Breakdown In Liquids Research Articles

DC Underwater Capillary Discharge With Symmetrical Hole: Diagnostics and Pumping Effect

Capillary discharge investigated by this work was created in the reactor using positive half-cycle of AC high voltage up to 2 kV. Electric field created between two electrodes which were separated by the dielectric ceramic barrier with a symmetrical cylindrical hole (diameter of 1 mm, length of 5 mm) in it. When voltage reached the value sufficient for liquid breakdown, the discharge appeared initially in bubbles of evaporated solution in the hole vicinity and spread further into the liquid volume. After the rise of discharge, two kinds of plasma channels (streamers) propagated towards electrodes from the pin-hole: longer positive streamers on the side with the cathode (analogically as in the positive corona discharge) and shorter negative streamers (like negative corona discharge). These streamer kinds differed especially in the energy dissipation originating from different electron velocities in plasma channels due to electron collisions with positive particles accelerating or decelerating electron avalanches and it gave rise of a significant pump effect.

Degradation of imidazolium‐based ionic liquids in aqueous solution by Fenton oxidation

AbstractBACKGROUNDThe breakdown of imidazolium ionic liquids (ILs) by Fenton oxidation has been demonstrated, but using unaffordable doses of hydrogen peroxide (H2O2). The literature studies so far do not report on the evolution of total organic carbon (TOC) and ecotoxicity. Therefore, the potential application of the process for the degradation of ILs still remains an open question. This work investigates the feasibility of the Fenton process for the treatment of aqueous effluents containing imidazolium‐based ILs of different structures, represented by the length of the alkyl chain and the nature of the anion. Special attention is paid to the evolution of the ecotoxicity and the consumption of H2O2 as a critical issue for the economy of the process. The experiments were performed at 70 °C.RESULTSWith the stoichiometric H2O2 dose, the complete conversion of the starting imidazolium‐based ILs was achieved in less than 5 min, at 70°C, with 60% of TOC reduction after 4 h, except in the [C4mim][CH3CO2] case, where iron precipitation was observed, thus inhibiting its catalytic action. Low ecotoxicity effluents were obtained at the stoichiometric H2O2 dose, even in the cases of the highly ecotoxic ILs of long alkyl chain with chloride anion.CONCLUSIONFenton oxidation is shown to be a promising solution for the degradation of imidazolium‐based ILs in water, allowing deep detoxification with the stoichiometric H2O2 dose. © 2014 Society of Chemical Industry

Ionic liquids breakdown by Fenton oxidation

Fenton oxidation has proved to be an efficient treatment for the degradation of ionic liquids (ILs) of different families viz. imidazolium, pyridinium, ammonium and phosphonium, in water. The intensification of the process, defined as the improvement on the efficiency of H2O2 consumption, by increasing the temperature is necessary to avoid high reaction times and the need of large excess of H2O2. In this work, temperatures within the range of 70–90°C have been used, which allowed an effective breakdown of the ILs tested (1gL−1 initial concentration) with the stoichiometric amount of H2O2 and a relatively low Fe3+ dose (50mgL−1). Under these conditions conversion of the ILs was achieved in less than 10min, with TOC reductions higher than 60% upon 4h reaction time, except for the phosphonium IL. The remaining TOC corresponded mainly to short-chain organic acids. The treatment reduced substantially the ecotoxicity up to final values below 0.01TU in most cases and a significant improvement of the biodegradability was achieved. Upon Fenton oxidation of the four ILs tested hydroxylated compounds of higher molecular weight than the starting ILs, fragments of ILs partially oxidized and short-chain organic acids were identified as reaction by-products. Reaction pathways are proposed.

Characterization and process evaluation of Ni–Ti–Fe shape memory alloy macro-spheres directly fabricated via rotating electrode process

The present work demonstrates the manufacture of spherical millimeter sized macro-spheres of Ni–Ti–Fe shape memory alloy (SMA) for self-seizing bearing application through rotating electrode process (REP) through a set of optimized REP parameters and subsequent characterization. Image analysis has revealed that these macro-spheres follow a bimodal size distribution with larger mass fraction is about 1.5mm and that of smaller mass fraction has a mean value of 0.4mm. Differential scanning calorimetry (DSC) and compressive load–deformation behaviour confirm that both chemical and mechanical properties are consistent before and after REP. Microstructural characterization was carried out using high-resolution scanning electron microscope with electron backscattered diffraction (EBSD) technique. Process evaluation shows that there spheres form through the liquid ligament breakdown mechanism.

Initiation stage of nanosecond breakdown in liquid

In this paper, based on a theoretical model (Shneider and Pekker 2013 Phys. Rev. E 87 043004), it has been shown experimentally that the initial stage of development of a nanosecond breakdown in liquids is associated with the appearance of discontinuities in the liquid (cavitation) under the influence of electrostriction forces. Comparison of experimentally measured area dimensions and its temporal development were found to be in a good agreement with the theoretical calculations. This work is a continuation of the experimental and theoretical works (Dobrynin et al 2013 J. Phys. D: Appl. Phys. 46 105201, Starikovskiy 2013 Plasma Sources Sci. Technol. 22 012001, Seepersad et al 2013 J. Phys. D: Appl. Phys. 46 162001, Marinov et al 2013 Plasma Sources Sci. Technol. 22 042001, Seepersad et al 2013 J. Phys. D: Appl. Phys. 46 3555201), initiated by the work in (Shneider et al 2012 IEEE Trans. Dielectr. Electr. Insul. 19 1597–82), in which the electrostriction mechanism of breakdown was proposed.

Note: Tesla based pulse generator for electrical breakdown study of liquid dielectrics

In the process of studying charge holding capability and delay time for breakdown in liquids under nanosecond (ns) time scales, a Tesla based pulse generator has been developed. Pulse generator is a combination of Tesla transformer, pulse forming line, a fast closing switch, and test chamber. Use of Tesla transformer over conventional Marx generators makes the pulse generator very compact, cost effective, and requires less maintenance. The system has been designed and developed to deliver maximum output voltage of 300 kV and rise time of the order of tens of nanoseconds. The paper deals with the system design parameters, breakdown test procedure, and various experimental results. To validate the pulse generator performance, experimental results have been compared with PSPICE simulation software and are in good agreement with simulation results.

Hydrocarbon level detection with nanosecond laser ablation

Nanosecond laser induced breakdown in liquid is used as a technique to detect hydrocarbon levels in water. A Q-switched Nd:YAG laser was focused to generate optical breakdown associated with shock wave generation. The shock wave was propagated at the speed of sound in the medium after travelling 1 μs outward from the center of optical breakdown. Different amplitudes of sound were traced with the aid of an ultrasonic probe. The optical properties of the hydrocarbon solution were quantified via fundamental refractive index measurement (the Snell law). A continuous mode diode pumped solid state laser with second harmonic generation was used as the illumination light source. A CCD video camera with Matrox version 4.2 software was utilized to analyze the recording image. Option line analysis was performed to analyze the intensity of optical breakdown at different input energies. Gray level analysis was also conducted on the scattering light after passing through the hydrocarbon solution at different concentrations. The hydrocarbon solution comprised impurities or particles that varied according to the concentration. The average of the gray level is assumed to present the size of the particle. Inherently, as the acoustic wave propagates outward, it transports the mass (particles or impurities) and impacts on the ultrasonic probe. As a result a higher concentration of hydrocarbons reveals a larger amplitude of sound waves. This phenomenon is identified as a finger print for hydrocarbon levels between 100 and 1000 ppm. The transient detection, without complicated sampling preparation and no hazardous chemical involvement, makes laser ablation a promising technique to detect in situ hydrocarbon levels in water.

Anisotropic breakdown of Fermi liquid quasiparticle excitations in overdoped La2−xSrxCuO4

High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La1.77Sr0.23CuO4 is highly anisotropic in momentum space. The quasiparticle scattering and residue behave differently along the Fermi surface and hence the Kadowaki-Wood's relation is not obeyed. This kind of Fermi liquid breakdown may apply to a wide range of strongly correlated metal systems where spin fluctuations are present.

Three-dimensional LBE simulations of a decay of liquid dielectrics with a solute gas into the system of gas–vapor channels under the action of strong electric fields

The three-dimensional simulations of an anisotropic decay of binary mixtures of a dielectric liquid with solute gas in a strong electric field are carried out. The Lattice Boltzmann Equation method (LBE) is exploited for computer simulations of the evolution of such systems with the newly arising interfaces between vapor and liquid phases. The parallel implementation of the LBE algorithm is realized on a large number of cores in the GPU. For the GPU programming, the CUDA technology is used.It is important that new regions of the low-density phase appear as thin quasi-cylindrical gas–vapor channels oriented along the electric field. The gas–vapor channels expand because of the diffusion of the solute gas from the mixture, evaporation of liquid into the channels and also due to the coalescence of channels with each other. The critical values of electric field necessary for such decay of a binary mixture are considerably lower than the critical electric field for pure dielectric liquids. Hence, if we take into account a solute gas, the electric fields for which the anisotropic mechanism of streamer channels generation and growth is operated, become considerably lower.Thus, at a breakdown of dielectric liquids in a strong electric field, the anisotropic instability is possibly the key mechanism of the generation of a gas phase, inception of conducting streamer structures, their fast growth in the form of thin filamentary channels, as well as branching of streamer structures during propagation.

Specific features of the acoustic emission upon the optical breakdown in liquid induced by the radiation of Nd:YAG laser

The acoustic emission from the zone of the optical breakdown in liquid is experimentally studied. The spectral characteristics and energy of the acoustic wave that is generated in liquid due to expansion of the plasma formation initiated by the optical breakdown at a wavelength of 532 nm are analyzed. Two spectral peaks that characterize the acoustic emission and the low-frequency shift of the low-frequency peak owing to an increase in the laser pulse energy are demonstrated. In general, the linear dependence of the acoustic pressure on the laser pulse energy is observed. The acoustic data can be used to reconstruct function R(t) that is in agreement with dependences R(t) resulting from the optical data. This circumstance is important for the study of breakdown in opaque media.

Cavitation in the vicinity of the high-voltage electrode as a key step of nanosecond breakdown in liquids

Fast shadowgraphy of nanosecond discharge in liquids with different dielectric permittivity, namely in water, ethanol and n-pentane, has been performed. Formation of a gas cavity at a nanosecond time scale was observed as a pre-breakdown phenomenon at amplitudes of the high-voltage pulse close to the breakdown threshold. This phenomenon is considered as a possible key step of high-voltage breakdown in polar liquids.

Enhancements in the lightning impulse breakdown characteristics of natural ester dielectric liquids

In this letter, we experimentally demonstrate that the lightning impulse breakdown characteristics of natural ester liquids can be significantly enhanced by dispersing molecular additives possessing lower values of both the ionization potential and 1st excitation energies as compared to the base liquid. One such additive contributed to an increase in the breakdown and acceleration voltage of the base liquid by 32% and 90%, respectively. Apart from the expected influence of the low ionization potential of the additives, results also indicate a positive effect of lower value of 1st excitation energy.

Electric Breakdown in Liquids: Faster Ignition Using Less Energy

The formation of vapor layers around an electrode immersed in a conducting liquid prior to generation of a plasma discharge is studied using numerical simulations. This study quantifies and explains the effects of the electrode geometry and applied voltage pulses, as well as the electrical and thermal properties of the liquids on the temporal dynamics of the pre-breakdown conditions in the vapor layer. This model agrees well with experimental data, in particular, the time needed to reach the electrical breakdown threshold. Because the time needed for discharge ignition can be accurately predicted from the model, the parameters such as the pulse shape, voltage, and electrode configuration can be optimized under different liquid conditions, which facilitates a faster and more energy-efficient plasma generation.

Breakdown and withstand strengths of ester transformer liquids in a quasi-uniform field under impulse voltages

This paper presents breakdown and withstand strengths of a synthetic ester, a natural ester and a mineral oil in a quasi-uniform sphere-to-sphere electric field under impulse voltages. The influences of impulse waveforms (lightning and switching), voltage polarities (positive and negative) and testing methods (rising-voltage, up-and-down and multiple-level) on the breakdown behaviour were investigated. The results indicated that breakdown voltages of both the ester liquids are comparable with those of the mineral oil under various test conditions. The withstand voltage at 1% breakdown probability was also obtained based on Weibull distribution fitting on the cumulative probability plot, which was built using the ~1000 impulse shots. It was found that the withstand voltages at 1% breakdown probability of the ester liquids are almost the same as that of the mineral oil. The results in this paper were further compared with other available test data in the literature. By comparing the performances between the ester liquids and the mineral oil, recommendations for insulation design were given to aid the application of ester liquids in high voltage large power transformers.

Bubble Motion in Transformer Oil under Non-Uniform Electric Fields

We investigate the bubble motion in transformer oil under a typical non-uniform electric field. Based on the principle of virtual work, an analytical equation is derived for the electric force putting on the bubbles. The bubble's visualized motion graphics are then obtained by using the finite-element method, which utilizes Comsol Multiphysics software. The effects of bubble radius and release position on the motion trajectory are studied. It is found that, under the non-uniform electric field, the bubbles have a long residence time between the electrodes. This accordingly can lead to a higher probability of forming a bubble-bridge along the bubble motion trajectory. The bubbles with a particular radius or release position are easier to form bubble-bridge between the electrodes. They can increase the chance of liquid breakdown.

Streamer-Like Electrical Discharges in Water: Part I. Fundamental Mechanisms

Electrical discharge plasma formed in liquid water is under intensive investigation for many possible applications in biomedical, environmental and chemical engineering as well as for general scientific issues in plasma chemistry and other engineering applications. The subject of pulsed breakdown of water has additionally begun to assume importance due to growing interest in decontamination, purification of water containing chemical impurities and industrial sludge, and also in the emerging area of bio-electrics. This review paper focuses on the plasma physics (Part I) and chemistry of electrical discharges in liquid water and the chemical effects of plasmas on the degradation of organic molecules (Part II). This part discusses dielectric liquid breakdown and its mechanisms, streamer propagation and the effect of electrode polarity on streamer dynamics.

Dielectric liquids [Editorial

Summary form only given. The present issue of the IEEE Transactions on Dielectric and Electrical Insulation is principally devoted to papers dealing with dielectric liquids. The subject is of great importance in the electrical insulation area, because insulating liquids are extensively used in the area of liquid filled power apparatus such as transformers and capacitors. Over the past decade there have been considerable efforts to improve the electrical and thermal properties of existing liquid dielectrics as well as to introduce new dielectric liquids with better insulating properties as well as develop improved testing methods for their evaluation. The opening paper in this issue is of a tutorial nature and addresses the development of standards for partial discharge (PD) measurements on dielectric liquids. Such information should be useful for standards oriented organizations that are currently engaged in the preparation of testing methods for PD detection and measurement on insulating liquids. In many respects partial discharges in insulating liquids must be viewed as a precursor stage of breakdown streamers in these liquids. The latter have received much attention in the past and continue to sustain interest in studies dealing with the mechanism of breakdown in dielectric liquids. Over the past decade there has been considerable effort directed towards researching the effects of copper sulfide in mineral oils. The problem gained attention when a significant number of transformers and reactors, that had undergone failure, were found to contain corrosive sulphur in their mineral oil. Subsequent research and testing of these oils indicated that they contained corrosive sulphur, which after its reaction with copper conductors gave rise to the formation of semi-conducting copper sulfide, resulting in overheating and the further formation of hot spots. Thereafter the work volume on this subject diminished considerably, as is evident in the present issue. Finally, mention should be made of the natural and synthetic esters, which are now being used in transformers. They have received considerable attention, because both types of esters are biodegradable and thus benign to the environment. Their electrical properties are found to be comparable to those of mineral oils. Both types of liquids are principally used primarily in distribution transformers. Evidently, the number of published papers in the Special Issue on Dielectric Liquids reflects the activity in this field of endeavor. It is surprising that in the past issues on Dielectric Liquids, the number of papers published tended to hover in the vicinity of 20, as it also does in the present issue. The tentative date of the next Special Issue on Dielectric Liquids is planned four years hence.

Electrical breakdown and volt-ampere characteristics in water vapor in microgaps

Although the physics for breakdown initiation in gases in generally well understood, the process of breakdown initiation in liquids is much less clear. A large number of experimental data on the breakdown in water revealed that the breakdown voltage in water is of the same magnitude as in the case of gases. This means that the breakdown in liquids can occur not at the extremely high electric fields required by the Paschen curve, but at those that only slightly exceed the breakdown electric fields in atmospheric-pressure molecular gases. This letter contains the results of experimental study on electrical breakdown characteristics in water vapor in microgaps between two parallel electrodes. Measurements were performed for the pressures of 24.15 torr, 20.85 torr and 14.55 torr with the gap size ranging from 40 μm to 900 μm. Considering that the atmospheric pressure sources operate in ambient air which unavoidably contains water vapor it is of great importance to investigate the basic processes and properties of discharges in water vapor.

Research on Dynamic of Wall Loading of the Discharge Chamber under Electro-Hydraulic Breakdown in Liquid

The underwater hydraulic discharge chamber is basic tools and method of research for many ocean issues. In this paper analyzed physical process and mechanical state of hydraulic discharge chamber, into account plasma, streamer and cavitation. The results of simulation and calculation compared with empirical data. On based the results analyzed the factors of effect of the underwater state and put forward improvement conditions of structural design for discharge chamber.

Effect of carbon allotropes on the structure and hydrogen sorption during reactive ball-milling of Mg–C powder mixtures

Hydrogen sorption by magnesium–carbon composites during reactive high-energy ball milling under hydrogen was investigated. Mg-based composites absorb up to 5 wt.% of hydrogen during milling. It was found that reactive ball milling of magnesium with the different carbon allotropes (graphite, ultrafine diamonds, carbon nanotubes and amorphous carbon powder produced by electric breakdown of organic liquids) leads to a significant increase of hydrogen sorption rate. Morphology and phase composition of the synthesized Mg–C composites have been studied by TEM and XRD. Special attention was focused on the study of structural changes in the carbon additives. The hydrogenation rate of magnesium-carbon composites during reactive ball milling is strongly affected by the nature of carbon additives and their dispersion in an initial state.