RESUMEN
The studies of polaronic centers in a homologous series of scheelite-type compounds AWO4 (A = Ca, Sr, Ba) were performed using the W L3-edge and Sr K-edge X-ray absorption spectroscopy combined with the reverse Monte Carlo simulations, X-ray photoelectron spectroscopy (XPS), and first-principles calculations. Protonated scheelites HxAWO4 were produced using acid electrolytes in a one-step route at ambient conditions. The underlying mechanism behind this phenomenon can be ascribed to the intercalation of H+ into the crystal structure of tungstate, effectively resulting in the reduction of W6+ to W5+, i.e., the formation of polaronic centers, and giving rise to a characteristic dark blue-purple color. The emergence of the W5+ was confirmed by XPS experiments. The relaxation of the local atomic structure around the W5+ polaronic center was determined from the analysis of the extended X-ray absorption fine structures using the reverse Monte Carlo method. The results obtained suggest the displacement of the W5+ ions from the center of [W5+O4] tetrahedra in the structure of AWO4 scheelite-type tungstates. This finding was also supported by the results of the first-principles calculations.
RESUMEN
Local distortions in perovskites can be induced by cation displacements and/or by the tilting and rotating of cation-anion octahedra. Both phenomena have been subject to intense investigations over many years. However, there are still controversies in the results obtained from experimental techniques that are sensitive to long-range order (X-ray, neutron, or electron diffraction) and those sensitive to short-range order (X-ray absorption spectroscopy). In this study, we probed the details of the local environment in AMoO3 perovskites (A = Ca, Sr, Ba) using extended X-ray absorption fine structure (EXAFS) in a wide temperature range (10-300 K). An advanced analysis of the EXAFS spectra within the multiple-scattering formalism using the reverse Monte Carlo method enhanced by an evolutionary algorithm allowed us (i) to extract detailed information on metal-oxygen and metal-metal radial distribution functions, and metal-oxygen-metal and oxygen-metal-oxygen bond angle distribution functions, and (ii) to perform polyhedral analysis. The obtained results demonstrate the strong sensitivity of the EXAFS spectra to the tilting of [MoO6] octahedra induced by the differences in the sizes of alkaline earth metal cations (Ca2+, Sr2+, and Ba2+).
RESUMEN
We present a heuristic procedure for determining key processing parameters (PPs) in materials-extrusion-based additive manufacturing processes. The concept relies on a design-of-experiment approach and consists of eleven "test objects" to determine the optimal combinations of key PPs values, starting with the PPs for printing the first layer and progressing to more complex geometric features, e.g., "bridges". In each of the test objects, several combinations of the known PPs' values are used, and only the values resulting in the best printed-part quality are selected for the following tests. The concept is intrinsically insensitive to different artefacts of the additive manufacturing machine (e.g., discrepancies between the nominal and actual nozzle diameters, and improper calibration of the feeding screws) and the optimal values of key PPs for manufacturing defect-free parts under the actual processing conditions can be determined. We validated the proposed procedure for two common commercial polymer feedstock materials, and we show that, by using the proposed procedure, it is possible to reduce the optimization time down to several hours, as well as to reduce the amount of consumed feedstock material. Tensile tests revealed a strong effect of amorphous and semi-crystalline nature of the polymer on the results of optimization. To the best of our knowledge, this is the first attempt to describe a systematic approach for optimizing PPs for materials extrusion-based additive manufacturing processes without relying on statistical data analysis or virtual simulations. The concept was implemented as a web-tool 3DOptimizer®.
RESUMEN
We investigate the influence of the annealing temperature on the evolution of the ice nanoclusters' geometry by means of low-temperature scanning tunneling microscopy. The clusters, grown at 110â K on Ag(100), gradually increase in height and their shape becomes more compact during annealing at 120â K, 125â K, and 130â K. The increase in height indicates an upward mass transport and reflects a stronger water-water than water-surface bonding. The change in shape, quantitatively expressed as an increase in fractal dimension, is driven by a reduction of the total energy of the step edge. The significant changes in geometry induced by a relatively mild temperature increase underline the importance of temperature for the shape and all properties influenced by this shape of hydrogen-bonded clusters of water ice.