Breakdown voltage and electronic current studies of tantalum-tantalum oxide electrolyte systems

Breakdown voltages and electronic current data (at constant voltage) of anodic tantalum oxide films in contact with aqueous, as well as non-aqueous, electrolytes of varying concentrations and compositions have been obtained at 298 K. Both breakdown voltage and electronic current depend on electrolyte concentration, resistivity and composition. A linear relation between breakdown voltage and logarithm of electronic current has been observed. The effect of electrolyte concentration, composition and resistivity on breakdown voltage has been discussed in terms of the Ikonopisov electron injecting avalanche model of electrical breakdown. The values of the parameters for impact ionization coefficient () and primary electronic current (j ₀) have been evaluated. The major factor contributing to the decrease in breakdown voltage with increasing electrolyte concentration is the increasing primary electronic current.     

Dielectric characteristics of miniature aluminium electrolytic capacitors under stressed voltage conditions

The effects of applying voltages higher than the nominal voltageV _N, for given periods of time, to miniature aluminium electrolytic capacitors has been investigated. The measurements of the current transients, theI–V characteristics, and the a.c. properties indicate that the main effect of subjecting the capacitors to the high voltages is an irreversible change in the capacitor dielectric characteristics as a consequence of a large increase in the resistance of the electrolyte and in the permanent leakage current through the anodic oxide. This current, very noticeable for voltages higher thanV _N, is attributed to an electronic conduction mechanism in an avalanche regime. The measured dielectric parameters and their evolution after exposing the capacitors to stressed voltage conditions are interpreted in terms of an extension of McLean's equivalent circuit for an electrolytic capacitor.     

Numerical analysis of thermal and electrochemical phenomena for anode supported microtubular SOFC

A 2D model considering momentum, heat/species transport and electrochemical phenomena, has been proposed for tubular solid oxide fuel cell. The model was validated using experimental polarization curves and the good agreement with the experimental data was attained. The temperature distributions show that temperature varies severely at the tube inlet than at the tube outlet. The heat generation and transfer mechanisms in electrodes, electrolyte and electrochemical reaction interface were investigated. The results show that the overall electrochemical reaction heat is produced at cathode/electrolyte interface, and a small portion of the heat is consumed at anode/electrolyte interface. The heat produced at cathode/electrolyte interface is about five times as much as that consumed at anode/electrolyte interface. Overwhelming part of the heat transfer between cell and outside occurs at cathode external surface. Most current flow goes into anode from a very small area where the current collectors locates. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Effects of a magnetic field on growth of porous alumina films on aluminum

The effects induced by a magnetic field on the oxide film growth on aluminum in sulfuric, oxalic, phosphoric and sulfamic acid, and on current transients during re-anodizing of porous alumina films in the barrier-type electrolyte, were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on Si wafer substrates. We could show that the duration of the anodizing process increased by 33% during anodizing in sulfuric acid when a magnetic field was applied (0.7 T), compared to the process without a magnetic field. Interestingly, such a magnetic field effect was not found during anodizing in oxalic and sulfamic acid. The pore intervals were decreased by ca. 17% in oxalic acid. These findings were attributed to variations in electronic properties of the anodic oxide films formed in various electrolytes and interpreted on the basis of the influence of trapped electrons on the mobility of ions migrating during the film growth. The spin dependent tunneling of electrons into the surface layer of the oxide under the magnetic field could be responsible for the shifts of the current transients to lower potentials during re-anodizing of heat-treated oxalic and phosphoric acid alumina films.     

Anodization of Al–Nd alloy films in nonaqueous electrolyte solutions for TFT-LCD application

The anodization behavior of Al–Nd alloys in nonaqueous electrolyte solutions and the electronic properties of the resultant anodic oxide films were studied for TFT-LCD application. Sputtered Al–Nd alloy films on glass substrates were anodized at 25 °C and 1 mA cm⁻² up to 100 V in ethylene glycol–water solutions containing 10 wt.% ammonium tartrate or salicylate to give uniform and flat oxide films. The incorporation of organic components into the anodic oxide films from the electrolyte solutions has lowered the relative permittivity and increased the breakdown electric field of the oxide films. This was performed by decreasing the water content in the electrolyte solutions. The tartrate solution caused higher carbon incorporation than the salicylate counterpart at the same water concentrations, giving lower relative permittivity, and higher forward breakdown electric field. The AlO stretching frequency of the oxide films decreased slightly as the amount of incorporated organic moieties increased. Nd was uniformly distributed in the oxide films and an increase in the Nd content was likely to increase both the relative permittivity and the forward breakdown electric field without any apparent change in the anodization behavior.     

The anodic oxidation of zirconium—II. Growth and morphology of anodic ZrO2 films

Measurements of the rate of growth of anodic barrier films, the stresses in these films, and the relative film thicknesses after anodising in electrolytes which may contain fluoride ions have been interpreted as showing that the incorporation of F^? ions into the oxide leads to a greater flux of zirconium ions which in turn may lead to tensile stresses in the oxide. These stresses can cause oxide fracture and the development of corrosion products.The incorporation of F^? ions into the oxide depends upon the F^? ion concentration in the electrolyte and the surface pH of the electrode. This latter is in turn dependent upon the bulk pH of the electrolyte and the applied current density. At high current densities the surface pH shifts to smaller values which leads to enhanced pick-up of F^? ions; at low current densities higher surface pH's can prevent the pick-up of fluoride ions.     

Modelling and simulation of self-ordering in anodic porous alumina

Real-time evolution of pre-textured anodic porous alumina growth during anodization is numerically simulated in two-dimensional cases based on a kinetic model involving the Laplacian electric field potential distribution and a continuity equation for current density within the oxide body. Ion current densities governed by the Cabrera–Mott equation in high electric field theory are formed by ion migration within the oxide as well as across the metal/oxide (m/o) and oxide/electrolyte (o/e) interfaces, and the movements of the m/o and o/e interfaces due to oxidation and electric field assisted oxide decomposition, respectively, are governed by Faraday's law. Typical experimental results, such as linear voltage dependence of the barrier layer thickness and pore diameter, time evolution of the current density, scalloped shape of the barrier layer, and the extreme difference in the reaction rates between pore bottoms and pore walls, are successfully predicted. Our simulations revealed the existence of a domain of model parameters within which pre-textured porous structures which do not satisfy self-ordering configurations are driven into self-ordering configurations through a self-adjustment process. Our experimental results also verify the existence of the self-adjustment process during anodization.     

The conditions for incorporation of electrolyte ions into anodic oxides

A model is developed to account for the variation in pick-up, in the oxide, of anions from the solution during anodic oxidation. According to this model, during constant current anodizing, the pick-up depends upon the [anion]/[OH^?] ratio at the electrode surface. This in turn is a critical function not only of the pH of the bulk electrolyte but also of the impressed current density which can alter the local concentration of ions, particularly at the electrode—electrolyte interface. The model explains the abrupt change of pick-up observed experimentally when in certain ranges, the pH of the bulk solution or the impressed current density are changed.     

The anodic oxidation of valve metals—II. The influence of the anodizing conditions on the transport processes during the anodic oxidation of zirconium

The transport numbers for metal and oxygen in anodic ZrO₂ have been measured as a function of the oxide film thickness, growth rate, electrolyte composition, metal surface finish and electric field using ²²²Rn as an inert marker. The field in the oxide has also been determined as a function of current density and electrolyte composition.

The metal transport number apparently diminishes with increasing oxide thickness but this is shown to arise from the formation of a hydrated layer on the oxide surface which leads to greater energy loss of the -particles and apparently greater burying. However, this effect only has significance for thin films. For thick films the measured transport numbers are independent of thickness but show increases from very low values as the current density increases and with changes in the anodizing electrolyte in the sequence: sodium hydroxide; ammonium borate; sodium sulphate. The maximum metal transport number observed was 0.22 for 0.5 M sodium sulphate at 50 mA current density. The field in the oxide also increased with increase in current density and with electrolyte in the same sequence.     

Design of SrZr0.1Mn0.4Mo0.4Y0.1O3-δ heterostructured with ZnO as electrolyte material: Structural,optical and electrochemical behavior at low temperatures

P-type semiconductor SrZr_0.1Mn_0.4Mo_0.4Y_0.1O_3-δ (SZMMY) is for the first time composited with n-type ZnO to prepare a solid oxide electrolyte used in fuel cell operable at low temperature. Prepared nanocomposite electrolyte material is considered as a novel material owing to the results obtained in terms of improved ionic conductivity, power density, and current density at lower operational temperature. The material has been analyzed as well crystalline material with a dual-phase as confirmed by X-Ray Diffraction (XRD). The structural morphology of designed electrolyte materials was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), including high-resolution TEM (HR-TEM). The electrochemical impedance spectra (EIS) showed a remarkably lower charge transfer resistance than conventional electrolyte materials. Obtained results illustrated that ionic conductivity increased, which lead to the acceleration of the electrode reactions. Heterostructure nanocomposite SZMMY-ZnO is beneficial to attain a high ionic conductivity due to the suppression of electronic conduction through the p-n junction. The maximum power density was noted as 841 mW cm^?2 at 550 °C with a maximum current density of 2287 mA cm^?2. Based on optical properties through the p-n junction, the internal electronic current was blocked, which further reduced the short circuit problem in the heterostructure. In addition, the performance and lifetime test indicated good stability of the cell at 550 °C with a very small degradation loss. The present study suggests that the SZMMY-ZnO is a promising electrolyte for low-temperature-SOFCs development.     

质子传导陶瓷电解质燃料电池特性分析

An electrolyte model for the solid oxide fuel cell (SOFC) with proton conducting perovskite electrolyte is developed in this study, in which four types of charge carriers including proton, oxygen vacancy (oxide ion), free electron and electron hole are taken into consideration. The electrochemical process within the SOFC with hydrogen as the fuel is theoretically analyzed. With the present model, the effects of some parameters, such as the thickness of electrolyte, operating temperature and gas composition, on the ionic transport (or gas permeation) through the electrolyte and the electrical performance, i.e., the electromotive force (EMF) and internal resistance of the cell, are investigated in detail. The theoretical results are tested partly by comparing with the experimental data obtained from SrCe0.05M0.05O3-α(M=Yb, Y) cells.     

Theory of electrical breakdown during formation of barrier anodic films

The anodic formation of barrier oxide films on valve metals can be carried on till the electrode potential has attained the “breakdown voltage” (U_B, after which a sustained sparking appears. The systematization of the available data on this phenomenon revealed that U_B depends fundamentally on the nature of the anodized metal, as well as on the composition and resistivity of the electrolyte. Many other factors like the current density, the surface topography of the electrode, the history of the film formation, etc do not affect U_B noticeably. This phenomenology of the breakdowns, as well as their appearance, was explained by a breakdown model. According to this model, sparking is considered as an avalanching in the bulk of the anodic film, the initial electrons being injected into the film from the electrolyte. The peculiar features of the sparking were found to arise both from the specific injecting behaviour of the electrolyte and from the simultaneous oxide film formation.     

The effect of cobalt oxide sintering aid on electronic transport in Ce0.80Gd0.20O2−δ electrolyte

Additions of 2 mol% CoO_1.333 into gadolinia-doped ceria (CGO) solid electrolyte considerably improve sinterability and make it possible to obtain Ce_0.8Gd_0.2O_2−δ ceramics with 95-99% density at 1173-1373 K. The effect of cobalt oxide on the total electrical conductivity in air is negligible if the sintering is performed at 1173 K, although p-type electronic conduction measured at 900-1200 K increases with doping by 10-30 times. When increasing the sintering temperature up to 1773 K, grain growth in Co-containing CGO ceramics is accompanied with a decrease in both ionic and electron-hole transport. The oxygen ion transference numbers under oxygen/air gradient vary in the range 0.89-0.99. The n-type conductivity measured by the ion-blocking technique is lower for Co-containing materials than for undoped CGO, suggesting that the electrolytic domain can, to some extent, be enlarged by cobalt oxide additions. The relative role of both p- and n-type electronic contributions to the total conductivity of CGO increases with increasing temperature. The results show that Co-doped materials can still be used as solid electrolyte for intermediate-temperature electrochemical applications, when the operation temperature is 770-970 K.     

Model for corrosion of metals covered with thin electrolyte layers: Pseudo-steady state diffusion of oxygen

A one-dimensional mathematical model is presented for the free corrosion of a bare metal surface (devoid of any oxide film) under a thin electrolyte layer using mixed potential theory where anodic metal dissolution is controlled by oxygen diffusion through the electrolyte layer and by the oxygen reduction at the metal surface. A pseudo-steady state is considered wherein the oxygen diffusion is at steady state while the metal and hydroxyl ions keep accumulating in the thin electrolyte layer due to a decoupling arising from the assumed Tafel laws for corrosion kinetics. Under free corrosion the oxygen diffusion is shown to depend on a non-linear boundary condition with a non-integer power on oxygen concentration at the metal surface which makes the model non-trivial. Analytical and numerical results for the oxygen concentration at the metal surface, corrosion potential, and corrosion current density are reported which depend on several kinetic, thermodynamic and transport parameters in the system. The model is applied to iron and zinc systems with input data taken from the literature. The experimental utility of the model for gathering thin-film corrosion parameters from a study of the corrosion current and potential as a function of the thickness of the electrolyte layer is discussed. Precipitation and passivity, though not the main object of study in this work, are briefly discussed.     

Improved electrode systems for reverse electro-dialysis and electro-dialysis

Reverse Electro-Dialysis, RED, is an electrochemical technique to extract work by mixing aqueous solutions of different salinities. Electro-Dialysis, ED, is an electrochemical technique to extract potable water from seawater. Both RED and ED typically employs noble metal oxide catalysts in the conversion between electronic current and ionic current. By testing several different electrode materials and red-ox salts, this paper demonstrates that these electrochemical reactions are controlled by mass transfer in the electrolyte rather than by the electrocatalytic properties of the electrode material. By comparing two different carbon materials and standard noble metal oxides, we show, for two red-ox salts, that the electrochemical performances of the inexpensive and expensive materials are similar. Relatively inexpensive graphite along with environmentally benign FeCl₂ and FeCl₃ in moderate concentrations was demonstrated to lower the electrode concentration overpotential.     

Sulphur potential measurements with a two-phase sulphideoxide electrolyte

The open circuit potentials of the galvanic cell,Pt (or Au)¦(Ar + H₂S + H₂)′∥CaS + ZrO₂(CaO)∥ (Ar + H₂S+ H₂)″£t (or Au) has been measured in the temperature range 1000 to 1660 K and P_H2S:P_{H 2} ratios from 1.73×10^?5 to 2.65×10^?1. The solid electrolyte consists of a dispersion of calcium sulphide in a matrix of calcia-stabilized zirconia. The surface of the electrolyte is coated with a thin layer of calcium sulphide to prevent the formation of water vapour by reaction of hydrogen sulphide with calcium oxide or zirconia present in the electrolyte. The use of a ‘point electrode’ with a catalytically active tip was necessary to obtain steady emfs. At low temperatures and high sulphur potentials the emfs agreed with the Nernst equation. Deviations were observed at high temperatures and low sulphur potentials, probably due to the onset of significant electronic conduction in the oxide matrix of the electrolyte. The values of oxygen and sulphur potentials at which the electronic conductivity is equal to ionic conductivity in the two-phase electrolyte have been evaluated from the emf response of the cell. The sulphide-oxide electrolyte is unsuitable for sulphur potential measurements in atmospheres with high oxygen potentials, where oxidation of calcium sulphide may be expected.     

Improved Tank in Series Reactor Model for Tubular Solid Oxide Fuel Cell Stacks

An improved tank in series reactor model for the tubular solid oxide fuel cell stack is presented. The model accounts for the charge balances in the electrodes and electrolyte in addition to the component and energy balances, and is used to simulate the potentiostatic operation of the stack. Moreover, the time required for simulating this model is much less compared to the more elaborate three‐dimensional models in the literature. The results demonstrate a strong coupling between the temperature, concentration and current density distributions.     

两步电解法回收废电子元器件中的银

研究了两步电解法回收废电子元器件中银的工艺条件。第一步将镀件作为阳极,电解法制得粗银;第二步将粗银作为阳极,放入含100 g/L AgNO3、10 g/L HNO3和40 g/L Cu(NO3)2组成的混合电解液中,电极间隙140 mm时,控制电压1.2 V,电流密度300 A/m2和46℃的电解温度时可获得纯度为99.8%的金属银。     

Electrochemical oxidation of isobutanol to isobutyric acid at nickel oxide electrode: improvement of the anode stability

Electrooxidation processes using nickel oxide anodes in alkaline electrolyte offer good alternatives to the chemical processes used for the oxidation of alcohols. However, they suffer from the low stability of the electrodeposited layer of nickel oxide, resulting in poor current efficiency. This requires periodic reactivation of the anode. In the present work, different parameters governing the stability of the nickel oxide layer have been identified and their influence on the current efficiency for oxidation has been investigated using oxidation of isobutanol (2-methyl-1-propanol) to isobutyric acid (2-methylpropionic acid) as a model system. Alternative anode activation procedures have been studied. The procedures employed resulted in a five fold increase in the anode stability over earlier reported procedures for anode activation and electrolysis.     

Characterisation of the surface films formed on Nb during electrodissolution in aqueous alkali

Anodic polarisation of Nb in warm and concentrated aqueous alkali results in sustained electrodissolution. The process is investigated in NaOH by voltammetric, steady-state and impedance techniques. The j-E steady-state curve is characterised by a current peak and a subsequent plateau extending up to the explored positive limit (8 V_SCE). Upon addition of tartrate to the electrolyte the dissolution current increases markedly. In some NaOH+Na tartrate electrolytes a dissolution-precipitation mechanism sets in, with formation of a surface layer which causes a gradual current decrease; this layer is identified by XRD and elemental analysis as a mixed Na and Nb oxide hydrate. Larger currents may be sustained in KOH+K tartrate solutions without any precipitation process. Impedance diagrams recorded in the plateau region are analysed on the basis of the equivalent circuit resulting from the surface charge model; the oxide formation ratio and the main model parameters are estimated and their temperature dependence is discussed.