Abstract: A CoCO3–polypyrrole composite (CC–PPy) for lithium ion battery anodes was prepared by first synthesizing urchin-like CoCO3 microspheres (CC) via a hydrothermal route and further modifying them with a PPy coating. The resulting CC–PPy exhibits excellent cycling stability, outstanding rate performance and a great recovery capability compared to CC, delivering a reversible capacity of 1070.7, 811.2, 737.6, 518.7, 504.5 and 559 mA h g−1 after 100 cycles at 0.1, 1, 2, 3, 4 and 5 C, respectively, and a recovery capacity of up to 1787 mA h g−1 after 500 cycles from 1 to 5 C. A more comprehensive lithium storage mechanism of CoCO3 has been proposed to support the experimental data, which includes two-step conversion reactions with a total theoretical value of 7 Li per CoCO3. The ‘first-order’ reaction involves reduction of CoCO3 to metallic Co and the formation of Li2CO3, and the second reaction involves the further reduction of Li2CO3 to LixC2 (x = 0, 1, 2), along with the formation of Li2O. The lithiation and delithiation processes of CC and CC–PPy have been compared based on their potential profiles and CV curves, which show clear two-order character. The kinetic factors for the superior performance of CC–PPy are analyzed based on the Nyquist plots. Furthermore, the transition from CoCO3 to Li2CO3 to Li2O and its reversibility is confirmed by ex situ IR spectra recorded at the different discharge–charge states of CC–PPy.

Abstract: The potentiodynamic polymerization of pyrrole is carried out on Pt electrodes using sodium dodecyl sulphate and the coated electrode is employed for sensing of urea non-enzymatically. Employing amperometric and impedimetric methods, urea is estimated in mildly acidic conditions. A detection limit of 40 μM is obtained along with a linearity of 80–1440 μM and a sensitivity of 1.11 μA μM−1 cm−2 from the amperometric data. The Nyquist plot is employed for the construction of the calibration curve from the impedance analysis. The interference of different compounds such as ascorbic acid, uric acid, sodium chloride and calcium chloride is also analyzed.

Abstract: This chapter is devoted to the analysis of the single degree-of-freedom oscillator, with and without viscous damping, in the time domain: Impulse response and convolution integral, and in the frequency domain: harmonic response, dynamic amplification, quality factor. The chapter also reviews various representations of the frequency response function: Bode plots and Nyquist plot. The Beat phenomenon resulting from an excitation close to the resonance frequency is analyzed. Finally, the state space forms of the equation of motion are introduced. The chapter ends with a set of problems.

Abstract: NiO nanoparticles were prepared by wet chemical precipitation and dielectric, AC conductivity, and impedance properties were studied by complex impedance spectroscopy as a function of frequency at different temperatures. Non-stoichiometric sample (Ni 40 O 60 ) shows diffraction peaks corresponding to cubic NiO and near stoichiometric sample (Ni 48 O 52 ) was found to be amorphous. Stoichiometry of NiO nanoparticles was determined using EDS analysis. The estimated dielectric constant was found to be 35 and 40 for Ni 40 O 60 and Ni 48 O 52 nanoparticles. Loss peak shifts to the higher frequency which is due to long range hopping of charge carriers. Impedance analysis reveals Debye type relaxation process. The presence of two semicircular arcs in Nyquist plot explains that the grain and grain boundary conduction prevails in the sample. Nyquist plot shows that NiO nanoparticles possess negative temperature coefficient of resistance (NTCR). The variation of AC conductivity as function of temperature indicates that the conduction is due to thermally activated charge carriers.

Abstract: Background: The electrical properties of agarose gel, namely impedance and capacitance, are relatively unexplored. Agarose gels are used as in vitro models in studies across numerous disciplines, including imaging, radiotherapy, infusion, and neurosurgery. Purpose: In this study, we seek to characterize the impedance response of low concentration agarose gels by relating the gel concentrations to Nyquist Plot phase in order to establish a baseline with which to modify the response of the gel to simulate that of in vivo brain tissue. This information is relevant to areas such as deep brain stimulation, and could have a significant impact on in vitro model design for such studies in the future. Methods: Ten agarose gels spanning four different concentrations were subjected to impedance spectroscopy using a Model 3387 DBS electrode. Phase angles were calculated and Cartesian Nyquist plots generated from the data. Results: Results suggest that an inverse relationship exists between agarose gel concentration and phase angle. In addition, the results indicate that agarose gel reasonably emulates a constant phase element, which portrays the electrode-electrolyte interface impedance of some equivalent circuit models of brain tissue. Conclusion: The data shows that agarose gel is a suitable substrate for a deep brain stimulation in vitro model, but requires modification. In the future, we plan to utilize this data to determine the modifications necessary in the current agarose gel model to make it scientifically applicable to studies of both deep brain stimulation and infusion due to their overlapping variables.

Abstract: ZnO nanorods were synthesized using a low-cost sol-gel spin coating technique. The synthesized nanorods were consisted of hexagonal phase having c-axis orientation. SEM images reflected perpendicular ZnO nanorods forming bridging network in some areas. The impact of different hydrogen concentrations on the Pd-sensitized ZnO nanorods was investigated using an impedance spectroscopy (IS). The grain boundary resistance (Rgb) significantly contributed to the sensing properties of hydrogen gas. The boundary resistance was decreased from 11.95 to 3.765 kΩ when the hydrogen concentration was increased from 40 to 360 ppm. IS gain curve showed a gain of 6.5 for 360 ppm of hydrogen at room temperature. Nyquist plot showed reduction in real part of impedance at low frequencies on exposure to different concentrations of hydrogen. Circuit equivalency was investigated by placing capacitors and resistors to identify the conduction mechanism according to complex impedance Nyquist plot. Variations in nanorod resistance and capacitance in response to the introduction of various concentrations of hydrogen gas were obtained from the alternating current impedance spectra.

Abstract: Micro-arc oxidation (MAO) films on AZ31 magnesium alloy were modified by high-intensity pulsed ion beam(HIPIB) irradiation with ion energy of 300 keV at 200 A/cm 2 with 1–10 shots. A compact and solidified layer was prepared on the irradiated MAO films. The electrochemical impedance spectrum (EIS) of the MAO films in 3.5% NaCl solution was measured with varying the immersion time. The Nyquist plots for EIS showed typical capacitive arc and inductive loop. The diameters for capacitive arc and inductive loop are both increased and then decreased with increasing irradiation shot number, the maximal value for diameters obtained at 200 A/cm 2 with 5 shots at immersion time of 5 h achieved to 6 × 10 7 Ω and 1.5 × 10 5 Ω, respectively. With immersion time augment, the diameters for capacitive arc and inductive loop of Nyquist plots both decreased. Based on the results of EIS measurements, appropriate equivalent circuits for the modified layers were proposed to fit the EIS for MAO films. It is found that the enhancement in corrosion property of ablated MAO films is mainly caused by improvement of continuity and compaction of films irradiated by HIPIB, which suppressed the corrosion process by holding back the transfer of electrolyte through the modified films to magnesium substrate during immersion.

Abstract: An analytical impedance model and a small-signal equivalent circuit are derived for the impedance spectra of Li-air batteries with porous cathodes. The model takes into consideration the effects of the oxygen diffusion, double layer, and faradaic processes in the cathode and can be applied to Li-air batteries with organic and aqueous electrolytes operating under d.c. discharge. It is shown that the cathode of Li-air batteries can create two slightly asymmetrical semicircles on the Nyquist diagram: one at low frequencies, where the oxygen diffusion dominates the operation of the cell and one at medium frequencies due to the combined effects of the double-layer capacitance and faradaic processes. The second semicircle becomes negligibly small at low values of the cathode width oroxygenconcentration. Bothsemicircles candegenerate intoone large semicircle whenthe doublelayer capacitance is large enough and masks the effects of the faradaic processes, which happens at large values of the specific area of the cathode and double layer capacitance, or when the oxygen diffusion coefficient in the electrolyte is relatively large. They also degenerate into one semicircle when the porosity is decreased, for instance during the final period of the discharge of Li-air batteries with organic electrolyte, when the cathode is partly clogged with the deposit reaction products. The elements of the small-signal equivalent circuit are expressed in terms of the oxygen diffusion coefficient, oxygen concentration, discharge current, and other material and kinetic parameters, which make our model instrumental for extracting information about the material structure, reaction processes, and diffusion in the cathode. Based on the derived analytical results, we also propose a method to extract the effective value of the oxygen diffusion coefficient and reaction rate constant from the experimental impedance spectra of the cells. A simplified small-signal equivalent circuit model is also presented. This model contains only elementary components such as resistors and capacitors and can be implemented numerically in circuit simulators.

Abstract: In the present work, the electrochemical behaviour of LiFePO4/C electrode has been reported. Specially, the electrochemical impedance spectroscopies (EIS) have been studied in detail. The discharge capacity is more than 120 mAh/g. There are two semicircles being found in the Nyquist plot for the cycled electrode and one semicircle for the as-prepared electrode. It is found that the interface capacitance is in an order of magnitude of 10 μF/cm2 for the high-frequency semicircle, while for the second semicircle the interface capacitance is 5.3~45.4 × 103 μF/cm2. It could be concluded that the high-frequency semicircle is to correspond to the charge transfer process. The function of the carbon layer is also briefly discussed.

Abstract: In recent years, significant efforts have been devoted to developing non-destructive techniques for damage identification in structures. This study investigated the effects of cracks and damages on the integrity of structures, with a view to detect, quantify, and determine their extents and locations. Previous works have used parameters, such as, changes in natural frequencies and mode shapes, as detectors. However, such parameters are not sensitive enough to detect early defects. In this paper, phase measurements are sought. Measurements of the acceleration frequency responses at different points on each rotor shaft were taken using a multi-channel frequency analyzer. The damage detection schemes used in this study depended on the changes in the phase of the measured acceleration frequency response functions. To study the changes of phases, it was interpreted in phase spectrum and Nyquist plot. Nyquist plot was used as it includes both real and imaginary parts of the amplitude and this was used to study phase shifts due to the presence of crack. The changes in phase depended on crack depth and how close the crack is to that mode shape node. Meanwhile, the changes in phase of lower eigenvectors were observed clearly. Thus, first mode shape was helpful in identifying the location of the crack. The vibration behavior of the rotor shaft was shown to be very sensitive to the crack depth, crack location and mode number. It is concluded that changes in phase as a function of crack depths and locations can be effective in crack detection methodology.

Abstract: An analytical framework to examine the finite-gain stability for a positive feedback interconnection between two stable, linear time-invariant systems where one system has “mixed” passivity, negative-imaginary and small-gain properties and the other system has “mixed” negative-imaginary, negative-passivity, and small-gain properties is proposed. A classical Nyquist argument is used to examine the stability of the interconnected systems and the usefulness of the proposed framework is illustrated by a numerical example.

Abstract: article i nfo Article history: Received 13 November 2012 Received in revised form 8 January 2013 Available online xxxx The structure, ionic conduction and electrochemical performance of a lead-molybdate-germanate glass with the 10MoO3∙90(7GeO2∙3PbO) composition were studied by means of cyclic voltammetry, electrochemical impedance spectroscopy, FTIR, UV-Vis and EPR spectroscopy. The cyclic voltammogram exhibits three distinct maxima attrib- uted to the reduction of Ag +1 to Ag 0 ,M o +5 to Mo +4 and Pb +2 to Pb 0 . Nyquist diagram of the complex impedance shows two semicircles which appear in the high and small frequency region an das maller arc in the intermediate frequency domain corresponds to the Warburg diffusion or/and mass transport impedance of the cell. Analyzing the Nyquist plots of the complex impedance for different silver nitrate electrolyte solution was found good sensing properties of these cells. Spectroscopic data investigations on glass after cyclic voltammetry show some modifications such as: i) the number of (PbO4) and (MoO6) structural units increases; ii) molybdenum ions appear in the different valence states (Mo +3 ,M o +4 ,M o +5 ,M o +6 ). Our results suggest a mixed ionic-electronic conduction process and a larger mobility of the ions species in the glasses. Then, the modifier lead and molybdenum ions are able to migrate under an applied electric field and are responsible for mixed ionic-electric conduction in ternary molybdenum-lead-germanate glasses.

Abstract: Frequency response analysis (FRA) is a technique for evaluating the mechanical integrity of power transformers. This paper proposes a methodology to locate inter-disc fault in a transformer winding through FRA measurements. A core-type three phase transformer is used for simulating faults on several locations along the interleaved winding, from top to bottom end. Correlation coefficient, vector fitting and Nyquist plot are employed to assist in the analysis. By drawing the transfer function of the winding response on Nyquist plot, key information is extracted and two equations are developed to locate the fault.

Abstract: This study focuses on the characterization of MWNT-epoxy composites for different MWNT concentrations of 0–7 wt% by correlating different dynamic analysis techniques, including DMA, impedance, and DEA. An optimum composition was established at 0.1 wt% MWNTs corresponding to the best MWNT dispersion which resulted in the formation of an optimum MWNT network. The addition of this low fraction of MWNTs in epoxy resulted in stiffening the molecular structure and suppressing certain molecular transitions, raising the dielectric constant especially in the low-to-medium frequency range, raising the electrical conductivity especially at the high frequencies, and increasing the electromagnetic shielding effectiveness. The 0.1% MWNT-epoxy nanocomposite switched the electromagnetic shielding behaviour from being a very effective absorber at low frequencies to being an effective reflector at high frequencies. Finally, the Nyquist plot derived from the dynamic impedance spectroscopy proved most useful at providing evidence of multiple size distribution of MWNT agglomerates.

Abstract: This study presents a new set of formulae for the design of discrete proportional-integral-derivative (PID) controllers under requirements on steady-state performance and robustness specifications, such as phase and the gain margins, as well as the gain crossover frequency. The proposed technique has the advantage of avoiding trial-and-error procedures or approximations connected to an a posteriori discretisation. This method can also be implemented as a graphical design procedure in the Nyquist plane. The plot of suitable regions can be used to check a priori if the problem leads to feasible values of the PID parameters.

Abstract: By transforming an exact stability condition, a new Nyquist-like plot is proposed to predict occurrences of three typical instabilities in DC–DC converters. The three instabilities are saddle-node bifurcation (coexistence of multiple solutions), period-doubling bifurcation (subharmonic oscillation), and Neimark bifurcation (quasi-periodic oscillation). In a single plot, it accurately predicts whether an instability occurs and what type the instability is. The plot is equivalent to the Nyquist plot, and it is a useful design tool to avoid these instabilities. Four examples are used to illustrate the accuracy of this new plot to predict instabilities in the buck or boost converter with fixed or variable switching frequency.

Abstract: Impedance measurement is a common method to study the electrical properties of thin film photovoltaics. For the first time, we use the MATLAB/Simulink environment to extract the complex impedance of the nanostructured heterojunction solar cells. The impedance magnitude, phase and Nyquist plot of the PV are simulated in LTI Viewer and Impedance versus Frequency analysis tools of SimPower GUI block of Simulink. We examined a variety of the equivalent circuits consisting of capacitance, series and shunt resistances representing the solar cell structure. The model uses the parameters with values reported in the literature at room temperature and zero bias. The effect of the additional capacitance and resistances in the equivalent circuits on the impedance components of the cells is considered by Simulink environment.

Abstract: Electrochemical impedance spectra (EIS) for lithium ion insertion and extraction in α-MoO3 cathode material were obtained at different potentials during initial discharge–charge cycle. A significant “three semicircles” were obtained at 0.5 V in the Nyquist diagram, and were assigned to lithium ion migration through solid electrolyte interphase (SEI) film, the electronic properties of the material as well as charge transfer step, respectively. An equivalent circuit that includes elements related to the electronic and ionic transport, in addition to the charge transfer process, is proposed to simulate the experimental EIS data. The variations of the resistance of SEI film, the electronic conductivity of the material and the resistance of charge transfer along with the increase and decrease of electrode polarization potential were quantitatively analyzed, and the reasonable explanation is given. Furthermore, the chemical diffusion coefficients of lithium ion in α-MoO3 cathode material were calculated.

Abstract: The effect of different surface preparation methods such as mechanical, chemical, and electrochemical surface preparation as well as ultrasonic cleaning on the formation, stability, and deterioration of surface films formed on austenitic 316 stainless steel was investigated in Tyrode’s solution. The methods of cyclic polarization, AC impedance measurements and surface techniques were used. A hysteresis loop in a cyclic polarization curve was obtained that indicates a delay in re-passivation of an existing pit when the potential is scanned cathodically. Nyquist diagrams consist of two strongly overlapped capacitive semicircles which are assigned to charge transfer and passive film resistance. Electrolytic polishing improves corrosion resistance by increasing the value of the corrosion potential and breakdown potential of the passive layer as well as the pit initiation potential. After chemical passivation and acid cleaning, no perfect passivation region was observed. Change in surface fractal is in good agreement with the result obtained from height roughness factor of AFM.

Abstract: E-mail: mgong@dankook.ac.krReceived April 8, 2013, Accepted June 17, 2013We examined the effect of electrode fingers and gaps of coplanar interdigitated electrode (IDE) structures tocharacterize the ammonium salt-containing polyelectrolyte film of resistance-based humidity sensors. IDEsdesigned for this purpose were flexible gold electrodes deposited on a polyimide substrate using a printingprocess because the geometry presents a potential for tunable sensitivity over other electrode designs. The basicdesign of the sensors consisted of IDEs with a different number of electrode fingers such as 3, 4, and 5 and gapsizes of 310, 360, 410, and 460 µm. Details of the AC complex impedance characteristics such as the Nyquistplot, Bode plot, and activation energy based on electrode construction were investigated. Key Words : Resistive humidity sensor, Bode plot, Nyquist plot, Interdigitated electrode, Electrode patternIntroductionHumidity sensors have attracted increasing attention becauseof the growing demand for accurate measurement and con-trol of environment humidity in many fields.

Abstract: The electrochemical behavior of a symmetrical cell, Pt/Ce0.8Sm0.2O1.9−δ/Pt, under reducing conditions and wide temperature range (250 – 600 °C) is detailed. In terms of the charge carriers transport through the electrolyte microstructure, AC impedance spectroscopy has been applied to address useful concerns about the transport properties over electrolytic and mixed conduction regimes. The impedance spectra at lower temperature and oxygen partial pressure show the electrochemical response of separated bulk and grain boundary contributions. The increase in the electronic conductivity from 250 to 400 °C shows that the electrochemical reduction Ce4+/Ce3+ is as kinetic as thermodynamically favorable in the experimental conditions. In a typical Nyquist plot of an impedance diagram, until temperatures as low as 400 °C, the high and low frequency arcs can be accessed and the influence of reducing atmosphere over both the components is presented. The apparent activation energy for the electronic process (ΔE) extracted from the total conductivity is 2.54 eV. Distinguished bulk (2.34 eV) and grain boundary (2.63 eV) activation energies point the latter as an energetic barrier in the redox reaction. The oxygen partial pressure dependence of individual capacitances suggests storage of electrical charge along grain boundaries which can potentially behave as a chemical capacitor.

Abstract: The aim of this study is to present a theoretical study of a silicon solar cell under frequency modulation. Solving the continuity equation lead to the analytical expressions of the minority carriers' density, the photocurrent and the photo voltage. The photocurrent and the photo voltage are plotted versus the modulation frequency in a semi-logarithmic scale for various incidence angles; the dynamic impedance of the solar cell is then derived. The Nyquist diagram of the impedance is plotted for various incidence angles, leading to the determination of the series and parallel resistances. The determination of the diffusion capacitance is also made based on the Bode diagram of the solar cell impedance. The influence of the modulation frequency and the incidence angle on the diffusion capacitance and the series and parallel resistances is then exhibited.

Abstract: This paper proposes a methodology to interpret frequency response measurements of a deformed transformer winding due to conductors tilt. To achieve that, geometrical parameters of a three phase prototype transformer is used to simulate the frequency response of one of the windings using the multi-conductor transmission line model. To simulate the response, inter-turn, inter-disc and inter-winding capacitances of the winding based on normal and deformed conditions are computed using finite element method. Using the computed values, the frequency response for both normal and deformed conditions are simulated. The simulated responses are studied by subdividing into four frequency regions and compared using the correlation coefficient. For further analysis, the vector fitting algorithm was implemented to approximate the transfer function of each response. All transfer functions are then represented in the Nyquist diagram to analyse the shape of each plot. From the Nyquist diagram, it was observed that the real minimum of each plot increases as the winding deformation increases. This finding can be used to estimate the winding deformation severity based on the plotted attributes.

Abstract: New inorganic-organic hybrid [(C3H7)4N]2Hg2Cl6 compound was obtained and characterised by single-crystal X-ray diffraction, infrared, and impedance spectroscopy. The latter crystallizes in the monoclinic system (space group C 2/c, ) with the following unit cell dimensions: (1) A, (6) A, (2) A, and (2). Besides, its structure was solved using 84860 independent reflections leading to . Electrical properties of the material were studied using impedance spectroscopic technique at different temperatures in the frequency range of 209 Hz to 5 MHz. Detailed analysis of the impedance spectrum suggested that the electrical properties of the material are strongly temperature-dependent. The Nyquist plots clearly showed the presence of bulk and grain boundary effect in the compound.