Publikationen

Microporous carbon materials are widely used in gas storage, sorbents, supercapacitor electrodes, water desalination, and catalyst supports. While these microporous carbons usually have a particle size in the 1-100 μm range, here the synthesis of porous carbide-derived carbon (CDC) with particle diameters around 30 nm by extraction of titanium from nanometer-sized titanium carbide (TiC) powder at temperatures of 200 °C and above is reported. Nanometer-sized CDCs prepared at 200-400 °C show a disordered structure and the presence of CN sp1 bonds. Above 400 °C, the CN bond disappears with the structure transition to disordered carbon similar to that observed after synthesis from carbide micropowders. Compared to CDCs produced from micrometer-sized TiC, nano-CDC has a broader pore size distribution due to interparticle porosity and a large contribution from the surface layers. The material shows excellent electrochemical performance due to its easily accessible pores and a large specific surface area.

Desalination by capacitive deionization (CDI) is an emerging technology for the energy- and cost-efficient removal of ions from water by electrosorption in charged porous carbon electrodes. A variety of carbon materials, including activated carbons, templated carbons, carbon aerogels, and carbon nanotubes, have been studied as electrode materials for CDI. Using carbide-derived carbons (CDCs) with precisely tailored pore size distributions (PSD) of micro- and mesopores, we studied experimentally and theoretically the effect of pore architecture on salt electrosorption capacity and salt removal rate. Of the reported CDC-materials, ordered mesoporous silicon carbide-derived carbon (OM SiC-CDC), with a bimodal distribution of pore sizes at 1 and 4 nm, shows the highest salt electrosorption capacity per unit mass, namely 15.0 mg of NaCl per 1 g of porous carbon in both electrodes at a cell voltage of 1.2 V (12.8 mg per 1 g of total electrode mass). We present a method to quantify the influence of each pore size increment on desalination performance in CDI by correlating the PSD with desalination performance. We obtain a high correlation when assuming the ion adsorption capacity to increase sharply for pore sizes below one nanometer, in line with previous observations for CDI and for electrical double layer capacitors, but in contrast to the commonly held view about CDI that mesopores are required to avoid electrical double layer overlap. To quantify the dynamics of CDI, we develop a two-dimensional porous electrode modified Donnan model. For two of the tested materials, both containing a fair degree of mesopores (while the total electrode porosity is [similar]95 vol%), the model describes data for the accumulation rate of charge (current) and salt accumulation very well, and also accurately reproduces the effect of an increase in electrode thickness. However, for TiC-CDC with hardly any mesopores, and with a lower total porosity, the current is underestimated. Calculation results show that a material with higher electrode porosity is not necessarily responding faster, as more porosity also implies longer transport pathways across the electrode. Our work highlights that a direct prediction of CDI performance both for equilibrium and dynamics can be achieved based on the PSD and knowledge of the geometrical structure of the electrodes.

Porous carbon electrodes have significant potential for energy-efficient water desalination using a promising technology called Capacitive Deionization (CDI). In CDI, salt ions are removed from brackish water upon applying an electrical voltage difference between two porous electrodes, in which the ions will be temporarily immobilized. These electrodes are made of porous carbons optimized for salt storage capacity and ion and electron transport. We review the science and technology of CDI and describe the range of possible electrode materials and the various approaches to the testing of materials and devices. We summarize the range of options for CDI-designs and possible operational modes, and describe the various theoretical–conceptual approaches to understand the phenomenon of CDI.

Self oscillations in homogeneous chemical systems have been studied extensively in diverse array of systems ranging from flow and non-flow, stirred and unstirred, for non-linear chemical reactions coupled with or without the effect of diffusion and multiple systems exhibiting synchronized oscillations. There are however not many examples in which specific geometric features of the reaction chamber turns an otherwise stable system oscillatory, importantly at very low Reynolds number regime. Here we describe such a phenomenon in regard to a microfluidic fuel cell for which the cell potential turns oscillatory when its smooth wall is replaced by one patterned with microscopic features. In essence, we have a membraneless fuel cell in which fuel and oxidant streams flow in laminar contact, side by side, forming a sharp interface. For smooth surface of the channel wall, with steady flow of fuel and oxidant, the cell generates a constant open circuit potential (OCP). However, when the surface of the channel is patterned with parallel triangular ridges, although the flow continues to remain temporally steady, time invariant oscillating OCP ensues within a small window of very low Reynolds number. We have presented a mechanism for occurrence of these oscillations.

Polymers capable of dynamic bonding/debonding reactions are of great interest in modern day research. Potential applications can be found in the fields of self-healing materials or printable networks. Since temperature is often used as a stimulus for triggering reversible bonding reactions, an analysis operating at elevated temperatures is very useful for the in situ investigation of the reaction mechanism, as unwanted side effects can be minimized when performing the analyses at the same temperature at which the reactions occur. A temperature-dependent size exclusion chromatographic system (TD SEC) has been optimized for investigating the kinetics of retro Diels−Alder-based depolymerization of Diels−Alder polymers. The changing molecular weight distribution of the analyzed polymers during depolymerization gives valuable quantitative information on the kinetics of the reactions. Adequate data interpretation methods were developed for the correct evaluation of the chromatograms. The results are confirmed by high-temperature dynamic light scattering, thermogravimetric analysis, and time-resolved nuclear magnetic resonance spectroscopy at high temperatures. In addition, the SEC system and column material stability under application conditions were assessed using thermoanalysis methods, infrared spectroscopy, nitrogen physisorption, and scanning electron microscopy. The findings demonstrate that the system is stable and, thus, we can reliably characterize such dynamically bonding/debonding systems with TD SEC.

Force-displacement measurements were performed on single macroscopic polydimethylsiloxane pillars. The pillars had diameters of 400 μm, aspect ratios between 1 and 5, and spherical, flat, and mushroom shaped tips. We present a method to derive numerous parameters, e.g. stress, strain, elastic modulus, and work of separation from these force-displacement measurements. We also show that the deformation of the pillar, their tips and the backing layers can be separately identified from the same experiments. This method can help to better characterize the mechanical properties of polymeric microstructures using a simple experimental approach.

We tested the adhesive response of polymer surfaces structured with arrays of cylindrical fibrils having diameters of 10-20 µm and aspect ratios 1-2.4. Fibrils had two different tip shapes of end-flaps and round edges. A preload-induced mechanical buckling instability of the fibrils was used to switch between the states of adhesion and non-adhesion. Non-adhesion in fibrils with round edges was reached at preloads that caused fibril buckling, whereas fibrils with end-flaps showed adhesion loss only at very high preloads. The round edge acted as a circumferential flaw prohibiting smooth tip contact recovery leading to an adhesion loss. In situ observations showed that, after reversal of buckling, the end-flaps unfold and re-form contact under prevailing compressive stress, retaining adhesion in spite of buckling. At very high preloads, however, end-flaps are unable to re-form contact resulting in adhesion loss. Additionally, the end-flaps showed varying contact adaptability as a function of the fibril-probe alignment, which further affects the preload for adhesion loss. The combined influence of preload, tip shape and alignment on adhesion can be used to switch adhesion in bioinspired fibrillar arrays.

Das INM – Leibniz-Institut für Neue Materialien in Saarbrücken erforscht und entwickelt neue Materialien vom Molekül bis zur Pilotfertigung. Nach einer externen Evaluierung im Jahre 2005 wurde das Institut als modernes, interdisziplinäres Forschungszentrum neu gestaltet, wobei auch strukturell neue Wege gegangen wurden. Der Artikel beschreibt diese Konzepte und deren erfolgreiche Umsetzung in den letzten Jahren. Entscheidend waren die internationale Rekrutierung von wissenschaftlichem Personal, die stärkere Einbeziehung junger Forscherinnen und Forscher, die Schaffung einer flachen Hierarchie sowie die Umsetzung wirksamer Kommunikationsstrategien. Die Balance zwischen hervorragender Grundlagenforschung und indus trieller Anwendung ist einem dauernden Optimierungsprozess unterworfen. Als in der Rechtsform einer gemeinnützigen GmbH geführte, selbstständige Einrichtung verfügt das INM über einen kaufmännischen Bereich, dessen Strukturen und Prozesse zur Unterstützung der Forschung, der Projektakquise sowie der Verwertung ebenfalls neu aufgestellt wurden und sich damit an den Anforderungen des heutigen Wissenschaftsmanagements orientiert.

Nanoparticles of heavy materials such as gold can be used as markers in quantitative electron microscopic studies of protein distributions in cells with nanometer spatial resolution. Studying nanoparticles within the context of cells is also relevant for nanotoxicological research. Here, we report a method to quantify the locations and the number of nanoparticles, and of clusters of nanoparticles inside whole eukaryotic cells in three dimensions using scanning transmission electron microscopy (STEM) tomography. Whole-mount fixed cellular samples were prepared, avoiding sectioning or slicing. The level of membrane staining was kept much lower than is common practice in transmission electron microscopy (TEM), such that the nanoparticles could be detected throughout the entire cellular thickness. Tilt-series were recorded with a limited tilt-range of 80° thereby preventing excessive beam broadening occurring at higher tilt angles. The 3D locations of the nanoparticles were nevertheless determined with high precision using computation. The obtained information differed from that obtained with conventional TEM tomography data since the nanoparticles were highlighted while only faint contrast was obtained on the cellular material. Similar as in fluorescence microscopy, a particular set of labels can be studied. This method was applied to study the fate of sequentially up-taken low-density lipoprotein (LDL) conjugated to gold nanoparticles in macrophages. Analysis of a 3D reconstruction revealed that newly up-taken LDL-gold was delivered to lysosomes containing previously up-taken LDL-gold thereby forming onion-like clusters.