Dc Impedance Research Articles

Extremely high magnetic-field sensitivity of charge transport in the Mn/SiO2/p-Si hybrid structure

We report on abrupt changes in dc resistance and impedance of a diode with the Schottky barrier based on the Mn/SiO2/p-Si structure in a magnetic field. It was observed that at low temperatures the dc and ac resistances of the device change by a factor of more than 106 with an increase in a magnetic field to 200 mT. The strong effect of the magnetic field is observed only above the threshold forward bias across the diode. The ratios between ac and dc magnetoresistances can be tuned from almost zero to 108% by varying the bias. To explain the diversity of magnetotransport phenomena observed in the Mn/SiO2/p-Si structure, it is necessary to attract several mechanisms, which possibly work in different regions of the structure. The anomalously strong magnetotransport effects are attributed to the magnetic-field-dependent impact ionization in the bulk of a Si substrate. At the same time, the conditions for this process are specified by structure composition, which, in turn, affects the current through each structure region. The effect of magnetic field attributed to suppression of impact ionization via two mechanisms leads to an increase in the carrier energy required for initiation of impact ionization. The first mechanism is related to displacement of acceptor levels toward higher energies relative to the top of the valence band and the other mechanism is associated with the Lorentz forces affecting carrier trajectories between scatterings events. The estimated contributions of these two mechanisms are similar. The proposed structure is a good candidate for application in CMOS technology-compatible magnetic- and electric-field sensors and switching devices.

Center conductor diagnostic for multipactor detection in inaccessible geometries.

Electron collecting current probes are the most reliable diagnostic of multipactor and radiofrequency (RF) ionization breakdown; however, stand-alone probes can only be used in test setups where the breakdown region is physically accessible. This paper describes techniques for measuring multipactor current directly on the center conductor of a coaxial RF device (or more generally, on the signal line in any two-conductor RF system) enabling global multipactor detection with improved sensitivity compared to other common diagnostics such as phase null, third harmonic, and reflected power. The center conductor diagnostic may be AC coupled for use in systems with a low DC impedance between the center conductor and ground. The effect of DC bias on the breakdown threshold was studied: in coaxial geometry, the change in threshold was <1 dB for positive biases satisfying VDC/VRF0<0.8, where VRF0 is the RF voltage amplitude at the unperturbed breakdown threshold. In parallel plate geometry, setting VDC/VRF0<0.2 was necessary to avoid altering the threshold by more than 1 dB. In most cases, the center conductor diagnostic functions effectively with no bias at all-this is the preferred implementation, but biases in the range VDC=0-10V may be applied if necessary. The polarity of the detected current signal may be positive or negative depending on whether there is net electron collection or emission globally.

Structure and Physical Properties of NiO/Co3O4 Nanoparticles

The thermal treatment method was employed to prepare nickel-cobalt oxide (NiO/Co3O4) nanoparticles. This method was attempted to achieve the higher homogeneity of the final product. Specimens of nickel-cobalt oxide were characterized by various experimental techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). X-ray diffraction results showed that there was no crystallinity in the predecessor, and it still had the amorphous phase. The formations of the crystalline phases of the nickel-cobalt oxide nanoparticles started from 350–500 °C, and the final products had different crystallite sizes ranging from 11–35 nm. Furthermore, the variation of DC conductivity (σdc), impedance, tangent loss (tgδ) and dielectric constant (ε′) of the calcined specimens with frequency in the range of 102–106 Hz was investigated. σdc showed a value of 1.9 × 10−6 S/m, 1.3 × 10−6 S/m and 1.6 × 10−6 S/m for the specimens calcined at 350, 400 and 450 °C, respectively. Additionally, a decrease in tgδ values with an increase in temperature was observed. Finally, the formed nanoparticles exhibited ferromagnetic behaviors, which were confirmed by using a vibrating sample magnetometer (VSM).

Ageing of Commercial 18650 Batteries Used in Tesla Model S Electric Vehicles

Li-ion batteries are attractive energy storage devices for years to come. They benefit from a high specific capacity combined with outstanding cyclability. In order to advance in conventional lithium-ion technology it is necessary to develop new materials and to improve existing concepts. In particular, the latter includes ageing studies on, e.g., commercially available batteries. Such studies help predict the lifetime of the batteries and assist in identifying the main failure mechanisms connected to lithium plating, passivating surface films, co-intercalation or dissolution of metal ions, for example. In the present study commercial 18650 lithium-ion battery cells were used to perform fundamental cycle and calendar ageing studies. For this purpose systematic electro­chemical tests were carried out at three different temperatures, viz. – 25 °C, 40 °C and 60 °C. The cells were charged and discharged via galvanostatic cycling at different C rates ranging from 0.3 C to 1 C. For comparison, calendar ageing was electrochemically monitored at four different SOCs (100 %, 80 %, 50 % and 20 %). After the various aging steps the capacity fade and the increase in AC and DC impedance was determined. It turned out that temperature has little effect on calendar ageing; the capacity fade (5 % at most) increases with increasing SOC. In contrast to this behavior, during cycle ageing temperature has a much larger effect on capacity fade and DC impedances. As an example, after 500 cycles at 25 °C (or 40 °C) the capacity fade is approximately 12 % of the initial value while at 60 °C the fade has reached already 22 %. DC impedances measured revealed the same trend. First post mortem analyses indicate that the ageing effects observed might be related to changes of the solid electrolyte interphase. Hence, the changes in electrochemical performance were correlated with the thickness and composition of the SEI formed. In particular, we quantitatively measured the formation of LiF via ion exchange chromatography. Interestingly, there is a clear interrelation between fluoride quantity in the SEI and DC impedance growth.

Effect of hydrogen and carbon dioxide on the performance of methane fueled solid oxide fuel cell

Recycling and reusing of exhaust gas from solid oxide fuel cell (SOFC) could enhance the fuel utilization of methane. In the present work, the effects of two main components of exhaust gas, viz. hydrogen and carbon dioxide, on the performance of exhaust gas reforming methane fueled cell were investigated. The cell with Ni/8 mol% Y2O3 doped zirconia (Ni/YSZ) as the anode was exposed to H2CO2CH4 atmospheres at 800 °C. DC techniques and impedance spectroscopy was used for characterization. Two kinds of cells with different fuel utilization factors (Uf) were used in experiments, which represented different relationship of power density/voltage with current density. The results indicated that electrooxidation of H* dominated the electrochemical reaction on the anode of the cell at low Uf value, while electrooxidation of C* becomes important at high Uf value. In the case of CO2H2 mixtures, CO2 improved the exchange current density, raised the value of Uf, and depressed anodic activation polarization. As for H2CH4CO2 system, H2, partly replacing CH4, reduced the heat loss in dry reforming, improved the exchange current density, raised Uf value, and depressed the activation and concentration polarization resistances of Ni/YSZ anode. But it had limited effect on the power density of the cell with high Uf value.

Enhancement of performance of pyridine modified polybenzimidazole fuel cell membranes using zirconium oxide nanoclusters and optimized phosphoric acid doping level

Nanocomposite proton conducting membranes incorporating zirconium oxide (ZrO2) nanoclusters into solution cast 2,6-pyridine polybenzimidazole (2,6-Py-PBI) films and doped with phosphoric acid (PA) were prepared. The content of ZrO2 nanoclusters in the membrane matrix was varied from 0 to 10 wt%. The parameters of the acid doping reaction were optimized by the response surface method (RSM). The membrane properties were evaluated by Fourier transform infrared (FTIR), scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), universal mechanical tester and DC impedance. The incorporation of 5 wt% ZrO2 nanoclusters increased the acid doping level and remarkably enhanced the proton conductivity of the membrane. A maximum proton conductivity of 60.0 mS/cm was recorded compared to 21.5 and 12.0 mS/cm for pristine and 10 wt% ZrO2 containing membranes at 150 °C and dry condition, respectively. The membrane with 5 wt% ZrO2 also showed the highest performance in a single cell at 120 °C under dry conditions. The results suggest that the composite membrane containing 5 wt% ZrO2 nanoclusters has a potential for application in high temperature proton exchange membrane fuel cell application.

Determination of Individual Contributions to the Ionic Conduction in Litf/Pegdme-150 Liquid Electrolyte

Current salt containing liquid lithium electrolytes show both cationic and anionic conductivity with lithium transference numbers below 50% leading to formation of the ion concentration gradients inside a lithium battery cell. Moreover, the possibility of indirect (vehicular) transport of lithium ions from one electrode to the other via ion pair diffusion is usually neglected when typical DC polarization measurements are performed.[1] If the concept of Conservative Ensembles [2] is employed, effective conductivity can be deconvoluted into transport contributions from free ions and ion pairs even when higher order aggregates are present.[3] We demonstrate experimentally that a combination of DC polarization, impedance spectroscopy and PFG-NMR can enable such analysis for an electrolyte composed of poly(ethylene glycol) dimethyl ether (PEGDME-150) and lithium trifluoromethanesulfonate (LiTf) salt at ambient temperature.[4] In this case, the influence of ion pairs is substantial at all concentrations with values exceeding pure lithium conductivity (in 1M LiTf/PEGDME-150, only 5% of the current is transported by solvated Li+ while 21% is transported via Li-Tf ion pair under applied voltage). By including IR spectroscopy in the arsenal of techniques, mobilities of all species as well as effective mass action constants are calculated for the first time. J. Maier, Advanced Functional Materials, 21, 1448, 2011.J. Maier, Journal of the American Ceramic Society, 76, 1212, 1993.J. Maier, Electrochimica Acta, 129, 21, 2014.J. Popovic, C. Pfaffenhuber, J. Melchior, J. Maier, Electrochemistry Communications, 60, 195–198, 2015.

Impedance-Based Analysis of Active Frequency Drift Islanding Detection for Grid-Tied Inverter System

Islanding detection is critical to the safety of grid-tied inverters. This paper presents an impedance-based analysis of the active frequency drift (AFD) islanding detection method. To study this method, the output impedance of a grid-tied inverter is modeled. The $Z_{qq}$ of the inverter impedance features a negative incremental resistor, whose magnitude is equal to the dc impedance magnitude of the local load defined by the test standard. With a large value of feedforward gain $N$ in the AFD method, the phase of $Z_{qq}$ is shown to drop below 180°. Under an islanding condition, due to a lack of phase margin, the inverter system becomes unstable with a frequency drift away from its steady state. The islanding condition is then identified by a frequency detection unit. Experimental results verify the analysis.

Adaptive DC Stabilizer with Reduced DC fault Current for Active Distribution Power System Application

This paper takes a systematic view on the control and protection of medium power DC networks in an active distribution power system considering fault current limiting, system control, and converter design. Reduced terminal capacitance and extra DC impedance are used to limit DC fault current and reduce the required converter current rating for medium power DC networks. An adaptive DC power stabilizer is proposed to alleviate possible system instability brought by the fault current limiting settings in the presence of constant power load. The effect of the current limiting method and the proposed stabilizer on DC fault current and stability enhancement are validated by simulation studies using a simple two-converter DC network and a multiterminal DC network in an active distribution power system.

Improved electrical properties of cadmium substituted cobalt ferrites nano-particles for microwave application

Cadmium substituted cobalt ferrites with formula CdxCo1−xFe2O4 (x=0.0, 0.2, 0.35, 0.5), have been synthesized by wet chemical co-precipitation technique. Electrical, morphological and Structural properties of the samples have been studied using DC electrical resistivity and Impedance analyzer, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD), respectively. XRD, SEM and AFM have been used to study the structural parameters such as measured density, lattice constant, X-ray density, crystallite size and morphology of the synthesized nano-particles. Debye–Scherrer formula has been used for the estimation of crystallite sizes. The estimated crystallite sizes were to be 15–19±2nm. Hopping length of octahedral and tetrahedral sites have been calculated using indexed XRD data. The porosity and lattice constant increased as Cd2+concentration increases. DC electrical resistivity was performed using two probe technique. The decrease of resistivity with temperature confirms the semiconducting nature of the samples. The dielectric properties variation has been studied at room temperature as a function of frequency. Variation of dielectric properties from 100Hz to 5MHz has been explained on the basis of Maxwell and Wagner’s model and hoping of electrons on octahedral sites. To separates the grains boundary and grains of the system CdxCo1−xFe2O4 the impedance analysis were performed.

Photovoltaic Hot-Spot Detection for Solar Panel Substrings Using AC Parameter Characterization

Hot spotting is a problem in photovoltaic (PV) systems that reduces panel power performance and accelerates cell degradation. In present day systems, bypass diodes are used to mitigate hot spotting, but it does not prevent hot spotting or the damage it causes. This paper presents an active hot-spot detection method to detect hot spotting within a series of PV cells, using ac parameter characterization. A PV cell is comprised of series and parallel resistances and parallel capacitance, which are affected by voltage bias, illumination, and temperature. Experimental results have shown that when a PV string is under a maximum power point tracking control, hot spotting in a single cell results in a capacitance increase and dc impedance increase. The capacitance change is detectable by measuring the ac impedance magnitude in the 10-70 kHz frequency range. An impedance value change due to hot spotting can be detected by monitoring one high-frequency measurement in the capacitive region and one low-frequency measurement in the dc impedance region. Alternatively, the dc impedance can also be calculated using dc operating point measurements. The proposed hot-spot detection method can be integrated into a dc-dc power converter that operates at the panel or subpanel level.

PREPARATION AND CHARACTERIZATION OF POLY(1-VINYL IMIDAZOLE)-GRAFT-ETFE/PHOSPHORIC ACID PROTON CONDUCTING MEMBRANES

Proton conducting membranes obtained by radiation-induced grafting (RIG) of 1-vinylimidazole (1-VIm) onto poly(ethylene-co-tetraflouroethene) (ETFE) films followed by phosphoric acid (PA) doping was prepared. The effect of grafting parameters on the grafted basic moiety was studied. The level of PA doping was controlled by manipulation of reaction parameters. The obtained membranes were investigated with Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), differential scanning calorimeter (DSC) and DC impedance spectroscopy. The obtained membranes proved to have less-water dependent proton conductivity with a value of 140 mS/cm at 120oC and 17 % RH% at acid doping level of of 6.54 mmol/repeat unit. The membranes have potential for application in high temperature polymer electrolyte membrane fuel cell (PEMFC).

DC Impedance-Model-Based Resonance Analysis of a VSC–HVDC System

Resonances can have a negative impact on a voltage-source converter-high voltage dc (VSC-HVDC) system. This paper develops dc impedance models for the rectifier and inverter stations for a VSC-HVDC system when they are viewed from a dc terminal. The impedance models take converter controllers into account. The derived impedance models are validated by comparing frequency responses of the analytical model and the impedance measured at the dc terminal from a detailed VSC-HVDC model simulated in a real-time digital simulator. Resonances are examined in the frequency domain (e.g., Bode plots and Nyquist plots) using the derived analytical impedance models. The analysis is verified by time-domain simulations. Real-time digital simulation in RT-Lab demonstrates that the dc capacitor has a significant impact on resonances while the power transfer level has an insignificant impact on resonances.

Comparing DC Offset and Impedance Readings in the Assessment of Electrode Connection Quality

EEG electrode impedance measurements of 5,000 ohms or less are required by common standards of practice to minimize artifacts due to electro-magnetic interference (EMI). Some manufacturers of amplifiers geared toward the neurofeedback market do not provide on-board impedance monitoring, but provide DC offset measurements. To discover if DC offset is a reliable measure of connection quality, measurements of DC offset and impedance, each independently taken by students in a university graduate level course in neurofeedback over a one year period were analyzed retrospectively. DC offset was not found to have predictive value of a standard impedance level. Additionally, 19 channel EEGs collected within manufacturer recommended parameters of DC offset using a high-impedance amplifier were analyzed to assess the level of EMI pollution of QEEG data. Visible peaks of EMI in the spectra in at least one channel in each of these recordings were identified. A sample of EMI pollution of QEEG results is presented. Together, these findings suggest that DC offset is not a reliable measure of electrode connection quality.

Preparation and Chemical Gas Sensing Behavior of ZnCo&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Nano-Crystals

Spinel ZnCo2O4 nanosized powders were prepared by decomposing of the metal nitrate solutions at 100 °C. Structural properties of the sample were investigated by using powder X-ray diffraction (XRD), UV-visible and High resolution transmission electron microscopy (HR-TEM) techniques. Spinel crystal structure of the sample is confirmed by XRD analysis. The average crystallite size of the samples is found about 60 nm by Williamson-Hall analysis. The average grain size and crystalline nature of the samples is further confirmed by using the HR-TEM analysis. The band-gap energy is found to be 2.67 (±0.03) eV. Electrical properties were studied by two probe ac impedance spectroscopy. Dc conductivity is temperature dependent and found to obey the Arrhenius behaviour. The gas sensing behaviors were investigated by using dc and ac impedance measurement methods. It shows the promising gas sensing behavior for liquefied petroleum gas.

Enhanced electrochemical performance and thermal stability of a CePO4-coated Li1.2Ni0.13Co0.13Mn0.54O2 cathode material for lithium-ion batteries

CePO4 coating via a simple precipitation method enhances the thermal stability of the layered Li1.2Ni0.13Co0.13Mn0.54O2 cathode material.

Microstructure and electrical properties of praseodymium oxide doped Bi2O3 based ZnO varistor films

Pr2O3 and Pr6O11 doped ZnO thin films were prepared by the sol–gel method respectively. The microstructure and electrical characteristics of the samples were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), varistor dc parameter analyser and impedance analyser. Two kinds of Pr-oxide doped ZnO thin film with similar phase which contain well crystallised ZnO. The grains of Pr2O3 doped ZnO thin films and Pr6O11 doped ZnO thin films are uniform and closely packed, and all samples have lower surface roughness. When the doping content of Pr2O3 is 0·90 mol.-%, the varistor properties of ZnO thin film are optimum, the leakage current is 487 μA, the potential gradient is 232 V mm−1 and the nonlinear coefficient is 2·5. The dielectric constant of ZnO thin films increases in a certain degree with different contents of Pr2O3 and Pr6O11 doping. The Pr2O3 doping reduces the dielectric loss effectively, but the Pr6O11 doping has the opposite effect. These results provide favourable theoretical and experimental basis to optimise and extend electrical application of ZnO thin film.

A novel highly sensitive humidity sensor based on poly(pyrrole-co-formyl pyrrole) copolymer film: AC and DC impedance analysis

A novel humidity sensor of poly(pyrrole-co-formyl pyrrole) (P(Py-co-FPy)) film was successfully prepared via spin coating during chemical polymerization. The chemical structure of the P(Py-co-FPy) film was confirmed using infrared spectroscopy (FT-IR). The distinctly different nanodomain morphology and surface roughness observed for materials produced using different spin-coater rotation speeds were analyzed by field emission scanning microscopy (FE-SEM) and atomic force microscopy (AFM). The electrical humidity response of the sensing film was studied in terms of impedance, capacitance, and complex impedance properties. The impedance properties of the sensing film were highly sensitive to the relative humidity (RH) and the frequency operation. The resistive-type humidity of the P(Py-co-FPy) sensing film was highly sensitive; the impedance changed 5461 times at 100Hz from RH values of 11 to 97%. The P(Py-co-FPy) sensing film responded rapidly when the RH changed from 30 to 90% within 5s. The response of sensing films generated a small hysteresis during the adsorption–desorption processes. The experimental data were calculated using an exponential function, revealing an excellent correlation between the impedance parameters and the %RH. Furthermore, the impedance response against humidity can be improved by modulating the speed of spin coating that had effect on regularity properties of the surface film. In addition, the relationships among continuous frequencies (40–200kHz) of impedance at different humidity (11–97%RH) were discussed in details.

Metal-oxide-semiconductor capacitors and Schottky diodes studied with scanning microwave microscopy at 18 GHz

We measured the DC and RF impedance characteristics of micrometric metal-oxide-semiconductor (MOS) capacitors and Schottky diodes using scanning microwave microscopy (SMM). The SMM consisting of an atomic force microscopy (AFM) interfaced with a vector network analyser (VNA) was used to measure the reflection S11 coefficient of the metallic MOS and Schottky contact pads at 18 GHz as a function of the tip bias voltage. By controlling the SMM biasing conditions, the AFM tip was used to bias the Schottky contacts between reverse and forward mode. In reverse bias direction, the Schottky contacts showed mostly a change in the imaginary part of the admittance while in forward bias direction the change was mostly in the real part of the admittance. Reference MOS capacitors which are next to the Schottky diodes on the same sample were used to calibrate the SMM S11 data and convert it into capacitance values. Calibrated capacitance between 1–10 fF and 1/C2 spectroscopy curves were acquired on the different Schottky diodes as a function of the DC bias voltage following a linear behavior. Additionally, measurements were done directly with the AFM-tip in contact with the silicon substrate forming a nanoscale Schottky contact. Similar capacitance-voltage curves were obtained but with smaller values (30–300 aF) due to the corresponding smaller AFM-tip diameter. Calibrated capacitance images of both the MOS and Schottky contacts were acquired with nanoscale resolution at different tip-bias voltages.

EPR, FTIR, thermal and electrical properties of VO2+ doped BaCl2⋅BaO⋅B2O3 glasses

Glasses with composition xBaCl2⋅(30-x)BaO⋅68B2O3⋅2V2O5 (x=0, 2, 5, 10, in mol%) were prepared by the conventional melt quenching technique. The Electron paramagnetic resonance (EPR) spectra have been recorded at room temperature and the Spin Hamiltonian Parameters (SHPs) of VO2+ ion were calculated. It was observed that with the increase in BaCl2:BaO ratio, the tetragonal symmetry of the V4+O6 complex was improved and there was an expansion in the 3dxy orbital. Infrared (IR) transmission spectra were recorded at room temperature using FTIR spectroscope and it was found that in glass structure, BO3 triangular units are being replaced by the BO4 tetrahedral units on addition of halide content. The glass transition temperature (Tg) and crystalline temperature (Tx) were determined using Simultaneous Thermal Analyzer (STA). The trends in Tg and Tx suggests that there is a decrease in glass stability and glass forming ability (GFA) with increasing Cl− ions content. The dc conductivity & impedance spectroscopy has been analyzed in the temperature region from 353K to 573K and in the frequency range from 10Hz to 7MHz. The frequency dependence of conductivity was found in accordance with the Jonscher’s relation.