Papers by Keyword: Porous Anodic Alumina

Abstract: Template/spin-coating method was presented for fabricating single-layer nano-pillar array polymethylmethacrylate (SL-PMMA) and double-layer nano-pillar array polymethylmethacrylate (DL-PMMA) membranes. The different mass ratio of PMMA/DiMethyl Formamide (DMF) solution was dripped on single-layer porous anodic alumina (SL-PAA) or double-layer porous anodic alumina (DL-PAA) membrane and spun at 4000rpm speed for 30 seconds. The SL-PAA and DL-PAA membranes had been put into vacuum oven at 150°C for 2 hours, before SL-PMMA and DL-PMMA were removed. Experimental results show that the regularity degree of PAA fabricated in OAS/PAS is higher than that of PAA fabricated in PAS by two-step anodization method. The ordered pores and clear double-layer outline can be observed from the surface and cross-section FESEM images of DL-PAA membranes. When the content of PMMA in mixture solution is 20 wt%, the top shape of nano-pillars is convex, because the solution was completely filled in the pore of SL-PAA, and the length of nano-pillars is equal to the height of pore of PAA. However, the top pore amount on nano-pillars at ascending speed 20°C per minute is more than that of at ascending speed 10°C per minute. The PMMA membranes with ordered mid-hollow and porous nano-pillars will have wide application prospect in biosensors, chemical sensors, microcapsules fabrication fields due to many advantages such as simple operation, low cost, high specific surface area, etc.

Abstract: In this study, porous anodic alumina was formed on aluminum alloy AA6061 by anodizing using mixture of 0.3 M oxalic acid and phosphoric acid with concentration ranged from 0.1 M to 1.0 M. AA6061 alloys were anodized at 40 V and 25°C for 60 minutes. FESEM images show that the uniformity of the pores arrangement of porous anodic alumina decreased with the increasing concentration of phosphoric acid in the electrolyte. Well-ordered porous anodic alumina was formed in mixture of 0.3 M oxalic acid and 0.1 M phosphoric acid while disordered porous anodic alumina were formed when the concentration of phosphoric acid were in the range of 0.3 M to 1.0 M. Pore size and interpore distance were found to increase with the concentration of phosphoric acid in the mixture. X-ray diffraction patterns show that to γ-Al₂O₃ were formed on the surface of AA6061 after the anodizing process, regardless of the concentration of phosphoric acid in the mixture electrolyte.

Abstract: In this study, porous anodic alumina was formed by anodizing of aluminum alloy AA6061 in oxalic acid with concentration ranged from 0.1 M to 1.0 M respectively. AA6061 alloys were anodized at 40 V and 25°C for 60 minutes. FESEM images show that the uniformity of the pores arrangement of porous anodic alumina depends significantly on the concentration of oxalic acid. Well-ordered porous anodic alumina was formed in oxalic acid of 0.3 M, 0.5 M and 0.7 M while disordered porous anodic alumina were formed when the oxalic acid of 0.1 M and 1.0 M were used as electrolytes. EDX analysis revealed that the only peaks corresponding to aluminum and oxygen were detected. Pore size was found to increase with the concentration of oxalic acid while the interpore distance remained almost unchanged although the concentration of oxalic acid increased from 0.1 M to 0.7 M. Atypical anodic alumina without pores arrangement was formed when 1.0 M oxalic acid was used for anodizing.

Abstract: The morphology of porous anodic alumina (PAA) formed by anodizing in inorganic electrolytes is reported. An impure aluminum was anodized in sulfuric acid, phosphoric acid and chromic acid at room temperature with a constant applied potential 2 – 30 V. The formation of porous anodic alumina was carried out by one and two steps anodization. It is clearly noted that anodizing impure aluminum at room temperature provide higher kinetic of oxide dissolution compared to oxide growth. Two steps anodizing aluminum in sulfate electrolyte always resulted in random porous alumina, while phosphate electrolyte provided strong anodization producing irregular porous alumina with average diameter of 61.6 nm. Two steps anodizing aluminum in chromate electrolyte produce better pore ordering with relatively large size pore distributions. The average pore diameter of alumina increases linearly with applied voltage, with proportionality factor l_p 0.83 nmV^-1. Annealing the sample increased the interpore distance, removed stresses providing lower activation energy for pore formation.

Abstract: In this study, porous anodic alumina was formed on aluminium alloy substrate with increasing manganese content, from high purity aluminium with 0 wt% Mn to aluminium alloy with 2.0 wt% manganese by anodizing. Substrates were anodized at 50 V in 0.3 M oxalic acid of 15°C for 60 minutes. Images from the optical microscope revealed that no secondary phase existed in high purity aluminium and aluminium substrate with 0.5 wt% manganese while two phases were observed when the manganese contents were higher than 0.5 wt%. Element dispersive X ray spectroscopy spot analysis suggested that the secondary phase consists of both aluminium and manganese. Well ordered porous anodic alumina was obtained on high purity aluminium and aluminium substrate with 0.5 wt% manganese while pore arrangement of porous anodic alumina was significant disturbed when aluminium alloys with manganese contents higher than 0.5 wt% were anodized.

Abstract: To obtain a large surface area, the aluminum electrode foils (AEF) were etched by direct current (DC) in hot chloride solution to get lots of vertical [001] tunnels. Whereas, because of the defects as rolling trace, distorted oxidation film, non-uniform organization, pollution layer on the surface of the aluminum foil, and the easy corrosion in hot Cl- acid solution, the etch pits are disorder and merged. These merged pits cause laterally directed dissolution and excessive dissolution on the aluminum surface, which limited the surface enlarging and decreased the strength of the aluminum foils. In this paper, we adopted anodic oxidation to fabricate the porous anodic alumina (PAA) on the high-purity electronic aluminum foils with 99.99% purity, and polarized by a negative voltage in the KCl solution. The result showed that several nanopores combined together to form a large open holes without barrier layers. With this membrane on, the foils were etched in 70°C HCl+H2SO4 solution, and the etch pits were just generated in the combining-nanopores. The etch pits grew into tunnels respectively without merged, and the surface was also smooth.

Abstract: Thin films of oxygen-conducting materials are expected to exhibit enhanced oxygen permeability, due to their reduced diffusion path. Two such materials, yttria-stabilised zirconia (YSZ) and ceria gadolinium oxide (CGO), were deposited onto anodic alumina substrates by electron beam evaporation of the parent materials. Continuous films 200-400 nm thick were characterised through SEM and XRD analysis. It was found that the substrate temperature during deposition strongly influenced the structure and stability of the films, with the original simple cubic structure being retained at deposition temperatures above 450 °C. Attempts to deposit thin films of a yttria-doped SrCoO_3-δ perovskite were unsuccessful, due to melting of the material during deposition and thermal diffusion into the substrate.

Abstract: A convincing interpretation to hexagonal prism ordered-arrangement and self-ordering cell in porous anodic alumina (PAA) is absent up to now. Based on the growth model of oxygen bubble mould effect (OBME) for PAA, a satisfactory explanation for the growth process of hexagonal cells is proposed. The columnar pores and hexagonal cells result from the oxide growth embracing oxygen bubbles. The avalanche electron multiplication at critical thickness dc leads to electronic current which gives rise to the evolution of oxygen gas under anion-contaminated alumina (ACA) layer. The holes on the surface are usually irregular whereas the pores under the surface layer (ACA layer) are big and regular. The thickness of the barrier oxide layer remains constant due to continuous releasing of the oxygen bubbles at the critical thickness. The self-ordering of cell arrangement and the ordered morphology are related to the dissolving process of the ACA layer on PAA surface.

Abstract: Ca-containing anodic alumina (CAA) has been successfully prepared by anodizing Al film in an alkali solution at a constant voltage and subsequently electro-depositing calcium salts on and into anodic alumina. This paper investigated the anodic behavior of Al, deposition behavior of calcium salt, and microstructure of CAA. The results show that the anodic behavior of Al in Na3PO4 electrolyte can be described as three continuous stages as of initial growth of the compact barrier layer, formation of porous alumina and further development of its pores and columnar holes. It is also found that cell voltage of electro-deposition process plays an important role at the deposition behavior of calcium slat and the microstructure of Ca-containing anodic alumina (CAA). The higher the cell voltage is, the faster the deposition rate, and the more calcium being deposited at the surface of anodic alumina and into the columnar holes or at the walls of the holes of anodic alumina. It is expected that Ca-containing anodic alumina films are promising substrates for fabricating functional bio-coatings for prosthetic applications.

Abstract: This paper examines processes for the preparation and characterisation of new ceramic membrane materials with potential for gas purification, based on nanostructured anodic alumina. The ultimate research goal is to develop a membrane capable of separating hydrogen from hot synthesis gas so a key factor is the ability of the membrane to operate successfully at temperatures in excess of 800°C. Two membrane materials are compared and contrasted: a commercial Whatman product and a membrane prepared in our laboratory. We have demonstrated that the fabrication conditions, most particularly the acid environment used during membrane fabrication, controls and directs the high temperature behaviour of the membranes. Membranes prepared using sulphuric acid electrolytes have been shown to withstand 800°C without distortion and without compromising their nanostructured pore array.