Small DC Research Articles

Unveiling the tradeoff between device scale and surface nonidealities for an optimized quality factor at room temperature in 2D MoS2 nanomechanical resonators

A high quality (Q) factor is essential for enhancing the performance of resonant nanoelectromechanical systems (NEMS). NEMS resonators based on two-dimensional (2D) materials such as molybdenum disulfide (MoS2) have high frequency tunability, large dynamic range, and high sensitivity, yet room-temperature Q factors are typically less than 1000. Here, we systematically investigate the effects of device size and surface nonidealities on Q factor by measuring 52 dry-transferred fully clamped circular MoS2 NEMS resonators with diameters ranging from 1 μm to 8 μm, and optimize the Q factor by combining these effects with the strain-modulated dissipation model. We find that Q factor first increases and then decreases with diameter, with an optimized room-temperature Q factor up to 3315 ± 115 for a 2-μm-diameter device. Through extensive characterization and analysis using Raman spectroscopy, atomic force microscopy, and scanning electron microscopy, we demonstrate that surface nonidealities such as wrinkles, residues, and bubbles are especially significant for decreasing Q factor, especially for larger suspended membranes, while resonators with flat and smooth surfaces typically have larger Q factors. To further optimize Q factors, we measure and model Q factor dependence on the gate voltage, showing that smaller DC and radio-frequency (RF) driving voltages always lead to a higher Q factor, consistent with the strain-modulated dissipation model. This optimization of the Q factor delineates a straightforward and promising pathway for designing high-Q 2D NEMS resonators for ultrasensitive transducers, efficient RF communications, and low-power memory and computing.

A novel approach to predict the electrical conductivity of nanocomposites by a weak interphase around graphene network

Herein, we offer a model for estimating the tunneling conductivity of polymer-graphene nanocomposites based on interfacial properties, the proportion of networked graphene, and the wettability value between the polymer medium and the filler. The interfacial properties are influenced by the minimum diameter of the nanosheets (Dc), whose conductivity can be transferred to the medium via interfacial conduction (τ). These parameters impact the actual aspect ratio and the volume proportion of the filler, which, in turn, control the onset of percolation and the proportion of nanosheets in the network. We apply all these parameters to develop a novel model for estimating the conductivity of graphene systems. The predictions obtained from this model across different parameter ranges are discussed. Additionally, experimental measurements are employed to evaluate the proposed equations. High filler conductivity enhances the nanocomposite’s conductivity by a strong interfacial conduction. However, the conductivity cannot be transferred to the polymer medium under condition of weak interfacial conduction. Furthermore, a robust interphase and a small Dc contribute to increased conductivity. Ultimately, the developed equations accurately predict the onset of percolation and conductivity, validated by real experimental data.

A Low-Cost Handheld Centrifugal Microfluidic System for Multiplexed Visual Detection Based on Isothermal Amplification.

A low-cost, handheld centrifugal microfluidic system for multiplexed visual detection based on recombinase polymerase amplification (RPA) was developed. A concise centrifugal microfluidic chip featuring four reaction units was developed to run multiplexed RPA amplification in parallel. Additionally, a significantly shrunk-size and cost-effective handheld companion device was developed, incorporating heating, optical, rotation, and sensing modules, to perform multiplexed amplification and visual detection. After one-time sample loading, the metered sample was equally distributed into four separate reactors with high-speed centrifugation. Non-contact heating was adopted for isothermal amplification. A tiny DC motor on top of the chip was used to drive steel beads inside reactors for active mixing. Another small DC motor, which was controlled by an elaborate locking strategy based on magnetic sensing, was adopted for centrifugation and positioning. Visual fluorescence detection was optimized from different sides, including material, surface properties, excitation light, and optical filters. With fluorescence intensity-based visual detection, the detection results could be directly observed through the eyes or with a smartphone. As a proof of concept, the handheld device could detect multiple targets, e.g., different genes of African swine fever virus (ASFV) with the comparable LOD (limit of detection) of 75 copies/test compared to the tube-based RPA.

Harmonic modelling and control of dual active bridge converter for DC microgrid applications

Due to the intermittent power availability from renewable sources, the bidirectional DC-DC converter (BDC) must integrate with the energy storage system for bidirectional power flow. Among many BDCs, the Dual Active Bridge (DAB) converter is the most promising because of the system's power handling capacity and efficiency. DAB bidirectional DC-DC converter is a topology with the advantages of a decreased number of devices, soft-switching commutations, low cost, and high efficiency. This work describes the guidelines for designing a DAB converter for small DC microgrid applications. It is shown that the judicial selection of reactive elements leads to soft switching for all the switches in the converter in varying load conditions. A guideline for the controller design is given and the operations are validated through the simulation. A harmonic modelling technique has been implemented to model the DAB converter based on an efficient closed-loop regulator. A solar PV system has also been implemented here with a SEPIC converter that is used as an interfacing element between the PV system and the load bus. At last, a DC Microgrid is simulated in a PSIM environment to showcase its various modes of operation. It is shown that the battery bank can support variable loads independently through the DAB converter. In inadequate solar power generation, load power demand is shared between the battery and PV system. In case of deep discharge of the battery, the PV system can charge the battery through a DAB converter.

Static Magnetic Properties and Morphological Behaviour of Mn and Zn Substituted Ca-hexaferrite by Ceramic Technic

Synthesis of M-type hexagonal ferrite compounds, Ca(CoTi)0.5(MnZn)x/2Fe11-xO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) prepared by ceramic technic. Mn and Zn ions substituted to find the magnetic characterizationat magnetic field of 10 kOe. XRD and SEM study have been done for getting the hexagonal structure of these samples. Due to the substitution of Mn and Zn ions saturation magnetization increases and magnetocrystalline anisotropy decreases. Due to the sub-latice site distribution the variations in magnetic parameters have been observed. Because of Mn and Zn ions substitution Curie temperature decreases by weakening of superexchange interaction. These materials can be used for various applications such aspermanent magnetic material, magnetic recording media, ferrofluids, sensors, microwave absorbing materials, ceramic magnets in loud speakers and rotors in small DC motors by studying various magnetic parameters from VSM characterization.

ИМПЕДАНСНЫЙ АНАЛИЗ ПРОЦЕССОВ ПЕРЕНОСА ЗАРЯДА В СТРУКТУРЕ МЕТАЛЛ/ПОЛИМЕР/МЕТАЛЛ

The reaserch is devoted to the experimental study of charge transfer processes in thin films of an electroactive polymer from the class of polyarylene phthalides – poly (diphenylene phthalide). The method of impedance analysis of thin polymer films was used in this work. The material is quite well studied using other methods, such as thermally stimulated methods, optical spectroscopy, etc. However, frequency measurements were carried out mainly only in the field of determining capacitance-voltage characteristics. And since the polymer is electroactive, we should expect to identify features associated with charge transfer in the alternating field mode. In this regard, the work carried out an impedance analysis of thin films of poly (diphenylene phthalide) in the frequency range from 0.1 to 106 Hz. Typically, in impedance analysis, in addition to an ac field, a small DC bias voltage is applied to the metal/polymer/metal structure. This makes it possible to obtain data on surface polarization, charge carrier mobility, and the Maxwell-Wagner effect in the case of studying multilayer films. However, taking into account the electrical activity of the studied material, even impedance analysis in an alternating field, but with an effective voltage range from 0.1 to 10 V, made it possible to evaluate some of the electrophysical characteristics of the behavior of thin films. It was found that the slope of the impedance curve depends on the mechanism of transport of charge carriers in the polymer film. As the voltage increases from 0.1 to 10 V, an increase in the slope of the graph is observed. With an increase in the magnitude of the effective voltage, a transition occurs from the ohmic to the injection mechanism of transport of charge carriers limited by traps. When reaching 2 V, a mode of complete filling of the traps is observed, as a result of which an increase in voltage ceases to affect the slope of the frequency response. The dependence of the transition frequency fc on voltage has been plotted. This frequency corresponds to the transition from the predominance of the active to the reactive component of the impedance. Here, an exponential dependence on the magnitude of the applied voltage is observed, which also indicates the dependence of the mechanisms of transport of charge carriers in the polymer film on voltage (electric field).

Optoelectronic Transimpedance Converter Based on MOS Photovaricap for High Resistive Gas Sensors

The gas detectors consist of two parts: metal-oxides gas sensor and transducer. The sensing material is made of a thin semiconductor layer. The transducer is responsible for converting the change in the several physical parameters of the thin sensitive layer into a measurable signal. Due to the high output resistance of high resistive gas sensors and the low input currents of amplifiers, electrometric cascades are employed, followed by the conversion of the gas sensors small direct current into an active current signal using a mechanical modulator (dynamic capacitor) to enhance sensitivity and stability. The aim of this research is to analyze the main characteristics of a novel type of dynamic capacitor that utilizes a surface metal-oxide-semiconductor (MOS) photovaricap (PV) as the regulating element when detecting signals from high resistive gas sensors. For the creation of surface MOS PVs, samples of p-Si with varying resistivity were chosen. A thin layer of SiO2 was deposited on these samples, onto which a translucent metallic electrode was applied. Modulation of the PV capacitance was achieved by illuminating it with a gallium arsenide LED. As a result, the capacitance of the PV is changed, producing a variable voltage across the load resistor, which was then amplified and recorded. It was observed that the settling time of the useful signal is determined by two factors. The first is the RC chain composed of the load resistance of the gas sensor and the input capacitance of the PV, and the second is slow relaxation phenomena in the structure of the MOS PV that lead to the screening of the induced charge. Characteristics of the MOS PV as a dynamic capacitor and the features of the photo capacitance as a semiconductor element, in turn, determine the transimpedance conversion coefficient. The conducted research showed that MOS PVs can be applied to register small DC currents and voltages from high resistive gas sensors. In this case, ordinary narrowband or wideband amplifiers with relatively low input resistance can be used instead of electrometric amplifiers with mechanical dynamic capacitors. The considered circuit has been experimentally tested with commercial resistors.

Slow relaxation of magnetization in the Co(II)‐phosphotungstate Keggin polyoxometalate

AbstractWe are reporting the single crystal structure of the tetrabutylammonium (TBA) salt of the old Co(II) Keggin phosphotungstate anion (Co(II)POM) and a study of the magnetic behaviour of this well‐known POM. This Co(II)POM crystallizes in a cubic cell and this high symmetry obscure a precise characterization of Co(II) coordination sphere metrics due to positional disordered. We employed DFT and SA‐SOC‐CASSCF as auxiliary tools to complement DC and AC magnetic data collected. The studied complex behaves as a field‐induced single ion magnet (SIM) showing slow relaxing magnetization below 5 K under small DC applied magnetic fields. Raman relaxation mechanisms dominates with different relaxation times observed when switching from the TBA salt to the K salt of the Co(II)POM.

Electrical Impedance Spectroscopy with Bacterial Biofilms: Neuronal-like Behavior.

Negative capacitance at low frequencies for spiking neurons was first demonstrated in 1941 (K. S. Cole) by using extracellular electrodes. The phenomenon subsequently was explained by using the Hodgkin-Huxley model and is due to the activity of voltage-gated potassium ion channels. We show that Escherichia coli (E. coli) biofilms exhibit significant stable negative capacitances at low frequencies when they experience a small DC bias voltage in electrical impedance spectroscopy experiments. Using a frequency domain Hodgkin-Huxley model, we characterize the conditions for the emergence of this feature and demonstrate that the negative capacitance exists only in biofilms containing living cells. Furthermore, we establish the importance of the voltage-gated potassium ion channel, Kch, using knock-down mutants. The experiments provide further evidence for voltage-gated ion channels in E. coli and a new, low-cost method to probe biofilm electrophysiology, e.g., to understand the efficacy of antibiotics. We expect that the majority of bacterial biofilms will demonstrate negative capacitances.

A quad DC source switched three-phase multilevel DC-link inverter topology

The concept of an isolated DC source cascaded multilevel inverter finds good solutions for generating quality output voltage for low-medium power applications. It shapes the output voltage from three levels into a number of steps closer to a sinusoidal shape using small DC sources or batteries. Several advantages have been sighted like lower voltage stress and bearing noise, and lesser THD. However, a common issue in the MLIs is the total components required which increase with the rise in voltage levels. This paper proposes a three-phase MLI design having several isolated quad voltage source modules including an H-Bridge inverter. The design suggested claims reduced switching components for current conduction paths showing improved output quality. The operational features of the suggested MLI have been analyzed using Matlab/Simulink software, furthermore, an experimental module is constructed for demonstrating the effectiveness of the simulated results.

Development of a Hardware-in-the-Loop Platform for the Validation of a Small-Scale Wind System Control Strategy

The use of renewable energies contributes to the goal of mitigating climate change by 2030. One of the fastest-growing renewable energy sources in recent years is wind power. Large wind generation systems have drawbacks that can be minimized using small wind systems and DC microgrids (DC-µGs). A wind system requires a control system to function correctly in different regions of its operating range. However, real-time analysis of a physical wind system may not be feasible. An alternative to counteract this disadvantage is using real-time hardware in the loop (HIL) simulation. This article describes the implementation of an HIL platform in an NI myRIO 1900 to evaluate the performance of control algorithms in a small wind system (SWS) that serves as a distributed generator for a DC-µG. In the case of an SWS, its implementation implies nonlinear behaviors and, therefore, nonlinear equations, and this paper shows a way to do it by distributing the computational work, using a high-level description language, and achieving good accuracy and latency with a student-oriented development kit. The platform reproduces, with an integration time of 10 µs, the response of the SWS composed of a 3.5 kW turbine with a fixed blade pitch angle and no gear transmission, a permanent magnet synchronous generator (PMSG), and a three-phase full-bridge AC/DC electronic power converter. The platform accuracy was validated by comparing its results against a software simulation. The compared variables were the PMSG currents in dq directions, the turbine’s angular speed, and the DC bus’s voltage. These comparisons showed mean absolute errors of 0.04 A, 1.9 A, 0.7 rad/s, and 9.5 V, respectively. The platform proved useful for validating the control algorithm, exhibiting the expected results in comparison with a lab-scale prototype using the same well-known control strategy. Using a well-known control strategy provides a solid reference to validate the platform.

Identification of Transfer Function to Model Brush Temperature of a Small DC Machine at No-Load

Using temperature measurement to provide insight into the health condition of an electrical machine at any operating point could be possible if a baseline temperature of the machine could be modelled in real-time and compared to the actual current temperature. In this study, transfer functions are being identified to be used as a baseline temperature response model for a small DC machine. As a preliminary study, the transfer functions are identified using experimental data of temperature responses at several no-load speed step inputs. The order of transfer function tested was between a range from 0 to 4. The third order transfer function was found to be the best followed by the first order transfer function with a model MSE error of less than 0.41 and 0.65 respectively. The slight variation on the poles of the system indicates that the thermal system of the electrical machine does not obey exactly the LTI hypothesis.

Validation of Electrical Noise of a DC Motor through Controlled Varistor Cracking: An Experimental Study

The varistor is an electronic component that protects the DC motor’s circuitry from electrical noise or transients that can cause damage. It works as a voltage-dependent resistor that can change its resistance according to the applied voltage. Once the voltage surpasses a specific threshold, the varistor conducts and directs the excess voltage away from the motor’s circuitry. In small DC motor manufacturing, ring varistors are vital for reducing electrical noise, minimizing spark-induced damage to the commutator and brush, and extending the motor’s lifespan. Additionally, they prevent damage to electronic parts in the customer’s mechanism set. The objective of this study is to investigate the impact of varistor cracks or chips that may occur during the soldering process of varistors to the commutator. To confirm the occurrence of cracks or chips, intentional damage will be inflicted on the varistors. The study aims to determine how the presence of cracked or chipped varistors affects the electrical noise produced by a DC motor during its operation. The resulting spark was observed through an oscilloscope, and it was found that the effect could be substantial, up to 5 to 10 times the rated voltage supplied to the motor. In the next phase of this study, further tests will be conducted on motors without varistors to provide a comparison.

Form-Stable Composite Phase Change Material Based Cooling of Small Brushless DC Motor Windings

Form-Stable Composite Phase Change Material Based Cooling of Small Brushless DC Motor Windings

Precision measurement of an electron pump at 2 GHz; the frontier of small DC current metrology

A well-characterised sample of silicon tunable-barrier electron pump has been operated at a frequency of 2 GHz using a custom drive waveform, generating a pump current of 320 pA. Precision measurements of the current were made as a function of pump control parameters, using a blind protocol, over a 7 week campaign. The combined standard uncertainty for each h measurement was in the range parts per million (ppm). The pump current exhibits a plateau along the exit gate voltage flat to approximately 0.15 ppm, but offset from ef by 0.2 ppm. This offset may be a sign of errors in the current traceability chain, indicating a limit to the accuracy of small current scaling using existing methods based on cryogenic current comparators.

Enhancement of the Voltage Control Response in Three-Phase Photovoltaic Inverters With Small DC Capacitors

In the case of photovoltaic (PV) inverters, an adequate dc voltage regulation is fundamental to maximize or limit the power injected into the grid. However, the traditional control requires a large dc capacitance to ensure stability in the whole operating range while the existing alternatives, despite achieving a stable control with a small capacitance, become too slow in the open-circuit area. This paper proposes two control methods to improve this performance. Firstly, a new voltage control with virtual impedance emulation is presented, showing that the response becomes faster in all operating points. Secondly, the control with impedance emulation is combined with a feed-forward compensation, further improving the dynamic response. Both methods are very simple to implement and their superior performance when using a small dc capacitance is verified by means of simulation results.

Effect of DC bias on dipolar response of yttrium iron garnet

Yttrium iron garnet (YIG) is a prominent magnetic (ferrimagnetic) insulator and an excellent material for wide ranging microwave and millimeter-wave applications. In polycrystalline form it exhibits high dielectric loss due to contributions coming from grain & grain boundaries, and Fe heterovalency. We have investigated the effect of dc bias voltage on the dielectric properties of YIG in a wide range of temperature (100 - 600 K) and frequencies (100 Hz - 1 MHz). Consistent with previous observations, without bias a dominant frequency dispersion due to hopping of the Fe2＋ & Fe3＋ is observed in low temperature region and a wider frequency dispersion is observed at higher temperatures arising due to grain-grain boundary contributions. Application of a small dc bias voltage increases the dielectric constant and reduces the dielectric loss significantly at higher frequencies. The DC bias-induced improvements are much more dramatic at higher temperatures, where grain boundary contributions dominate. Our detailed analysis of dc and ac conductivity data show that the dc bias suppresses the grain boundary contributions significantly and at the same time it boosts the polarizability due to the Fe2＋/Fe3＋ hopping.

Wafer-Level Fabricated Tight-Coupling Dual-Solenoid Transformer Chips With Watt-Scale Power Transfer

Reported are a novel kind of tight-coupled dual-solenoid transformers (DSTs) which are on-chip integrated by using wafer-level fabrication including a low melting-point zinc-aluminum alloy rapid microcasting technique. Without magnetic core used, the silicon-chip integrated transformer is with the two metal solenoids tightly interwound to form a highly-coupled 3-D structure, thereby performing high inductance density and featuring no concern in magnetic flux saturation. Fabricated with the wafer-level thick-metal 3-D microcasting technique, the DST exhibits sufficient inductance value and low dc resistance while maintaining tiny footprint. Various DST structures are designed, fabricated and characterized for performance optimization. The optimally designed DST chip demonstrates a compact footprint of 2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , a high inductance of 193.14 nH (i.e., inductance density = 96.57 nH/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and a small dc resistance of 1.09 Ω. The test shows that the chip achieves an ultrahigh coupling coefficient of 0.95 and superior maximum transformer efficiency of 87.8% at 100 MHz. More importantly, the DST achieves <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Watt</i> -scale power transfer capability at 100 MHz, where no any heat dissipation treatment to the chip is used. Therefore, the on-chip DSTs are very promising for power/signal transmission in various electronic microsystems.

Implementation of Modified SEPIC Converter for Renewable Energy Built DC Microgrids

The nonavailability of fossil fuels and the shortcomings of nonconventional energy sources taking place in the environment lead the research and development towards alternative and clean energy sources such as renewable energy sources. Renewable or nonconventional energy resources are being used to meet ever-increasing energy demand. The photo voltaic (PV) energy is the right choice of renewable energy for small voltage DC distribution systems, due to their advantages. But this energy source can produce low output power at the utility grid. Hence, to step up this low input voltage to high value for a range of high-voltage applications, DC-DC converters are integrated to the DC microgrids by means of PV system. The present work elaborates the modified SEPIC converter (MSC) designed based on the traditional SEPIC with a boost-up module. In comparison with conventional or traditional SEPIC converter, the proposed MSC produces high voltage gain and continuous current to the DC microgrids. Furthermore, MSC is operated with only one controlled switch. The proposed converter design improves the efficiency, output voltage, and continuous output current of the DC microgrids. This entire work is completed with PSIM, and finally, numerical simulation results prove the possibility of the MSC with PV-powered DC microgrids, and also the dynamic response of MSC for DC microgrid loads enhances the regulated output voltage and continuous output current of DC loads.

Green Energy: An Ideal Energy Solution for Sustainable Development of Afar Region, Ethiopia

Green energy is the primary concern for the sustainable development of Ethiopia’s Afar region. The study’s goals are to present scientific evidence of the Afar region’s energy potential to researchers and industry sectors. We used solar shortwave, radiation transfer model, miniscale meteorological model for the Weather Research Forecast (WRF), and spatial and temporal simulation as research techniques. The data show that the Afar region has an energy potential of 239.9 W/m2 average solar radiation flux, 2.102 MW·h/m2 average annual solar density, 131.18 W/m2 average wind power density at h = 10 m , and 204.5 W/m2 average wind power density at h = 50 m . We discovered that solar energy and wind energy are potential energy sources in the Afar region for energy consumption such as solar cooking, solar lighting, and small DC applications.