Direct Current Bias Research Articles

Direct current (dc) bias effect on the dielectric constant of Dy and Ho-doped BaTiO3-based ceramic and MLCCs

With the increasing demand for the miniaturization, high capacitance and high reliability of the Multilayer Ceramic Capacitor (MLCC), the thickness of single dielectric layer is less than 1 μm and the original partical size of the powder has reached the scale less than 300 nm. In this paper, BaTiO3 powder of various partical sizes (150–500 nm) was modified with Dy and Ho through a chemical coating method. The dense and uniform ceramics in accordance with EIA X7R-type temperature stability were sintered under low oxygen partial pressure and good dielectric properties of dielectric constant >2000 and dielectric loss <1% were obtained. The grain size effect on the dielectric properties under various dc-bias field were systematically investigated. It is found that the fine-grain ceramic exhibits a lower absolute change of the dielectric constant and a higher dc-bias stability. Furthermore, the modified 150 nm BaTiO3 powders was made into 1210-type MLCCs, and the dielectric behavior of the MLCCs with different dc-fields and ac-fields were study over −55 °C–125 °C. It is found that the ac electric field had a greater influence on the dielectric properties at room temperature and below, and the dielectric constant variation under a dc-bias field was also greatly advanced for the modified fine-grain MLCCs. The results provide a reference for the design of the next generation of BME-MLCCs.

Investigation of transport properties of perovskite single crystals by pulsed and DC bias transient current technique

Time-of-flight (ToF) transient current method is an important technique to study the transport characteristics of semiconductors. Here, both the direct current (DC) and pulsed bias ToF transient current method are employed to investigate the transport properties and electric field distribution inside the MAPbI3 single crystal detector. Owing to the almost homogeneous electric field built inside the detector during pulsed bias ToF measurement, the free hole mobility can be directly calculated to be about 22 cm2⋅V−1⋅s−1, and the hole lifetime is around 6.5 μs–17.5 μs. Hence, the mobility-lifetime product can be derived to be 1.4 × 10−4 cm2⋅V−1–3.9 × 10−4 cm2⋅V−1. The transit time measured under the DC bias deviates with increasing voltage compared with that under the pulsed bias, which arises mainly from the inhomogeneous electric field distribution inside the perovskite. The positive space charge density can then be deduced to increase from 3.1 × 1010 cm−3 to 6.89 × 1010 cm−3 in a bias range of 50 V–150 V. The ToF measurement can provide us with a facile way to accurately measure the transport properties of the perovskite single crystals, and is also helpful in obtaining a rough picture of the internal electric field distribution.

Low-pressure hollow cathode plasma source carburizing of AISI 304L austenitic stainless steel at low temperature

A carburizing treatment of AISI 304L austenitic stainless steel was performed at a low temperature of 450 °C using the low-pressure hollow cathode plasma source technique. The hollow cathode plasma was produced in a cathodic cage made of stainless steel screen inlaid with a stainless steel tubes array. A pulsed high voltage power, with a peak voltage of 800 V and a pulse frequency of 30 kHz, was applied at a low gas pressure of 50 Pa. The austenitic stainless steel samples were carburized with a direct current bias of −200 V under the CH4/H2 ratio changing in a range from 0.11 to 9, for a carburizing time of 4 h. Under the lower CH4/H2 ratio of 0.11, a diamond-like carbon film of approximately 260 nm thick formed on the stainless steel surface. As the CH4/H2 ratio increased to 0.67 and 1, both carburizing cases possessed a similar thickness of about 11 μm, with the surface carbon concentration being 6 to 6.5 at.%. A layer of carbon supersaturated face-centered-cubic (γC) phase formed, together with carbon-rich (Fe,Cr)5C2 carbide precipitation. Under the highest CH4/H2 ratio of 9, a single γC phase layer was obtained in a carburizing case of similar thickness, with a high surface carbon concentration of 7.5 at.%. The maximum surface hardness value of HV0.25 N 7.1 GPa occurred on the single γC phase layer. No pitting corrosion occurred on the single γC phase in the polarization curve. The higher passive film resistance and the lower donor and acceptor concentrations and flat band potential indicated that a more compact passive film is grown on the γC phase layer. The (Fe,Cr)5C2 carbide precipitates in the γC phase matrix degraded the passivity of the γC phase layer due to the reduced compactness of the passive film. A carbon transport competition mechanism between carbon adsorption onto the surface and carbon inward diffusion that was mainly dependent on the heavier CHn+ ion bombardment, was proposed to explain the transition with increasing CH4/H2 ratio from carbon film deposition, to a carburizing case composed of ae single γC phase, with both increased hardness and higher corrosion resistance.

Performance analysis of three‐phase five‐leg transformers under DC bias using a new frequency‐dependent reluctance‐based model

This paper presents a reluctance-based model considering the frequency-dependent loss nature of the windings for the analysis of three-phase five-leg transformers under grid voltages with direct current (DC) bias. This is very important especially for proper determination of their harmonic current distortion and maximum loading capability (MLC) under DC-biased grid voltage conditions. To figure out the developed model's validity under sinusoidal and DC-biased grid voltage cases, it is comparatively analyzed with the model based on 2D finite element method (FEM). Thus, for the considered transformer type operated under DC bias, the excitation current's harmonic pollution, losses, and reactive power demand parameters are analyzed by using the developed model. Additionally, by regarding these performance parameters, the DC susceptibilities of the considered-type transformer and the single-phase shell-type transformer are comparatively evaluated. Finally, for the studied grid voltage conditions, the effects of two important design considerations as (i) magnetic core material selection and (ii) legs’ cross-sectional area sizing on the MLC are investigated. It is concluded from these investigations that under saturation conditions, the transformers, which have the core material with higher permeability or lower reluctance, draw higher excitation current, and have lower MLC ratio when compared to ones having the core material with lower permeability or higher reluctance. However, for unsaturated transformers, which work under DC bias, the case is the opposite to that in saturation conditions. On the other hand, under DC bias conditions, the effect of cross-sectional area sizing on the MLC ratio is much more for the transformer with high permeable magnetic core material with regards to ones with low permeable magnetic core material.

A high voltage capacitance measurement method based on alternating coupled signal injection.

As high voltage pulse power capacitors, ceramic capacitors are widely used in high voltage pulse generators, trigger circuits, laser generators, and other fields. The capacitance of ceramic capacitors is closely related to the direct current (DC) bias voltage. However, the current capacitance measurement methods can only achieve a DC bias of 1 kV, which cannot meet the measurement requirements in high voltage environments. This paper proposes a capacitance measurement method that can accurately measure the capacitance under a DC bias of 3 kV. This method decouples the high DC bias voltage and high frequency alternating small signals and realizes low voltage calibration and high voltage isolation. The experimental results show that the proposed method measures the capacitance under a DC bias of 3 kV with a relative error within ±1%, which makes it possible to accurately quantify the capacitance hysteresis deviation in the process of increasing and decreasing back the voltage.

Peak-tracking scanning capacitance force microscopy with multibias modulation technique

Scanning capacitance force microscopy (SCFM) is a good method for capacitance measurements using electrostatic force detection. However, to obtain an entire capacitance–voltage (C–V) curve by SCFM, a sweep of a direct current (DC) bias voltage is required at a certain fixed point on a sample surface during scan suspension, and thus the measurements become very time-consuming when we want to observe some types of image related with C–V characteristics. In this paper, we propose peak-tracking scanning capacitance force microscopy (PTSCFM) for the purpose of extracting the main feature of the C–V curve without DC voltage sweep. In PT-SCFM, alternating current voltages at three different angular frequencies, ω 1, ω 2, and , are applied together with DC voltage, , to generate an electrostatic force, and high-order components at the angular frequencies of and , which represent a voltage derivative of a capacitance () and a second-order derivative of the capacitance (), respectively, are extracted from the electrostatic force. Then, a DC voltage, , giving the peak of is determined from to be adjusted to nullify the component using a feedback controller. From the obtained values of and at , the C–V curve can be outlined. In PT-SCFM, the distributions of those values are simultaneously imaged together with a topography without sweep, and when we operate PT-SCFM under various modulation frequency conditions, analyses similar to those based on the frequency dependence of the C–V property are realized. We have applied the PT-SCFM to a microcrystalline material to discuss the effects of surface depletion and deep-level states, from which the validity of PT-SCFM has been examined.

Research on power transformer vibration and noise under DC bias condition

In the process of Ultra high voltage direct current (UHVDC) transmission, the direct current (DC) bias of power transformer is easily induced, which makes the transformer exciting current distorted, the ferromagnetic material saturated and the magnetic leakage increased, and then leads to the increase of core vibration and noise. Aiming at this problem, taking a 240 MVA, 330 kV three-phase five-column power transformer as an example, the coupling of the electromagnetic field, structural force field and acoustic field is studied, and the influence of DC bias on vibration and noise of power transformer core is analyzed in this paper. According to the magnetic density and electric density of transformer core under different magnetic bias degree, the structural force field is solved, and the displacement and surface acceleration of core are obtained, which can be as the excitation of sound field to determine the noise distribution of transformer. In order to avoid the natural frequencies which easily cause resonance, the modal analysis is needed to obtain the natural frequencies and modal modes of the core. The transformer noise under no-load and DC bias conditions of the prototype is tested experimentally and compared with the theoretical calculation, the results prove the accuracy of the simulation calculation method in this paper.

Light-controlled metasurface with a controllable range of reflection phase modulation

The reconfigurable metasurface (MS) with tunable electromagnetic scattering properties has had an impact on innovative communication applications. However, the widely studied MS controlled by direct-current (DC) bias requires complex physical control circuits in array wavefront encoding, while the light-controlled MS does not. In this paper, we report a new approach for a light-controlled MS with a controllable modulation range of reflection phase, and its unit is composed of a reflection phase element based on the varactor and an optical interrogation network (OIN) based on the photoresistor so that the phase distribution of each MS unit could be independently programmed optically. To illustrate our approach, a 2-bit reflection phase MS with 10 × 10 units was fabricated and tested to achieve pencil beam scanning and orbital angular momentum beam at microwave frequencies. The simulated and measured results verify the feasibility of the proposed design. Furthermore, all the OINs on the MS are connected in parallel and powered by the same DC voltage source, which simplifies the difficulty of array expansion in MS design with a large-scale array.

Effects of Radio Frequency Bias on the Structure Parameters and Mechanical Properties of Magnetron-Sputtered Nb Films

Due to its highly unreactive nature and advanced biocompatibility, niobium (Nb) coating films are increasingly being used to improve the corrosion resistance and biocompatibility of base implant materials. However, Nb films have relatively low yield strengths and surface hardness; therefore, it is necessary to explore a simple and low-cost method to improve their mechanical properties. Magnetron sputtering is a commonly used tool for Nb film deposition. Applying substrate bias can introduce Ar+ bombard to the film surface, which is effective to improve the film’s mechanical properties. As the direct current (DC) bias-sputtering tool requires an extra DC power supply, applying the negative bias by a radio frequency (RF) power source (usually installed in the sputtering system to conduct substrate pre-cleaning) will be more economical and convenient. Moreover, the RF bias was accompanied with higher ion density and energy compared to the DC bias. In this study, Nb films were deposited on silicon wafers by magnetron sputtering under different RF bias powers. The effects of the RF bias on the structural parameters and mechanical properties of the films were studied via stress measurements, X-ray diffraction, and indentation tests. The results show that the RF bias can change the crystal distribution, grain size, and lattice parameter of the film, as well as the mechanical properties. The stress of the Nb film was compressive; it increased markedly when an RF power was applied and saturated when the RF power was over 40 W. The hardness of the film increased from 4.17 GPa to 5.34 GPa with an elevating RF power from 0 W to 60 W. This study aimed to enhance the mechanical properties of the Nb films deposited by RF-biased sputtering, which provides wider potentials for Nb film as protective coatings for medical–biological implant bodies. Although the research was carried out on Si substrates to facilitate the study of film stress, we believe that the evolution trends of our results will also apply to other metal substrates, because the measured film mechanical properties are intrinsic.

A Micromechanical Transmitter with Only One BAW Magneto-Electric Antenna.

Implantable medical devices have been facing the severe challenge of wireless communication for a long time. Acoustically actuated magnetoelectric (ME) transducer antennas have attracted lots of attention due to their miniaturization, high radiation efficiency and easy integration. Here, we fully demonstrate the possibility of using only one bulk acoustic wave (BAW) actuated ME transducer antenna (BAW ME antenna) for communication by describing the correspondence between the BAW ME antenna and components of the traditional transmitter in detail. Specifically, we first demonstrate that the signal could be modulated by applying a direct current (DC) magnetic bias and exciting different resonance modes of the BAW ME antenna with frequencies ranging from medium frequency (MF) (1.5 MHz) to medium frequency (UHF) (2 GHz). Then, two methods of adjusting the radiation power of the BAW ME antenna are proposed to realize signal amplification, including increasing the input voltage and using higher order resonance. Finally, a method based on electromagnetic (EM) perturbation is presented to simulate the transmission process of the BAW ME antenna via the finite element analysis (FEA) model. The simulation results match the radiation pattern of magnetic dipoles perfectly, which verifies both the model and our purpose.

Reconfigurable of current-mode differentiator and integrator based-on current conveyor transconductance amplifiers

The reconfigurable of the differentiator and integrator based on current conveyor transconductance amplifiers (CCTAs) have been presented in this paper. The proposed configurations are provided with two CCTAs and grounded elements. The configurations can be operated in the differentiator and integrator by selecting external passive elements. The input and output currents have low and high impedances, respectively; therefore, the configurations can be cascaded without additional current buffer. The proposed configurations can be electronically tuned by external direct current (DC) bias currents, and it also has slight fluctuation with temperature. An application of universal filter is demonstrated to confirm the ability of the proposed configurations. The results of simulation with Pspice program are accordance with the theoretical analysis.

Design and Optimization of a BAW Magnetic Sensor Based on Magnetoelectric Coupling.

Magnetic sensors actuated by bulk acoustic wave (BAW) have attracted extensive attention due to the fact of their high sensitivity, GHz-level high frequency, and small size. Different from previous studies, suppression of energy loss and improvement in energy conversion efficiency of the BAW magnetoelectric (ME) sensor were systematically considered during the device design in this work. Finite element analysis models of material (magnetic composite), structure (ME heterostructure), and device (BAW ME magnetic sensor) were established and analyzed in COMSOL software. Additionally, the magnetic composite was prepared by radio frequency magnetron sputtering, and its soft magnetism was characterized by magnetic hysteresis loop and surface roughness. The research results demonstrate that after inserting four layers of 5 nm Al2O3 films, a performance of 86.7% eddy current loss suppression rate, a less than 1.1% magnetostriction degradation rate, and better soft magnetism were achieved in 600 nm FeGaB. Furthermore, compared with other structures, the two-layer piezomagnetic/piezoelectric heterostructure had a better ME coupling performance. Eventually, the design of the BAW ME magnetic sensor was optimized by the resonance-enhanced ME coupling to match the resonance frequency between the magnetic composite and the BAW resonator. When a 54,500 A/m direct current bias magnetic field was applied, the sensor worked at the first-order resonance frequency and showed good performance. Its linearity was better than 1.30%, the sensitivity was as high as 2.33 μmV/A, and the measurement range covered 0–5000 A/m.

Dielectric rod loaded tunable substrate integrated waveguide slot antenna

This paper presents a novel passive tuning technique to mechanically tune the resonant frequency of a substrate integrated waveguide (SIW) slot antenna, by using cylindrical rods of different dielectric materials in the air holes. A full-wave 3-dimensional Electromagnetic simulator software is used to estimate tunability in the resonant frequency of the SIW slot antenna, which is then analysed using its equivalent circuit, and then verified experimentally. Tunability is obtained by changing relative permittivity of dielectric rods. Measurements show good frequency tunability even with a limited number of dielectric rods and gains over 5 dBi. The similarity in the radiation patterns with tuning, demonstrates a good characteristic for tunable antennas. The technique is simple but effective and does not require external direct current bias circuitry, which removes parasitic effects and less power consumption. The low field perturbation of dielectric rods provides better loss performance and a higher Q-factor over a wider bandwidth. The initial design demonstrates a cost-effective tuning technique achieving return loss (>10 dB) in the X-band. This technique is also suitable for practically overcoming the performance deviation due to fabrication inaccuracies and temperature variation.

Position–sensitive electric property of flash–sintered 3Y–TZP ceramics based on DC bias assisted impedance analysis

In this study, impedance spectroscopy with direct current (DC) bias was used to analyze the electric property and oxygen vacancies of flash–sintered 3 mol% Y 2 O 3 doped tetragonal zirconia polycrystalline (3Y–TZP) ceramics at anode, middle, cathode and no field vicinity, respectively. The electric property was obtained using modified–Debye circuit model and the oxygen vacancies concentration was calculated using double Schottky barrier model. Interestingly, a phenomenon of position–sensitive electric property and oxygen vacancies concentration was observed. It means after flashing the tested samples become ununiform electrically at anode, middle and cathode vicinity. The neutral oxygen vacancies generated at cathode vicinity are responsible for this unique phenomenon fundamentally. The finding here is useful to explore the mechanism of flash sintering and tune the electric property of ZrO 2 ceramics electrolyte.

Fabrication and characterization of In doped Li13.9Sr0.1Zn(GeO4+δ)4 electrolytes for proton-conducting solid oxide fuel cells

Li13.9Sr0.1Zn(GeO4+δ)4 (LSZG) materials can exhibit proton conduction by Li+/H+ ion exchange in hydrogen atmosphere. It can be used in solid oxide fuel cells (SOFCs) as an electrolyte. In this study, In3+ doped LSZG powders are synthesized by sol-gel method. X-ray diffraction, scanning electron microscopy, thermal gravimetric analyzer, and electrochemical impedance spectroscopy are used to investigate the effects of In doping on LSZG. All Li13.9-xInxSr0.1Zn(GeO4+δ)4 (LISZG, 0 ≤ x ≤ 0.3) ceramics exhibit the same phase with LSZG. The dopant of In promotes the sintering activity and Li+/H+ ion exchange rate of LSZG. The optimum doping of In is x = 0.2. At 600 °C, Li13.7In0.2Sr0.1Zn(GeO4+δ)4 (0.2LISZG) shows a proton conductivity of 0.094 S/cm under 0.9 V direct current bias voltage. In addition, the single cell based on 0.2LISZG electrolyte is prepared, and it has been demonstrated that the practical utilization of 0.2LISZG in IT-SOFCs is feasible.

Joint NOMA for Improved SER of Cell-Edge Users in Multi-Cell Indoor VLC

In this letter, we propose a non-orthogonal multiple access (NOMA) based scheme for multi-cell indoor visible light communications (VLC), where cell-edge user is jointly served by multiple cells. Unlike the typical designs in NOMA-VLC literature, the proposed scheme doesn’t involve complex domain and direct current bias addition, which makes the design simple and feasible for real time implementation in indoor VLC. The symbol error rate (SER) of the proposed NOMA scheme is analysed using simulation results, and is compared with orthogonal multiple access (OMA). It is observed that the average SER of the users with the proposed NOMA is marginally degraded as compared to OMA. However, the SER of cell-edge user with the proposed NOMA is significantly improved with joint maximum likelihood decoding, and outperforms successive interference cancellation based decoding and OMA, with trade-off on computational complexity.

Electrochemical study on molten iron-carbon electrodes in slag and their decarburization by slag/metal reaction

An electrochemical study on molten iron-carbon electrodes in slag and their decarburization during anodic polarization was performed at 1600 °C to obtain information relevant to steelmaking. Carbon concentration was varied from 4.5 to 0.2 wt% to cover the range of interest between pig iron tapped from blast furnaces and steel tapped from oxygen converters or electric arc furnaces. Impedance spectra were measured at the rest potential and at applied direct current biases, and results were fitted to a faradaic decarburization reaction proceeding through two adsorbed intermediates. At the rest potential, carbon significantly increases faradaic activity at the metal/slag interface as evidenced by a strong increase in exchange current and interfacial capacitance with carbon activity. The apparent standard heterogeneous rate constant of the anodic decarburization half-cell was determined using the dependence of exchange current on carbon activity. During electrochemical decarburization, interfacial capacitance decreases significantly which is consistent with changes in surface concentrations of intermediates. Using a comprehensive kinetic model, surface concentrations of intermediate adsorbed species were determined during anodic polarization in the Tafel region. Results from impedance spectroscopy were compared with those obtained by gas chromatography during constant-current decarburization.

End-to-End Performance Optimization of a Dual-Hop Hybrid VLC/RF IoT System Based on SLIPT

In order to enhance the service provisioning to users in the indoor environment, a hybrid visible light communication (VLC)/radio-frequency (RF) Internet of Things (IoT) system is proposed based on simultaneous lightwave information and power transfer (SLIPT). A mobile user equipment, which serves as an off-the-grid relay, is able to extract information from the light-emitting diode source and then forward the processed information to the destination far away, thus dividing the signal transmission into two hops. Specifically, the optical signal received at the relay is separated into alternating current and direct current components for information decoding and energy harvesting, respectively, in the first hop. Then, the energy harvested is used to forward the processed source information to the destination by using RF in the second hop. Subject to the constraints imposed on both the average and the peak powers of the source, the end-to-end outage probability of the system is analytically derived in a closed form. On this basis, the minimization of the end-to-end outage probability is formulated as an optimization problem. This problem is then solved with a joint design of the peak amplitude and the direct current bias of the source transmitter, by trading-off between the performance of two successive hops. Simulation results demonstrate that by using the proposed optimal solution, the information flow and energy flow can be dynamically balanced, resulting in significant performance gains in terms of outage probability and throughput.

Switchable instantaneous frequency measurement by optical power monitoring based on DP-QPSK modulator

We propose and analyze an instantaneous frequency measurement system by using optical power monitoring technique with improved resolution. The primary component adopted in the proposal is a dual-polarization quadrature phase shift keying (DP-QPSK) modulator which is used to modulate the microwave signal that has a designed time delay and phase shifting. The generated optical signal is sent to polarization beam splitter (PBS) in DP-QPSK modulator. Owing to the complementary transmission nature of polarization interference introduced by PBS, the frequency information is converted into the optical power and the relationship between the amplitude comparison function (ACF) and microwave frequency to be measured is established. Thus, the frequency of the microwave signal can be easily measured through monitoring the optical powers of the two output ports of the PBS. Furthermore, by adjusting the direct current (DC) biases of the DP-QPSK modulator instead of changing the electrical delay, the measurement range and resolution can be switched. In this paper, the basic principle of the instantaneous frequency measurement system is derived in detail, and simulation has been performed to investigate the resolution, the measurement range, and the influence of imperfection devices. The proposed scheme is wavelength-independent and its measurement range is switchable, which can avoid the laser wavelength drifting problem and thus greatly increasing the system flexibility.

Fabrication and characterization of AlGaN/GaN HEMTs with high power gain and efficiency at 8 GHz

State-of-the-art AlGaN/GaN high electron mobility structures were grown on semi-insulating 4H-SiC substrates by MOCVD and X-band microwave power high electron mobility transistors were fabricated and characterized. Hall mobility of 2291.1 cm2/(V·s) and two-dimensional electron gas density of 9.954 × 1012 cm–2 were achieved at 300 K. The HEMT devices with a 0.45-μm gate length exhibited maximum drain current density as high as 1039.6 mA/mm and peak extrinsic transconductance of 229.7 mS/mm. The f T of 30.89 GHz and f max of 38.71 GHz were measured on the device. Load-pull measurements were performed and analyzed under (–3.5, 28) V, (–3.5, 34) V and (–3.5, 40) V gate/drain direct current bias in class-AB, respectively. The uncooled device showed high linear power gain of 17.04 dB and high power-added efficiency of 50.56% at 8 GHz when drain biased at (–3.5, 28) V. In addition, when drain biased at (–3.5, 40) V, the device exhibited a saturation output power density up to 6.21 W/mm at 8 GHz, with a power gain of 11.94 dB and a power-added efficiency of 39.56%. Furthermore, the low f max/f T ratio and the variation of the power sweep of the device at 8 GHz with drain bias voltage were analyzed.