Effect of processing parameters on pore opening and mechanism of voltage pulse detachment of nanoporous anodic alumina

• Through-hole PAMs were fabricated by anodization and voltage detachment process.
• Voltage detachment process is affected by anodization conditions, applied voltage and sample storage period.
• One voltage pulse is sufficient to detach effectively PAM.
• Feasible and detailed mechanism of the voltage pulse detachment was proposed.

The free-standing through-hole porous alumina membranes (PAMs) were fabricated by two-step self-organized anodization of aluminum in oxalic acid followed by the subsequent voltage pulse detachment performed in an environmental-friendly solution based on HClO4 and C2H5OH. The effects of oxalic acid concentration used for anodic oxide synthesis and the chemical composition of the barrier oxide layer on the effective detachment of PAMs were studied. On the other hand, the voltage detachment conditions such as applied voltage, number of pulses, and temperature of electrolyte were tested as parameters which might affect the detachment of PAMs and their bottom side morphology. It was found that the voltage detachment process is affected by applied detachment voltage while the bottom surface morphology of detached PAMs is influenced by oxalic acid concentration used for anodization, the chemical composition of the barrier oxide layer, and the time gap between anodization and detachment (storage period). Furthermore, it was revealed that number of applied pulses and electrolyte temperature maintained during the detachment have a little effect on the morphology of PAM films. The most feasible and detailed mechanism of the voltage pulse detachment, considering the individual stages of the process, was proposed. At the beginning, the mechanism was related to an electric field assisted dissolution of oxide enhanced by its chemical dissolution induced by Joule's heating. These processes occur at the pore bottoms upon application of the voltage pulse and results in pore widening and thinning of the barrier layer. Then, after the electrical breakdown of the barrier layer, the intensive electropolishing of aluminum at the metal/oxide interface takes place.

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