Low Impedance Characteristics Research Articles

Synergistically magnetic and dielectric properties of two dimensional Fe3Al@PPy lamellae exhibiting broadband and strong electromagnetic wave absorption

Addressing the issue of low impedance characteristics is essential to improve the electromagnetic wave absorption performance of magnetic materials. Herein, two dimensional Fe3Al@polypyrrole (PPy) lamellae with synergistic magnetic and dielectric properties are fabricated by a mechanical alloying, ordering transformation and polymerization process, which exhibits excellent electromagnetic wave absorption performance. By carefully controlling the thickness of the PPy shell, the optimized Fe3Al@PPy lamellae show a minimum reflection loss of −45.6 dB and an effective absorption bandwidth of 9.1 GHz at a thickness of only 1.5 mm. The conformal growth of Fe3Al@PPy lamellae can induce strong interfacial polarization, dipole polarization, multiple scattering effect and magnetic loss behaviors for the attenuation of electromagnetic waves. This study demonstrates a facile strategy for the development of efficient Fe3Al@PPy composite absorbents showing great potential for practical applications.

Estimation of the Transverse Wave Velocity in Siliceous Carbonate Reservoirs of the Dengying Formation in the Gaoshiti–Moxi Area, Sichuan Basin, China

Siliceous minerals of the Dengying Formation in the Gaoshiti–Moxi area in the central Sichuan Basin exhibit four types of quartz crystals (cryptocrystalline quartz, chalcedony, microcrystalline quartz, and megacrystalline quartz) and three structural types: cryptocrystalline, microcrystalline, and mosaic (laminated mosaic, window-hole interrupted mosaic, and arc-laminated mosaic). Siliceous minerals have a great influence on the storage performance of the reservoirs in the Dengying Formation. According to the petrophysical parameters of the Dengying Formation and porosity intersection diagrams, the siliceous dolomite and the reservoirs have low impedance characteristics, which makes it difficult to distinguish between them and leads to difficulties in the characterization and prediction of the reservoirs. The transverse wave velocity is favorable for reservoir characterization. Currently, the main method used to estimate the transverse wave velocity is petrophysical modeling, which establishes a relationship between the elastic and physical parameters of the reservoir. In this paper, the siliceous minerals in the dolomite in the study area are regarded as solid inclusions, and the calculation method of the rock matrix modulus is improved by using solid replacement. Then, an improved petrophysical model is constructed by combining the KT (Kuster–Toksöz) model, the DEM (Discrete Element Method) model, the Gassmann equation, and the Wood equation. The transverse wave velocity is estimated using the improved model under the constraint of the longitudinal wave velocity. The shapes of the transverse wave velocity curves obtained by the improved model and the deviations from the measured velocities are significantly better than those of the Xu–Payne model and other models. The results show that the improved model can effectively estimate the transverse wave velocity of the reservoir in this area, which provides a basis for future reservoir predictions in this area.

Wireless, Fully Implantable and Expandable Electronic System for Bidirectional Electrical Neuromodulation of the Urinary Bladder.

Current standard clinical options for patients with detrusor underactivity (DUA) or underactive bladder─the inability to release urine naturally─include the use of medications, voiding techniques, and intermittent catheterization, for which the patient inserts a tube directly into the urethra to eliminate urine. Although those are life-saving techniques, there are still unfavorable side effects, including urinary tract infection (UTI), urethritis, irritation, and discomfort. Here, we report a wireless, fully implantable, and expandable electronic complex that enables elaborate management of abnormal bladder function via seamless integrations with the urinary bladder. Such electronics can not only record multiple physiological parameters simultaneously but also provide direct electrical stimulation based on a feedback control system. Uniform distribution of multiple stimulation electrodes via mesh-type geometry realizes low-impedance characteristics, which improves voiding/urination efficiency at the desired times. In vivo evaluations using live, free-moving animal models demonstrate system-level functionality.

600-GHz high-temperature superconducting sub-harmonic mixer coupled using a double-Y-type slot integrated lens antenna

In this paper, we present a quasi-optically coupled 600-GHz high-temperature superconducting (HTS) sub-harmonic mixer for communication and sensing applications. The mixer features an innovative double-Y-type slot integrated lens antenna, which can efficiently couple the radio frequency (RF) and local oscillator (LO) signals with a small frequency ratio by exciting the half-wave and full-wave resonant current modes on the slot, respectively. Considering the low impedance characteristics of HTS Josephson junctions, a coplanar-waveguide stepped impedance transformer is utilized for minimizing the mismatching loss. A cascaded filter network is designed to prevent the high-frequency signal leakage at both bands while coupling the intermediate-frequency (IF) signal output efficiently. Based on this antenna design and an established HTS step-edge junction technology, a 600-GHz mixer prototype was designed, fabricated and measured, which was compared with the simulation results. The achieved conversion gain and noise temperature are the best performance specs as reported to date for HTS harmonic mixers at comparable frequencies and operating temperatures.

A Fault Current Limiting Hybrid DC Circuit Breaker

Due to the low impedance characteristic of the high voltage direct current (HVDC) grid, the fault current rises extremely fast after a DC-side fault occurs, and this phenomenon seriously endangers the safety of the HVDC grid. In order to suppress the rising speed of the fault current and reduce the current interruption requirements of the main breaker (MB), a fault current limiting hybrid DC circuit breaker (FCL-HCB) has been proposed in this paper, and it has the capability of bidirectional fault current limiting and fault current interruption. After the occurrence of the overcurrent in the HVDC grid, the current limiting circuit (CLC) of FCL-HCB is put into operation immediately, and whether the protected line is cut off or resumed to normal operation is decided according to the fault detection result. Compared with the traditional hybrid DC circuit breaker (HCB), the required number of semiconductor switches and the peak value of fault current after fault occurs are greatly reduced by adopting the proposed device. Extensive simulations also verify the effectiveness of the proposed FCL-HCB.

Impedance Control Considering Velocity Saturation of a Series Elasticity System with a Motor

Human-machine cooperative robots are required to drive their arms with low impedance and high torque. As a compact mechanism that generates a large torque and has low impedance characteristics, the series elastic drive system, in which an elastic element is inserted between the motor and driving unit, has been proposed. In this paper, we propose a method of applying impedance control to a series elasticity system with a torque-compensating motor that uses a torsion bar as an elastic body that enables its use under high loads. The stability of the system was verified via simulation and experiment by considering the allowable speed and maximum torque of the motor. The experimental results from the conventional system and the proposed system were compared. The proposed system was confirmed to be superior to the conventional system in terms of both stability and tracking performance. Consequently, the effectiveness of our proposed system was confirmed.

Synthesis of ZnS Nanorods Coated by MoS2/N-Doped Carbon Nanosheets with Enhanced Sodium Storage Properties

In order to obtain an excellent ZnS-based anode material for sodium-ion batteries (SIBs), a designed hierarchical nanostructure formed by ZnS hollow nanorods and uniform nanosheets was synthesized via hydrothermal method in the presence of carboxymethyl chitosan. The nanosheets were composed of amorphous nitrogen doped carbon and molybdenum disulfide (MoS2-NC). Notably, the one-dimensional tubular framework possessed low impedance characteristics. Through the way of combining it with nanosheets which had abundant defects, it could boost the charge transfer and improve the sodium storage performance effectively. When tested as the anode material of SIBs, this ZnS@MoS2-NC composite exhibited excellent cycling and rate performance. Moreover, it was further assembled into sodium-ion full batteries and showed good cyclic stability. This work provides a valuable option for preparing promising SIBs anode materials by combining structural design with multi-component coordination, which can be extended to other metal sulfide electrode materials.

Improved measurement techniques for high-power transistor modeling.

Measurement for modeling of the high-power transistors is difficult due to its high-power and low-impedance characteristics. In this paper, novel methods and devices were designed and applied to achieve precise measurements of the high-power transistors. Fixtures capable of withstanding high voltage and current were designed to replace traditional radio frequency (RF) probes for higher power capacity. To reduce the impact of capacitive and inductive components of traditional bias tees on the rising/falling edge, two wideband 90° hybrid couplers that were connected back-to-back were designed for pulsed measurements. The measurement system of stable S-parameters with the Vector Network Analyzer (VNA) was reported, which could protect the devices and laboratory equipment from damage of self-oscillation. Application of several innovative approaches enabled accurate I-V characteristic and S-parameters measurements of high-power transistors in DC or pulsed mode. Experimental results of a 30 W gallium nitride high-electron-mobility transistor verified the validity.

Significantly enhanced lithium storage by in situ grown CoS2@MoS2 core–shell nanorods anchored on carbon cloth

MoS 2 becomes one of the ideal anode materials for lithium-ion batteries (LIBs) because of its unique two-dimensional layered structure and high theoretical capacity. However, the poor conductivity and volume change during reaction hinder its application. In this work, we demonstrate a two-step hydrothermal method to synthesis the unique structure of core–shell CoS 2 @MoS 2 nanorods anchored on carbon cloth (CC). The one-dimensional Co(OH) 2 nanowire array on CC forms a carbon-based framework, which increases the reaction surface area for subsequent in situ generation of MoS 2 . The MoS 2 layer with rich holes enhances lithium storage capacity and increases the active sites. Compared with pure MoS 2 electrode, CoS 2 @MoS 2 /CC (CMCC) exhibits a stronger electrochemical performance as a binder-free electrode for LIBs. The structure has a high discharge specific capacity of 1175 mA h g −1 at the current density of 0.1 A g −1 . The further experiments prove that the CMCC structure has outstanding ion transport ability and low impedance characteristics, which are attributed to its special structure and the introduction of Co ions. The present work provides a new idea for the design of stable flexible electrode with high electrochemical performance for lithium-ion batteries. • The core–shell structure grown in situ is formed by constructing a precursor template. • The core–shell structure is synthesized by one-step hydrothermal method, which effectively improves the reaction rate. • The porous structure on the surface effectively prevents the accumulation of MoS 2 . • The synergistic effect of the structure significantly improves the electrical properties and stability of the material.

Lithium-Ion Capacitor with Three-Dimensional Porous HAC/SP/PVDF as Positive Electrode

Activated carbon (AC) was acidified with dilute sulfuric acid (H2SO4) as an acidifying agent to obtain acidified activated carbon (HAC). The positive electrode was prepared by mixing HAC with a conductive agent (SP) or polyvinylidene fluoride. Carbon nanotubes and mesocarbon microbeads, as negative materials, were prelithiated and used as the negative eletrode. The positive electrode and negative electrode were assembled into the lithium ion capacitor. Materials and electrodes were characterized by scanning electron microscopy; elemental analysis, chemical bond analysis and specific surface area analysis of acidified activated carbon were made by x-ray energy spectrum analysis, an intelligent Fourier transform infrared spectrometer and specific surface space as well as a porosimetry analyzer (BET); The electrochemical capability of lithium ion capacitors was tested by constant current charge and discharge as well as electrochemical impedance. From the results, it can be concluded that the activated carbon acidified successfully grafted with hydroxyl (OH) and carboxyl (COOH) functional groups has increased the surface area by about 60% compared with AC. Under this situation, the charge and discharge current density is 50 mA/g. Also the capacitor has a mass ratio of 40.17 F/g, whose maximum power density is 0.98 kW/kg (700 mA/g) and maximum energy density is 52.34 Wh/kg (50 mA/g). Impedance tests showed that it has low impedance characteristics; the capacitance retention rate is above 60% after 2000 cycles of charge and discharge; the energy density can still reach 21.68 Wh/kg with a power density of 0.98 kW/kg. The electrochemical capability of the lithium ion capacitor was improved with acidified activated carbon as positive electrode.

Dual-band negative-permittivity metamaterial using crossed loop resonator

This paper presents a novel dual-band negative-permittivity metamaterial (MTM). The MTM is based on a crossed loop resonator (CLR) which exhibits negative-permittivity property at 4.85–5.58 GHz and 9.34–15.48 GHz frequency bands under the normal incidence of EM wave. The MTM shows epsilon-very-large (EVL) and mu-near-zero (MNZ) properties near the resonance frequencies (4.85 GHz and 9.34 GHz). Thus, low-impedance characteristics are obtained around the resonance frequencies of the CLR. The CLR MTM is insensitive to the polarization and incident angle of the imposed EM wave (for incident angle < 20°). This MTM, which is polarization and incident angle independent, can be used for gain enhancement of magnetic dipole antennas, design of filters and ultrathin microwave absorbers.

Effect of Reducing Agent on Preparation and Electroactivity of MnO₂/Graphene Composite Electrode for Capacitors.

Supercapacitor electrodes materials with improved electrochemical properties were prepared by synthesizing manganese dioxide (MnO2) in an aqueous solution of graphene having a wide specific surface area and high electrical conductivity. MnO2/graphene composites were synthesized by reducing potassium permanganate with three kinds of reducing agent (ethanol, ethylene glycol, DMF). TEM-image confirmed that the MnO2/graphene composite which was reduced by ethanol had a small average particle size. Electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge-discharge test in 1 M Na2SO4 aqueous electrolyte solution. The MnO2/graphene composite reduced by ethanol showed higher specific capacitance than the MnO2/graphene composite reduced by ethylene glycol or DMF. It was concluded that the manganese dioxide reduced by ethanol having a small average particle size, resulting in a large specific surface area and low impedance characteristics.

A simple method for the construction of a recording-injection microelectrode with glass-insulated microwire

A rapid method for the production of a glass-insulated microwire electrode is described. A microwire was threaded into a glass capillary which was then pulled on a vertical pipette puller. A conical tip of the microwire was formed when the strongly heated glass capillary broke together with the wire in it. A tight seal of the glass-insulated microwire electrode between the glass and the metal was accomplished with silicone glue. The manufactured electrode performed consistently at different immersion depths, and yielded stable recordings of single units in the cerebral cortex and the medulla of rats. The strength and low impedance characteristics of the glass-insulated microwire electrode may make it useful for the recording of single units in deep brain structures. Furthermore, the electrode can be easily combined with another glass micropipette to form a dual recording-injection microelectrode unit.

An electrochemical investigation into the lithium insertion properties of Li xCoO 2

The thermodynamic, kinetic and interfacial properties of lithium insertion in Li x CoO 2 have been probed by ac and dc electrochemical techniques. The electrochemical voltage spectroscopy method, which was used to cycle the cells, indicated that a significant coulombic inefficiency was present in the first charge-discharge cycle. The second cycle data, however, were close to 100% coulombic efficiency and demonstrated that around x = 0.64 in Li x CoO 2, lithium could be inserted reversibly. This value is significantly higher than has been previously reported. In agreement with the work carried out by Reimers and Dahn[14]we also find two additional reversible differential capacity features at cell potentials of approximately 4.08 V and 4.20 V vs. Li on charge. The kinetic investigation revealed the average diffusion coefficient for the composite Li x CoO 2 cathode to be in the range 10 −9 cm 2 s −1 indicating the relatively facile reaction kinetics for the lithium insertion reaction. The dc impedance data collected by the current interrupt method were broadly consistent with the ac impedance spectra and showed the presence of two distinct impedance regions dependent on the cell state-of-charge. One region exists below approximately x = 0.6 where the system generates relatively low impedance characteristics. The second region occurs above about x = 0.7 where significantly higher impedance properties are found. We suspect morphological changes at the lithium/electrolyte interface to be responsible for these impedance variations.

Integrator circuit with bipolar transistor for Gbit/s applications

A novel continuous time integrator circuit suitable for Gbit/s signal processing is presented. It takes advantage of the inherent low impedance characteristic of broadband transistors. Instead of charging a capacitor from a high impedance current source, basically an inductance is used which is driven by a low impedance voltage source.

Low-impedance operational characteristics of toroidal cores used in magnetic amplifiers

The need for knowing the low-impedance operating characteristics of a toroidal core or cores to be used in a magnetic amplifier operating under low-impedance control conditions is shown in this paper. A method is then presented for determining these low-impedance characteristics without first winding turns on the core. This method is presented for the edification of persons who are working on the development of new core-testing techniques.