28,057 research outputs found

    Effects of Surface Roughness on the Electrochemical Reduction of CO₂ over Cu

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    We have investigated the role of surface roughening on the CO₂ reduction reaction (CO₂RR) over Cu. The activity and product selectivity of Cu surfaces roughened by plasma pretreatment in Ar, O₂, or N₂ were compared with that of electrochemically polished Cu samples. Differences in total and product current densities, the ratio of current densities for HER (the hydrogen evolution reaction) to CO₂RR, and the ratio of current densities for C₂₊ to C₁ products depend on the electrochemically active surface and are nearly independent of plasma composition. Theoretical analysis of an electropolished and roughened Cu surface reveals a higher fraction of undercoordinated Cu sites on the roughened surface, sites that bind CO preferentially. Roughened surfaces also contain square sites similar to those on a Cu(100) surface but with neighboring step sites, which adsorb OC–COH, a precursor to C₂₊ products. These findings explain the increases in the formation of oxygenates and hydrocarbons relative to CO and the ratio of oxygenates to hydrocarbons observed with increasing surface roughness

    Suppression of Subsynchronous Vibration in the SSME HPFTP

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    Space Shuttle Main Engine (SSME) High Pressure Fuel Turbopump (HPFTP) hot-fire dynamic data evaluation and rotordynamic analysis both confirm that two of the most significant turbopump attributes in determining susceptibility to subsynchronous vibration are impeller interstage seal configuration and rotor sideload resulting from turbine turnaround duct configuration and hot gas manifold. Recent hot-fire testing has provided promising indications that the incorporation of roughened damping seals at the impeller interstages may further increase the stability margin of this machine. A summary of the analysis which led to the conclusion that roughened seals would enhance the stability margin is presented along with a correlation of the analysis with recent test data

    Selective tube roughening increases heat transfer capability

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    Selectively roughening inside surfaces of tubes increases the heat transfer capabilities, but minimizes the pressure drop. This technique is used to construct roughened test sections for hydrogen heat transfer studies

    On the Friction Coefficient for Turbulent Flow ThroughSectionally Roughened Square Ducts

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    The friction coefficient of sectionally rough pipes has not been studied yet. The friction coefficient for turbulent flow through sectionally roughened square ducts is experimentally studied. Four arrangements of rough surfaces are used to obtain the sectionally roughened square ducts. It is attempted to predict the friction coefficient for sectionally roughened square ducts from the friction coefficients for alloverly roughened and smooth square ducts

    Influence of Annealing on the Optical and Scintillation Properties of CaWO4_4 Single Crystals

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    We investigate the influence of oxygen annealing on the room temperature optical and scintillation properties of CaWO4_4 single crystals that are being produced for direct Dark Matter search experiments. The applied annealing procedure reduces the absorption coefficient at the peak position of the scintillation spectrum (430\sim430 nm) by a factor of 6\sim6 and leads to an even larger reduction of the scattering coefficient. Furthermore, the annealing has no significant influence on the \emph{intrinsic} light yield. An additional absorption occurring at 400\sim400 nm suggests the formation of O^- hole centers. Light-yield measurements at room temperature where one crystal surface was mechanically roughened showed an increase of the \emph{measured} light yield by 40\sim40 % and an improvement of the energy resolution at 59.5 keV by 12\sim12 % for the annealed crystal. We ascribe this result to the reduction of the absorption coefficient while the surface roughening is needed to compensate for the also observed reduction of the scattering coefficient after annealing

    3D modelling of enhanced surface emission using surface roughening

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    Damping seal for turbomachinery

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    A damping seal between a high speed rotor member and stator member that separates pressurized fluid compartments is described. It is characterized by the rotor member having a smooth outer surface and the stator member having its bore surface roughened by a plurality of pockets or depressions
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