolution. However, 3D reconstruction of specular reflecting technical surfaces still remains a challenge to optical measurement principles. In this paper we present a measurement principle based on structured light optical microscopy which enables 3D reconstruction of specular- and diffusive reflecting technical surfaces. It is realized using two light paths of a stereo microscope equipped with different magnification levels. The right optical path of the stereo microscope is used to project structured light onto the object surface. The left optical path is used to capture the structured illuminated object surface with a camera. Structured light patterns are generated by a Digital Light Processing (DLP) device in combination with a high power Light Emitting Diode (LED). Structured light patterns are realized as a matrix of discrete light spots to illuminate defined areas on the object surface. The introduced measurement principle is based on multiple and parallel processed point measurements. Analysis of the measured Point Spread Function (PSF) by pattern recognition and model fitting algorithms enables the precise calculation of 3D coordinates. Using exemplary technical surfaces we demonstrate the successful application of our measurement principle.

This work presents a new type of high-resolution dry-film resist laminates for the fabrication of extremely thick epoxy based polymer structures. Via a lamination process a resist thickness of up to d=360@mm can be applied by one single process step. Afterwards, the dry-film resist is patterned by standard UV-lithography. An epoxy based resist mixture which contains epoxy resins, reactive diluents, additives, and a photo acid generator is deposited onto a PET supporting film by doctor blading for fabrication of the resist film. The films can be cut defect-free into favoured sheet sizes. Currently, dry-film resists with a layer thickness of up to 400@mm can be produced reproducible. With the tested d=360@mm thick dry-film resist an aspect ratio of 15:1 (height:width) has been achieved. By lamination and UV-patterning of two layers resist a master for 700@mm thick electroplated Nickel structures was fabricated. Thereby, steep side walls with an angle deviation of @a