This
paper describes the fundamental mechanism for the formation of a 3-dimensional
porous template during the anodization of Al with less than 1 atom
% Cu percentages. It is known that the presence of Cu impurities in
an Al film introduces horizontal pores interconnecting the vertically
aligned porous structure of the anodized aluminum oxide (AAO) template.
We show that the formation of these horizontal pores is accompanied
by current density oscillations when the anodization is performed
at a constant voltage. The frequency of these oscillations is directly
related to the horizontal interpore distance. We propose a mechanism
that links the current density oscillations to the Cu accumulation
at the metal/oxide interface through the cyclic change in anode potential.
The distance between the horizontal pores is found independent of
the current density, temperature, and electrolyte concentration. Instead,
it was found that the spacing between the vertical pores, and thus,
the anodization voltage determines the spacing between the horizontal
pores. A model based on the plastic flow of the alumina barrier layer
was suggested to link the spacing between the horizontal and the vertical
pores. These results provide important insights in the formation of
3D AAO templates. In addition, we show the fabrication of rigid 3D
metal nanomeshes by electrochemical deposition into these 3D porous
templates.
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