Moment Of Breakdown Research Articles

Efficiency of spark discharge in electrohydraulic lithotripsy.

Electrohydraulic lithotripters and SWT devices generate shock waves by discharge of a high-voltage capacitor through submerged electrodes. As the electrodes age, the interelectrode gap widens. How this affects the efficiency of spark generation was studied using a research HM3-clone lithotripter. Widening of the interelectrode gap (∼0.3 mm with new electrodes; ∼2.5 mm after 4000 discharges) increased the lag-time to breakdown (∼0 to ∼30 μs, respectively). Increased lag-time as electrodes aged was associated with partial discharge of the capacitor (leakage promoted by ∼0.6 mS conductivity of the surrounding water), such that the average energy remaining at the capacitor at the moment of breakdown was reduced four times compared to new electrodes. However, with new electrodes almost 90% of the energy was lost in the circuitry rather than in the spark, as the resistance of the spark (R∼0.03 Ω) was much smaller than the resistance of the remainder of the circuit—including the high-voltage switch and connective wires (r∼0.23 Ω). With old electrodes, most of the energy was released in the spark, as the electric efficiency R/(R+r) approached ∼90%. The product of electric efficiency and capacitor energy agreed closely with both the acoustic energy and the volume of the primary bubble produced at spark discharge. [Work supported by NIH-DK43881.]

High Voltage Marx Generator Implementation using IGBT Stacks

High voltage Marx generator implementation using IGBT (Insulated Gate Bipolar Transistor) stacks is proposed in this paper. To protect the Marx generator at the moment of breakdown, AOCP (Active Over-Current Protection) part is included. The Marx generator is composed of 12 stages and each stage is made of IGBT stacks, two diode stacks, and capacitors. IGBT stack is used as a single switch. Diode stacks and inductors are used to charge the high voltage capacitor at each stage without power loss. These are also used to isolate input and high voltage negative output in high voltage generation mode. The proposed Marx generator implementation uses IGBT stack with a simple driver and has modular design. This system structure gives compactness and easiness to implement the total system. Some experimental and simulated results are included to verify the system performances in this paper.

Study on breakdown characteristics of a low-pressure dielectric barrier discharge

AbstractThe breakdown characteristics of a low-frequency dielectric barrier discharge (DBD) at low pressure are investigated experimentally in He, Ne and Ar. The current waveform of this DBD, a series of pulses, is caused by the results of the electron avalanches under the action of the applied field and the quenching effect of the wall-charge field on the avalanches. Taking into consideration the diffusion loss of the charged particles in the breakdown processes, the experimental investigation and the theoretical analysis of its breakdown characteristics indicate that its breakdown voltage is higher than the calculation by the Paschen law and depends on the filled-gas pressure and the distance between the electrodes, instead of their product. The ion-induced secondary electron emission coefficient and the mean electron energy at the moment of breakdown can also be determined approximately by measuring the breakdown characteristics of the discharge tubes with different distances between the electrodes and combining the theoretical deductions. The experimental results and the theoretical analysis of the breakdown characteristics of this DBD are discussed.

On the Grain Boundary Grooving in Thin Filaments

We considered a polycrystalline cylindrical wire of the initial radius R0 composed of identical cylindrical grains of the length L0, strained uniaxially by an external stress P. At the temperatures at which some surface and grain boundary diffusion are allowed the thinning of the nanowire in the vicinity of grain boundaries occurs due to the phenomenon of grain boundary grooving. We calculated the equilibrium shapes of the nanowire achieved after long annealing times. Our calculations demonstrated that for any given L0/R0 ratio some critical value of the applied stress exists above which the nanowire is unstable and breaks down into the string of isolated spherical particles, in full analogy with the Rayleigh instability of long cylinders. The kinetics of the shape change was calculated numerically. It was shown that the rate of thinning of unstable wires diverges as the moment of breakdown is approached. We also demonstrated that the breakdown may occur even for nominally stable wires “on the way” to achieving their equilibrium shape. Therefore, the stability of nanowire is determined by a combination of geometric (L0/R0), thermodynamic (grain boundary energy), and kinetic (ratio of grain boundary and surface diffusivities) parameters. An application of external tensile stress accelerates the breakdown of nanowires.

Thermal stability and creep of polycrystalline nanowires

We considered a polycrystalline cylindrical nanowire with bamboo microstructure strained uniaxially by an external load. Our estimates demonstrated that for sufficiently thin nanowires the evolution of their morphology and the creep rate are controlled by surface and grain boundary self-diffusion. We calculated the equilibrium shapes of the nanowire achieved after long annealing times. Our calculations demonstrated that for any given grain aspect ratio some critical value of the applied load exists above which the nanowire is unstable and breaks up into a string of isolated spherical particles. The kinetics of the shape change were calculated numerically. It was shown that the rate of thinning of unstable nanowires diverges as the moment of breakdown is approached. We also demonstrated that the break up may occur even for nominally stable nanowires “on the way” to achieving their equilibrium shape. The external tensile load accelerates the breakdown of nanowires.

Trafficking

In cities the world over, we are able to determine stability in daily existence, to identify with our social spaces, because modes of transport have become essential components of subjective autonomy. But would it not be just as accurate to say that in transit, modern life puts the self in abeyance? The author argues that the ways we allow ourselves to be moved around in “traffic space” create a passivity that renders almost invisible the complex mechanics of movement, which we become alert to only at the moment of breakdown, precisely when they become a threat to autonomy. Our trafficking has an almost narcotic effect, rendering us immobile against the continual movements that constitute urban life, one that also magnifies out of all proportion the accidents or aberrations that sometimes disturb our traffic space, making it seem as if we may easily descend into an uncontrollable chaos.

Breakdown Modes and Their Evolution in Ultrathin Gate Oxide

Post-breakdown current–voltage characteristics of metal-oxide-semiconductor (MOS) devices with an ultrathin gate oxide layer have been carefully studied. Several breakdown modes were identified. Specifically, it was found that the typical soft-breakdown mode induced in an oxide layer thinner than 3 nm is actually quite different from that in an oxide layer thicker than 3 nm. Based on these findings, we have also proposed a model to explain the evolution of different breakdown modes. The model takes into consideration the thermal runaway process at the breakdown moment, and is substantiated by a number of experimental findings. Impacts of each breakdown mode on device switching behavior are also discussed.

Independence of the Soft Breakdown Phenomenon from the Gate Material: Soft- vs. Hard-Gate Capacitors

The effect of the gate material on soft breakdown is studied by using Hg instead of poly-Si as gate material. It is demonstrated that soft breakdown occurs also in Hg-gate capacitors with a probability that exhibits the same dependence on the oxide thickness and the stress voltage as observed in poly-Si gate capacitors. These results indicate that soft breakdown is a phenomenon independent of the gate material, i.e., influenced more by the details of the energy discharge in the conduction spot at the moment of breakdown than by the details of charge injection and defect accumulation in the oxide. © 2002 The Electrochemical Society. All rights reserved.

Evolution from soft to hard breakdown in thin gate oxides: effect of oxide thickness, capacitor area and stress current

The breakdown of oxide films with thickness ranging from 7 to 4 nm is studied as a function of the stress current and of the capacitor area. The tests were carried out using Hg- and poly-Si-gated capacitors. It is found that the probability of soft breakdown (SBD) decreases if the capacitor area is decreased or if the stress current is increased. It is shown that these experimental observations can be explained if (a) SBD is considered as an incomplete hard breakdown, i.e. a breakdown mode in which the energy dissipated into the percolative path is not enough to trigger its thermal expansion; (b) the SBD probability depends not only on the energy stored in the capacitor and measurement system at the moment of breakdown, but also on the discharge current flowing in the conductive spot.

Analysis of breakdown in ferromagnetic tunnel junctions

Due to their very thin tunnel barrier layer, magnetic tunnel junctions show dielectric breakdown at voltages of the order of 1 V. At the moment of breakdown, a highly conductive short is formed in the barrier and is visible as a hot spot. The breakdown effect is investigated by means of voltage ramp experiments on a series of nominally identical Co/Al2O3/Co tunnel junctions. The results are described in terms of a voltage dependent breakdown probability, and are further analyzed within the framework of a general model for the breakdown probability in dielectric materials, within which it is assumed that at any time the breakdown probability is independent of the (possibly time-dependent) voltage that has been previously applied. The experimental data can be described by several specific forms of the voltage breakdown probability function. A comparison with the models commonly used for describing thin film SiO2 breakdown is given, as well as suggestions for future experiments.

Optical breakdown of helium in Bessel laser radiation beams

Numerical simulation is used to investigate the dynamics of formation of a helium plasma in Bessel beams, shaped by an axicon and a phase converter from a laser radiation pulse with Gaussian temporal and radial intensity profiles. The beam intensities at the breakdown threshold are determined as a function of the pulse duration for various radial field distributions in a beam characterised by Bessel functions of order m (m = 0 — 5). It is shown that in the investigated range of parameters the threshold intensity is independent of m. The temporal and spatial evolution of the resultant plasma, and the dependence of the plasma characteristics on the pulse parameters are considered. Conditions are found for the formation of tubular plasma channels in beams of orders m≥1. The adopted model of the optical breakdown of helium is shown to be satisfactory because of a good agreement between the results of calculations of the moment of breakdown in a zeroth-order Bessel beam and experimental results.

Do interviews really assess students’ knowledge?

Interviews are important assessment means. Most studies assume that interviews can be used to assess a conceptual framework that remains relatively stable in the interview situation. We provide evidence here that questions this assumption. Using a situated cognition framework, we reanalysed pre‐test and post‐test interviews from a large classroom study. Our analyses show that cognition, rather than being a stable property of individuals, is a dynamic process which changes in different ways over shorter and longer time scales. We also observed considerable interactions between the complexity of the questions asked by the interviewer and the complexity of cognitive processes that interviewees enacted. Finally, our analysis reveals moments of breakdown when interviewers’ and interviewees’ expectations of the situation were mismatched. Implications are drawn for organizing interviews intended to elicit information about cognition.

Masculinity, rationality, and family therapy

Contemporary accounts of masculinity have revealed masculine identity to be a fragile achievement. The ‘deconstruction’ of masculinity apparent in Western culture might also be seen in a powerful way in therapy, with the appearance of moments of breakdown in rationality, followed by the gradual articulation of an apparently nonrational position. The particular nature of this position will be as an expression of the ‘semiotic’: a breakthrough of the body into consciousness, an experience of being govened by something outside oneself which is nevertheless representative of the ‘truth’ of one's emotional state. Two illustrations are given of moments in family therapy where masculine assumptions appear to give way to a more confusional and fluid mode of experience.

Erosion jet breakdown accompanying pulsed irradiation of metals by a neodymium laser

A calculation was made of the surface temperature of a metal at the moment of breakdown accompanying interaction with rectangular neodymium laser radiation pulses of microsecond duration and of ~ 500 MW/cm2 intensity. The surface temperature at the moment of plasma formation was found to be weakly dependent on the incident radiation intensity, to be practically constant for each metal, and to exceed the metal boiling point by several hundred degrees. Calculations were performed for a number of metals and the results were compared with experiment. A comparison with previous studies of the breakdown of metal vapors accompanying the interaction of millisecond laser pulses having an intensity of the order of 1 MW/cm2 snowed that the sharp fall in the surface temperature (at the moment of breakdown) on increase in the intensity, which was discovered in these studies, was due to a different dependence of the ionization coefficient on the intensity.

Scheduling on a single machine with a single breakdown to minimize stochastically the number of tardy jobs

Jobs with known processing times and due dates have to be processed on a machine which is subject to a single breakdown. The moment of breakdown and the repair time are independent random variables. Two cases are distinguished with reference to the processing time preempted by the breakdown (no other preemptions are allowed): (i) resumption without time losses and (ii) restart from the beginning. Under certain compatible conditions, we find the policies which minimize stochastically the number of tardy jobs. Copyright

Dynamics of vaporized electrode material during initiation of vacuum breakdown-laser-induced fluorescence

Results of laser-induced-fluorescence measurements of prebreakdown vapor concentration of electrode materials are reported. It is shown that the breakdown moment can be determined with an accuracy of 80 to 100 mu s. At a laser pulse duration of 1 mu s, the probability of coincidence of the pulse with the breakdown is only 2%, which strongly hinders the measurements. In addition, the technique allows the vapor concentration dynamics just before the breakdown to be measured only in a statistical sense. When the growth rate of the initiating voltage is increased, the time needed for experiments is reduced but it becomes doubtful whether the electrode processes remain similar to those taking place under constant voltage. >

Investigation of formation of an optical breakdown plasma near a solid surface subjected to microsecond 10.6 µ radiation

Theoretical and experimental investigations were made of the dependences of the delay time of plasma formation in a gas near the surface of a solid and of the energy density released at the target up to the moment of breakdown on the intensity of microsecond CO2 laser radiation, subject to suitable allowance for the pulse profile. The appearance time of the optical breakdown was found to be determined both by the time needed to heat the target surface to the temperature of advanced evaporation and by the time needed to form a plasma in the gas. The plasma formation times were determined experimentally for the optical breakdown near the surfaces of targets made of graphite and tungsten grids. A study was made of attenuation of radiation by a breakdown plasma as a function of the intensity of laser radiation, as well as of the nature and pressure of the gas.

Temporal and spatial spectroscopy of the plasma formation in crossed electric and magnetic fields

The formation of argon plasma in crossed electric and magnetic fields in a cylindrical diode with an incandescent cathode has been studied by means of the delay time of the anode current pulse and photon pulse (corresponding to the optical transitions) and the shape of the voltage collapse. The working conditions were: pressure p=10−5–10−3 Torr, anode voltage Ua=800 V, and maximum magnetic field Bmax=1200 Gs. Photoelectrical recording of spectral lines was performed with a monochromator in the wavelength range 3600–6000 Å, and the total optical spectrum was recorded simultaneously on a photoplate in a separate spectrograph in the wavelength range 2000–10 000 Å. The delay time of the anode current pulse and photon pulse are approximately the same and are in the millisecond range. The delay time of the photon pulse does not depend on the wavelength. Simultaneously the spectral lines of the working gas (A II), residual gas (O II), and tungsten (W I) appear. Tungsten atoms appear at the moment of breakdown as a result of ion bombardment of the cathode. Neutral atomic lines of the working gas (A I) have not been observed. Radial analysis shows that the delay time of the photon pulse does not depend on the radius. Spectroscopic results have been analyzed in terms of excitation and ionization processes during the formation time. The shape of the voltage collapse suggests the streamer breakdown mechanism.

Measurement of the stickiness of powders of antibiotics

In a number of technological processes the strength properties of powders, with characterize a powder as a whole as a physicochemical system, are of great importance. An important strength characteristic is the resistance of the powder to breakdown -stickiness. This characteristic is very important for process of the packing of powders into bottles that are widely used in the antibiotics industry. Stickiness may also be important for grinding, sieving, granulation, and other processes. The principle of measuring stickiness consists in the application of a force increasing with time directed to breaking down a column of powder, and measuring this force at the moment of breakdown. Various designs of instruments for determining the stickiness of powders based on the use of this principle have been described in the literature [1-4]. The aim of the present work was to determine the stickiness of pharmaceutical powders and to estimate the stickiness of some commercial antibiotics powders. In view of the fact that in a number of cases antibiotics are additionally ground during the production process, we measured the stickiness of powders obtained by jet grinding and also by grinding in an agate ball mill.

Optical Changes in Oxide Films on Iron prior to Breakdown

A RECENT investigation1 using the optical technique of ellip-sometry2 to study the localized breakdown of a passive oxide film on iron by chloride ions revealed the existence of an induction period. During this induction period prior to breakdown, no changes in the film's properties were found using a wavelength of 5461.0 A. Those results indicate that the processes leading to breakdown occur only at the moment of breakdown. This could mean that chloride has no effect on the passive film until the completion of a process at the surface which instantaneously initiates breakdown. An example of this would be the formation of the chloride-metal ion complex suggested by Hoar3 for the breakdown of a passive film on stainless steel. Or, conversely, it would indicate that although subtle changes are taking place in the film prior to breakdown (the slow penetration of the chloride ion through the film's lattice, for example), these changes are not detected with ellipsometry when using the single wavelength of 5461.0 A.