Anode Edge Research Articles

How Hyperpolarization and the Recovery of Excitability Affect Propagation through a Virtual Anode in the Heart

Researchers have suggested that the fate of a shock-induced wave front at the edge of a “virtual anode” (a region hyperpolarized by the shock) is a key factor determining success or failure during defibrillation of the heart. In this paper, we use a simple one-dimensional computer model to examine propagation speed through a hyperpolarized region. Our goal is to test the hypothesis that rapid propagation through a virtual anode can cause failure of propagation at the edge of the virtual anode. The calculations support this hypothesis and suggest that the time constant of the sodium inactivation gate is an important parameter. These results may be significant in understanding the mechanism of the upper limit of vulnerability.

Novel composite contact design and fabrication for planar Gunn devices for millimeter‐wave and terahertz frequencies

AbstractContacts to planar Gunn devices on the semiconductor surface suffer from high electric field stress and current crowding leading to premature burn out. We have developed a new composite contact design using a Schottky extension of the conventional Ohmic contact. The composite contact structures have been fabricated and tested to mitigate the effects of high concentrations of charge and high electric fields at the anode edge of the planar Gunn diode. Monte Carlo simulations confirm these experimental results in composite contacts. These results suggest a mechanism of increased lifetime when using composite contacts as compared to conventional Ohmic contacts. This is of significant importance for planar Gunn device operation at millimetre wave and terahertz frequencies where high doping density (∼1017cm‐ 3) and short cathode to anode separation (1µm or less) is required. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Grayscale Junction Termination for High-Voltage SiC Devices

In silicon carbide devices used above around 2.4 kV, effective anode edge termination usually requires a high-resolution floating guard ring implant or multiple lithography/implant cycles to effect a multi-zone junction termination extension. In general the goal is to produce a smoothly tapered field profile to prevent high-voltage field-crowding that causes premature breakdown at the edge of the high voltage electrode. Using a much simpler grayscale photolithographic technique and a single termination implant, we directly produce the desired tapered doping profile. The effectiveness of this termination is shown by the near-ideal (6.1 kV) breakdown measured in PiN diodes made with a 38 µm intrinsic layer. The simple method is applicable to the fabrication of many high-voltage devices.

Numerical Study of Plasma Behavior in a Magnetoplasmadynamic Thruster Around Critical Current

A self-field magnetoplasmadynamic thruster is numerically studied for an argon mass flow rate of 0.8 g/s. We focus on the plasma behavior around the critical current, at which full propellant ionization is theoretically expected. At a discharge current higher than the critical current, the Hall parameter is large at the anode surface, which leads to an obliquely skewed current profile. This skewed current profile, together with the voltage―current characteristics, show that the Hall effect plays an important role around the critical current. When operating the magnetoplasmadynamic thruster above the critical current, it is shown that the current concentration and the gas density depletion at the anode edge induce a shortage of current carriers. To circumvent the carrier shortage at high discharge currents, we employ a segmented anode. It is found that a segmented anode is effective for suppressing the current concentration and the plasma density depletion on the anode surface without degrading the performance of the thruster.

A Mechanism for the Upper Limit of Vulnerability

The strongest shock that induces reentry in the heart is the upper limit of vulnerability (ULV). In order to understand defibrillation, one must know what causes the ULV. The goal of this study was to examine the mechanism of the upper limit of vulnerability. Numerical simulations of cardiac tissue were performed using the bidomain model. An S2 shock was applied during the refractory period of the S1 action potential, and results using a smooth curving fiber geometry were compared with results using a smooth plus random fiber geometry. When using a smooth fiber geometry only, no ULV was observed. However, when a random fiber geometry was included, the ULV was present. The difference arises from the fate of the shock-induced break wave front when it reaches the edge of the tissue hyperpolarized by the shock (the virtual anode). Our numerical simulations suggest that local heterogeneities throughout the tissue may be crucial for determining the fate of the shock-induced wave front at the edge of the virtual anode, and therefore play an important role in the mechanism underlying the ULV.

Gap-Dependent Terahertz Pulses from Mid-Size-Gap Multi-Energy Arsenic-Ion-Implanted GaAs Antennas

We investigated the characteristics of sub-picosecond radiation pulses from mid-size-gap multi-energy arsenic-ion-implanted GaAs photoconductive antennas in terahertz time-domain spectroscopy experiments. At fixed fluence excitation, both the bandwidth and the negative peak of terahertz waveform minimum reveal increasing trends with increases in the antenna's gap. We reproduce both the measured terahertz waveforms and increasing trends numerically, by calculating a model that incorporates the trap-enhanced bias field with wave equation. We deduce from our simulation that the gap-dependent behavior stems from the fact that an antenna with a larger gap has a stronger trap-enhanced bias field near the anode edge and thereby induces a larger space-charge screening effect and bandwidth.

Optical Investigation of Surface Flashover Plasma Across Silicon Stimulated by Pulsed High Voltage in Vacuum Based on ICCD

Optical investigation of flashover phenomena across silicon is performed in vacuum. A pulse waveform of ~0.4/2.5 mus is employed as stimulation source. The p100-type silicon wafers are the samples. The images of flashover growing are recorded by intensified charge couple device, and the corresponding waveforms of voltage and current are given. The results primarily suggest that the flashover across p-type silicon begins with the distortion of electric field at the edge of anode. Injection of minors may play an important role in the formation process of filament current. It is regarded that the plasma produced by ionized desorbed gas will appear at the final stage of surface flashover.

Second-order optical nonlinearity in thermally poled Pyrex borosilicate glass

The thermal poling method was utilized to create second-order optical nonlinearity in Pyrex borosilicate glass. The distribution and amplitude of the induced nonlinearity were characterized with second harmonic microscopy. The induced optical nonlinearity was found in a thin layer around 1.9μm under the anode surface with a magnitude as high as 0.24pm/V, comparable to that observed in fused silica samples. SEM observation of the cross-section of the poled glass region, after it had been etched in diluted hydrofluoric acid for several minutes, revealed an etched trench, ∼1.8μm under the anode edge and ∼0.3μm in width; while in post-annealed samples, no such etched trench could be observed. The effect of poling voltage on the magnitude of the induced nonlinearity was also studied, where the results showed that higher poling voltage resulted in higher nonlinearity with a threshold of ∼0.9kV.

Deposition of Homogeneous Amorphous Carbon Film at High Growth Rates by Vacuum Arc Deposition

Although vacuum arc deposition can deposit amorphous carbon (a-C) film at high growth rates, the problem is that it leads numerous droplets than other PVD. Proposed methods for decreasing droplets, such as shield plate and magnetic filter, can actually decrease droplets, however they reduce growth rate. This study focuses on deposition of a-C films by vacuum arc deposition to achieve both higher growth rates and homogeneity of a-C films by decreasing droplets. It is necessary to control the vaporizing process of solid carbon target by improving discharge conditions for both higher growth rates and fewer droplets. Deposition of a-C film with low density of droplets at high growth rates was attempted by optimizing discharge voltage, electric charge and the structure of arc discharge unit. The optimized conditions, such as higher discharge voltages, larger electric charges and shorter distances between cathode spot and anode edge, decreased the number density of droplets per film thickness. The a-C film deposited under the optimized condition had the number density of droplets of 4.2/mm2·nm at the growth rate of 3.5 nm/count. XPS analysis showed that deposited a-C films had sp3 ratio of about 80%, therefore its density was estimated to be approximately 3.0g/cm3.

Thermo‐Mechanical Model of Solid Oxide Fuel Cell Fed with Methane

AbstractInternal steam reforming induces an inhomogeneous temperature distribution in SOFC cell. A thermo‐mechanical model has been developed to determine both the temperature gradient and the stress field within an anode supported cell structure fed with methane. Residual stresses due to manufacturing process and thermal stresses induced by the mismatch in thermal expansion coefficients have been taken into account. Mechanical external loading of the cell has been neglected.The results of this study have shown a cooling area at the inlet of the cell due to the endothermic steam reforming reaction. However, in our conditions of simulation, the temperature gradient in the cell has been found relatively low (0.2 °C mm–1). Consequently, thermal stresses in the three layers of the cell are mainly due to the heating from room temperature to the operating one.Mechanical calculations have been performed considering a low compressive residual stress in the thin electrolyte at room temperature. In these conditions, a high tensile stress has been calculated in the thin electrolyte at operating temperature. Its value is nearly homogenous all over the layer and exceeds the limit strength of the Yttria Stabilised Zirconia. The brittle fracture of electrolyte layer in one thermal cycle has been analysed by a Weibull distribution. The failure probability reaches 66% for a 60 micrometer electrolyte thickness. However, this result depends strongly on the layer thickness. Moreover, a singular stress field analysis has allowed determining an important tensile stress at the anode edge (on the circumferential free surface). It has been shown that fracture can be initiated at defects in this tensile surface, if the fracture toughness of the anodic cermet is inferior to 0.41 MPa √m.

Thermo-Mechanical Aspects of SOFC Operations

Mechanical properties of SOFCs appear to affect drastically their durability. In this work, special attention has been paid to the mechanical loading of a cell upon heating from room temperature to operation temperature and during operation. In the latter case, both hydrogen and methane feeding have been considered. A thermo-mechanical model has been developed to determine both the temperature gradient and the stress field within an anode supported cell structure. Residual stresses due to manufacturing processes and thermal stresses induced by the mismatch in thermal expansion coefficients have been taken into account. Mechanical calculations have been performed considering low compressive residual stresses in the thin electrolyte at room temperature and neglecting any mechanical external loading of the cell. After a heating step, during H 2 operation, a high tensile stress is calculated in the thin electrolyte. Its value is nearly homogenous all over the layer and is found to exceed the limit strength of the yttria stabilised zirconia. Moreover, a singular stress field analysis has allowed determining a high tensile stress at the anode edge (on the circumferential free surface).,The occurrence of brittle fracture of the electrolyte layer upon one thermal cycle is analysed using a Weibull distribution. Depending on the cell material characteristics and on the cell geometry, a critical electrolyte thickness ranging around 50 μm is determined which shows the highest rupture probability. In addition internal steam reforming induces an inhomogeneous temperature distribution in the cell including a cooled area at the inlet of the cell due to the endothermic steam reforming reaction. However, in representative simulation conditions, the temperature gradient in the cell is found to be low (0.2°C/mm). Thus, it cannot be involved in any mechanical degradation process of the cell upon operation.

Three-dimensional diagnostics of dynamic behaviors of excited atoms in microplasma for plasma display panels

A three-dimensional (3-D) observation cell with realistic AC-type structure and size was fabricated and measurements were done, for the first time, on the spatiotemporal behaviors of excited Xe atoms by using optical emission and laser absorption spectroscopic techniques. As a typical characteristic feature, it is seen that the discharge starts from the temporal anode edge, stretching toward the cathode, and a striated pattern of excited atoms appears on the anode side while a single broader peak distributes on the cathode side, traveling to its far end. In addition, the dependence on the applied voltage was clearly seen in the distribution of excited atoms, which shows the influence of the charge accumulation on the walls. With the two-dimensional projection images obtained from side and front views in both the emission and absorption measurements, an iterative method was tried preliminarily to derive the peak values of excited Xe atoms in the 3-D space.

Copper dissolution phenomena along a vertical plane anode in CuSO 4 solution

The concentration profile of Cu 2+ ion and the velocity profile of natural convection near a vertical plane copper anode immersed in 0.6 M CuSO 4 solution were measured under the electrolytic condition of uniform current density distribution. Holographic interferometry and tracer technique were applied. The current density distribution was examined with a miniature probe method. It was fairly uniform at the average current density less than 4 mA cm −2. The experimental values of the concentration difference of Cu 2+ ion between anode surface and bulk-electrolyte, Θ, the thickness of the diffusion layer, δ c, the maximal velocity of natural convection, u m, and the distance between anode surface and the position of maximal velocity, τ, were determined. Θ and u m increased while δ c and τ, on the other hand, decreased, with increasing anodic current density. These four parameters increased with the vertical distance from the upper edge of anode. The similarity principle was applied to explain the dependence of these parameters on the current density and the vertical distance from the upper edge of anode.

Dynamic shape of the depletion layer of a submillimeter-wave Schottky varactor

Most frequency multipliers at submillimeter wavelengths are based on the Schottky varactor. The main problem of these multipliers is the output power, which remains low at frequencies above 500 GHz. The aim of this work is to help the situation by studying the usability of the conventional equivalent circuit during a fast voltage modulation. The anode edge effects play an important role in this voltage modulation. While the fringing fields, due to the edge effects, reduce the capacitance modulation in small submillimeter-wave varactors, the edge effects also lessen the effect of electron velocity saturation compared with an ideal varactor with a pure parallel plate capacitance. The usefulness of the static model can be estimated by comparing the three-dimensional shape of the depletion layer to the shape given by the dynamic model. The dynamic shape can be obtained by solving the potential and electron conduction currents in the epitaxial layer of the Schottky varactor. In this work the potential and the electron currents have been calculated from simplified device physics by using numerical methods.

Morphology of Damage in Al Films Tested Under Electromigration Conditions Using the Drift Velocity Method

AbstractPure Al films deposited by the partially ionized beam technique at bias Iavels of 0, 3 and 6 kV onto TiN sublayers were patterned for measurement for direct drift velocities. Then films were annealed in a vacuum 10−4 Pa for 1 hour. Electromigration tests were performed in an air atmosphere at a temperature of 280°C. The current density during tests was 6·105 A/cm2. Examinations in the SEM showed that the morphology of the damage on cathode and anode ends of the Al stripes depends on the bias during deposition. In the case of 0 kV bias, voids nucleate on the back and side edges of the cathode end and propagate toward the anode end. Hillock and whisker formation was observed on the anode end. The height and diameter of the whiskers were about 50 and 1–2 μm, respectively. In the case of 3 and 6 kV bias the drifting edge was smooth. Only hillocks (no whiskers) grew on the anode edge. The morphological differences of these films are discussed on the basis of self-ion bombardment effects.

Surface electrical breakdown with white-light emission on semi-insulating GaAs substrates

Electrical breakdown on surface passivated semi-insulating GaAs substrates is shown to be accompanied with white-light emission from ohmic electrode edges. It cannot be explained by the previous space-charge-limited current model. A new model is proposed in which filling of states near the surface produces an intense electric field near the anode edge and triggers an avalanche.

Nonlinear propagation of void front during electromigration in alloy films

Void formation during electromigration has been observed near the anode edge of a Cu-Al zone in an Al stripe. Growth of new voids was continuous with a void front propagating along the electron flow direction at a nonlinear rate. Such motion of the void front can be quantitatively correlated to the electromigration of Cu at grain boundaries. The measured displacements of the void front were analyzed to yield the grain-boundary diffusivity and effective charge for Cu electromigration.

The Focalograph

MANY methods have been proposed for the examination of the target of an x-ray tube. It must be conceded that any method which provides a permanent record for comparison has advantages over methods which do not have this feature. The device described, the Focalograph, as proposed by Sergeant Black is a simple, economical, and efficient apparatus for: (a) Providing a record of the appearance of the target of a roentgen tube. (By enlargement obtained by increasing the grid-film distance it is possible to see minute defects plainly.) (b) Demonstrating the effect of angular position of the target of a roentgen tube upon the effective ray. (c) Checking the effective area, actual area and minimum projected area of the focal spot of a roentgen tube. This is especially valuable in checking performance of tubes against specification claims. (d) Determining the maximum ray divergence and minimum focal film distance for various types of tubes as they are affected by the angle of the target face. The apparatus described consists of a simple wood-constructed tube holder for the roentgen tube with a special accurately perforated lead diaphragm placed on an arc between the tube and a film which is to be exposed for record (Fig. 1-A). It is also possible to study the lateral divergence by use of grids constructed in that plane. It is conceivable that many methods of grid construction may be applied to the basic apparatus in the future. The grid shown is the best found as yet for comparison studies, as it is possible to obtain several exposures on the same film. The tube is enervated from a regular x-ray machine in the usual manner. The arc of the film and of the lead diaphragm is so constructed that the appearance of the film exposed is the same as the composite image obtained by many horizontally placed films exposed with the target placed at various angles. The saving in film for experimental purposes in this procedure is obvious. It is recommended that the following technic be used in making the records described: kv.p., 40; ma., 10; sec. 10. The distance of the target to the lead grid is not to exceed 4 inches; the usual film distance is set by the apparatus. Cardboard film holders are used. Enlargements may be made by increasing the distance to the film. Under no circumstances may the kilovolt peak be increased above 40, as the danger of spark-over through the lead grid is imminent. Descriptive Data 1. Westinghouse Treatment Tube, tungsten disc (Fig. 1): Dark ring— Actual focal spot. Lighter halo— Margin of focal spot. Wide halo— Outside edge of entire anode. 2. Universal Broad Focus, solid tungsten anode (Fig. 2): Dark center— A crater on surface. Light ring— Flat surface. Dappled outer dark ring— Melted raised areas on target. Outer halo— Outer margin of anode. 3. Universal Medium Focus, solid tungsten anode: good condition (Fig. 3): Dark— Actual focal spot. Halo— Anode. 4. Universal Fine Focus, solid tungsten anode: good condition (Fig. 4): Dark— Actual focal spot.

Striated Discharge in Hydrogen

Potential distribution, electron concentration and mean electron energy at eleven points were determined for a discharge through a tube 15 cm long at pressures of.625,.20,.90 and.02 mm, using Langmuir's probe method. The total potential difference was 270-300 volts. The potential dropped to a minimum within 2 mm of the cathode, of 125, 60 and 20 volts for pressures.20,.09 and.02 respectively, giving a reverse gradient in a region 1 to 2 cm long through which the current must have been carried by diffusion of positive ions and electrons. The gradient was also reversed at the anode edge of a striation for pressures of.20 and.09 mm---the lower pressure.02 mm gave a uniform positive column. The electron concentration reached a maximum 1 to 2 cm in front of the cathode; the values, in general, are lower the higher the pressure. The mean electron energy decreases sharply at the cathode to a minimum of 1 to 2 volts, then rises gradually to about 10 volts at the beginning of the positive column; it also reaches 10 volts at a striation. Such electrons have sufficient energy to cause the ionization which is associated with the luminosity in the positive column. The anode drop within 2 mm was only 5 to 15 volts, the potential distribution depending on the position of the nearest striation.