Gyözö G. Láng

On the electrochemical applications of the bending beam method

Abstract Measuring the bending of a plate to determine the stress in thin films coated on one side of a substrate is a common technique. The surface stress changes can be estimated by using an appropriate form of Stoneys equation, if the thickness of the film is sufficiently less than that of the substrate. The changes in the bending of the substrate are usually calculated from the displacement of the reflected laser beam measured, e.g., with a position sensitive photo detector (PSD). Since the displacement of the light spot on the PSD is measured in air outside the liquid phase (where the mirroring surface is located), the measured values should be corrected according to Snells law of refraction. A relation between the reciprocal of curvature radius of the substrate and the displacement of the laser beam on PSD was derived by taking into account the bending of the laser beam due to refraction at the optical window. It is shown that the neglect of the refraction at the optical window may cause an error of 25–30% in the estimation of the stress changes.

Preparation and Magnetoresistance Characteristics of Electrodeposited Ni‐Cu Alloys and Ni‐Cu/Cu Multilayers

Galvanostatic electrodeposition was used to produce Ni-Cu alloys and Ni 81 Cu 19 /Cu multilayers by direct current (dc) plating and two-pulse plating, respectively, from a sulfate/citrate electrolyte. For the dc-plated Ni-Cu alloys, the deposition rate and the alloy composition were established as a function of the deposition current density, from which the appropriate deposition parameters for the constituent sublayers of the multilayers could be established. By measuring the resistivity at room temperature in magnetic fields up to H = 7 kOe, anisotropic magnetoresistance (AMR) was found for Ni 81 Cu 19 electrodeposits, whereas both giant magnetoresistance (GMR) and AMR contributions were observed for most Ni 81 Cu 19 /Cu multilayers. Finally, Ni-Cu alloys were also prepared by conventional pulse plating, varying the length of the deposition pulse (on-time) with constant separation (off-time) between the pulses. Clear evidence of a GMR contribution also appeared in these pulse plated Ni-Cu alloys which may be explained by the formation of a Cu enriched layer between the ferromagnetic layers deposited during the cathodic pulses. A quartz crystal microbalance experiment confirmed that an exchange reaction takes place during the off-time. These findings provide useful information on the formation mechanism of multilayers by the two-pulse plating technique.

PROBLEMS RELATED TO THE SPECIFIC SURFACE ENERGY OF SOLID ELECTRODES

Abstract The literature concerning specific surface energies of solids and the thermodynamics of solid electrodes is critically reviewed. It is pointed out that assumptions resulting in superficial energy and surface stress (S. Trasatti and R. Parsons, Pure Appl. Chem., 58 (1986) 451) being independent contributions to a general specific surface energy do not correspond to complete equilibrium with constant chemical potentials of the components throughout the system. An irreversible, usually time-dependent part of the specific surface energy is due to differences in the chemical potentials of the components in the interphase and in the bulk of a solid. Consequences of the irreversible part for the charge number of adsorbed species and the frequency dependence of the double-layer capacitance are outlined.

An advanced model of the impedance of polymer film electrodes

Abstract A new model for the impedance of polymer film electrodes has been developed in that three mobile charge carriers are considered. It has been observed especially in the case of organic conductive polymer that beside the electron diffusion and counterion transport very often hydrogen ions also participate in the charge transport and charge transfer processes in the course of redox transformation of these polymers, i.e. a model describing this case is essential. It has been demonstrated that by the help of reasonable assumptions some of the mathematical problems can be overcome and an analytical solution can be obtained for the complex impedance in the case of low amplitude variation of the electrode polarization. The equations derived for the charge transport and charge transfer impedances are tested by CNLS fitting data obtained for an Au∣poly( o -phenylenediamine) electrode.

Impedance analysis of polymer film electrodes

Abstract A new model is elaborated which can explain some anomalies of the impedance spectra of polymer films on electrodes. The model assumes two capacitances associated with the double layers at the substrate/film and at the film/solution interfaces, respectively. The latter capacitance is considered as a constant phase element due to the inhomogeneity of the polymer network and to energetically non-equivalent bonding sites for counterions. The effect of a low dc current, ie a slow reaction of the redox sites in the film with solution species, is also investigated. The electrochemical impedances characteristic of the model have been derived. The simulated impedance spectra were compared with those obtained experimentally.

The electrodeposition of CdSe from alkaline electrolytes

Polarization curves of CdSe electrodeposited from aqueous ammonia solutions containing selenosulfate and the EDTA complex of cadmium ions follow Tafel lines for current densities smaller than cathodic and anodic reaction limited current densities. Reaction orders for electrodeposition are one with respect to selenosulfate and zero with respect to cadmium ions. At high flow velocities of the electrolyte, only thin layers of CdSe are obtained. Very small effective diffusion coefficients determined from diffusion limited currents indicate diffusion into the electrolyte of a reaction product formed in an equilibrium reaction. Three-dimensional nucleation on the thin layer occurs at low flow velocities. A mechanism modeling the nucleation and growth kinetics is proposed. The Gibbs energy of formation of CdSe is calculated using measurements of the equilibrium potential of CdSe.

Laser techniques for the study of electrode processes

Experimental Determination of Surface Stress Changes in Electrochemical Systems.- Thermodynamic Theory of Solid/Liquid Interfaces.- Problems related to the Measurement of Surface Stress Changes of Solid Electrodes.- Basic Principles of the Experimental Methods.- Optical Detection of the Deformation.- Detection by Microscopy.- Applications.- Related Fields.- Determination of Mobile Species Fluxes Driven by Electrochemical Processes using Optical Methods: Basic Principles of the Experimental Methods (Probe Beam Deflection, Interferometry).- Application to Surface Confined Systems.- Application to Soluble Systems.- Photothermal Deflection.- Comparison with other Methods to Monitor Fluxes.

Surface energies of solid electrodes

Anisotropic specific surface energies of solids changed in different ways after a change of state depending on the possibility of mass transport between the equilibrium surfaces. If mass transport is impossible, the solid is deformed by a nonhydrostatic stress field and the chemical potentials of the components become anisotropic. In order to establish full equilibrium with constant chemical potentials throughout the whole system, mass transport is necessary. Since it is slow for solids, there will be irreversible contributions to the specific surface energy. Changes of specific surface energy were measured as functions of electrode potential and electrolyte composition at solid electrodes by Koesters laser interferometry and compared to changes of mass and charge. In all cases investigated so far, electrocapillary curves of polycrystalline gold in aqueous solutions of potassium sulphate, potassium chloride, lead perchlorate, thallium perchlorate and zinc sulphate, and in partially miscible solutions in water and isobutyl alcohol of hydrochloric acid slowly changed with time.

The electrochemical degradation of poly(3,4-ethylenedioxythiophene) films electrodeposited from aqueous solutions

Abstract In this review, results of recent studies on the electrochemical stability and degradation properties of poly(3,4-ethylenedioxythiophene) films are summarized, with particular emphasis on the structural changes induced by overoxidation and electrochemical degradation. The most important electrodeposition methods for the preparation of PEDOT films in surfactant free aqueous media have also been summarized, and several experimental techniques suitable for monitoring the degradation process have been discussed. Morphological changes in PEDOT films during overoxidation have been analyzed. Overoxidation mechanisms proposed in the literature have been surveyed.

An alternative explanation of confined copper dissolution by an STM tip

Problems connected with the interpretation of spatially confined copper dissolution by an STM tip are discussed. By discussing the influence of dissolved oxygen on the system, an alternative for the explanation is attempted in the light of literature data on copper dissolution.