Laser Nitriding of Iron,Stainless Steel,and Plain Carbon Steel Investigated by Mössbauer Spectroscopy

Nitriding is a common method for improving the hardness, mechanical properties, wear and corrosion resistance of metals. Laser nitriding of metals is an efficient process, where the irradiation of surfaces in air or nitrogen atmospheres with short laser pulses leads to a fast take-up of nitrogen into the irradiated surfaces. This process has been extensively investigated for pure iron, but usually, no tools or functional parts are made of pure iron. Mainly steel or cast iron is used as a base material. Therefore, when looking for technical applicability, also the influence of alloying elements on the laser nitriding process is of great interest. Besides the pure iron various carbon steels and an austenitic stainless steel were studied in laser nitriding experiments in order to investigate the influence of the material itself. Here, the process is investigated via Conversion Electron and X-ray Mössbauer Spectroscopy (CEMS and CXMS), Resonant Nuclear Reaction Analysis (RNRA), and X-Ray Diffraction (XRD). It appears that carbon steels are even better suited for the laser nitriding process than pure iron, and the laser nitriding also works efficiently for the stainless steel which is normally difficult to be nitrided.     

Chemical force titration of plasma polymer-modified PDMS substrates by using plasma polymer-modified AFM tips

Plasma polymerization has gained increasing attention in surface functionalization. We use here chemical force titration to characterize PDMS (polydimethylsiloxane) substrates modified by maleic anhydride-pulsed plasma polymerization. The coating is hydrolyzed to promote the formation of dicarboxylic acid groups. To enhance the variation of the adhesion forces as a function of pH, we use AFM tips modified in the same way as the substrates. The pH-dependent adhesion measurements are performed at different KCl concentrations. The dicarboxylic nature of the maleic acid groups clearly emerges from the force titration curves. The surface pK(a) values (pK(a1) = 3.5 +/- 0.5 and pK(a2) = 9.5 +/- 0.5) of the dicarboxylic acids are evaluated from low electrolyte concentration solutions. The values are shifted toward higher pK(a) values when compared to maleic acid in solution. The first pK(a) appears in the titration force curve for low salt concentration as a peak. This peak changes to a sigmoidal shape at higher salt concentrations. The appearance of a peak is attributed to the formation of strong hydrogen bonds between the tip and the substrate as reported in the literature. The effect of the ionic strength on the force curves is explained by the condensation of counterions on the carboxylate groups. At high pH, the adhesion force almost vanishes. On the approach, at high pH, one first observes repulsion between the tip and the substrate, which varies exponentially with the tip/substrate distance. The decay length of this repulsion force is in good agreement with theoretical predictions of the Debye length, attesting to the electrostatic nature of the interactions. We also find that the replacement of monovalent cation K(+) by the divalent cation Ca(2+) leads to significant changes in the force titration curve at high pH where the dicarboxylic groups are fully ionized. We observe that the adhesion force no longer vanishes at high pH but even slightly increases with pH, an effect that is explained by Ca(2+) ions bridging between two carboxylate groups.     

Exfoliated 2D Layered and Nonlayered Metal Phosphorous Trichalcogenides Nanosheets as Promising Electrocatalysts for CO2 Reduction

Two-dimensional (2D) materials catalysts provide an atomic-scale view on a fascinating arena for understanding the mechanism of electrocatalytic carbon dioxide reduction (CO₂ ECR). Here, we successfully exfoliated both layered and nonlayered ultra-thin metal phosphorous trichalcogenides (MPCh₃) nanosheets via wet grinding exfoliation (WGE), and systematically investigated the mechanism of MPCh₃ as catalysts for CO₂ ECR. Unlike the layered CoPS₃ and NiPS₃ nanosheets, the active Sn atoms tend to be exposed on the surfaces of nonlayered SnPS₃ nanosheets. Correspondingly, the nonlayered SnPS₃ nanosheets exhibit clearly improved catalytic activity, showing formic acid selectivity up to 31.6 % with −7.51 mA cm⁻² at −0.65 V vs. RHE. The enhanced catalytic performance can be attributed to the formation of HCOO* via the first proton-electron pair addition on the SnPS₃ surface. These results provide a new avenue to understand the novel CO₂ ECR mechanism of Sn-based and MPCh₃-based catalysts.     

Reversible Soft Mechanochemical Control of Biaryl Conformations through Crosslinking in a 3D Macromolecular Network

Tuning the dihedral angle (DA) of axially chiral compounds can impact biological activity, catalyst efficiency, molecular motor performance, or chiroptical properties. Herein, we report gradual, controlled, and reversible changes in molecular conformation of a covalently linked binaphthyl moiety within a 3D polymeric network by application of a macroscopic stretching force. We managed direct observation of DA changes by measuring the circular dichroism signal of an optically pure BINOL-crosslinked elastomer network. Stretching the elastomer resulted in a widening of the DA between naphthyl rings when the BINOL was doubly grafted to the elastomer network; no effect was observed when a single naphthyl ring of the BINOL was grafted to the elastomer network. We have determined that ca. 170 % extension of the elastomers led to the transfer of a mechanical force to the BINOL moiety of 2.5 kcal mol⁻¹ Å⁻¹ (ca. 175 pN) in magnitude and results in the opening of the DA of BINOL up to 130°.     

PB- Und Sr-Isotopensignaturen AlS Herkunftsindikatoren Für Anthropogene Und Geogene Kontaminationen

Abstract

In isotope geochemistry, natural differences in isotope abundance ratios of heavy elements (e.g. Sr, Nd, Pb) allow the use of specific isotopic signatures as tracers for these and genetically related elements. Examples of such applications in the field of anthropogeochemistry will be presented for lead and strontium.     

Supported Catalytically Active Supramolecular Hydrogels for Continuous Flow Chemistry

Inspired by biology, one current goal in supramolecular chemistry is to control the emergence of new functionalities arising from the self‐assembly of molecules. In particular, some peptides can self‐assemble and generate exceptionally catalytically active fibrous networks able to underpin hydrogels. Unfortunately, the mechanical fragility of these materials is incompatible with process developments, relaying this exciting field to academic curiosity. Here, we show that this drawback can be circumvented by enzyme‐assisted self‐assembly of peptides initiated at the walls of a supporting porous material. We applied this strategy to grow an esterase‐like catalytically active supramolecular hydrogel (CASH) in an open‐cell polymer foam, filling the whole interior space. Our supported CASH material is highly efficient towards inactivated esters and enables the kinetic resolution of racemates. This hybrid material is robust enough to be used in continuous flow reactors, and is reusable and stable over months.     

Simultaneous Triple Radiation Mössbauer Spectroscopy (STRMS)

The setup for Simultaneous Triple Radiation Mössbauer Spectroscopy (STRMS) is described. The arrangement allows an independent and simultaneous recording of conversion electron Mössbauer spectra (CEMS) and of conversion X-ray Mössbauer spectra (CXMS), both in backscattering geometry, and-in addition-of γ-ray absorption spectra in transmission (TMS). Due to the different escape or penetration ranges of the three radiations involved, the spectra give information on phases, depth and orientation. From a practical point of view the counter for γ-rays, X-rays and electrons must be separated and shielded to minimize the mutual perturbation.     

Ion-beam mixing in Fe/Si bilayers by singly and highly charged ions: evolution of phases,spike mechanism and possible effects of the ion-charge state

Applied Physics A - Silicide formation and ion beam mixing of Fe/Si bilayers due to Ar-, Xe- and Au-ion irradiations at room and liquid-nitrogen temperatures were investigated. For the study of...     

Surface investigation of excimer laser irradiated phosphatized steel plates

Steel plates (St 14-05) of 1.5 mm thickness and coated with 1.5 m of ironzinc-phosphatehydrate (ASTM 29-1429) were irradiated with an XeCl-excimer laser (Siemens XP 2020) at energy densities of 20–80 mJ/mm² and with 2–32 pulses per spot. Depth-sensitive Mössbauer spectroscopy was carried out by means of conversion electron (CEMS) and conversion X-ray (CXMS) Mössbauer spectroscopy in order to determine the phases produced by the excimer laser treatment. Although the phosphate layer is mainly ablated during the laser treatment, there is a significant formation of Fe₂P. The phosphorous phase and the wüstite, with changing stoichiometries, were found in the very surface (CEMS). In deeper layers and in correlation with the energy density and the number of pulses, austenite was found in surprisingly high amounts (CEMS and CXMS). The austenite was found to be nitrogen austenite. The high Fe-N austenite content as well as the presence of some ferromagnetic Fe-N phase (-Fe_2+xN) must be ascribed to an unexpectedly high nitriding effect during the laser treatment.