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A laser-operated, angle-tunable transducer was employed to excite selectively elastic waves guided along the apex of a solid wedge. The propagation of wedge waves at anisotropic monocrystalline silicon edges with different symmetry properties was studied by optical detection. The reduced symmetry in crystals, as compared to isotropic media, causes a number of new features, such as the existence of supersonic leaky wedge waves, tilted spatial pulse profiles, and other peculiarities of their localization. Experimental and theoretical results are presented for three different types of symmetry configurations: the wedge symmetric about its midplane, the wedge symmetric about the plane normal to its apex line, and the wedge symmetric about one of its faces. The experiments include accurate measurements of the phase velocity and the wave field distribution, providing information on localization and coupling of wedge waves with other waves. Theoretically, the wedge waves were treated by the Laguerre function method, extended to modes that are not localized at the tip of the wedge. This approach allowed an accurate description of the observed localized and leaky wedge waves in anisotropic wedges.
Pure component sorption isotherms of n-butane, isobutane, 1-butene and isobutene on the metal–organic framework (MOF) 3∞[Cu4(μ4-O)(μ2-OH)2(Me2trz-pba)4] at various temperatures between 283 K and 343 K and pressures up to 300 kPa are presented. The isotherms show a stepwise pore filling which is typical for structurally flexible materials with broad adsorption–desorption hysteresis loops. Gate opening pressures in their endemic characteristic depend on the used hydrocarbon gases. From all investigated gases only the isotherms of 1-butene present a second step at a relative pressure above p/p0 = 0.55. As a consequence, only 1-butene can fully open the framework resulting in a pore volume of 0.54 cm3 g−1. This result is in good agreement with the value of 0.59 cm3 g−1 calculated based on single crystal structure data. The isosteric heat of adsorption was calculated from the experimental isotherms for all C4-isomers. At low loadings the isosteric heat is in a narrow region between 41 and 49 kJ mol−1. Moreover, in situ XRD measurements at different relative hydrocarbon pressures were performed at 298 K for the C4-isomers. The differences in the pressure-depending powder diffraction patterns indicate phase transitions as a result of adsorption. Similar diffraction patterns were observed for all C4-hydrocarbons, except 1-butene, where the second step at higher relative pressure (p/p0 > 0.55) is accompanied by an additional phase transition. This powder pattern resembles that of the as-synthesized MOF material containing solvent molecules in the pore system. The resulting structural changes of the material during guest and pressure induced external stimuli are evidenced by the new coupled XRD adsorption equipment.
Private households constitute a considerable share of Europe's electricity consumption. The current electricity distribution system treats them as effectively passive individual units. In the future, however, users of the electricity grid will be involved more actively in the grid operation and can become part of intelligent networked collaborations. They can then contribute the demand and supply flexibility that they dispose of and, as a result, help to better integrate renewable energy in-feed into the distribution grids.
A wide range catalyst screening with noble metal and oxide catalysts for a metal–air battery with an aqueous alkaline electrolyte was carried out. Suitable catalysts reduce overpotentials during the charge and discharge process, and therefore improve the round-trip efficiency of the battery. In this case, the electrodes will be used as optimized cathodes for a future lithium–air battery with an aqueous alkaline electrolyte. Oxide catalysts were synthesized via atmospheric plasma spraying. The screening showed that IrO2, RuO2, La0.6Ca0.4Co3, Mn3O4, and Co3O4 are promising bi-functional catalysts. Considering the high price for the noble metal catalysts further investigations of the oxide catalysts were carried out to analyze their electrochemical behavior at varied temperatures, molarities, and in case of La1−x Ca x CoO3 a varying calcium content. Additionally all catalysts were tested in a longterm test to proof cyclability at varied molarities. Further investigations showed that Co3O4 seems to be the most promising bi-functional catalyst of the tested oxide catalysts. Furthermore, it was shown that a calcium content of x = 0.4 in LCCO has the best performance.
Aqueous lithium–oxygen batteries are promising candidates for electric energy storage. In this paper we present and discuss a multiphase continuum model of an aqueous lithium–oxygen single cell including reactions and transport in a porous gas diffusion electrode (GDE). The model is parameterized using in-house half-cell experiments and available literature data on aqueous electrolytes. We validate our transport model with cyclic voltammetry and electrochemical impedance spectroscopy measurements over a wide range of temperatures (25, 40, 55 °C) and electrolyte concentrations (0.1–2 M). We observe very good agreement between simulations and measurements during oxygen reduction conditions. A sensitivity analysis of the validated model demonstrates the influence of the porous structure on GDE performance and gives directions for the future development of electrodes.
Packed beds serve as thermal energy storages (TES) and heat exchangers (HEX) in different technological applications. In this paper, a general heterogeneous model of heat transfer in packed beds is developed. It is implemented by lumped element formulation in object-oriented modeling language Modelica and is successful validated with data sets taken from two different experiments reported in literature.
The main advantages of the introduced model are the general, theory-based approach and the lumped element formulation in Modelica. The first point mentioned above should allow to simulate a packed bed TES/HEX without the necessity applying measured data for model calibration or to apply specific heat transfer correlations with restricted application. The second point establishes the possibility to integrate the TES/HEX model within plant models of larger scale without increasing the simulation time drastically.