Electron Microscopy of Bubbles and Dislocations in Helium Implanted Metals

<p>The theoretical contrast in transmission electron microscope of a superlattice of helium gas bubbles in copper is computed using the two-beam and many-beam dynamical theories of electron diffraction with the aim the aim of measuring the density and size of dislocation loops associated with the bubble array. A wide range of parameters (foil thickness, diffraction vector, excitation error, defocus, and depth, radius, and strain-field of the bubble) is considered to considered to construct a library of theoretical images and intensity profiles for a single, isolated bubble. Various criteria are applied to obtain a measurement of the bubble radius from the simulations but the results are inaccurate because of the sensitive dependence of the intensity profile on the imaging parameters. A better measurement is profiles from a single stack of bubbles are modeled and electron diffraction from superlattices simulated. The results obtained suggest that the bubble ordering is of limited extent. A library is made of the theoretical contrast when imaging a system of dislocation loops punched out along the <110> directions by the growth of gas bubbles ordered on a superlattice aligned with the host fcc matrix. These image simulations use the displacement fields surrounding loops and bubbles predicted by isotropic elasticity theory. For a variety of structures involving loops and bubbles, the following imaging parameters were investigated: beam direction, foil normal, diffracting vector, excitation error, number of beams, and defocus, These simulations indicate that it should be possible to image the small dislocations at high density thought to be present in the bubble lattice, provided well focused micrographs taken under strong two-beam conditions can be obtained. In Practice it proved difficult to tilt specimens containing superlattices to strong two-beam conditions because of the deterioration in crystallinity resulting from the implantation. However, the lower concentrations by low dose implantations.</p>

Electron Microscopy of Bubbles and Dislocations in Helium Implanted Metals

<p>The theoretical contrast in transmission electron microscope of a superlattice of helium gas bubbles in copper is computed using the two-beam and many-beam dynamical theories of electron diffraction with the aim the aim of measuring the density and size of dislocation loops associated with the bubble array. A wide range of parameters (foil thickness, diffraction vector, excitation error, defocus, and depth, radius, and strain-field of the bubble) is considered to considered to construct a library of theoretical images and intensity profiles for a single, isolated bubble. Various criteria are applied to obtain a measurement of the bubble radius from the simulations but the results are inaccurate because of the sensitive dependence of the intensity profile on the imaging parameters. A better measurement is profiles from a single stack of bubbles are modeled and electron diffraction from superlattices simulated. The results obtained suggest that the bubble ordering is of limited extent. A library is made of the theoretical contrast when imaging a system of dislocation loops punched out along the <110> directions by the growth of gas bubbles ordered on a superlattice aligned with the host fcc matrix. These image simulations use the displacement fields surrounding loops and bubbles predicted by isotropic elasticity theory. For a variety of structures involving loops and bubbles, the following imaging parameters were investigated: beam direction, foil normal, diffracting vector, excitation error, number of beams, and defocus, These simulations indicate that it should be possible to image the small dislocations at high density thought to be present in the bubble lattice, provided well focused micrographs taken under strong two-beam conditions can be obtained. In Practice it proved difficult to tilt specimens containing superlattices to strong two-beam conditions because of the deterioration in crystallinity resulting from the implantation. However, the lower concentrations by low dose implantations.</p>

Stress-driven surface swell and exfoliation of copper as the plasma-facing materials in NBI ICP source

Neutral beam injection (NBI) heating is a significant auxiliary heating method used in Tokamak fusion devices. The material of faraday shield (FS) and accelerator grids in the NBI inductively coupled plasma (ICP) source can be damaged during operation by the high-density hydrogen plasma irradiation, and thus affecting the stability of the NBI system. In this paper, a series of hydrogen plasma exposure experiments are performed on oxygen-free copper (OFC) specimens at 400-850 K with ion energy of 20-200 eV and irradiation fluence up to 1.0×1025 /m2. Meanwhile, the rate equation model is adopted for numerical simulation of the bubble growth and hydrogen retention. The influence of OFC surface temperature, hydrogen ion energy and fluence on OFC damage are experimentally and numerically investigated. Surface observations show that swell and exfoliation are formed on the OFC samples at 400 K and 600 K by scanning electron microscopy (SEM). The hydrogen ion energy varying from 20 to 200 eV at 400 K is observed to have little effect on OFC surface microstructure. The simulative results show that there exist different critical temperatures when the initial bubble radius changes. The bubble surface density rises and the bubble size decreases with increasing temperature (below the critical temperature). In addition, adjacent bubbles get closer to each other with the growth of hydrogen bubbles, and the strong tensile stress is produced inside the surrounding material of hydrogen bubbles. Some cracks caused by hydrogen bubbles appear on the surface of the OFC to relax the pressure-induced stress, ultimately leading to OFC FS/grids material damage. This investigation helps to understand hydrogen retention and failure mechanisms of OFC materials under extreme operation conditions in the NBI devices.

Shapes of a filament on the surface of a bubble

The shape assumed by a slender elastic structure is a function both of the geometry of the space in which it exists and the forces it experiences. We explore, by experiments and theoretical analysis, the morphological phase space of a filament confined to the surface of a spherical bubble. The morphology is controlled by varying bending stiffness and weight of the filament, and its length relative to the bubble radius. When the dominant considerations are the geometry of confinement and elastic energy, the filament lies along a geodesic and when gravitational energy becomes significant, a bifurcation occurs, with a part of the filament occupying a longitude and the rest along a curve approximated by a latitude. Far beyond the transition, when the filament is much longer than the diameter, it coils around the selected latitudinal region. A simple model with filament shape as a composite of two arcs captures the transition well. For better quantitative agreement with the subcritical nature of bifurcation, we study the morphology by numerical energy minimization. Our analysis of the filament’s morphological space spanned by one geometric parameter, and one parameter that compares elastic energy with body forces, may provide guidance for packing slender structures on complex surfaces.

Investigation of Cavitation Bubble Dynamics Considering Pressure Fluctuation Induced by Slap Forces

Cavitation erosion is induced by the penetrating pressure from implosion of cavitation bubbles nearby solid boundary. The bubble evolution and the subsequent collapse pressure are especially important to evaluate the erosion degradation of solid boundary materials. The bubble dynamics equation taking into account the influence of distance between bubble and solid boundary is formulated to investigate the effect of boundary wall on bubble evolution process. The pressure fluctuation induced by slapping forces is adopted to evaluate the bubble dynamic characteristics. Negative pressure period which reflects the effect of vibration velocity and gap clearance also has large influence on bubble dynamics. The effects of standoff distance, initial radius and negative pressure period on bubble evolution and collapsing shock pressure are discussed. Maximum bubble radius increases with standoff distance and initial radius, while shock pressure increases with distance and decreases with bubble initial radius, and both of them increase with negative pressure period.

Static Characteristics and Leakage Rates of Smooth Annular Seals Based on a New Solution Method for Gas-Liquid Two-Phase Conditions

This paper proposes a new solution method for the leakage and static characteristics of smooth annular seal under a homogeneous gas-liquid two-phase flow based on a bulk-flow model. In this solution method, the Rayleigh–Plesset equation is introduced into the governing equations to describe the behavior of bubbles considering mixture compressibility. Detailed comparisons between Childs’ experimental leakage rates and predicted ones based on the proposed method are conducted, and the predicted results show good agreement with the experimental results, with a maximum error of 11.2%. Moreover, static characteristics of the seal, including leakage rates, gas volume fraction (GVF) distribution, pressure distribution, mixture density, and viscosity within the seals, are investigated based on the present method. The results show that as the inlet gas volume fraction increases from 0% to 10%, the local gas volume fraction of each axial position will increase, however, the seal leakage, mixture density, and mixture viscosity will decrease. Bubble radius has little effect on the leakage rates and the static characteristics of the seals. Additionally, comparisons between the characteristics of the model seals with different clearances show that the leakages of the seals with bigger clearance behave more sensitively to the inlet GVF changes.

Gravitational waves from colliding vacuum bubbles in gauge theories

We study production of gravitational waves (GWs) in strongly supercooled cosmological phase transitions in gauge theories. We extract from two-bubble lattice simulations the scaling of the GW source, and use it in many-bubble simulations in the thin-wall limit to estimate the resulting GW spectrum. We find that in presence of the gauge field the GW source decays with bubble radius as $$\propto R^{-3}$$ ∝ R - 3 after collisions. This leads to a GW spectrum that follows $$\Omega _{\mathrm{GW}} \propto \omega ^{2.3}$$ Ω GW ∝ ω 2.3 at low frequencies and $$\Omega _{\mathrm{GW}} \propto \omega ^{-2.9}$$ Ω GW ∝ ω - 2.9 at high frequencies, marking a significant deviation from the popular envelope approximation.

The Numerical Investigation on Bubble Interaction Dynamics in Hydrodynamic Cavitation

Bubble-bubble interactions are of great importance for bubble dynamics. A mathematical model describing the dynamics of a cluster in hydrodynamic cavitation is presented. The interaction strength (i.e. the number density of bubbles) is introduced into Keller-Misis equation. Using this model, numerical investigations of bubble dynamics (i.e. radial motion and internal energy) of single bubble in a cluster have been made due to linear pressure gradient. With the increase of interaction strength, the times of bubble reaching the maximum and minimum radii are delayed. The more of bubbles are in a cluster, the more significant of the delay effect is. The maximum internal energy inside the bubble is closely related to interaction strength (i.e. positive correlation). Furthermore, the initial bubble radius and final recovered pressure of the orifice on bubble dynamics are quantitatively discussed. Based on numerical results, some references are put forward for optimize and manipulate of hydrodynamic cavitation reactor.