Fabrication Of The Anodic Aluminum Oxide Films With Large-Scale Tunable Pore Intervals

Anodic aluminum oxide (AAO) is a typical self-organized porous membrane materialfabricated in an acidic electrolyte by applying voltage to anodize the metal aluminum. Thepore interval (Dint) of the AAO membrane is dependent on the applied voltages (Ua); thehigher the voltage is, the larger the Dintof AAO membrane is. The pore arrangement of AAOmembrane is dependent on the Ua, the anodizing current, the type and concentration ofelectrolyte, and the temperature. For a specific acidic electrolyte at constant temperature, theself-organized voltage window is fixed. Inside the voltage window, AAO membranes growwith highly ordered hexagonal pore arrangement. Currently, the most preferred AAOmembranes are usually fabricated in0.3M sulfuric electrolyte at25V,0.3M oxalic acidelectrolyte at40V, and1wt%phosphoric acid electrolyte at195V, giving the pore intervalsof63nm,100nm, and500nm, respectively. However, obtaining the highly ordered AAOmembranes with continuous and adjustable Dintis difficult, how to achieve the AAO with Dintover a large scale remains unresolved.In this dissertation, anodization in oxalic acid electrolyte was studied firstly, and highlyordered AAO membranes were successfully fabricated in0.3M oxalic acid at40V and0oC.Based on this result and the previous reports of the Dintwith500nm and100nm in1wt%phosphoric acid electrolyte at195V and in0.3M oxalic acid electrolyte at40V, respectively,we tries to make the Dintof highly ordered AAO membranes adjustable over a large scale inphosphoric acid and oxalic acid mixed electrolytes. Experimental results show that theanodization mode will convert when the Uachanges. And the critical voltage will increasewith the increase of the phosphoric acid concentration in the mixed electrolyte. In the mixedelectrolyte with4:1volume ratio of0.3M oxalic acid solution and0.3M phosphoric acidsolution at50V, highly ordered AAO membrane can be fabricated over a larger area.However, in the mixed electrolytes with any other volume ratio of0.3M oxalic acid solutionand0.3M phosphoric acid solution, the pore arrangement is disordered, even in theelectrolyte only with phosphoric acid solution.In order to fabricate highly ordered AAO membranes with large Dintin phosphoric acid electrolyte, we introduce aluminum oxalate (Alox) to the electrolyte. As a result, highlyordered AAO membranes are fabricated in1wt%phosphoric acid electrolyte with0.1MAlox addition at205V,0.5wt%phosphoric acid electrolyte with0.005M Alox addition at215V and0.27wt%phosphoric acid electrolyte with0.0054M Alox addition at230V,respectively. In addition, the pore widening and barrier layer removal technologies for theAAO membranes fabricated at205V are studied in detail by taking5wt%phosphoric acidand0.06M sodium citrate as etching solutions, respectively. The relationship between thepore diameter and the etching time in5wt%phosphoric acid at30-50oC is achieved, and theequation of the etching rate and the temperature is obtained. Based on the results of barrierlayer removing in5wt%phosphoric acid at30-60oC and0.06M sodium citrate at60-80oC,respectively, it is confirmed that the cells of AAO membranes has a double structure. Theetching rate is fast in phosphoric acid for the inner layer but in sodium citrate acid for theouter layer.On the basis of the AAO membrane fabrication in mixed electrolyte and phosphoric acidelectrolyte with Alox addition mentioned above, we tries to make the Dintof highly orderedAAO membranes adjustable over a large scale in0.1M phosphoric acid/0.01M Aloxelectrolytes by adding different volume content of0.3M oxalic acid solution. The AAOmembranes with Dintfrom100to470nm are fabricated successfully as the volume ratiomodification. Experimental results demonstrate that the optimum voltages decrease linearlywith the volume content of oxalic acid in the mixed electrolytes. However, the porearrangement of AAO membranes does not totally obey this ruler. When the volume content ofoxalic acid in the mixed electrolyte is lower, highly ordered AAO membranes are fabricated ata specific Uo. But highly ordered AAO films are difficult to obtain in the electrolytes withhigher volume content of oxalic acid at a range of Ua. The detailed analyses on theanodization current-time curves are helpful to understand the influence of oxalic acid additionon the intrinsic mechanism of anodization process.