Evolution Of Cavities Research Articles

Plasma expansion and evolution of density perturbations in the polar wind: Comparison of semikinetic and transport models

Comparisons are made between transport and semikinetic models in a study of the time evolution of plasma density perturbations in the polar wind. The situations modeled include plasma expansion into a low‐density region and time evolution of localized density enhancements and cavities. The results show that the semikinetic model generally yields smoother profiles in density, drift velocity, and ion temperature than the transport model, principally because of ion velocity dispersion. While shocks frequently develop in the results of the transport model, they do not occur in the semikinetic results. In addition, in the semikinetic results, two ion streams, or double‐humped distributions, frequently develop. In the transport model results the bulk parameters, at a given time, often have a one‐to‐one correspondence in the locations of their local minima or maxima. This is a consequence of the coupling of the fluid equations. There is, however, no such relationship among the moments produced by the semikinetic model where the local moment maxima and minima are often shifted in altitude. In general, incorporation of enhanced heat fluxes in the transport model leads to somewhat improved agreement with the semikinetic results.

A Computational Study of Defects Evolution during Irradiation in Single-Phase Austenitic Alloys

A composite model of the point defect processes and the extended defect evolution in irradiated materials was composed for a theoretical investigation of the response of fusion first wall materials to neutron bombardment. The point defects model calculates the concentration changes of single point defects including a transmutant helium, simple point defect clusters and complex clusters. The extended defect model consists of individual rate theory models of evolution of cavities, Frank faulted loops, network dislocation and other microstructural features. The model was calibrated based on the dual-ion experimental data on an Fe-15Cr-20Ni ternary austenitic model alloy. Using the calibrated model, the effects of helium generation on point defect processes, cavity nucleation, dislocation evolution and mechanical property change were investigated

Cavity Evolution During Tensile Creep of Si3N4

ABSTRACTWe have characterized the evolution of cavities during tensile creep of a Y2O3-hot isostatically pressed Si3N4, using precision density measurements, small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). The cavities are bimodally distributed in size. Lenticular, 200 nm-size cavities are common, and lie primarily on two-grain boundaries. Irregularly shaped 500-1000 nm-size cavities are rare and lie at multi-grain junctions, but comprise approximately half of the total volume fraction of cavities. Although the material shows a continuous decrease in strain rate with strain, the cavity volume fraction evolves linearly with strain. Cavities account for approximately 85% of the total strain at any point during creep.

Numerical study of cavity nucleation kinetics

The numerical method of calculation of cavity nucleation kinetics under the time dependent stress has been developed. The results of computations are not in full accord with the recent concept of classical steady-state solution. The cavity nucleation kinetics in the system of cavities and the time evolution of individual cavities in the nucleation and early growth stages at intergranular inclusions under the time dependent stress have been simulated numerically.

Forced and Natural Convection Effects on the Shape Evolution of Cavities during Wet Chemical Etching

The effect of forced and natural convection on the shape evolution of deep cavities during wet chemical etching was investigated. Etching was assumed to be limited by removal of the dissolution products away from the vicinity of the active surface. Finite element methods were employed to solve for the fluid velocity and product concentration distributions in cavities of irregular geometries resulting from etching. Forced convection was found very ineffective for rinsing deep cavities. The etching rate decreased sharply with time as the cavity became deeper during etching. At the same time, the etching rate distribution along the active surface became nearly uniform, degrading etch anisotropy. In contrast, natural convection was effective for rinsing the dissolution products out of the cavity. Both the etching rate and the etch factor remained at relatively high values throughout etching, even at later times when the cavity became deeper. The cavity wall profiles and the corresponding flow and concentration fields showed some interesting features, especially for the case of natural convection. The results have important implications for deep anisotropic etching and other related processes.

Nucleation and growth theory of cavity evolution under conditions of cascade damage and high helium generation

The evolution of helium-filled cavities during neutron irradiation is analyzed in terms of the stochastic theory of atomic clustering. The conventional separation of nucleation and growth is replaced by a self-consistent evolution model. Starting from kinetic rate (master) equations for the clustering of helium and vacancies, helium mobility, helium-vacancy cluster stability, and cavity nucleation and growth are all included in the model. Under typical fusion irradiation conditions (cascade damage and high helium-to-dpa ratios), the following is suggested: (1) Helium mobility decreases with the evolution of the microstructure. At quasi-steady state, it is mainly controlled by interstitial replacement or thermal desorption. (2) Gas re-solution from cavities by cascades increases nucleation at high fluences. (3) The cavity size distribution is broadened because of cascade-induced fluctuations. (4) The majority of helium-filled cavities are in a nonequilibrium thermodynamic state.

Unified thermodynamic treatment of cavity nucleation and growth in high temperature creep

Existing models describing the creep nucleation and growth are briefly outlined in orderss to obtain a starting point for a new physical framework which unifies the stochastic treatment of nucleation with the deterministic treatment of growth. For this purpose an original hypothesis was postulated to develop the equations of motion of a thermodynamic system in non-equilibrium state at constant temperature and pressure. It is shown the application of this hypotthesis to a system of creep cavities provides the unified equations describing the evolution of cavities during their nucleation and growth stages. Particular attention is devoted to the role of the elastic relaxation of local normal stress due to deposition of atoms from a cavity its nucleation.

Effect of Transport and Reaction on the Shape Evolution of Cavities during Wet Chemical Etching

The effect of fluid flow, transport, and reaction on the shape evolution of two‐dimensional cavities during wet chemical etching was studied. Finite element methods were employed to solve for the fluid velocity profiles and for the etchant concentration distribution in cavities of arbitrary shape. A moving boundary scheme was developed to track the shape evolution of the etching cavity. In the case of pure diffusion and under mass‐transfer control, a mask with finite thickness resulted in significantly better etch factor (etch anisotropy) as compared to an infinitely thin mask, albeit the etch rate was essentially unaffected. With fluid flow past the cavity, the etch rate increased fourfold and the etch factor increased by 40% as compared to pure diffusion, under the conditions examined. In addition, the etch rate, etch factor, and cavity wall profiles showed a strong dependence on etch time as the cavity aspect ratio (depth/width) increased with time during etching.

Evolution of diamagnetic cavities in the solar wind

Two‐dimensional hybrid (particle ions and fluid electrons) simulations of a finite plasma beam moving through a collisionless plasma parallel to an ambient magnetic field are considered. When the relative drift speed exceeds twice the local Alfven velocity, an electromagnetic ion beam instability is excited which produces an energetic plasma in the interaction region with β larger than unity. This energetic plasma pushes out the magnetic field, creating a diamagnetic cavity and launching outgoing Alfven waves. Application of the results to recent observations of hot diamagnetic cavities in the solar wind is discussed.

Modelling of cavitation in two phase superplastic alloys under uniaxial stress systems

Superplastic materials are characterized by their ability to undergo exceptionally large deformation under low stresses. Many alloys, however, are prone to cavitation, which leads to premature failure even though deformation is within the superplastic regime. A model is presented which analyses the evolution of pre-existing cavities in two phase alloys subjected to uniaxial stress systems. It is seen that this model adequately describes the cavitation phenomenon of α–β brasses and microduplex Cu-Zn-Ni alloys in the tensile test. Closure of cavities in the compression test may also be described by this model. The results obtained thus enable the formation of an overall view of the behaviour of a two phase superplastic alloy subjected to the uniaxial tension test, and the difference in cavity evolution of different materials.

Frequency-shift of self-trapped light

A decrease in the frequency of self-trapped light by as much as a factor of two has been observed in computer simulations of several physical situations. The frequency shift is due to a decrease in the resonant frequency of the plasma cavity as it deepens and expands shortly after the light becomes trapped. Three examples are presented: a model problem in an initially uniform plasma, trapping of laser light in cavities at critical density, and modulational instability and self-trapping of Raman-scattered light at quarter-critical density in a nonuniform, expanding plasma. This effect is essential in the analysis of the evolution and stability of such cavities, and such large frequency changes limit the applicability of models using a nonlinear Schrödinger equation to represent the high-frequency fields.

A numerical study of cavity growth controlled by coupled surface and grain boundary diffusion

The diffusive growth of both two dimensional and axisymmetric cavities initially having equilibrium shapes and located on grain boundaries loaded in tension is studied using finite difference techniques. The shape evolution and growth kinetics of individual cavities as well as the time required for adjacent cavities to grow together is studied as a function of applied stress and the ratio of grain boundary to surface diffusivity. A key feature of this treatment is that the diffusional processes in the grain boundary and on the cavity surface are coupled by boundary conditions at the tip of the cavity. When surface diffusion is much slower than grain boundary diffusion, the cavities become crack-like during growth, and the fracture time varies reciprocally with the third power of the applied stress. When grain boundary diffusion is the slower process, the cavities remain rounded during growth, and the fracture time varies reciprocally with the first power of the stress. The transition between these limiting kinds of behavior is described and the results are compared with previous treatments of these problems.

Overview 14 Creep fracture in ceramic polycrystals—I. creep cavitation effects in polycrystalline alumina

Fine grained polycrystalline alumina has been deformed in creep at high temperatures, to examine the evolution of cavities at grain boundaries. Cavities with equilibrium and crack-like morphologies have been observed, distributed non-uniformly throughout the material. The role of these cavities during creep has been described. A transition from equilibrium to crack-like morphology has been observed and correlated with a model based on the influence of the surface to boundary diffusivity ratio and the local tensile stress. The contribution of cavitation to the creep rate and total creep strain has been analyzed and excluded as the principal cause of the observed non-linear creep rate.

Electrical Breakdown Due to Discharges in Different Types of Insulation

By comparing the propagation of electrical trees in polyethylene with the evolution of artificial air-filled cavities in epoxy, it is possible to envisage the dielectric breakdown as a treeing process. Experiments were carried out on epoxy resin specimens with voids of 0.250 mm thick and 3 mm in diameter. The propagation of electrical trees was studied in low density polyethylene in which a needle was inserted, a metallic plane in close contact with the polymer being the grounded electrode. For a cavity, the delocalization of discharges was observed by recording the corresponding pulses against time and was confirmed by optical observations. On the other hand, the study of the partial discharge characteristics of electrical trees has shown the existence of several distinct phases. They give a good idea of the nature and the localization of trees. To take into account the decentralization of the discharges, the classic equivalent circuit of a cavity has been modified. The model explains the behavior of the discharge number but does not take into account the permanent change in the dimensions and the electrical characteristics of the medium, particularly in the case of treeing.

The effect of dose on the evolution of cavities in 500-keV 4He +-ion irradiated nickel

Transmission electron microscopy has been used to investigate the effect of total dose on the depth distribution of cavities (voids or bubbles) in nickel irradiated at 500°C with 500-keV 4He + ions. A transverse sectioning technique, which allows one to obtain the entire depth distribution of cavities and of damage from a single specimen, was utilized. The size, number density and volume fraction of bubbles or voids were measured from the micrographs taken from samples sectioned parallel to the direction of the incident beam. The results for the dose range studied (2 × 10 19 to 1 × 10 21 ions/ m 2) show that the average cavity diameter, number density, and the volume fraction (i.e. swelling) increases with increasing dose. The peak in the swelling distribution occurs at depths 8 to 15% deeper than the peak in the calculated projected range profile.

Characteristics of the space-time evolution of vapor-gas cavities generated by an underwater spark discharge

Characteristics of the space-time evolution of vapor-gas cavities generated by an underwater spark discharge

Burrowing Habit and the Early Evolution of Body Cavities

RECENT researches on burrowing1 have substantiated Clark's suggestion that the method of penetration into a soft substrate is essentially the same in all soft-bodied animals2. The basis of this mechanism (Fig. 1) is the formation of two types of anchors, first by dilation of part of the body above the distal extremity to form what may be termed the “penetration anchor”, and second, by the distension of the extremity, the “terminal anchor”. These terms are here introduced to describe the anchorages from a functional aspect in all soft-bodied animals and respectively supersede terms used in previous accounts for restricted groups of animals3,4. When the circular muscles contract, the tip of the animal elongates and the penetration anchor enables this to be extended downwards by preventing the animal from being thrust upwards out of the sand. The terminal anchor, which is formed by part of the body-wall at the lower extremity of the animal pressing outwards against the substrate, allows the upper part to be drawn into the burrow by the contraction of longitudinal or retractor muscles. These anchorages are successively applied until burial is complete. Before the animal has entered the burrow, the penetration anchor cannot be applied and the animal is only prevented from being pushed backwards by its weight, while making rather weak penetrative or scraping movements.

Coxal Cavities and the Classification of the Adephaga (Coleoptera)12

The origin and evolution of coxal cavities are discussed. Each major type occurring in the Adephaga is described and illustrated, and its distribution within the suborder is listed. Because of extensive parallel and convergent evolution, no classification can be based on the variations of the coxal cavities of a single thoracic segment, as has been attempted in the past; but when used in combination with other characters, the coxal cavities do offer evidence about phylogeny. A natural classification is proposed. Jeannel's Isochaeta and Anisochaeta are accepted as the primary subdivisions of the Adephaga. The Isochaeta are subdivided into Septisternia and Glabricornia; the Anisochaeta, into Isopleuri, Hemipleuri, and Lobopleuri. A few genera do not fit into this classification, and are discussed separately.

Evolution of the paranasal cavities of man. Phyletic and embryological study

L'auteur etudie le developpement des cavites paranasales chez les ver-tebres, en particulier chez les mammiferes, les grands primates, et chez l'homme.Il fait ensuite une etude embryologique des cavites sinusiennes de l'homme, et il montre comment la station verticale de l'homo sapiens a conduit au developpement special de l'ethmoide, et du complexe ethmoido-frontal humain.

The evolution of bullous cavities in adequately treated experimental pulmonary tuberculosis.

The evolution of bullous cavities in adequately treated experimental pulmonary tuberculosis.