Abstract: A solid-state electrochemical NOx sensor was fabricated by using a closed-one-end yttria-stabilized zirconia (YSZ) tube, an oxide sensing electrode (SE) and a Pt counter electrode (CE). The complex impedance of the device using each of several oxide SEs was measured in the frequency and the temperature ranges of 0.1 Hz to 100 kHz and 600–700 °C, respectively. In most cases, a large semicircular arc was observed in complex impedance spectra (Nyquist plots) in the lower frequency range examined in dry-air flow. Only in the case of the device using ZnCr2O3 SE, the semi-arc shrank to some extent upon exposure to NOx gas. The resistance value (Z′) at the intersection of the semi-arc with the real axis at lower frequencies (around 0.1 Hz) varied with concentration of both NO and NO2 in the sample gas. The impedance value at 1 Hz of the present device was found to vary almost linearly with the concentration of NO (or NO2) from 50 to 400 ppm. The 90% response and 90% recovery times were as short as less than few seconds at 700 °C. Furthermore, it is noted that the sensitivity of NO is almost equal to that of NO2. This indicates that the present device can detect the total NOx at higher temperatures.

Abstract: A new tuning method for PID controller design is proposed for a class of unknown, stable, and minimum phase plants. We are able to design a PID controller to ensure that the phase Bode plot is flat, i.e., the phase derivative w.r.t. the frequency is zero, at a given frequency called the "tangent frequency" so that the closed-loop system is robust to gain variations and the step responses exhibit an iso-damping property. At the "tangent frequency", the Nyquist curve tangentially touches the sensitivity circle. Several relay feedback tests are used to identify the plant gain and phase at the tangent frequency in an iterative way. The identified plant gain and phase at the desired tangent frequency are used to estimate the derivatives of amplitude and phase of the plant with respect to frequency at the same frequency point by Bode's integral relationship. Then, these derivatives are used to design a PID controller for slope adjustment of the Nyquist plot to achieve the robustness of the system to gain variations. No plant model is assumed during the PID controller design. Only- several relay tests are needed. Simulation examples illustrate the effectiveness and the simplicity of the proposed method for robust PID controller design with an iso-damping property.

Abstract: A negative characteristic of low-emission flames is their weak dynamic flame stability, which leads in many cases to periodic pulsations of heat release and combustor pressure. Due to the extremely complex nature of the forcing and feedback mechanisms in flows with heat release, reliable prediction of the stability limits of combustors has not been achieved yet with satisfactory accuracy. The dynamics of complicated thermoacoustic systems, which are modeled on the basis of linear acoustics and a representation of the system as a network of acoustic elements, can be analyzed using methods derived from control theory. It was shown in a companion article (Sattelmayer and Polifke, Combustion Science and Technology , vol. 175, pp. 453-476, 2003) that the commonly employed open-loop Bode plot stability analysis can lead to erroneous results. Similar problems may occur also for a Nyquist diagram, if the interpretation of the open-loop gain curve follows the standard rules of traditional control theory. A lack of...

Abstract: In this paper, the response to the first harmonic component (2f) of the electrostatic force in single terminal driven electrostatic comb-drive and parallel-plate drive was used as a signal to extract device parameters, namely, the Q-factor and resonant frequency instead of the fundamental (1f) resonance response. It is shown that the difficulty in motional measurement due to electrical cross-talk (parasitics) using 1f measurement can be overcome with a higher signal-to-noise ratio of the 2f signal. Both atmospheric (low-Q) and reduced pressure environment were investigated using off-chip electronics and lock-in amplifier. The measurements were done on the electrostatic comb-drive and capacitive parallel plate sensing plates that form the two core modules of a yaw rate sensor (dual-axis resonator). The effects of AC and DC bias voltages on the measured response have been investigated. Experimental amplitude and phase response data have been analysed using the Lorentzian curve-fit, Resonance Curve Area (RCA) method, the half-power bandwidth method (3 dB) and the Nyquist plot for data fitting and determination of the Q-factor and resonance frequency.

Abstract: This paper deals with impedance measurement of gas sensors with active layers prepared by pulsed laser deposition (PLD) technology. The deposition was carried out from tin dioxide (SnO 2 ) and tin acetylacetonate (SnAcAc)-based targets by KrF (248 nm) excimer laser. The main interest was focused to following problems: measurements of real and imaginary part of complex impedance for frequencies ranging from 100 Hz to 10 MHz (Nyquist diagram), comparison of layers based on both organic and inorganic materials, the influence of dopants (NiO, Fe 2 O 3 , FeAcAc) and catalyst (Pd). As a result of analysis of Nyquist diagrams an alternative phase-angle sensitivity S pa (based on phase shift) to atmosphere containing reducing gas in “pure” synthetic air was proposed.

Abstract: A new approach of data simulation using neural networks (NN) has been proposed for electrochemical impedance spectroscopy, applied for copper electrodeposition from sulphate acidic electrolytes. The trained NN, with data obtained in different experimental conditions (electrode potential and thiourea concentrations), have been used to generate impedance spectra for new electrode potential values, within the investigated potential range, as well as to estimate the maximum frequency value on Nyquist plot, by generating supplementary output data for new frequency values, inside the investigated domain of frequencies.

Abstract: A probe having a pair of closely spaced electrodes is immersed in the lubricant and one electrode is excited with a relatively low voltage AC current. The frequency is swept over a range of about 1-10,000 Hertz and the current and phase angle measured at selected frequency intervals. The reactive (Z″) and resistive (Z″) impedances are computed for each current measurement and values of Z″ plotted as a function of Z′ as a Nyquist plot. The center of curvature of the plot between the origin and the minimum value of Z″ is located; and, the angle of depression of a line from the origin through the center of curvature ⊖ is determined from the plot. Samples of lubricant having known concentration of soot are measured and the angle ⊖ determined for each sample. The angle is then plotted as a function of soot concentration and a smooth curve fitted to the data plots. The curve may be programmed into a microcomputer to be used with the sensor for real time determination of soot concentration.

Abstract: Copper electrodeposition in a gel electrolyte has been studied by an electrochemical impedance spectroscopy (EIS). The electrochemical impedance was measured in a gel electrolyte containing various chloride concentrations. An inductive loop and a second capacitive loop were described on the Nyquist plot of electrochemical impedance at low frequency. These are called Faradaic impedance, which originates from the elementary steps of an electrode reaction. Warburg impedance was observed at low frequency in the diffusion limiting current region, indicating the contribution of the Cu(II) diffusion from the solution bulk to the electrode surface. The electrodeposition model of copper in the gel electrolyte was proposed on the basis of above-mentioned results. The numerical simulations were performed to confirm the proposed electrodeposition model and to obtain the kinetic parameters. Moreover, the influence of chloride was discussed from the obtained kinetic parameters.

Abstract: The electrical properties of polycrystalline Bi6Fe2Ti3O18 are investigated by impedance spectroscopy in the temperature range 30–550°C. The imaginary part of impedance as a function of frequency shows Debye like relaxation. Impedance data are presented in the Nyquist plot which is used to identify an equivalent circuit and the fundamental circuit parameters are determined at different temperatures. The grain and grain-boundary contributions are estimated. The results of bulk a.c. conductivity as a function of temperature and frequency are presented. The activation energies for the a.c. conductivity are calculated. The polaron hopping frequencies are estimated from the a.c. conductivity data.

Abstract: In the present work, impedance spectra of a pitted Inconel alloy 600 electrode were analysed in aqueous 0.1 M Na2S2O3+0.1 M NaCl solution at elevated solution temperatures of 298–573 K and at pressures of 0.1–8 MPa in terms of pit size distribution, fractal dimension and surface roughness in sequence as the solution temperature rose. From impedance spectra of the Nyquist plot obtained from the pitted specimen exposed to solutions at temperatures of 60–150°C, a constant phase element (CPE) was observed in the frequency range from 103 to 1 Hz. Especially, it was found that the impedance spectra were divided into two sections, i.e. a first CPE with a smaller slope in the higher frequency range and a second CPE with a larger slope in the lower frequency range. The occurrence of the two kinds of CPE results from a transition of ion diffusion through the pit to ion accumulation at the pit bottom, which is caused by double-layer charging at the pit bottom and prior double-layer charging at the pit wall. Impedance spectra were simulated in terms of pit size distribution and values of the fractal dimension of the pits as a function of solution temperature to compare with those spectra measured experimentally. The spectra were simulated based upon the conventional transmission line model for a cylindrically shaped pit at 60 °C and on the basis of a modified transmission line model for the triadic Koch construction at 100 °C and for the quadratic Koch construction at 150 °C. The modified transmission line model takes into account the resistive and capacitive elements in the lateral direction as well as those in the downward direction of the three-dimensional Koch constructions. Above 200 °C the Nyquist plots were found to be depressed more noticeably from a perfect semicircular form with increasing solution temperature. This is due to the increase in the surface roughness of the specimen by the formation and growth of the pits.

Abstract: The study of a plain carbon steel (AISI 1020) in Na2SO4 aqueous solutions at different concentrations was carried out by electrochemical impedance spectroscopy (EIS) in order to determine the corrosion mechanism and to obtain representative corrosion rates of the system. EIS was used to measure corrosion current densities at high concentrations in the range 0.1-1 wt% Na2SO4, but in the low concentration range, from 0.001 to 0.01 wt%, a scattered Nyquist plot was obtained. Other electro- chemical techniques, such as polarization resistance (PR), Tafel plots and electrochemical noise (EN), were also used in this analysis. The charge transfer resistance was de- termined and compared with the PR and noise resistance.

Abstract: The use of solid electrolyte cells to detect specific gases is widely applied in the electronic control systems. Ionic conduction in this type of sensors is normally studied through impedance spectroscopy, since the data obtained with this technique provide valuable information about the electrical properties of the solid electrolyte and electrode behavior. This work addresses the influence of inter-electrode capacitance on the impedance characterization of small structures. Parasitic capacitance was determined by solving Laplace equation and integrating the potential over a Gauss surface including the electrode. Computer simulations of electrode impedance behavior of a solid cell were carried out using a well-known electronic simulator. It is concluded that parasitic capacitances introduce a small distortion on the high-frequency portions of Nyquist plot that are important when miniaturized sensor are electrically characterized.

Abstract: We apply the Nyquist method to analyze the stability of small amplitude electrostatic waves in quantum plasmas described by the Wigner-Poisson system. The technique is presented in detail to assess the stability of a threestream equilibrium distribution function. The topology of the Nyquist diagrams is substantially changed with increasing quantum diffraction effects.