Animation of fiber fuse damage, demonstrating periodic void formation

A series of optical micrographs showing the front region of fiber fuse damage were obtained to reveal the periodic void formation process. They were collected from a number of samples and were sorted in order of increasing distance between the top of the first large void and the top of the first regular void. The micrographs clearly show that the first large void sheds its tail, which shrinks to form a regular void. This mechanism leads to the formation of bullet-shaped regular voids as the result of the balance between the internal pressure of the optical discharge and the increasing viscosity of the surrounding glass that occurs during pinching off.     

Standing electron plasma wave mechanism of void array formation inside glass by femtosecond laser irradiation

We investigate the mechanism of formation of periodic void arrays inside fused silica and BK7 glass irradiated by a tightly focused femtosecond (fs) laser beam. Our results show that the period of each void array is not uniform along the laser propagation direction, and the average period of the void array decreases with increasing pulse number and pulse energy. We propose a mechanism in which a standing electron plasma wave created by the interference of a fs-laser-driven electron wave and its reflected wave is responsible for the formation of the periodic void arrays. PACS 61.80.Ba; 42.65.Re; 42.70.Ce; 61.72.Qq; 52.35.Mw     

Theory of dust voids in plasmas

Dusty plasmas in a gas discharge often feature a stable void, i.e., a dust-free region inside the dust cloud. This occurs under conditions relevant to both plasma processing discharges and plasma crystal experiments. The void results from a balance of the electrostatic and ion drag forces on a dust particle. The ion drag force is driven by a flow of ions outward from an ionization source and toward the surrounding dust cloud, which has a negative space charge. In equilibrium the force balance for dust particles requires that the boundary with the dust cloud be sharp, provided that the particles are cold and monodispersive. Numerical solutions of the one-dimensional nonlinear fluid equations are carried out including dust charging and dust-neutral collisions, but not ion-neutral collisions. The regions of parameter space that allow stable void equilibria are identified. There is a minimum ionization rate that can sustain a void. Spatial profiles of plasma parameters in the void are reported. In the absence of ion-neutral collisions, the ion flow enters the dust cloud's edge at Mach number M=1. Phase diagrams for expanding or contracting voids reveal a stationary point corresponding to a single stable equilibrium void size, provided the ionization rate is constant. Large voids contract and small voids expand until they attain this stationary void size. On the other hand, if the ionization rate is not constant, the void size can oscillate. Results are compared to recent laboratory and microgravity experiments.     

Material transport in Al-metallizations of power-loaded SAW structures

Material transport in Al/Ti-finger electrodes on LiNbO₃ substrates of power-loaded surface acoustic wave (SAW) structures were investigated under microscopic observation with respect to stress-induced material transport. Additionally, investigations were carried out before and after SAW loading during lifetime experiments. For the experiments, a special power SAW test structure was applied realizing travelling SAWs. The results show that the microstructure of the electrodes was damaged by void and hillock formation even at moderate input power. This changes the electrical and acoustical properties of the SAW structures irreversibly. The logarithmic time-to-failure (TTF) of damaged SAW structures depends linearly on loading time and rf power.     

基于单孔洞近似的高纯铝部分层裂实验的数值模拟研究

基于单孔洞近似,对不同撞击速度下高纯铝的部分层裂实验进行了数值模拟研究,讨论了微孔洞长大对波传播的影响及其在自由面速度波剖面上的表现. 通过分析微孔洞周围的应力场变化,认识到实测自由面速度波剖面出现"回跳"特征并不能说明材料发生完全层裂,其直接原因是样品内部微孔洞长大所引起的局部卸载效应. 将计算得到的自由面速度波剖面和微孔洞相对体积与实验结果进行了对比分析,发现两者均符合很好,表明采用单孔洞增长来近似描述部分层裂样品中随机损伤发展及其对波传播的影响是可行的. 关键词： 层裂 孔洞增长 自由面速度波剖面 微孔洞相对体积     

Bubble Velocity and Size Measurement with a Four‐Point Optical Fiber Probe

The possibility to measure the velocity and size of individual bubbles in a high‐void fraction bubbly flow is investigated by using a four‐point optical fiber probe. The air bubbles have an initial spherical equivalent diameter ranging from 4 to 10 mm and the void fraction is up to 0.3. Firstly, single bubble experiments show that intrusiveness effects, i.e. bubble deformations due to the probe, are negligible provided that the bubble approaches the probe at the axis of the central fiber. A selection criterion is utilized for multiple bubble experiments. A good compromise can be found between the required accuracy, the duration of the measurements and the number of validated bubbles required for reliable statistical averaging. In an air‐water high‐void fraction vertical bubbly pipe flow, the void fraction obtained with the instrument is found to be in good agreement with both local single‐fiber probe measurements, and with the volume average void fraction obtained from pressure gradient measurements. The area average volumetric gas flow rate, based on the bubble velocity and void fraction as measured with the four‐point probe, agree with the measured gas flow rate. Also, the liquid velocity is measured by means of a laser‐Doppler anemometer, to investigate the slip velocity. The results show that reliable and interesting measurements can be obtained by using a four‐point optical fiber probe in high void fraction flows.     

Atomic simulations to evaluate effects of stacking fault energy on interactions between edge dislocation and spherical void in face-centred cubic metals

In this study, molecular dynamics simulations were performed to elucidate the effects of stacking fault energy (SFE) on the physical interactions between an edge dislocation and a spherical void in the crystal structure of face-centred cubic metals at various temperatures and for different void sizes. Four different types of interaction morphologies were observed, in which (1) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the trailing partial; (2) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the leading partial; (3) the partial dislocations detached from the void almost simultaneously without jog formation; and (4) the partial dislocations detached from the void almost simultaneously with jog formation. With an increase in void size or SFE, the interaction morphology changed in the above-mentioned order. It was observed that the magnitude of the critical resolved shear stress (CRSS) and its dependence on the SFE were determined by these interaction morphologies. The value of the CRSS in the case of interaction morphology (1) is almost equal to an analytical one based on the linear elasticity by employing the Burgers vector of a single partial dislocation. The maximum value of the CRSS is also obtained by the analytical model with the Burgers vector of the two partial dislocations.     

Void formation in Nickel during high temperature proton irradiation

Pure Ni foils, doped with He from 0 to 28 appm, were irradiated with protons at temperatures in the range 0.3–0.6 T_m (T_m = melting point in °K) and void formation was studied. The influence of He doping, irradiation temperature and alloying were investigated. For constant He content and proton fluence, void number density and swelling are maximum at about 400°C, while the void size increases with temperature. Most voids are octahedral in shape with no sign of truncation. Helium is required to nucleate voids, and lowering the stacking fault energy by alloying suppresses void formation completely. Present results suggest that void nucleation is inhomogeneous. Some implications of these findings are discussed.     

Numerical Simulation of Dust Void Evolution in Complex Plasmas with Ionization Effect

We develop the nonlinear theory of dust voids [Phys. Rev. Lett. 90 （2003） 075001], focusing particularly on effects of the ionization, to investigate numerically the void evolution under cylindrical coordinates [Phys. Plasmas 13 （2006） 064502]. The ion velocity profile is solved by a more accurate ion motion equation with the ion convection and ionization terms. It is shown that the differences between the previous result and the one obtained with ionizations are significant for the distributions of the ion and dust velocities, the dust density, and etc., in the void formation process. Furthermore, the ionization can slow down the void formation process effectively.     

Hydrogen induced voids in hydrogenated amorphous silicon carbon (a-SiC:H): Results of effusion and diffusion studies

The void formation in Si-rich a-SiC:H films deposited with dc magnetron sputtering is studied by effusion measurements of hydrogen and of implanted rare gases and secondary ion mass spectrometry (SIMS). Rare gas atoms were incorporated into the material by ion implantation. The results suggest a widening of the network openings with increasing alloy concentration. However, the void formation is mainly attributed not to an increase in carbon concentration but to an increase in hydrogen incorporation.     

Deformation Processes in Materials under Radiation Exposure

Russian Physics Journal - The paper focuses on the void swelling processes in materials subjected to radiation, that occur due to the formation of the void ensemble in the crystal lattice....     

Size of dust voids as a function of the power input in dusty plasma

A dust void is a dust-free region in dusty plasma. Theory demonstrates that the void results from the balance of the electrostatic and plasma (such as the ion drag) forces acting on a dust particle. In dusty plasma experiments, physical properties of the void show clear dependence on the power input into the plasma (in particular, its size increases with the increase of the applied power). Here, the theory and numerical results are presented for such a dependence. The text was submitted by the authors in English.     

An Experimental Study of the Two-Phase Flow Adjacent to a Wire in Nucleate Boiling Under an Electric Field

An experimental study of the two-phase flow near a heated wire in nucleate pool boiling is presented. A nonuniform electric field of cylindrical geometry was imposed to the heater. The two-phase flow parameters were measured using specifically developed local phase-detection optical probes. The study includes experiments varying the vertical distance from the heater (z) and measurements of the void fraction and the impact rate in a plane perpendicular to the heater (y-z). The results show that the void fraction decreases with applied voltage and with z. The dependence of the void fraction and the impact rate with heat flux is complex, with a strong decrement in the impact rate and void fraction near the critical heat flux when voltage is applied. The maximum void fraction and impact rate move from the vertical centerline (z) to a position 3 heater radii away.     

Film formation from PS latex doped PNIPAM hydrogels at various heating and cooling rates

Film formation from polystrene (PS) latex doped poly(N-isopropylacrylamide) (PNIPAM) hydrogels was studied by using photon transmission technique. The transmitted light intensity, I _tr was monitored during film formation process. Films were prepared by annealing, 10 wt% PS doped PNIPAM particles at five different heating and cooling rates at temperatures ranging from 10 to 100°C. I _tr presented a hysteresis loops during heating–cooling cycles, which were explained by void closure and void reconstruction processes. The corresponding activation energies were measured during reversible film formation process. Void closure and void reconstruction models were introduced to produce the activation energies.     

Phase change and stress wave in picosecond laser–material interaction with shock wave formation

When background gas is present in pulsed laser–material interaction, a shock wave down to the nanoscale will emerge. The background gas will affect the phase change and explosion in the target. This study is focused on the void dynamics and stress wave in a model material (argon crystal) under picosecond pulsed laser irradiation. Our results show that existence of ambient gas and the shock wave significantly suppresses the void formation and their lifetime. Void dynamics, including their growing rate, lifetime, and size under the influence of ambient gas are studied in detail. All the voids undergo an accelerating and decelerating process in the growth. The collapsing process is almost symmetrical to the growing process. Higher laser fluence is found to induce an obvious foamy structure. Stress wave formation and propagation, temperature contour, and target and gas atom number densities are studied to reveal the underlying physical processes. Although the interaction of the plume with ambient gas significantly suppresses the void formation and phase explosion, no obvious effect is found on the stress wave within the target. Very interestingly, secondary stress waves resulting from re-deposition of ablated atoms and void collapse are observed, although their magnitude is much smaller than the directly laser-induced stress wave.     

Nonlinear theory of void formation in colloidal plasmas

A nonlinear time-dependent model for void formation in colloidal plasmas is proposed. For experimentally relevant initial conditions, the model describes the nonlinear evolution of a zero-frequency linear instability that grows rapidly in the nonlinear regime and subsequently saturates to form a void. A number of features of the model are consistent with experimental observations under laboratory and microgravity conditions.     

Void Closure in Complex Plasmas under Microgravity Conditions

We describe the first observation of a void closure in complex plasma experiments under microgravity conditions performed with the Plasma-Kristall (PKE-Nefedov) facility on board the International Space Station. The void--a grain-free region in the central part of the discharge where the complex plasma is generated--has been formed under most of the plasma conditions and thought to be an inevitable effect. However, we demonstrate in this Letter that an appropriate tune of the discharge parameters allows the void to close. This experimental achievement along with its theoretical interpretation opens new perspectives in engineering new experiments with large quasi-isotropic void-free complex plasma clouds in microgravity conditions.     

Electrochemical effects of isolated voids in uranium dioxide

We present a model to study the electrochemical effects of voids in oxide materials under equilibrium conditions and apply this model to uranium dioxide. Based on thermodynamic arguments, we claim that voids in uranium dioxide must contain oxygen gas at a pressure that we determine via a Kelvin equation in terms of temperature, void radius and the oxygen pressure of the outside gas reservoir in equilibrium with the oxide. The oxygen gas within a void gives rise to ionosorption and the formation of a layer of surface-charge on the void surface, which, in turn, induces an influence zone of space charge into the matrix surrounding the void. Since the space charge is carried in part by atomic defects, it is concluded that, as a part of the thermodynamic equilibrium of oxides containing voids, the off-stoichiometry around the void is different from its remote bulk value. As such, in a uranium dioxide solid with a void ensemble, the average off-stoichiometry level in the material differs from that of the void-free counterpart. The model is applied to isolated voids in off-stoichiometric uranium dioxide for a wide range of temperature and disorder state of the oxide.     

Mechanism of coarsening and bubble formation in high-genus nanoporous metals

Coarsening of crystalline nanoporous metals involves complex changes in topology associated with the reduction of genus via both ligament pinch-off and void bubble formation. Although void bubbles in metals are often associated with vacancy agglomeration, we use large-scale kinetic Monte Carlo simulations to show that both bubble formation and ligament pinch-off are natural results of a surface-diffusion-controlled solid-state Rayleigh instability that controls changes in the topology of the porous material during coarsening. This result is used to find an effective activation energy for coarsening in nanoporous metals that is associated with the reduction of topological genus, and not the reduction of local surface roughness.