## 1 Introduction  
High-quality AAO (anodic aluminum oxide) films provide ordered straight channels, with a diameter of 10-500 nm, pore density of \(10^{7}\)-\(10^{11}\) pore/cm\({}^{2}\), and thickness of 1-300 \(\upmu\)m [1; 2]. With large surface areas, high mechanical strength, and flexibility, AAO can be used in medical or energy applications, such as drug delivery and detection [3; 4]. The large AAO surfaces can be utilized to absorb the bio-indicators or drugs, and the releasing behavior can also be controlled based on the heat sensitivity. AAO has also found applications in energy conversion between carbon dioxide (CO\({}_{2}\)) and methanol (CH\({}_{4}\)) [5; 6]. By loading photocatalyst particles on the AAO surface, such photocatalytic systems can be used to recycle carbon dioxide into organic compounds. Based on the features of larger surface areas and nanochannels for mass delivery and gas diffusion, three-dimensional (3D)structure of AAO films have practical advantages over two-dimensional (2D) AAO films for medical and energy applications. AAO has a lower melting point than pure alumina because of the inclusions in the porous AAO structure. [PERSON] [7] presented the following compositions of alumina film anodized using sulfuric acid as electrolyte: Al\({}_{2}\)O\({}_{3}\) (78.9 wt. %), Al\({}_{2}\)O\({}_{3}\cdot\) H\({}_{2}\)O (0.5 wt. %), Al\({}_{2}\) (SO\({}_{4}\))\({}_{3}\), and H\({}_{2}\)O (0.4 wt. %). According to [PERSON]'s work [8], the melting point of AAO is near 1200 \({}^{\circ}\)C, and AAO template retains stable, at around 1000 \({}^{\circ}\)C [9], which is lower than that of bulk alumina (2017 \({}^{\circ}\)C for Al\({}_{2}\)O\({}_{3(\gamma)}\)). The AAO structure maintaining temperature of 1000 \({}^{\circ}\)C is stable enough to be a template for CaO-CaCO\({}_{3}\) reaction at 894 \({}^{\circ}\)C.  
In the past, tubular AAO has attracted attention. Several methods were proposed for the fabrication and applications of tubular AAO films. According to [PERSON]'s report [10], the large area (50 cm\({}^{2}\)) free-standing AAO membranes was obtained by sputtering carbon onto AAO surface for conductive AAO biosensor applications, especially tubular AAO for filtering. [PERSON] [11] showed that AAO tubular membranes were fabricated from aluminum alloy tubes in sulfuric and oxalic acid electrolytes, the pore sizes were ranging from 14 to 24 nm, and the wall thicknesses was as high as 76 \(\upmu\)m, which increased the mechanical strengths for handing. The pore density can be calculated by determining the number of pores according to the area fraction: \(P=A_{p}/[(\pi/4)D^{2}]\), where \(P\) is the number of pores, \(A_{p}\) is the area fraction of pores, and \(D\) is the average pore diameter. [PERSON] [12] presented that AAO membrane was prepared in 0.2 M oxalic acid electrolyte under 25 to 40 V applied for 11 to 18 h; additionally, the control in drug delivery by using nanoporous alumina tube capsules with pores of 25 to 55 nm was demonstrated. [PERSON] [13] further reported that the purity of Al is also a matter of great concern for AAO fabrication. Some applications, such as nano-templates for semiconductor industry, require a regular pore arrangement with a honeycomb structure, which cannot be achieved from low grade Al. Moreover, AAO membrane in tubular form can further satisfy the demand in both diffusive and convective filtration of hemodialysis. The control in preparing tubular AAO is more challenging than flat AAO film on Al sheet because of film cracking issues. Since the AAO processes are sensitive to the operation conditions, defects may appear in AAO membranes if unsuitable conditions are applied. How to manage the variable conditions is pivotal, including electrolyte temperatures, applied voltages, electrolyte compositions, cooling stirring, and current density distribution. [PERSON] [14] reported that more cracks were generated in smaller diameter (2 mm) tube than larger one (19 mm).