677 research outputs found
Reducing Hydrogen Permeation through Metals
No full textMetal–hydrogen systems are of great basic and technological interest in connection to the
role of hydrogen as a clean energy carrier. Frequently, metal systems are involved in hydrogen
purification, storage, and engines making use of this fuel. The presence of hydrogen in a metallic
matrix gives rise to modifications of electrical, optical and mechanical properties. Hydrogen
accumulation in metals may cause damage to the material by also producing fracture, thus limiting
operating lifetime. Reducing the hydrogen permeation is an important task also for the fusion
reactors: it is well known, indeed, that tritium is radioactive so that it is very important to be able to
confine tritium during the nuclear fusion process. The theoretical study of permeation is thus of
fundamental importance to obtain efficient barriers to permeation. Hydrogen trapping sites have a
great influence on the hydrogen permeation through a slab sample. The diffusion of the hydrogen in
a crystal is generally described by a parabolic partial differential equation with appropriate boundary
conditions. The numerical simulation code PHM (Permeation of Hydrogen through Metals),
realized for the study of the permeation of hydrogen in presence of trapping sites, is here described
and utilized for the analysis of the influence of reversible and irreversible traps on the diffusion of
hydrogen in a metal
Simulation of phase explosion in the nanosecond laser ablation of aluminum
No full textVaporization, spallation and phase explosion are considered to be the main mechanisms contributing to the nanosecond laser ablation of metals. The theory of homogeneous nucleation, together with the dynamics of target heating, allows a space-time resolved simulation of the phase explosion mechanism. The thermal phenomena occurring at the target surface are studied within the framework of a thermodynamic continuum approach. A 20 ns laser pulse of variable fluence and Gaussian time dependence was assumed. The temperature profile in the target external layers is studied through the heat diffusion equation. The vaporization from the surface is modeled assuming unsteady adiabatic expansion (UAE) of the vapor and a Monte Carlo (MC) method is used to describe the formation of liquid nanodroplets through phase explosion. Liquid nanodroplets in the ablated material are studied at different laser fluences. The size distribution of the nanodroplets formed in the phase explosion process is here reported and connections with experiments are discussed
Surface Effects Controlling Electron-stimulated Oxidation of Silicon
No full textPreviously reported experimental results on the electron-stimulated oxidation of Si are quantitatively described. The framework of the analysis is a macroscopic continuum model which includes transport processes through the oxide layer, and surface effects mainly connected to the O//2 sticking coefficient. The existence of the chemisorbed precursor states is pointed out and a simple picture of the potential energy experienced by the O//2 molecule approaching the surface of the oxide layer is proposed
Radiation Enhanced Diffusion In Glasses
No full textWe discuss a possible new mechanism leading to stimulated ionic transport processes in electron-irradiated glasses. By invoking percolation through structurally relaxed units following defect recombination we can give a unified picture of various experimental results. Peculiarities in the transport properties are shown to give some insight into the nature of the irradiation-induced defects in glasses
Hydrogen permeation through a slab sample in the case of high hydrogen concentration
No full textHydrogen trapping sites have a great influence on the hydrogen permeation through a slab sample. The diffusion of the hydrogen in a crystal is generally governed by a parabolic partial differential equation: a numerical simulation code, realized for the study of the permeation of hydrogen in presence of trapping sites, has been utilized for the analysis of the influence of reversible and irreversible traps on the diffusion of hydrogen in a metal for the case of high (not negligible) hydrogen concentration with boundary conditions which cannot be treated analytically
Network Relaxation Processes Governing Alkali-metal Transport In Electron-irradiated Glasses
No full textWe study the transport of alkali-metal ions in oxide glasses at low temperature under electron irradiation in connection with the onset of percolation through structurally relaxed SiO2 units. The relaxation of the SiO2 units forming a path for cation motion and the obtained low value for the critical exponent governing the ion diffusion coefficient are discussed
Control of cluster synthesis in nano-glassy carbon films
No full textCarbon films were prepared by pulsed laser deposition (PLD), changing buffer gas nature and pressure and laser power density. Nanometer- sized cluster assembled (CA) films, resulting from direct aggregation of carbon clusters in the ablation plume, were obtained. Visible Raman spectroscopy shows that all films are trigonally co-ordinated and structurally disordered, with a dependence of the degree of disorder on the deposition parameters. The microstructure and morphology of the films were studied in a complementary way by scanning electron microscopy (SEM) both in plane and in cross-section, and by atomic force microscopy (AFM). Different growth modes are found in the deposited CA films, depending on the interplay of laser fluence and nature-pressure of the buffer gas. Threshold fluences of increasing value separate dense columnar growth from sponge like morphology, from an open dendritic structure. AFM pictures show that our glass-like carbon films consist of agglomerates of nanometer-sized clusters. Cluster formation in the plume is modeled, allowing to estimate the average number of carbon atoms per cluster. The calculated size of the clusters depends mainly on ambient gas pressure. Cluster sizes obtained by model predictions agree with those directly observed by transmission electron microscopy (TEM) imaging and with the deduced film coherence length from Raman spectroscopy conducted herein
Laser-Surface Interactions for New Materials Production - Tailoring Structure and Properties
No full textMICROSCOPIC MECHANISMS GOVERNING ALKALI-METAL TRANSPORT IN ELECTRON-IRRADIATED GLASSES
No full textWe propose that alkali-metal migration in oxide glasses at low temperature occurs through SiO2 units structurally relaxed by irradiation. With a combination of percolation and diffusion equations, we are able to explain all experimental data obtained by Pantano et al
Dynamics of liquid nanodroplet formation in nanosecond laser ablation of metals
No full textThe laser ablation mechanisms of metallic targets leading to liquid nanodroplet ejection are of wide interest both from a fundamental point of view and for applications in various fields, especially when nanoparticle synthesis is required. The phase explosion process was recognized as the driving mechanism of the expulsion of a mixture of vapor and liquid nanodroplets in the short pulse laser ablation of metals. A model based on thermodynamics that links the theory of homogeneous vapor bubble nucleation to the size distribution of the generated liquid nanoclusters has been recently proposed. The present work aims to take a step ahead to remove some assumptions made in previous work. Here an improved computational approach allows us to describe time-dependent nucleation in a homogeneous system with no temperature spatial gradients under nanosecond laser irradiation. Numerical results regarding the size distribution of formed liquid clusters and the time evolution of the process are shown for aluminum, iron, cobalt, nickel, copper, silver and gold. Connections with experimental data and molecular dynamics simulations, when available from literature, are reported and discussed
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