Central Angle of Porous Alumina Nanocells Formed at Different Anodic Voltages

Formation of Alumina Nanotubes and Jet Effect during High‐Voltage Local Anodization of Aluminum

“…Geometrical parameters of the structures formed were derived from their surface SEM images before and after etching off alumina in the water solution containing 1 M H 3 PO 4 and 3 wt% CrO 3 at 70 C. These are the diameter of the pores (d por ), the interpore distance (D int ), the peak central angle (α p ), and the concave central angle (α c ) shown in Figure 3a. [15,16] They were used to calculate the porosity P ¼ ð2π= ffiffi ffi 3 p Þðd por =2D int Þ 2 and cell-forming ratio (proportionality constant) ζ ¼ D int =U a , where U a stands for the anodic voltage.…”

“…These are the diameter of the pores (), the interpore distance (), the peak central angle (), and the concave central angle () shown in Figure a. [ 15,16 ] They were used to calculate the porosity and cell‐forming ratio (proportionality constant) , where stands for the anodic voltage.…”

“…The volume expansion factor of the material in the tubular structures increases two to three times, and diameters of the pores and the cell-forming ratio decrease, confirming that the new portions of the synthesized material occupy the inner pore surface rather than increasing the total thickness of the porous layer. [15,16] An increased temperature and a high electric field increase the mobility of the substrate aluminum atoms at the interface with the alumina barrier layer, which results in the observed formation of sharper aluminum peaks and increased peak central and concave angles. The nonmonotonous I-V curves characterizing local anodic oxidation of aluminum (Figure 1) and the jet effect can be explained by specific processes initiated by high electric fields.…”