Nonlinear least-squares analyses of complex impedance and admittance data for solid electrolytes

A nonlinear least-squares curve fitting technique, suitable for use with a laboratory minicomputer, is described for the analysis of a complex impedance (admittance) spectrum in which distortions of the various shapes constituting the complex plane plot occur because of overlapping time constants. With this technique, estimates of the parameter values occurring in an equivalent circuit are first obtained by a geometrical-fitting procedure and these are then used as the initial parametric values in a complex least-squares method which simultaneously fits the real and imaginary parts of the impedance (admittance) to a model (equivalent circuit), thereby yielding best-fit values of the circuit parameters as well as their confidence intervals. The method is illustrated by analyzing: (1) artificial, computer-generated impedance (admittance) data constructed for some simple RC circuits commonly used to model solid electrolyte behavior; and (2) measured admittance response of the fast sodium ion conductor NASICON at 486 K.     

On the deviation from the theoretical behavior in applying Wagner's polarization method to the solid electrolyte cuprous iodide

An interpretation of the deviations from the theoretical behavior by the application of Wagner's polarization method to CuI is presented. It is based on the effect of high polarization potentials on the defect concentration, which leads to a modification of the slope of the logarithm of the hole current versus applied potential curves.     

交流阻抗谱方法对钇稳定氧化锆晶界电性能的研究

采用交流阻抗谱技术测量了多晶钇稳定的氧化锆固体电解质的晶界离子电导率,并根据砖层模型估算了多晶陶瓷样品的晶界体积分数,分析了晶界体积分数与电性能的关系.     

Study of ammonia decomposition in a proton conducting solid electrolyte cell

The reaction of NH₃ decomposition was studied at 500–700 °C and atmospheric total pressure using a single-chamber cell-reactor. The proton conductor was a strontia-ceria-ytterbia (SCY) perovskite of the form: SrCe_0.95Yb_0.05O_3−α. Silver films served as cathodic and anodic electrodes. The effects of imposed current, temperature and inlet gas composition on the reaction rate were examined. Paper presented at the 8th EuroConference on Ionics, Carvoeire, Algarve, Portugal, Sept. 16–22, 2001.     

Investigation of interfacial states in MIS structures by a single-frequency admittance method

Two models of tunneling charge exchange on interfacial states buried in the insulator are examined. In one model the spatial distribution of these states is assumed to be uniform and in the other model the volume density of traps decreases exponentially with increasing depth. Analytical expressions are obtained for the width and position of the peak in the normalizedconductance curves in both models, and the accuracy and limitations of these expressions are indicated. A new procedure for investigating interfacial states by the admittance method is proposed, based on the use of G-V characteristics of metal-insulator-semiconductor (MIS) structures measured at a fixed frequency. Zh. Tekh. Fiz. 67, 55–59 (October 1997)     

Study of silicon-organic interfaces by admittance spectroscopy

An admittance spectroscopy technique has been developed for the interfaces between organic monolayers and silicon. The present work involves the development of an effective equivalent circuit to represent the silicon/organic-monolayer system, and the development of a parameter extraction procedure, which yields the monolayer capacitance and the monolayer thickness, the flat-band voltage, the silicon doping density, the silicon surface potential, the interface trap density, the interface trap capture cross-section and the interface trap energy. This technique was applied to three types of silicon/organic-monolayer system.     

Evanescent imaging with induced polarization by using a solid immersion lens

Image contrast enhancement, high lateral resolution, and height information are obtained with induced polarization evanescent imaging using a solid immersion lens. Experiments are conducted by imaging features on a patterned Si substrate. Imaging theory is used to predict optimum orientation of high-spatial-frequency samples, and a topographical image is derived from the induced polarization image through a calibration procedure. A numerical aperture of 1.5 is used in the experiment. Height accuracy of +/-2 nm is demonstrated with a known sample.     

Measuring the complex admittance of a carbon nanotube double quantum dot

We investigate radio-frequency (rf) reflectometry in a tunable carbon nanotube double quantum dot coupled to a resonant circuit. By measuring the in-phase and quadrature components of the reflected rf signal, we are able to determine the complex admittance of the double quantum dot as a function of the energies of the single-electron states. The measurements are found to be in good agreement with a theoretical model of the device in the incoherent limit. In addition to being of fundamental interest, our results present an important step forward towards noninvasive charge and spin state readout in carbon nanotube quantum dots.     

Kinetics of metal-oxygen reaction by solid oxide electrolyte cells

Chronopotentiometric curves, generated by galvanostatic single steps, applied to solid oxide electrolyte cells, have been analysed on the basis of a dimensionless equation derived on the assumption that a scale of oxide grows at one of the electrode-electrolyte interfaces. This process is rate limiting for developing the charge transfer-diffusion overvoltage, and Wagner's theory on tarnishing under retarding electric field conditions, has been assumed for treating the kinetics of the growing scale. At constant temperature, the oxidation rate and oxidation rate constant have been measured as a function of the oxygen partial pressure in the range of pressures near the equilibrium pressure of the metal-oxide system.     

Production of synthesis gas in a solid electrolyte cell

The production of synthesis gas from methane was studied at 800-950°C in an yttria-stabilized zirconia cell, using iron as a catalyst-anode and platinum as cathodic electrode. The effect of gaseous O₂ vs that of ionically transported O^2- on CO selectivity and yield was studied. In general, O^2- gave higher CO yields with maximum of 73%. The side reaction of hydrogen oxidation was studied separately to find that the catalyst is usually in the oxide phase and only at very low , metallic iron is the stable phase. Reduced Fe is catalytically much more active than any of its oxides. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994.     

Dynamical polarization of protons in some polymers using the solid effect method

Experimental results on the dynamical polarization of protons in various polymers irradiated by neutrons are recapitulated. The measurements were performed at liquid nitrogen temperatures. The dependence was investigated of the degree of polarization on the microwave power, on the irradiation dose and on the change in frequency of the nuclear magnetic resonance. The maximum enhancement of polarization was obtained for polyethylene of a density 0·95 g/cm³ and was equal to 30.

---

SOLID EFFECT

. . , . , 30, 0,95 g/cm³.

---

---

In conclusion we thank R. Tichý for assistance in the measurements and S. Roda for help in the construction of the apparatus.     

Sizeable pure tensor polarization of deuterons in a solid

A new effect of dynamic nuclear orientation has been observed in deuterated 1, 2-ethanediol, (CD₂OH)₂, at 0.12 K in a magnetic field of 25 kG: a strong RF field applied at a frequency slightly different from the proton Larmor frequency changes the deuteron tensor polarization. Mechanisms for this effect are proposed.     

Enhanced dynamic nuclear polarization by the integrated solid effect

A spectacular gain in efficiency of DNP by the differential solid effect is demonstrated in a Si:B sample using inversion of the polarization of electron spins. The best results are obtained using an adiabatic fast passage through the ESR line.     

Direct evidence for conduction pathways in a solid electrolyte

The emission of silver ions from the apex of an amorphous electrolyte tip has been investigated by field ion microscopy. The ion emission patterns show discrete nanometer-sized spots. We present evidence that they represent the termination of bulk ion conduction pathways at the solid-vacuum interface. The analysis of the signals from individual emission sites suggests the existence of a network of such pathways in the solid. Auto- and cross-correlation measurements of the currents from individual sites provide quantitative information on the microscopic dynamics of charge transport in solid electrolytes as well as on the lateral extent of the pathway network.     

Utilization of a dilute solid electrolyte in an oxygen gauge

An electrochemical cell involving a SrCl₂-based solid electrolyte either doped or undoped with SrO was investigated. The measuring electrode was of graphite or vitreous carbon and the reference electrode was formed by dissolving Ag⁺ ions and embedding a silver wire. A simple fabrication technique is described. The reported results indicate that an electrode reaction involving dissolved oxide ions: O₂(gas)+4e⁻=2O²⁻ (SrCl₂) can take place and be utilized in an oxygen gauge down to 200°C. The same cell also functions as a chlorinesensor. At least down to 10⁻³ atm, interference with oxygen is negligible.     

Direct electro-oxidation of iso-octane in a solid electrolyte fuel cell

The present work aims to explore the activity of Cu/CeO₂ composites as anodic electrodes in direct iso-octane SOFCs. When the cell was operated as a membrane reactor, the effect of temperature, P_i-C8H18 and applied anodic overpotentials on the electrocatalytic activity and products' distribution, at both open and closed circuit conditions, was examined. Additionally, in situ DRIFT spectroscopy was carried out in order to correlate the performance of Cu/CeO₂ with its surface chemistry during iso-octane decomposition. Under the “fuel cell” mode of operation, the electrochemical performance and stability of Cu/CeO₂ were investigated by voltage-current density-power density and AC impedance measurements. The results reveal that at high anodic polarization conditions, carbon formation can be noticeably restricted (verified also by EDAX analysis), while H₂ production was enhanced due to partial oxidation, steam reforming, dehydrogenation and water gas shift reactions. Achieved power densities were found to substantially increase both with temperature and P_i-C8H18, while minor performance degradation was indicated in the step-change tests, where the overall activity of Cu-CeO₂ electrodes remained essentially unaffected.     

Evaluation of solid electrolyte cells with a versatile electrochemical technique

Voltage losses in fuel cells and other solid electrolyte systems are due to several mass transport and kinetics processes at the electrode/electrolyte interface as well as to ohmic contributions from the electrolyte, electrodes, current collectors and contact resistances. Electrochemical impedance spectroscopy (EIS) has been in use for several decades in fuel cell research and is quite effective in determining the contribution of individual electrode and electrolyte processes. However, data acquisition and analysis can be time-consuming and the technique has many limitations whilst cell performance and operating conditions are varying rapidly with time especially when the cells are under current load. The galvanostatic current interruption (GCI) technique is fast and can be used under a wide range of operating as well as for rapidly varying loads and cell performance conditions. In this paper a totally new and very simple way of adapting commercially available equipment has been described to perform high quality, reliable and fast GCI measurements over a range of different currents in one sequence without having to use an electronic switch or a solid state relay or a separate fast data logging system. Its versatility has been demonstrated with a number of standard RC circuits simulating slow electrode and fast electrolyte processes and by evaluating a number of solid oxide fuel cell materials. The GCI technique has been shown to be able to determine the composition of all standard test circuits within ±1 % of those determined from the EIS technique and actual values of circuit components. The technique has been applied to investigating solid electrolyte cells and produced excellent results.     

Measurement of cut-off wavelength in monomode fibres by a polarization method

A method for the measurement of cut-off wavelength of higher order mode in monomode fibres, based on the different polarization properties of the modes, is discussed. It permits one to derive the fraction of power guided in the fundamental mode, at each wavelength. Experimental results that prove the reliability and the accuracy of the method are provided. Criteria for the choice of the sample length for the measurement are also discussed. Finally a very simplified, yet precise, measurement procedure is suggested.     

Spectral decomposition at a complex laser polarization configuration

We study the role of laser polarization in the diamagnetic spectrum for the transition from the ground state to the highly excited Rydberg states through a single photon absorption. For simplicity, one usually polarizes the irradiation laser to the selected main quantum axis, which is along the applied external electric or magnetic field. The transition selection rule is simply expressed as m = 0, which corresponds to the π transition. When the polarization is circularly polarized around the main axis, the σ+ or σ- transition occurs, corresponding to the selection rule of m = 1 or m = 1, respectively. A slightly more complex case is that the laser is linearly polarized perpendicular to the main axis. The numerical calculation shows that we can decompose the transition into the sum of σ+ and σ- transitions, it is noted as the σ transition. For the more complex case in which the laser is linearly polarized with an arbitrary angle with respect to the main axis, we have to decompose the polarization into one along the main axis and the other one perpendicular to the main axis. They correspond to π and σ transitions, respectively. We demonstrate that these transitions in the diamagnetic spectrum and the above spectral decomposition well explain the experimentally observed spectra.