Interelectrode Gap Width Research Articles

Achieving high replicability for electrochemical machining via the stamp flushing method in stationary electrolyte

Traditional high-speed electrolyte flow for removing bubbles and sludges from the inter-electrode gap during electrochemical machining (ECM) usually results in the non-uniform distribution of temperature, bubbles, and sludges, thus deteriorating machining accuracy. Therefore, ECM performed in stationary electrolyte via an efficient flushing method may solve this problem. For this reason, the previous study proposed the stamp flushing method to squeeze out the bubbles and sludge from the inter-electrode gap by shortening the gap width. However, the effectiveness of the stamp flushing method in achieving high replicability has not been systematically investigated. In the present work, the stamp stroke motion was first optimized to improve the flushing efficiency of the stamp flushing method. To obtain higher stamping velocity and to avoid the damage due to the collision between the tool electrode and workpiece, a cushion mechanism was newly installed on the equipment. Based on the optimized stamp stroke motion, the periodical micropatterns fabricated on the tool electrode surface using wire electrical discharge machining (WEDM) were replicated onto the SK4 workpiece using ECM in stationary electrolyte. The dimensional deviation of the workpiece from the tool electrode after ECM was less than 4 μm in the feed direction and approximately 30 μm in the side direction when the initial width of the frontal gap was 10 μm. Outside the inter-electrode gap, the electrolyte flowing along the axial direction (EFAAD) was supplied to rapidly flush out the bubbles and sludge squeezed out by stamp motion to prevent them from backflowing into the inter-electrode gap during the widening phase of the gap after the stamp motion. Since the EFAAD does not affect the electrolyte in the inter-electrode gap, the ECM remains performed in a stationary electrolyte. High replicability was obtained uniformly over the working surface, which originates from the combined effect of small inter-electrode gap width and uniform distribution of temperature, bubbles, and sludge in stationary electrolyte. This work proves the potential of the proposed stamp-flushing method for manufacturing metal parts with high replicability.

Investigation of electropolishing characteristics of tungsten in eco-friendly sodium hydroxide aqueous solution

In this study, an eco-friendly electrolyte for electropolishing tungsten and the minimum material removal depth on the electropolished tungsten surface are investigated using an electrochemical etching method. Using a concentrated acid electrolyte, the polarization curve and current density transient are observed. For a NaOH electrolyte, the effects of interelectrode gap and electrolyte concentration on electropolishing are investigated. The differences in electropolishing characteristics are compared among different electrolyte types. Microholes are etched on the electropolished tungsten surface to determine the minimum material removal depth on the tungsten surface. Experimental results indicate the color effect due to a change in the thickness of the oxide film on the tungsten surface after electropolishing with a concentrated acid electrolyte. The surface roughness decreases with the interelectrode gap width owing to the increased current density when using the NaOH electrolyte. However, the electropolishing effect is less prominent with a significantly smaller gap because the generated bubbles are unable to escape from the narrow working gap in time. A material removal depth of less than 10 nm is achieved on the tungsten surface in an area of diameter 300 µm, using the electrochemical etching method.

Numerical Investigation on the Transient Evolution Mechanisms of Nonlinear Phenomena in a Helium Dielectric Barrier Discharge at Atmospheric Pressure

In this paper, the evolution and transition mechanisms of typical nonlinear phenomena in an atmospheric pressure helium dielectric barrier discharge (DBD) are investigated from the perspective of transient evolution properties through fluid modeling. By gradually up-regulating the interelectrode gap width, the discharge (steady state) evolves from symmetric single-period (SP1), through asymmetric single-period (AP1), into Period-2 (P2) state. Similar evolution trace can also be obtained when increasing the driving frequency. Right after the initial breakdown, the discharge goes through several transient periods featuring unstable asymmetric current pulses before the establishment of steady-state discharge. As the bifurcation parameter increases in excess of the critical value, the discrepancy between the seed electron level of positive and negative discharges accumulates over the transient periods, switching the discharge into AP1 state. After then, a further elevation in the gas gap width (or driving frequency) leads to the generation of a “moderate intensity” positive pulse. With the “moderate intensity” positive pulse and a stronger positive pulse emerging alternatively in the temporal waveform sequence, the discharge evolves into P2 state. Finally, based on the transient evolution mechanisms, a preliminary state-controlling method regarding the use of a first-peak-leveled applied voltage is proposed, which may shed light on the modification of nonlinear states in some practical DBD application scenarios.

Improving machining accuracy in wire electrochemical micromachining using a rotary helical electrode

Wire electrochemical micromachining (WECMM) is becoming more important in fabricating complex microstructures. The machining accuracy of WECMM is usually evaluated by the interelectrode gap width. Faster electrolyte refreshment increases the uniformity of the electrolyte electrical conductivity distribution in the machining gap, the machining stability and the machining accuracy. We propose a method of WECMM using a rotary helical electrode that improves the machining accuracy by increasing the electrolyte refreshment and achieving a faster electrode feed rate and consequently decreasing the machining gap. Simulations of the electric field indicated that the current density was localized for the rotary helical electrode. Experiments verified that the rotary helical electrode decreased the machining gap from 94 to 19 μm compared with a cylindrical electrode of the same diameter. In addition, the effects of the process parameters on the interelectrode gap width were experimentally investigated. Finally, using optimal parameters, two precision microstructures with clean-cut edges were fabricated.

Measuring the robson angle in vacuum arcs of various length

It is established that the Robson angle, which determines the direction of motion of a cathode spot of vacuum arc in a homogeneous electric field sloped relative to the cathode, depends mostly on the inter-electrode gap width. The dependence of the Robson angle on the field slope at various gap widths has been measured for molybdenum and tungsten cathodes. The knowledge of this angle is necessary for correct choice of the optimum magnetic field configuration controlling the cathode spot dynamics in setups for vacuum-arc deposition of various coatings and in vacuum commutation devices.

Note: Measurement of extreme-short current pulse duration of runaway electron beam in atmospheric pressure air

This note reports the time-amplitude characteristic of the supershort avalanche electron beam with up to 20 ps time resolution. For the first time it is shown that the electron beam downstream of small-diameter diaphragms in atmospheric pressure air has a complex structure which depends on the interelectrode gap width and cathode design. With a spherical cathode and collimator the minimum duration at half maximum of the supershort avalanche electron beam current pulse was shown to be ~25 ps. The minimum duration at half maximum of one peak in the pulses with two peaks can reach ~25 ps too.

The temporal structure of a runaway electron beam generated in air at atmospheric pressure

Supershort avalanche electron beams (SAEBs) generated in air at atmospheric pressure have been studied with picosecond time resolution. It is established that an SAEB has a complicated structure that depends on the interelectrode gap width and cathode design. In a gas-filled diode with a small gap width, an SAEB current pulse with a full width at half maximum (FWHM) of ∼25 ps has been observed behind a collimator with a hole diameter of 1 mm. As the gap width is increased or decreased relative to the optimum value that corresponds to the maximum beam current, the SAEB current pulse shape changes and pulses with two peaks are more likely detected. The two-peak SAEB current pulse shape is retained behind aluminum foil with a thickness of 60 and 110 μm.

Runaway electrons and x-rays from a corona discharge in atmospheric pressure air

The characteristics of a corona discharge in atmospheric pressure air are studied using pulsed power generators that produce voltage pulses of different durations, polarities and shapes. The characteristics are measured in the single pulse, batch, and repetitively pulsed modes. It is shown that no matter what the voltage pulse polarity is, a corona discharge starts developing as a conical diffuse discharge near the electrode tip with a voltage rate of increase of ∼1015 V s−1 across an electrode of small curvature radius. With lower voltage rate of increase (∼1013 V s−1 or lower), one or several diffuse jets develop from this electrode. The diameter of the jets at their front is less than 1 mm and depends on many factors (voltage pulse amplitude and increase, inter-electrode gap width, pulse repetition rate, etc). It is found that at long voltage pulse durations, the radiation spectrum of the corona discharge changes, and the bands and lines of the material of the electrode appear in the UV region at 200–300 nm. It is demonstrated that a runaway electron beam in a corona discharge is generated and detected at a distance several times greater than the brightly glowing plasma region of the corona discharge. It is shown that x-rays are generated from a corona discharge at high pulse repetition rates of up to 1 kHz.

Stochastic Nature of Electrical Breakdown in Vacuum

This paper presents measurement results for the random variable "vacuum breakdown voltage", obtained by the application of pulse, dc, and ac voltage. Parameters varied during the measurements were: 1) vacuum pressure and 2) inter-electrode gap width. The measurements were performed under strictly controlled laboratory conditions. Conclusions about the influence that the variable parameters in the experiment exert on the parameters of theoretical statistical distributions have been drawn, based on the statistical analysis of the obtained experimental results. The obtained results have been explained by the mechanisms constituting the process of electrical breakdown in vacuum.

The Townsend coefficient and electron runaway characteristics in nitrogen

The results of analytical description and numerical simulation show that the Townsend ionization mechanism (characterized by a constant mean electron drift velocity and energy and by exponential growth in the number of electrons) is operative even in rather strong electric fields, for which the ionization drag of electrons can be ignored. The universal function Ucr(pd) calculated for nitrogen establishes the relationship between the critical interelectrode voltage (separating the regions of effective multiplication and significant runaway of electrons) and the product pd of the gas pressure and interelectrode gap width. The Ucr(pd) curve has two branches—the upper, characterizing the runaway of electrons, and the lower, describing their loss by drift. A minimum pd value on the Ucr(pd) curve corresponds to the maximum Townsend coefficient as a function of E/p. Using the Ucr(pd) function, an analog of the well-known Paschen curve characterizing self-sustained discharge in nitrogen has been constructed.

Microwave losses in electrodes of distributed and lumped ferroelectric-based elements

The microwave losses in electrodes of a ferroelectric-based transmission line and a lumped ferroelectric-based element have been theoretically analyzed. The results of calculations of the effective loss tangent for the transmission line are confirmed by a two-dimensional electrodynamic model analysis and by experiments with a coplanar phase shifter. It is established that the use of microwave devices based on ferroelectric lumped elements is expedient in the 1–50 GHz frequency range (that is, in most microwave applications). The microwave losses in electrodes are significantly lower for lumped elements than for a long transmission line. These losses are determined by the ratio of the depth of electromagnetic field penetration into a conductor (superconductor) to the interelectrode gap width and are independent of the dielectric permittivity.

A theoretical analysis of electrochemical arc machining

Electrochemical arc machining (ECAM) involves the removal of metal from an anodically polarized workpiece by both erosion arising from discharges produced in an aqueous electrolyte and electrolytic dissolution. A theoretical model is derived for the process and analysed for two specific applications, fine-hole drilling and the finishing of components by smoothing of their initially rough surfaces. In the second of these examples, a perturbation procedure for obtaining approximate solutions is used; the model so developed encorporates the effects of current density on current efficiency which are known from experimental electrochemical machining (ECM) studies to influence the rate and mode of smoothing. For fine-hole drilling by ECAM, the analysis predicts that the interelectrode gap width increases with the applied voltage and inversely with the square root of the mechanically driven anode. In the case of smoothing, ECAM is found to remove the surface irregularities at a much faster rate and with lower loss of stock metal than ECM alone, when electrolytes such as sodium chloride solution yielding 100% current efficiency are used for the latter process. The analysis shows that an electrolyte solution with a current density-dependent current efficiency is needed if parent metal loss by ECM is to approach that of ECAM, and even then, machining by the latter is still much faster. Attention is drawn to experimental evidence in support of these predictions of ECAM behaviour. Finally, results from the model are used to verify the practical use of ECM for rapid finishing of the surfaces of components left rough by electrodischarge machining.

Theoretical Analysis of the Electroforming Process

A theoretical model is developed for electroforming with a shaped, soluble anode-electrode. Cases are considered for which the electrode is either flat or conforms initially to the shape of the cathode-mandrel. The model is first analysed by a perturbation procedure for a case where the amplitude and slope of the surfaces of sinusoidally-shaped electrodes are small compared with the inter-electrode gap width. The analysis shows that for d.c. electroforming, when the soluble anode is initially flat, the upper surface of the electroformed metal can become fiat at some stage. After that stage has been passed, the amplitude of the metal layer increases indefinitely with time of electroforming. A uniform thickness for the electroformed metal layer cannot, therefore, be achieved, unlike the case for insoluble plane anodes. When a soluble anode which conforms initially to the cathode-mandrel is used, a fiat upper surface for the metal layer can be obtained. When periodic reversal of polarity is used with soluble anodes, a uniform thickness for the electroformed layer cannot be achieved. A numerical method of analysis of electroforming is proposed which removes the restrictions on the size of shape treated by the perturbation method. The latter method is then applied to the practical case of electroforming of a mould for a rubber ‘O’ ring. Conditions are investigated where the anode is (i) plane and insoluble, (ii) plane and soluble, (iii) and (iv) conforming initially to the mandrel-shape and either insoluble or soluble. The numerical method has revealed that a variation in the metal layer thickness occurs for all these types of anode, and that little marked improvement arises if the simplest type of anode—plane and insoluble—is replaced by any one of the others.

Analysis of Electroforming with Direct Current

The process of electroforming with direct current is analysed by a perturbation method for a case in which the amplitude and slope of the electroformed metal layer are small compared with the inter-electrode gap width. Conditions are established for which the metal layer has a uniform thickness. The converse problem of electroforming a metal layer, whose lower face is irregular but whose upper face is effectively flat, is also investigated.