Publikationen

Strain (stress-free) relaxation in mechanically prestrained bone has a time constant of 75 s. It occurs by a reorganization of the proteoglycan-glycoprotein matrix between collagen fibers, which requires ionic interactions. Dissolving and relinking the ionic bonds is thus an important tool of nature to enable plastic deformation and to develop self-healing tissues. A way to transfer this approach to technical materials is the attachment of ionic end groups to polymeric chains. In these classes of materials, the so-called polymeric ionic liquids, structural recovery of thermally disorganized material is observed. A time constant between minutes and a week could be achieved, also by ionic rearrangement. The same mechanism, rearrangement of ionic bonds, can lead to vastly different relaxation times when the ionic interaction is varied by exchange of the cationic end groups or the anions.

Mechanical properties of plants and underlying structure-property relationships are important for agricultural purposes as well as for biomimetic concepts. In this study, the effect of mechanical stimulation on morphology and bending properties of the stalk was investigated for Sorghum bicolor (Poaceae), a widely used drought-tolerant biomass grass. An experimental set-up allowing for defined growth and mechanical perturbation (flexing) during a defined growth period was designed. Mechanical properties of individual internodes of the stalk were determined by three-point bending tests. We found that the three investigated lines showed differences in their general bending strength in the non-stimulated condition. However, similar high range of bending strength values was measured for all plant lines after they underwent the mechanical stimulation procedure. The anatomy of internode cross-sections was examined to evaluate structure-property relationships. An increased thickness of the outer sclerenchymatous tissue was observed for internodes with higher bending strength values. Dried internodes fail under lower strains but showed higher bending strength. These findings show that mechanosensitivity in sorghum is dependent on genetic as well as environmental factors. The experimental system presented here offers new straight-forward possibilities for S. bicolor line selection for applications requiring mechanical strength of the stalk.

A microfluidic biosensor with surface acoustic wave technology was used in this study to monitor the interaction of calcium carbonate with standard carboxylate self-assembled monolayer sensor chips. Different fluids, with and without biomolecular components, were investigated. The pH-dependent surface interactions of two bio-inspired cationic peptides, AS8 and ES9, which are similar to an extracellular domain of the chitin synthase involved in mollusc shell formation, were also investigated in a biological buffer system. A range of experimental conditions are described that are suitable to study non-covalent molecular interactions in the presence of ionic substances, such as, mineral precursors below the solubility equilibrium. The peptide ES9, equal to the mollusc chitin synthase epitope, is less sensitive to changes in pH than its counterpart AS8 with a penta-lysine core, which lacks the flanking acidic residues. This study demonstrates the extraordinary potential of microfluidic surface acoustic wave biosensors to significantly expand our experimental capabilities for studying the principles underlying biomineralization in vitro.

High-resolution synchrotron X-ray powder diffraction (XRD) combined with the Rietveld refinement method and confocal laser scanning microscopy (CLSM) were utilized in this study to elucidate the interaction between a recombinant biomineralization protein (perlucin) fused to green fluorescent protein (GFP) and synthetic calcite. Although recombinant perlucin is insoluble, its solubility was increased via fusion to the highly soluble GFP. We demonstrate that GFP-perlucin derivatives become incorporated into the calcite structure and induce concentration-dependent anisotropic lattice distortions along the host?s c-axis. In contrast, GFP alone is hardly incorporated at all. The observed lattice distortions and peculiar microstructure of the crystals are comparable to those previously observed in biogenic calcite. Taking advantage of biotechnology to optimize individual protein properties, such as the solubility of an otherwise insoluble protein derivative, is a promising route toward the synthesis of new and improved biocomposite materials.

Electrical energy storage (EES) is one of the most critical areas of technological research around the world. Storing and efficiently using electricity generated by intermittent sources and the transition of our transportation fleet to electric drive depend fundamentally on the development of EES systems with high energy and power densities. Supercapacitors are promising devices for highly efficient energy storage and power management, yet they still suffer from moderate energy densities compared to batteries. To establish a detailed understanding of the science and technology of carbon/carbon supercapacitors, this review discusses the basic principles of the electrical double-layer (EDL), especially regarding the correlation between ion size/ion solvation and the pore size of porous carbon electrodes. We summarize the key aspects of various carbon materials synthesized for use in supercapacitors. With the objective of improving the energy density, the last two sections are dedicated to strategies to increase the capacitance by either introducing pseudocapacitive materials or by using novel electrolytes that allow to increasing the cell voltage. In particular, advances in ionic liquids, but also in the field of organic electrolytes, are discussed and electrode mass balancing is expanded because of its importance to create higher performance asymmetric electrochemical capacitors.

Nuclear magnetic resonance (NMR) spectroscopy is increasingly being used to study the adsorption of molecules in porous carbons, a process which underpins applications ranging from electrochemical energy storage to water purification. Here we present density functional theory (DFT) calculations of the nucleus-independent chemical shift (NICS) near various sp2-hybridized carbon fragments to explore the structural factors that may affect the resonance frequencies observed for adsorbed species. The domain size of the delocalized electron system affects the calculated NICSs, with larger domains giving rise to larger chemical shieldings. In slit-pores, overlap of the ring current effects from the pore walls is shown to increase the chemical shielding. Finally, curvature in the carbon sheets is shown to have a significant effect on the NICS. The trends observed are consistent with existing NMR results as well as new spectra presented for an electrolyte adsorbed on carbide-derived carbons prepared at different temperatures.

Template-based chemical vapor deposition is an efficient one step process to synthesize carbon nanotubes (CNTs) for a wide range of applications. In this process, the choice of template dictates certain physical features of the CNT, such as length and outer diameter, while the process itself affects other features, such as tube wall thickness, carbon deposition rate and carbon morphology. Although it is generally understood that the process affects important CNT properties, little is known about how parameters affect synthesized CNTs. In this report, a systematic parametric study was conducted to determine how three key process parameters (deposition time, temperature, and gas flow rate) affect overall carbon mass deposition rate and CNT wall thickness and morphology. The findings show that process parameters can be independently utilized to produce CNTs with similar or differing cross-sectional dimensions and other useful features, each with distinct advantages.

The expansion of porous carbons used in electrochemical double layer capacitors during positive or negative charging has been investigated in several studies, however, the underlying mechanisms are not well understood yet. We therefore investigated dimensional changes of three different carbons in various electrolytes including ionic liquids e.g. [EMIM][BF4] with and without solvent and [EMIM][TFSI]. For all carbon/electrolyte combinations the expansion during positive charging was smaller than for negative charging. The expansion increased with decreasing average pore size of the carbon. Addition of solvents, such as acetonitrile or propylene carbonate, significantly increased the expansion for negative charging. These observations are discussed in the view of various existing models for charge induced strain of carbons.

This study investigates carbons onions (∼400 m2g-1)as a conductive additive for supercapacitor electrodes of activated carbon and compares their performance with carbon black with high or low internal surface area. We provide a study of the electrical conductivity and electrochemical behavior between 2.5 and 20 mass% addition of each of these three additives to activated carbon. Structural characterization shows that the density of the resulting film electrodes depends on the degree of agglomeration and the amount of additive. Additions of low surface area carbon black (∼80 m2g-1) enhances the power handling of carbon electrodes but significantly lowers the specific capacitance even when adding small amounts of carbon black. A much lower decrease in specific capacitance is observed for carbon onions and the best values are seen for carbon black with a high surface area (∼1390 m2·g-1). The overall performance benefits from the addition of any of the studied additives only at either high scan rates and/or electrolytes with high ion mobility. Normalization to the volume shows a severe decrease in volumetric capacitance and only at high current densities nearing 10 A g-1 we can see an improvement of the electrode capacitance.

Herein we show that heating 2D Ti3C2 in air resulted in TiO2 nanocrystals on thin sheets of disordered graphitic carbon structure that can handle extremely high cycling rates when tested as anodes in lithium ion batteries. Oxidation of 2D Ti3C2 in either CO2 or pressurized water also resulted in TiO2/C hybrid structure. Similarly, other hybrids can be produced, as we show here for Nb2O5/C from 2D Nb2C.