Alternating Current Mode Research Articles

AC/DC Thermal Nano-Analyzer Compatible with Bulk Liquid Measurements

Nanocalorimetry, or thermal nano-analysis, is a powerful tool for fast thermal processing and thermodynamic analysis of materials at the nanoscale. Despite multiple reports of successful applications in the material sciences to study phase transitions in metals and polymers, thermodynamic analysis of biological systems in their natural microenvironment has not been achieved yet. Simply scaling down traditional calorimetric techniques, although beneficial for material sciences, is not always appropriate for biological objects, which cannot be removed out of their native biological environment or be miniaturized to suit instrument limitations. Thermal analysis at micro- or nano-scale immersed in bulk liquid media has not yet been possible. Here, we report an AC/DC modulated thermal nano-analyzer capable of detecting nanogram quantities of material in bulk liquids. The detection principle used in our custom-build instrument utilizes localized heat waves, which under certain conditions confine the measurement area to the surface layer of the sample in the close vicinity of the sensing element. To illustrate the sensitivity and quantitative capabilities of the instrument we used model materials with detectable phase transitions. Here, we report ca. 106 improvement in the thermal analysis sensitivity over a traditional DSC instrument. Interestingly, fundamental thermal properties of the material can be determined independently from heat flow in DC (direct current) mode, by using the AC (alternating current) component of the modulated heat in AC/DC mode. The thermal high-frequency AC modulation mode might be especially useful for investigating thermal transitions on the surface of material, because of the ability to control the depth of penetration of AC-modulated heat and hence the depth of thermal sensing. The high-frequency AC mode might potentially expand the range of applications to the surface analysis of bulk materials or liquid-solid interfaces.

Synthesis of Copper Nitride Layers by the Pulsed Magnetron Sputtering Method Carried out under Various Operating Conditions.

Copper nitride shows various properties that depend on the structure of the material and is influenced by the change in technical parameters. In the present work, Cu–N layers were synthesized using the pulsed magnetron sputtering method. The synthesis was performed under different operating conditions: direct current (DC) or alternating current (AC) power supply, and various atmospheres: pure Ar and a mixture of Ar + N2. The structural properties of the deposited layers were characterized by X-ray diffraction measurements, and Raman spectroscopy and scanning electron microscopy have been performed. Optical properties were also evaluated. The obtained layers showed tightly packed columnar grain features. The kinetics of the layer growth in the AC mode was lower than that observed in the DC mode, and the layers were thinner and more fine-grained. The copper nitride layers were characterized by the one-phase and two-phase polycrystalline structure of the Cu3N phase with the preferred growth orientation (100). The lattice constant oscillates between 3.808 and 3.815 Å for one-phase and has a value of 3.828 Å for a two-phase structure. Phase composition results were correlated with Raman spectroscopy measurements. Raman spectra exhibited a broad, diffused, and intense signal of Cu3N phase, with Raman shift located at 628–635 cm−1. Studies on optical properties showed that the energy gap ranged from 2.17 to 2.47 eV. The results showed that controlling technical parameters gives a possibility to optimize the structure and phase composition of deposited layers. The reported changes were discussed and attributed to the properties of the material layers and technology method.

Sinusoidal small-signal (AC) and steady-state (DC) analysis of large-area solar cells

Beside fabrication challenges, efficiency loss factors of solar cells such as shunts and an increasing series resistance caused by the sheet resistance of the electrodes, are issues to be tackled when scaling novel photovoltaic devices up from laboratory to industrial size. We present a FEM (Finite Element Method) software that supports the upscaling process from small- to large-area devices. Considering Ohm’s law in the top and bottom electrodes, which are coupled by a vertical current, the software solves for the electric potential distribution in the 2D electrode domains. In addition to steady-state simulations, we introduce a small-signal analysis that allows us to compute the influence of resistive electrodes and defects on the frequency-dependent impedance response. Herein, we describe the implemented numerical model for the AC (alternating current) mode. The steady-state model was validated with measurements using monocrystalline silicon solar cells of several sizes and one cell was intentionally shunted with a laser to demonstrate the fingerprints of these defects in the DC (direct current) and AC response. In a further step, we verify the numerical simulation of the AC model with an analytical solution to a one-dimensional AC model for a simplistic quadratic domain and linearized coupling law. Overall, the presented AC model is able to reproduce and predict the behavior of the measurements of the original and later shunted silicon solar cell. Thereby we have demonstrated that the presented AC model is a powerful tool to study devices in the frequency domain which complements characterization in steady-state.

Optical and frequency degradation behavior of GaN-based micro-LEDs for visible light communication.

In this study, optical power and frequency response degradation behavior of GaN-based micro-LEDs with bandwidth up to 800MHz were investigated under different modes, including direct current (DC) mode, alternating current (AC) mode and DC plus AC small signal mode at room temperature. The electroluminescence (EL), current-voltage (I-V) characteristics and small signal frequency response were measured during the stress. The results show that micro-LEDs under AC mode have better reliability because of the decreased junction temperature, but the high current density would still generate some defects within or around the active region, which can increase the trap-assisted tunneling (TAT) current and non-radiative recombination. The electrical stress-related defects not only reduce the effective carrier concentration injected into QWs but also increase the carrier lifetime for radiative recombination and Auger recombination and decrease the modulation bandwidth. These results will help to understand and improve the reliability of micro-LEDs operated under high current density and promote the application of micro-LEDs for visible light communication.

An Alterable Structure Power Router with General AC and DC Port for Microgrid Applications

This paper proposes an alterable structure power router (PR) topology which shares the ability of general Alternating Current (AC) and Direct Current (DC) port. This alterable structure PR aims to better implement the interconnections among micro-grids, renewable energy, and traditional grids. The PR’s converter structure is alterable according to the system power. Based on the operation analysis of the PR, the mathematic model of the power router is established and the relationship between switch states and port voltage is analyzed. According to the PR mathematical model, the control method is designed to realize current control in both AC and DC mode. With the proposed power router topology, the PR’s port can be used in both AC and DC situations, which saves the cost of traditional PRs and increases economic efficiency. The simulation and experimental results prove the good steady-state and dynamic performance of the proposed power router topology.

Evaluation of direct and alternating current on nitrate removal using a continuous electrocoagulation process: Economical and environmental approaches through RSM

This study aims to investigate the effects of alternating current (AC) and direct current (DC) for nitrate removal and its operating costs by using a continuous electrocoagulation (CEC) process. For this purpose, two series of 31 experiments, which were designed by response surface method (RSM), were carried out in both cases of the AC and the DC modes. In each series, the effect of selected parameters, namely, initial nitrate concentration, inlet flow rate, current density and initial pH along with their interactions on the nitrate removal efficiency as well as its operating costs, as responses, were investigated separately. According to the analysis of variance (ANOVA), there is a reasonable agreement between achieving results and the experimental data for both responses. The nitrate removal in the AC mode was slightly more efficient than that of the DC mode. In addition, the average operating costs of the DC mode, including the energy and the electrode consumption for the CEC process were achieved 54 US$/(kg nitrate removed); whereas this amount was calculated 29 US$/(kg nitrate removed) for the AC mode. Therefore, the average of the operating costs was improved more than 40% using the AC mode, which was mainly related to reduction of aluminum electrode consumption.

Hopping-Mode Scanning Ion-Conductance Microscopy Resolution during Synaptic Imaging

Scanning ion conductance microscopy (SICM) is a non-contact scanning-probe topography imaging technique, in which current measured between an electrolyte-filled nanopipette and an external bath electrode is monitored through closed-loop feedback to maintain a constant distance between the probe and the sample. Topography is mapped by scanning this nanopipette laterally across the sample in either direct-current or alternating-current mode. Both modes suffer from limitations when imaging in the presence of large morphological undulations, such as with neuronal bodies. The hopping-probe scanning (HP-SICM) mode, however, is ideally suited for imaging neuronal topography. In HP-SICM, the pipette current reduces upon approaching the sample, and height is recorded when the current reaches a pre-determined threshold (i.e. a fraction of the far-away value). The probe then retracts by a distance larger than the tallest structural feature, moves laterally by a pre-defined step, and reapproaches the sample. Here, using detailed finite-element simulations incorporating the Poisson-Boltzmann and Nernst-Planck formalisms, we examine the factors that affect HP-SICM resolution when imaging synapses in live hippocampal cultures. We find that currents originating from the imaged membrane can affect the recorded sample height, thereby introducing artifacts that can range from a few nm to ∼100nm. This feature of HP-SICM resolution critically depends on the set threshold value, pipette concentration and pipette geometry but provides an opportunity to extracellularly map activity from small dendritic spines and pre-synaptic terminals (diameter < .5um) that are practically inaccessible with conventional electrophysiology. Last, we explore the current-clamp response near the membrane and find that pipette geometry and local ion gradients affect the voltage response. Funding: U. S. Army Research Laboratory and U. S. Army Research Office. Contract number W911NF-12-1-0594 (MURI).

Improving the corrosion properties of magnesium AZ31 alloy GTA weld metal using microarc oxidation process

In this work, the morphology, phase composition, and corrosion properties of microarc oxidized (MAO) gas tungsten arc (GTA) weldments of AZ31 alloy were investigated. Autogenous gas tungsten arc welds were made as full penetration bead-on-plate welding under the alternating-current mode. A uniform oxide layer was developed on the surface of the specimens with MAO treatment in silicate-based alkaline electrolytes for different oxidation times. The corrosion behavior of the samples was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy. The oxide film improved the corrosion resistance substantially compared to the uncoated specimens. The sample coated for 10 min exhibited better corrosion properties. The corrosion resistance of the coatings was concluded to strongly depend on the morphology, whereas the phase composition and thickness were concluded to only slightly affect the corrosion resistance.

Organic Electrochemical Transistors for Bio-Sensing Applications

Organic electrochemical transistors (OECT) have been demonstrated and extensively studied as an effective platform for the design of chemical and biological electronic sensing applications, based on their advantages on low-voltage operation, good biocompatibility and interaction with aqueous medium, facile chemical modification of semiconductors and electrodes, and applicability on varied substrates including flexible plastics.[1] However, conventional OECTs are normally fabricated in the scale of millimeter or larger, resulting in a slow response time for device operation. To overcome this problem, here we develop a novel approach to miniaturize OECT by using only lithographic and magnetron sputtering technique. The channel of OECTs could be reduced to several to twenty microns therefore the speed of device response could be raised up to order of 10-5 s in aqueous medium. Due to the improvement in transient response time of the micro OECT device, alternating current (AC) is possible to be introduced for device operation. Combined with lock-in amplifier, the small AC channel signal could be precisely extracted from an extremely noisy environment, which provides the possibility to enhance the sensing capability of the OECT devices in complex situations. A new detection method is then established based on AC signal driven micro OECT. The detection limit of dopamine, an important neuro transmitter within the central nervous system, is down to 10-8 M, which is comparable to previous OECT sensor in steady current mode[2]. Furthermore, the pre waiting time to stabilize the device under the newly established AC mode is much shorter (tens of seconds) than conventional device needs (more than 600 s), indicating that the AC signal driven micro OECT could be a promising candidate for rapid sensing and diagnosis applications. In conclusion, we introduce a simple and easy-to-fabricate process for miniaturization of OECT devices to the cellular dimensions. The advantage of fast transient response makes the AC-driven micro OECT capable of in-situ cell activity monitoring and associated chemical sensing applications.

Rapid Transformation of Hexagonal to Cubic Silicon Carbide (SiC) by Electric Discharge Assisted Mechanical Milling

Silicon carbide powder was successfully transformed from hexagonal SiC into cubic SiC using the electric discharge assisted mechanical milling (EDAMM) method. The milling process was conducted in nitrogen plasma at atmospheric pressure. The effects of pulsed alternating current (AC) and direct current (DC) discharge on product formation were investigated. Products were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and nanohardness. It was found that hexagonal SiC can be transformed into cubic SiC under EDAMM processing, near complete transformation occurring within 5 min when applying AC mode electrical pulses, and within 10 min when applying DC mode discharges.

An Alternating Current Electroosmotic Pump Based on Conical Nanopore Membranes.

Electroosmotic flow (EOF) is used to pump solutions through microfluidic devices and capillary electrophoresis columns. We describe here an EOF pump based on membrane EOF rectification, an electrokinetic phenomenon we recently described. EOF rectification requires membranes with asymmetrically shaped pores, and conical pores in a polymeric membrane were used here. We show here that solution flow through the membrane can be achieved by applying a symmetrical sinusoidal voltage waveform across the membrane. This is possible because the alternating current (AC) carried by ions through the pore is rectified, and we previously showed that rectified currents yield EOF rectification. We have investigated the effect of both the magnitude and frequency of the voltage waveform on flow rate through the membrane, and we have measured the maximum operating pressure. Finally, we show that operating in AC mode offers potential advantages relative to conventional DC-mode EOF pumps.

Effects of electric current type and electrode configuration on the removal of Indigo Carmine from aqueous solutions by electrocoagulation in a batch reactor

The aim of this study is to investigate several electrode configurations and establish the effects of alternating current (AC), alternating pulse current (APC), and direct current (DC) on an electrocoagulation (EC) process conducted in a batch reactor. Indigo Carmine (IC) was considered as pollutant model. The investigated electrode configurations consisted in aluminum/aluminum, mild steel/mild steel, and aluminum/mild steel pairs. The effects of electrode configurations and current regimes on the removal efficiency, energy consumption, electrical operating costs, and electrode dissolution rate are discussed. The most powerful electrode configuration for IC removal from aqueous solutions by EC consisted in an anode of mild steel and a cathode of aluminum. EC operated in AC mode resulted in low efficiencies in any electrode configuration of those considered. Contrariwise, APC provided higher performance to the EC process in comparison with DC. Also, it led to higher dissolution rates in case of aluminum electrodes and to a more stable removal of dye when mild steel anode was employed. Energy-Dispersive X-Ray spectroscopy, Fourier transform infrared (FTIR), and scanning electron microscopy (SEM) analytical methods were used to characterize the sludge separated by sedimentation and flotation in case of EC in APC mode using Al–Fe configuration. Thus, it was shown that IC dye was adsorbed on aluminum and iron hydroxides.

Alternating current electrophoretic deposition (EPD) of TiO2 nanoparticles in aqueous suspensions

TiO2-nanostructured coatings from aqueous suspensions have been successfully prepared by the application of alternating current (AC) instead of direct current (DC) during electrophoretic deposition (EPD). No organic additives in suspension were required for successful EPD. The quality of the AC-EPD TiO2 coatings in terms of homogeneity and extent of microcracking, upon drying, observed by SEM, was superior to that of DC-EPD coatings made from the same type of suspensions. The main difference between AC- and DC-EPD was the suppression of bubble formation. The absence of water electrolysis at the electrodes can be explained by the particular distribution of the electric field during AC mode, which prevents the nucleation of bubbles. The preparation of TiO2 coatings from aqueous suspension and without the aid of organic stabilizers opens the possibility for co-deposition of sensitive materials such as biomolecules and even cells for biomedical applications, given the high biocompatibility of TiO2. The deposition of TiO2 coatings from aqueous suspensions is also attractive from environmental and economical points of view.

Deep nickel metal deposition on multiscale feature sizes of a small hole and large sector cavities using electroforming

The deep nickel metal deposition on multiscale features with a single small hole and large sector cavities with depth of 1 mm has been investigated using direct current (DC) and pulse alternating current (AC) electroforming. The diameter of the hole is around 240 µm, and the size of the sector cavity is several mm to cm corresponding to the various aspect ratios from 0.07 to 4.17. The successful metal electroforming into both the hole and cavity features can be obtained by the AC mode, while the DC mode leads to metal electroforming only in sector cavities rather than in the small hole. This is because the surface concentration of the cathode in pulse electroforming is higher than that in DC electroforming, allowing large cations to diffuse into different multiscale features at lower overpotential. The increment factor, i.e., percentage increase in concentration used as an index of the varied surface concentration increases from 0.903 (10−2%) at 1 Hz to 1.077 (10−2%) at 100 Hz. This achievement can be used for the high aspect ratio metal electroforming of a single hole as a micro-sensing tip in different applications.

Alternating current anodic stripping voltammetry in the study of cadmium complexation by a reference Suwannee river fulvic acid: a model case with strong electrode adsorption and weak binding

The possibilities of anodic stripping voltammetry (ASV) using an alternating current (AC) scan in the stripping step have been checked through the study of the complexation of cadmium by Suwannee river fulvic acid (SRFA), a reference fulvic acid from the International Humic Substances Society. Because of the strong electrode adsorption of SRFA, AC mode appears to be a good approach to the study when proper selection of the phase angle is made. The goodness of AC mode in ASV has been demonstrated, and the complexation constant of 3.71 +/- 0.04 determined is in good agreement with the value of the constant obtained by the reference technique of reverse pulse polarography. Some particularities of SRFA have been observed, among them its homofunctional and strongly heterogeneous behaviour in cadmium complexation and the impossibility of avoiding electrode adsorption problems in ASV measurements at very low metal concentrations. Figure DP anodic stripping and AC anodic stripping voltammograms at -12 degrees and -65 degrees during the titration of a 10(-7) mol L(-1) Cd(II) solution with SRFA at pH 7.5 in 0.05 L(-1) Tris.

Voltage mode and current mode Tow Thomas bi‐quadratic filters using inverting CCII

AbstractA voltage mode Tow Thomas bi‐quadratic filter using the inverting second‐generation current conveyor (ICCII) is given. The filter has high input impedance, employs two grounded capacitors, and has independent control on Q, independent control on the band‐pass and low‐pass response gain. Three alternative current mode filters are generated from the voltage mode circuit. The three circuits have zero input impedance, employ grounded capacitors and have independent control on Q. Two of the circuits have also all resistors grounded and the other uses only ICCII−and has only one floating resistor. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Electrochemical determination of 2,4-D at a mercury electrode

An indirect electrochemical determination of 2,4-dichlorophenoxyacetic acid (2,4-D), has been presented. The method is based on the adsorption and desorption of 2,4-D on mercury electrode. Also, the electrochemical behavior of 2,4-D in aqueous solutions at different pH values and different 2,4-D concentrations were studied. A simple and rapid method has been developed for its extraction from water and soil. The subsequent determination was carried out by a tensammetric method. Three calibration curves could be obtained from different parts of voltammogram. Under the optimum conditions (pH = 2.3; E acc = −1100 mV; t acc = 60 s; alternative current mode; ν = 40 mV s −1; pulse height = 20 mV; modulation frequency = 60 Hz; phase angle = 90°) the limit of detection was 50 μg L −1. The proposed method was applied to the determination of 2,4-D in real samples such as soil and water.

Characteristics of Peltier current lead system for alternating current mode

Since the Peltier current lead (PCL) was proposed to reduce heat leakage in low-temperature systems, numerical calculations and experimental studies have been performed. However, these studies were done for the direct current (DC) mode because the applications of superconducting systems are mainly used in DC mode. On the other hand, since commercial electric power is mainly used in the alternating current (AC) mode, future studies for superconducting application should move towards the AC mode. The topics to study in alternating current PCL (AC-PCL) are switching devices, transportation phenomena in thermoelectric semiconductors in the AC mode, and thermal transportation of current lead under AC operation. We study the AC-PCL numerically.

Operating Current Mode Dependence of Luminescence Properties of Rubrene-doped Yellow Organic Light Emitting Diodes

The operating current mode dependence of luminescence properties of rubrene-doped yellow organic light-emitting diodes (OLEDs) was investigated. The devices were driven by three types of constant current mode: a simple direct current (DC) mode, a simple alternating current (AC) mode and a pulsed current (PC) mode. The device driven by the AC mode showed low energy efficiency because of the existence of time intervals with no emission in spite of energy consumption. The device driven by the PC mode showed the highest efficiency and better stability compared with the devices driven by the other modes.