Breakdown Region Research Articles

Mitigating the Corrosion of Anodized 304 and 316 Stainless Steel with GOx/PEDOT Coating for Bioelectrochemical Saline Water Applications

Steels are the most used alloys worldwide as they have superior mechanical properties, such as ductility and elasticity. Stainless steel (SS) is used far and wide from infrastructure, households, vehicles, surgical equipment, and medical implants, to electrochemical applications such as supercapacitors, fuel cells, lithium-ion batteries, aqueous rechargeable batteries, and bioelectrochemical systems (BES)1 such as microbial fuel cells (MFCs)2 and microbial electrolysis cells (MECs)3. Even though oxidized stainless steel is a very effective electrode material for BES, it has a high risk of corrosion due to the removal of the protective Cr-based passive oxide layer4.Hereby, this research aims to investigate the corrosion behavior of anodized stainless steel (SS) 304 and 316 mesh in saline water for potential use in BES with high salinity and try to mitigate that corrosion behavior using biocompatible materials. The tested electrodes include SS304 and SS316 mesh before and after anodization (anodized SS, AN-SS), in comparison to AN-SS coated with double layers of graphene oxide (GOx) and poly(3,4-ethylenedioxythiophene) (PEDOT). SS 304 and 316 were anodized using selective leaching protocol to enhance the removal of the bioincompatible and insulating Cr-based layer. The removal of that layer makes SS more susceptible to dissolution and hence further protection is needed. Both GOx and PEDOT were selected to enhance the surface conductivity, biocompatibility, and corrosion resistance. GOx was synthesized using the electro-exfoliation of a graphite sheet at a constant voltage of 10.0 V in terephthalic acid + NaOH solution, followed by centrifuging at 3000 rpm to remove the large particles.5 After several cycles of washing, the freeze-dried GOx was mixed with Nafion for ink preparation, which was used to coat the An-SS using the spray coating method. The coated electrode was further covered with a PEDOT layer, using the chronpotentiometry electropolymerization method (2 mA/cm2, for 10, 30, or 60 min). Several characterization techniques such as SEM, EDX, XPS, FT-IT, Raman spectroscopy, XRD, and TEM, were used to characterize the physical and chemical structure of the coating materials and the the fabricated electrodes. The corrosion behavior was evaluated using potentiodynamic and potentiostatic methods. Based on the anodic polarization results, the corrosion, passivation, and breakdown (due to dissolution) regions were identified. In this presentation, the corrosion behavior (Ecorr, Icorr, breakdown potential, etc) of the studied electrodes will be correlated with their chemical and physical structures. References K.-B. Pu, J.-R. Bai, Q.-Y. Chen, and Y.-H. Wang, in Novel Catalyst Materials for Bioelectrochemical Systems: Fundamentals and Applications, ACS Symposium Series., vol. 1342, p. 165–184, American Chemical Society (2020) https://doi.org/10.1021/bk-2020-1342.ch008.A. A. Abbas, H. H. Farrag, E. El-Sawy, and N. K. Allam, Journal of Cleaner Production, 285, 124816 (2021).Y. Zhang, M. D. Merrill, and B. E. Logan, International Journal of Hydrogen Energy, 35, 12020–12028 (2010).P. Ledezma, B. C. Donose, S. Freguia, and J. Keller, Electrochimica Acta, 158, 356–360 (2015).H. S. Wang et al., Green Chem., 20, 1306–1315 (2018).

Structuring of the Surface of Thin Carbon Films During Activation by Microsecond Current Pulses

The influence of current activation by electric pulse breakdown on changes in surface morphology and emission characteristics of a field emission cathode made on the basis of carbon films obtained by deposition in a microwave gas discharge plasma was studied. Current activation of these films was carried out by applying voltage pulses of microsecond duration until an electrical breakdown occurred. It is shown that during activation, the morphology of the film surface in the breakdown region changes with the formation of a micro-sized emitting structure, which significantly improves the field emission characteristics of cathodes based on carbon films.

The design of a linear voltage divider based on metal oxide arresters.

In recent years, linearization technology for nonlinear devices has become a hot topic in many fields. In this study, a linear voltage divider based on metal oxide arresters was designed by combining linearization technology and electrical measurement technology to solve the objective problems of online voltage monitoring. These problems include high difficulty in equipment installation, low measurement accuracy, and poor economic benefits. Based on a summary of linearization theory, the sufficient and necessary conditions for the linearization of the voltage divider were deduced in detail. The relevant circuit simulations were conducted, along with voltage divider experiments under power frequency AC voltage, operating overvoltage, and lightning overvoltage. The results revealed that the voltage divider was able to realize linearized measurements and meet the relevant standards of online voltage monitoring. The measurement errors were concentrated in the transition region between the pre-breakdown region (small current region) and the breakdown region (nonlinear region) in the volt-ampere characteristic curve. The main influencing factor of errors was the consistency of the nonlinear characteristics of the high- and low-voltage arms of the voltage divider. The voltage divider designed in this study can be applied in many scenarios, such as power plants, substations, high-voltage electrical equipment manufacturing plants, and high-voltage laboratories.

Adhesion, roughness, wettability, and dielectric strength of elastomers liquid blends for high-density fiberboard wood adhesive

In the present study, the physical characteristics of elastomer (EL) blend with natural polymers such as polyvinyl alcohol (PVA), Dexrin (D), Arabic gum (AG), and corn starch (CS) based on high-density fiberboard wood adhesives were investigated. The EL blends were prepared by dissolving AG, D, PVA, and CS in deionized water at 70 °C for 1 h under magnetic stirring continuously until the solution was clear, and blends were made with a weight of 60/40 (w/w); then were cast into a mold with a 20 cm diameter and left at room temperature for 24 h to ensure complete water removal and drying of the samples. The prepared EL and EL blend structures, adhesion strengths, roughness, wettings, and dielectric strengths, were investigated. The modified EL blend reveals a 144 MPa for pull-off strength and 770.8 N for shear strength for high-density fiberboard (HDF) wood as a substrate to the EL/AG, respectively. The surface roughness and contact angle of the EL/PVA mixture were found to be high, measuring 4.57 µm for roughness and shows the water contact angles for the samples. An increase in the contact angle of the El/AG blend where reached to(83.94˚) was observed due to the decrease in the –COOH and OH groups present in the backbone of the arabic gum. The greatest dielectric strength for EL/AG was reported to be 18.62 kV/mm at 0.5 kV/s and 22.77 kV/mm at 5 kV/s.and optical microscoby image for break down region was shown the carbonization in the break down point as aresult of carashing polymers chains, also micro cracks occuring forspecimens and this cracks extends directly from the breakdown region

Feasibility of macroscopic parameters for NS to DSMC solver switching in micronozzle simulations

Enhancing the design and performance of micronozzles could lead to novel applications and advancements in propulsion systems, making the exploration of micronozzles crucial for the future. This paper critically examines the feasibility of utilizing macroscopic property-based Kn as indicator for defining the breakdown region during the transition from the NS solver to the DSMC solver in micronozzle simulations. The aim is to specify a parameter that can be calculated from both NS and DSMC simulations, making it suitable for implementation in hybrid simulations that dynamically switch between the two solvers. The results show that the density-based Kn accurately represents the continuum breakdown, and it exhibits an earlier breakdown compared to pressure and temperature-based Kn values. The study also analyzes the rarefaction effects and introduces the rarefaction parameter (RP), quantifying the increase in Kn for a unit change in the non-dimensionalized distance. The findings demonstrate that at very low exit pressures, the rarefaction effects increase rapidly as the flow moves towards the nozzle exit, leading to a transition from the continuum to the rarefied regime. The hybrid NS-DSMC simulations show good agreement with experimental data, validating the proposed approach. Additionally, the research examines the effect of back pressure on the RP and identifies the transition regime based on the slope of the RP curve. Therefore, the manuscript provides detailed insights into novel elements, such as the quantification of rarefaction within the nozzle using the RP, the classification of the nozzle into different regimes (continuum, slip, and transition), the definition of an easily obtainable parameter for switching between NS and DSMC methods, and an examination of the contributions of the shear stress term and heat addition term to non-equilibrium conditions.

(Invited) Recent Progress in Medium-Voltage Vertical GaN Power Devices

Vertical gallium nitride (GaN) power devices continue to garner interest in multiple power conversion applications requiring a medium-voltage (1.2 – 20 kV) capability. Currently, silicon carbide (SiC) is addressing this voltage range, however, with a comparable critical electric field and superior mobility, GaN is expected to offer advantages in applications where fast switching and avalanche breakdown response times are desired. While uses in electric vehicles, solid-state transformers, and renewable energy conversion are being actively explored, the potential of a vertical GaN device for electric grid protection in the form of an electromagnetic pulse arrestor is a unique proposition that requires very fast transient capabilities (<1 µs pulse widths with rise-times on the order of 10 ns). However, vertical GaN devices are significantly less mature than present SiC offerings. Specifically, low-doped, thick epitaxial growth of GaN via metal-organic chemical vapor deposition (MOCVD) still presents many challenges, and advancements in processing, manufacturability, and failure analysis are needed.In this work, we describe our efforts to address the above issues and advance the state-of-the-art in vertical GaN PN diode development. We have successfully demonstrated an MOCVD-grown, 50 µm thick, low-doped (<1015 cm-3) drift region on a GaN substrate that was processed into relatively large-area (1 mm2) PN diodes capable of achieving a 6.7 kV breakdown. Temperature-dependent breakdown was observed, consistent with the avalanche process. The devices consisted of a 4-zone step-etched junction termination extension (JTE), where the breakdown region was visualized via electroluminescence (EL) imaging. Ongoing work aims to scale the current capability of the medium-voltage diodes through a parallel interconnect design that negates defective or poor performing diodes. Further investigation of edge termination structures was explored using a bevel approach, where we studied the relationship between the bevel angle and p-doping. It was found that a very shallow angle of only 5° accompanied by a 500 nm p-region consisting of 3×1017 cm-3 Mg concentration resulted in a consistent 1.2 kV breakdown for an 8 µm thick, 1.6×1016 cm-3 doped drift region. EL imaging confirmed uniform breakdown, and temperature dependence was demonstrated. The bevel approach was then implemented on a diode structure with a 20 µm thick drift region capable of 3.3 kV breakdown, where an unclamped inductive switching (UIS) test was performed to evaluate the impact of a field plate design on avalanche uniformity and ruggedness.A parallel effort to establish a foundry process for vertical GaN devices has been underway. Initially, this focus was on comprehensive studies of GaN wafer metrology using capacitance-voltage (C-V) mapping, optical profilometry, and x-ray diffraction (XRD) mapping. A machine learning algorithm was implemented to identify defective regions and produce a yield prediction for each GaN wafer prior to processing. A hybrid edge termination structure consisting of implanted guard rings (GR) and JTEs was developed in coordination with a controlled experiment that varied the anode thickness, and therefore the remaining p-GaN after implantation. It was observed that thinner p-GaN regions under the JTE/GR region resulted in a significant (>100x) reduction in leakage current under reverse-bias conditions. This process has resulted in 1.2-kV-class devices with up to 18 A forward current for a 1 mm2 device with a specific on-resistance of 1.2 mOhm-cm2. The foundry effort has since been extended to 3.3-kV-class devices that utilize 25 µm thick drift layers with ~2-4×1015 cm-3 doping. These devices have demonstrated up to 3.8 kV breakdown with leakage currents <1 nA up to 3 kV. More than 40 wafers have been processed to date, resulting in >20,000 devices. Statistical variations in I-V and C-V characteristics will be discussed. Packaging process development and analysis are underway to develop electrical stress procedures and identify fundamental failure mechanisms. Finally, a pulse arrested spark discharge (PASD) setup, capable of up to 15 kV pulsed operation in 100 V steps, was implemented to quantify the time response of avalanche breakdown. Initial results on a packaged 800 V device showed a ~1 ns response time during breakdown, which reinforces the potential EMP grid protection applicability. This work was supported by the ARPA-E OPEN+ Kilovolt Devices Cohort directed by Dr.Isik Kizilyalli. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy of the United States Government.

Explainability analysis of neural network-based turbulence modeling for transonic axial compressor rotor flows

Model–consistent training has become trending for data-driven turbulence modeling since it can improve model generalizability and reduce data requirements by involving the Reynolds–averaged Navier–Stokes (RANS) equation during model learning. Neural networks are often used for the Reynolds stress representation due to their great expressive power, while they lack interpretability for the causal relationship between model inputs and outputs. Some post–hoc methods have been used to explain the neural network by indicating input feature importance. However, for the model–consistent training, the model explainability involves the analysis of both the neural network inputs and outputs. That is, the effects of model output on the RANS predictions should also be explained in addition to the input feature analysis. In this work, we investigate the explainability of the model–consistent learned model for the internal flow prediction of NASA Rotor 37 at its peak efficiency operating condition. The neural–network–based corrections for the Spalart–Allmaras turbulence model are learned from various experimental data based on the ensemble Kalman method. The learned model can noticeably improve the velocity prediction near the shroud. The explainability of the trained neural network is analyzed in terms of the model correction and the input feature importance. Specifically, the learned model correction increases the local turbulence production in the vortex breakdown region due to non–equilibrium effects, which capture the blockage effects near the shroud. Besides, the ratio of production to destruction and the helicity are shown to have relatively high importance for accurately predicting the compressor rotor flows based on the Shapley additive explanations method.

Investigation of Tip Leakage Vortex Breakdown in a High-Speed Multistage Axial Compressor

AbstractIn this article, an unsteady tip leakage flow instability is identified and investigated for an axial compressor at near-surge conditions. We describe the results of experimental verification of a new compressor developed by improving the blade geometry based on the criterion for the occurrence of this unsteady phenomenon. In a high-speed multistage axial flow compressor having a subsonic high stagger rotor blade, a surge test was carried out by changing the tip clearance. Under a condition of large tip clearance, a drastic decrease in the static pressure rise coefficient near the surge point was observed. At this operating condition, large, unsteady pressure fluctuation at the blade tip was confirmed, and the occurrence of tip leakage vortex breakdown was clarified by unsteady multipoint pressure measurement and detailed unsteady numerical simulations. Due to the blockage effect caused by vortex breakdown of the tip leakage, double leakage and axially reversed flow near the trailing edge were observed. It was found that the vortex breakdown region of the tip leakage vortex propagated in the circumferential direction and caused the rotating instability. In order to investigate the relationship among this unsteady flow phenomenon, tip clearance size, and flow pattern, unsteady calculation was conducted by changing the blade tip stagger and tip clearance size. A new concept of tip clearance of staggered pitch reference was proposed, which makes it possible to include the effect of blade loading on the clearance and clarifies that there exists a threshold at which vortex breakdown occurs/does not occur. On the basis of the aforementioned results, a high-speed multistage improved compressor was designed and manufactured to prevent tip leakage vortex breakdown. A clearance change test using active clearance control technology was conducted, and an increase in the static pressure rise coefficient near the surge point was confirmed for each clearance. The design concept of the improved blade, which suppressed the unsteady tip leakage flow instability, was tested and verified, and the effectiveness of the design guideline in actual gas turbines for power generation was confirmed.

Applicability of Flours from Pigmented and Glutinous Rice in Gluten-Free Bread Baking

The flours from pigmented and glutinous rice have a great potential to increase the nutritional quality of gluten-free breads. The characteristics of whole-meal fine flours, slurries, doughs, and breads prepared from black, red, and white glutinous rice were, therefore, compared with commercially used refined fine and semi-coarse white rice flours. The pasting properties of different flours were strongly influenced by the type of rice they were made from. Slurries with red and glutinous flours exhibited a shift to a lower pasting temperature T0, lower values of ηPeak and ηFinal, as well as higher values of the breakdown and setback region than the slurries with refined flours. The slurry with black flour exhibited high values of viscosity η0 and breakdown, together with low values of ηFinal, setback region and total setback. Bread characteristics were strongly correlated with the pasting properties. The presence of bran particles did not negatively impact loaf volume, crumb hardness, springiness, and chewiness. Some defects were observed in glutinous and red rice bread. Fine flour exhibited better baking performance than semi-coarse flour. Glutinous flour has the potential to become an ingredient in gluten-free baking. The applicability of various black and red rice flours may be limited by the flavor and the taste.

High speed and broadband fiber-integrated WS2/Bi2O2Se avalanche photodetector

Photodetectors based on two-dimensional(2D) materials have attracted tremendous attention in recent years because of their potential for development in compact and integrated devices. Owing to the high carrier mobility and air stability, the newly discovered 2D bismuth oxyselenide (Bi2O2Se) exhibits outstanding optoelectronic properties. However, the high dark current inevitably restricts the performance of the Bi2O2Se-based photodetectors. Here, we propose and demonstrate a fiber-integrated photodetector with a low dark current of ∼10 −10 A by assembling a WS2/Bi2O2Se Van der Waals heterostructure onto the end-face of a standard optical fiber. Benefiting from the narrow bandgap of the 2D materials, the fiber-integrated photodetector exhibits a broadband detection range from 650 to 1630 nm along with a responsivity of 1.13 AW−1. The fast rise time is measured as ∼410μs, while the fall time is ∼600μs. Taking advantage of the built-in potential in the WS2/Bi2O2Se heterostructure, the photodetector can work on self-powered mode. Applied a certainly high reverse voltage, the WS2/Bi2O2Se fiber-integrated photodetector can operate at avalanche breakdown region with an external quantum efficiency of 141% and an avalanche gain of 44 under the input power of 56.7μW. Finally, the fiber-integrated photodetector is applied to measure the wrist bending owing to its high photoresponsivity. These findings indicate that our proposed device has intriguing capabilities in terms of photodetection and on-line power monitoring, which enable our proposed device to have superb application potential in a fiber-integrated multi-function system.

Triple Threshold Transitions and Strong Polariton Interaction in 2D Layered Metal-Organic Framework Microplates.

Room-temperature interaction between light-matter hybrid particles such as exciton-polaritons under extremely low-pump plays a crucial role in future coherent quantum light sources. However, the practical and scalable realization of coherent quantum light sources operating under low-pump remains a challenge because of the insufficient polariton interaction strength. Here, at room temperature, a very large polariton interaction strength is demonstrated, g≈ 128 ± 21 µeV µm2 realized in a 2D nanolayered metal-organic framework (MOF). As a result, a polariton lasing at an extremelylow pump fluence of P1 ≈ 0.01 ± 0.0015 µJ cm-2 (first threshold) is observed. Interestingly, as pump fluence increases to P2 ≈ 0.031 ± 0.003 µJ cm-2 (second threshold), a spontaneous transition to a polariton breakdown region occurs, whichhas notbeen reported before. Finally, an ordinary photon lasing occurs at P3 ≈ 0.11 ± 0.077 µJ cm-2 (third threshold), or above. These experiments and the theoretical model reveal new insights into the transition mechanisms characterized by three distinct optical regions. This work introduces MOF as a new type of quantum material, with naturally formed polariton cavities, thatis a cost-effective and scalable solution to build microscale coherent quantum light sources and polaritonic devices.

Prediction of the breakdown voltage and breakdown area of gas switches based on ionization integral

Gas spark switches have a simple structure and high current capacity and are widely used in the field of pulsed power. The breakdown path of gas switches affects their jitter and lifetime. Particle simulations are used to analyze the breakdown path, which requires a large amount of computation resources. In this paper, in order to calculate the ionization integral value of electrode gaps, a simulation-based method using a simplified model is studied. The integration of the ionization coefficient along the E-field curves is calculated for the electrode gap and is used to determine if breakdown occurs for a given curve. The breakdown region of two-electrode self-breakdown switches is estimated using the distribution of breakdown curves, and the trigger voltage of a three-electrode gas-trigger switch is estimated by analyzing the curves. By applying the ionization integral value to different paths to reach the streamer formation condition as the breakdown criterion, the operating voltage and erosion region of the electrodes are estimated. The numerical results for an environmental pressure of 0.2 MPa are in good agreement with the experimental results from a triggering experiment that uses a three-electrode gas-triggered switch. This ionization integral model can be used to predict the breakdown voltage and breakdown region of gas switches, which is conducive to improving the performance of gas switches.

Breakdown Spectroscopy Induced by Nonlinear Interactions of Femtosecond Laser Filaments and Multidimensional Plasma Gratings

Breakdown spectroscopy is a valuable tool for determining elements in solids, liquids, and gases. All materials in the breakdown region can be ionized and dissociated into highly excited fragments and emit characteristic fluorescence spectra. In this sense, the elemental composition of materials can be evaluated by detecting the fluorescence spectrum. This paper reviews the recent developments in laser-induced breakdown spectroscopy. The traditional laser-induced breakdown spectroscopy, filament-induced breakdown spectroscopy, plasma grating, and multidimensional plasma grating-induced breakdown spectroscopy are introduced. There are also some proposals for applications of plasma gratings, such as laser ablation, laser deposition, and laser catalysis of chemical reactions in conjunction with research on the properties of plasma gratings.

Numerical simulation of gas breakdown characteristics in porous dielectric and theoretical analysis based on the capillary network model

Gas discharge in heterogeneous porous dielectric (PD) is becoming a reliable and popular low-temperature plasma technique in surface modification and plasma catalysis. However, the exact breakdown characteristics in the PD are not well studied due to experimental and diagnostic limitations. In this paper, a fluid model is then used to simulate the gas breakdown characteristics in real PD. It is found that the breakdown will selectively occur in PDs. The spatial distribution of PD determines regions of local breakdown. Variations in pressure result in the rearrangement of breakdown probability in each region of PD so that breakdown regions will change. In order to explain the selective breakdown phenomenon, based on the assumption that a PD can be simplified into a network consisting of multiple tortuous capillaries, we developed a theoretical model of the breakdown in a capillary network. Three geometrical factors, i.e. capillary tortuosity, capillary radius, and line porosity, are taken into the model to account for the effect of electric field distortion and electron loss on the dielectric walls in the presence of PD. The calculated results explain the selective breakdown phenomenon occurring in the numerical simulation. This paper simulates the breakdown in a real PD and provides a quantitative theoretical model to analyze the geometrical effect of PD on the breakdown. This paper will also provide insights into the design of optimal parameters for porous dielectric discharge in surface modification and plasma catalysis.

Efficient Dynamic Mesh Refinement Technique for Simulation of HPM Breakdown-Induced Plasma Pattern Formation

Numerical simulation of the complex plasma dynamics associated with high power, high-frequency microwave breakdown at high pressures, leading to the formation of filamentary plasma structures such as self-organized plasma arrays, is a computationally challenging problem. The widely used 2-D electromagnetic (EM)–plasma fluid model, which accurately captures the experimental observations, requires a runtime of several days to months to simulate standard problems due to stringent numerical requirements in terms of cell size and time step. This article presents a self-aware mesh refinement (MR) algorithm that uses a coarse mesh and a fine mesh that dynamically expand based on the plasma profile topology to resolve the sharp gradients in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula> -fields and plasma density in the breakdown region. The dynamic MR (DMR) technique is explained in detail, and its performance has been evaluated using a standard benchmark microwave breakdown problem. We observe a speedup of 8 (of the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$O(r^{3})$ </tex-math></inline-formula> , when the refinement factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$r$ </tex-math></inline-formula> ) is 2) compared with a traditional single uniform fine-mesh-based simulation. The technique is scalable and performs better when the problem size increases. We also present a comprehensive spatio-temporal visual analysis to explain the complex physics of high-power microwave (HPM) breakdown, leading to self-organized plasma filaments as an application of the DMR technique.

Polyimide Films Impregnated with Epoxy Resin Demonstrating Superior Self-Healing Properties for Thermally Stable Energy Storage Capacitors.

Metallized polymer films (MPFs) with superior self-healing properties are extremely attractive for application in energy storage capacitors. Self-healing behaviors allow MPFs to keep insulating between the local electrical breakdown region and the electrode, thereby reserving long-term operational viability of the capacitors. Polyimide (PI) is a type of well-developed polymer material with excellent mechanical and thermal stabilities, but it is deficient in intrinsic self-healing capabilities. This work reports a facile surface engineering strategy to endow metalized PI films with self-healing capabilities. By simple immersion of bare PI films in the solution of epoxy resin (ER) accompanied by curing of ER, PI films impregnated with ER (P-E films) not only show enhanced dielectric characteristics but also obtain excellent self-healing abilities upon multiple cycles of electrical breakdowns, even at a high temperature. For example, in comparison to bare PI films, PI films impregnated in ER solution with a solid content of 1 wt % (P-1%E) display improved initial Weibull breakdown strength (αb1 of 353.0 versus 310.9 kV/mm), maximum discharging energy density (Ud of 2.1836 versus 0.8254 J/cm3), and charging/discharging efficiency (η of 95.72 versus 55.19%) at 150 °C. After 5 breakdown cycles, P-1%E films could maintain a much higher breakdown strength (αb5 of 338.1 versus 21.3 kV/mm). When subjected to a constant electrical strength of 350 kV/mm at 150 °C, P-1%E films show merely <6% decline in both Ud and η values after 5 breakdown cycles. On the contrary, bare PI films would undergo dramatic performance decay after 1 or 2 breakdowns under similar conditions. In view of their outstanding self-healing properties at a high temperature, P-E films can serve as a promising candidate to fabricate thermally stable MPF capacitors for long-term operation.

Investigation of Four-Port Characteristic Dependence on Gate Length for MOSFETs in the Breakdown Region

In this article, gate length-dependent RF four-port characteristics of MOSFETs are investigated in the avalanche regime by using the double feedback and chain rule methods for the first time. The obvious decline of the imaginary part of source input impedance at high drain voltage for short-channel MOSFETs is investigated through the double feedback instead of only the series feedback for the origin of the inductive source impedance. The dominant terms causing the reduction of inductive source impedance due to parallel feedback are determined by this chain rule method. When compared with the conventional analysis due to short-channel effects on the direct-current (DC) characteristics of threshold voltage and substrate current, short-channel MOSFETs resulting in the aspect of RF characteristics are further analyzed by using different gate length MOSFETs. Besides, as gate length reduces, drain-to-source transmission raises significantly in the avalanche regime due to the RF avalanche network according to this analysis. This mechanism results in lower isolation at the breakdown for the shorter gate length MOSFETs. Good agreement of the four-port scattering parameter (S-parameter) between simulation and measurement is obtained for the MOSFETs operating in the both breakdown and saturation regions. The methods can be utilized for other advanced technology nodes and are helpful for the design of RF CMOS amplifiers in the impact ionization region.

Experimental Investigation of Discharge Path Selectivity of a Long Air Gap Containing a Floating Conductor

Live-line maintenance is an effective means to ensure the reliability of the power system. When linemen float between the high-voltage conductors and the ground potential towers, the discharge characteristics of air gaps are significantly affected. In this article, simplified experiments are carried out to simulate the linemen by a floating conductor. The discharge characteristics of air gaps when the floating conductor is in different positions are studied. An experimental platform with a long air gap containing a floating conductor is used to obtain the breakdown voltage and the discharge path of air gaps under a positive switching impulse. The experimental results show that the existence of a floating electrode affects the air gap breakdown voltage and the discharge path. When the transverse displacement is 80 cm, almost all the discharge paths directly connect the high voltage electrode and plane electrode. Furthermore, the influencing factors of discharge path and breakdown voltage are analyzed, and the fitting equations of breakdown voltage and discharge path probability considering spatial position coordinates are established. Finally, the easy breakdown region considering discharge path is proposed, which provides a theoretical reference for quantitative analysis of the path of linemen entering high potential safely, and to improve the working efficiency of live-line maintenance.

Lightning parameters of sprites and diameter of halos over South Africa

Transient Luminous Events (TLEs) above thunderclouds have been previously associated with variables such as the lightning Charge Moment Change (CMC), charge height, charge transfer, and lightning current rise-time. We show for the first time a comparison of the CMC, rise-time, fall-time, peak electric field, and peak current of the lightning discharges associated with 11 column, 11 carrot, and 18 sprites with halo. We found that carrot sprites are induced by a lightning discharge with CMC, peak electric field, and peak current greater and less than that for column sprites and sprites with halo, respectively. Sprites with a halo are initiated by a lightning discharge with a longer rise-time and fall-time than that for column and carrot sprites. Column sprites top altitude and carrot sprites brightest region altitude positively correlate with lightning rise-time. For carrot sprites top altitude, the results suggest that the electrical breakdown region decreases in altitude for a longer fall-time, greater peak electric field, and greater peak current. For the altitude of the sprites brightest region, column sprites correlate negatively with lightning fall-time, peak electric field, and CMC, and column sprites top altitude also correlates negatively with lightning peak electric field. For sprites with a halo top altitude increased with lightning fall-time and peak current, and sprites with a halo brightest altitude increased with an increase in lightning CMC. Halo diameters correlate positively with lightning fall-time, peak electric field, and peak current. The investigated lightning parameters can be used to identify the initiated sprites morphological type when optics are not available.

Control of first-mode oblique breakdown in a supersonic boundary layer using micro-groove coating strips

This paper describes a method for controlling the first-mode oblique breakdown in a Mach 2.0 supersonic boundary layer using micro-groove coating strips. The analysis is performed using the nonlinear parabolized stability equations. Micro-groove coating strips with various combinations of the groove half-width, porosity coefficient, and strip width are placed at different streamwise locations to control the transition. The results show that the micro-groove coating strip significantly suppresses the first-mode oblique breakdown. This effect is enhanced as the groove half-width, porosity coefficient, and strip width increase. When the micro-groove coating strip is located in the linear development region of the oblique breakdown, the suppression effect comes from the stabilization of the fundamental oblique mode by the micro-groove coating. When the strip is employed in the early nonlinear or late nonlinear development regions, a second stabilization mechanism, in addition to the stabilization effect of the micro-groove coating strip, is produced by the high-spanwise-wavenumber steady modes. Compared with the stabilization effect of the micro-groove coating strip, the second stabilization effect is more effective. The combination of these two stabilization mechanisms pushes the breakdown farther downstream.