Fast fabrication of self-ordered anodic porous alumina on oriented aluminum grains by high acid concentration and high temperature anodization

Abstract

Anodic porous alumina, which exhibits a characteristic nanohoneycomb structure, has been used in a wide range of nanotechnology applications. The conventional fabrication method of mild anodization (MA) requires a prolonged anodization time which is impractical for batch processing, and self-ordered porous structures can only be formed within narrow processing windows so that the dimensions of the resultant structures are extremely limited. The alternative hard anodization (HA) may easily result in macroscopic defects on the alumina surface. In this work, by systematically varying the anodization conditions including the substrate grain orientation, electrolyte concentration, temperature, voltage, and time, a new oxalic acid based anodization method, called high acid concentration and high temperature anodization (HHA), is found, which can result in far better self-ordering of the porous structures at rates 7-26 times faster than MA, under a continuous voltage range of 30-60 V on (001) oriented Al grains. Unlike HA, no macroscopic defects appear under the optimum self-ordered conditions of HHA at 40 V, even for pore channels grown up to high aspect ratios of more than 3000. Compared to MA and HA, HHA provides more choices of self-ordered nano-porous structures with fast and mechanically stable formation features for practical applications.

Massive, eco-friendly, and facile fabrication of multi-functional anodic aluminum oxides: application to nanoporous templates and sensing platforms

“…51 Cheng and Ngan demonstrated that anodization under higher acid concentration and temperature than standard MA conditions could improve growth rate of AAO and arrangement of the hexagonal unit cells. 52 Xu and co-workers reported that there existed on the optimal amount of ethanol addition into the acidic electrolyte, which was typically used as a coolant for HA, [46][47][48][49] for increasing AAO growth rate under MA condition. 53 For separating an AAO from the Al substrate, toxic chemicals containing heavy metal ions, such as mercury chloride 5,6,19,21,37,[54][55][56][57] or copper chloride (CuCl 2 ) 42,48,58,59 are commonly used for dissolving the remaining part of the Al substrate, which is another timeconsuming part in the conventional AAO fabricating methods.…”

Theoretical Pore Growth Models for Nanoporous Alumina

“…demonstrated that under hard anodization (HA) conditions in which the oxide growth rate is tens of micrometers per hour, the voltage dependency becomes 0. [90]. HHA can result in much better self-ordering AAO compared with MA in a voltage range from 30 to 60 V in oxalic acid based electrolyte, and reduce the From the top-view of self-ordered AAO, the pores are arranged in a quasi-hexagonal pattern.…”

“…Nanoporous structured AAO can be easily fabricated by anodization of aluminum in different kinds of electrolytes, such as sulfuric acid [89], oxalic acid [90][91][92][93], phosphoric acid [94], and chromic acid [94,95]. However, for neutral electrolytes with pH in the range of 5-7, only barrier-type alumina thin film can be formed.…”

Controllable Fabrication of Networked Three-Dimensional Nanoporous Anodic Alumina Films on Low-Purity Al Materials

“…17 However, most studies so far have used Al with a purity greater than 99.99% as starting material to produce highly ordered porous structures. From the viewpoint of practical applications, using low-purity Al or Al alloy materials to fabricate nanoporous alumina nanostructures is highly desirable in terms of lowering costs and gaining satisfied mechanical strength.Recently, much effort has been dedicated to fabricating tailored nanoporous alumina films with varying pore diameters gradually or step-wise from high-purity Al foils (e.g., 99.997% Al), by means of changing the anodizing voltages or current densities, the electrolytes, temperature [18][19][20] or by using oscillatory currents, followed by chemical etching.21 However, to the best of our knowledge, there has been no report so far on the fabrication of 3D networked nanoporous alumina films with both vertical and transverse pores, whether from high-or low-purity Al materials. Moreover, Skeldon et al recently reported the generation of defects or branched pores on the pore walls of PAA films by anodizing a commercial Al sheet (A1050-99.5%Al) and a Al-0.05%Cu sputtered film in phosphoric electrolytes, and proposed a mechanism that the formation of branched pores was related to the Cu component in Al base materials.…”

“…Recently, much effort has been dedicated to fabricating tailored nanoporous alumina films with varying pore diameters gradually or step-wise from high-purity Al foils (e.g., 99.997% Al), by means of changing the anodizing voltages or current densities, the electrolytes, temperature [18][19][20] or by using oscillatory currents, followed by chemical etching.…”