Prevention of burning phenomenon in fabrication of anodic aluminum oxide membranes using a constant current method
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Prevention of burning phenomenon in fabrication of anodic aluminum oxide membranes using a constant current method Chih -Yao Chen and I-Chen Chen* Institute of Materials Science and Engineering, National Central University, Jhong Li, Taiwan
ABSTRACT In this study, we have developed a constant current method for fabrication of AAO membranes with a large interpore distance in order to avoid the burning phenomenon. From our preliminary results, the average growth rate of AAO membranes could increase up to 6 μm/hr with an applied current density of 6 mA/cm2 and the burning phenomenon could be totally avoided at a relatively high anodizing voltage of 175 V. The effect of current density on the growth rate and burning phenomenon was also investigated. INTRODUCTION Nanosize templates are a low-cost and promising approach for deposition of low-dimensional nanomaterials or transfer of nanopatterns onto a substrate without using costly lithography equipments [1-3]. Among them, anodic aluminum oxide (AAO) membrane is one of the most attractive nanopore materials due to its highly ordered structure, high pore density and tunable pore size[4], which make them potentially for a variety of applications[5-8]. The pore size and interpore distance of AAO membranes are linearly dependent on the applied voltage during the anodization process[4]. In the conventional constant voltage method, the AAO membranes with a large interpore spacing of 300~500 nm could be obtained using an anodizing voltage of 150~195 V [9-11]. When applying a high voltage over 170~180 V, however, the burning phenomenon, which would cause structural damages, usually appears owing to the high current flows concentrated at the some weak areas [12-17]. Although the anodic aluminum oxide membrane has been prepared using constant current anodization mode, the method failed to control the consistent pore sizes from bottom to top of AAO membranes [18]. In this study, a two-step constant current method for fabrication of large interpore AAO membranes was developed in order to avoid the burning phenomenon. The surface and cross-sectional morphologies of the AAO membranes were analyzed by field-emission scanning electron microscopy. The effect of current density on the growth rate and burning phenomenon was also investigated.
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EXPERIMENTAL The AAO membranes were fabricated using high purity aluminum foils (99.999%, Aldrich) through an anodization procedure in phosphoric acid (H3PO4) solution. Prior to the anodization process, the Al foils were degreased with acetone and then electrochemically polished in a mixed solution of H2SO4 and H3PO4. For the two-step constant current process, the Al foils were first anodized to form a uniform oxide layer on the top under a constant current density of 0.5 mA/cm2 for 30 min, and then the current density was adjusted to 2.5 and 6 mA/cm2 for 3h, respectively. The remaining Al foils were etched in a CuSO4-based solution. For comparison, the AAO membranes were also prepared using the conventional constant voltage and two-step consta
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