The accumulation of helium atoms in metals or metal tritides is known to result in the formation of helium bubbles in the lattice and to cause degradation of the material. Helium is introduced either through neutron transmutation reaction or via the radioactive decay of tritium. We have performed first-principles calculations of interstitial helium inside Pd and Pd tritide using density functional theory (DFT) and the projector augmented-wave (PAW) method within the generalized gradient approximation (GGA). We model the growth process of an interstitial helium cluster and find that when the size of the cluster reaches to five atoms, the cluster can induce an energetically favorable vacancy with a self-trapping mechanism. The cluster growth mechanism of interstitial helium is addressed by investigating the associated energetics, cluster configurations, and electronic structural properties. In addition, we study the diffusion properties of helium in palladium-based compounds by performing the nudged elastic band (NEB) calculations. Our computational models propose that by loading the lattice with hydrogen atoms at certain concentration, or substituting with alloying metals can modify the diffusivity by increasing its migration barrier, which may impede the cluster formation in the beginning stage.

Neutron irradiated copper-boron alloys are employed to study the mutual interaction between metallic crystalline lattices and inert gases. Inert gases precipitate to form gas bubbles and their growth induces dilation of the matrix. This dilation, technically designated as swelling, affects the structural integrity of nuclear fuels during their service. The estimated enthalpy of solution of helium in copper is 5.5 eV/atom. As a Consequence, its solubility in the copper matrix is extremely poor and it cannot enter a copper based matrix via any thermo-chemical route. Hence, recourse to a nuclear reaction is taken to impregnate copper with helium. Helium is produced in situ through neutron irradiation in copper-boron alloys as a result of (n, alpha) nuclear reaction between boron atoms and neutrons. The characteristic feature of the growth of helium gas bubbles driven by isothermal annealing of the metallic matrix is that their rate of growth is highly sensitive to the distance of the bubbles from the external surface of the specimen. The growth of gas bubbles as a function of time and temperature is modulated by the flow of vacancies from the free surface of the specimen. A theory for the surface induced growth of helium gas bubbles in the neutron irradiated copper-boron alloys is presented here.

The rates of dissolution of heavy crude oil in liquid solvents and rates of desorption of solvents from oil have been measured. The crude oil used is a non-volatile heavy oil of 4253 mPa.s viscosity at room temperature. The solvents used are hexane, heptane and toluene. When the oil (black) is contacted with a solvent (transparent) an interface is seen which moves with time and takes a very long time to become fuzzy. The rate of movementof the front is measured. The dissolution experiments give very consistent results, but there are two parameters involved, Do, the diffusivity at infinite dilution and alpha which determines the concentration dependence. As a result it is necessary to do desorption experiments to be able to calculate both constants from the rate of movement of the front data. However, desorption experiments could not be performed under conditions of low concentrations suitable for the present case because of the very viscous nature of the oil. As a result, although the desorption experiments also showed good results, they could not be used to obtain good values of the parameters. When Stokes-Einstein equation was used to calculate Do, excellent results were obtained with alpha ~ 10 for the dissolution experiments and good deal smaller for the desorption experiments. That result is used to conclude that the above form for concentration dependent diffusivity is correct and concentration dependence is very high at low solvent concentrations explaining the sharp interfaces during dissolution.Other evidences have also been offered.

Inert gases in metals can occur due to ion implantation, from a plasma in a magnetron device or as a result of being by-products of nuclear reactions. Mainly because of the nuclear applications, the properties of the inert gases, helium, argon and xenon in the body centred cubic (bcc) iron crystal are examined theoretically using a combination of molecular dynamics, static energy minimisation and long time scale techniques using empirical potential functions. The same techniques are also used to investigate argon and xenon in aluminium. The primary interest of the work occurred because of He produced in nuclear fission and its effect on the structural materials of a fission reactor. This structure is modelled with perfectly crystalline bcc Fe. In bcc iron, helium is shown to diffuse rapidly forming small bubbles over picosecond time scales, which reach a certain optimum size. In the initial phase of He accumulation, Fe interstitials are ejected. This occurs instantaneously for bubbles containing 5 He atoms and as the more He accumulates, more Fe interstitials are ejected. The most energetically favourable He to vacancy ratios at 0 K, vary from 1 : 1 for 5 vacancies up to about 4 : 1 for larger numbers of vacancies. An existing He bubble can be enlarged by a nearby collision cascade through the ejection of Fe interstitials, allowing more He to be trapped. Ar and Xe in bcc Fe prefer to be substitutional rather than interstitial and there are large barriers to be overcome for the inert gas atoms to diffuse from a substitutional site. Bubbles that form can again be enlarged by the presence of a nearby collision cascade or at very high temperatures. In this case the most energetically favourable vacancy ratios in the bubbles is 1: 1 for Ar and from 0.6: 1 to 0.8: 1 for Xe. For Ar and Xe, bubble formation is more likely as a direct result of radiation or radiation enhanced diffusion rather than diffusion from a substitutional site. Ar in aluminium is also studied. Ar atoms in fcc Al prefer to be substitutional rather than interstitial and evolution into substitutional occurs over picosecond time scales at room temperature. Bubble formation can occur more easily than in bcc iron, mainly because the barriers for vacancy diffusion are much lower but the time scales for bubble accumulation are much longer than those for He. A vacancy assisted mechanism is found which allows Ar to diffuse through the lattice. Finally some preliminary results on the energetics of different geometrical structures of larger Xe bubbles in Al are investigated since experiment has indicated that these can become facetted.

Ce travail de thèse porte sur l'exploitation de la spectroscopie des pertes d'énergie des électrons,(EELS) résolue spatialement pour l'étude de systèmes de dimensions nanométriques. Cette technique étant réalisée avec un microscope électronique en transmission et à balayage, on parle d'expérience STEM-EELS dont le principal avantage est de permettre à la fois une caractérisation morphologique à l'échelle nanométrique et l'accès à une grande variété d'informations sur les propriétés physiques et chimiques des systèmes étudiés. Nous présentons ainsi deux cas d'étude portant sur : des nanobulles d'hélium confinées dans des matrices à base de fer et des nanoparticules métalliques or-argent de structure coeur- coquille. Ces deux études se basent sur le couplage entre imagerie microscopique à l'échelle nanomètrique et spectroscopie EELS en perte proche (Low-loss EELS).La première étude permet de sonder les densités et les pressions des bulles d'hélium en fonction de leur tailles en s'appuyant sur la détection et l'étude, bulle par bulle, du seuil K de l'hélium. Les résultats sont globalement en accord avec d'autres études dans le domaine, mais des densités très basses en hélium (<10 at nmˉ³) ont été mesurées de façon fiable pour la première fois.la deuxième étude concerne l'exploration des propriétés plasmoniques des nanoparticules bimétalliques Au@Ag en sondant les énergies et les localisations des modes plasmon de surface. On trouve un accord excellent entre les expériences et les calculs par éléments finis pour les positions des modes plasmon. La coquille en Ag semble déterminer leurs énergies, avec le coeur en Au ayant apparemment peu d'effet.Une part importante de ce travail concerne le développement des méthodologies d'acquisition et de traitement des données. Tout particulièrement l'utilisation des techniques d'analyses mutivariés (MVA) pour améliorer la détection de l'hélium dans les bulles. / This thesis describes the application of high spatial resolution electron energy-loss spectroscopy (EELS) to the nanometre scale analysis of two rather di#erent types of sam- ple. EELS performed in the scanning transmission electron microscope (STEM) is known as STEM-EELS and offers the possibility of performing morphological characterisations simultaneously with access to a wide variety of information on the physical and chemical properties of the systems under study. Our two areas of study are nano-bubbles of helium in iron-based alloys and core-shell Au-Ag nano-particles. Both systems are studied via electron microscopic imaging and low-loss EELS (i.e. the spectral range below 50 eV). In the first case the helium densities and pressures in the bubbles are measured as a function of their size and the conditions in which they are generated, via the examination of the intensity and spectral position of the He-K excitation. The results are in broad agreement with othe studies in the field, but particularly lowhelium densities (< 10 at nmˉ³) have been reliably measured here for the first time.The second case concerns the exploration of the bi-metallic nano-particles plasmonic properties via the study of the surface plasmon mode energies and spatial localisations. Excellent agreement is found between experiment and finite element calculations for the plasmon modes. The outer Ag shell essentially fixes their energies, the Au core apparently having little effect.An important part of this work concerns the application of novel acquisition and data analysis methods. In particular the use of multivariate statistical analysis (MVA) is shown to facilitate the detection and quantification of helium in the bubbles.

Electrons are weakly attracted to liquid helium due to the small but finite polarizability of helium atoms. However, they cannot enter the liquid unless their energy is more than 1 eV, due to the Pauli exclusion principle. As a result, electrons are bound perpendicular to the surface but are free to move parallel to the surface i.e., they form a two-dimensional electron system (2DES). If the electron density of the 2DES is increased above a critical value ( 1013 per m2) the surface becomes un-stable leading to the formation of charged bubbles known as multielectron bubbles (MEBs). In MEBs the electrons are confined to the inner bubble surface and hence we have a 2DES on a curved surface. The critical density of electrons on the bulk surface is too low to study the quantum dominated phases of the 2DES. In contrast, due to the enormous surface defects and impurities, the electronic density of 2DES in semiconductors cannot be lowered below 1015 per m2, which is high enough such that the 2DES is always in a quantum liquid phase. Alternatively, the possibility of varying the electron density over a wide range and the effects of curvature implies that MEBs can be used to probe new phases of 2DES like Wigner crystallization with strong electron-ripplon coupling, quantum melting, superconductivity etc. In this thesis the experiments done on MEBs in liquid helium are described. In the initial experiments we generated MEBs which were observed to shrink in size. We saw a difference in their collapse behaviour: MEBs in super fluid helium though initially bigger in size collapse much faster than MEBs generated in normal fluid. The vapour present in the MEBs cannot condense fast in normal fluid due to the lower thermal conductivity. In subsequent experiments, we could trap these MEBs, generated in normal fluid and stabilised by their vapour content, in a linear Paul trap. We measured the charge and radius of these trapped MEBs by analysing their dynamics. Interestingly, the stably trapped MEBs were found not to lose charge as they shrink and disappear in hundreds of milliseconds, implying that the charge density inside increases at least two orders of magnitude from the initial value. MEBs so trapped can be used to study the properties of 2DES in the high electron density limit where the quantum confinement energy dominates. Later, we measured the charge of the MEB with respect to time when it was held on a solid substrate. We propose a charge loss mechanism as the tunneling of electrons across a thin lm of helium formed between the MEB and the substrate. We estimated the density of electrons on this thin lm by using a numerical model. We found that the maximum electron density (about a few 1015 per m2) which could be held on a thin lm is limited by tunneling. Moreover, the substrate surface roughness did not affect the charge loss due to the microscopic contact of MEBs with the substrate, resolving the complications in charge loss observed in previous experiments on macroscopic thin films on metallic substrates. Finally, we describe the experiments and the results on the stability of MEBs generated in super fluid helium. Highly charged MEBs (with more than 104 electrons which have an equilibrium radius that is easily visible) are found to be unstable against fission into smaller bubble showing a type of electro-hydrodynamic instability. However, the stability of bubbles with radius less than our detection limit ( 1 m) is still an open question.