Short noteCost-effective technique to fabricate a tubular through-hole anodic aluminum oxide membrane using one-step anodization
Graphical abstract
Introduction
Aluminum is the most abundant metal element on the surface of the earth's crust, and can be mixed with other elements to form aluminum alloys of varying strengths. Due to its light weight, good thermal conductivity and corrosion resistance, and easy processing or recycling characteristics, aluminum alloy is widely used in household goods, industrial machinery, and even the aerospace industry. Under normal circumstances, a thin oxide layer is naturally formed on the surface of aluminum as it is a highly reactive metal. However, the mechanical strength of the oxide layer against abrasion is limited. Therefore, the oxidation treatment can be used to further enhance the aluminum oxide layer by increasing the thickness or changing the structure of the surface.
Aluminum anodic oxidation process is a common treatment that can form a porous alumina structure on the surface of aluminum [1]. The different morphologies of anodic aluminum oxide (AAO) micro/nanostructures can be controlled by adjusting the anodic oxidation process parameters, such as electrolyte type/concentration, applied voltage or current density, aluminum substrate purity, anodizing or etching time, temperature, etc. [[2], [3], [4]]. The structural features of AAO such as interpore distance and pore size usually increase in proportion to the increase in applied voltage. The anodizing time and etching time will affect the increase and decrease of AAO structure thickness respectively. By changing the physical roughness of the solid surface, the wettability of the liquid on the solid surface can be further adjusted [5,6]. Therefore, the wettability switching techniques on the solid surfaces through the AAO micro/nanostructure have been introduced by several studies [[7], [8], [9], [10]]. On the other hand, some studies have further investigated the phenomenon of electrowetting/magnetowetting on the AAO surface by electrically/magnetically controlled fluids [11,12]. In bionic applications, in order to mimic the surface morphology of dragonfly wings, Wang et al. grew Ag nanorods on AAO surfaces to form a composite structure [13]. After surface modification by immersion perfluorodecanethiol, the multilayer structure exhibits a superhydrophobic characteristic. Large AAO surfaces also have considerable potential for development in the field of biomedicine such as drug delivery devices or sensors [[14], [15], [16], [17]]. Song et al. used AAO nanopore thin film as a device to control drug loading and release [18]. The drug release process was accomplished by immersing the device in flowing deionized water. It was showed that the drug release time and the released dose could be further enhanced by increasing the nanopore size and density. By combining with other substances, the releasing behavior of AAO surface can also be controlled based on the temperature, pH, ionic strength, ultrasound, light, magnetic field, electric field, etc. [19,20]. Yen et al. investigated the influence of surface structure on the aluminum template for cell culture [21]. It was found that the surface of the aluminum template, which had been micro-power blasted and anodized, had a superhydrophilic property, and its micro/nano-complex structure provides a suitable environment for cell culture.
In addition, the micro/nano-textured morphology of the AAO surface enhances its specific surface area. Weng & Yang generated an AAO structure on the inner wall surface of the evaporator section aluminum alloy gravity heat pipes, and through the thermal performance test system, it was found that the gravity heat pipe with AAO wall surface could significantly improve the heat transfer effect compared to the ordinary gravity heat pipe, and the heat transfer performance inside the pipe could be further improved with the increase of AAO nanotube diameter and length [22]. Furthermore, AAO can also be used as a template for structural transfer and as a through-hole membrane for dialysis [23]. Masuda & Fukuda transferred the AAO structure onto the PMMA (poly(methyl methacrylate)) using a two-step replication technique, and then used the PMMA template to form a platinum or gold nanohole array [24]. Attaluri et al. anodized the tubular aluminum alloys to obtain the tubular AAO membranes, which were then applied to hemodialysis. It was found that AAO membranes have better hydraulic conductivity and inulin sieving coefficient than the commonly used PES (polyethersulfone) membranes, and the higher solute clearances and albumin leakage make AAO membranes an effective substitute for dialysis membranes [25]. The AAO membrane was fabricated by etching the aluminum substrate or detachment of the AAO from aluminum substrate [26]; the through-hole AAO membrane was further removal of the barrier layer alumina (BLA) on the AAO membrane bottom. Yanagishita & Masuda and Yanagishita et al. were additionally fabrication the through-hole AAO membranes by two-layer anodization [27,28]. Recently, Yanagishita et al. further used the two-layer anodization to prepare the tubular through-hole AAO membranes with tapered shape [29].
Compared with two-dimensional (2D) structure of the flat AAO membranes, the three-dimensional (3D) structure of the tubular AAO membranes has more comprehensive advantages in geometry and mechanical strength. In 1996, the fabrication of tubular anodic aluminum oxide membrane through an external anodizing method was firstly introduced by Itoh et al. [30]. The tubular AAO membrane is formed on the outside surface of an aluminum tube. However, the growth of AAO is usually accompanied by the volumetric expansion of the alumina layer during anodizing. The tubular AAO membrane prepared by the external anodizing method is prone to generate tension in the alumina layer and thereby cause crack formation. In contrast, the tubular AAO membrane prepared by an internal anodizing method provides an internal compressive stress in the alumina layer. Therefore, Itoh et al. [31] further used this method to improve the mechanical strength of the tubular AAO membrane. Belwalkar et al. [32] investigated the influence of processing parameters on morphology and thickness of the tubular AAO membranes. The results showed that in the sulfuric and oxalic acid electrolytes, the pore size and interpore distance was linearly proportional increased with increase the applied voltage. Moreover, in the sulfuric acid electrolyte, the highest acid concentration displays the largest membrane thickness. The tubular AAO membranes with circle, square, and triangle geometries were fabricated by Yue et al. [33]. During the anodic oxidation process of an aluminum block, some cracks are formed at the edges. Thus, Kasi et al. [34] further exploited this property to produce the tubular and rectangular AAO membranes. However, this method consumes a large amount of aluminum base material when making the large tubular AAO membranes, so the authors recommend it for the small tubular AAO membranes. To understand the relationship between aluminum tube diameter and external surface cracks, Kasi et al. observed the formation of cracks in different tube diameters for various anodization times [35]. The results showed that the smaller diameter tubes produce more cracks, and the number of cracks increases with increasing the AAO membrane thickness. On the other hand, the chemical etching aluminum substrate has used the mercuric chloride or mixture of cupric chloride and hydrogen chloride and the removing BLA has adopted phosphoric acid or mixture of chromic acid and phosphoric acid. Impurities in the residual chemical etching solution will contaminate the AAO membrane, and there will be pore widening problems when removing the BLA. Moreover, both external anodizing method and internal anodizing method require a single side isolation when removing the aluminum substrate, which makes the fabrication more complicated. Thus, Kasi et al. introduced a new method for etching aluminum substrate and BLA in a single step, which called continuous voltage detachment and etching [36]. As with the electrochemical polishing process, the double-sided anodized aluminum tube was dipped directly into the electrochemical polishing solution, and then a continuous voltage was applied until all the aluminum was completely etched. Nevertheless, although this method is easy to prepare the tubular AAO membrane, in practice it may be difficult to install on the system due to the lack of a both end aluminum metal connection. Recently, in order to improve the cost-effective of production, Hun et al. developed a process for batch production of the tubular AAO membranes using a commercial 6061aluminum alloy tube [37].
Referring to the literature on the preparation of tubular AAO membranes, most processes use electrochemical polishing to remove the surface scratches and obtain a smooth with shiny surface before anodizing the aluminum tube. However, the electrolyte used in electrochemical polishing usually contains perchloric acid, ethanol, and phosphoric acid, which will be dispersed into the environment as the temperature rises during the reaction. After electrochemical polishing, the recycling of electrolytes with metal ions is also another problem. On the other hand, most authors use two-step anodization process to form AAO on aluminum tubes, which means that it takes more time to complete the tubular AAO membrane. In this study, a mirror-like aluminum tube surface is made through physically wet grinding and buff wheel polishing, and AAO membranes are fabricated from aluminum alloy tubes using a one-step anodization process. Then, the removing of the BLA is observed at different etching times to obtain a tubular through-hole AAO membrane.
Section snippets
Materials
6063 aluminum alloy (Mg: 0.45–0.90%; Si: 0.20–0.60%; Al: the base) tubes with an outer diameter of 19 mm and a thickness of 1 mm from Shin-Li Metals Co. (Taiwan) were used as the testing material for preparing AAO membranes. All the other chemicals, including acetone (99.5%, SEEDCHEM, Australia), alcohol (95%, ECHO, Taiwan), sulfuric acid (95%, Fisher, Canada), oxalic acid (99.8%, SHOWA, Japan), phosphoric acid (85%, Scharlau, Spain), hydrochloric acid (37%, Fisher, Canada), and copper chloride
Results and discussion
Fig. 3 presents the FE-SEM images of the AAO membrane after dissolving inner aluminum substrate. In Fig. 3a, numerous nanotubes are observed at the top view of the AAO membrane. The average pore size is 35 nm. Fig. 3b shows the bottom view of the AAO membrane. It can be seen that the BLA on the AAO membrane bottom has an orderly and compact structure. In the fabrication of a tubular through-hole AAO membrane, the process of removing BLA is an important part. However, pores widening problem
Conclusions
In this study, a one-step anodization process has been used to make a tubular through-hole AAO membrane from commercial aluminum alloy tubes to reduce the required manufacturing time. First of all, a smooth and shiny outer wall surface of the tube was obtained by several processes of annealing, physically wet grinding, buff wheel polishing and ultrasonic cleaning. Then, the AAO membrane was formed on the tube outer wall by the one-step anodization process. After etching the inner substrate of
Author contributions
Conceptualization, Y.-C.C. and H.C.W.; methodology, Y.-C.C. and H.C.W.; formal analysis, Y.-C.C. and H.C.W.; writing—original draft preparation, Y.-C.C. and H.C.W.; writing—review and editing, Y.-C.C. and H.C.W. All authors have read and agreed to the published version of the manuscript.
Funding
This work was supported by the Ministry of Science and Technology, Taiwan [grant number 108-2221-E-033-014].
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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