G. Garcia-Belmonte

Impedance of constant phase element (CPE)-blocked diffusion in film electrodes

Abstract We construct a model of ac impedance response to blocked linear diffusion that has a sloped low frequency region in the impedance plot. The approach is based on the transmission line analogy to linear diffusion, and it is equivalent to solving Fick’s law with a boundary condition that allows us to set an arbitrary impedance response at low frequencies. We argue that roughness at the blocking interface gives rise to constant phase element (CPE) response at low frequencies, and we give an impedance model function that can fit data along the whole frequency range when such a CPE is found. This is tested in an experiment of Li + insertion in Nb 2 O 5 . The model should be of significance for metal oxide thin film electrodes and modified polymer electrodes.

Mott-Schottky Analysis of Nanoporous Semiconductor Electrodes in Dielectric State Deposited on SnO2 ( F ) Conducting Substrates

This paper analyzes the dark capacitance of nanostructured electrodes in the dielectric state, with particular emphasis on TiO 2 electrodes deposited over a transparent conducting substrate of SnO 2(F). It is shown that at those potentials where the TiO 2 nanostructure is in the dielectric state, the capacitance is controlled by the contact SnO 2(F)/~electrolyte, TiO2). The partial or total covering of the substrate by a dielectric medium causes a modification of the Mott-Schottky plot of the bare substrate. We provide a mapping of the various Mott-Schottky curves that will appear depending on the film characteristics. If the dielectric nanoparticles completely block part of the substrate surface, the slope of the Mott-Schottky plot increases ~with the same apparent flatband potential! as an effect of area reduction. The covering of a significant fraction of the surface by a thin dielectric layer shifts the apparent flatband negatively. Measurements on several TiO 2 nanostructured electrodes show that the capacitance contribution of the semiconductor network in the dielectric state is very low, indicating that the field lines penetrate little into the TiO2 network, not much further than the first particle. The different surface covering observed for rutile-anatase and pure anatase colloids is explained by lattice matching rules with the substrate. By comparing different electrodes, the Helmholtz capacitance at the SnO2(F)/solution interface was calculated and the apparent flatband potential was corrected for the effect of band unpinning.

Nature of the Schottky-type barrier of highly dense SnO2 systems displaying nonohmic behavior

The electrical characteristics of highly dense SnO2 ceramic varistors are believed to be caused by the existence of potential barriers at the grain boundary. A complex plane analysis technique (to eliminate the influence of trapping activity associated with the conductance term observed via depression angle of a semicircular relaxation in the complex capacitance plane), allied with an approached Mott–Schottky model, are used to demonstrate that the potential barriers at the grain boundary are Schottky-type barriers in SnO2 varistors such as those observed in the traditional ZnO varistor.

Analysis of the admittance-frequency and capacitance–voltage of dense SnO2⋅CoO-based varistor ceramics

This article describes the admittance-frequency feature of a class of SnO2⋅CoO-based polycrystalline ceramics with high nonlinear current–voltage characteristics (nonlinear coefficients above 50). Broad relaxation peaks caused by the presence of deep trap states were characterized based on the admittance response of different systems doped with La2O3, Pr2O3, and CeO2. The calculation of the energy of this deep trap level revealed not only that all the compositions share the same value but also that this value could be attributed to an oxygen vacancy or to CoSn▾ like defects. The values of barrier height and density of states obtained from a capacitance–voltage analysis are in good agreement with the nonlinear coefficients. The highest nonlinear coefficients are found in compositions with greater barrier height values and higher density of deep trap states at the grain boundary interface.

Dynamic Processes in the Coloration of WO 3 by Lithium Insertion

Electrochemical impedance spectroscopy was used in the study of lithium insertion in a WO 3 film Spectra obtained at different potentials showed the characteristic shape of a diffusion process, with a Warburg regime at high frequencies and a capacitive branch at low frequencies arising from the charge storage associated with the intercalation of Li + . But the low frequency wing showed, as in many other reports, a pronounced dispersion which could not be accounted for solely by the diffusion capacitance. The possibility of an additional kinetic process was investigated. A number of models accounting for such effect were considered, including the constant phase element restricted diffusion model that differentiates diffusion from blocking effects and another model with fast and slow charging processes. The evolution of the impedance with charge-discharge cycles was considered as well It was found that a suitable interpretation of the experimental results is provided by a new model that considers the coupling of diffusion and the solid-state reaction whereby inserted ions are incorporated in fixed sites of the lattice. This model could be further elaborated on the basis of a diffusion and multiple trapping scheme, which provides an ordinary or Warburg diffusion region in the range of highest frequencies, and additional components at lower frequencies which reflect the kinetics of the ion immobilizing process and which in some cases show an anomalous diffusion pattern. This explanation was found consistent with the results of complementary techniques of measurement such as the electro-optical transfer function.

DIGITAL IMPLEMENTATION OF FILTERS FOR NUCLEAR APPLICATIONS USING THE DISCRETE WAVELET TRANSFORM

Abstract This paper presents a novel digital pulse processing technique based on fast implementations of a modern signal analysis method known as the wavelet transform (WT). From the point of view of standard nuclear filtering, the whole analysis may be understood as the action of a bank of gaussian shapers. The algorithm permits the evaluation of relevant parameters on each pulse and, making use of this information, a spectral improvement is achieved in the response of HgI 2 detectors constructed in our laboratories. As the performance of these detectors is mainly limited by the hole trapping phenomenon, the introduction of a charge loss correction making use of the WT has been considered. In this work, the pulse processing has been carried out by transferring the digital recorded pulses to a computer where a software version of the algorithm is performed.

Gaussian filter bank use for improving CZT-γ-ray detector spectroscopic response

Abstract This letter summarizes the application of a bank of digital Gaussian filters, previously reported [1], to the analysis of pulses from CZT detectors using different γ-sources. This study makes plain the hole trapping effect repercussions on the spectroscopic response. Finally, a significant spectral improvement is achieved by regarding a simple charge-loss correction model.

Observation of ionic thermocurrents in zirconia-based solid electrolytes

In ionic conductors, long range-migrating charges are a main cause of polarization processes. This has complicated, up to date, the study of ionic thermocurrents (ITC) in solid electrolytes. However, the method is appealing, as it probes directly charge-formation phenomena that are important both from a scientific point of view and for applications.This work reports on the observation of ITC in solid electrolytes. Under appropriate experimental conditions, the ITC response of a zirconia sample electroded with platinum is a reproducible one, thus opening the way to a new characterization method that may complement other well established methods, such as Impedance Spectroscopy and a number of electrochemical techniques. The general trends of the response, which is composed of two well resolved ITC peaks, is discussed. One of them, taking place at higher temperatures, conforms to the standard shape of a first order kinetics depolarization process.

Nuclear pulse discrimination using statistical detection theory

Abstract The introduction of modern materials, like HgI 2 , into the field of nuclear detection is not fulfilling previous expectations. Due to technological problems, a significant percentage of HgI 2 detectors do not respond to the theoretical behaviour in the detection of radiation. This is mainly the case when high efficiency is desired and thick detectors must be employed. In particular, standard discriminators are sensitive to evolution in time of the pulse parameters, such as amplitude or rise time. This phenomenon has been related to polarization effects. In order to improve the response of these “bad quality” detectors, we have studied the possibility of using statistical discrimination methods implemented by means of digital processing techniques, aiming to keep the counting behaviour stable in time when thick HgI 2 detectors (1 cm) are employed. Some theoretical aspects of these procedures are presented and a discrimination algorithm is implemented with the help of a real-time fast digital processor.

Cathode effect on current-voltage characteristics of blue light-emitting diodes based on a polyspirofluorene

Current-voltage characteristics of polyspiroblue SB -based light-emitting diodes with the structure: ITO/PEDOT:PSS/SB/cathode have been analyzed. Several cathodes were used (Al, LiF with different thicknesses, and Ba) in order to change the barrier for electron injection. As expected, the inclusion of a thin (0.5-1 nm) LiF layer between SB and Al, or the use of Ba, modifies the electron barrier as derived from the increment in the turn-on voltage (related to the built-in potential) with respect to that observed for Al cathode. For hole-only devices (Au cathode) J-V characteristics are interpreted in terms of bulk-limited SCLC transport with hole mobility of the order of 10-6 cm2/V s. When J-V characteristics obtained using different cathodes are compared the current level observed are consistent with the mobility observed for the hole only device. This implies that the device operation is mainly determined by the hole conduction. However, the electroluminescence observed for these devices employing different cathodes differs over four orders of magnitude. Our results suggest that the electron mobility is much smaller than the hole mobility and that the recombination process is confined to a thin layer near the cathode. Additionally, the results obtained from simple device modeling are also presented.