No abstract

No abstract

The general problem of the incorporation of IR compensation in a potentiostatic circuit is considered. The manner in which the number of time constants needed to describe the system affect the amount of stable positive feedback at short times or high frequencies is discussed and a theoretical model developed for present state of the art potentiostats and operational amplifiers. It is shown that the use of a model in which all series nonfaradaic resistance has been removed to establish a reference for complete IR compensation indicates that stable overcompensation cannot take place in any practical system. The use of positive feedback in pulse and a-c techniques is compared and it is concluded that great caution must be exercised in the latter with regard to the proper detection of optimum compensation. Interpretation of results using IR compensation is considered and it is shown that the use of frequency domain conversion to obtain the impedance of the system under, e.g., pulse conditions allows potentiostat parameters to be eliminated such that meaningful results may be obtained at times of the order of 10 nsec.

A new adaptation of the technique of mechanical friction is proposed for the study of metal solution interfaces in electrochemistry. The coefficient of friction, especially sensitive to surface-state changes, is measured by means of a micro-slider, which does not disturb the i-v electrochemical curves. This method, called polaromicrotribometry, is easy to employ and of general application. It permits the measurement of electrochemical transient phenomena as well as stationary ones, in aqueous and organic electrolytes. The results obtained using different operating modes are described from the study of gold oxide formation and reduction. Different friction-potential behaviors were found for three different rubbing contacts, against gold in the same electrolyte. Gold/gold friction is elevated at cathodic potentials, indicating metalmetal bond formation, but at anodic potentials friction is reduced by chemisorbed oxygen: the friction-potential curve is in agreement with the currentpotential curve. Teflon/gold friction is independent of potential, indicating that in this last case the slider is inert over the potential range studied. As opposed to gold/gold friction, silica/gold friction is low at cathodic potentials, but elevated at anodic potentials with appearance of stick-slip (high static friction). This high static friction is attributed to the electrochemical formation of bonds or bridges which require the presence of an adsorbed oxygen layer. When the electrode is covered by oxygen the friction variations depend only on the applied potential.Measurement of friction for practical engineering applications is a classical technique of physics. The coefficient of fraction is defined by f = T/N, where N is the normal force to the contact surface of two bodies, and T is the tangential force required to cause sliding. Two measures exist: the coefficient of static friction ]s is the force required to initiate sliding, while the coefficient of sliding friction f smaller than fs, is the force required to keep the bodies sliding on each other. The friction coefficients are ordinarily independent of the force normal to the surface.Friction and adhesion are phenomena which are very sensible to the physicochemical nature of the contact surface. One quarter of an oxygen monolayer on clean tungsten is sufficient to divide the coefficient of friction by half (1). Conversely any change of friction must be connected to a modification of the surface.This sensitivity can be explained in the following way. Solids are sticking only on a small part of the apparent contact area (2), and it is on this real contact surface that adhesive forces are working. During the displacement of the slider, the junctions are stretched and broken and then built again; the stronger the adhesive forces the greater the frictional work dissipated. In the presence of adsorbed layers, the slider is always in contact with this superficial layer, and consequently it appears that friction measurement depends directly on the physico-chemical surface state, wha...