Slow Sweep Rates Research Articles

Radiotracer study of the sorption of counter- and co-ions in tetracyanoquinodimethane and poly(vinyl ferrocene) modified electrodes

Sorption of Ca 2+ and SO 2− 4 ions into TCNQ and PVF surface films has been studied under potentiostatic steady state conditions and during potential cycling using a radiotracer technique. It has been shown that the sorption of the counter- and co-ions follows the charge and discharge processes occurring in the film. No sorption of ions has been found in the case of virgin films. No equilibrium in regard with the electrolyte is attained during potential cycling even at slow sweep rates. In the case of TCNQ electrodes the equilibrium concentration of counter-ions in the film is associated with the equilibrium concentration of different dimerized species.

Time domain response of an optically frequency swept Fabry-Perot interferometer

Time domain techniques are used to analyze the response of a Fabry-Perot interferometer to an optically swept carrier. A technique which increases the sensitivity or dynamic range of a fiber-optic sensor Fabry-Perot interferometer is analyzed. At slow optical sweep rates, classical Fabry-Perot fringes are produced when a linearly swept optical carrier interrogates a Fabry-Perot sensor interferometer. As the optical sweep rate is increased, the finesse of the Fabry-Perot fringes begins to decrease. Fringe finesse continues to deteriorate with increasing sweep rates until the fringes are destroyed, unless certain discrete sweep rates are selected. The analysis shows how the discrete sweep rates are selected. The sensitivity or dynamic range of a Fabry-Perot sensor system is increased by placing a second Fabry-Perot of appropriate length in series with the sensor Fabry-Perot. The phase information of the sensor interferometer is preserved in time domain multiplication in a manner that is analogous to the radio frequency processing of phase information.

The square scheme with the electrochemical reactions at equilibrium A study by diffusion or thin layer cyclic voltammetry and by scanning potential coulometry

The four-member square scheme with the electrochemical reactions at equilibrium is analyzed for semi-infinite or thin layer cyclic voltammetry and for scanning potential coulometry. First order or pseudo-first order chemical reactions are considered. At slow sweep rates a monoelectronic reversible system is observed, situated at a potential E° K which is a function of the two equilibrium constants of the chemical reactions. These equilibrium constants can be calculated from the difference E° 1 - E° K or E° 2 - E° K; E° 1 and E° 2, the redox potentials of the electrochemical reactions, can be obtained for large sweep rates. The rate constant can be deduced from the theory of the EC mechanism when the chemical reaction is irreversible, applied to each half of the square scheme.

A voltammetric investigation of the charge storage reactions of hydrous iridium oxide layers

The unusual potential/pH behaviour of hydrous oxide layers grown on iridium under potential cycling conditions is interpreted on the basis of a model which describes the interaction between the acid-base and redox properties of this type of system. It is pointed out that the suggested disperse nature of the material in the film is in agreement with earlier conclusions as to the nature of transition metal hydrous oxides in general based on magnetic susceptibility data. Kinetic studies show that at low film thicknesses and slow sweep-rates in 1.0 mol dm −3 H 2 SO 4 the main anodic and cathodic charge storage reactions occur in a reversible manner. However with thicker films and faster sweep-rates a marked diffusion limitation is observed for the cathodic process; slow transfer of electrons between redox sites in the dispersed oxide material is assumed to be involved here. Since, for the same film thickness, diffusion limitation during the positive sweep arises only at much higher sweep-rates (there appears to be a second mode of electron transfer in this case due to the fact that in the higher, Ir(IV), oxidation state the oxide is slightly non-stoichiometric), the anodic peak current densities for thick film/fast sweep-rate conditions are generally much large. The large displacement of the anodic peak potential from its reversible value (limiting Tafel slope value ≈200 mV decade −1 ) demonstrates that some other form of inhibition (possibly related to chemical rearrangement within the surface layer) is involved here.

A voltammetric investigation of the charge storage reactions of hydrous iridium oxide layers

The unusual potential/pH behaviour of hydrous oxide layers grown on iridium under potential cycling conditions is interpreted on the basis of a model which describes the interaction between the acid-base and redox properties of this type of system. It is pointed out that the suggested disperse nature of the material in the film is in agreement with earlier conclusions as to the nature of transition metal hydrous oxides in general based on magnetic susceptibility data. Kinetic studies show that at low film thicknesses and slow sweep-rates in 1.0 mol dm −3 H 2SO 4 the main anodic and cathodic charge storage reactions occur in a reversible manner. However with thicker films and faster sweep-rates a marked diffusion limitation is observed for the cathodic process; slow transfer of electrons between redox sites in the dispersed oxide material is assumed to be involved here. Since, for the same film thickness, diffusion limitation during the positive sweep arises only at much higher sweep-rates (there appears to be a second mode of electron transfer in this case due to the fact that in the higher, Ir(IV), oxidation state the oxide is slightly non-stoichiometric), the anodic peak current densities for thick film/fast sweep-rate conditions are generally much large. The large displacement of the anodic peak potential from its reversible value (limiting Tafel slope value ≈200 mV decade −1) demonstrates that some other form of inhibition (possibly related to chemical rearrangement within the surface layer) is involved here.

Using magnetic field pulses with a slow sweep rate to produce uniform current distribution in multifilamentary superconducting wire

A detailed discussion is presented on the change in current distribution which takes place in a twisted multifilamentary superconducting wire induced by successive magnetic field pulses with a slow sweep-rate. The number of the field pulses required for a localized current-distribution to become uniform is estimated. Some modification of existing theories is necessary to describe the phenomenon. The results obtained are confirmed by systematic measurements of the net flux penetration into the wire during each cycle of field pulses.

Low power nonlinear effects in the ferromagnetic resonance of Zn2Y and MnZnY hexagonal ferrites

The room temperature ferromagnetic resonance (FMR) absorption at 8.9–11.2 GHz in two planar hexagonal ferrites, Zn2Y and MnZnY, has been found to exhibit strong nonlinear behavior at low power levels corresponding to microwave field amplitudes on the order of 10 mOe. For somewhat larger amplitudes, 20–50 mOe, the asymmetric resonance curves show pronounced foldover effects. With increasing static field, the absorption increases gradually to a steep cutoff at a field several hundred Oe above the position of the low power FMR. With decreasing field, a sharp spike occurs coincident with the high field tail of the low power FMR line, followed by a gradual decrease which tracks the upsweep absorption. The detailed shapes of these curves depend strongly on sweep rate. Steady-state absorption curves at the highest powers employed, with 350 mW incident power and a microwave field amplitude of 58 mOe, result only for very slow sweep rates of 20 Oe/min or less.

Cupric oxalate-magnesium seawater cells

Cells with cupric oxalate cathodes bonded with polystyrene and magnesium alloy AZ61 anodes were discharged at room temperature. The cathode current efficiency was approximately 75% to a 0.8 V cut-off at an apparent current density of 10 mA cm−2. Cells with sulphur-cupric oxalate cathodes showed discharge performance at higher voltages with higher efficiences than those without sulphur. The use of different oxalate samples appeared to result in no significant difference in performance. The samples were either obtained commercially or prepared by precipitation. The cathodic behaviour of cupric oxalate was also investigated by potential sweep voltammetry with a slow sweep rate. In sodium chloride solutions a double peak was obtained on the voltammogram. When the stoichiometric quantity of sulphur was added to the cupric oxalate active material, it gave a main and two smaller peaks. The possible reactions are discussed.

Cyclic voltammetry of supported “cluster” catalysts containing Pt, Ru and Cu

Cyclic voltammetric studies in the potential range - 0.1 V to + 1.6 V at various sweep rates (10 m V s -1 to 100 m V s -1) have been conducted on finely dispersed mono-, bi- and tri-metallic catalysts containing Pt, Ru and Cu on a conducting carbon support. The behavior of dispersed ruthenium on carbon is very similar to that reported recently for bulk ruthenium electrodes with the surface oxidation processes markedly more irreversible than at comparable noble metal electrodes and the hydrogen region, comprising adsorption and absorption, only resolved at slower sweep rates which favour complete reduction of the oxide. The presence of copper in bimetallic and trimetallic catalysts eliminates the so-called “hydrogen region” due to an interaction between copper and both platinum and ruthenium at the electrode surfaces. The concept of an alloyed catalyst surface is thus confirmed.

Photogalvanic cells: Part XII. The dark electrochemistry of the iron-thionine system at platinum

The dark electrochemical behaviour of the thionine/leucothionine (Th/L) and Fe(III)/Fe(II) couples in acid aqueous sulphate solutions is investigated at “bare” platinum and at platinum covered with an irreversibly bound thionine multilayer. Thionine adsorption on “bare” platinum reaches saturation at roughly one monolayer. The rate constant for reaction of the Th/L couple at this electrode is fast (>7×10 −2 cm s −1). The cyclic voltammograms of thionine-coated platinum show behaviour characteristic of a transition from surface to diffusion control; at slow sweep rates the multilayer is completely reduced and oxidised in the time of a dual sweep, while at higher sweep rates it is only partially reacted and a diffusive process, possibly electron hopping, becomes rate determining. The Fe(III)/Fe(II) couple has quasi-reversible electrode kinetics at platinum covered with submonolayer reversibly adsorbed thionine, and is very irreversible at a thionine-coated electrode.

The predictability of stress corrosion cracking susceptibility of steels in acetate solutions from potentiodynamic polarization curves

Stress corrosion cracking susceptibilities of mild and low alloy steels in acetate solutions, assessed using slow strain-rate testing techniques, correlate well with those predicted on the basis of data from fast and slow sweep rate potentiodynamic polarization curves. Accurate prediction of cracking susceptibility necessitates allowances being made for changes of solution pH which occur during stress corrosion and polarization tests. A predictive technique which allows for these pH changes has been developed, and when implemented yields polarization data that indicates the influence of potential, steel composition and solution temperature on cracking susceptibility. The stress corrosion cracking of steels in acetate solutions is intergranular and occurs only when the electrochemical conditions correspond to those of a predicted potential-pH domain that is associated with the Fe 2+/Fe 3O 4 transition.

Electrochemical studies of mercury(II)

The reduction of Hg2+ is studied by linear sweep voltammetry, rotated disk electrode voltammetry, and chronoamperometry in a non-complexing medium at a vitreous carbon electrode (VCE). At the VCE which is completely free of any mercury deposit, the reduction of Hg2+ is found to be of first order, involving two electrons and reversible at slow sweep rates. When the VCE is partially covered with mercury droplets, the reduction mechanism is different and occurs in two steps. At the most active sites on the VCE where mercury droplets are formed during a previous cathodic sweep, Hg2+ undergoes disproportionation to Hg22+ which is subsequently reduced to Hg. The second step involves the simple two-electron, diffusion-controlled reduction of Hg2+ to Hg at the bare electrode surface.

Electrochemical studies of mercury(II)

The reduction of Hg 2+ is studied by linear sweep voltammetry, rotated disk electrode voltammetry, and chronoamperometry in a non-complexing medium at a vitreous carbon electrode (VCE). At the VCE which is completely free of any mercury deposit, the reduction of Hg 2+ is found to be of first order, involving two electrons and reversible at slow sweep rates. When the VCE is partially covered with mercury droplets, the reduction mechanism is different and occurs in two steps. At the most active sites on the VCE where mercury droplets are formed during a previous cathodic sweep, Hg 2+ undergoes disproportionation to Hg 2 2+ which is subsequently reduced to Hg. The second step involves the simple two-electron, diffusion-controlled reduction of Hg 2+ to Hg at the bare electrode surface.

The electrochemical behaviour of different graphite/cryolite alumina melt interfaces under potentiodynamic perturbations

The anodic reaction in molten cryolite alumina systems has been studied in a wide range of potentials by applying both repetitive and single pulse linear sweep voltammetry, varying the sweep rate between 0.04 V/s and 600 V/s, and the alumina concentration between 0.05 and 1.5 wt%. The experimental results obtained at slow sweep rates were interpreted taking into account that the hydrodynamic conditions which prevail at the anodic interface are defined by the gas flow caused by the anodic reaction. At fast sweep rates, under conditions of linear diffusion, a mechanism for the oxide ion discharge is postulated. The low value of the diffusion coefficient of the oxide ion species, calculated from fast sweep rate experiments indicates a prior chemical equilibrium step. At fast sweep rates, a new voltammetric peak is reported, probably associated with the COF 2 formation.

Capacitance-voltage and surface photovoltage measurements of pyrolytically deposited SiO 2 on InP

Capacitance-voltage measurements at 300 and 85 K of metal-SiO 2-InP structures are reported in the frequency range from d.c. to 450 MHz. A continuous decrease in the apparent oxide capacitance occurred over this frequency range. Deep depletion of the semiconductor was obtained at 300 K with pulsed bias and at 85 K with slow sweep rates. Quasistatic capacitance-voltage measurements indicate movement of the surface potential over a range of approximately 1 V. The surface photovoltage measurements are in qualitative agreement with this in that they suggest that the surface can be modulated from near flat band to the onset of strong inversion.

Inversion and accumulation layer formation at elevated temperatures in n-type GaAs-anodic oxide MIS devices

At room temperature n-type GaAs (n-GaAs)-anodic oxide MIS devices typically exhibit some form of deep depletion mode operation which is characteristic of a low thermal generation rate for minority carriers in the semiconductor. In addition, for accumulation mode bias voltages the small signal capacitance often does not approach the oxide capacitance and instead exhibits a dispersion in which this capacitance varies with the frequency of the a.c. signal and the sweep rate of the d.c. bias voltage. In this paper, capacitance-voltage measurements taken at elevated temperatures with varying d.c. bias voltage sweep rates are reported for n-GaAs-anodic oxide MIS devices. The data indicate that a stable inversion layer is formed even at high sweep rates, and no accumulation mode capacitance dispersion is observed for a capacitance-voltage measurement frequency of up to 1 MHz at temperatures above room temperature. At high temperatures and slow sweep rates, considerable mobile ion motion in the oxide is observed. A discussion of this observation based on recently developed theories for oxide growth is presented.

Electrochemical oxidation of uric acid and xanthine: An investigation by cyclic voltammetry, double potential step chronoamperometry and thin-layer spectroelectrochemistry

The electrochemical oxidation of uric acid and xanthine has been studied at various graphite electrodes and at a gold electrode by a variety of techniques including cyclic voltammetry, double potential step chronoamperometry, and thin-layer spectroelectrochemistry. It has been confirmed that the primary electrooxidation product formed from uric acid or xanthine is a diimine species. This diimine is very unstable in solution and is rapidly hydrated to an imine-alcohol in a first order reaction having an apparent rate constant of ca. 30 s −1 at pH 8.0. Both below and above pH 8.0 the rate of the latter reaction appears to increase. Although the diimine primary electrooxidation product is very unstable in homogeneous solution, having a half-life at pH 8.0 of ca. 23 ms, a voltammetric reduction peak due to reduction of the diimine may be observed on cyclic voltammetry of uric acid or xanthine at quite slow sweep rates (e.g., 200 mV s −1 at pH 8.0) at graphite electrodes provided that the electrode has a rough surface (large surface area). This is so because the diimine is strongly adsorbed at the rough electrode surface and in the adsorbed state is appreciably more stable than in solution. The imine-alcohol, formed by partial hydration of the diimine, may be observed as a further, unstable, u.v.-absorbing intermediate upon electrochemical oxidation of uric acid or xanthine at pH 7–9 by means of thin-layer spectroelectrochemical studies. The imine-alcohol is hydrated in a first order reaction to give uric acid-4,5-diol, characterized by an observed rate constant of 8×10 −3 s −1 between pH7 and 9. Uric acid-4,5-diol further decomposes to allantoin and CO 2 in neutral and weakly alkaline solution.

Stress Corrosion Cracking of C-Mn Steel in Phosphate Solutions

Abstract Annealed C-Mn steel has been shown to fail by transgranular stress corrosion cracking (SCC) in sodium phosphate solutions over a range of potentials and pH's at room temperature. The observed SCC domain is reasonably accurately predicted from fast and slow sweep rate polarization curves. Moreover, the cracking domain boundaries can be calculated with reasonable precision from the thermodynamic properties of the reactants and products involved in the reactions responsible for the various modes of failure at different potentials and pH's.

Electrochemical cleavage of sulphones(II) — the influence of the electrogenerated bases in the cathodic reduction of allylic and benzylic sulphones

Allylic and benzylic sulphones were reduced in aprotic solvents on a stationary mercury electrode. For slow sweep rates, it was shown that electrogenerated bases were able to deprotonate the substrate and consequently to form inactive anions. A comparison between the electrochemical behaviour of the allylic and the vinylic sulphones in protic and aprotic solvents is presented: the results of the electrolysis have to take in account the deprotonation reaction and the isomerization of some of the substrate.

Studies on flux jump of superconductor in magnetic field at slow sweep rate

Studies on flux jump of superconductor in magnetic field at slow sweep rate