Observation and Analysis of the Tunnel Formed on D. C. Etched Highly Ordered Aluminum

Dependence of the Tunnel Shape on the Electrolyte Composition

Abstract

The observation of the tunnel formed on D. C. etched {100} highly ordered aluminum and analysis of the tunnel shape have been performed by using the oxide replica method. The tunnel widths (W_i) were measured as a function of the tunnel depth (l_i) for the tunnels obtained by D. C. etching in AlCl₃, HCl+AlCl₃, NaCl+AlCl₃, H₂SO₄+AlCl₃, LiCl, NaCl and KCl aqueous solutions. As has already been reported regarding HCl solution, Wi tapered exponentially with l_i, i. e., log (W_i/W₁₀)=a·l_i, where W₁₀ is the tunnel width at the depth of 10μm from the tunnel entrance, in all solutions examined in this study. In HCl+AlCl₃ solution, W_i tapered rapidly with l_i compared to the case of HCl or AlCl₃ solutions. The rapid decrease in W_i with l_i, in HCl+AlCl₃ solution was considered to be caused by the increase in the concentration of Al³⁺and Cl^- ions in the tunnel, followed by the hindrance of electrochemical dissolution of Al. On the other hand, for the tunnel obtained in LiCl, NaCl, or KCl solutions, W_i tapered with l_i, at the same rate. It was considered that Li⁺, Na⁺, and K⁺ ions play the same role in the formation of the tunnel.