Journal articles and book contributions

This work reports the formation of nanoflowers after annealing of Au/Ni bilayers deposited on SiO2/Si substrates. The cores of the nanoflowers consist of segregated Ni silicide and Au parts and are surrounded by SiOx branches. The SiO2 decomposition is activated at 1050 ˚C in a reducing atmosphere, and it can be enhanced more by Au compared to Ni. SiO gas from the decomposition of SiO2 and the active oxidation of Si is the source of Si for the growth of the SiOx branches of the nanoflowers. The concentration of SiO gas around the decomposition cavities is inhomogeneously distributed. Closer to the cavity border, the concentration of the Si sources is higher, and SiOx branches grow faster. Hence, nanoflowers present shorter and shorter branches as they are getting away from the border. However, such inhomogeneous SiO gas concentration is weakened in the sample with the highest Au concentration due to the strong ability of Au to enhance SiO2 decomposition, and nanoflowers with less difference in their branches can be observed across the whole sample.

In this paper, a miniaturized weighing cell that is based on a micro-electro-mechanical-system (MEMS) is discussed. The MEMS-based weighing cell is inspired by macroscopic electromagnetic force compensation (EMFC) weighing cells and one of the crucial system parameters, the stiffness, is analyzed. The system stiffness in the direction of motion is first analytically evaluated using a rigid body approach and then also numerically modeled using the finite element method for comparison purposes. First prototypes of MEMS-based weighing cells were successfully microfabricated and the occurring fabrication-based system characteristics were considered in the overall system evaluation. The stiffness of the MEMS-based weighing cells was experimentally determined by using a static approach based on force-displacement measurements. Considering the geometry parameters of the microfabricated weighing cells, the measured stiffness values fit to the calculated stiffness values with a deviation from -6.7 to 3.8% depending on the microsystem under test. Based on our results, we demonstrate that MEMS-based weighing cells can be successfully fabricated with the proposed process and in principle be used for high-precision force measurements in the future. Nevertheless, improved system designs and read-out strategies are still required.

Understanding the connection of molecular structure and optical properties of rare earth doped luminescent materials is essential for fabrication of state-of-the-art active laser media. On the other hand, rare earth ions can be used as a probe ion for the molecular structure of the host material if the structure-property correlations are known. Therefore, this work combines molecular dynamics simulations, Judd-Ofelt theory and UV-vis-NIR absorption spectroscopy including the behavior of the structure-sensitive hypersensitive absorption transitions of Er3+ to expand the knowledge on the local molecular structure in the immediate vicinity of the doped rare earth ions in dependence of glass composition. For this purpose, glasses of the compositions (35-x) BaO &hahog; x MgO &hahog; 10 Al2O3 &hahog; 55 SiO2 (mol%) (x = 0, 7.5, 15, 25, 35) and (20-x) BaO &hahog; x MgO &hahog; 20 Al2O3 &hahog; 60 SiO2 (mol%) (x = 0, 10, 20), doped with 2 × 10^20 ions/cm^3 Er3+ were prepared and analyzed. Clear differences in the absorption spectra between glasses of different BaO/MgO ratios, i.e. different network modifier field strengths, and different network modifier oxide to Al2O3 ratios are found and discussed in detail. Glasses with high BaO concentrations and high network modifier oxide to Al2O3 ratios provide lower rare earth coordination numbers with oxygen in general but higher coordination probabilities with non-bridging oxygen, which results in notably increased splitting of the optical transitions of the doped rare earth ions and higher hypersensitivity / lower local site symmetry for the doped rare earth ions in the investigated compositions. Based on our results and results from other publications the local rare earth site symmetry in glasses can in general be correlated with the rare earth coordination number.

Miniaturized on-chip micropumps with no moving parts are intriguing components for advanced lab-on-chip systems. Magnetohydrodynamic pumping is one possibility but requires further research with respect to microsystems design and fabrication. In this paper, the design and fabrication of a magnetohydrodynamic micropump is discussed using a composite of patterned glass and stacked dry film photoresist as demonstrator platform. The magnetohydrodynamic pumping effect is achieved by the superposition of an electric ion current generated by integrated electrodes and an external magnetic field provided by a permanent magnet. As test electrolytes, potassium chloride with potassium hexacyanoferrate (III) and potassium hexacyane iron (II) were used. Seamless fluid channel sidewalls were achieved from stacked dry film resists, which appear to be cast from a single mold. A liquid-tight sealing of the microchannels was realized by covering them with a thermally bonded laser-structured glass lid. Although, a complete characterization of the pump performance was not yet realized, the micropump in its current state serves as a technology demonstrator for further research of microfluidic on-chip micropumps that utilize the magnetohydrodynamic effect and also for other microfluidic systems.

Reactive multilayer systems are nanostructures of great interest for various technological applications because of their high energy release rate during the self-propagating reaction of their components. Therefore, many efforts are aimed at controlling the propagation velocity of these reactions. Herein, reactive multilayer systems of Al/Ni in the shape of free-standing foils with a wavelike surface morphology prepared by using sacrificial substrates with well-aligned waves are presented and the propagation of the reaction along different directions of the reproduced waves is analyzed. During the ignition test, the propagation front is recorded with a high-speed camera, and the maximum temperature is measured using a pyrometer. The propagation of the reaction is favored in the direction of the waves, which points out the influence of the anisotropy generated by this morphology and how it affects the propagation dynamics and the resulting microstructure. Furthermore, compared to their counterparts fabricated on flat substrates, these reactive multilayers with wavelike morphology exhibit a remarkable reduction in the propagation velocity of the reaction of about 50%, without significantly affecting the maximum temperature registered during the reaction.