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Electrochemical oscillations accompanying the formation of anodic silica have been shown in the past to be correlated with rather abrupt changes in the mechanical stress state of the silica film, commonly associated with some kind of fracture or porosification of the oxide. To advance the understanding on the origin of such oscillations in fluoride-free electrolytes, we have revisited a seminal experiment reported by Lehmann almost two decades ago. We thereby demonstrate that the oscillations are not stress-induced, and do not originate from a morphological transformation of the oxide in the course of anodisation. Alternatively, the mechanical features accompanying the oscillations can be explained by a partial relaxation of the field-induced electrostrictive stress. Furthermore, our observations suggest that the oscillation mechanism more likely results from a periodic depolarisation of the anodic silica.

Seemingly, contradictory results have been reported so far for electrostriction in anodic oxides. Furthermore, no definitive agreement could be obtained with theory. In this paper, in situ techniques are combined to elucidate electrostriction in anodic niobium oxide. The dependence of strain, internal stress, and dielectric constant on the electric field is measured by, respectively, spectroscopic ellipsometry, curvature, and impedance measurements. The through-thickness strain is tensile and proportional to the square of the electric field. The in-plane internal stress is compressive and proportional to the square of the electric field at low field values. The internal stress is predicted relatively well by the Maxwell stress because of the weak dependence of the dielectric constant on the volume change of the oxide. The dielectric constant decreases with the electric field, the dependence being quadratic. While the evolution of the strain and stress with the electric field can be ascribed to the dependence of the dielectric constant on strain, the dependence of the dielectric constant on the electric field contains an explicit strain and electric field dependence. A mechanism for the latter is proposed

This study investigates the mechanism responsible for the electrochemical oscillations during silicon galvanostatic anodizing. Two oscillatory regimes of anodic silica formation in dilute fluoride electrolyte are monitored by combined in-situ curvature measurement and ellipsometry. At lower applied current density, a dense silica film is formed and the oscillations features are similar to those observed in fluoride-free electrolytes. At higher applied current density, a porous silica film is formed and the oscillations progressively reappear after a transition regime without oscillations. The disappearance and the reappearance of the oscillations are associated to variations of the degree of synchronization between the self-oscillating domains. In addition, the similarities between the oscillatory regimes and the persistence of sustained oscillations during the formation of thicker silica indicate that all oscillations arise from a same mechanism that survives the oxide accumulation at the silicon surface.