Localization of the Surface Acoustic Longitudinal Pseudomode of Silicon In A Buried Sio2 Layer

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On the structural integrity of the container for a liquid metal spallation target under high power pulsed proton irradiation

Neutrons are an ideal probe for understanding the microscopic structure and dynamics of the matter and its behaviour. They are mainly produced by the fission chain reaction in reactors or by some accelerator-based reactions such as the spallation. An increase of the neutron flux of reactors for a better instrumental resolution is limited by heat transfer problems. Even if pulsed reactors may partially overcome this limits, a more effective way to produce neutrons seems to be the spallation reaction because the amount of energy released per available neutron is smaller by an order of magnitude. Profiting of the significant advances in the accelerator technology during the past 20 years, a new spallation source has been planned. The specifications given for the European spallation source (ESS), a 2 x 5 MW linear accelerator as the power source, two target stations with different pulse repetition rates: short pulse target station (SPTS) at 50 Hz repetition rate, 1 #mu#s proton pulse length, long pulse target station (LPTS) at 16_2_/_3 s"-"1 repetition rate, 2 ms proton pulse length, a peak neutron flux up to 2 x 10"1"7 n cm"-"2s"-"1 for the SPTS, will, besides assuring the availability of a general purpose neutron source for the research, also enlarge its actual application field. A liquid metal target appeared to be the best choice in order to fulfil the given specifications for the neutron production and lifetime. In order to identify and solve the problems connected with the structural integrity of the liquid metal target within the specified operative conditions the international ASTE (AGS spallation target experiment) collaboration was created. Within this collaboration a liquid mercury target with a simplified geometry was built. In different experiments which took place between 1997 and 2001 various efforts in order to measure relevant quantities as pressure, strain or temperature under realistic conditions were done. Considerable experience was gained concerning the experimental techniques necessary to measure such quantities in a highly radioactive environment. The finite elements simulations of the problem besides giving results in good agreement with the experimental strain data, provided a better insight as far as the pressure measurements in the mercury are concerned. The estimated maximum stress values under the ESS operative conditions in the first critical instants after the beam energy deposition are still within the elasticity limits for the materials under examination. Nevertheless, the modifications in the mechanical properties induced by the irradiation and also by the probable corrosion and cavitation need further investigations. (orig.)SIGLEAvailable from TIB Hannover: RA 831(4039) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

QWIP-LED Pixelless thermal imaging device

NRC publication: Ye

Dendritic Assembly of Gold Nanoparticles during Fuel-Forming Electrocatalysis

We observe the dendritic assembly of alkanethiol-capped gold nanoparticles on a glassy carbon support during electrochemical reduction of protons and CO2. We find that the primary mechanism by which surfactant-ligated gold nanoparticles lose surface area is by taking a random walk along the support, colliding with their neighbors, and fusing to form dendrites, a type of fractal aggregate. A random walk model reproduces the fractal dimensionality of the dendrites observed experimentally. The rate at which the dendrites form is strongly dependent on the solubility of the surfactant in the electrochemical double layer under the conditions of electrolysis. Since alkanethiolate surfactants reductively desorb at potentials close to the onset of CO2 reduction, they do not poison the catalytic activity of the gold nanoparticles. Although catalyst mobility is typically thought to be limited for room-temperature electrochemistry, our results demonstrate that nanoparticle mobility is significant under conditions at which they electrochemically catalyze gas evolution, even in the presence of a high surface area carbon and binder. A careful understanding of the electrolyte- and polarization-dependent nanoparticle aggregation kinetics informs strategies for maintaining catalyst dispersion during fuel-forming electrocatalysis