Unipolar Current Research Articles

Pulse electrodeposition of titanium on carbon steel in the LiF-NaF-KF eutectic melt

Electrodeposition of titanium was carried out in the K3TiF6–LiF–NaF–KF melt using both direct (DC) and unipolar pulse current (PC) techniques. Dense and smooth titanium coatings were obtained by PC plating at 750 °C whereas DC plating led to rough and dendritic deposits. The best results were obtained using a 100 μC cm−2 pulse charge and a cathodic current density of 50 and 75 mA cm−2. The cathodic current efficiency was in the range 60–65%. The titanium deposits obtained under such conditions behaved similarly to CP-titanium in NaCl and HNO3 solutions at room temperature.

Electrohydrodynamic flow associated with unipolar charge current due to corona discharge from a wire enclosed in a rectangular shield

Unipolar charge current can be generated through corona discharge from a thin wire enclosed in a shield electrode. Except for an ionization sheath adjacent to the coronating wire surface, most parts of the region in the enclosing shield contain drifting ions of a single polarity in response to the electric field. Momentum transfer as a consequence of collisions between drifting ions and electrically neutral air molecules gives rise to the electrohydrodynamic flow known as “corona wind.” Although primarily driven by the Coulomb force due to unipolar charge in the electric field, the electrohydrodynamic flow cannot simply follow the direction of electric field lines because of the confinement of the solid walls of the shield. Therefore, the structure of the electrohydrodynamic flow can vary significantly depending on the system configuration. In the present work, the electrohydrodynamic flow in a rectangular shield is studied by solving the nonlinearly coupled governing equations via the Galerkin finite-element method. A highly symmetric system with the wire positioned at the center of a square shield is shown to contain eight equal-sized, two-dimensional recirculation vortices. The number of recirculation vortices tends to be reduced by a slight asymmetry in the system. The flow structure of two major counter-rotating recirculation vortices is found to be most common in systems where the wire is positioned off the center of the rectangular shield in a two-dimensional domain. The results reported here may be brought to bear upon the “corona wind” effect in charging devices such as corotrons and scorotrons used in electrophotographic printing processes.

Electrohydrodynamic behaviour of a drop subjected to a steady uniform electric field at finite electric Reynolds number

The electrohydrodynamic flow associated with a fluid drop in an electric field is a consequence of the tangential electric stress at the fluid interface. The tangential viscous stress due to the electrohydrodynamic flow arises to just balance the tangential electric stress at the fluid interface so that the traction boundary condition is satisfied. Influenced by both the local electric stress and viscous stress, the drop interface may exhibit various shapes. The presence of fluid flow also leads to charge convection phenomena. The relative significance of the charge convection effect is usually measured in terms of the electric Reynolds number, Re E , defined as the ratio of the timescales of charge convection by flow and that for charge relaxation by ohmic conduction. This work presents a quantitative analysis of the charge convection effects in a framework of the leaky dielectric model at finite Re E , which has not been considered in previous investigations. Axisymmetric steady flows driven by an applied uniform electric field about a deformable fluid drop suspended in an immiscible fluid are studied by computational means of the Galerkin finite–element method with supplementary asymptotic analysis. The results of finite–element computations are in general agreement with the prediction by the asymptotic analysis for spherical drops at vanishingly small Re E . A common effect of charge convection is found to reduce the intensity of electrohydrodynamic flow. As a consequence, oblate drops are predicted to be less deformed in an electric field when charge convection is taken into account. The prolate drops are often associated with an equator–to–pole flow, which convects charges toward the poles to form a charge distribution resembling that in a highly conducting drop immersed in an insulating medium. Therefore, charge convection tends to enhance the prolate drop deformation. In many cases, charge convection effects are found to be significant even at apparently small Re E , corresponding to the charge relaxation time–scale about 10 −3 s, suggesting that many experimental results reported in the previous publications could have been influenced by charge convection effects.

Application of Galerkin Finite-Element Method with Newton Iterations in Computing Steady-State Solutions of Unipolar Charge Currents in Corona Devices

The phenomenon of corona discharge from thin-wire or sharp-point electrodes has found various important applications especially when effective charging devices are needed. In most parts of the corona charge transport region, the unipolar charge current is established as a consequence of the drifting of ions of single polarity along the electric field lines. With negligible diffusion effects, the equation governing charge transport appears to be of hyperbolic type and has been commonly dealt with by the method of characteristics or with upwind treatments. The Poisson's equation governing the electric potential distribution in the presence of unipolar charge, on the other hand, is of elliptic type and has been conveniently dealt with by the finite-element method or the equivalent for typical boundary-value problems. The first-principle based modeling of the corona device behavior has often involved either back-and-forth iterations of solving for one of the variables with others fixed or using time integrations even when steady states are sought. In the present work, the Galerkin finite-element method is applied uniformly to all the equations in the mathematical system and the Newton iteration method is utilized to obtain quadratically converged steady-state solutions in a few steps. Straightforward application of a typical Galerkin finite-element procedure to all the equations is shown to be quite adequate, because no mechanism for boundary layer formation is present in the unipolar charge current in corona devices. Wiggle-free numerical solutions can be obtained without invoking the upwind schemes or excessive mesh refinements.

An analysis of corona currents between two concentric cylindrical electrodes

The electric field and space charge in a corona device are governed by nonlinear differential equations. With the symmetry present in the configuration involving concentric cylindrical electrodes, exact mathematical solution becomes tractable. However, some commonly used formulas in the literature have lacked rigorous mathematical derivation and analysis. In the present work, a general exact solution is derived for the problem of the unipolar charge current between two concentric cylindrical electrodes. In addition to rigorous mathematical analysis, an approximate expression based on the empirical form of well-established Warburg's formula for current–voltage relationship is derived systematically. For typical wire–cylinder corona devices, the presently derived coefficient in Warburg's formula turns out to be numerically very close to that in the commonly used Townsend's formula, despite their rather different mathematical forms. Moreover, the validity of the commonly used, quadratic form of Townsend's formula is shown to lack concrete mathematical reasoning.

Characteristic Analysis of Bi- and Uni-polar Drives of Reluctance Motors.

The comparative studies on the torque characteristics of the reluctance motor for bi- and uni-polar sinusoidal drives are given. The 3-phase test motor with 6/8 stator and rotor pole combination is used for the non-linear FEM analysis and experiments. The analysis shows that the different flux paths are produced, depending on bi-and uni-polar current drives, which makes two significant differences in torque-current characteristics. Since the unipolar current drive produces shorter flux path and, therefore, higher air gap flux density, higher torque for the given winding current can be expected. Another important point is the effect of magnetic saturation. The examination of the yoke flux with respect to various rotor angles suggests the yoke is apt to be saturated in the bipolar current drive.The experimental verification has been made by using the test motor. From these experiments and analysis, it is concluded that the unipolar sinusoidal current drive is preferred to have a linear relation between torque and current and to avoid the effect of magnetic saturation.

Hardening and reconditioning machine components by electromagnetic coating

Electromagnetic coating (EMC) is one of the progressive methods concerned with the preparation of a piece surface layer with specific physical and mechanical properties. It is accomplished with the aid of an electromagnetic head mounted, for instance, on a lathe that provides rotation and displacement of a hardened surface relative to a pole piece. As a source of discharge unipolar pulse current, use is made of welding transformers with a single-half-period rectification circuit. EMC optimal conditions for processing cylindrical coatings are as follows: operating current, 120 A; magnetic induction in a working gap, 0-7 T; the working gap, 1-5 mm; rotation speed, 15r.p.m.; ferropowder feed rate, 20-25gmin−1. Under these conditions the coating thickness is 0-7 mm and the hardened transition layer thickness is 0-9 mm.

Current Injection and Light Emission in Liquid Argon and Xenon in a Divergent Electric Field

Unipolar injection currents originating from a point electrode immersed in LAr or LXe have been observed as functions of voltage and polarity. The dependence was found to be governed by space charge limitation. Ion mobilities were deduced from the analysis of the positive injection currents. Time resolved measurements of the light emission demonstrated that the negative injection current consisted of pulses, similar to Trichel pulses in air corona. Positive currents produced continuous light emission. The results are discussed on the assumption that a gaseous discharge is formed around the point electrode.

Radiation-induced charge transport in polymer electrets

Recently, a new physical model has been developed describing the charge dynamics in dielectrics during irradiation. Experimental investigations of the charge transport in polymer electrets were carried out in a modified electron-beam microscope on various materials (FEP, PETP, PVDF). A qualitative comparison of the theoretical results with experimental data shows that, in FEP, electrons are practically immobile, whereas positive charge carriers cause a trap-modulated unipolar hole current. For PETP, analogous results are found except that now the mobility of the electrons dominates. In PVDF however, both charge carriers must be mobile.

Unipolar currents and electrostatic probe characteristics in a gas discharge plasma

Measurements have been made of the current circulating through a plasma and an adjacent electrically isolated conductor containing an electron-emitting surface region. This ‘‘unipolar’’ current (so called since it requires only a single electrode) was measured in a dc mercury–argon positive column plasma (7 mTorr Hg, 3 Torr Ar, 1.8 cm radius, 400 mA). The unipolar conductor consists of metal disks located at the boundary of the plasma; these disks include indirectly heated dispenser cathodes which act as the electron-emitting regions, providing external control of the emission level. The disks can be connected externally in various combinations to form a conductor of variable distribution and area or they can be used as independent electrostatic probes. Measured unipolar currents are in good agreement with values predicted from individual experimental probe V-I characteristics for the disk emitters and collectors, demonstrating that the unipolar current circulation process can be interpreted in terms of electrostatic probe characteristics.

Simple approximate treatment of unipolar space-charge-dominated coronas: The Warburg law and the saturation current

Unipolar space-charge distribution ρ(t) drifting with constant ion mobility μ in gas under time and space dependent electric fields will evolve according to the unipolar charge drift formula [1/ρ(t)]−[1/ρ(t0)] = (μ/ε0)(t−t0), neglecting diffusion effects. This general formula is especially useful as a starting point for approximate treatment of space-charge effects, both in ion drift and diffusion experiments, and in electrical coronas. A first simple derivation of an equivalent of the Warburg cos5ϑ current-density distribution for point-to-plane coronas is given, together with a formula for the maximum possible unipolar ion current (the unipolar saturation current) Is ≊2με0 V2/d, for a given corona voltage V and point-to-plane distance d. It is demonstrated that corona currents in excess of this limit always involve free electron or bipolar conduction phenomena, like streamers.

An experimental study of electrostatic precipitator performance with pulse excitation

Pulse excitation was applied to a laboratory electrostatic precipitator (ESP). Measurements were made to obtain experimental data which would assist in understanding and evaluating the basic concepts of pulse excitation. The electrical characteristics of interest were the peak voltage d.c. voltage, corona current and current distribution over the collector plates. These characteristics were measured at 150° C (300° F) under both clean and high resistivity dust (6 × 10 12 ohm cm −1 ) conditions. The results show the changes in the voltage-current relationship due to high resistivity dusts and also provide a basis for selecting operating conditions. The charging and collecting characteristics of the ESP with pulse and d.c. excitation were compared by measuring charge/mass ratios, mass penetration and opacity. Although the d.c. field strength was less with pulse excitation than with d.c. excitation, the charging and collecting of high resistivity dust were improved. This was due to a combination of the peak voltage and the evenly distributed unipolar current.

Field‐controlled ionic charging of ice crystals

The charging curves and the saturation values of both charge and potential have been determined for six metal models with typical ice crystal shapes (bullet, columns, needle, plate, and dendrite) with different orientations in a unipolar ion current, in conditions that make the charging process field controlled. Metal spheres of several sizes were also used, as a reference. It was found that the theory of field‐controlled charging for spheres adequately describes the process for ice shapes, provided the variable (charge or potential) is multiplied by a factor depending on shape and orientation. Thus the saturation charge per unit cross section exposed to the ion current varied from 0.4 to 15 times that for a sphere, for the models used. The results are relevant to the formation of shielding layers at the upper level of thunderclouds. The measurements also provide estimates of a parameter (numerically equal to capacitance) that controls the rate of growth by sublimation.

Joule heating of Io's ionosphere by unipolar induction currents

The unexpectedly large scale height of Io's ionosphere (Kliore, A., et al., 1975, Icarus 24, 407–410) together with the relatively large molecular weight of the likely principal constituent, SO 2 (Pearl, J., et al., 1979, Nature 280, 755–758), suggest a high ionospheric temperature. Electrical induction in Io's ionosphere due to the corotating plasma bound to the Jovian magnetosphere is one possible source for attainment of such high temperatures. Accordingly, unipolar induction models were constructed to calculate ionospheric joule heating numerically. Heating rates produced by highly simplified models lie in the range 10 −9 to 10 −8 W/m 3. These heating rates are lower than those determined from uv photodissociative heating models (Kumar, S., 1980, Geophys. Res. Lett. 7, 9–12) at low levels in the ionosphere but are comparable in the upper ionosphere. The low electrical heating rate throughout most of the ionosphere is due to the power limitation imposed by the Alfvén wings which complete the electrical circuit (Neubauer, F.M., 1980, J. Geophys. Res. 85, 1171–1178). Contrary to the pre-Voyager calculations of Cloutier, P. A., et al. (1978, Astrophys. Space Sci. 55, 93–112), our numerical results show that the J × B force density due to unipolar induction currents in the ionosphere is much less than the gravitational force density when the combined mass of the neutral species is included. The binding and coupling of the ionosphere is principally due to the relatively dense (possibly localized) neutral SO 2 atmosphere. In regions where the ions and neutrals are collisionally coupled the ionosphere will not be stripped off by the J × B forces. However at a level above that (to which the ions move by diffusion only) the charged species would be removed. Thus there appears to be no need to postulate the existence of an intrinsic Ionian magnetic field as suggested by Kivelson, M. G., et al. (79, Science 205, 491–493) and Southwood, S. J., et al. (1980, J. Geophys. Res., in press) in order to retain the observed ionosphere.

Laparoscopic sterilization with electrocautery, silastic bands and spring-loaded clips: Report of our experience with 790 patients

A comparative study was carried out in 790 women undergoing one of the 3 most frequently used laparoscopic sterilization techniques: high-frequency unipolar electrocoagulation of the tubes, application of silastic rings or spring-loaded clips to the tubes. The technical problems and per- and postoperative (early) complications are assessed. The use of high-frequency unipolar current has been superseded by today's mechanical and non-electrical sterilization methods. The spring-loaded clip method appears to be a sterilization technique with a minimum of severe complications, and theoretically the greatest chances for possible refertilization.

Reliability and reversibility of female sterilisation.

The technique of laparoscopy has become popular in Britain, requiring effective methods of occluding the tubes. High-frequency unipolar current, bipolar current and thermocoagulation are all very destructive procedures. A surgeon attempting reversal after electrocoagulation may find extensive tissue damage or other complications such as contact burns; when equally effecitve procedures are available, it becomes difficult to justify continued use of the method. A popular alternative is the variety of occlusive devices. The British Rocket clip has a failure rate of only about 2/1000, it destroys only about .3 cm of tube, and the applicator has a safety catch mechanism allowing removal before final closure if placement is in question. The silicone (Falope) ring causes infarction and obliteration of the lumen of the 2-3 cm of tube ensnared in the ring. Although the need to draw a loop of tube into the instrument delivering the ring has resulted in injuries to the tube and mesosalpinx, this has occurred less frequently with experience. Successful reversal after the use of these devices has been reported. Any study of reliability of a method of sterilization should consider the following variables: operator experience, the timing of the procedure, the route and technique used, the means of achieving occlusion, the site of tube selected, the method of analyzing results, and an account of the time of exposure when expressing failure rates. Pregnancies are most likely in the first 2 years, ectopic pregnancies are most likely with electrocoagulation, and 12-month life table pregnancy rates on data from 60 centers have shown identical rates of 2/1000 for laparoscopy and minilaparotomy. For both closure and reversibility the narrowest part of the tube is the best choice, and minilaparotomy using a Pomeroy operation or a device would have advantages over laparoscopy in underdeveloped societies. Now that devices have proved as effective as electrocoagulation, or surgical occlusion, they can be recommended by any route with preference give to those destroying the least tissue.

Space-charge effects upon unipolar conduction in semiconductor regions

The effect of space charge upon unipolar drift currents in uniformly doped semiconductor regions is studied. The geometry is assumed one dimensional and diffusion is neglected. Both majority- and minority-carrier injection is considered. First, the carrier mobility is taken to be constant. The current and voltage are expressed as functions of the injection level and the normalized carrier transit time, not regarding the actual boundary condition (at the injecting electrode). A space-charge factor which indicates the nonuniformity of the electric field is also defined and computed. Then, various physical situations, such as the space-charge-limited current diode, the shallow-barrier contact diode, the Schottky emission diode, and the emitter-current-limited injection can be studied. Universal diagrams for majority- and minority-carrier injection, respectively, allow evaluation of the space-charge effects and calculation of the J-V dependence by taking into account the proper boundary condition. Finally, an arbitrary velocity-field dependence and the possibility of a negative conductance on the static characteristic is discussed.

Small-signal theory of unipolar injection currents in solids

Small-signal behavior of unipolar currents injected in semiconductors or insulators is theoretically studied. The one-dimensional problem of plane-parallel electrodes is considered. Analysis neglects diffusion but takes into account carrier mobility field dependence, ionized impurities, and trapping. Arbitrary boundary conditions are assumed. The present theory describes the small-signal response by a set of four complex quantities recalling the Llewellyn-Peterson coefficients for vacuum tubes. The present paper represents a generalization of previous results obtained by Yoshimura in the following respects: new physical phenomena (hot carrier effects and trapping) are taken into account, and new applications of the derived coefficients are suggested. This paper also contains a brief comment on physical situations and devices that can be studied by the present theory, paying particular attention to high-frequency negative-resistance devices.

The read-write disturb problem in plated wire memories

Data illustrating the read-write disturb problem in plated wire memories are reported. It is shown that the application of a dc hard axis field of the order of 1 Oe acting in the same direction as the word pulse can alleviate the disturb. Read-write disturb threshold curves are presented for a variety of pulse sequences incorporating either unipolar or bipolar bit currents. It is found that the second pulse of a bipolar disturb bit current is, for long pulse widths, the most influential. A model, which is based upon an extension of the modified lever mechanism used to explain conventional creep, is proposed. This model can explain all the data. In addition, it predicts that as the bit pulse length is decreased, the bipolar disturb sequence should become less severe. This is observed and, for a pulse width of 50 ns, the disturb threshold moves well above the wall motion current. Finally, a number of methods for increasing operating margins is presented.

Solar wind torque on the geomagnetic cavity

It is argued that the rotation of the ionospheric plasma drives unipolar induction currents through the solar wind plasma in the boundary layer of the geomagnetic cavity. The consequences of this current include a torque on the earth, Joule heating of the ionospheric plasma, and a magnetic field that is mainly eastward in the northern hemisphere and westward in the southern hemisphere. Assuming that all of the magnetic flux from the regions above the auroral zones threads the solar wind plasma in the boundary layer, the torque on the earth at magnetically quiet times is in the neighborhood of 1020 g cm2/sec2;, corresponding to a decrease in the earth's rotation rate of about one part in 1013 per year. The Joule heating rate is of order 1016 ergs/sec compared with the 1018 ergs/sec presently estimated for the dissipation rate associated with the magnetospheric convection.