Open-circuit relaxation

Fig. 10.6 — A comparison of absorbance-time curves for a potential step followed by open circuit relaxation for the ece and disp 1 mechanisms. Reproduced with permission from A. Bewick, J. M. Mellor, B. S. Pons, Hlectrochem. Acta, 23, (1978), 77.

Open circuit relaxation of thin PANI films prepared by chemical and electrochemical oxidation from both the oxidized and the reduced state was investigated with UV-vis and infrared spectroscopy by Chinn et al. [497]. In both cases, the emeraldine state is obtained. The process is controlled by disproportionation reaction and semiquinone radical formation. 

Total charge leaked through circuit during open-circuit relaxation... 

Stress Relaxation. Copper alloys are used extensively in appHcations where they are subjected to moderately elevated temperatures while under load. An important example is the spring member for contacts in electrical and electronic coimectors. Critical to rehable performance is the maintenance of adequate contact force, or stabiUty, while in service. Excessive decrease in this force to below a minimum threshold value because of losses in spring property can lead to premature open-circuit failure (see Electrical connectors). 

The piezoelectric constant of polymer films is usually a function of the frequency of the applied strain, and the constant is expressed by a complex quantity. In other words, the open-circuit voltage across the film surfaces is not in phase with the applied strain and the short-circuit current is not in phase with the strain rate. This effect, first pointed out by Fukada, Date and Emura (1968) and designated piezoelectric relaxation or dispersion, will be discussed in this review in terms of irreversible thermodynamics and composite-system theory. 

The time range of the electrochemical measurements has been decreased considerably by using more powerful -> potentiostats, circuitry, -> microelectrodes, etc. by pulse techniques, fast -> cyclic voltammetry, -> scanning electrochemical microscopy the 10-6-10-1° s range has become available [iv,v]. The electrochemical techniques have been combined with spectroscopic ones (see -> spectroelectrochemistry) which have successfully been applied for relaxation studies [vi]. For the study of the rate of heterogeneous -> electron transfer processes the ILIT (Indirect Laser Induced Temperature) method has been developed [vi]. It applies a small temperature perturbation, e.g., of 5 K, and the change of the open-circuit potential is followed during the relaxation period. By this method a response function of the order of 1-10 ns has been achieved. 

In electrochemistry, potentials between two phases, originating from adsorption-desorption processes, i.e. potentials at relaxed Interfaces, are sometimes called open circuit potentials. As the experiments should be carried out In excess electrolyte, the activity coefficients are determined by the carrier electrolyte, and therefore are Independent of the concentration of cd electroljrte, i.e. dlna - dlnc j, which is actually measured. So,... 

An adaptation of the temperature-jump method, named indirect laser-induced temperature jump [29], was used in studies of distance dependence of electron transfer at electrodes. A pulsed Nd YAG laser was used to cause a sudden (<5 ns) change in temperature (<5 K) at an electrode/electrolyte interface. The increase in temperature causes a change in the open-circuit potential. The relaxation step is a function of the dissipation of thermal energy and the rate of electron transfer between the electrode and its redox partners. 

Fig. 23. Cyclic voltammogram for the first cycle of the solid-state secondary cell described in the text. The potential steps were confined to the range 1.3-3.0 V. Voc open circuit potential relaxed to only 1.4 V after reducing to 1.3 V.

The rapid temperature change of the electrode perturbs the equilibrium at the electrode-solution interface and causes a change in the potential of the electrode measured with respect to a reference electrode. The change in the open-circuit potential, A t, and its relaxation with time are used to obtain kinetic information about the electrode reaction. A number of different phenomena come into play to cause the potential shift with temperature (e.g., temperature dependence of the double-layer capacitance and the Soret potential arising from the temperature gradient between the electrode and the bulk electrolyte), but the response can be treated by a general master equation (40) ... 

Nanoscopic Investigations of Dealloyed Surfaces Erom the background of competitive models of selective alloy dissolution as described above, a closer microscopic examination of this process with the ultimate objective of atomic resolution and chemical information on an atomic scale appears mandatory. Ex situ transmission electron microscopy (TEM) of thin, corroded alloy films provides lateral resolution at the nanometer scale, but suffers from poor depth resolution and from structural relaxation processes that may occur after termination of the anodic polarization and transferring the samples into high vacuum. Classical TEM investigations in this field were performed under open circuit conditions in oxidizing environments (that is, at > Eq) [51,... 

Measurements are made by applying a current and monitoring the potential after a relaxation time and repeating the procedure for different anodic and cathodic currents. The experimental procedure for obtaining the polarization diagram of a corroding system requires the initial measurement of the open circuit potential of the system. The open circuit potential value falls between the equilibrium potentials of the anodic and cathodic reactions. When there is no current in the external circuit, an open circuit potential value is equal to the corrosion potential. 

When an external electric field is imposed on an electrolyte solution by electrodes dipped into the solution, the electric current produced is proportional to the potential difference between the electrodes. The proportionality coefficient is the resistance of the solution, and its reciprocal, the conductivity, is readily measured accurately with an alternating potential at a rate of 1 kHz in a virtually open circuit (zero current), in order to avoid electrolysis at the electrodes. The conductivity depends on the concentration of the ions, the carriers of the current, and can be determined per unit concentration as the molar conductivity Ae. At finite concentrations ion-ion interactions cause the conductivities of electrolytes to decrease, not only if ion pairs are formed (see Sect. 2.6.2) but also due to indirect causes. The molar conductivity Ae can be extrapolated to infinite dilution to yield Ae" by an appropriate theoretical expression. The modern theory, e.g., that of Fernandez-Prini (1969), takes into account the electrophoretic and ionic atmosphere relaxation effects. The molar conductivity of a completely dissociated electrolyte is ... 

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