Critical Field Strength Research Articles

Internal magnetic fields in 13 red giants detected by asteroseismology

Context. Magnetic fields affect stars at all evolutionary stages. While surface fields have been measured for stars across the Hertzsprung–Russell (HR) diagram, internal magnetic fields remain largely unknown. The recent seismic detection of magnetic fields in the cores of several Kepler red giants has opened a new avenue to better understand the origin of magnetic fields and their impact on stellar structure and evolution. Aims. The goal of our study is to use asteroseismology to systematically search for internal magnetic fields in red giant stars observed with the Kepler satellite, and to determine the strengths and geometries of these fields. Methods. Magnetic fields are known to break the symmetry of rotational multiplets. In red giants, oscillation modes are mixed, behaving as pressure modes in the envelope and as gravity modes in the core. Magnetism-induced asymmetries are expected to be stronger for gravity-dominated modes than for pressure-dominated modes, and to decrease with frequency. Among Kepler red giants, we searched for stars that exhibit asymmetries satisfying these properties. Results. After collecting a sample of ∼2500 Kepler red giant stars with clear mixed-mode patterns, we specifically searched for targets among ∼1200 stars with dipole triplets. We identified 13 stars exhibiting clear asymmetric multiplets and measured their parameters, especially the asymmetry parameter a and the magnetic frequency shift δνg. By combining these estimates with best-fitting stellar models, we measured average core magnetic fields ranging from ∼20 to ∼150 kG, corresponding to ∼5% to ∼30% of the critical field strengths. We showed that the detected core fields have various horizontal geometries, some of which significantly differ from a dipolar configuration. We found that the field strengths decrease with stellar evolution, despite the fact that the cores of these stars are contracting. Additionally, even though these stars have strong internal magnetic fields, they display normal core rotation rates, suggesting no significantly different histories of angular momentum transport compared to other red giant stars. We also discuss the possible origin of the detected fields.

Physical insights in thermo-elastic damping and critical magnetic field of an embedded micro/nanobeam resonator

This article investigates the behavior of thermoelastic damping in a micro/nano beam subjected to an external magnetic field and an elastic matrix via small-scale theory. The material properties of the beam, such as Young’s modulus, thermal conductivity, specific heat capacity, and coefficient of thermal expansion, are considered to analyze the physical aspects of thermoelastic damping. Further, the critical magnetic field strength of the nanobeam, which affects magnetic properties, is studied in relation to temperature, magnetic material properties, modenumber, and boundary conditions, leading to important findings for magnetomechanical systems and materials design.

Discharge Characteristics and Mechanisms of Electrolytic Discharge Processing by Jet Mask

As a novel microfabrication method, electrochemical discharge machining has remarkable effects on the forming and processing of brittle and hard materials and non-conductive materials, but little research has been done on the electrochemical discharge mode in the jet state. To fulfil the potential of this technology, innovative research on the discharge characteristics and mechanism of electrochemical discharge machining in the jet mask is proposed. A high-speed camera observation experiment was set up to record the process of the jet flow column discharge formation and penetration. Changes in the electric field of the electrolytic jet channel were analysed by simulation software, and the morphology of the machined micro-pits was observed using a microscope. A mathematical derivation of the dielectric electric field in the gas–liquid two-phase jet column reveals the mechanism of discharge channel formation in the jet state. The experiments show that when the processing voltage is 400 V, a stable continuous spark appears, realizing the unique characteristics of a large-gap long-distance discharge and a flat small circle-shaped discharge mark produced at the bottom of the crater. The actual field strength within the bubble of this model obtained by mathematical derivation is approximately 61.5 kV/cm greater than the critical field strength for air bubble breakdown in the standard state, where bubble breakdown occurs in the discharge.

Thermodynamic and magnetotransport properties of EuAg2Ge2 single crystals

Here, we report the crystallographic, thermodynamic, and magnetotransport properties of flux grown EuAg2Ge2 single crystals using X-ray diffraction, anisotropic magnetic susceptibility, and magnetization, heat capacity, electrical resistivity, and magnetoresistance measurements. The room temperature powder X-ray diffraction pattern confirms that EuAg2Ge2 crystallizes in a body-centered tetragonal ThCr2Si2–type structure (space group I4/mmm) with lattice parameters, a = 4.401 Å and c = 10.845 Å. The magnetic susceptibility data reveal two subsequent antiferromagnetic transitions at TN1 ∼ 8.3 K, and TN2 ∼ 6.2 K. For H‖c at T = 5 K, the magnetization data show the spin-flop like metamagnetic transition around the critical field of 1.4 T. The existence of antiferromagnetic transitions is further confirmed by the heat capacity measurements, which display two anomalies around TN1 = 8.3 K and TN2 = 6.1 K. The electrical resistivity data show the metallic behavior of the compound, which is best described by the Bloch-Grüneisen model above 30 K. At low temperatures, the antiferromagnetic transition manifests through a slight upturn around 8 K, followed by a sharp drop in the resistivity data. The transverse magnetoresistance (MR) measured for H‖c below TN first increases, showing a broad maximum at critical field strength that shifts towards lower fields with increasing temperature. With further increasing fields beyond the maximum, MR decreases, and changes its value from positive to negative. The evolution of MR from positive to negative could be associated with the metamagnetic transition in EuAg2Ge2. In the paramagnetic region, MR remains negative across the entire field range.

Mode and mechanism of water droplet breakup in oil under high-voltage and high-frequency pulsed electric fields

In industrial processes, the application of pulsed electric fields for electrostatic demulsification may cause droplet breakup, reducing demulsification efficiency. In this paper, a comprehensive approach combining microscopic experiment, theoretical derivation, and molecular dynamics simulation, were employed to explore the water droplet breakup in water-in-oil (W/O) under high-voltage and high-frequency pulsed electric fields. The results showed that droplet breakup would occur when the electrostatic tensile stress of the droplet surpassed the additional compressive stress. Three main modes of droplet breakup were observed: breakup at one end, breakup at both ends, and breakup in the middle part. By applying Maxwell stress equation, critical field strengths were derived to predict breakup locations. In the pulsed electric field, the droplets underwent a sequence of stretching, retraction, and eventual breakup. The formation and migration of hydrated ions was of crucial importance in the droplet breakup. A decrease in electrostatic attraction between H2O and an increase in dispersion attraction effect between H2O and C6H14 promoted the droplet breakup. The number of hydrogen bond inside the droplet was reduced, weakening the stability of droplet. The energy of electric field was transformed into the deflection of dipole moment and the velocity of water molecules. The time required for droplet breakup decreased when the pulsed electric field strength increased. The research findings in this paper uncovered the mechanism behind the breakup of water droplets in pulsed electric fields, and offered theoretical guidance for optimizing demulsification processes.

Generation and optimization of cross-sections for electron-C4F7N collisions

Abstract A complete and consistent set of electron-neutral collision cross-sections for the novel insulating gas C4F7N is reported. The set is composed of a combination of cross-sections previously reported in literature, optimized via a genetic algorithm in conjunction with a multi-term Boltzmann equation solver, and calculated ab initio using the R-matrix code Quantemol-EC. The finalized set accurately reproduces reported macroscopic rate and transport coefficients as well as Townsend coefficients and critical electric field strengths in C4F7N and its mixtures with nitrogen, carbon dioxide, and argon.

Simulation analysis of synthetic electric field of UHV transmission line under mountain fire condition

Factors such as high temperature, charge density and charged particles have a major impact on the insulation strength of the transmission line gap. When the DC transmission line is in normal operation, the surface of the conductor is easy to adsorb dirt due to the environmental influences, so the local electric field strength is larger than the critical field strength of the corona discharge, and the gas molecules in the air are ionized to form space charges. The electric field of the space charge is superimposed with the electric field of the wire and the earth (nominal electric field) to form a synthetic electric field. In this work, the calculation model of the synthetic electric field is created and the influence of mountain fires on the synthetic electric field of the transmission line is simulated and analyzed. The influence of flame temperature, charge density, particle type and shape on the electric field is simulated and investigated. The law of variation of the electric field of the transmission line in mountain fire under different factors is obtained. The distorting effect of particles on the electric field under the UHV DC transmission line is simulated. The distorting effect of particle size and particle chain on the synthetic electric field is obtained and the simulation results are compared to the calculation results.

(Invited) Gallium Oxides Devices for Grid-Scale Power Electronics and High Power Switched Mode RF Amplifiers

Within a decade of the first demonstration of a MESFET device, monoclinic β-Ga2O3 (Ga2O3) devices have made incredible progress in breakdown voltages, power device figure of merit and high-speed performance. It has emerged as a promising ultra-widebandgap semiconductor for next generation power, GHz switching and RF applications. In this talk, we will present an overview of our studies on fundamental transport properties, kilovolt class power MOSFETs and aggressively scaled RF devices. The large bandgap of Ga2O3 leads to a high critical field strength. This high field strength in combination with demonstrated room temperature mobility and calculated electron velocity leads to a higher Baliga’s Figure of Merit (BFoM) and Johnston Figure of Merit (JFoM) than current commercially available widebandgap technologies. Additionally, the large bandgap also enables high temperature operation and radiation hardness making it attractive for space applications such as Mars and Venus missions.In the first part, we will discuss the deep insights gained from the first-principles based transport calculations in Ga2O3. We will discuss the limits to low field mobility, saturation velocities and impact ionization coefficients. The impact of the plasmon-phonon coupling on the phonon limited transport will be discussed. We will present the experimental signatures of plasmon-phonon coupling and anti-screening behavior in ionic gated accumulation layers.Next, I will describe our work on the lateral MOSFETs with improved field plate design and beyond-kV breakdown. Temperature dependent analysis and device simulation suggest an extrinsic breakdown mechanism outside the channel. A simple and yet effective SU-8 polymer passivation technology was developed that results in significant improvement in breakdown voltages. The higher field strength of the SU-8 polymer enables a significant increase in breakdown voltage to 8 kVs in lateral MOSFETs. However, these devices show a high Ron, which is due to the depletion caused by RIE of the channel. We will present the use of ultra-high vacuum annealing techniques to improve the on-resistance of the devices still maintaining the multi-kilo-volt rating of the devices.Due to its high JFoM, Ga2O3 is a potential candidate for high power density RF amplifiers. We will also present the performance advancement that can be obtained in RF power performance using Ga2O3 technology. We will present the challenges in the technology and potential solutions. We will present the experimental DC and pulsed measurements on laterally scaled MOSFETs with 100 nm gate length. Severe DC-RF dispersion was observed which is attributed to the RIE damage and surface states on the oxide. Passivation of the devices shows reduced current collapse. Using aggressive gate lengths and gate-source spacing scaling, (AlxGa1-x)2O3/Ga2O3 heterostructure FETs were fabricated that show record high RF performance. The device transconductance was limited by the regrowth interface resistance. Ex-situ silicon nitride passivation was shown to be effective to reduce DC-RF dispersion. Recently, thin channel MOSFETs with deep-sub-micron gate lengths and source/drain regrowth have demonstrated devices with simultaneously low on resistance, high frequency, high voltages.

Probing the coalescence mechanism of water droplet and Oil/Water interface in demulsification process under DC electric field

Under electric fields, coalescence of water droplets in oil media and bulk water (droplet-interface coalescence) is a significant phenomenon during the electrostatic process of water-in-oil (W/O) emulsion. In order to reveal the microscopic mechanism of droplet-interface coalescence, MD simulations and high-speed imaging were performed to study the influence of droplet size (r0) and electric field strength (E) on the coalescence process between water layer and droplet in oil. The findings suggested that the molecular arrangement of water molecules was altered at applied electric fields, resulting in a weakened interaction force and an increased spacing between water molecules. The droplet-interface coalescence process could be classified into five temporal regimes, under the mutual effect of the associated intermolecular interactions (Coulomb interaction and van der Waals interaction) and external electric field. The interfacial charge density reached a high value of 0.15 e·nm−3 due to the charge orientation, leading to dipole–dipole interaction between the droplet and water layer. Driven by both applied electric field and polarized charge, when E = 0.44 V·nm−1, the droplet with a diameter of 6 nm had the shortest coalescence period, only 370 ps. The critical field strength (Ec) required to prevent complete coalescence between nano-droplets and oil–water interface increased approximately linearly with r0−0.5. This work has revealed the molecular interaction mechanism of electro-coalescence between water droplets and oil/water interface, and provides useful insights into the electrostatic demulsification.

Searching for magnetic fields in pulsating A-type stars: the discovery of a strong field in the probable δ Sct star HD 340577 and a null result for the γ Dor star HR 8799

ABSTRACT Numerous δ Sct and γ Dor pulsators are identified in the region of the Hertzsprung–Russell diagram that is occupied by chemically peculiar magnetic Ap stars. The connection between δ Sct and γ Dor pulsations and the magnetic field in Ap stars is however not clear: theory suggests for magnetic Ap stars some critical field strengths for pulsation mode suppression by computing the magnetic damping effect for selected p and g modes. To test these theoretical considerations, we obtained PEPSI spectropolarimetric snapshots of the typical Ap star HD 340577, for which δ Sct-like pulsations were recently detected in Transiting Exoplanet Survey Satellite data, and the γ Dor pulsator HR 8799, which is a remarkable system with multiple planets and two debris discs. Our measurements reveal the presence of a magnetic field with a strength of several hundred Gauss in HD 340577. The measured mean longitudinal field would be the strongest field measured so far in a δ Sct star if the pulsational character of HD 340577 is confirmed spectroscopically. No magnetic field is detected in HR 8799.

Early Detection of Health Condition Degradation of Circuit Breaker Based on Electrical Quantity Monitoring

Circuit breakers on the filter bank branches in converter stations are vulnerable to contact wear and mechanical deterioration caused by frequent operations, which can lead to circuit breaker breakdowns and explosions. It is imperative to conduct research on the early detection of abnormal states in circuit breakers. Existing electrical quantity-based detection methods are constrained by a priori assumptions, and their measurement methods are susceptible to interference, leading to misjudgments. To address this issue, this paper examines the influence of changes in critical breakdown field strength and contact spacing on circuit breaker operation states. It also proposes a technical scheme that employs breakdown current values to comprehensively characterize circuit breaker operation states, replacing the use of critical breakdown field strength and contact spacing. An early detection method for abnormal circuit breaker states based on a sequence of breakdown current ratios at different times is proposed, and its effectiveness is verified through simulation and field recording data.

Mechanical properties and application of superconducting materials of Nb3Sn

Nb3Sn is one of the most promising low temperature superconducting materials and its high critical current density, high critical magnetic field strength and high critical transition temperature have attracted a lot of attention in many application areas. However, the superconductivity of Nb3Sn is strongly sensitive to its mechanical behaviour. The strain and torque arise from tension during cabling and electromagnetic forces during working operation can affect the critical performance of the strand. In this paper, the development of superconducting materials was firstly introduced in a brief way. Subsequently, the paper focused on several experiments to test the mechanical properties. Devices such as Pacman and TRASIS are used to measure the axial strain/stress, transverse force, spatial periodic contact loading cycles and periodic bending. In the next section, the paper introduced the applications and finally drew a conclusion. These results shed light on guiding future studies focusing on Nb3Sn and lay a foundation for subsequent mechanical studies of Nb3Sn.

Boundary quantum phase transitions in the spin- 12 Heisenberg chain with boundary magnetic fields

We consider the spin $\frac{1}{2}$ Heisenberg chain with boundary magnetic fields and analyze it using a combination of Bethe ansatz and density matrix renormalization group (DMRG) techniques. We show that the system exhibits several different ground states which depend on the orientation of the boundary magnetic fields. When both the boundary fields take equal values greater than a critical field strength, each edge in the ground state accumulates a fractional spin which saturates to spin $\frac{1}{4}$, which is similar to systems exhibiting symmetry protected topological phases (SPT). Unlike in SPT systems, the fractional boundary spin in the Heisenberg spin chain is not a genuine quantum number since the variance of the associated operator does not vanish, this is due to the absence of a bulk gap. The system exhibits high energy bound states when the boundary fields take values greater than the critical field. All the excitations in the system can be sorted out into towers whose number depends on the number of bound states exhibited by the system. As the boundary fields are varied, in addition to the ground state phase transition, we find that the system may undergo an eigenstate phase transition (EPT) where the number of towers of the Hilbert space changes. We further inquire how the EPT reflects itself on local ground state properties by computing the magnetization profile $\langle S^z_j \rangle$ using DMRG. We identify a clear qualitative change from low edge fields to high edge fields when crossing the critical field. We though are unable to conclude on the basis of our data that EPT corresponds to a genuine phase transition in the ground state.

Tutorial: Metalorganic chemical vapor deposition of β-Ga2O3 thin films, alloys, and heterostructures

β-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor with a bandgap energy of ∼ 4.8 eV and a predicted high critical electric field strength of ∼8 MV/cm, enabling promising applications in next generation high power electronics and deep ultraviolet optoelectronics. The advantages of Ga2O3 also stem from its availability of single crystal bulk native substrates synthesized from melt, and its well-controllable n-type doping from both bulk growth and thin film epitaxy. Among several thin film growth methods, metalorganic chemical vapor deposition (MOCVD) has been demonstrated as an enabling technology for developing high-quality epitaxy of Ga2O3 thin films, (AlxGa1−x)2O3 alloys, and heterostructures along various crystal orientations and with different phases. This tutorial summarizes the recent progresses in the epitaxial growth of β-Ga2O3 thin films via different growth methods, with a focus on the growth of Ga2O3 and its compositional alloys by MOCVD. The challenges for the epitaxial development of β-Ga2O3 are discussed, along with the opportunities of future works to enhance the state-of-the-art device performance based on this emerging UWBG semiconductor material system.

Impact of Dimensions and Doping on the Breakdown Voltage of a Trench 4H-SiC Vertical JFET

In this paper we study the feasibility of the design/fabrication of a vertical trench 4H-SiC Junction Field Effect Transistor (JFET), assuming realistic constraints of the depth of the P+ implantation. The P+ doping profile is obtained using a Monte Carlo implantation simulation. The calculation used a drift-diffusion approach. The JFET aims to achieve a threshold voltage of-3V. We found that this constraint in concomitance with the proposed structure limits the breakdown voltage to approximately 200V. This is the result of a premature breakdown induced by short channel effects, namely Drain Induced Barrier Lowering (DIBL). However, a negative increase in the gate bias represses this short channel effect and improves the breakdown voltage to roughly 1800V. At this gate bias, the breakdown is induced by reaching the critical field strength of 4H-SiC at the gate P+/N junction, which causes avalanche generation of carriers. In addition, we have calculated the dependence of the threshold voltage on the drift doping and pillar width. This work also shows the vulnerability of the design to random fluctuation in the doping profile.

Electric field-induced deformation and breakup of water droplets in polymer-flooding W/O emulsions: A simulation study

Polymers have a significant impact on the electrostatic demulsification efficiency of water-in-oil (W/O) emulsions by altering the motion characteristics of water droplets. To uncover the underlying micro-mechanism behind electric field-induced deformation and breakup of polymer-containing water droplets, we employed the molecular dynamics (MD) method to investigate the effect of different polymer concentrations (cp) on droplet motion under a DC electric field. The simulation results indicated that polyacrylamide (PAM) molecules possess a prominent electrostatic potential (ESP), resulting in a strong electrostatic attraction between PAM and water. The interface film of the droplet was found to exhibits high strength due to the formation of hydrogen bonds with a lifetime of 1 ps between polymers. The addition of polymers increased the polarity of the droplets, thereby promoting their deformation and breakup under an electric field. Both polarized polymers and ions migrated by carrying water molecules, which ultimately led to droplet breakup. At cp = 0.0221 mol·L−1, the critical field strength (Ec) for droplet breakup was the lowest, at only 0.672 V·nm−1. As cp > 0.0221 mol·L−1, the steric hindrance of PAM molecules and the strong interfacial film impeded the breakup of the droplets. These findings provided a theoretical basis for enhancing the electrostatic demulsification efficiency of polymer-flooding crude oil.

Ion energy dependence of dry etch damage depth in Ga2O3 Schottky rectifiers

The β-polytype of Ga2O3 has a high critical electric field strength (∼8 MV.cm−1) and can be grown as large diameter (6 in.) single crystal boules by inexpensive melt-growth techniques. This makes it promising for next generation power electronics and solar-blind UV photodetectors. Since wet etching is not feasible for patterning such devices due to the chemical inertness of Ga2O3, attention is focused on dry etching, with the attendant issue of surface modification due to chemical or ion-induced changes. In this work, we demonstrate that dry etch damage in β-Ga2O3 is manifested by a reduction in near-surface carrier concentration, possibly due to the introduction of Gav acceptor states. The depth to which the carrier density is affected depends on the interplay between ion energy and etch rate, with faster etch rates reducing the depth of the remaining damaged region. The maximum damage depth observed experimentally from capacitance-voltage profiling on Schottky rectifiers was ∼ 110 nm, well beyond the range of the ions in the Inductively Coupled Plasmas and emphasizing that rapid diffusion of point defects occurs into the sample during the etch step. Using Schottky rectifier structures as a platform to understand the effect of dry etch damage on device parameters, we found that on-state resistance was more affected than ideality factor or breakdown voltage, with a large increase (5x) for ion energies ≥ 325 eV. Reverse current density characteristics showed the presence of trap-assisted space-charge-limited conduction in the damaged layers. The implications for high power rectifier fabrication are discussed.

Equivalence between C5F10O Addition and PTFE Vapor Contamination in the Particle Composition and Critical Electric Field Strength of High‐Temperature CO2/O2 Gas Mixture

AbstractSF6 has been introduced as an insulating and extinguishing gas in gas circuit breakers. However, its global warming potential is significantly high at approximately 25 200. Therefore, development of alternative gases is essential. We have been investigating the dielectric breakdown characteristics of high temperature CO2 gas contaminated by various gases. This study discusses the equivalent condition between high temperature CO2/O2/C5F10O gas mixture and CO2/O2 gas mixture contaminated with PTFE (C2F4) vapor because both gas mixtures consist of C, O, and F atoms. It is concluded that when the mole fractions of every atom are equivalent between both gas mixtures, both gas mixtures have the same characteristics as each other, such as the particle composition and critical electric field strength. © 2023 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Numerical Simulation of Electrode Structure on High Voltage Pulse Discharge Characteristics

For the study of the different electrode structures of the high voltage pulse discharge breakdown process of the insulating medium, the numerical simulation software is compiled to establish two kinds of the electrode structure of high voltage pulse discharge three-dimensional model, finite element difference method to simulate the expansion of plasma channel, the discharge characteristics of electrode structure internal voltage, and current of plasma channel and the impact of energy consumption. The simulation results show that, compared with the needle-needle electrode structure, the needle-plate electrode structure has a shorter time to reach the critical field strength of distilled water smaller breakdown voltage, shorter time to establish the discharge channel, smaller equivalent resistance, faster extension speed of the discharge channel, and less energy consumption for breakdown. And other conditions are the same. The results of this study can provide a theoretical reference for the structural design of high-voltage pulse-crushing electrodes.

Coalescence and Break-Up Behaviors of Nanodroplets under AC Electric Field.

Water must be separated from water-in-oil (W/O) emulsion because of the corrosion it brings to the relative equipment in the process of transportation and storage. It is an effective method to apply external electric field to achieve high performance of separating small, dispersed water droplets from W/O emulsion; however, the coalescing micromechanism of such small salty droplets under AC electric field is unclear. In this paper, molecular dynamics simulation was adopted to investigate the coalescence and separation process of two NaCl-aqueous droplets under AC electric field and discuss the effect of AC electric field frequency, as well as the time required for contacting, the critical electric field strength, the dynamic coalescence process and the stability of the final merged droplet. The results show that the critical electric field strength of the droplet coalescence increases with the increase of frequency, while the time required for droplet contacting becomes shorter. The shrinkage function curve was applied to characterize the droplet coalescence effect and it was found that the droplets coalescence and form a nearly spherical droplet under the AC electric field with a frequency of 1.25 GHz and strength of 0.5 V/nm. When the electric field frequency is 10 GHZ, the merged droplet presents a periodic fluctuation with the same period as the AC electric field, which mainly depends on the periodic movement of cations and anions under the AC electric field. The results can provide theoretical basis for the practical application of electrostatic demulsification technology in the petroleum or chemical industry from the microscopic perspective.