Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Eingeladener Vortrag
- nein (1)
Absorption edge tomography is a method which exploits the sudden change of the attenuation coefficient, when the photon energy crosses the absorption edge of an element. The beamline BAMline at BESSY II, which is operated by the Federal Institute for Materials Research and Testing, can provide a monochromatized beam in a photon energy range from 5 keV up to 80 keV with a bandwidth of 2%. Together with the microtomography setup, this enables differential tomography sensitive to any element with N >= 24 (Cr) by using an appropriate K- or L-edge in this range. Here, absorption edge tomography at the Yttrium edge is employed to perform a non-destructive 3D characterization of the microstructure of a high strength Mg-Y-Zn alloy. The long period stacking ordered (LPSO) phase which forms fibres in this material was extracted based on the Yttrium content and the fibre length distribution was analysed.
Mechanochemistry is increasingly used for synthesizing soft matter materials including metal organic compounds and cocrystals. The ever-increasing interest in this method is contrasted by a limited mechanistic understanding of the mechanochemical reactivity and selectivity. Time-resolved in situ investigations of milling reactions provide direct insights in the underlying mechanisms. We recently introduced a setup enabling in situ investigation of mechanochemical reactions using synchrotron XRD combined with Raman spectroscopy. The specific combination allows to study milling processes comprehensively on the level of the molecular and crystalline structure and thus obtaining reliable data for mechanistic studies. In situ Raman spectroscopy coupled with thermography revealed a low temperature increase during milling reactions due to the mechanical impact and clear temperature increases because of the reaction heat.
Based on the data, temperature rises as postulated in the magma plasma and hot spot theory can be excluded for soft matter milling syntheses. Our results indicate that in situ investigation of milling reactions offer a new approach to tune and optimize mechanochemically synthesized compounds.
In this study, series of coatings with the goal of sensitising gold SPR sensors towards CO, NH4, etc. were prepared. To better understand the mechanism behind the sensitizing effect and to enable the targeted production of optimized layers, the wide range of coatings was studied by multiple methods from spectroscopic ellipsometry for their dielectric function, by XPS for chemical composition and SEM, TEM to determine their microscopic structure. The coatings were deposited at inert Ar gas and reactive Ar/O2 gas mixture by RF magnetron sputtering or DC pulsed magnetron sputtering with settable RF bias on the substrates. The plasma processes were monitors by mass spectrometry.
The metal oxide coatings in SPR sensors have to be prepared reproducible with thickness about 5 nm therefore lower deposition rate conditions were advantages.
Ever since increasing a reaction’s yield while shortening the reaction time is the main objective in synthesis optimization. Microwave reactors meet these demands. In literature however their usage is under discussion due to claims of the existence of non-thermal effects resulting from the microwave radiation. Especially for nano-material syntheses it is of crucial importance to be aware of influences on the reaction pathway. Therefore, we compare ultra-small silver nanoparticles with mean radii of 3 nm, synthesized via conventional and microwave heating. We employed a versatile one-pot polyol synthesis of poly(acrylic acid) (PAA) stabilized silver nanoparticles, which display superior catalytic properties. No microwave specific effects in terms of particle size distribution characteristics, as derived by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS), are revealed. Due to the microwave reactor’s characteristics of a closed system, syntheses can be carried out at temperatures beyond the solvent’s boiling point. Particle formation was accelerated by a factor of 30 by increasing the reaction temperature from 200 °C to 250 °C. The particle growth process follows a cluster coalescence mechanism. A post-synthetic incubation step at 250 °C induces a further growth of the particles while the size distribution broadens. Thus, utilization of microwave reactors enables an enormous decrease of the reaction time as well as the opportunity of tuning the particles’ size. Possibly, decomposition of the stabilizing ligand at elevated temperatures results in reduced yields. A temperature of 250 °C and a corresponding reaction time of 30 s represent a compromise between short reaction times and high yields.
Silver nanoparticles are one of the most widespread consumer related nanoparticles worldwide. Since the particles show special optical and antibacterial properties they are used for a wide range of applications from biological investigations over medical applications and catalysis. Especially the outstanding question of applicable alternatives for catalysts in diverse reactions can be addressed with the design of versatile system of small silver nanoparticles. In this study we present the synthesis and application of ultra-small silver nanoparticles with a narrow size distribution (R = 3.1 nm, σ = 0.6 nm). The particles are thoroughly characterized by small angle X-ray scattering, dynamic light scattering and UV/Vis spectroscopy. As a representative test reaction the reduction of 4-nitrophenol to 4-aminophenol was chosen. The particles show a catalytic activity of (436 ± 24) L g-1 s-1, which is two orders of magnitude higher than for other silver particles in the literature. The particles surrounding shell, composed of poly(acrylic acid), provides the particles with a good accessibility for the reactants. Since the catalytic activity strongly depends on the surrounding ligand, the particles shell can also be exchanged by other ligands enabling a tuning of the catalytic activity to a desired value. This shows the high flexibility of this system which can also be applied for other catalytic reactions.
During Rutherford cable production the wires are plastically deformed and their initially round shape is distorted. Using X-ray absorption tomography we have determined the 3D shape of an unreacted Nb3Sn 11 T dipole Rutherford cable, and of a reacted and impregnated Nb3Sn cable double stack. State-of-theart image processing was applied to correct for tomographic artefacts caused by the large cable aspect ratio, for the segmentation of the individual wires and subelement bundles inside the wires, and for the calculation of the wire cross sectional
area and shape variations. The 11 T dipole cable cross section oscillates by 2% with a frequency of 1.24 mm (1/80 of the transposition pitch length of the 40 wire cable). A comparatively stronger cross sectional area variation is observed in the individual wires at the thin edge of the keystoned cable where the wire aspect ratio is largest.
Influence of deposition hatch length on residual stress in selective laser melted Inconel 718
(2017)
Additive Manufacturing (AM) by Selective Laser Melting (SLM) offers ample scope for producing geometrically complex parts in comparison to the traditional subtractive manufacturing strategies. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
Damage Analysis in Metal Matrix Composites by means of Synchrotron Radiation Computed Tomography
(2017)
The damage evolution after compression tests of two types of MMC, consisting of eutectic AlSi12CuMgNi alloy and reinforced with 15vol% of Al2O3 fibers and with 7vol% of Al2O3 fibers+15vol% of SiC particles was studied by synchrotron CT. Internal damage at different pre-strain conditions in eutectic Si, intermetallic phases and Al2O3 fibers was observed, as well as debonding of SiC particles.
Multielectrodes are arrays of single electrodes arranged in a particular geometry. In our work, all the single electrodes are identical stainless steel X5CrNi18-10 (1.4301) wire electrodes. Using a multielectrode analyser all single electrodes are connected via zero resistance ammeters, simulating a galvanically coupled single electrode surface. The advantage of the multielectrode analyser (MMA) is that the currents flowing between single electrodes can be measured. Thus, real-time maps can be generated indicating where anodic and cathodic areas lie on the surface of the multielectrode and how they behave.
The combination of the multielectrode analyser with the scanning electrochemical microscopy (SECM) enables the identification of corrosion sites and the detailed electrochemical analysis.