Onset Of Breakdown Research Articles

Break-down of a planar liquid-solid interface during directional solidification; influence of convection

The influence of convection on the development of morphological instability at the liquid-solid interface during directional solidification in a positive thermal gradient has been examined in Pb-10wt%Sn and succinonitrile-1.9wt%acetone. The onset of interfacial breakdown occurs at higher growth speeds in the presence of convection. The linear stability analysis due to Favier and Rouzaud which uses the “deformable” mass flow boundary layer concept shows a good agreement with the experimentally observed behavior.

GaAs/AlGaAs delta-doped staircase avalanche photodiode with separated absorption layer

A GaAs/AIGaAs staircase SAM-APD containing a 10 stage undoped staircase multiplication region is demonstrated. Doping dipoles were inserted in GaAs/Al0.45Ga0.55As interfaces to increase the effective conduction band offset. Typical dark current at 0.9VB was 70pA. The maximum avalanche gain of 1500 was measured near the onset of breakdown. An effective ionisation rate ratio kaff = 0.2 was obtained from multiplication noise measurements.

Navier-Stokes simulation of burst vortex flowfields for fighter aircraft at high incidence

A computational study has been conducted to investigate vortex breakdown on a generic fighter conf iguration operating at high incidence. The flow simulations are based on solutions of the full Reynolds-averaged Navier-Stokes equations using an implicit finite-difference algorithm. Results of the analysis have been correlated with an experimental data base obtained using a 3-D Laser Velocimeter system. Good correlation has been obtained between the analysis and the data base in terms of the vortex core path, cross-flow velocity fields and the approximate vortex breakdown onset location.

Harmonic generation of microwaves in semiconductors in the presence of deep repulsive traps

The characteristics of second-harmonic generation (SHG) of microwaves in a semiconductor at low lattice temperatures are investigated theoretically over a wide range of the dc bias field, beginning from a weakly heated carrier ensemble up to the onset of impurity breakdown giving due consideration to the effect of recombination of the nonequilibrium carriers in the presence of deep repulsive traps. The calculations are carried out for a parabolic law of dispersion and a scalar effective mass. In comparison to what can be obtained under the condition where the nonequilibrium carriers are captured by attractive traps or in the absence of recombination effects, the results of the quantum trapping of the nonequilibrium carriers by the repulsive traps bring in interestingly both qualitative and quantitative changes in the characteristics throughout the entire range of the bias field. The curves for the efficiency of SHG versus the bias field show a number of optimum fields for which the generation would be most efficient. The inadequacies of the present theory are pointed out and the scope for refinement of the same is discussed.

Effect of nonequilibrium interface kinetics on cellular breakdown of planar interfaces during rapid solidification of Si-Sn

During rapid solidification, nonequilibrium interface kinetics alter the predictions of the Mullins-Sekerka theory for the stability of a planar interface against cellular breakdown. The velocity-dependence of the partition coefficient and of the Sn concentration at the onset of cellular breakdown have been measured during pulsed laser melting of Si-Sn alloys. The Mullins-Sekerka theory is modified by inserting a velocity-dependent partition coefficient and a velocity-dependent slope of the “kinetic liquids”, both of which are extracted from the continuous growth model for interface kinetics. These nonequilibrium interface kinetic effects increase the predicted critical concentration for cellular breakdown by two orders of magnitude for Sn in Si, and account fairly well for the experimental results.

Classification of spontaneous oscillations at the onset of avalanche breakdown in p-type germanium.

We present a detailed study on the oscillatory behavior of p-type germanium at the onset of avalanche breakdown at low temperatures. Measuring the frequency and amplitude of these oscillations as a function of different control parameters, such as average current, perpendicular magnetic field, and outer circuit conditions, we can clearly distinguish two different types of oscillations. For one, we found an oscillatory mechanism that depends on the outer electrical circuit. It describes the behavior in detail and also clarifies the connection between the dynamical behavior and the actual appearance of the current-voltage characteristic. Whereas this first type is the result of a global mechanism involving sample voltage and sample resistance, the second type results from a local oscillatory mechanism. Therefore, the more complicated dynamical structure can be understood as a result of the coupling between different localized oscillatory modes.

Numerical simulation of vortex breakdown onset

Based on an improved inviscid multiple line-vortex model (MLVM), numerical simulation for vortex flows over a pointed-nose body of revolution at high angles of attack is used to study the vortex breakdown onset. The result indicates that vortex breakdown occurs at angle of attack 38° with the special initial locations of the free vortex lines given, which coincides with the result of flow visualization in a water tunnel. The calculations also show that the adverse pressure gradient may be one of the reasons which result in vortex breakdown.

Circuit-limited oscillation at the onset of avalanche breakdown in semiconductors

Experimental evidence of one simple type of oscillation, arising in the instability region of electric breakdown, is given. Presenting different semiconductor systems (p-Ge and two types of n-GaAs), we show conditions under which this instability is favored to other competing instabilities.

Fracto-emission-induced electrical breakdown in vacuum

Electrical breakdown was induced in a vacuum directly across a crack. These breakdown events are explained in terms of fracto-emission (the emission of charged particles, neutral species and photons due to fracture) serving as a trigger which induced breakdown. The effect of creating a crack in a dielectric suspended between two electrodes, the dependence of the observed discharges on the magnitude of the applied voltage and on the nature of the fracto-emission, and the temporal relationship of the onset of breakdown to the fracture event were examined. The materials studied were polymethylmethacrylate and a Kevlar-fiber reinforced composite. >

Realization of a staircase photodiode: Towards a solid-state photomultiplier

Abstract A few years ago a low noise avalanche photodiode with a near unity excess noise factor was proposed. This detector achieves a high ratio of ionization coefficient κ via a suitable staircase band diagram. This APD can in principle exhibit photomultiplier-like noise behavior. In this paper we report the first realization of a staircase APD, grown in AlGaAs/GaAs by MBE, consisting of a ten-stage undoped staircase high-field region between p+ and n+ GaAs layers. Typical dark currents were 200 pA at the breakdown onset. The electron initiated avalanche gain was greater than 100 at the same voltage. Ionization ratios greater than 10 were estimated. Pulse responses with a full width at half maximum (FWHM) of 200 ps were demonstrated.

Lensless imaging through phase-conjugated backscattering in SF_6

A phase-conjugate mirror based on stimulated Brillouin scattering in gaseous SF(6) at 248 nm was used for pattern transfer. We measured the pressure dependence of backscattering efficiency of the Brillouin mirror, which approached 40% reflectivity and was limited by the onset of gas breakdown. Measurement of the image contrast ratio showed high statistical fluctuation due to the threshold behavior of Brillouin scattering.

A HYDRODYNAMICAL STUDY OF THE INSTABILITY OF A PLANAR LIQUID METAL ION SOURCE (SUMMARY)

Liquid metal ion sources (LMIS) are of interest in diverse areas of technology since they provide a high brightness, quasi-point source of ions for high resolution ion beam lithography, microfabrication, surface analysis and other potential applications[l]. The technical difficulties of building and operating stable sources have been largely overcome. The basic physics of source operation and ion formation is, however, still incompletely understood. Krohn and Ringo first described the fundamental processes of ion emission from liquid metal tips in a strong electric field[2]. Subsequently, Gomer[3] and others[4-81 analyzed the mechanism of LMIS and developed theoretical models to explain the shape and size of the ion emitting region. The electrohydrodynamical effect, that is, the onset of instability and breakdown, which appears for a critical voltage applied on the tips was first studied by Zeleny[9] and Taylor[lO]. Recently, Chung et. a1.[7] made an analytical and numerical study of the equilibrium shape and stability of an electrically stressed fluid with an axially symmetric arbitrary shaped surface in the quasi-electrohydrostatic limit. They did a stability analysis required for physically acceptable shapes using a general mechanical criterion applied earlier by Zeleny[9] in his analysis of the stability of charged droplets. Chung, et. a1.[7,8] used their results to argue that Taylor had not utilized any proper stability criterion. Furthermore, it was demonstrated by them that, in fact, Taylor used only an equilibrium condition for the establishment of a infinite cone in the hydrostatic limit. Explicity, Chung, et. al. have shown that it is not possible to physically form a liquid (i.e., deformable) structure with a Taylor or any other conical configuration by the application of an increasing electric field under quasi-hydrostatic conditions. When a proper stability analysis is made, it is found that the Taylor cone is not a stable equilibrium shape in either the hydrostatic or hydrodynamic limit[8]. This has been confirmed by Zheng and Linsu from a numerical solution of the Navier-Stokes equation[ll], and also discussed by Gabovich[4]. Miskovsky et. al.[12] subsequently developed an electrohydrodynamic capillary wave theory for ion and droplet emission in electrically stressed conducting viscous fluids based on a mathematical fromalism introduced by Melcher, et. a1.[13,14] and Grossmann et. a1.[15]. As the simplest analytical application of this theory we have chosen a model consisting of a planar fluid surface supported on a rigid electrode a distance 'a' below the unperturbed surface. A parallel planar counterelectrode is at a distance 'b' from the unperturbed surface. The same model was recently used by Pregenzer[l6], to study high power liquid metal ion diodes for inertial confinement fusion experiments at Sandia National Laboratories. She included graviational effects, but neglected the viscosity of the fluid. In our study, we apply the electrohydrodynamic capillary wave theory to a planar model of a LMIS, which also includes viscosity. Instead of the Bernoulli equation, as used by Pregenzer, we have solved the Navier-Stokes equation subject to a time-dependment Laplace-Young stress boundary condition, which now includes the frictional tensor. The effects of surface tension, viscosity and gravity on the critical electric field were analyzed. Although it was found that the surface tension dominates both gravity and viscosity in determining the critical electric field for breakdown, viscous effects are important, and significantly so for the higher mass liquid metals.

N-channel depletion-mode InP FET with enhanced barrier height gates

The fabrication of an n-channel depletion-mode InP field-effect transistor (FET) with enhanced barrier height gates, using a surface passivation technique that substantially increases the barrier height ( Phi /sub b/=0.83 eV) of InP, is reported. The transistors demonstrate characteristics with excellent pinch-off, flat saturation, transconductance in the range of 60-68 mS/mm, and no indication of the onset of breakdown for drain-source biases in excess of 35 V. They are shown to be highly stable, with no observable drain current drift over a period of more than 24 h of testing under DC bias. The high stability and performance of these devices demonstrate the potential for the gate metallization of InP. >

Properties of a laser-initiated pseudospark discharge

A short-pulse Nd:YAG laser is used to illuminate the back electrode surface in the hollow cathode region of a pseudospark discharge. A fast rising step voltage pulse is applied across the electrodes at various times relative to the pulsed laser illumination. The delay and jitter to the onset of gas breakdown at various filling gas pressures are measured for different laser energies and wavelengths. It is observed that the millijoule range of a green laser applied to illuminate the hollow cathode surface at up to tens of microseconds before the application of voltage would significantly reduce both delay and jitter to discharge initiation. Higher laser energy applied after voltage rise is observed to cause gas breakdown in under 100 ns.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nuclear envelope breakdown and mitosis in sand dollar embryos is inhibited by microinjection of calcium buffers in a calcium-reversible fashion, and by antagonists of intracellular Ca 2+ channels

Transient elevations in intracellular free Ca2+ are believed to signal the initiation of mitosis. This model predicts that mitosis might be arrested prior to nuclear envelope breakdown (NEB) or anaphase onset if intracellular Ca2+ concentration is buffered or dampened. Microinjection of a discrete dose of Ca2+ into the cell might then release the cell to resume mitotic cycling. Experimentally, one blastomere of two cell sand dollar (Echinaracnius parma) embryos was microinjected with Ca2+ buffers, Ca2+ solutions, or Ca2+ channel antagonists; the uninjected blastomere was the control. Cells were loaded with 10 pl doses of the Ca2+ buffer antipyrylazo III (ApIII) at specific times in the cell cycle to attempt a competitive inhibition of Ca2+-dependent steps in NEB and initiation of mitosis. Injection of 50 microM ApIII 6 min prior to NEB blocked NEB and further cell cycling. Injections of solutions between 0 and 30 microM ApIII were without observable effect. Control injections had no observable effect on the injected cell. Cells injected with 50 microM ApIII 2 min prior to the onset of anaphase in control cells were blocked in metaphase. Cells were sensitive to Ca2+ buffer injections 6 min prior to NEB (with a 40- to 45-sec duration), and 2 min prior to anaphase onset (with a 10- to 20-sec duration). Vital staining of these cells with H33342 demonstrated that they contained only one nucleus that had the same fluorescence intensity as seen prior to microinjection, and thus did not undergo DNA synthesis following the imposition of the Ca2+ buffer block to mitosis. Cells arrested in this fashion did not spontaneously resume mitotic cycling. This Ca2+ buffer-induced mitotic arrest was, however, experimentally reversible. Cells arrested with 50 microM ApIII 6 min prior to NEB could be returned to mitotic activity by injecting 300 microM CaCl2 5 min after the ApIII injection. The double injected cells resumed cycling, NEB, and mitosis after a delay of one cell cycle period, and remained one cell cycle out of phase with the sister (control) cell. Microinjection of antagonists of endomembrane Ca2+ channels inhibited NEB and anaphase onset in a concentration- and time-dependent fashion. The effective doses of compounds tested were 7 micrograms/ml ryanodine and 500 micrograms/ml TMB-8. These results indicate that a transient elevation of intracellular Ca2+ from endomembrane stores is required to initiate mitotic events, namely NEB and anaphase onset.(ABSTRACT TRUNCATED AT 400 WORDS)

On the electrical breakdown of gaseous dielectrics-an engineering approach

Following an examination of the basic gas-discharge concepts, the Townsend breakdown criterion is derived for nonuniform fields and a reformulation of the streamer criterion is undertaken. A direct application of the Townsend criterion to a practical situation is hardly possible, whereas the streamer criterion leads to applicable criteria for the onset of breakdown in air as well as in strongly electronegative gases. In this approach, a knowledge of the relevant ionization coefficients is not required, because these are replaced by parameters obtained from uniform-field Paschen curve data. In addition, the criteria developed do not contain any arbitrary constants. By introducing the electrode surface mean curvature, it is shown that the calculation of breakdown data for widely different electrode shapes can be addressed through a single formula. It is emphasized that the inherent roughness of practical electrode surface demands that caution be exercised in all design criteria.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Onset of Electrical Breakdown in Dust Layers: II. Effective Dielectric Constant and Local Field Enhancement

Part I of this work has shown that electrical breakdown in dust layers obeys Paschen's Law, but occurs at applied field values which appear too small to initiate the breakdown. In this paper we will show how an effective dielectric constant characterizing the dust layer can be determined from ac dielectric measurements and the theory of Debye. When combined with an expression for the enhanced local electric field in the void spaces between particles in the layer, field strengths which are large enough to initiate electrical breakdown in the layer are predicted at relatively low values of applied field. The effect of temperature and dust layer thickness on the onset of electrical breakdown within the dust layer can also be explained by the dependence of the effective dielectric constant on these parameters.

The Onset of Electrical Breakdown in Dust Layers: I, Microsparking Described by Paschen’s Law

This paper describes a study of the onset of electrical breakdown in dust layers, for hand-deposited dust layers in a parallel-plate geometry. It was found that the breakdown was an ordinary electron-avalanche process originating in voids within the dust layer and obeying Paschen's Law. The size of voids where breakdown occurs was in the range of 10-20 micrometers for the layers used. The distribution of particle sizes in a sample influences its breakdown through changes in the average void dimension where breakdown takes place. Water vapor in the test environment, which affects the electrical conduction mechanism prior to breakdown, lowered the average electric field required to initiate breakdown. Moderate compaction of the sample had little or no effect on its breakdown behavior.

Harmonic generation in nondegenerate semiconductors at low lattice temperature

The characteristics of second harmonic generation (SHG) in covalent semiconductors at low lattice temperatures are studied here over a wide range of the bias field, beginning from a weakly heated carrier ensemble up to the onset of impurity breakdown giving due consideration to the effect of recombination of the nonequilibrium carriers in the presence of shallow attractive traps. The calculations are carried out for a parabolic law of dispersion and a scalar effective mass. The results confirm that the recombination effects bring in both qualitative and quantitative changes in the SHG characteristics throughout the entire range of the bias field. Like the situation around room temperatures, the curves for the efficiency of SHG versus the bias field at low lattice temperatures also exhibit maximum, thus illustrating the optimum bias field for which the generation can be most efficient. Depending upon the lattice temperature and the material, the recombination effects may bring in a second maximum in the characteristics of SHG. The results are found to be useful for the determination of some capture parameters. The inadequacies of the theory are pointed out and the scope for further refinement is discussed.

Mass spectroscopic analysis of excimer laser ablated material from human corneal tissue

The clinical feasibility of photorefractive keratectomy depends on the surface structure of the ablated cornea. Two factors that influence the remodeled surface are the homogeneity of the energy distribution and the properties of the laser source (energy and wavelength). Currently, the homogeneity of the beam is difficult to control. The second factor, laser source properties, was the focus of this study. We investigated the effect of laser wavelength and energy by analyzing the reaction products of photoablation. We monitored the fragments produced by UV-laser ablation of human corneas using mass spectroscopy in the range of 0 to 100 atomic mass units. At 248 nm (KrF), average photon energy was 5 MJ/cm2, increased to 90 MJ/CM2 by intervals of 5 mJ/cm2. At 193 nm (ArF), photon energy was increased by the same interval from 5 mj/cm2 to 80 MJ/CM2. Our experiments showed that there was a fluence threshold of approximately 40 mJ/cm2 at 193 nm (ArF) and 50 MJ/CM2 at 248 nm (KrF). Exceeding this threshold led to sudden increases in the number and relative intensity of fragment peaks in ma spectroscopy. This indicates the onset of multiple-photon processes and effective photochemical breakdown. There was a significant difference between both wavelengths in the distribution of mass peaks, indicating higher ionization power at 193 nm.