Breakdown Mechanism Research Articles

Cysteine Nucleophiles in Glycosidase Catalysis: Application of a Covalent β-l-Arabinofuranosidase Inhibitor.

The recent discovery of zinc-dependent retaining glycoside hydrolases (GHs), with active sites built around a Zn(Cys)3 (Glu) coordination complex, has presented unresolved mechanistic questions. In particular, the proposed mechanism, depending on a Zn-coordinated cysteine nucleophile and passing through a thioglycosyl enzyme intermediate, remains controversial. This is primarily due to the expected stability of the intermediate C-S bond. To facilitate the study of this atypical mechanism, we report the synthesis of a cyclophellitol-derived β-l-arabinofuranosidase inhibitor, hypothesised to react with the catalytic nucleophile to form a non-hydrolysable adduct analogous to the mechanistic covalent intermediate. This β-l-arabinofuranosidase inhibitor reacts exclusively with the proposed cysteine thiol catalytic nucleophiles of representatives of GH families 127 and 146. X-ray crystal structures determined for the resulting adducts enable MD and QM/MM simulations, which provide insight into the mechanism of thioglycosyl enzyme intermediate breakdown. Leveraging the unique chemistry of cyclophellitol derivatives, the structures and simulations presented here support the assignment of a zinc-coordinated cysteine as the catalytic nucleophile and illuminate the finely tuned energetics of this remarkable metalloenzyme clan.

Cysteine Nucleophiles in Glycosidase Catalysis: Application of a Covalent β‐l‐Arabinofuranosidase Inhibitor

AbstractThe recent discovery of zinc‐dependent retaining glycoside hydrolases (GHs), with active sites built around a Zn(Cys)3(Glu) coordination complex, has presented unresolved mechanistic questions. In particular, the proposed mechanism, depending on a Zn‐coordinated cysteine nucleophile and passing through a thioglycosyl enzyme intermediate, remains controversial. This is primarily due to the expected stability of the intermediate C−S bond. To facilitate the study of this atypical mechanism, we report the synthesis of a cyclophellitol‐derived β‐l‐arabinofuranosidase inhibitor, hypothesised to react with the catalytic nucleophile to form a non‐hydrolysable adduct analogous to the mechanistic covalent intermediate. This β‐l‐arabinofuranosidase inhibitor reacts exclusively with the proposed cysteine thiol catalytic nucleophiles of representatives of GH families 127 and 146. X‐ray crystal structures determined for the resulting adducts enable MD and QM/MM simulations, which provide insight into the mechanism of thioglycosyl enzyme intermediate breakdown. Leveraging the unique chemistry of cyclophellitol derivatives, the structures and simulations presented here support the assignment of a zinc‐coordinated cysteine as the catalytic nucleophile and illuminate the finely tuned energetics of this remarkable metalloenzyme clan.

Mechanism of radio frequency breakdown on metal surfaces in relativistic backward wave oscillator

Radio frequency (RF) breakdown can result in pulse shortening and severely limits the performance of the relativistic backward wave oscillator (RBWO). In this study, segmented slow-wave structure (SWS) rings, which can be observed directly using a scanning electron microscope without any post-processing, are used to study the breakdown in a RBWO. After undergoing high-power microwave experiments, the working surface of the SWS rings was found to exhibit corrugated morphologies at the sub-mm to mm scale, and a large number of craters in the vicinity of the corrugated morphologies and a mass of scattered droplets were visible on the surface. It was deduced that the unusual morphologies resulted from electron bombardment of the working surface. The observed structures and droplets provide new evidence that helps to explain the mechanism for RF breakdown in the RBWO.

Electrical Breakdown Mechanism of Transformer Oil with Water Impurity: Molecular Dynamics Simulations and First-Principles Calculations

Water impurity is the essential factor of reducing the insulation performance of transformer oil, which directly determines the operating safety and life of a transformer. Molecular dynamics simulations and first-principles electronic-structure calculations are employed to study the diffusion behavior of water molecules and the electrical breakdown mechanism of transformer oil containing water impurities. The molecular dynamics of an oil-water micro-system model demonstrates that the increase of aging acid concentration will exponentially expedite thermal diffusion of water molecules. Density of states (DOS) for a local region model of transformer oil containing water molecules indicates that water molecules can introduce unoccupied localized electron-states with energy levels close to the conduction band minimum of transformer oil, which makes water molecules capable of capturing electrons and transforming them into water ions during thermal diffusion. Subsequently, under a high electric field, water ions collide and impact on oil molecules to break the molecular chain of transformer oil, engendering carbonized components that introduce a conduction electronic-band in the band-gap of oil molecules as a manifestation of forming a conductive region in transformer oil. The conduction channel composed of carbonized components will be eventually formed, connecting two electrodes, with the carbonized components developing rapidly under the impact of water ions, based on which a large number of electron carriers will be produced similar to “avalanche” discharge, leading to an electrical breakdown of transformer oil insulation. The water impurity in oil, as the key factor for forming the carbonized conducting channel, initiates the electric breakdown process of transformer oil, which is dominated by thermal diffusion of water molecules. The increase of aging acid concentration will significantly promote the thermal diffusion of water impurities and the formation of an initial conducting channel, accounting for the degradation in dielectric strength of insulating oil containing water impurities after long-term operation of the transformer.

Study on the Mechanism of Microparticle-Initiated Breakdown in the Air Gap

The microparticle-initiated breakdown plays a key role in the failure of gas switches and has become an active area of investigation in gas discharge recently. The mechanism of the microparticle-initiated breakdown in a 2-mm air gap is studied by experiments and simulations in this article. The dynamics of the microparticle in the electric field is obtained, and four breakdown modes are acquired from the picture of the high-speed camera. The probabilities of breakdown for the 200-, 300-, and 500- $\mu \text{m}$ Cu microparticles under different voltages are presented, and the corresponding lowest breakdown voltages reduce to 65%, 56%, and 47% of the static-breakdown voltage. The relation between the breakdown voltage and position of the microparticle is calculated based on the Townsend theory. The simulated results agree well with the experimental results. The breakdown process and evolution of the plasma are also revealed with PIC simulations.

Vortex impingement onto an axisymmetric obstacle – subcritical bifurcation to vortex breakdown

Abstract

Non-twist invariant circles in conformally symplectic systems

Dissipative mechanical systems on the torus with a friction that is proportional to the velocity are modeled by conformally symplectic maps on the annulus, which are maps that transport the symplectic form into a multiple of itself (with a conformal factor smaller than 1). It is important to understand the structure and the dynamics on the attractors. It is well-known that, with the aid of parameters, and under suitable non-degeneracy conditions, one can obtain that there is an attractor that is an invariant torus whose internal dynamics is conjugate to a rotation. By analogy with symplectic dynamics, a natural question is establishing appropriate definitions for twist and non-twist invariant tori in conformally symplectic systems.The main goals of this paper are: (a) to establish proper definitions of twist and non-twist invariant tori in families of conformally symplectic systems; (b) to interpret these definitions in terms of dynamical properties; (c) to derive algorithms to compute twist and non-twist invariant tori; (d) to implement these algorithms in examples; (e) to explore the mechanisms of breakdown of twist and non-twist invariant tori. Hence, the last part of the paper is devoted to implementations of the algorithms, illustrating the definitions presented in this paper, and studying robustness properties of invariant tori.

Research progress of high-power waveguide window

High-power waveguide window is a critical component of high-power klystron and accelerator. The RF breakdown of the waveguide window always cause the failure of the high-power klystron. In this paper, the research progress of high-power waveguide window in the international vacuum electronics field is introduced. The research status, manufacturing requirement, and RF breakdown mechanism of the classical pill-box window are presented at first. Then the characters of some new type waveguide windows such as the tapered window, travelling wave in ceramic window, mixed-mode window, and over-mode window are summarized. Some breakdown suppression technologies like changing the material character of the window disk, changing the surface configuration of the window disk, using the positive dielectric angle, applying DC electric field or DC magnetic field, changing the waveform of transverse electric field are emphasized at the end.

Modification Effect of TiO<sub>2</sub> Hollow Nanoparticles on Insulating Properties of Transformer Oil

With the promoting of transmission voltage level and the continuous fluctuation of operating conditions by connecting renewable energy into the power system, more stringent requirements have been placed on the operation reliability of transformers and other large electrical equipment. Transformer oil, as the main liquid insulating medium in large power equipment, the modification of its insulating performance has always been research focus in dielectric field. Studies have proved that TiO₂ nanomaterials can not only enhance the AC and positive impulse breakdown voltage of transformer oil, but also significantly improve the aging resistance of transformer oil. However, the DC breakdown voltage of transformer oil has not been greatly increased yet, and sometime even weakened. In this paper, TiO₂ hollow nanoparticles were prepared by hard template method to modify transformer oil. AC and DC breakdown voltage and dielectric permittivity of transformer oil before and after modification were measured and analyzed. The results show that the AC breakdown voltage of the transformer oil can be increased by 32.5%. DC breakdown voltage was increased first and then reduced with the increasing of nanoparticle concentration, while no obvious improvemnet was observed. Based on breakdown mechanism and permittivity of transformer oil, we think the hollow structure of the nanoparticles can significantly enlarge the interface between particle and oil, which greatly promotes the capturing capability to fast electrons and inhibit their migration in the oil, so the AC breakdown voltage are significantly increased. Under DC stress, nanoparticles can capture electrons and meanwhile migrate along fixed direction, which increased electrical conductivity of oil and resulted in lowering of DC breakdown voltage with the increase of nanoparticle concentration.

Breakdown mechanisms of whey protein gels during dynamic in vitro gastric digestion.

Breakdown of solid foods during gastric digestion plays a major role in the release and absorption of nutrients in the gastrointestinal tract. The breakdown mechanisms of foods during gastric digestion may be influenced by composition, particle geometry, and the resulting moisture uptake and gastric emptying. The extent of breakdown may have implications on the pH, pepsin activity, and subsequent protein hydrolysis. This study aims to identify the influence of particle geometry on pH, buffering capacity, and breakdown mechanisms during in vitro dynamic gastric digestion. Whey protein gels made in different geometries (small, medium, and large cubes with side lengths of 3.1, 5.2, and 10.3 mm, respectively, and spheres with a diameter of 6.5 mm) were subjected to gastric digestion using the Human Gastric Simulator (HGS) over a 180 min period. Particle size in the bulk digesta showed the breakdown mechanism of spheres was primarily by erosion, whereas breakdown of cubes was by fragmentation at the beginning of digestion, followed by erosion. Moisture uptake and gastric emptying of dry matter were significantly influenced by digestion time, particle geometry, and their interaction (p < 0.001). Initial buffering capacity of the gels was highest in small cubes and lowest in large cubes, causing the pH to decrease faster in large cubes. There was a higher pepsin activity in the liquid phase of the digesta in large cubes compared to the rest of the treatments, which was hypothesized to be due to a diffusion limitation of pepsin, resulting in less diffusion into large cubes due to their lower total specific surface area. Further work is needed to develop quantitative connections between food initial properties, breakdown mechanisms, and their implications on pH, pepsin activity, and nutrient digestibility for future food design.

Mechanism of Femtosecond Laser-Induced Breakdown Mediated by Al/SiO2 Core/Shell Nanostructures

Mechanism of Femtosecond Laser-Induced Breakdown Mediated by Al/SiO2 Core/Shell Nanostructures

A Novel Current-Source-Based Gate Driver With Active Voltage Balancing Control for Series-Connected GaN HEMTs

The voltage rating of the commercial Gallium Nitride (GaN) power devices are limited to 600/650 V due to the lateral structure. Stacking the low-voltage rating devices is a straightforward approach to block higher dc link voltage. However, the unbalanced voltage sharing can occur due to the discrepancies in the gate driving loops, the device parameter tolerance and the device-to-ground displacement currents for the series-connected devices in the stack. The voltage imbalance may cause the over-voltage breakdown, in particular for GaN devices, which do not have the avalanche breakdown mechanism. In this article, a novel controllable current source gate driver is proposed, which addresses the voltage imbalance issue of series-connected GaN HEMTs for both hard switching and soft switching scenarios. The proposed current source gate driver controls the device switching timing and the dv/dt with fine accuracy by directly regulating the device gate current. Without the employment of the lossy snubber circuit or the external Miller capacitor, the switching energy and the switching speed are almost not compromised for each individual device. Meanwhile, the current mirror circuits are utilized as the discontinuous pulsed current sources, which produce negligible additional gate driving loss. A series-connected GaN-based multiple pulse tester is built to validate the proposed current source gate driver and the voltage balancing strategies. It is demonstrated that the drain-to-source voltage difference of the series-connected GaN devices is below 10% for different load current and different switching speed (dv/dt) conditions. Moreover, it is found that the series-connected GaN solution can save 33.6% switching energy compared with the benchmark SiC solution under the same operating condition.

Electrical discharges in ferrofluids based on mineral oil and novel gas-to-liquid oil

Ferrofluids consisting of stabilized iron oxide nanoparticles and insulating oils have emerged as a promising substitute for liquid dielectrics in electrical engineering applications. Recent enhancements of electrical insulating liquids rely on preparation of ferrofluids on emerging insulating oils available on the market. The present work reports on a comparative experimental study of insulating properties of a conventional mineral oil (MO)-based ferrofluid and a ferrofluid based on novel insulating oil produced by a gas-to-liquid (GTL) technology. The ferrofluids prepared on the two oils are subjected to rigorous experimental investigation of dielectric breakdown and partial discharges. The experiments are conducted on 4 MO-and 4 GTL-based ferrofluids with equal concentrations of magnetite nanoparticles. Measurements of partial discharges according to IEC 60270 are complemented with high frame–rate photography with positive streamer analysis. Based on the statistical analysis, it is found that MO-based ferrofluids exhibit superior breakdown performance to GTL-based ferrofluids, even though the pure GTL oil exhibits slightly higher mean breakdown voltage than the pure MO. A deeper analysis revealed a significantly greater number of partial discharges in GTL ferrofluids. The positive streamer size is lowered with increasing nanoparticle concentration only in MO-based nanofluids. Differences in the physical properties of the two oils, such as density, viscosity and permittivity, are considered in the interpretation of the different dielectric performance of the two ferrofluids. The experimental comparison leads to better understanding of the breakdown mechanism, and lay the foundations for proper selection of physical properties of a base oil for high performance insulating ferrofluids.

Temperature-Dependent Field Emission and Breakdown Measurements Using a Pulsed High-Voltage Cryosystem

Field emission and vacuum breakdown limit performance in many classes of electronic devices, including rf systems in particle accelerators. Although there are general explanations of both processes, a large correction factor is systematically needed to explain observations, and experimental evidence to establish the breakdown mechanism is limited. This study elucidates both issues, using a high-voltage electrode system that can operate down to cryogenic temperatures. Measurements of temperature-dependent field emission and breakdown with this system reveal remarkable effects that yield insight into both processes, which in turn will enable further development of high-field technology.

An experimental study of the effect adding locally manufactured shear studs on raising the punching of the flat slabs

The slabs are one of the important structural elements in the buildings and constitute approximately 60% of the concrete size used. The flat slabs are an ideal solution that guarantees the speed and ease of implementation, especially in multi-storey buildings. The phenomenon of punching is a fundamental defect in these slabs, and the Syrian code addressed it to treat it in traditional ways such as making crowns for columns or falls of beams, which creates architectural and technological defects. The research includes conducting laboratory experiments on reinforced concrete slabs and analyzing the results with the aim of strengthening the flat slabs punching within the same thickness of the approved slab by linking the lower and upper reinforcement steel which provides a working principle similar to the trusses. The strength was made using shear studs (distribution case, matrix or diagonal), which in turn raised the efficiency of flat concrete slabs on the punching in terms of durability, deformations, and the breakdown mechanism in this case. The importance of the research lies in the use of locally manufactured elements within the workshop without the need for ready-made shear studs, which reflects positively on the cost and time of implementation.

Theoretical and numerical study on breakdown mechanism of nitrogen spark switch

Compared with the two-electrode gas spark switch, the three-electrode gas spark switch has high controllability, low working voltage and small jitter, so the three-electrode gas spark switch is widely used in pulse power technology. The discharge of gas spark switch is high pressure gas discharge, which is characterized by high electron collision frequency (10&lt;sup&gt;12&lt;/sup&gt; Hz), small mean free path (10&lt;sup&gt;–6&lt;/sup&gt; m), short breakdown time (10&lt;sup&gt;–9&lt;/sup&gt; s), and complex physical process (including the secondary electron emission, the generation of seed electrons, the space charge effect and various collision processes between electrons and nitrogen molecules, etc). At present, it is difficult to quantitatively describe the breakdown process of the three-electrode gas switch, and the detailed theoretical research is lacking. Therefore, the breakdown mechanism of atmospheric pressure nitrogen spark switch, including two-electrode and three-electrode, is studied theoretically and numerically in this paper. The purpose of this study is to compare the simulation results of the two different gas spark switches, and obtain the characteristics of stream breakdown in different gas spark switches. Firstly, the numerical simulation and theoretical analysis of two-electrode gas spark switch are carried out. According to theoretical and numerical calculation, it can be found that for the plate-plate two-electrode switch, the stream breakdown cannot be generated under low voltage (less than 6.3 kV), while under high voltage (more than 6.3 kV), first the anode-directed streamer is formed, and then the cathode-directed streamer is created. In addition, the simulation results show that the plasma generated by the trigger can effectively reduce the breakdown voltage. Finally, the three-electrode gas spark switch is studied theoretically and numerically. It can be seen that in the breakdown process of the three-electrode gas spark switch, the breakdown first occurs between the trigger and the insulator, and then this plasma channel expands to the anode and cathode, finally forming the arc channel between the anode and the cathode. Under the calculation conditions in this paper, if the cathode-trigger and the anode-trigger are required to break down simultaneously, the applied voltage between the cathode-trigger should be greater than 1.18 kV, while the applied voltage between the anode-trigger should be greater than 3 KV. When the field emission of the trigger is considered, the breakdown threshold can be significantly reduced.

BREAKDOWN VOLTAGE OF MICRON RANGE AIR INCLUSIONS IN CAPACITOR PAPER

Purpose. To substantiate the breakdown mechanism of capacitor paper on the basis of numerical-field models with segmented cross-sections of cylindrical volumes of air and water, and also use the proposed models to determine the breakdown strength of air in micron-sized gaps under normal conditions. Methodology. The model bases on a finite element solution to an electrostatic problem in a volume of capacitor paper consisting of cellulose and pores with air and water. First, the possible scenarios for the growth of breakdown in capacitor paper are analyzed and to the conclusion is made, that complete breakdown developed from a partial breakdown in the air cavity. A brand of capacitor paper is chosen in such a way that when its thickness changed, the breakdown strength of the electric field changed over a wide range. Then, for the paper with the lowest average electric field intensity the possibility of explaining the complete breakdown by the breakdown of air segments on the basis of the Paschen dependence is checked. Further points of the obtained dependence by constructing models of papers of the same brand and a different thickness under the assumption of the similarity of electrostatic fields are determined. As such a criterion, the constancy of the equivalent effective permittivity are taken. Results. The dependence of the breakdown strength of the air in the range of 1.36...5.54 μm under normal conditions is determined. The obtained relationship is between the Peschot and Taev curves. Originality. For the first time, the possibility of indirectly estimation the breakdown strength of an insulating material using an electrostatic field model is indicated. Practical value. The proposed method for the numerical calculation of the breakdown voltage of air inclusions in the presence of water inclusions in the thickness of solid insulation can be applied to other types of solid thin-layer insulation.

A Review on Synthetic Ester Liquids for Transformer Applications

Synthetic esters have become more and more popular in last few decades, explaining the increasing number of units filled with this liquid year by year. They have been investigated under different aspects, both from the fundamental point of view and breakdown mechanisms, well as from the application point of view. However, their use in high voltage equipment is always a challenge and deeper knowledge of the various aspects that can be encountered in their exploitation is needed. The intent of this review paper is to present the recent research progress on synthetic ester liquid in relation to the selected issues, most important for ester development in the authors’ opinion. The described issues are the breakdown performance of synthetic esters, lightning impulse strength and pre-breakdown phenomena of synthetic esters, synthetic esters-based nanofluids, combined paper-synthetic ester based insulating systems, application of synthetic ester for retro-filling and drying of mineral oil-immersed transformers, DGA(dissolved gas analysis)-based diagnosis of synthetic esters filled transformers as well as static electrification of synthetic esters. The different sections are based both on the data available in the literature, but above all on the authors’ own experience from their research work on synthetic ester liquids for electrical application purposes.

Hydrogen Bonds Sites of Amylose or Amylopectin from Starch at the ONION Level (B3LYP/6-311++G [d, p]: AM1)

The research aims to help reduce the enormous post-harvest losses of rice or bananas in Cote d’Ivoire. It focuses on the breakdown mechanism of their starch. This process contributes to their decay. This study wants to elucidate it; to do this, it analyzes the tetra or penta saccharide reactions with the water or dioxide carbon. It calculates the geometric, energetic and spectroscopic parameters at the ONIOM level (B3LYP/6-311++G [d, p]: AM1). These quantities allow establishing that oxygen represents the privileged hydrogen bond (HB) site for amylose with four or five D-glucose building blocks. They suggest and are respectively those of amylopectin when the latter consists of four and five D-glucose synthons. They prove that amylose deteriorates before it; the degradation of starch begins with its alteration into disaccharides. It continues with that of amylopectin into tri or tetra-saccharides with four or five building blocks.

Transitions between electron emission and gas breakdown mechanisms across length and pressure scales

This Perspective outlines theoretical, simulation, and experimental studies linking electron emission and gas breakdown. Many studies have investigated field emission-driven microscale gas breakdown, including recent reviews [Garner et al., IEEE Trans. Plasma Sci. 48, 808–824 (2020); Fu et al., Plasma Res. Express 2, 013001 (2020)]. This Perspective focuses on generalizing field emission-driven microscale gas breakdown to consider the contribution of other forms of electron emission, specifically thermionic and space-charge limited. Recent theoretical studies have unified thermionic, field, and space-charge limited emission with and without collisions to derive “nexuses” where the individual solutions match, indicating transitions in the mechanisms. Reducing device size to nanoscale at atmospheric pressure leads to a transition from field emission to space-charge limited emission for nitrogen at ∼250 nm. This Perspective summarizes the derivation of these nexuses and future extensions. We next describe simulation and theoretical studies for field emission-driven microscale gas breakdown and highlight how the nexus theory may be integrated to account for temperature, space-charge, and pulse parameters. Finally, we summarize the development of optical techniques to assess microscale gas breakdown and recent nanoscale experiments at atmospheric pressure that suggest that space-charge may begin to contribute to field emission prior to gas breakdown. We highlight the combination of theory, simulation, and experiment to link electron emission and gas breakdown mechanisms across length, pressure, and temperature scales for applications that include vacuum electronics, pulsed power, and medicine.