Moving Boundaries in Other Than Chemical Fields

3 Moving Boundaries in Other Than Chemical Fields 

A gradient of electrical potential constitutes the classic (external) force field for ionic solids. Let us study the effect of this electric field on the interface morphology and stability. The thermodynamic driving force in ionic crystals is Vi/,(= + 

For the formal treatment, we note that the divergence of the total electric current vanishes, that is, V = 0. In a closed system, the condition of (local) electro- 

With an open system to which electrodes are attached, we can study the stability of interface morphology in an external electric field. A particularly simple case is met if the crystals involved are chemically homogeneous. In this case, Vfij = 0, and the ions are essentially driven by the electric field. Also, this is easy to handle experimentally. The counterpart of our basic stability experiment (Fig. 11-7) in which the AO crystal was exposed to an oxygen chemical potential gradient is now the exposure of AX to an electric field from the attached electrodes. In order to define the thermodynamic state of AX, it is necessary to apply electrodes with a predetermined 

A similar process occurs if we electrolyze the phase sequence AX/AY, using A-metal electrodes. AX and AY are immiscible ionic crystals. This time we focus on the AX/AY interface. Since there is always a finite electronic partial conductivity and the very small transference numbers te (AX) and te (AY) are normally different, the AX side of the AX/AY interface serves either as an anode (oxidizing) or as a cathode (reducing). The difference (te(AY)-te(AX)) is proportional to the anodic (cathodic) current in AX. The cathodic interface is expected to obtain similar morphologies as have been described for the A-metal cathode in the previous paragraph. It is immobile as long as Dx,Dy DA. The morphological instability is therefore due to the A precipitates which cause the perturbations. 

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