Simultaneous Fabrication of Porous Alumina Lines By 3D Printed Multi-Capillary Sf-MDC
Authors: Sakairi Masatoshi , Muhammad Bilal 
Published: December 2020

Introduction
Porous-type anodic aluminum oxide (porous alumina) films with an ordered nanostructure have attracted much attention in recent years. It is well known that the size and length of the pores are controlled by the anodizing conditions, including the concentration and composition of the used electrolyte.
There are demand for formation of porous alumina film at selected area, and are a number of techniques for the formation of porous alumina at the desired size and shape. However, commonly used local anodized techniques present disadvantages in the complex processes involved.
One technique that gets around the these problems is the droplet cell techniques. The droplet cell used electrolyte droplet that formed at the tip of capillary tube uses as electrochemical cell. One of the authors reported the application of solution flow type micro-droplet cell with co-axial dual capillary tubes (Sf-MDC) to form porous alumina film at selected area 1, 2, 3, 4. The thickness and width of the oxide film increases with decreasing droplet moving speed, and the growth rate of the oxide film is greatly affected by the specimen temperature. Using this cell makes it possible to form porous alumina lines with layered structure. However previously used Sf-MDC has drawback of slow alumina line fabrication. One technique that gets around this problem is using multi-capillary Sf-MDC. It is difficult to increase number of capillaries using previous making technique for Sf-MDC.
3D printer is widely used and recent advancements in 3D printing technology allows us to produce complex structures with great accuracy and minimal production time. Therefor 3D printing technique is apply to make multi-capillary Sf-MDC. The purpose of this experiment is to design and fabricate a novel Sf-MDC using 3D printer and form multi porous alumina lines on the substrate.
Fabrication of multi-capillary Sf-MDC
A Sf-MDC was designed and photopolymer resin was used for 3D printing the parts. Optimal orientation was chosen to print the parts in order to minimize the number of supports, number of layers, time and volume required for printing. Using optimized printing parameter, multi-capillary Sf-MDC was made. Before using the Sf-MDC for anodizing, the support structure was trimmed and Pt wire was inserted inside the capillaries.
Experimental
Specimen and electrolyte: High purity Al sheets (99.99 %, 10 × 30 mm, 110 µm in thickness) were used as specimens. The specimens were ultrasonically cleaned with highly purified water for 300 s, followed by ultrasonic cleaning in ethanol for 300 s. Then, the specimens were electropolished in 13.6 kmol m-3 CH3COOH /2.56 kmol m-3 HClO4 at a constant voltage of 28 V for 150 s at 278 K. After the polishing process, specimens were cleaned with highly purified water and acetone.
The solutions used as the electrolyte for the anodizing were 0.22 kmol m-3 (COOH)2, 0.22 kmol/m-3 H3PO4, 0.3 kmol/m-3 H2SO4solutions.
Anodizing: An electropolished specimen was set on a computer-controlled pulse-XYZ stage, and the specimen temperature during the anodizing was controlled by a Peltier device at 323 K. The cell was filled with electrolyte via a solution pump, then a droplet of electrolyte was formed at the tip of the inner capillary by adjusting the solution flow rate and a vacuum pressure. After the droplet of electrolyte was formed, thereafter, the droplet was put in contact with the specimen surface, and a constant voltage was applied. The moving speed of the specimen was controlled at 2.0 μms-1, and distance was 4 mm for the formation of the oxide film lines.
Observation:
Specimen surfaces and cross-sections after anodizing were examined by an optical microscope and scanning electron microscope, SEM. The specimens for the cross-sectional observation were prepared by simple curving and breaking. Before the SEM observations, a thin Au layer was sputtered on the specimens.
Results
Three uniformly sized porous alumina lines on the Al specimen can be formed by the Sf-MDC. From SEM observation, formed anodic oxide lines have a porous structure and average pores arrangement and pore diameter is nearly same for each lines.
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