Scientific publications

The influence of titanium dioxide (TiO2) nanoparticles on the crystallization behavior of polypropylene was investigated by conventional differential scanning calorimetry (DSC) and fast scanning DSC measurements. The data obtained from both methods were estimated for the first time using the Lauritzen-Hoffmann equation to analyze the behavior over a wide cooling range under nonisothermal conditions. This provides more reliable values of nucleation parameters (Kg) and surface free energy (σe). The variation of the effective energy (ΔE) was determined with the Kissinger method. Regardless of the cooling rate, both Kg and σe indicate the role of titania as a nucleating agent enhances the crystallization rate. However, the ΔE denotes that TiO2 acts as an obstacle to the mobility of chain segments at cooling rates below 150 °C/s, while, in contrast, the presence of titania enhances the chain mobility at cooling rates above 150 °C/s.

In this study, the influence of surface morphology, reagent ions and surface restructuring effects on atmospheric pressure laser desorption/ionization (LDI) for small molecules after laser irradiation of palladium self-assembled nanoparticular (Pd-NP) structures has been systematically studied. The dominant role of surface morphology during the LDI process, which was previously shown for silicon-based substrates, has not been investigated for metal-based substrates before. In our experiments, we demonstrated that both the presence of reagent ions and surface reorganization effects – in particular, melting – during laser irradiation was required for LDI activity of the substrate. The synthesized Pd nanostructures with diameters ranging from 60 to 180 nm started to melt at similar temperatures, viz. 890-898 K. These materials exhibited different LDI efficiencies, however, with Pd-NP materials being the most effective surface in our experiments. Pd nanostructures of diameters >400-800 nm started to melt at higher temperatures, >1000 K, making such targets more resistant to laser irradiation, with subsequent loss of LDI activity. Our data demonstrated that both melting of the surface structures and the presence of reagent ions were essential for efficient LDI of the investigated low molecular weight compounds. This dependence of LDI on melting points was exploited further to improve the performance of Pd-NP-based sampling targets. For example, adding sodium hypophosphite as reducing agent to Pd electrolyte solutions during synthesis lowered the melting points of the Pd-NP materials and subsequently gave reduced laser fluence requirements for LDI.

In this study, the impact of analyte ablation and substrate surface acidity on ion yield in surface-assisted laser desorption/ionization-mass spectrometry (SALDI-MS) from Pd nanoparticles was examined. The extent of analyte removal from the Pd material was investigated by X-ray analysis and compared to conventional desorption/ionization on silicon (DIOS) and matrix-assisted laser desorption/ionization (MALDI). Despite the different surface morphologies of the investigated materials and the widely varying ionization yields observed during the experiments, virtually the same amount of analyte was removed by laser irradiation from each of the investigated substrates at a given laser fluence. Our experiments therefore suggested that the extent of analyte ablation from the substrate was less important for SALDI efficiency than the chemical properties of the surface.

The charge storage mechanism and ion arrangement inside electrically charged carbon nanopores is a very active research field with tremendous importance for advanced electrochemical technologies, such as supercapacitors or capacitive deionization. Going far beyond the state of art, we present for the first time a comprehensive study of tracking ion electrosorption in aqueous electrolytes during charging and discharging of porous carbon electrodes using in situ X-ray scattering. We provide novel and quantitative insights into the local concentration of anions and cations and demonstrate that the global number of ions within the pores does not vary during charging and discharging. In addition, we have unique access to the spatial arrangement of ions inside carbon nanopores by using a simple, yet powerful two-phase model. Applying this model to our data, we show that double-layer formation is accomplished by a unique combination of preferred counter-ion adsorption directly at the pore wall which drains ions from their local surrounding inside carbon nanopores. Effectively, this leads to a situation which globally appears as ion swapping.

Mixtures of polyvinylpyrrolidone/polyvinyl butyral (PVP/PVB) are attractive binders for the preparation of carbon electrodes for aqueous electrolyte supercapacitors. The use of PVP/PVB offers several key advantages: They are soluble in ethanol and can be used to spray coat or drain cast activated carbon (AC) electrodes directly on a current collector. Infrared spectroscopy and contact angle measurements show that the PVP-to-PVB ratio determines the degree of binder hydrophilicity. Within our study, the most favorable performance was obtained for AC electrodes with a composition of AC + 1.5 mass% PVP + 6.0 mass% PVB; such electrodes were mechanically stabile and water resistant with a PVP release of less than 5% of total PVP while PVB itself is water insoluble. Compared to when using PVDF, the specific surface area (SSA) of the assembled electrodes was 10% higher, indicating a reduced pore blocking tendency. A good electrochemical performance was observed in different aqueous electrolytes for composite electrodes with the optimized binder composition: 160 F g−1 at 1 A g−1 for 1 M H2SO4 and 6 M KOH and 120 F g−1 for 1 M NaCl. The capacitance was slightly reduced by 2.5% after cycling to 1.2 V with 1.28 A g−1 in 1 M NaCl for 10,000 times.

Ultrafast high energy electron diffraction in reflection geometry is employed to study the structural dynamics of self-organized Germanium hut-, dome-, and relaxed clusters on Si(001) upon femtosecond laser excitation. Utilizing the difference in size and strain state the response of hut- and dome clusters can be distinguished by a transient spot profile analysis. Surface diffraction from {105}-type facets provide exclusive information on hut clusters. A pixel-by-pixel analysis of the dynamics of the entire diffraction pattern gives time constants of 40, 160, and 390 ps, which are assigned to the cooling time constants for hut-, dome-, and relaxed clusters.

The mechanics of detachment is analysed for 2 D flat-bottomed planar pillars and 3 D cylindrical pillars from a dissimilar elastic substrate. Application of an axial stress to the free end of the pillar results in a singularity in stress at the corner with the substrate. An eigenvalue analysis reveals that the stress field near the corner is dominated by two singular eigenfields having eigenvalues (λ1,λ2) with corresponding intensities (H1,H2). The asymptotic stress field σij is of the form σij = H1rλ1-1fij(λ1, θ)+H2rλ2−1fij(λ2, θ) , where fij describe the angular dependence θ of σij, and r is the radial distance from the corner. The stress intensities (H1,H2) are calculated numerically, using a domain integral approach, as a function of the elastic mismatch between the pillar and substrate. The singular zone extends across approximately 10% of the pillar diameter (in 3 D) or pillar width (in 2 D). Interfacial failure is predicted for an assumed crack emanating from the corner of pillar and substrate. For the case of an interfacial crack that resides within the domain of corner singularity, a boundary layer analysis is performed to calculate the dependence of the interfacial stress intensity factor K upon (H1,H2). When the crack extends beyond the domain of corner singularity, it is necessary to consider the full geometry in order to obtain K. A case study explores the sensitivity of the pull-off stress to the flaw size and to the degree of material mismatch. The study has implications for the optimum design of adhesive surface micropatterns, for bonding to either stiffer or more compliant substrates.

Nature uses hierarchical fibrillar structures to mediate temporary adhesion to arbitrary substrates. Such structures provide high compliance such that the flat fibril tips can be better positioned with respect to asperities of a wavy rough substrate. We investigated the buckling and adhesion of hierarchically structured adhesives in contact with flat smooth, flat rough and wavy rough substrates. A macroscopic model for the structural adhesive was fabricated by molding polydimethylsiloxane into pillars of diameter in the range of 0.3–4.8 mm, with up to three different hierarchy levels. Both flat-ended and mushroom-shaped hierarchical samples buckled at preloads one quarter that of the single level structures. We explain this behavior by a change in the buckling mode; buckling leads to a loss of contact and diminishes adhesion. Our results indicate that hierarchical structures can have a strong influence on the degree of adhesion on both flat and wavy substrates. Strategies are discussed that achieve highly compliant substrates which adhere to rough substrates.

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten?carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation.

Carbon fibers are widely used as reinforcements in poly-ether-ether-ketone (PEEK). In recent years, these materials have also been used for tribological applications. For further optimization of these tribo-materials, the contribution and action mechanisms of carbon fiber reinforcements to the tribological performance of PEEK composites need to be understood. Toward this goal, we have studied carbon fibers in a PEEK composite by scratching experiments using Berkovich and conical indenters and friction imaging using contact atomic force microscopy. For comparison, scratching was extended into the PEEK matrix surrounding the carbon fibers. It is found that shearing dominates the friction and wear behavior of carbon fibers alone, while both shearing and plowing contribute to the overall friction of PEEK composites. There is no local variation in friction across a carbon fiber surface. The wear reduction by carbon fibers originates from their effective load-bearing capability. For the first time, fatigue of individual carbon fibers is revealed, as well as the dependence of interfacial debonding or delamination on the contact configuration between fibers and scratching asperities.