Showing papers on "Insulator (electricity) published in 2019"

A Highly Stretchable Liquid Metal Polymer as Reversible Transitional Insulator and Conductor

Abstract: Materials with a temperature-controlled reversible electrical transition between insulator and conductor are attracting huge attention due to their promising applications in many fields. However, most of them are intrinsically rigid and require complicated fabrication processes. Here, a highly stretchable (680% strain) liquid metal polymer composite as a reversible transitional insulator and conductor (TIC), which is accompanied with huge resistivity changes (more than 4 × 109 times) reversibly through a tuning temperature in a few seconds is introduced. When frozen, the insulated TIC becomes conductive and recovers after warming. Both the phase change of the liquid metal droplets and the rigidity change of the polymer contribute directly to transition between insulator and conductor. A simplified model is established to predict the expansion and connection of liquid metal droplets. Along with high stretchability, straightforward fabrication methods, rapid triggering time, large switching ratio, good repeatability, the TIC offers tremendous possibilities for numerous applications, like stretchable switches, semiconductors, temperature sensors, and resistive random-access memory. Accordingly, a system that can display numbers and letters via converting alternative TIC temperature to a binary signal on a computer is conceived and demonstrated. The present discovery suggests a general strategy for fabricating and stimulating a stretchable transitional insulator and conductor based on liquid metal and allied polymers.

Ultrathin calcium fluoride insulators for two-dimensional field-effect transistors

Abstract: Two-dimensional semiconductors could be used to fabricate ultimately scaled field-effect transistors and more-than-Moore nanoelectronic devices. However, these targets cannot be reached without appropriate gate insulators that are scalable to the nanometre range. Typically used oxides such as SiO2, Al2O3 and HfO2 are, however, amorphous when scaled, and 2D hexagonal boron nitride exhibits excessive gate leakage currents. Here, we show that epitaxial calcium fluoride (CaF2), which can form a quasi van der Waals interface with 2D semiconductors, can serve as an ultrathin gate insulator for 2D devices. We fabricate scalable bilayer MoS2 field-effect transistors with a crystalline CaF2 insulator of ~2 nm thickness, which corresponds to an equivalent oxide thickness of less than 1 nm. Our devices exhibit low leakage currents and competitive device performance characteristics, including subthreshold swings down to 90 mV dec−1, on/off current ratios up to 107 and a small hysteresis. High-performance MoS2 transistors can be created using 2-nm-thick CaF2 as a gate insulator, which forms a quasi van der Waals interface with the 2D semiconductor.

Resonant Wireless Charging System Design for 110-kV High-Voltage Transmission Line Monitoring Equipment

Abstract: A novel high-voltage operation featured wireless power transfer system for monitoring equipment charging on a 110-kV high-voltage transmission line based on magnetic resonant coupling is studied and designed in this paper. With consideration of operation environment on transmission line and electrical transmission tower, an overall scheme is proposed through installation position, coupling structure, and driving topology design. In order to improve the system's suitability in high-voltage environment, related optimization methods including constraining power flow path, improving quality factor, and coupling have been adopted. Since the coupling coils are fixed at both ends of an insulator string, a barrel-shaped high-permeability material layer is added to constrain the magnetic field distribution. Moreover, cross impacts between transmission line and the charging system are analyzed. The influence of a power frequency magnetic field on the charging system is calculated and electric distribution of the insulator string with the charging system is simulated. Analysis results indicate that cross impacts can be negligible. Experimental results verify that the transmission power can meet power supply requirements and the designed charging system can be operated stably under the high-voltage condition.

Evidence for an axionic charge density wave in the Weyl semimetal (TaSe4)2I

Abstract: An axion insulator is a correlated topological phase, predicted to arise from the formation of a charge density wave in a Weyl semimetal. The accompanying sliding mode in the charge density wave phase, the phason, is an axion. It is expected to cause anomalous magneto-electric transport effects. However, this axionic charge density wave has so far eluded experimental detection. In this paper, we report the observation of a large, positive contribution to the magneto-conductance in the sliding mode of the charge density wave Weyl semimetal (TaSe4)2I for collinear electric and magnetic fields (E||B). The positive contribution to the magneto-conductance originates from the anomalous axionic contribution of the chiral anomaly to the phason current, and is locked to the parallel alignment of E and B. By rotating B, we show that the angular dependence of the magneto-conductance is consistent with the anomalous transport of an axionic charge density wave.

3D printing fabrication of conductivity non-uniform insulator for surface flashover mitigation

Abstract: Solid insulators applying functionally graded material (d-FGM) have spatially non-uniform dielectric properties. The d-FGM insulator is effective on insulation performance improvement without complicating the structure; however, the fabrication of such insulators remains a challenge. To investigate the feasibility of 3D printing technology on d-FGM, we designed and fabricated a conductivity non-uniform insulator and tested its surface flashover characteristics. First, a modified genetic algorithm is employed to design the conductivity distribution in the non-uniform insulator. The designed insulator is then fabricated using a 3D printing process named fused deposition modeling (FDM), in which we verified the applicability of 3D printing materials on electrical insulation. Finally, compared to the uniform insulator, the surface flashover voltages of non-uniform insulators were improved by 23% in SF6 and 20% in vacuum. From above, we envision potential application feasibility for 3D printing of d-FGM in practical insulation design.

Topology optimization of truncated cone insulator with graded permittivity using variable density method

Abstract: Dielectrics with functionally graded material (d-FGM), which have spatially nonuniform dielectric properties, is effective to improve the insulation performance without complicating the structure. In the application of d-FGM, searching the optimal distribution of dielectric properties (permittivity or conductivity) is the most important goal, which however is difficult for conventional insulation design approaches. In this paper, the topology optimization technique is introduced to design a truncated cone insulator with permittivity FGM, which is to relieve local electric field (E-field) enhancement at the triple junctions. Firstly, a multi-objective topology optimization model is proposed using the variable density method. Secondly, the optimal values of several model parameters are determined based on the analysis of E-field uniformity. Finally, finite elements calculation demonstrates that the E-field uniformity is significantly improved for the topology-optimized permittivity FGM, and the E-field enhancement at triple junctions is strongly inhibited. It is thus expected that by applying the topology-optimized permittivity FGM insulator, the breakdown strength in gas or vacuum would be effectively improved.

Mechanical stress distribution and risk assessment of 110 kV GIS insulator considering Al2O3 settlement

Abstract: The growing application of gas insulated switchgear (GIS) in electric power system asks for higher reliability of epoxy (EP) insulators, thus it is important to seek effective methods to improve the operating reliability of GIS insulators. On the basis of the 110 kV GIS insulator, this study studied the aluminium oxide (Al2O3) settlement in the curing process and the stress distribution of the horizontally laid GIS. Besides, the risk coefficient was defined to evaluate the operating reliability of insulators laid by different methods (I–V). Some conclusions are shown below: the Al2O3 settlement in the curing process causes the insulator density to have a large variation from 2.13 to 2.32 g/cm3. The Al2O3 content has a great influence on the mechanical performances of EP/Al2O3system. The first principle stress is localised on the upper interface between the insulator and the conductor, which is the main threat to the mechanical damage of insulator. The laying method V appears to be optimal with a pretty low risk coefficient of 31.5. In actual application, for the horizontally laid GIS, the insulator should be laid with the mould gate downward to improve its mechanical reliability.

Topological Anderson insulator in electric circuits

Abstract: In this paper, we investigate the realization of topological Anderson insulators in electric circuits. A disordered Haldane model is constructed through electric circuit networks composed of capacitors and inductors, where the disorder is introduced through the random induction of the grounding inductors. Based on the noncommutative geometry method and transport calculations, we confirm that such kind of disorder can drive a phase transition from a normal insulator to a topological Anderson insulator. Besides, such a disorder also possesses unique characteristics which are absent for the usual Anderson disorder. Therefore distinct features are exhibited by the topological Anderson transition in electric circuits. Finally, the topological Anderson insulator in circuits holds additional advantages for microelectronic technology that can be easily detected by measuring the quantized transmission coefficients and the edge state wave functions.

Semitransparent Polymer-Based Solar Cells with Aluminum-Doped Zinc Oxide Electrodes

Abstract: With the usage of two transparent electrodes, organic solar cells are semitransparent and may be combined to parallel-connected multi-junction devices or used for innovative applications like power-generating windows. A challenging issue is the optimization of the electrodes, in order to combine high transparency with adequate electric properties. In the present work, we study the potential of sputter-deposited aluminum-doped zinc oxide (AZO) as an alternative to the widely used but relatively expensive indium tin oxide (ITO) as cathode material in semitransparent polymer-fullerene solar cells. Concerning the anode, we utilized an insulator/metal/insulator structure based on ultra-thin Au films embedded between two evaporated MoO$_3$ layers, with the outer MoO$_3$ film (capping layer) serving as a light coupling layer. The performance of the ITO-free semitransparent solar cells is systematically studied as dependent on the thickness of the capping layer and the active layer, as well as the illumination direction. These variations are found to have strong impact on the obtained photocurrent. We performed optical simulations of the electric field distribution within the devices to analyze the origin of the current variations and provide deep insight in the device physics. With the conventional absorber materials studied herein, optimized ITO-free and semitransparent devices reached 2.0% power conversion efficiency and a maximum optical transmission of 60%, with the device concept being potentially transferable to other absorber materials.

Novel insulator with interfacial σ-FGM for DC compact gaseous insulated pipeline

Abstract: Electric field optimization plays a significant role in the insulation design of gaseous insulated pipeline (GIL) for the DC power system. Functionally graded material (FGM) with spatial distribution of conductivity σ (σ-FGM) provides a potential method to uniform the electric field and thus reduces flashovers to some extent. However, researches at present are nearly at a standstill for the complicated fabrication techniques and huge bulk conduction losses. In this paper, a novel cone-type insulator with surface conductivity gradient was proposed based on direct fluorination treatment. Theoretical analysis shows that the electric field distortion near the high voltage electrode can be effectively suppressed with increasing the thickness and conductivity of the fluorinated layer. Epoxy cone-type insulators with homogeneous and graded surface conductivity were then prepared by different fluorination treatments to verify the effectiveness of σ-FGM design. As for the homogeneously fluorinated cone-type insulators, the DC flashover characteristics were improved with the rise of fluorination time. Especially, the flashover voltage of the insulators with σ-FGM layers can be raised by up to 36.3 % compared with that of the traditional ones, owing to a more uniform electric field distribution. Theoretical and experimental results prove that the insulator with surface σ-FGM is a promising choice for DC compact GIL.

Tunable Dynamic Black Phosphorus/Insulator/Si Heterojunction Direct-Current Generator Based on the Hot Electron Transport

Abstract: Static heterojunction-based electronic devices have been widely applied because carrier dynamic processes between semiconductors can be designed through band gap engineering. Herein, we demonstrate a tunable direct-current generator based on the dynamic heterojunction, whose mechanism is based on breaking the symmetry of drift and diffusion currents and rebounding hot carrier transport in dynamic heterojunctions. Furthermore, the output voltage can be delicately adjusted and enhanced with the interface energy level engineering of inserting dielectric layers. Under the ultrahigh interface electric field, hot electrons will still transfer across the interface through the tunneling and hopping effect. In particular, the intrinsic anisotropy of black phosphorus arising from the lattice structure produces extraordinary electronic, transport, and mechanical properties exploited in our dynamic heterojunction generator. Herein, the voltage of 6.1 V, current density of 124.0 A/m2, power density of 201.0 W/m2, and energy-conversion efficiency of 31.4% have been achieved based on the dynamic black phosphorus/AlN/Si heterojunction, which can be used to directly and synchronously light up light-emitting diodes. This direct-current generator has the potential to convert ubiquitous mechanical energy into electric energy and is a promising candidate for novel portable and miniaturized power sources in the in situ energy acquisition field.

Joule heating effects in optimized insulator-based dielectrophoretic devices: An interplay between post geometry and temperature rise.

Abstract: Insulator-based dielectrophoresis (iDEP) is the electrokinetic migration of polarized particles when subjected to a non-uniform electric field generated by the inclusion of insulating structures between two remote electrodes. Electrode spacing is considerable in iDEP systems when compared to electrode-based DEP systems, therefore, iDEP systems require high voltages to achieve efficient particle manipulation. A consequence of this is the temperature increase within the channel due to Joule heating effects, which, in some cases, can be detrimental when manipulating biological samples. This work presents an experimental and modeling study on the increase in temperature inside iDEP devices. For this, we studied seven distinct channel designs that mainly differ from each other in their post array characteristics: post shape, post size and spacing between posts. Experimental results obtained using a custom-built copper Resistance Temperature Detector, based on resistance changes, show that the influence of the insulators produces a difference in temperature rise of approximately 4°C between the designs studied. Furthermore, a 3D COMSOL model is also introduced to evaluate heat generation and dissipation, which is in good agreement with the experiments. The model allowed relating the difference in average temperature for the geometries under study to the electric resistance posed by the post array in each design.

Classification of common discharges in outdoor insulation using acoustic signals and artificial neural network

Abstract: Condition monitoring of outdoor insulation systems is crucial to the integrity of distribution and transmission overhead lines and substations. The objective of this study is to use a commercial acoustic sensor along with artificial neural network (ANN), to classify different typical types of discharges in outdoor insulation systems. First, ANN was used to distinguish between five common electrical discharges that were generated under controlled conditions. Next, this approach was extended to include outdoor ceramic insulators. Three types of defects were tested under laboratory conditions, i.e. a crack in the ceramic disc, surface pollution discharge, and corona near the insulator surface. Both a single disc, and three discs connected in an insulator string were tested with respect to these defects. For both controlled samples and full insulators, a recognition rate of more than 85% was achieved.

Surface charge distribution on DC basin-type insulator

Abstract: Surface charge accumulation on insulators is one of the bottlenecks for the development of DC electric power system. This paper studies a ±200 kV DC basin-type insulator designed for GIL (Gas-insulated Transmission Line). A surface charge measurement device is made consisting of a 3-dimension, 4-axes manipulating device, a HV shielding conductor installation system and data acquisition system. A new scaling method of the probe with an inversion algorithm using Cholesky decomposition method is proposed to obtain charge density distribution. The dynamic process of surface charge accumulation and dissipation are studied under DC voltages with different polarities, amplitudes and time durations. Charge distributions are obtained and theoretically analyzed. A novel gas-side normal electric field model is proposed for regular charge distribution and methods of suppressing surface charge are suggested, including a recommended ratio of surface and volume electric conductivity of the insulator at 1∼10, structure design, and direct fluorination of the surface.

Reliability of scalable MoS2 FETs with 2 nm crystalline CaF2 insulators

Abstract: Two-dimensional (2D) semiconductors are currently considered a very promising alternative to Si for channel applications in next-generation field-effect transistors of sub-5 nm designs. However, their huge potential cannot be fully exploited owing to a lack of competitive insulators which are required to effectively separate the channel from the gate, while being scalable down to few nanometers thicknesses. Recently we have made an attempt at addressing this issue by using crystalline CaF2 insulators and demonstrated competitive MoS2 devices with the insulator thickness of only about 2 nm. Here we report a detailed study of the performance, reliability and thermal stability of these devices. We demonstrate that, in contrast to SiO2 and other amorphous oxides, CaF2 has a very low density of insulator defects which are responsible for the hysteresis and long-term drifts of the transistor characteristics. At the same time, CaF2 exhibits smaller leakage currents and higher electric stability compared to hBN. By comparing our MoS2 transistors with CaF2 fabricated using MoS2 films of different quality, we show that the major limitations on the performance and reliability of these devices come from the bare channel rather than from the superior CaF2 insulator. Finally, we perform the first study of degradation mechanisms only observed for tunnel-thin gate insulators in 2D devices. While these degradation mechanisms are similar for hBN and CaF2, they are less pronounced in CaF2. Our results therefore present a solution to a very important roadblock on the way towards fully scalable 2D nanoelectronics with competitive performance and reliability.

Surface coating affecting charge distribution and flashover voltage of cone-type insulator under DC stress

Abstract: In order to scale down the dimensions of gas insulated line (GIL) insulators and to improve their reliability at ultra high voltage (UHV), considerable research has been conducted to raise the flashover voltage of the insulators. Usually, a non-uniform electric field distribution is an important factor triggering flashovers. Under AC and impulse voltages, the application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity (e-FGM) has been shown to be an effective solution. But at DC, the electric field distribution depends not only on the conductivity of the dielectrics but also the charge distribution on the surface. Based on the cone-type insulator, this paper reports on the surface charge accumulation and DC flashover of epoxy (EP)/graphene (GR) coated insulators in air. The EP/GR coated insulators with different filler amounts were prepared to test their surface charge accumulation and flashover characteristics. The results show that the uniformly distributed homopolar surface charge is helpful in reducing the peak electric field thereby raising the flashover voltage. The 0.1% EP/GR composite has the slowest surface potential decay process. Accordingly, the 0.1% coated insulator has the highest flashover voltage among the other EP/GR coated insulators and uncoated insulator. The bipolar surface charge regions caused by wire-type metal particles distort the electric field distribution. Flashovers of the 0.1% coated insulator occurs at lower voltage than the uncoated insulator when a wire-type particle attaches to the surface.

Metal particle movement and distribution characteristics under AC voltage and ball-plane electrodes

Abstract: Metal particles, difficult to be eliminated in gas-insulated metal-enclosed switchgear (GIS), can cause GIS discharge and breakdown between electrodes, or flashover on the insulator surface. It influences the development of ultra-high voltage (UHV) projects. Therefore, the work optimised the model of metal particle movement under AC voltage, studying the metal particle movement and distribution characteristics between ball-plane electrodes through experiment and simulation. Under AC voltage, the particle jumps on a small scale on the plane surface. With the increase of voltage, the jump amplitude increases. However, the collision frequency decreases until the particle collides with the ball electrode. When the initial phase angle of power changes, the particle-moving pattern is symmetrical in the angle ranging from 0 to 180°, and from 180 to 360°. The collision frequency changes slightly with the increase of jump amplitude when the angel ranges from 0 to 120°.

Quantum Oscillations of Electrical Resistivity in an Insulator

Abstract: In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here we report a notable exception in an insulator, ytterbium dodecaboride (YbB12). Despite much larger than that of metals, the resistivity of YbB12 exhibits profound quantum oscillations. This unconventional oscillation is shown to arise from the insulating bulk, yet the temperature dependence of their amplitude follows the conventional Fermi liquid theory of metals. The large effective masses indicate the presence of Fermi surface consisting of strongly correlated electrons. Our result reveals a mysterious bipartite ground state of YbB12: it is both a charge insulator and a strongly correlated metal.

Optical conductivity of black phosphorus with a tunable electronic structure

Abstract: Black phosphorus (BP) is a two-dimensional layered material composed of phosphorus atoms. Recently, it was demonstrated that external perturbations such as an electric field close the band gap in few-layer BP, and can even induce a band inversion, resulting in an insulator phase with a finite energy gap or a Dirac semimetal phase characterized by two separate Dirac nodes. At the transition between the two phases, a semi-Dirac state appears in which energy disperses linearly along one direction and quadratically along the other. In this work, we study the optical conductivity of few-layer BP using a lattice model and the corresponding continuum model, incorporating the effects of an external electric field and finite temperature. We find that the low-frequency optical conductivity scales a power law that differs depending on the phase, which can be utilized as an experimental signature of few-layer BP in different phases. We also systematically analyze the evolution of the material parameters as the electric field increases, and the consequence on the power-law behavior of the optical conductivity.

Verification of Charge Transfer in Metal-Insulator-Oxide Semiconductor Diodes via Defect Engineering of Insulator.

Abstract: In a MIS (Metal/Insulator/Semiconductor) structure consisting of two terminals, a systematic analysis of the electrical charge transport mechanism through an insulator is essential for advanced electronic application devices such as next-generation memories based on resistance differences. Herein, we have verified the charge transfer phenomenon in MIOS (Metal/Insulator/Oxide Semiconductor) diodes through a defect engineering of the insulator. By selectively generating the oxygen vacancies in the insulator (Al2O3), the MIOS diode rectification of the P++-Si anode/Al2O3/IGZO cathode reached 107 at 1.8 V and considerably suppressed the leakage current. Studying the current-voltage characteristics of MIOS diodes shows that the charge carrier transport mechanism can vary depending on the defect density as well as the difference between the CBM (conduction band minimum) of the semiconductor and the oxygen vacancy energy level of the insulator.

Influence of pollution chemical components on AC flashover performance of various types of insulators

Abstract: The pollution on insulators is composed of various chemical components, which threaten the stability of external insulation. However, the existing insulation designs have not taken this issue into consideration. This work aimed to study the influence of pollution chemical components on flashover performance of insulators with various materials. Flashover tests of porcelain, glass and silicone rubber insulators polluted by eight types of common contaminants were conducted. Based on the test data, the influence of chemical components and their solubility on equivalent salt deposit density (ESDD) and insulator flashover performance were analysed; and the electrical properties of different types of insulators were compared. Results show that the ESDD contribution rate changes with chemical components. The dissolution characteristic has a significant influence on the measurement of ESDD; slightly soluble contaminants need to be considered separately. The corresponding insulator flashover gradient for ions can be summarised as Cl− < NO3 − < SO4 2−, K+ < Na+, NH4 + < Ca2+. For a glass insulator, the flashover gradient is 4.9–17.0% higher than that of a porcelain insulator. For a silicone rubber insulator, this gap reaches 14.6–24.4 and 83.3–90% for freely soluble pollution and CaSO4, respectively, this phenomenon can be explained by the combined action of the slightly soluble character of CaSO4 and hydrophobicity transference of silicone rubber.

Surface flashover characteristics of epoxy insulator in C 4 F 7 N/CO 2 mixtures in a uniform field under AC voltage

Abstract: This paper reports on an investigation into the surface flashover of an insulator in a C4F7N/CO2 mixture which is used as an alternative to SF6. To date, no studies on this aspect have been reported in the literature. This study was done for a uniform field using a 220 kV AC experimental platform. The influence of molar fraction, gas pressure and creepage distance is reported and compared to that in SF6. The results show that small quantities of C4F7N mixed in CO2 can increase the surface flashover voltage but, as the fraction of C4F7N increases, saturation of the surface flashover occurs. The relative surface flashover voltage of 13.0% C4F7N/87.0% CO2 is approximately 80% that of SF6. The empirical formulae on the critical flashover electrical field has been obtained, which may be referenced in the design of environment friendly high voltage equipment.

Interfacial E-field self-regulating insulator considered for DC GIL application

Abstract: The non-uniform electric field (E-field) distribution is typically considered a key factor in triggering surface flashover inside gas insulated lines (GILs). To improve the E-field distribution along GIL insulators, this study proposes the concept of an interfacial E-field self-regulating (IER) insulator. The capability of the IER insulator with a resistivity-nonlinear (ρ-nonlinear) skin layer to regulate the E-field distribution is validated via theoretical analysis. In this paper, cone-type insulators are coated with epoxy (EP)/SiC composites to fabricate EP/SiC coated IER insulators. Next, an electrical simulation model is built to calculate the E-field distributions and energy losses of the insulators. As the coating thickness or SiC content increases, the E-field regulation function of the EP/SiC coated insulator becomes increasingly significant with increasing leakage current and energy loss. Under DC voltage, nearly 24 hours is required for the E-field distribution along the conventional insulator to reach the steady state. When the SiC content in the coating layer is increased, the transient time of the EP/SiC coated insulators decreases because of the reduced volume resistivity. Furthermore, coating thickness measurements and DC flashover tests are conducted. The coating thickness of the EP/SiC coated insulator is found to increase with increases in the SiC content because of the higher viscosity of the liquid-state coating. The DC flashover voltage of the EP/SiC coated insulator is found to be higher than that of the conventional insulator, with the flashover voltage increasing with the SiC content in the coating layer.