Stable Voids Research Articles

Periodic index-modulated plasma waveguide

We demonstrate a wire-obstructed cluster flow technique for making periodically modulated plasma waveguides in hydrogen, nitrogen, and argon with sharp, stable voids as short as 50 microm with a period as small as 200 microm. These gaps persist as the plasma expands for the full lifetime of the waveguide. We demonstrate guided propagation at intensities up to 2 x 10(17) W/cm(2), limited by our laser energy currently available. This technique is useful for quasi-phase matching applications where index-modulated guides are superior to diameter modulated guides.

Atomistic Simulation of Voids Effect on Nanoindentation

In this paper, spherical voids effect on the first dislocation emission of single-crystal Cu in nanoindentation is investigated by means of the atomistic simulation. The results show that there are two opposite effects on dislocation emission resulting from the stable voids in matrix. The existence of voids can lower or heighten the load needed to nucleate the first dislocation. The relative distance between the void and the indenter, and the void size play an important role in the yield load of the first dislocation emission in nanoindentation.

Contemporary analysis of erectile, voiding, and oncologic outcomes following primary targeted cryoablation of the prostate for clinically localized prostate cancer

To evaluate erectile function (EF) and voiding function following primary targeted cryoablation of the prostate (TCAP) for clinically localized prostate cancer (CaP) in a contemporary cohort. We retrospectively reviewed all patients treated between 2/2000-5/2006 with primary TCAP. Variables included age, Gleason sum, pre-TCAP prostate specific antigen (PSA), prostate volume, clinical stage, pre-TCAP hormonal ablation, pre-TCAP EF and American Urologic Association Symptom Score (AUASS). EF was recorded as follows: 1 = potent; 2 = sufficient for intercourse; 3 = partial/insufficient; 4 = minimal/insufficient; 5 = none. Voiding function was analyzed by comparing pre/post-TCAP AUASS. Statistical analysis utilized SAS software with p < 0.05 considered significant. After exclusions, 78 consecutive patients were analyzed with a mean age of 69.2 years and follow-up 39.8 months. Thirty-five (44.9%) men reported pre-TCAP EF level of 1-2. Post-TCAP, 9 of 35 (25.7%) regained EF of level 1-2 while 1 (2.9%) achieved level 3 EF. Median pre-TCAP AUASS was 8.75 versus 7.50 postoperatively (p = 0.39). Six patients (7.7%) experienced post-TCAP urinary incontinence. Lower pre-TCAP PSA (p = 0.008) and higher Gleason sum (p = 0.002) were associated with higher post-TCAP AUASS while prostate volume demonstrated a trend (p = 0.07). Post-TCAP EF and stable AUASS were not associated with increased disease-recurrence (p = 0.24 and p = 0.67, respectively). Stable voiding function was observed post-TCAP, with an overall incontinence rate of 7.7%. Further, though erectile dysfunction is common following TCAP, 25.7% of previously potent patients demonstrated erections suitable for intercourse. While long-term data is requisite, consideration should be made for prospective evaluation of penile rehabilitation following primary TCAP.

A limit analysis of the kinematics of void collapse using the discrete element method

Discrete element modelling of oedometer tests has been performed to investigate the kinematics of void collapse by measuring the void reduction as a result of completely removing particles in the oedometer one after another. For smooth conditions, it is found that volume change due to a ball being removed in an oedometer is approximately equal to the volume of the ball removed plus its associated void space. For frictional balls, this finding is only true once the formation of soil arching in the oedometer reaches a constant condition. Soil arching forms in samples with frictional particles and creates stable voids within the oedometer sample. The extent of soil arching generally increases with increasing coefficient of particle friction. However, it is found that, at high coefficients of particle friction, the correlation with the extent of soil arching is weak. It has been shown that if void reduction in an oedometer is to be evaluated accurately, at least 20 particles need to be deleted to obtain a good average of void reduction per particle deleted. It is also important to allow soil arching to reach a constant condition before extracting any void reduction data.

Stress-induced deceleration of electromigration-driven void motion in metallic thin films

We report effects of biaxially applied stress on the electromigration-driven motion of morphologically stable voids in elastically deforming metallic thin films based on self-consistent numerical simulations of void evolution. We find that under certain electromechanical conditions, the applied stress can cause substantial deceleration of the electromigration-driven void motion, as measured by the migration speed of morphologically stable voids translating along the metallic thin film. This effect reveals a new aspect of the complex evolution of voids in metallic thin films under the action of external fields: deceleration of electromigration-driven void motion and even complete inhibition of such motion may be possible through application of mechanical stress.

Theoretical analysis of current-driven interactions between voids in metallic thin films

We analyze the electromigration-induced interactions between voids in metallic thin films using self-consistent numerical simulations of current-driven void morphological evolution. We focus on the interactions between two voids that, if isolated, they would be morphologically stable and migrate at constant speed along the metallic thin film. It is demonstrated that, under certain electromigration conditions, two such different-sized voids migrating in the same direction along a metallic thin film can lead to stable time-periodic states characterized by wave propagation on a void’s surface or cause failure prior to or following their coalescence. Moreover, various complex phenomena are revealed, including void breakup into two morphologically stable voids following the coalescence of the original two voids and voids attempting to pass each other. In numerous cases, it is noteworthy that the void-void interactions examined arise due to a larger void migrating faster than an originally leading smaller one in a finite-width conducting film with anisotropic material properties, contrary to the conventional notion that smaller voids migrate faster than larger ones for electromigration-driven void motion. These results set the stage toward a fundamental understanding of the current-driven dynamics of populations of interacting voids in metallic interconnect lines. Finally, our analysis predicts that void breakup preceded by void coalescence can cause an abrupt increase in the electrical resistance of an interconnect line, in qualitative agreement with observations from accelerated electromigration testing experiments.

Stability of dust voids

Dust voids are frequently observed as dust-free regions in a dusty plasma. Experiments demonstrate a variety of dust void structures such as stable or unstable voids, global “heatbeat” modes of oscillations of voids, dust voids in the center of the chamber or near its walls. Theory shows that a dust void generally results from the balance of the electrostatic and the plasma (such as the ion drag) forces acting on a dust particle. Here, the stability theory of a void is developed and the void behavior is modeled. It is shown that sequences of stable and unstable void sizes can exist. The dynamics of dust in a plasma follows these stability characteristics leading to various stable and/or unstable dust void structures.

Simulations of the effects of 1-d interstitial diffusion on void lattice formation during irradiation

This paper describes the use of simulation techniques to examine some of the processes involved in the alignment of voids under the influence of one-dimensional self interstitial atom (1-d SIA) transport. The work follows the paper of Heinisch and Singh [1] on this topic but a different and simpler methodology is used. Besides repeating the scenarios studied by Heinisch and Singh, the effects of re-nucleation and the influence of vacancies have been introduced. One of the important processes that emerged from the results was the barrier to precise void alignment caused by the SIA-induced coalescence of aligned voids. This appears to prevent the formation of stable void lattices by any 1-d SIA transport mechanism, a point supported by the initial void alignment in the mechanism requiring swelling values well above those found experimentally. A full consideration of the void lattice phenomenon shows that the one-dimensional diffusion of self-interstitials central to the production bias model of irradiation damage cannot be the only mode of anisotropic diffusion available under irradiation.

Analysis of Electromigration-Induced Void Motion and Surface Oscillations in Metallic Thin-Film Interconnects

AbstractAn analysis is presented of electromigration-induced migration and oscillatory dynamics of morphologically stable void surfaces based on self-consistent numerical simulations of morphological evolution of voids in metallic thin films. As the morphological stability limit is approached, the migration speed of a stable void deviates substantially from being inversely proportional to the void size, a well-known result that is rigorously valid in an infinite conductor with isotropic material properties. A non-linear shape function that includes both current crowding and diffusional anisotropy effects is used to rescale properly the void migration speed resulting in a universally valid relationship for the migration speed as a function of void size. Furthermore, in grains characterized by high symmetry of surface diffusional anisotropy, our analysis predicts the onset of stable time-periodic states for the void surface morphology that correspond to waves propagating on surfaces of voids that migrate along the metallic film at constant speeds.

Analysis of Electromigration- and Stress-Induced Dynamical Response of Voids Confined in Metallic Thin Films

AbstractA theoretical analysis based on self-consistent dynamical simulations is presented of electromigration- and stress-induced surface morphological response of voids confined in metallic thin films. The analysis predicts the onset of stable time-periodic states for the void surface morphological response, which is associated with current-driven wave propagation on the void surface. This time-periodic response is demonstrated under certain electromigration conditions and detailed response diagrams are presented which map the corresponding parameter space to regions of steady, time-periodic, and unstable surface morphological response. The evolution of the electrical resistance of these thin films also is computed, providing an interpretation for experimentally observed time-periodic response of the electrical resistance of metallic interconnect lines on the basis of current-driven void morphological evolution. In addition, we demonstrate significant effects on the electromigration-induced morphologically stable void migration of mechanical stress application in a metallic thin film. Specifically, we find that under certain electromechanical conditions, elastic stress can cause substantial retardation of void motion, as measured by the constant speed of electromigration-induced translation of morphologically stable voids. More importantly, this effect suggests the possibility for complete inhibition of void motion under stress.

Electromigration-driven motion of morphologically stable voids in metallic thin films: Universal scaling of migration speed with void size

The dependence on void size of the migration speed of morphologically stable voids that translate along metallic thin films due to surface electromigration is analyzed in finite-width films through self-consistent numerical simulations taking surface diffusional anisotropy into account. It is shown that, as the morphological stability limit is approached, the void migration speed deviates substantially from being inversely proportional to the void size. A nonlinear “shape function” that includes both current crowding and diffusional anisotropy effects is derived and incorporated into the well-known theoretical result that is valid for infinite-conductor domains and isotropic surface diffusivity. Rescaling the void migration velocity with the corresponding, numerically evaluated shape function results in a universally valid relationship for the migration speed as a function of void size. This result is important in understanding electromigration-induced void dynamics in metallic interconnect lines.

Stability of voids formed in cavities at liquid–solid interfaces

A thermodynamic model is developed of the free energy of gas-filled voids formed within cavities on solid surfaces covered by a liquid. Capillary effects are assumed to be the only important contributions to the free energy, and expressions are derived for the free energy of the system as a function of the void size, the relative surface free energy densities involved, and the geometry of the cavity. The results of the model are (1) construction of a stability diagram that maps the most stable void configuration versus the wetting properties of the various solid surfaces involved, and (2) rough estimates of the work required to liberate a void of a given size and position. The model can give qualitative insight into the stability of coating defects on uneven surfaces, and also can be used to prescribe possible surface treatments for reducing the work required to remove voids from the system.

Probe induced voids in a dusty plasma

An experimental study of the formation of voids (dust-free regions) around negatively biased probes in a dusty plasma is described. Stable voids are maintained by the balance of electric and ion drag forces on the dust particles. A theoretical model is proposed to explain how the size of the void scales with the probe bias potential.

Damage accumulation and failure of HSLA-100 steel

Damage accumulation in the form of the volume fractions and number densities of strain-induced voids has been experimentally characterized for HSLA-100 steel subjected to tensile failure over a range of stress-states (stress triaxiality ratios of 0.8 to 1.4). The dependence of void volume fraction on strain indicates the presence of a void growth stage that is sensitive to stress-state in a manner that can be described by a relationship with the form that can be predicted by Rice and Tracey but with an increased dependence on stress triaxiality. The damage results also suggest a transition of stable void growth to rapid void growth and imminent material fracture at a critical void volume fraction that decreases slowly with increasing stress triaxiality ratio. A straight-forward analysis, based on the experimental observations, relates the observed experimental dependence of failure strains on stress triaxiality for this steel. The damage accumulation behavior of this steel, which fails due to the growth and coalescence of equiaxed voids, is also contrasted to that of HY-100 steel which forms elongated voids.

Kinetic Monte Carlo simulations of void lattice formation during irradiation

Over the last decade, molecular dynamics simulations of displacement cascades have revealed that glissile clusters of self-interstitial crowdions are formed directly in cascades and that they migrate one-dimensionally along close-packed directions with extremely low activation energies. Occasionally, under various conditions, a crowdion cluster can change its Burgers vector and glide along a different close-packed direction. The recently developed production bias model (PBM) of microstructure evolution under irradiation has been structured specifically to take into account the unique properties of the vacancy and interstitial clusters produced in the cascades. Atomic-scale kinetic Monte Carlo (KMC) simulations have played a useful role in understanding the defect reaction kinetics of one-dimensionally migrating crowdion clusters as a function of the frequency of direction changes. This has made it possible to incorporate the migration properties of crowdion clusters and changes in reaction kinetics into the PBM. In the present paper we utilize similar KMC simulations to investigate the significant role that crowdion clusters can play in the formation and stability of void lattices. The creation of stable void lattices, starting from a random distribution of voids, is simulated by a KMC model in which vacancies migrate three-dimensionally and self-interstitial atom (SIA) clusters migrate one-dimensionally, interrupted by directional changes. The necessity of both one-dimensional migration and Burgers vectors changes of SIA clusters for the production of stable void lattices is demonstrated, and the effects of the frequency of Burgers vector changes are described.

Observations related to healing of interface damage and cavity nucleation during superplastic flow

Two critical observations related to the nucleation of superplastic cavities are reported. Cavities preexisting in a fine grain Al–Mg alloy prior to superplastic deformation have been minimized, but not completely eliminated by a high temperature annealing treatment. This has reduced the density of emerging cavities during subsequent deformation, however nucleation of cavities occur continuously with superplastic strain, starting possibly from partially healed defects of imperceptible (nanoscale) size. The observations raise doubts about the often-used cavity nucleation criterion based on a thermodynamically stable void size. Diffusional cavity growth model, used in conjunction with this assumed “stable” size artificially enlarges its perceived contribution, but still predicts less growth than the observed cavitation levels in superplastic metals. Alternate growth mechanism for submicron size cavities is suggested.

Micromechanisms of slow crack growth in polyethylene under constant tensile loading

Circumferentially notched specimens of a first generation and a third generation pipe-grade of high density polyethylene with similar weight average molar masses have been subjected to constant tensile loads at 80°C. A transition from full ligament yielding to failure by stable sub-critical crack growth was observed as the applied load was decreased. The specimen lifetimes in this latter regime were dependent on the initial stress intensity factor, K i, and failure was associated with slow crack propagation preceded by formation of a wedge-shaped cavitational deformation zone at the notch tip. The fibril diameters in the deformation zones decreased with stress intensity factor near the transition, the limiting behaviour of a relatively slow crack growth resistant third generation grade at the lowest K i being inferred from testing in Igepal™ to be the breakdown of diffuse zones of interlamellar voiding. This regime was not directly accessible to testing in air within the allotted experimental times. However, comparison with the results of accelerated testing in cyclic fatigue has indicated stable interlamellar voiding in the third generation grade not to necessitate the presence of Igepal. Moreover, in both grades, very similar modes of deformation were observed in air and in Igepal at relatively high K i. Igepal was therefore inferred not to lead to qualitative changes in the range of mechanisms that are characteristic of slow crack growth in polyethylene.

Direct observation of voids in the vacancy excess region of ion bombarded silicon

The results reported in this letter indicate that the spatial separation of the vacancy and interstitial excesses which result from ion bombardment gives rise to stable voids upon annealing at 850 °C even for implants where the projected ion range is only of the order of a few thousand Ångstrom. Such voids have been observed directly by transmission electron microscopy. Furthermore, in cases where both voids and interstitial-based defects are present at different depths, it is found that Au has a strong preference for decorating void surfaces and hence Au can, indeed, be used as a selective detector of open volume defects in Si.

AUTOMATED NONINVASIVE MEASUREMENT OF CYCLOPHOSPHAMIDE-INDUCED CHANGES IN MURINE VOIDING FREQUENCY AND VOLUME

We developed an automated noninvasive method for studying bladder function in mice. Changes in voided volume and frequency accompanying cyclophosphamide-induced cystitis were measured using computerized digital balances. Eight CRL CD-1 mice were given a solution of glucose and saccharin to increase urine output and studied during the dark cycle, when most voiding occurs. Urine fell directly onto electronic balance pans. The time and weight of each void was recorded. Computer programs adjusted for evaporative loss analyzed voiding data within and across sessions. After establishing stable voiding patterns 300 mg./kg. cyclophosphamide were administered intraperitoneally. The Wilcoxon signed rank test was done to compare median voided volumes, frequency and gm. per hour of urine produced before and after cyclophosphamide. We implemented an automated method for voiding studies in mice. After cyclophosphamide administration the number of voids per hour increased and voided volume decreased. Some mice had as much as a 70% decrease in bladder volume and a tripling of urinary frequency. Mice responded by a sustained elevation in frequency and decreased voided volume as early as 24 hours after cyclophosphamide administration or by a pattern of delayed toxicity. This noninvasive technique measures changes in mouse voiding patterns with great sensitivity and minimal effort. The method is applicable to murine models of interstitial cystitis, detrusor instability and other abnormal voiding states. It may be used for evaluating potential therapies for such conditions.

Simulating formation of voids in charged colloids by brownian dynamics

Using Brownian dynamics simulations and Sogami's effective pair potential with a long-range attractive term for the colloidal particles, we report that the mechanism of making voids in charged colloids is the long-range attractive interaction between particles. The present results are in good agreement with the experimental observations of stable voids in equilibrium with ordered structures, which may not be explained by repulsive potentials without a long-range attractive interaction such as the Derjaguin-Landau-Vervey-Overbeck potential. Our results are also in a good agreement with Monte Carlo simulation results. The voids have been reported with ordered structure in charged colloids by Brownian dynamics simulations.