Mechanisms current oscillation

On the basis of this argument, the mechanism for the current oscillation and the multilayer formation can be explained as follows. First note that U is kept constant externally with a potentiostat in the present case. In the high-current stage of the current oscillation, the tme electrode potential (or Helmholtz double layer potential), E, is much more positive than U because E is given hy E=U —JAR, where A is the electrode area, R is the resistance of the solution between the electrode surface and the reference electrode, andj is taken as negative for the reduction current. This implies that, even if U is kept constant in the region of the NDR, is much more... 

Karagueuzian, H.S. and Katzung, B.G. (1982). Voltage-clamp studies of transient inward current and mechanical oscillations induced by ouabain in ferret papillary muscle. J. Physiol, 327, 255-271. 

Samjeske G, OsawaM. 2005. Current oscillations during formic acid oxidation on a Pt electrode Insight into the mechanism by time-resolved IR spectroscopy. Angew Chem 44 5694-5698. 

As mentioned in the introduction, it is difficult to explain the characteristics of the oscillation based on the mechanisms which have been proposed so far for the potential oscillations with systems similar to Eq. (16) [4,7,35-38]. A precise investigation on individual ion transfers and adsorptions at two interfaces is necessary for the elucidation of the oscillation mechanism, although the spontaneous oscillation might be realized by the combination of a much larger number of ion transfer reactions and adsorptions than those in the case of the oscillation under the applied potential or current. 

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

A number of models have been proposed for the mechanism of current oscillation on silicon electrode. A brief description of the major models is given below. 

FIGURE 5.58. Voltage versus time curve (solid line) for an n -type silicon electrode anodized with a constant current density in NH4F. The thickness of the anodic oxide was measured by ellipsometry (open circles, dashed line fitted as a guide to the eye). The OCP dissolution time of the anodic oxide in the electrolyte was measured (values above arrows) at different points of the oscillation. The bar graph visualizes the proposed oscillation mechanism. After Lehmann. (Reproduced by permission of The Electrochemical Society, Inc.)... 

The complexity of the system implies that many phenomena are not directly explainable by the basic theories of semiconductor electrochemistry. The basic theories are developed for idealized situations, but the electrode behavior of a specific system is almost always deviated from the idealized situations in many different ways. Also, the complex details of each phenomenon are associated with all the processes at the silicon/electrolyte interface from a macro scale to the atomic scale such that the rich details are lost when simplifications are made in developing theories. Additionally, most theories are developed based on the data that are from a limited domain in the multidimensional space of numerous variables. As a result, in general such theories are valid only within this domain of the variable space but are inconsistent with the data outside this domain. In fact, the specific theories developed by different research groups on the various phenomena of silicon electrodes are often inconsistent with each other. In this respect, this book had the opportunity to have the space and scope to assemble the data and to review the discrete theories in a global perspective. In a number of cases, this exercise resulted in more complete physical schemes for the mechanisms of the electrode phenomena, such as current oscillation, growth of anodic oxide, anisotropic etching, and formation of porous silicon. 

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