Electrochemical anodization of aluminum at room temperature by electronically controlled direct current circuit

Abstract

Anodic aluminum oxide (AAO) films have been investigated and used in numerous products for more than 50 years. The morphologies of the AAO films can be classified into barrier-type and porous-type. Barrier-type films consist of compact amorphous alumina, while porous-type films comprise of a thin barrier layer next to the metal and an outer layer of porous alumina. There are limitations in the way nanoporous alumina has been fabricated. The limitations are the freezing of the electrolyte with a temperature controlled bath, very high potential and prolong anodizing time have also been used during AAO fabrication. In order to overcome these shortcomings associated with the fabrication of this porous alumina film at room temperature, this study aim to create a novel process to fabricate nanoporous alumina film with controlled pore diameters that is externally controlled by direct current circuit. The steps followed during the electronic circuit design set-up involved specification, design/cost, verification and testing of the circuit. The specification of the design is to address the issue of temperature, low potential anodization and the value of the most critical process parameter which is current, and the less critical parameters which are temperature and concentration of electrolyte. The next step of the design was the identification of important components i.e. capacitors and resistors that are crucial during the preliminary design stage. The components are cheap and provide a form of stability and control of the circuit. The verification and testing was done by computer simulation (PhET software kit) and practical testing of the circuit design. The computer simulations provided the information about the most suitable circuit design under different simulated conditions. The simulation identified a series connection of a single 60 V rated capacitor and a 500 ohms resistor to be the most suitable circuit design to limit the influence of the process parameters. The practical testing using different electrolyte concentration (i.e. 0.7 M, 1.5 M, and 2.2 M), different low potentials (i.e. 10 to 50 V) operating at room temperature identified the most critical current value to be less or equal to 150 mA. Any current value above this failed to produce a nanoporous alumina structure. A similar testing was also performed at higher temperature (50oC) to confirm the suitability of the circuit to operate at higher temperatures. The functionality of the circuit design is also equation based controlled by standard equations which can be operated via a direct circuit supply or alternating current supply, making the entire process flexible, accurate and precise. The room temperature and low potential anodization of aluminum were anodized under different electrolyte conditions for 1 hour, 3 hours, and 5 hours respectively. The scanning electron microscope (SEM) results show that the pores formed at different potentials and concentrations of electrolytes is within the range of 10 – 200 nm. The pores are randomly distributed all over the surface of the aluminum. As the applied voltage is increased with time subsequently the pores also increase in diameter. This results show that with the aid of the upgraded electrochemical cell controlled by the external dc circuit, nanoporous alumina were successfully fabricated at room temperature and high temperature with low potentials using a single step anodization technique which was suitable for AAO fabrication