Tunnel morphology of aluminum foil etched by a two-step DC etching method

A two-step DC etching of aluminum foil for capacitor is proposed. The first step DC etching was processed in a solution of HCl + H2SO4, the second was finished in a solution containing NaCl with some addition or no addition. Experiments indicated that the tunnel morphology was variable with the composition of electrolyte during the second step DC etching. When the solution of NaCl was used as electrolyte, the density of tunnels was increased. However, when some organic compound was added in the electrolyte of NaCl, the tunnels’ length was increased, and many branch tunnels were fabricated.