Frédéric Blaffart

Electromechanical coupling in anodic niobium oxide: Electric field-induced strain, internal stress, and dielectric response

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.

On the origin of damped electrochemical oscillations at silicon anodes (revisited)

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.