Anodic Aluminum Oxide With Large Pore Interval And Its Double-layer Structure: Research And Application

AAO （anodic aluminum oxide） membranes have received considerable interests due totheir controllable pore structure, stable preparation technology, and low cost of production. Inthis dissertation, the AAO membranes with large pore interval and tunable pore interval in alarge scale are fabricated successfully in a modified electrolyte. In addition, the porousα-Al₂O₃ceramic membranes and the aluminum oxyα-hydroxide （AlOOH） nanorod/nanotubearray are fabricated using the inner layer and outer layers of the AAO membranes, respectively.Besides, the humidity sensitive characteristics of the AAO membranes with large poreintervals were investigated.According to the linear relation of anodization voltage and pore interval, high anodizationvoltage is necessary for fabrication of the AAO membranes with large pore interval. However,high anodization voltage easily results in the breakdown of AAO membranes. To avoid thebreakdown, aluminum oxalate （Alox） is introduced as a buffer agent to modify the phosphoricacid electrolyte, and the highly ordered AAO membranes with the pore interval of530nm arefabricated successfully. Experimental results show that the pore order degree in AAOmembranes is simultaneously dependent on several factors, including anodization voltage,reaction temperature, and electrolyte concentration.In order to adjust the pore interval of AAO membranes in a large scale, the mixedelectrolytes with different volume ratio of0.3M oxalic acid and1wt%phosphoric acid/0.01M Alox are used for fabrication of AAO membranes. The experimental results indicate that theoptimal anodization voltage nearly equals to the critical breakdown voltage. And the poreinterval increases linearly from100nm to500nm with the decrease oxalic acid volumepercent. The theoretical analysis demonstrates that the anodization current has a close relationwith the ionization constant of acid radical ions. Since the secondary ionization constant ofphosphoric acid radical ion is much smaller than that of oxalic radical ion, the optimalanodization voltage increases and the steady anodization current decreases with the decreaseof oxalic acid volume percent.As a research hotspot, fabricating porous α-Al₂O₃membranes by taking AAO membranesas the starting materials has attracted much attention for a long time. Previously, all the effortstrying to sinter the AAO membranes directly at high temperature are failed due to that thebilayer cell structure of AAO membranes easily leads to the pores aligned disorderedly and themembranes curly and cracked on a macro level. To solve this problem, a novel strategy isproposed. By hydrothermal preα-treatment of the AAO membranes and subsequent chemical etching of the outer layer selectively and then sintering, the highly ordered hexagonal porousα-Al₂O₃membranes are fabricated successfully. The experimental results demonstrate that thehydrothermal temperature and the chemical etching time have great influence on the finalmorphologies of porous α-Al₂O₃membranes. This fabrication method might be a newapproach for preparation of porous α-Al₂O₃membrane. The obtained membranes areanticipated to become a new generation of filter and supports for catalysts or absorbents.The aluminum oxyα-hydroxide （AlOOH） nanoα-materials have been widely used in thefield of catalysis, adsorption, refractory materials, and the application performance has beenproved to be influenced by their morphologies. In order to control the morphologies of AlOOHproducts, AAO membrane is often used as a good starting material candidate. Here, aiming atthe hydrothermal treatment of AAO membranes, we propose a possible mechanism on theAlOOH growth. Based on this mechanism, AlOOH products with different morphologies areobtained by properly controlling the outer layer thickness of AAO membrane and the reactiontemperature. The obtained AlOOH nanorod and nanotube array are anticipated to improvetheir performances as catalysts and absorbents and broaden their applications in the fields ofbiology and nanotechnology.Since the large specific surface area and the excellent hydrophilic characteristics, theAAO membrane is an ideal material for fabrication of humidity sensors. In this dissertation,the humidity sensors based on the large pore intervals AAO membranes with throughα-holestructure and single hole structure were fabricated by a classical semiconductor technology（RF magnetron sputtering, lithography, and electrochemical plating）. The effects of componentstructure on the humidity sensing property were investigated. Experimental results show thatfor the single hole structure, the sensitivity of humidity sensors with full electrode is superiorto the humidity sensor with interdigital electrode, and the sensitivity of humidity sensors withfull electrode could be improved by the pore widening. For the throughα-hole structure, thehumidity sensors have excellent sensitivity and stability, and their practical applications arefacilitated due to the low impedances. Moreover, the electrode material also plays an importantrole in deciding the humidity sensing property. The humidity sensors with Au electrode aresensitive in the high humidity area （above75%RH） while the humidity sensors with Cuelectrode show excellent sensitivity in the low humidity area （less than40%RH）. We believethat the AAO membranes with throughα-hole structure not only can be used as the host materialof humidity sensor, but also can be used as gas sensors and in gas separation by filling the pores with catalysts or various gas sensitive materials.