Preparation And Properties Of The Nano-porous Tubular AAO Membranes

In recent years, micro-chemical technology has become a new developing direction and research focus for chemical engineering. Microchannels with diameter ranging from nanometer to millimeter are main sections of the micro-chemical equipments, Microfluidic reactors have a number of advantages over conventional chemical laboratory batch methodologies, such as short molecular diffusion distance, large specific surface area, high mass and heat transfer capacity, which will be expected to promote highly effective chemical reactions in the microfluidic reactors. Compared with the traditional reactor, micro-channel reactor often adopted the technology of the catalyst immobilized on the tube wall, so the selection of catalyst carrier was critical.The anodization of aluminum and its alloys, particularly porous-type anodization, has received considerable attention in the industry because of its extensive practical applications. Many desirable engineering properties such as excellent hardness, corrosion, and abrasion resistance can be obtained by anodizing aluminum in acid electrolytes. In addition, due to its high porosity, the porous oxide films formed on the metals can serve as good adhesion bases for electroplating, painting, and semi-permanent decorative coloration. The anodized products can be easily found in electronic gadgets, electrolytic capacitors, cookware, outdoor products, plasma equipment, vehicles, architectural materials, machine parts, etc. Tubular AAO membrane can be used as a good catalyst carrier in the microchannel reactor.This paper first adopted the method of the anodic oxidation outside the electrobath to prepare the tubular membrane of nano-porous AAO, the anodic alumina films with ordered nan-pore array structure and pore diameters of 20-160nm were prepared by anodic oxidation of aluminum under the optimum conditions in phosphoric acid electrolyte. The formation mechanism and the self-organized process of the anodic alumina films with ordered nano-pore array structure were studied thoroughly by ESEM, XRD, TG and XPS. The effects of the technical conditions, such as electrolyte, voltage, concentration and temperature on the self-organized process were studied systematically.The main contents and conclusions are as follows:1. In this paper, anodic alumina membranes were fabricated by one-step anodizing process in orthogonal experiment. The effects of oxidation time, electrolyte concentration and voltage on membrane thickness, pore diameter and orderly degree were investigated. Experimental results showed that the maximum influencing factor on membrane thickness was electrolyte concentration, followed by oxidation time and voltage; the maximum influencing factor on pore diameter was time, followed by electrolyte concentration and voltage; the maximum influencing factor on orderly degree was electrolyte concentration, followed by oxidation time and voltage. The technique parameters of thick membrane were obtained as follows: 8hours,125 V,5wt%, the technique parameters of thin membrane were obtained as follows:2hours,165V,15wt%; the technique parameters of big diameter membrane were obtained as follows: 8hours,165V,12.5wt%, the technique parameters of small diameter membrane were obtained as follows:2hours,105V, 5wt%; the best parameters of the techniques were obtained as follows:4hours,105V,10wt%.2. After the appropriate hot water treatment and calcination, great changes have taken place in the tubular membrane of nano-porous AAO, such as the transition of crystal structure, increase of specific surface area. The crystal structure of the nano-porous AAO membrane is amorphous without any hot water treatment. However, under the conditions of hot water treatment (85℃) and calcination (600℃), the amorphous nano-porous AAO transforms to be γ-Al2O3. TGA indicated that almost no thermal decomposition behaviors were happened at 700℃, and the exothermic peak also did not appear in the weight-loss step (700-900℃). The reason is that the γ-Al2O3, which was formed by the dehydration of hydrated alumina, possesses high thermal stability, and doesn’t transform to be other crystal form along with the increase of temperature. Therefore, after the hot water treatment, nano-porous AAO can transformed to be γ-Al2O3 at the lower temperature.3. After the hot water treatment, the tubular membrane of nano-porous AAO membrane has the same outstanding features as γ-Al2O3 because the γ-Al2O3, which has more excellent acid resistance and corrosion resistance than amorphous nano-porous AAO, nano-porous AAO membrane is formed by the amorphous aluminium under the conditions of hot water treatment and calcination.4. Outside the electrobath, the single parameter of anodic oxidation are 10 wt%,4h,105 V and 3℃, respectively. In order to analyze various conditions, we only change a single parameter, all else being equal. Investigations revealed that nano-porous morphology is optimal under the single condition:the reaction time is 4 h, the oxidation voltage is 105V, electrolyte concentration is 10 wt%, and the temperature of electrolyte is 3℃, respectively. Moreover, the pore size of the tubular membrane of nano-porous AAO becomes larger with the increase of the reaction time and oxidation voltage. However, electrolyte concentration is almost no influence on the pore size of the tubular membrane. What is more, we find that the most important factor of the pore morphology is temperature of electrolyte.