Interelectrode Gap Research Articles

Characteristics of evolution and collapse in spark-induced cavitation bubbles

Numerous scientists have thoroughly researched cavitation bubble dynamics using experimental methods such as tube arrest, underwater discharge, ultrasound, and laser focusing. In this study, with the aid of high-speed photography, the formation mechanism of the electrode-contact spark-induced cavitation bubble is discovered, i.e., electrolysis results in non-condensable gases wrapping the small inter-electrode gaps, and the non-condensable gases are broken down by discharge to form plasma, which then induces the cavitation bubble. Since the cavitation bubbles already contain a certain amount of non-condensable gases during the discharge process, the differences in the spatiotemporal evolution and collapse characteristics of the cavitation bubbles with varying amounts of non-condensable gases are further analyzed. The results show that underwater electrode-contact discharge system has an optimal voltage if the capacitance and discharge electrode size remain constant, and the cavitation bubbles generated under the optimal voltage condition are not only morphologically closest to the laser-induced cavitation bubbles, but also the change in radius over time during collapse is quite consistent with the Rayleigh bubble. Furthermore, compared to cavitation bubbles generated under varying voltages, those induced by the optimal voltage have a lower amount of non-condensable gases. This leads to the minimum first contraction radius and the maximum rebound radius being close to the corresponding values of the laser-induced cavitation bubbles. These new findings are of great significance for the improvement of experimental technology in the study of cavitation bubble dynamics, obtaining precise and dependable experimental data, and validating numerical simulations.

Electric field simulation in the electrochemical machining of a thin-walled part cavity

The article is aimed at simulating an electric field in the interelectrode gap during the electrochemical machining of a thin-walled part cavity for aerospace equipment. The study involved simulating the process of electrochemical cavity machining at a constant voltage in a steady-state mode in the COMSOL Multiphysics environment. The simulation was carried out for the scheme of electrochemical machining with a movable cathode and vertical and horizontal feeding to the workpiece surface undergoing machining while maintaining a constant interelectrode gap. The following simulation conditions were adopted: 12Cr18Ni10Ti stainless steel as the material of the cathode tube; AlMg6 aluminum alloy as the material of the thin-walled part; NaNO3 solution as the electrolyte. When simulating the electric field in the interelectrode gap, the heat exchange process was taken into account. The simulation of the electric field in the electrochemical cavity machining of a thin-walled part yielded a macro that allows the process simulation to be adapted to different input process conditions. As a result of the simulation, the following distribution patterns were obtained: current density in the cathode, potentials, electric field in the interelectrode gap and adjacent area, and process temperature of electrochemical machining. The simulation results show that the electric field lines are directed toward the cathode from the workpiece periphery. This means that anodic dissolution of material occurs in a given region, which characterizes the law concerning the distribution of potentials in an electrochemical cell. The temperature distribution pattern obtained in the simulation revealed that a temperature increase in the machining zone is insignificant. An increase in electrolyte temperature is shown to result in a proportional increase in wall temperature. Thus, the conducted study provides a theoretical insight into the examined process.

Design, Validation, and Fabrication of a Tailored Electrochemical Reactor Using 3D Printing for Studies of Commercial Boron-Doped Diamond Electrodes.

Boron-doped diamond (BDD) electrodes are the most effective and resistant electrodic materials to perform advanced oxidation processes. Having a reactor that can provide adequate hydrodynamic conditions is mandatory to use these electrodes effectively. In this work, the diamond anode electrochemical reactor (E3L-DAER) is designed to fulfill this necessity. Several features are included to improve its efficiency, like conic inlet/outlet, flow enhancers, and a reduced interelectrode gap. The fluid dynamic validation has been performed using computer fluid dynamics (CFD) calculations, residence time distribution (RDT) curves, and mass transfer analysis. The reactor has been made using a three-dimensional (3D) printing stereolithography (SLA) technique, which allows us to build chemical-resistant reactors with nonstandard and tailored features in a cheap and fast way. The obtained results demonstrate that the designed reactor has the required fluid dynamics properties to perform reliable BDD electrode studies and applications. Finally, a BDD electrode was used to test the production of different oxidants such as persulfate, peroxophosphate, and chlorine-derived species.

Method for controlling the area of the electrolyte outlets in the flow field simulation and experimental verification of electrochemical machining

Electrochemical machining (ECM) is regarded as a promising and cost-effective manufacturing method for difficult-to-cut materials with complex shapes and structures. Many methods have been successfully developed for the flow field state of machining gaps, which is a key factor affecting machining performance in ECM engineering practice. However, little attention has been given to the area of the electrolyte outlets. This study attempts to present a method for controlling the area of the electrolyte outlets in the flow field simulation and experimental verification of electrochemical machining. The simulation results indicate that the convergent outlet mode can effectively enhance the turbulent effect of the fluid in the inter-electrode gap, especially in the near-wall region, and improve the mass transfer effect of electrolyte. Meanwhile, the simulation results also showed that better material removal rate (MRR) and surface roughness can be obtained when the convergent degree is about 50–70% combined with the effects of bubble volume compression and flow velocity reduction. The experimental results confirmed that the convergent outlet mode can obtain better MRR and surface roughness. Energy-dispersive X-ray spectroscopy results of machined specimens showed that oxides with strong adhesion on the specimen’s surface were effectively eliminated due to stronger turbulence effect under convergent outlet conditions. This should be considered as the main reason for improving machining performance by changing the electrolyte outlet area. These conclusions can provide some useful assistance for ECM engineering practice.

Improving flatness of machined surface in rotating cathode electrochemical milling

Electrochemical milling has broad application prospects for machining large aeronautical thin-walled components. This is because this technique has no cutting force, has no heat-affected zones, and is considerably flexible. In contrast to the non-rotating tool cathode of traditional electrochemical milling, the rotating cathode used in electrochemical milling has more promising prospects because it has the same motion mode as the milling cutter employed in mechanical milling. However, because of the uneven electric quantity supply from the cylindrical rotating cathode, the machined surface typically has a curved profile with inadequate surface flatness; this is the main drawback of rotating-cathode electrochemical milling. To improve the flatness of surfaces processed by rotating-cathode electrochemical milling, a novel multichannel cathode with the outlet hole arrangement that can improve the uniformity of electric quantity supply is proposed. Numerous large outlet holes are bored at the bottom center of multichannel rotating cathode. Numerical simulation results indicate that these holes cause a decrease in the electric quantity supply from the inner area at the bottom of the cathode from the perspectives of electric and flow fields, minimizing the unevenness of the electric quantity supply distribution. Validation experiments prove that compared with previously reported rotating cathodes, the straightness index and depth error of the surface machined by novel multichannel cathode are reduced by 80.9 % and 69.4 %, respectively. After obtaining grooves with flat surfaces, the step-over distance required to machine a plane using a multichannel rotating cathode increases; meanwhile, the flatness of the machined plane significantly improves. With the applied voltage of 25 V, electrolyte pressure of 0.2 MPa, initial inter-electrode gap of 0.2 mm, and cathode rotational speed of 500 rpm, a 42 × 40 mm plane is finally machined, whose planeness index (Δhp) is only 0.047.

Features of numerical modeling electrical coalescence in a droplet-layer system using the arbitrary Lagrangian-Eulerian method

The electrocoalescence of a water drop at the water-oil interface was numerically analyzed using the finite element method by solving a set of equations including hydrodynamics and electrostatics. The simulation was performed using COMSOL Multiphysics software, the interface movement was implemented using an arbitrary Lagrangian-Eulerian method (also called the moving-mesh method). The influence of the initial distance from the drop to the layer, as well as the size of the entire domain on the outcome of the process was studied. The electrical coalescence and non-coalescence of a single uncharged droplet with a layer under the action of the DC field were calculated in a wide range of electric field strength. The created computer model makes it possible to calculate the voltage dependence of the detached droplet volume and charge. Also, it can help to determine features of non-coalescence, including the stretching of the droplet along the interelectrode gap and spraying.

Generalization of numerical simulation results on the electrical coalescence threshold for two conducting droplets based on non-dimensional parameters

Electrocoalescence, the physical process underlying the demulsification of a dielectric dispersion medium containing small conductive droplets (e.g., water), involves droplet merging at low electric fields and splashing at higher voltages. Understanding the physics of electrocoalescence is crucial for optimizing industrial electrocoalescers. However, mathematical modeling of these complex, multiphysics phenomena is challenging, and many published results are questionable. In this study, we utilized a previously developed reliable model for computing the threshold between electrical coalescence and non-coalescence. We investigate the applicability of dimensionless parameters, such as the Ohnesorge number and Weber electric number, to describe the coalescence threshold for uncharged droplets of equal size. Using COMSOL Multiphysics software, we analyze the dependency of the threshold electric field strength on water droplet radius and establish an equivalent dimensionless relationship. Our findings reveal that a universal Weber number quite accurately describes the threshold over a wide range of droplet radii, regardless of changes in liquid viscosity and inter-electrode gap. Direct mathematical simulations using up-to-date numerical models enable us to determine non-dimensional parameter values corresponding to the threshold electric field strength, providing generalizable results.

The Optimal Operating Point for Linearizing an Integrated Optical Lithium Niobate Directional Coupler Modulator

The influence of an operating point on the linearity of an integrated optical lithium niobate directional coupler modulator was studied. It was found that the optimal setting for the position of an operating point for suppressing the third-order intermodulation distortion depended on the power of the high-frequency modulation signal. Thus, despite the simple design of the device, the directional coupler modulator requires a complex algorithm for setting an operating point to achieve a high linearity of operation. An active system for setting an operating point based on the low-frequency pilot signal and zeroing its third harmonic was used to demonstrate the possibility of linearization when the amplitude of the modulation signal changes. The use of an operating point control system became possible after limiting the drift of the operating point by etching the dielectric buffer layer in the interelectrode gap. The results obtained look promising for high-performance analog optical links.

Editors' Choice—Improving Quality of EDMed Micro-Holes on Titanium via In Situ Electrochemical Post-processing: A Transient Simulation and Experimental Study

Electrical discharge micromachining (EDM) poses challenges to the fatigue-life performance of machined surfaces due to thermal damage, including recast layers, heat-affected zones, residual stress, micro-cracks, and pores. Existing literature proposes various ex situ post-processing techniques to mitigate these effects, albeit requiring separate facilities, leading to increased time and costs. This research involves an in situ sequential electrochemical post-processing (ECPP) technique to enhance the quality of EDMed micro-holes on titanium. The study develops an understanding of the evolution of overcutting during ECPP, conducting unique experiments that involve adjusting the initial radial interelectrode gap (utilizing in situ wire-electrical discharge grinding) and applied voltage. Additionally, an experimentally validated transient finite element method (FEM) model is developed, incorporating the passive film formation phenomenon for improved accuracy. Compared to EDM alone, the sequential EDM-ECPP approach produced micro-holes with superior surface integrity and form accuracy, completely eliminating thermal damage. Notably, surface roughness (Sa) was reduced by 80% after the ECPP. Increasing the voltage from 8 to 16 V or decreasing the gap from 60 to 20 μm rendered a larger overcut. This research’s novelty lies in using a two-phase dielectric (water-air), effectively addressing dielectric and electrolyte cross-contamination issues, rendering it suitable for commercial applications.

Study of mass transfer during electric spark processing of steel with tungsten anode material

Changes in the specific masses of electrodes were studied during electric spark processing of SCMnCr3 steel with anode material made of WP tungsten on an Alier-31 installation. Predominant erosion at high cathode processing energies is due to the difference in the melting/evaporation temperatures of the cathode substrate (iron) and anode (tungsten) materials. The influence of the energy of the electric spark discharge is associated with a change in the volume of the electrodes converted into liquid and gaseous states. The amplitude of the electrode-anode oscillations is influenced by changing the average volume of the interelectrode gap.

Raising the curtain: Bubble size measurement inside parallel plate electrolyzers

Alongside vertical gas-evolving electrodes, gas bubbles nucleate, grow, coalesce, detach, and ultimately form a bubble curtain, affecting the fluid dynamic, electrochemical reaction, mass transport, and ohmic drop. Due to the small inter-electrode gap in parallel plate electrolyzers, the size determination of detached bubbles inside the bubble curtain close to the electrodes is challenging. In particular, the size determination of detached oxygen bubbles needs to be addressed. We present an optical setup to record images from detached oxygen bubbles inside the bubble curtain of a membrane-separated, parallel plate electrolyzer. The images are postprocessed using an adopted region-based convolutional neuronal network to quantify the bubble sizes. Next, the electrolyte flow velocity, concentration, current density, and catalysts are varied, and the effect on the oxygen bubble size distribution is studied. The median size ranges from 140 to ▪, doubling values from literature measured in artificial electrolyzers.

The effect of space charge neutralization on the parametric design of photon enhanced thermionic solar converters

Photon-enhanced thermionic emission (PETE) is a promising method for increasing current density at relatively low cathode temperatures, but is susceptible to the space charge effect. In this study, we propose a novel PETE device model in which the space-charge barrier is relaxed by photon-excited cesium ions. The decrease in maximum efficiency with interelectrode gap is more gradual compared to previous studies. We investigate the effects of cathode materials properties (band gap and electron affinity) and operating conditions (cathode temperature and interelectrode gap) on PETE performance, and identify optimal performance conditions. Our findings show that the optimal gap range exceeds 5 μm and near field radiation loss can be minimized, providing an important opportunity for designing PETE device with larger interelectrode gaps and higher solar flux concentrations.

Microplasma characteristics of direct-current atmospheric pressure glow discharge in dependence of gap distance and discharge current

Microplasma at atmospheric pressure has been widely used in many fields due to the lower power consumption, higher plasma density, as well as better uniformity and stability. In this work, the microplasma characteristics of direct current atmospheric pressure glow discharge, including discharge morphology and electrical properties have been investigated with various interelectrode gaps (10–600 μm) and discharge currents (1–6 mA), and simultaneously, a finite element simulation has been conducted to obtain the distribution of the electric field and particle density. The evolution of cathode layer, anode layer and Faraday dark space at this scale was captured with a higher spatial resolution (∼1 μm) for the first time, demonstrating that the cathode layer, especially the cathode sheath rather than the positive column, plays a dominant role in the transition of the microplasma. As the gap shrinks to a size less than cathode layer (∼40 μm), the cathode sheath with a high electric field is compressed, leading to a rapid decrease of the discharge voltage. The discharge voltage remains basically unchanged regardless of the discharge current, because the electric field in the cathode sheath is limited by the accumulated space charge. The experimental results are well verified and explained by the simulation results. This study provides an in-depth understanding of the glow discharge mechanism at microscale, and of the stability of glow discharge at atmospheric pressure, and benefits to future research on the atmospheric pressure large-area microplasma and its related application.

Specific energy consumption of electro-contact-chemical treatment of metals with a vibrating tool in the electrolyte

According to the specific energy consumption, the shaping processes are arranged in three energy levels. Electrophysical and electrochemical methods of metal processing are at the third level, where the specific energy consumption is more than 6∙104 J/cm3. The analysis of the literature data showed the inconsistency of the specific costs of some authors. The specific energy consumption of electrical contact processing cannot be commensurate with the costs of electrochemical processing due to the different sizes of the particles removed from the surface of the workpiece. There are no literature data on the specific energy consumption of electro-contact-chemical treatment of metals with a vibrating instrument in the electrolyte, therefore, experiments have been carried out with the fixation of current, voltage and interelectrode gap oscillograms. The method of calculation of specific energy consumption according to the oscillograms of the process is given. The energy costs for vibration of the electrode-tool are calculated, which are an order of magnitude less for electro-contact-chemical treatment. When the vibration amplitude decreases or the voltage on the electrodes increases, the process in the interelectrode gap turns into dimensional arc processing. When electro-contact-chemical treatment of metals with a vibrating tool in water, the specific energy consumption is equal to (3.5–3.8) · 105 J/cm3, which corresponds to electrocon-tact treatment. It is assumed that the use of aqueous solutions of neutral salts will lead to a reduction in energy costs.

Process capability of electrochemical discharge machining: A review

Electrochemical discharge machining (ECDM) is a hybrid machining formed by combining the principles of electrochemical machining and electrical discharge machining. The ECDM is unconventional micromachining that can fabricate holes, blind holes, micro-cavities and grooves on brittle insulating materials such as quartz, glass, silicon wafers and different composites that are difficult to machine. These non-conducting materials have sundry applications such as Micro Electric-Mechanical Systems, biomedical and electronics items. The ECDM provides maximum material removal rate and good surface quality with minor tool wear. The study describes how different process parameters affect machining. The assessment mainly focuses on various tool electrode geometries and materials that are used in ECDM. The present article investigates the ramifications of different types of electrolytes and additives mixed in electrolytes used during the ECDM process on the machined zone. This paper comprehensively reviews the effect of change in applied voltage, machining gap and an inter-electrode gap in the ECDM and it also analyses the fabrication of different materials including glass, quartz, ceramics and composites.

Simultaneously measuring the dichroism and the dielectric response of an azobenzene-doped organic glass former

We have designed an experimental setup allowing to simultaneously measure both the dielectric response of a supercooled liquid and the dynamics of azobenzene chromophores dispersed in it. Both the azobenzene chromophores and the organic glass former have been synthesized with similar reaction paths: they are chemically similar, apart from the azobenzene group responsible for the strong optical absorption in the [350; 450 nm] range for the chromophores, while the embedding supercooled liquid is optically transparent. This material is deposited on transparent electrodes with an inter-electrode gap as small as 4 µm—obtained thanks to optical lithographic techniques. We show that our setup is sensitive enough to measure the coupling between the dielectric macroscopic response and the isomerization dynamics of 1% of chromophores excited by a 0.5–5 mW/cm2 light beam. We demonstrate that this coupling neither comes from the heating of the sample due to the light absorption nor from changes of the sample shape with light. Finally, we discuss the few physical effects, which may give rise to this coupling, and show that our experiment could test some recent predictions done in the framework of random first order transition theory of the glassy state.

Eco-friendly tool-based electrochemical polishing of additively manufactured metallic components

The elevated surface roughness of metal additive manufacturing (MAM) parts detrimentally affects wear resistance, diminishes fatigue strength, and hampers cooling efficiency, which mandates post-processing. Electrochemical polishing is a non-contact and non-thermal post-processing technique but uses non-ecofriendly acidic baths and is ineffective in removing unmolten or partially molten metal particles. To address these challenges, we propose a novel approach using a flat nano-polished cylindrical tool as the cathode and an eco-friendly electrolyte for finishing a MAM component fabricated via atomic diffusion additive manufacturing (ADAM). Our study evaluates the effectiveness of this approach through numerical simulation and optimises the process through experimental analysis. The numerical simulation developed in COMSOL incorporates the surface roughness data of the ADAM part as the anode surface profile in the 2D simulation domain. The current density distribution, viscous layer formation, reduction in surface roughness and mass material removal rate (MRRg) are analysed from the simulation results. The experimental optimisation of parameters, including polishing time, inter-electrode gap (IEG), electrolyte flow, and electrolyte composition, resulted in a substantial decrease in the average surface roughness (Ra) value of the ADAM component by 95.91 %. The polished surface is more levelled, exhibiting a glossy finish with no waviness and fewer surface cracks. The EDS and XRD analysis portrayed the presence of passive films, indicating improved corrosion resistance. Repeating the experiment for rolled and milled surfaces with the same process parameters resulted in a similar reduction in the Ra value by 97.59 % and 94.73 %, respectively. The comparative analysis, of ECP on MAM, rolled, and milled surfaces, indicates the potential to achieve a similar notable improvement in surface finish, irrespective of the process history of the manufactured part for the same ECP parameters. Comparing the anode surface profiles, Ra values, and mass of material dissolved showed a close fit between experimental and simulation results. Our study highlights the feasibility of ECP with the flat tool electrode and eco-friendly electrolytes to reduce the surface roughness of an MAM component significantly.

FACTORS INFLUENCING THE EFFICIENCY OF MODERN ELECTRO-EROSION PROCESSING

The article describes the factors of increasing the efficiency of processing products on EDM equipment. due to the use of linear drives with high dynamic accuracy. Also, the inter-electrode gap in the processing zone, its influence on optimal energy regimes and maximum material removal was investigated in the work. The electrode is fed into the spark formation zone 5-10 times more often, the relative duration is several times longer than in machines with KGP drives. Sludge removal due to the pumping effect from rapid movements of the electrode instead of pumping. The use of linear motors, servo drives, precision linear position sensors, devices for automatic wire feeding, hole search, wire breakage prevention - will increase the productivity of wire-cutting processing by 40-100%.A comparative analysis of the performance of parts processing on equipment with the use of linear and KGP (ball-screw pairs) drives was carried out.

Effect of inter-electrode gap on surface characteristics in conventional electrochemical machining

Conventional electrochemical machining (CECM) is an established toolroom process for the machining of conductive workpiece (W/P) substrates. In CECM, the material removal takes place through an electrochemical reaction between the cathode and anode. Significant studies have reported process parametric optimization of CECM with input parameters like voltage, feed rate, type of electrolyte and electrolyte concentration, etc., and their effect on the material removal rate (MRR), and surface roughness (Ra). But hitherto little has been reported on the effect of inter-electrode gap (IEG) and other CECM parameters in combination with a different tool and work materials. This study outlines the experimental investigations on CECM with a different tool and work materials based on Taguchi L18 orthogonal array (OA) on surface characteristics of machined surface for tool room applications in line with sustainable development goal (SDG) (with a special target on the industry, innovation, and infrastructure). The results suggest that for Ra, IEG is the most significant factor at a 95% confidence level. The results have been supported by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Ra analysis (with amplitude distribution function (ADF), and peak count (PC).

Investigation of the Change in Hydrophobicity of Polycarbonate in Atmospheric Pressure Dielectric Barrier Discharge (APBDB) Generated with a 50 Hz Power Supply

Dielectric Barrier Discharge (DBD) of disc electrodes with diameter 5 cm and thickness 1 cm of each has been used for treatment of the samples. The discharge was produced at fixed inter-electrode gap of 4.5 cm for the study of effectiveness of treatment on the surface of the Polycarbonates (PCs). After the treatment in varying time periods the hydrophobic properties of sample changed to the hydrophilic. To investigate the effect of plasma treatment, contact angle was measured by using goniometer with water as a testing liquid. Effect on the sample due to treatment time, applied voltage and working gas were studied. Treatment carried out in Argon environment produced better wettability compared to atmospheric air. Aging effect was also studied to see the evolution of the wettability after time elapsed by measuring the contact angle up to one month after the treatment. Contact angle was increased from 39.35° C to 56.4° C, which was still very lower than the contact angle 81.2° of untreated sample.