Study On The Process Of Metal Ions A.C. Deposited Into Porous Anodic Alumina Film

Porous anodic alumina films are used as the ideal templates to synthesize the nanowire arrays materials. However, the common synthetic processes are long and complex through direct current(D.C) deposition into the porous alumina films. If the direct currentdeposition method is replaed by an alternative current (A.C).ones, a simple process will be obtained for some procedures are canceled, such as separation from the barrier layer and aluminum, spray conductive metal on the film. However, the reactions become more complex during the application of A.C. deposition. Therefore, it is of great significance to study the process of metal ions A.C. deposited into the porous alumina film.In this paper, electrochemical technique is applied on aluminum and its alloy to anodize in sulfuric, oxalic and phosphoric acid solution to form porous anodic alumina films. During the constant voltage process of anodize, the current densities decline rapidly in the initial stage, then rise and finally become steady.When the high pure aluminum is anodized in the given solution at 20℃, current densities i change with voltage V are index relationship. For sulfate acid: i = 1.056e^0.1676V; for the oxalic acid: i= 2.763e^0.037V; and for phosphoric acid: i= 0.446e^0.024V.After electrochemical polishing and anodization, the anodic alumina films of high pure aluminum appear orderly porous structure. The average pore diameter is about 20nm, about 40nm, and 50-120nm, respectively, for anodizing in sulfuric, oxalic and phosphoric acid solution. Analysis by XRD, the film is the amorphousα-Al₂O₃ for anodizing in sulfate acid solution.U-I curve testt finds that the porous anodic alumina film has a semiconductor performance. The cathode polarization process applied on the films can be divided into three stages. Initially, current densities change slightly and potentials shift to negative rapidly. It is the stage of overcoming the barrier resistors of oxide film. Secondly, current densities grow quickly while potentials increase little. It is the hydrogen evolution stage. And the last stage is the current densities are nearly changed but potentials shift to negative rapidly. Is is the diffusing control step of hydrogen ions. The impedance of alumina film depends on the thickness of the barrier layer. The impedance increases with the barrier layer becoming thicker. The anodizing voltage proportional to the thickness of the barrier layer, and the anodizing ratio is about 1nm / V in sulfuric, phosphoric and oxalic acid solutions. The composite film contained zinc particle is prepared by A.C.deposition method. The solution contains 25g/LZnSO₄ ? 7H₂O, 25g/LH₃BO₃ and 1g/L additives((NH₄)₂SO₄ and N(CH₂COOH)₃ ) . When the alumina film is applied 15V A.C. deposition in the given solution for 300s the deposition sediment in the pores of oxide film is metal zinc, which is mainly distributed at the bottom of porous anodic alumina film with the thickness about 2um. And the amount of deposited zinc is about 31.404μg/cm². During A.C. deposition process, the amount of sediment C and the time s are fit to the formula: C = 7.6537 ln(s) -12.388.It is the first time to prepare Ce composite film in porous anodic alumina film by A.C. deposition method. Process conditions are as follows: the solution containing 1g/LCeCl₃·7H₂O and 10 mL/L H₂O₂, 10V(A.C) voltage applied at room temperature for 5 min. It is found that to obtain a uniform Ce composite film the thickness of the porous anodic alumina film must over 6.8μm. Ce sediment is distributed evenly on the inner surface of porous alumina film with the average content of 1.70 (wt)%. And Ce is mainly at the form of the amorphous Ce³⁺ and Ce⁴⁺ hydroxide, with the depth about 1.5μm.Ag and Cu composite films are prepared by A.C. deposition method. The metal of Ag and Cu are mainly deposition at the bottom of porous alumina film and there are some metal being distributed in the other place. It is found in the process of A.C. deposition, the region of the peak current dramatic changes, reduction of metal ions and hydrogen ions take place at the same time. When the peak current stable the mainly reaction is metal ions reduction, while, hydrogen ions reduction would be inhibited, In addition, during A.C. deposition process, the cathodic peak current is larger than that of anodic current.