Publikationen

Suction based attachment systems for pick and place handling of fragile objects like glass plates or optical lenses are energy-consuming and noisy and fail at reduced air pressure, which is essential, e.g., in chemical and physical vapor deposition processes. Recently, an alternative approach toward reversible adhesion of sensitive objects based on bioinspired dry adhesive structures has emerged. There, the switching in adhesion is achieved by a reversible buckling of adhesive pillar structures. In this study, we demonstrate that these adhesives are capable of switching adhesion not only in ambient air conditions but also in vacuum. Our bioinspired patterned adhesive with an area of 1 cm2 provided an adhesion force of 2.6 N ± 0.2 N in air, which was reduced to 1.9 N ± 0.2 N if measured in vacuum. Detachment was induced by buckling of the structures due to a high compressive preload and occurred, independent of air pressure, at approximately 0.9 N ± 0.1 N. The switch in adhesion was observed at a compressive preload between 5.6 and 6.0 N and was independent of air pressure. The difference between maximum adhesion force and adhesion force after buckling gives a reasonable window of operation for pick and place processes. High reversibility of the switching behavior is shown over 50 cycles in air and in vacuum, making the bioinspired switchable adhesive applicable for handling operations of fragile objects.

The present work proposes the use of a TiO2 electrode coupled to a one-dimensional photonic crystal (1DPC), all formed by the sequential deposition of nanocolumnar thin films by physical vapor oblique angle deposition (PV-OAD), to enhance the optical and electrical performance of DSCs while transparency is preserved. We demonstrate that this approach allows building an architecture combining a non-dispersive 3 µm of TiO2 electrode and 1 µm TiO2-SiO2 1DPC, both columnar, in a single-step process. The incorporation of the photonic structure is responsible for a rise of 30% in photovoltaic efficiency, as compared with a transparent cell with a single TiO2 electrode. Detailed analysis of the spectral dependence of the photocurrent demonstrates that the 1DPC improves light harvesting efficiency by both back reflection and optical cavity modes confinement within the TiO2 films, thus increasing the overall performance of the cell.

Four new praseodymium alkoxo and amido compounds ([Pr3(µ3-OtBu)2(µ2-OtBu)3(OtBu)4(HOtBu)2] (1), [Pr{OC(tBu)3}3(THF)] (2), [PrCl{N(SiMe3)2}2(THF)]2 (3), and [PrCl{OC(tBu)3}2(THF)]2 (4)) were synthesized and structurally characterized by single-crystal X-ray diffraction analysis. Application of these compounds in solvothermal synthesis of praseodymium oxide/hydroxide nanostructures showed their templating influence on the morphology and phase composition of the resulting solid-state materials. Differential reactivity of the chosen alkoxide ligands toward water and the different arrangements of metal-oxygen units in the studied precursor compounds strongly influenced the kinetics of hydrolysis and cross-condensation reactions as manifested in the morphological changes and phase composition of the final products. Thermal decomposition studies of 1- 4 confirmed their conversion into the corresponding oxide phases. Activation of compounds 1, 2, and 4 by either a base or a stoichiometric amount of water showed the distinct influence of their chemical configuration on the obtained nanopowders: whereas 1 solely produced nanorods of Pr(OH)3, 2 predominantly formed a mixture of rod-shaped and spherical particles. The solvothermal decomposition of 4 resulted in Pr(OH)2Cl or PrOCl due to the presence of Cl ligands in the molecular precursor. The resultant materials were thoroughly characterized to demonstrate the relationship between precursor chemistry and the processing parameters that are clearly manifested in the morphology and phase of the final ceramics.

A novel synthesis of a nanostructured cell adhesive surface is investigated for future stent developments. One-dimensional (1D) Al2O3 nanostructures were prepared by chemical vapor deposition of a single source precursor. Afterwards, recombinant filamentous bacteriophages which display a short binding motif with a cell adhesive peptide (RGD) on p3 and p8 proteins were immobilized on these 1D Al2O3 nanostructures by a simple dip-coating process to study the cellular response of human endothelial EA hy.926. While the cell density decreased on as-deposited 1D Al2O3 nanostructures, we observed enhanced cell proliferation and cell-cell interaction on recombinant phage overcoated 1D Al2O3 nanostructures. The recombinant phage overcoating also supports an isotropic cell spreading rather than elongated cell morphology as we observed on as-deposited Al2O3 1D nanostructures.

Polypropylene (PP)-based nanocomposites compounded by a twin-screw extruder and injection molded into plates those were then joined by linear vibration welding. The mechanical performances of the welds and bulk materials were examined. While the incorporation of rigid particles slightly improves the impact strength of the bulk PP, the mechanical properties of the welds decrease with increasing nanoparticle contents. The best weld quality is obtained at low weld pressure without nanoparticles. The fracture surfaces and microstructure of the welds showed that the reduced weld quality is caused by the orientation of nanofillers parallel to the weld plane, the destruction of interphase between fillers and matrix, and the reduction of molten-film thickness by incorporation of nanoparticles.

1D Al/Al2O3 nanostructures have been synthesized by chemical vapour deposition (CVD) of the molecular precursor [tBuOAlH2]2. The deposited nanostructures grow chaotically on the substrate forming a layer with a high porosity (80%). Depending on the deposition time, diverse nanostructured surfaces with different distribution densities were achieved. A three-dimensional (3D) reconstruction has been evaluated for every nanostructure density using the Focus Ion Beam (FIB) tomography technique and reconstruction software tools. Several structural parameters such as porosity, Euler number, geometrical tortuosity and aspect ratio have been quantified through the analysis with specified software of the reconstructions. Additionally roughness of the prepared surfaces has been characterized at micro- and nanoscale using profilometry and AFM techniques, respectively. While high aspects ratio around 20-30 indicates a strong anisotropy in the structure, high porosity values (around 80%) is observed as a consequence of highly tangled geometry of such 1D nanostructures. 1D nanostructures have tube or wire-like shapes with diameters below 100 nm and lengths of several micrometers. There are various methods to fabricate such ultrafine structures. The growth out of a chemical synthesis is the most elegant and simplest approach for fabrication of these ultra-small wires. Some of these methods are known as wet chemical syntheses since the material fabrication takes place within a solution. The chemical synthesis may also be carried out in the gas phase. Chemical Vapour Deposition (CVD) is such a chemical method, which leads to the fabrication of solid materials through accumulation of the vapour phase chemical species on a solid substrate. In this work, we have used a special CVD process to fabricate the nano-wires. The obtained wires have core-shell geometry. The inner core is made of aluminium and the outer surrounding shell is made of aluminium oxide. These ultrafine and extremely long wires form an assembly, which is similar to a bundle of spaghettis. It is difficult to analyse their shape, assembly, voids and other structural properties easily, just by looking from the top-view using a high-resolution electron microscopy. This microscopic method can give extremely high-resolution images (more than 500,000 times magnification) to analyse any fine detail. On the other hand, the gathered information is limited only to the planar 2D surface of the material. To get 3D information, we cut several slices perpendicular to the surface of the nano-wire assembly by edging and analysed every slice in terms of their morphology (destructing process). Afterwards, all these 2D projection images are brought together in order to get a reconstructed 3D image. This method allows observing every fine detail in the assembly just by rotating 3D image along any axis of interest.

Polyvinylpyrrolidone (PVP) is presented as a "greener" alternative to commonly used supercapacitor binders, namely polyvinylidenedifluoride (PVDF) or polytetrafluoroethylene (PTFE). The key advantages of using PVP are that it is non-toxic and soluble in ethanol and it can be used to spray coat or drain cast activated carbon (AC) electrodes directly on a current collector such as aluminum foil – in contrast to PTFE that requires rolling or PVDF that requires toxic N-methylpyrrolidone (NMP). The electrodes with the best mechanical stability incorporated 3.5 mass% of 1.300.000 g mol−1 PVP. Compared to PTFE or PVDF, the resulting pore volume was significantly higher and the specific surface area significantly larger when using PVP (normalized to the amount of AC). A good electrochemical performance was observed in organic electrolytes for AC–PVP electrodes: 112 or 97 F g−1 at 0.1 A g−1 in 1 M TEA–BF4 in propylene carbonate or acetonitrile, respectively. The performance stability was comparable to PTFE-bound electrodes when adjusting the maximum cell voltage to 2.5 V while preserving the manufacturing features of PVDF–AC films. (Electro)chemical stability is shown by electrochemical testing and infrared vibrational spectroscopy for propylene carbonate and acetonitrile.

The thermal conductivity of supercapacitor film electrodes composed of activated carbon (AC), AC with 15 mass% multi-walled carbon nanotubes (MWCNTs), AC with 15 mass% onion-like carbon (OLC), and only OLC, all mixed with polymer binder (polytetrafluoroethylene), has been measured. This was done for dry electrodes and after the electrodes have been saturated with an organic electrolyte (1 M tetraethylammonium-tetrafluoroborate in acetonitrile, TEA-BF4). The thermal conductivity data was implemented in a simple model of generation and transport of heat in a cylindrical cell supercapacitor systems. Dry electrodes showed a thermal conductivity in the range of 0.09-0.19 W K-1 m-1 and the electrodes soaked with an organic electrolyte yielded values for the thermal conductivity between 0.42 and 0.47 W K-1 m-1. It was seen that the values related strongly to the porosity of the carbon electrode materials. Modeling of the internal temperature profiles of a supercapacitor under conditions corresponding to extreme cycling demonstrated that only a moderate temperature gradient of several degrees Celsius can be expected and which depends on the ohmic resistance of the cell as well as the wetting of the electrode materials.