Abstract: CuFeO2 single crystal, synthesized by the flux method, is a narrow band gap semiconductor crystallizing in the delafossite structure with a direct optical transition of 1.63 eV. The oxide exhibits a good chemical stability; the semi-logarithmic plot gave an exchange current density of 0.60 µA cm−2 in KCl (0.5 M) electrolyte. CuFeO2 shows p-type conductivity; the origin of acceptors Cu2+ results from oxygen insertion in the layered lattice where most of excess holes are trapped in surface-polaron states. The electrochemical study is confined in the (a,b) plane and reversible oxygen intercalation is evidenced from the intensity potential characteristics. The detailed photoelectrochemical studies have been reported for the first time on the single crystal. The photocurrent is ascribed to the transfer Cu+:3d→3d. The capacitance measurement (C−2–V) shows a linear behavior from which a flat band potential of +0.54 VSCE and a density N A of 1.60 × 1018 cm−3 were determined. The valence band, located at 5.33 eV below vacuum, is made up of Cu-3d orbital typical of delafossite oxides. The Nyquist plot shows a semicircle attributed to a capacitive behavior with a low density of surface states within the gap. The centre is localized below the real axis with an angle of 16.2° ascribed to a constant phase element (CPE), a single barrier of the junction CuFeO2/electrolyte and one relaxation time of the electrical equivalent circuit.

Abstract: This paper reports the establishment and interpreting of an equivalent electrical circuit representing the humidity-sensitive electrical properties of TiO 2 –WO 3 thick films. Prototype sensors have been prepared by depositing an emulsion of TiO 2 and WO 3 powders in acetone with cellulous glue onto an alumina substrate, by a spin coating technique using a low spreading speed (250 rpm). Films were prepared with five different atom proportions of Ti and W: 100:0, 100:1, 100:6, 100:18 and 100:36. The oxides thick films were successively fired at the temperatures of 600, 700, 900, 1100 and 1300 °C, for 2 h at each of these temperatures. The variation of the electrical signal with humidity in ceramic sensors is originated by the chemical and physical sorptions of water molecules existing in the atmosphere. For all the sensors, the same type of circuit was established based both on circuit theory and on the physical mechanisms underlying conduction and polarization. The best fitting for the Nyquist plots obtained at the temperature of 25 °C and various relative humidities (RHs), in the frequency range 0.1 Hz to 40 MHz, was achieved with two R , C parallel circuits in series with two parallels of constant-phase elements (CPEs) and capacitances. As examples, for two of the sensors, TiW2 600 and TiW2 700, the values of the electrical components are presented in tables and graphs. The evaluated values of the components are interpreted and compared, and conclusions could be drawn about the mechanisms of conduction and polarization taking place in the adsorbed water.

Abstract: This paper presents a predictive model for electrochemical impedance Nyquist plots using artificial neural network. The proposed model obtains predictions of imaginary impedance based on the real part of the impedance as a function of time. The model takes into account the variations of the real impedance and immersion time of steel in a corrosive environment, considering constant carboxyamido-imidazoline corrosion inhibitor concentrations (5 and 25 ppm). For the network, the Levenberg–Marquardt learning algorithm, the hyperbolic tangent sigmoid transfer function, and the linear transfer function were used. The best-fitting training data set was obtained with five neurons in the hidden layer for 5 ppm of inhibitor and two neurons in the hidden layer for 25 ppm of inhibitor, which made it possible to predict the efficiency with accuracy at least as good as that of the theoretical error, over the whole theoretical range. On the validation data set, simulations and theoretical data test were in good agreement with an R value of 0.984 for 5 ppm and 0.994 for 25 ppm of inhibitor. The developed model can be used for the prediction of the real and imaginary parts of the impedance as a function of time for short simulation times.

Abstract: A polycrystalline sample of Ba5DyTi3V7O30 (BDTV) was prepared by a mixed-oxide method at 1,000 °C (sintering temperature). Preliminary X-ray structural analysis confirmed a single-phase formation of the compound. Scanning electron micrograph of the material show uniform distribution of grains of different shape and size on the surface of the sample. Detailed studies of dielectric properties in a wide range of temperature (30–400 °C) and frequencies (103–106 Hz) exhibit ferroelectric (diffuse) phase transition, which was confirmed by appearance of hysteresis loop at room temperature. Electrical properties (impedance properties) of the material were obtained using a complex impedance technique. The Nyquist plots confirmed the contribution of grain in the material. Studies of electrical conductivity (both dc and ac) over a wide temperature range suggest that the compound exhibits the negative temperature coefficient of resistance behavior. The ac conductivity spectra were found to obey Jonscher’s universal power law.

Abstract: The corrosion resistance of 12CrMoV and SS304 steels in contact with a molten mixture of (55–45) mol.% ZnCl2–KCl, similar to that found in waste-incineration plants, has been examined in air at 400 °C by the electrochemical-impedance-spectroscopy (EIS) technique. The initial Nyquist plots of 12CrMoV showed a semicircle at high frequency and a line at low frequency indicating a diffusion-controlled reaction. At a later stage, the Nyquist plots are composed of a small semi-circle at high frequency, a line at medium frequency and a large semi-circle at low frequency, similar to that shown by SS304 during the whole experimental test. The larger impedance of SS304 as compared to 12CrMoV may be attributed to the presence of Ni and to the higher Cr content of SS304. Equivalent circuits representing the features of the corrosion of 12CrMoV and SS304 are proposed to fit the corresponding impedance spectra, and the electrochemical parameters in the equivalent circuits are calculated.

Abstract: Thin sensitive layers of polypyrrole (PPY) are deposited onto alumina sensor substrates by matrix-assisted pulsed laser evaporation (MAPLE) technology. The depositions are carried out with the KrF excimer laser from dimethylsulfoxide (DMSO) matrix. After 1-month “ageing” period the complex impedance of prepared sensors is measured in the frequency range from 15 Hz to 10 MHz in following atmospheres: “pure” synthetic air and synthetic air with variable (23% or 90%) relative humidity. The general character of Nyquist diagrams, plots of complex admittance versus frequency, parameters of sensor equivalent circuit and phase-angle sensitivities are then evaluated from the obtained impedance data. While equivalent circuit capacity ( C ) of PPY-based sensors remains from 1.7 to 2.0 pF in all the above-mentioned atmospheres, equivalent circuit resistance ( R ) varies widely from 6 to 140 MΩ, mostly in dependence on surrounding atmosphere humidity. The differences in charge-transport mechanisms between PPY and tin dioxide sensitive layers and also influence of surrounding atmosphere to electric properties of PPY are discussed.

Abstract: Polythiophene (PTP) and poly (vinyl acetate) (PVAc) composite films were prepared by chemical oxidative polymerization method with FeCl3 as an oxidant, in methanol at room temperature. Their dc conductivities as a function of temperature (313–358 K) were measured. An attempt has been made to investigate the effect of temperature and concentration of ferric chloride oxidant on the conductivity of polythiophene–poly(vinyl acetate) composite films. For fixed wt% of PVAc, the dc electrical conductivity of the film initially increases with the molar concentration of FeCl3 and then decreases with the further increase in the concentration of FeCl3. The temperature dependence of conductivity showed Arrhenius behavior. The conductivity isotherms show VTF behavior which is typical for ion conducting polymers. The Nyquist plot of Z′ vs. Z″ were plotted for frequency range 100 Hz–200 kHz and temperature range of 313–358 K. These plots consist of semicircles for higher temperatures. This suggests Debye type relaxation mechanism. The equivalent circuit consists of parallel combination of bulk resistance RB and bulk capacitance CB. The bulk resistance RB of the samples decreased with increase in temperature.

Abstract: Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry were used to investigate the oxide layer formed on a copper disc electrode and the changes that took place when treated potentiostatically in the range of -0.3 V to 0.9 V in aqueous buffer solution of pH 9.2. Equivalent circuits were used to model the response of the electrode, initially at equilibrium to an applied potential. These circuits were proposed for different potential ranges in order to illustrate the Cu/oxide/electrolyte system and its properties in terms of 2 interfaces. A criterion for the applicability of equivalent circuit models was discussed. Changes in the film/metal interface as a function of potential were probed at 30 mHz from Nyquist plots. Diffusion coefficient calculated for the ionic movement in the film at 2 potential values using EIS data was of the order of 10-9 cm2 s-1.

Abstract: This paper presents the dynamic electrothermal simulation of a rectangular resistor integrated on a semi-conductor substrate. Due to the temperature dependence of the electrical conductivity of the resistive sheet, self-heating provokes a coupling between the electrical and thermal problem and gives rise to nonlinear phenomena. We introduce a time stepping iterative method to perform the calculations. The electrical and thermal solvers are based on FEM and Green's functions techniques, respectively. An extensive dynamic analysis of the device will be presented. The results include heating and cooling curves, Nyquist plot (complex locus) of the thermal impedance, time constant spectrum and structure function. Comparisons with the linear case, i.e. a temperature independent resistor, are made and accompanied by analytical approximations if possible. One key observation is that the nonlinearity may easily be overlooked: its detection is only possible in particular characteristics.

Abstract: The paper presents development of a program in the MATLAB for the analysis of Fractional Order Control Systems (FOCS). Using this program, frequency response plots such as Nyquist plot, Bode plots and Nichols plot can be obtained. Gain and phase margins of FOCS can be estimated and suitable controller can be designed for the related control system.

Abstract: Electrochemical impedance spectroscopy (EIS) and Kramers-Kronig (K-K) transforms were made on American Iron and Steel Institute (AISI) 304 stainless steel (SS) in naturally aerated sulfuric acid solution at room temperature. The K-K relations testify that the systems under investigation comply with the linearity, causality, and stability constraints of linear system theory (LST) and thereby validate the EIS data. The polarization resistance decreased with removing of passive film and pitting formed in the passive layer, due to layer thinning or pitting caused by chloride. The impedance data for 304 SS with passive films can be accurately modeled using individual components of the equivalent circuits. The polarization resistances (Rp) of the 304 SS can be confirmed by Nyquist plots and estimated from the anodic polarization curves. The reaction model of the dissolution-passivation process of 304 SS in acid solution is proposed.

Abstract: An exact analytical expression for the complex thermal impedance Z of multi-finger microelectronic components is presented in this paper. The integral transform technique has been used to obtain this expression and solve the three dimensional heat conduction equation directly in the frequency domain. Calculations were first performed for a single-finger on a single-layer structure in order to compare the results with those available in the literature and hence validate the solution. Generally, the comparison shows good agreement between our results and those given in most publications. When the structures are composed of several layers, the thermal impedance changes with the thermal conductivities and the thicknesses of the different layers. It is also affected by the thermal contact resistance between the layers. Some results illustrate the influence of these parameters. The case of a multi-finger component is then treated and the influence of distances between fingers is investigated. For all cases, the Nyquist diagram (i.e. Im(Z) versus Re(Z) for different pulsation values ω) is plotted. Mainly two zones are observed: one for the high frequencies and the other for the lower ones. The substrate dimensions are found to largely influence the scale of the low frequency zone whereas the distance between the fingers influences the higher one. Finally, the solution is applied to a multi-finger device in contact with a heat sink.

Abstract: The Nyquist criterion is a valuable design tool with applications to control systems and circuits. In this paper, it was shown that many Nyquist plots are classical plane curves. Surprisingly, this connection seems to have gone unnoticed. It was determine that the precise shapes of several Nyquist curves and relate them to the shapes of the classical plane curves. Some classical plane curves are related to exactly proper or improper loop transfer functions, which do not roll off at high frequencies and thus are not physical.

Abstract: Stability analysis in compliant mechanism (CM) design is of utmost importance. From a practical point of view, a CM that is unstable is of no significance (has no practical value). Three useful plots were considered in the evaluation of each of the dynamic models of nine configurations of compliant constant-force compression mechanisms (CCFCMs) for their stability characteristics, which includes the polar plot based on the Routh-Hurwitz stability criterion, the Bode plot, and the Nyquist diagram which considers stability in the real frequency domain. Frequency-domain stability criterion is very useful for determining suitable approaches to adjusting the CCFCM parameters in order to increase its relative stability. The results obtained show that the CCFCMs investigated do exhibit higher relative stability for higher values of damping ratio, and for zero damping ratio, all the CCFCMs investigated were unstable. The result also show that for the CCFCMs investigated to be stable, damping ratio must be greater than 0.03 (ξ > 0.03) and depending on what attributes are most desirable, the CCFCM parameters can be optimized to achieve the desired results. Nyquist criterion provides us with suitable information concerning the absolute stability and furthermore, can be utilized to define and ascertain the relative stability of a system. Keyword

Abstract: Poly(3-methylthiophene) (PMT) doped with perchlorate anion was prepared electrochemically in the presence of sodium perchlorate (ClO4 ) salt as the supporting electrolyte. The effect of current density, various initial monomer concentrations (0.1, 0.2, 0.5 M) and electrochemical impedance spectroscopy (EIS) on polymer films has been investigated. Cyclic voltammetry of PMT has been examined on carbon fiber microelectrode ( ~7 μm radius) in 0.1 M acetonitrile medium containing NaClO4 in the range of 0.0 to 2.0 V versus Ag in the presence and absence of 3-methylthiophene. Thin films were characterized by Fourier transform infrared attenuated transmission reflectance (FTIR-ATR) spectroscopy and scanning electron microscopy (SEM). SEM revealed a globular, branched and fibrous morphology of poly(3-methylthiophene). After surface modification of carbon fiber microelectrode (CFME), capacitance properties were checked by using electrochemical impedance spectroscopy. Capacitive behaviors of coated CFMEs were defined via Nyquist plot, Bode – magnitude plot, Bode – phase definitions; besides, conductance of modified microelectrodes was studied with Admittance plot. PMT / CFME exhibits high specific capacitance of ~ 4.12 F g -1 in the initial monomer concentration of 0.1 M, in comparison with a value of ~ 3.75 F g -1 for 0.2 M and ~ 2.32 F g -1 for 0.5 M.

Abstract: In this paper, a bi-facial silicon solar cell under white light illumination and under the application of constant magnetic field is studied in frequency modulation. To determine parameters such as series resistance, parallel resistance and capacitance, we utilized the Nyquist diagram by plotting the imaginary versus real part of the solar cell impedance. The use of the Bode diagram p ermits us to extract the minority carrier lifetime. Thus, all electric and electronic parameters depend on the ap plied magnetic field.

Abstract: This study focuses on the performance characteristics of polymer electrolyte membrane fuel cell (PEMFC) using the AC impedance technique. The experiment was carried out to investigate the optimal operating conditions of PEMFC such as cell temperature, flow rate, humidified temperature and back-pressure. The fuel cell performance was analyzed by DC electronic-loader with constant voltage mode and expressed by voltage-current density. Additionally, AC impedance was measured to analysis of ohmic and activation loss and expressed by Nyquist plot. The results showed that the cell performance increased with increase of cell temperature, air flow rate, humidified temperature and backpressure. Also, the activation loss decreased as the increase of cell temperature, air flow rate, humidified temperature and backpressure.

Abstract: The gold electrode was modified by noisomes, prepared by sonicating the mixed solution of PEG 6000/Tween 80/Span 80/H2O. Electrochemical cyclic voltammetry (CV) and impedance spectroscopy (EIS) were performed for characterizing the modification. The construction of multilayer films of octadecanethiol (ODT)-niosomes had almost no pinholes, which elicited that niosomes-coated electrode through ODT assembly was more effective immobilization compared to direct modification method. A large semicircle formation in the entire range of frequency indicated the complete electron-transfer control for the redox reaction, implying a perfect blocking behavior. The Nyquist plot was consisting of two depressed semicircles after storage for 36 h in air, which implied a progressive and uniform construction manner, as expected for porous coatings. The modified electrode was used to investigate the interaction between 1-anilino-8-naphthalene-sulfonate (ANS) and noisomes. It was found that the low-frequency semicircular arcs increase in diameter, indicating that the increase in the resistance Rp attributed mainly to the niosomes/solution interface. It was due to the binding of ANS to the niosomes-modified electrode surface, exhibited against diffusing species through the pores. Copyright © 2009 John Wiley & Sons, Ltd.

Abstract: The sticking of a soft polystyrene colloidal particle to a planar glass plate was studied by a microrheological technique using an optical tweezer to trap the particle and a piezoelectric-stage to position the plate and to sinusoidally drive it in-planeThe sticking process was well-described by a crossover of the mechanical coupling between the plate and the particle from being dominantly viscous to becoming dominantly elastic. By varying three parameters in the experiment, namely, the strength of the tweezer potential, the strength of the particle-plate interaction and the strength of the oscillatory drive, three different regimes of dynamics - stuck, non-stuck and ageing were observed, corresponding to the coupling that is elastic, viscous and viscoelastic, respectively. The observations of [1] can be illustrated better by a parametric Nyquist plot of the real and imaginary parts of the effective rigidity modulus of the coupling medium. Upon contact between the particle and the plate, these parameters change abruptly from their bulk values, regardless of whether the particle is stuck or non-stuck in the terminal steady state. In the ageing regime, where sticking evolves gradually in time, the system shows a parametric plot significantly different from what would be expected from a single-relaxation process. A tendency of clustering of the data in the Nyquist plot provides an interesting contrast between truly many-body systems and one with a few degrees of freedom.

Abstract: The kinetics of lithium-ion transfer at a Li0.35La0.55TiO3 (LLT)/liquid electrolyte interface were investigated by four-probe ac impedance spectroscopy. In Nyquist plots for a system consisting of ...

Abstract: The Australian wine industry has been undergoing an unprecedented growth during the last decade in both winery establishments and export volumes. Though automated controlling processes are useful for maintaining high quality products and reducing costs, the techniques are not widely adopted by small to medium sized wineries due to their relative high initial costs. In this paper, we investigate the dielectric properties of ethanol and organic acids, two major chemical ingredients that need to be monitored during wine fermentation using electrochemical impedance spectroscopy. The relationships between solution concentrations and parameters derived from the measured impedance spectra were investigated by two methods, the Nyquist plot approach and the equivalent circuit modeling approach. The investigation was carried out for both pure solutions including ethanol, malic acid and tartaric acid, the major constituents of grape must, and their various mixed compound solutions to simulate the grape must under fermentation. Strong linear as well as inverse-squared relations were found between the concentrations of the pure solutions and the derived dielectric parameters, e.g., the diameter of the Nyquist plot for ethanol has a correlation with concentrations with a coefficient of determination of 0.9796 and the R s value obtained from electric circuit modeling also shows a strong inverse-squared relationship with the concentration of both malic and tartaric acids. A further investigation suggests that the individual concentration of mixed acid solutions can be handily determined by measuring the impedance of the mixed acid solutions and using the initial concentration value of one of the mixed acids. Furthermore, the dielectric properties of mixed ethanol, malic acid and tartaric acid show a combined effort from mixed acid solutions and pure ethanol. These findings suggest that the electrochemical impedance, which can be easily measured with satisfactory accuracy, is a good indicator of the concentration change of the essential ingredients of grape must and thus can be further utilized to develop a feedback loop for wine fermentation automatic control.

Abstract: In designing and controlling fuel cell systems, it is advantageous to have models which predict fuel cell behaviour in steady-state as well as in dynamic operation. This work examines the use of electro-chemical impedance spectroscopy (EIS) for characterising and developing an impedance model for a high temperature PEM (HT-PEM) fuel cell stack. A Labview virtual instrument has been developed to perform the signal generation and data acquisition which is needed to perform EIS. The typical output of an EIS measurement on a fuel cell is a Nyquist plot, which shows the imaginary and real parts of the impedance of the measured system. The full stack impedance depends on the impedance of each of the single cells of the stack. Equivalent circuit models for each single cell can be used to predict the stack impedance at different temperature profiles of the stack. The information available in such models can be used to predict the fuel cell stack performance, e.g. in systems where different electronic components introduce current harmonics.