Search

We study PdPt bimetallic clusters in both free and supported phases. These clusters have been produced with a laser vaporization source. Free clusters directly produced by the source are studied by time of flight mass spectrometry and photofragmentation technique. We observed a sequential evaporation of Pd atoms in the mixed clusters consistent with a palladium segregation process. This tendency has been also observed on supported particles from which the structure and the composition are determined by high resolution transmission electron microscopy and energy dispersive x-ray analysis. A main result is that each particle has the composition of the massic rod vaporized in the source. The supported particles are well crystallized and exhibit truncated octahedron shapes. Experimental observations are well explained using a modified tight binding model. Indeed, within this model, we found that the equilibrium shape is strongly related to the variation of the cohesive energy with atomic coordination number. Also, some preliminary results on the specific reactivity of these bimetallic clusters are presented. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Low-frequency inelastic light scattering of water is observed from a temperature equal to 80 down to -20 °C in the supercooled regime. For energies higher than 3 cm-1 it is shown that the major part of the light scattering is Raman scattering. A broadband with a maximum at 50 cm-1 is interpreted as scattering from transverse acoustic modes of a disordered network. A weaker scattering which shifts towards the Rayleigh line when the temperature decreases is well explained by Raman scattering from the oscillations of water molecule aggregates which grow when the temperature decreases. [ABSTRACT FROM AUTHOR]

Major challenges in the area of wet transportation of radioactive materials are reliability and safety of transportation casks. In most cases, the bottom part of the cask is filled with water whereas a gaseous mixture is contained in the upper part. In such a configuration, water radiolysis leads to the formation of hydrogen and oxygen, which continuously enriches the gaseous mixture. Among the functions to be satisfied, wet transportation systems shall thus allow the control of the hydrogen content below its flammability limit. This is currently achieved by limiting the transportation duration so as to reopen the cask before the critical hydrogen concentration is reached. Development of new technologies that would mitigate the hydrogen risk is all the more motivated because it would allow an extension of the transportation duration. AREVA-TN International and the Institut de recherches sur la catalyse et l'environnement de Lyon have developed a catalytic system which aims at buffering the hydrogen concentration far below the flammability limit. The principle of this catalyst is to recombine the hydrogen with the oxygen formed by water radiolysis. The present paper gives an overview of the development of this catalytic recombining system. It describes the laboratory qualification tests undertaken for the evaluation of the recombining efficiency. Particular attention is placed on the recombining efficiency after immersion of the catalyst in borated water, which would occur in a nuclear reactor pool during loading of used fuel. Laboratory investigations, carried out in an autoclave simulating a transportation cask, showed that, after immersion in borated water, the catalytic system allows the recombination of 3% hydrogen in less than 24 h at temperatures as low as 35°C. [ABSTRACT FROM AUTHOR]

The optical properties of mixed (Ni0.5Ag0.5)n and Pt/Ag clusters are investigated in the size range 2-5 nm. Low Energy Ion Spectroscopy (LEIS) experiments show that the cluster surface is entirely covered by silver atoms for the two systems. The optical spectra of Ni/Ag clusters exhibit a large Surface Plasmon Resonance (SPR), damped and widened when the cluster size decreases, in agreement with a classical model assuming a core-shell geometry and including the reduction of the conduction electron mean-free path in the silver shell. For Pt/Ag clusters, no SPR emerges in the size range 2-5 nm, although it is predicted within a classical model, a pronounced SPR appearing only for clusters larger than 10 nm in diameter. [ABSTRACT FROM AUTHOR]

Optical properties of mixed clusters (AuxAg 1 - x ) n and (NixAg 1 - x ) n, produced by laser vaporization and embedded in an alumina matrix, are reported. The size effects are investigated for different concentrations ( x = 0.25, 0.5 and 0.75) in the diameter range 2-4 nm. For alloyed clusters (AuxAg 1 - x ) n of a given size an almost linear evolution of the surface plasmon frequency ω s with the concentration is observed (between those of pure gold and pure silver clusters). Moreover the blue-shift and the damping of the resonance with decreasing size is all the more important as the gold concentration in the particles increases. Such results are in agreement with theoretical calculations carried out in the frame of the time-dependent local-density-approximation (TDLDA) including an inner skin of ineffective screening and the porosity of the matrix. The optical response of (NixAg 1 - x ) n clusters exhibits a surface plasmon resonance in the same spectral range as the one observed for pure silver clusters, but considerably damped and broadened. For a given mean cluster size 3.0 nm, a blue-shift of the resonance is observed when increasing the nickel concentration (between x = 0.25 and x = 0.75). The results are in good qualitative agreement with classical predictions in the dipolar approximation, assuming a core-shell geometry. [ABSTRACT FROM AUTHOR]

Amorphous silicon carbide films have been prepared by low-energy cluster beam deposition. Transmission electron microscopy and tapping mode atomic force microscopy reveal the structure of the film which is formed by a random distribution of SiC nanoparticles. The size of the particles was about 8-10 nm and it is greater than the size of the incident free particles (less than 1 nm). However, the inner structure of the supported particles does not reveal long-range order. Both Raman spectroscopy and Fourier-transformed infrared spectroscopy show clearly limited chemical ordering and phase separation evidenced by the presence of silicon-rich, carbon-rich and SiC-rich areas. [ABSTRACT FROM AUTHOR]