Quasi DC Research Articles

Interfacial and Bulk Polarization of P(VDF-HFP) Investigated by Dielectric Spectroscopy

Capacitors in electrical and electronic systems composed of thin film dielectrics operate under extremely high electric fields. Dynamics of charge transport and relaxation are closely correlated to the energy storage properties and electrical degradation of thin film dielectrics for capacitive applications. In this paper, the dielectric relaxation properties of poly (vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer films are investigated over a broad frequency range from 10 mHz to 10 MHz and temperature range from -130 °C to 130 °C. The dielectric responses correlated to bulk and interfacial processes are clarified by varying the thickness and interfacial contact status of P(VDF-HFP) films. A low frequency process originating from the quasi-DC (QDC) process and a dielectric dispersion process correlated to the interfacial polarization are identified based on the Dissado-Hill dielectric response model and the equivalent circuit analysis. The high permittivity and high operation electric field of P(VDF-HFP) for capacitive energy storage give rise to improved blocking capacitance associated with the interface, suggesting the enhanced interfacial polarization. This work provides insights into the bulk and interface charge dynamics of polymer thin film dielectrics.

Modes of plasma-stabilized combustion in cavity-based M = 2 configuration

This paper considers the modes of plasma-stabilized supersonic combustion in a cavity-based flameholding configuration. The testing was performed in the SBR-50 supersonic facility at the University of Notre Dame under the following conditions: Mach number M = 2, stagnation pressure and temperature P0 = 1–3.2 bar, T0 = 295–750 K, and an ethylene injection rate of ṁC2H4 = 0–8 g/s. A rail type electric discharge is employed in this work, which represents a modification of the previously explored quasi-DC (Q-DC) configuration. Two principally different cases are realized and described in detail: in- or over-the-cavity combustion, and bulk combustion characterized by a significant increase of pressure downstream of the fuel injection ports. Rail discharge performance is compared to the performance of Plasma-Injection Modules (PIMs) using a 1D analysis. Dimensionless analysis of the plasma-assisted combustor performance indicates reasonable agreement of the in-cavity combustion mode with the Ozawa stability diagram, while the plasma-assisted bulk combustion is essentially distinct from this formalism.

Field experience of small quasi-DC bias on power transformers

Low-frequency currents (LFC) or quasi-DC (QDC) in the electrical power transmission network, and especially in power transformers, are causing negative effects such as an increase in noise level, in reactive power consumption and in power losses. Currently, no classification of LFC is available to identify a possible source. In order to identify the origin of undesired LFC, classifications of LFC in current and audio measurements are defined. They are based on a spectrum analysis of current and audio measurements. These classifications are successfully tested in laboratory and field measurements. Consequently, LFC sources are identified by field and laboratory measurements and analytical approaches. For power transformer operators, a user-friendly and fast method is presented to identify LFC in the transformers. The method is based on audible measurements and serves as a first estimator for low-frequency currents in power transformers.

Manifestation of Interactions of Nano-Silica in Silicone Rubber Investigated by Low-Frequency Dielectric Spectroscopy and Mechanical Tests.

Silicone rubber composites filled with nano-silica are currently widely used as high voltage insulating materials in power transmission and substation systems. We present a systematic study on the dielectric and mechanical performance of silicone rubber filled with surface modified and unmodified fumed nano-silica. The results indicate that the different interfaces between the silicone rubber and the two types of nano-silica introduce changes in their dielectric response when electrically stressed by a sinusoidal excitation in the frequency range of 10−4–1 Hz. The responses of pure silicone rubber and the composite filled with modified silica can be characterized by a paralleled combination of Maxwell-Wagner-Sillars interface polarization and DC conduction. In contrast, the silicone rubber composite with the unmodified nano-silica exhibits a quasi-DC (Q-DC) transport process. The mechanical properties of the composites (represented by their stress-strain characteristics) reveal an improvement in the mechanical strength with increasing filler content. Moreover, the strain level of the composite with a modified filler is improved.

Flow Control in Supersonic-Cavity-Based Airflow by Quasi-Direct-Current Electric Discharge

This experimental study examines flow control authority of a quasi-DC (Q-DC) electric discharge in a cavity-based supersonic flow. Testing was performed with Mach 2 airflow using the Research Cell 19 facility at the U.S. Air Force Research Laboratory while varying pressure (), temperature (), gaseous fuel-injection rate [ standard L/min (SLPM)], and power of the electric discharge (). Schlieren imaging and pressure sensors were used to study changes to the flowfield and wall pressure distribution over the test section. The Q-DC electric discharge generates a shock wave that impinges on a rear-facing cavity and increases the pressure throughout the cavity up to 5%. A gas was injected upstream of the cavity to represent fuel injection in a scramjet engine. The flowfield changes due to the Q-DC discharge interaction with the fuel-related bow-shock wave, and shifts the shock train upstream. For injection rates of SLPM, the Q-DC discharge shifts the bow-shock reflection forward to impinge the cavity, thus showing a synergistic effect of the Q-DC discharge and the upstream injection. The maximum relative pressure increase during Q-DC operation with injection was . The control effectiveness parameter was defined and is approximately constant across the tested flow conditions.

Dielectric response of nano aluminium tri-hydrate filled silicone rubber

The dielectric response of a nano-ATH (Aluminium Tri-hydrate) filled silicone rubber is investigated over the frequency range 10−4 to 103 Hz and compared with that of the micro-ATH composite. An equivalent circuit has been used to decompose the dielectric response into: a loss peak process, a quasi-dc (Q-DC) process, a dc-conductance and constant high frequency capacitance. It was found that the Q-DC and loss peak were a result of the ATH filler with the loss peak more clearly resolved in the nano-ATH filled material than in the micro-ATH material where it is weaker and partly obscured by the Q-DC process. In contrast to the micro-ATH material the characteristic frequencies of all the dielectric responses in the nano-ATH filled samples have the same activation energy. The physical mechanisms of the Q-DC process and loss peak are discussed in terms of long range transport between nano-ATH clusters and the dipolar behaviour of isolated clusters respectively.

Supersonic Cavity-Based Flow Control Using a Quasi-DC Discharge

The Quasi-DC (Q-DC) discharge is studied as an active flow control authority on a rear-facing cavity in a supersonic duct by creating an oblique shockwave that impinges the cavity. This geometry simulates the geometry of a typical scramjet flameholding scheme. The tests were performed at the University of Notre Dame in the SBR-50 supersonic blowdown rig with dried air at M=2. Schlieren imaging is used to view the flow field with and without the Q-DC discharge in operation. A significant change in the flow field structure is observed. Pressure sensors detect a pressure increase throughout the entire rear-facing cavity while the Q-DC discharge is operating. This reveals that the cavity redistributes the pressure increase from the shockwave as a result of the flow within the cavity being subsonic. As a result of this pressure absorption and redistribution, the impinging shockwave created by the Q-DC is almost completely absorbed. This absorption is confirmed by the schlieren images. The data reveal that the discharge power is the dominating influence, as compared to electrode/discharge geometry, on the pressure increase produced in the cavity. There is a nearly linear correlation between the power of the discharge and the pressure increase produced directly behind the discharge, in the cavity, and on the ramp of the cavity (to varying magnitudes). It is suggested that the 11 electrode system may be slightly more effective than the 7 electrode system.

Quasi-DC electrical discharge characterization in a supersonic flow

A Quasi-DC (Q-DC) electrical discharge generates a highly transient filamentary plasma in high-speed airflow. Major specific properties of this type of discharge are realized due to a strong coupling of the plasma to the moving gas. The plasma, supplied by a DC voltage waveform, demonstrates a pulsed-periodic pattern of dynamics significantly affecting the flow structure. In this study, the dynamics and plasma parameters of the Q-DC discharge are analyzed in the Supersonic Test Rig (SBR-50) at the University of Notre Dame at Mach number M = 2, stagnation pressure P 0 = (0.9–2.6) × 105 Pa, stagnation temperature T 0 = 300 K, unit Reynolds number ReL = 7–25 × 106 m−1, and plasma power W pl = 3–21 kW. The plasma parameters are measured with current–voltage probes and optical emission spectroscopy. An unsteady pattern of interaction is depicted by high-speed image capturing. The result of the plasma-flow interaction is characterized by means of pressure measurements and schlieren visualization. It is considered that the Q-DC discharge may be employed for active control of duct-driven flows, cavity-based flow, and for effective control of shock wave–boundary layer interaction.

Dielectric response of filled high temperature vulcanized silicone rubber

The dielectric response of three kinds of filled high temperature vulcanized (HTV) silicone rubber - SiO 2 based nanocomposite with different alumina tri-hydrate (ATH) micro-filler contents are investigated over a wide range of frequency and temperature. The results are analyzed in terms of the many-body Dissado-Hill dielectric response theory. In all three kinds of silicone rubber, a quasi-DC (QDC) process, a Dissado-Hill loss peak process and a diffusion process are observed. Specifically, the ATH filler contributes a QDC electrical transport/hopping process, which at higher temperatures causes the formation of a diffusion layer at the electrodes. The nano-SiO 2 particles combined with rubber matrix give a Dissado-Hill loss peak dielectric relaxation. These two dispersion processes are shown to be associated with the interior interfaces of filled HTV silicone rubber. The effects of ATH filler content and temperature on the dielectric response are also analyzed in depth.

The role of bulk charge transport processes in electrical tree formation and breakdown mechanisms in epoxy resins

Electrical treeing experiments have been conducted at different temperatures and levels of absorbed moisture in Araldite CY1311 epoxy resin samples above their glass transition temperature, i.e. when the resin was in a flexible state. The fractal dimension of the electrical trees obtained and the rate of tree growth were found to depend on the environmental factors: temperature and humidity. It has also been found that at certain levels of temperature and moisture absorbed in the samples, a transition occurs from electrical treeing degradation to breakdown by thermal runaway. Complementary investigations of the dielectric properties of the same epoxy resin system have revealed that a bulk quasi-dc (QDC) charge transport mechanism takes place above the glass transition temperature, and we show that the characteristic features of the dielectric response are related to the shape of the electrical treeing degradation and the transition to thermal breakdown. This is explained qualitatively through the effect of the bulk QDC charge transport process in modifying the local space charge electric field distribution.

The research progress of the medium voltage DC integrated power system in China

With the constant development of modern marine technologies, the vessel energy now can be distributed through an Integrated Power System, which is known as the third vessel power revolution. In this paper, the technical features of the first and second generation integrated power system are introduced. Next, based on the present domestic technical status, the research progress of the quasi second generation medium voltage DC integrated power system in China is presented, with the corresponding difficulties and problems analyzed, including the system model, electrical-magnetic transient simulation, the connected op-erator between the gas turbine generator set and the diesel generator set, the system steady analysis, and system layered protection. Finally, the solutions are proposed, which indicates that the development are re-quired on the medium voltage DC breaker, system energy storage, system safe operation, multi-time scale, and multi object system energy regulation.

Enhanced Electrorheological Performance of Nb-Doped TiO2 Microspheres Based Suspensions and Their Behavior Characteristics in Low-Frequency Dielectric Spectroscopy.

Titanium dioxide and Nb-doped titanium dioxide microspheres with the same size were fabricated by a simple sol-gel method, and the formation mechanism of Nb-doped titanium dioxide microspheres was proposed. Titanium dioxide and Nb-doped titanium dioxide microspheres were adopted as dispersed materials for electrorheological (ER) fluids to investigate the influence of the charge increase introduced by Nb doping on the ER activity. The results showed that Nb doping could effectively enhance the ER performance. Combining with the analysis of dielectric spectroscopy, it was found that the interface polarization of Nb-doped TiO2 ER fluid was larger than that of TiO2 ER fluid, which might be caused by more surface charges in Nb-TiO2 microspheres due to Nb(5+) doping and resulting in enhancement of electric field force and strengthening of fibrous structure. In addition, by comparing and analyzing the permittivity curves of Nb-TiO2/LDPE solid composite and Nb-TiO2/silicone-oil fluid composite, it could be concluded that the enhancement of permittivity at low frequency resulted from the increase of the order degree of dispersed particles in ER fluid rather than from the quasi-dc (QDC) behavior. Moreover, the absolute value of slope of permittivity curves (K) at 0.01 Hz could be utilized as the standard for judging the ability to maintain the chainlike structure. The relationships between polarizability of dispersed particles, dielectric spectrum, parameter K, and ER properties were discussed in detail.

Everything You Wish to Know About Memristors But Are Afraid to Ask

This paper classifies all memristors into three classes called Ideal, Generic, or Extended memristors. A subclass of Generic memristors is related to Ideal memristors via a one-to-one mathematical transformation, and is hence called Ideal Generic memristors. The concept of non-volatile memories is defined and clarified with illustrations. Several fundamental new concepts, including Continuum-memory memristor, POP (acronym for Power-Off Plot), DC V-I Plot, and Quasi DC V-I Plot, are rigorously defined and clarified with colorful illustrations. Among many colorful pictures the shoelace DC V-I Plot stands out as both stunning and illustrative. Even more impressive is that this bizarre shoelace plot has an exact analytical representation via 2 explicit functions of the state variable, derived by a novel parametric approach invented by the author.

Re-examination of the dielectric spectra of epoxy resins: bulk charge transport and interfacial polarization peaks

The dielectric properties of two amine cured bisphenol-A epoxy resin systems, Araldite CY1301 and Araldite CY1311 have been characterized using dielectric spectroscopy over the frequency range 1 mHz to 100 kHz. These two epoxy resin systems were chosen to allow the dielectric response to be studied from above and below the glass transition, as Araldite CY1311 is a modified version (with added plasticizer) of Araldite CY1301. The dielectric response was found to comprise both bulk and interfacial features. Above the glass transition temperature, two processes were identified, a low frequency process usually ascribed to interfacial polarization is shown to be a bulk process termed Quasi-DC (QDC) conduction and a dielectric dispersion usually taken to be a bulk process has been shown to be an interfacial effect.

The HITRAP project at GSI: trapping and cooling of highly-charged ions in a Penning trap

A decelerator will be installed at GSI in order to provide and study heavy nuclei without or with only few electrons at very low energies or even at rest. Highly-charged ions will be produced by stripping at relativistic energies. After electron cooling and deceleration in the Experimental Storage Ring (ESR) the ions are ejected out of the storage ring at 4 MeV/u and further decelerated in a combination of linear accelerator structures operated in reverse. Finally, they are injected into a Penning trap where the ions are cooled to 4 K by electron cooling in combination with resistive cooling. From here, the ions can be transferred in a quasi DC or in a pulsed mode to different experimental setups. This article describes the technical concepts of this project focused on the Penning trap.

Vortex melting and decoupling transitions in YBa 2Cu 4O 8 single crystals

The vortex correlation along the c-axis of Y124 single crystals has been investigated by using the quasi DC flux transformer configuration. A simultaneous sharp drop associated with the vortex lattice melting is observed in both the primary and secondary voltages. Our results suggest that the vortices just above the melting temperature are three-dimensional line liquid with its correlation length along the c direction l c ≤ t, the sample thickness. At a temperature T d, the vortices lose their correlation along the c direction and dissolve into 2D pancake vortices. An H-T phase diagram for the Y124 single crystal is obtained.

Modeling of dielectric relaxation spectra of polymers in the condensed phase

Discusses models and their possible implication in the interpretation of the relaxation spectra of polymers, particularly involving the concept of order/disorder systems. Furthermore, some examples of the application of the Dissado-Hill model have been provided in this work to explain quasi-dc (QDC) conduction processes, which have been observed in crosslinked polyethylene (XLPE) and low density polyethylene (LDPE), showing an ageing behavior with continuing use with an ac field in a humid environment. It is also shown in this paper how a measurement of dielectric relaxation properties on a wide frequency range may be used as a tool to investigate the ageing of polymeric power distribution cables with an ac field in a humid environment. Equivalent electrical analog circuits of dielectric relaxation spectra are also shown.

Critical currents in polycrystalline (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 systems: a comparative magnetisation study

We report on the quasi DC magnetisation of various polycrystalline (Bi,Pb) 2Sr 2C 2Cu 3O 10 systems. Using a vibrating sample magnetometer the magnetic response of PIT tapes, hot rolled bars and sintered pellets was measured as a function of temperature and electric field. Comparing measurements on both bulk and powdered samples, we can distinguish between the inter- and intra-granular contribution to the magnetisation. Differences and similarities in the extracted critical current density and magnetic relaxation rate of these systems are discussed in terms of grain-to-grain connectivity and intra-granular pinning properties.

Behavior of high-voltage gaps in the presence of large space-charge fields

It is shown in this paper that a high-voltage gap exhibits a nonlinear capacitance behavior in the presence of large space-charge fields. The existence of this nonlinear capacitor is quasi dc in nature and does not result from transit time effects, i.e., high-frequency effects. This nonlinear behavior can be used to tailor the shape of bunched intense relativistic electron beams.

Production of a dense low energy positive hydrogen ion beam

We present measurements and computer calculations concerning the generation of a dense, quasi dc (10 s) positive hydrogen ion beam of energies between 1 and 5 keV. Maximum beam current densities ranging from 200 to 230 mA cm −2 have been transported through a three-electrode extraction-deceleration system with divergencies between 9.5 and 3.5°. The measurements are compared to trajectory calculations, carried out with a modified version of the SLAC code. The highest transmission for a low energy beam with a small divergence is obtained when the calculated beam trajectories exhibit a cross-over in the region between the extraction and the decel electrode. This cross-over can be achieved when the extraction slit has a shaping angle (45°) smaller than the Pierce angle (67.5°).