Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte

The overall goal of this procedure is to efficiently fabricate anodic aluminum oxides, or AAOs, by combining simultaneous multi-surface anodization with direct AAO detachment, through the application of stair-like reverse biases. This method can help alter key issue in anodization and contribute to development of AAOs for normal technologic applications. The main advantages of this procedure are:it is in felicity, it is efficient use of resources, and that it avoids the use of toxic chemicals, like mercuric dichloride.

This technique forms AAOs on all exposed surfaces of on aluminum substrate. After AAO detachment, the remaining aluminum substrate can be reused, allowing fabrication of many AAOs from a single aluminum specimen. First, cut and polish an aluminum specimen into a parallelepiped.

Pour 350 milliliters of a four to one solution of absolute ethanol and 60%perchloric acid in to a double jacket beaker connected to a low temperature bath circulator. Based on the dummy aluminum specimen, adjust the solution levels, so that 4/5 of the substrate will be immersed. Set the bath temperature to slightly below seven degrees Celsius and start flowing the circulating medium through the outer section of the beaker, while stirring.

Then, ultrasonicate the aluminum specimen in acetone for 30 to 40 minutes. Rinse the substrate with acetone and deionized water and thoroughly dry it under a stream of air or nitrogen gas. Clamp the dry aluminum substrate and a platinum wire to electrode pass throughs 40 millimeters apart on an electrolytic cell lid.

Ensure that the platinum wire is inline with the wide face of the substrate. Once the perchloric acid solution has cooled, stop stirring. Place the lid on the beaker, to immerse the substrate and the platinum wire.

Connect the aluminum working electrode to the positive terminal of a programmable DC power supply. Connect the platinum counter electrode to the negative terminal. Apply a forward bias of positive 20 volts to the aluminum working electrode with respect to the platinum counter electrode for two to four minutes, depending on the condition of the aluminum surface.

Monitor the substrate for residues peeling off and sliding down in to the perchloric acid solution. When electropolishing is complete, stop applying the forward bias and disconnect the programmable power supply. Carefully remove the aluminum substrate and the platinum wire from the solution.

Rinse the substrate 95%ethanol and deionized water several times to remove residual perchloric acid. Store the electropolished substrate in 95%ethanol. To begin the AAO fabrication process:Pour 650 milliliters of a 0.3 molar aqueous oxalic acid solution in to a one liter double jacket beaker connected to the low temperature bath circulator.

Based on the dummy aluminum specimen, adjust the solution level, so that 3/4 of the substrate will be immersed. Set the bath temperature to slightly below 15 degrees Celsius, and start stirring the solution. Thoroughly dry the electropolished aluminum substrate under a stream of air or nitrogen gas.

Then, connect the dry substrate and a platinum wire to an electrolytic cell lid 50 millimeters apart. Place the lid on the beaker and verify that the platinum wire and 3/4 of the aluminum substrate are submerged in the oxalic acid electrolyte. Connect the positive terminal to the electropolished aluminum working electrode and the negative terminal to the platinum counter electrode.

Apply an anodic bias of positive 40 volts to the working electrode with respect to the counter electrode for more than three hours while stirring at 100 to 150 RPM. Then, stop applying the bias and disconnect the clips. Carefully remove the substrate from the solution.

Rinse the substrate with acetone and deionized water several times, to remove residual oxalic acid. Following pre-SMSA, warm an aqueous chromic acid solution to 60 to 65 degrees Celsius on a hotplate. Immerse the substrate in this solution for one to two hours to remove pre-AAOs.

Then, rinse the substrate with acetone and deionized water several times. Use a digital multimeter to measure the resistance across the substrate, to verify the complete removal of the insulating pre-AAOs. Next, reinstall the substrate and the platinum wire on the electrolytic lid and immerse them in the previously used oxalic acid electrolyte.

Connect the electrodes to the programmable DC power supply in the same configuration as the pre-SMSA. While stirring, apply an anodic bias of positive 40 volts, to the aluminum working electrode with respect to the platinum counter electrode. Continue applying the anodic bias, until the AAO layer has reached the desired thickness.

Once the main SMSA has finished, stop applying the bias and stop stirring the solution. Connect the negative terminal to the aluminum electrode and the positive terminal to the platinum electrode. Apply stair-like reverse biases and monitor the edges and surfaces of the aluminum substrate for bubbling effects and detachments.

When all the AAOs have successfully detached, as indicated by the AAOs only being attached to the substrate at the top, stop applying the SRBs and disconnect the power supply. Remove the aluminum substrate from the electrolyte being careful not to damage the partially detached AAOs. Gently rinse the substrate and the partially detached AAOs, with acetone and deionized water several times.

Carefully, dry the sample under a stream of air or nitrogen gas. Use a razor blade or a stationary cutter blade to manually break off the AAOs from the aluminum substrate. Place the AAOs in a petri dish and store them in a desiccator.

Then, immerse the AAO detached aluminum substrate in the warm chromic acid solution for 30 minutes to etch residual alumina from its surface. Rinse and dry the etched substrates as described in the text protocol. Repeat the fabrication unit sequence with the same aluminum substrate to continue preparing AAOs in the multi-surface process, as desired.

AAOs were simultaneously fabricated from the five surfaces of an aluminum substrate exposed to the electrolyte. This multi-surface process was repeated five times with the same substrate. Open pore and barrier side scanning electron microscopy images, showed the same patterns and dimensions for each AAO.

Current time data were collected during the pre-SMSA, the main SMSA, and the stair-like reverse bias steps. The current level decreased gradually with increased anodic voltage application time for both SMSAs. The current level also decreased, as the number of unit sequences increased.

This was attributed to the reduction of the total anodizing area with each repetition, and to the accumulation of mechanical stresses induced by volume expansion from anodic oxidation on multiple surfaces. Anodic aluminum oxides have accelerated arranged on the pores with a relatively high aspect ratio. Their application include nonporous templates for one dimensional structures, membrane filters, evaporation or etching masks, and other strategy biases.

Conventional method for fabricating AAOs on an aluminum mono surface have not had a good production yield, especially in two step anodizing procedures. AAOs are usually separated from the aluminum substrate by disturbing the remaining aluminum with toxic chemicals such as mercury dichloride. This technique used more eco-friendly an efficient methods, that are potentially applicable to mass production.

Stair-like reverse bias detachment is a significant reimburse synergistic with multi-surface anodization than conventional disturbing technique or constant reverse bias detachment. Once mastered, this technique can be adapted to fabricate AAOs with different features targeted for specific applications. If it is performed properly.

Future directions include finding a proper way to dissipate your hitting, which we allow this technique to be applied to other anodizations, further improving the yield.