Aluminum-anodized alumina

Anodic oxidation of aluminum opens another way to production SPM tip characterizers. Namely it is known that aluminum/anodic alumina interface has an appropriate geometry for this purpose. On the other hand aluminum is a plastic material that may be damaged during scanning. Our results show that despite of proposed mechanical instability anodized aluminum surface is excellent tip characterizer that may be used for a long time. 

Hydrogen produced from the decomposition of anunonia (600 °C) can be often used for feeding alkaline fuel cells, particularly suited for portable power applications. The performance of this reaction was developed by Ganley et al. [79] over a proper MSR configuration (Ru supported on aluminum-anodized alumina microchannels) resulting in a H2 production equivalent to 60W with an ammonia conversion of 99%, all in a volume of 0.35 cm, which exceeded the specifications for practical use laid out until that moment. 

Thin layers Anodic alumina films on aluminum 4.211... 

Electrolytic aluminum production is the most important process in both volume and significance. World production is about 15 megatons per year, consuming about 240 billion kilowatthours of electrical energy. Aluminum oxide (alumina), AI2O3, is subjected to electrolysis at a temperature of 950°C to this end it is dissolved in molten cryolite NujAlFg, with which it forms a eutectic melting at about 940°C. Carbon anodes that are anodically oxidized to CO2 in the process are employed. The overall electrolysis reaction can be written as... 

Anodic oxidation of valve metals, particularly, aluminum, has attracted considerable attention because of its wide application in various fields of technology. Traditionally, aluminum is anodized in order to protect the metal against corrosion, to improve its abrasion and adsorption properties, etc.1 The more recent and rapidly growing applications of anodic aluminas in electronics are due to their excellent dielectric properties, perfect planarity, and good reproducibility in production. Finally, ways have recently been found to use the energy potential of aluminum oxidation for chemical power sources of the metal-air type2,3 and other electrochemical applications. 

Anodic alumina oxides find steadily growing application in various spheres of technology. Traditionally, they are most popular in civil industrial engineering for producing protective and decorative surface finish in panels and different objects. These applications are well reviewed in the literature.321 Anodic alumina is also widely used in the aircraft and aerospace industry for adhesive bonding of aluminum structures,322-324 composite materials, etc. 

There are also several proposals to use anodic aluminum oxides in producing optoelectronic devices. Porous oxides may find use as antireflecting coatings for optical pathways. Anodic alumina films doped by Eu and Tb are promising for application in electroluminescent cells for TEELs.28... 

A very early use of anodic alumina as a template involved colonization of the alumina by depositing nanometals in the pores [39]. Somewhat later, Kawai and Ueda templated cobalt and nickel in alumina by electrodeposition [40]. Other metals were deposited by Andersson et al. [41] and Patel et al. [42]. The use of anodic alumina as a template increased after Furneaux et al. developed a convenient voltage-reduction method for detaching the porous anodized alumina from the underlying aluminum [38]. 

A novel method of preparing metal membranes with nearly suaight pores has been described by Masuda et al. [1993]. Basically it involves a two-step molding process (Figure 3.15). A porous anodized aluminum structure without the aluminum substrate removed is used as a template and the pores of the anodic alumina is filled with a monomer such as methylmethacrylate (MMA) and an initiator (e.g., benzoyl peroxide)... 

A test structure for SPM cantilever tip shape deconvolution is described. The structure is based on aluminum with ordered tip-like surface. This structure is created by anodic oxidation of aluminum with subsequent selective etching of anodic alumina film. The developed structures consist of aluminum base with sharp tips of alumina. It is found that curvature radius of the tips are as small as 2 nm. Various types of tip shapes were charaterized by this structure. Experimental studies of the developed test structure containing an array of sharp tips may be used for three-dimensional imaging of the SPM tips. 

Porous anodic alumina films were formed by a two-step anodic oxidation of aluminum foil (99.99% purity) (thickness 100 jum) or of thin aluminum film sputtered onto silicon substrate. First step was performed under lOmA/cm constant current density in 40 g/1 aqueous solution of (COOH)2 during 60 min. After first anodization the formed anodic oxide was removed in the aqueous solution of 0.35 M H3PO4 and 0.2 M CrOs at 90°C. The second anodization was performed in the same regimes as the first one. The formed oxide was removed from the specimen after the first anodization. Nanostructured aluminum samples were rinsed in deionized water and dried in an argon flow. 

After anodization and subsequent selective dissolution of anodic oxide the aluminum surface is a replica of back side of anodic alumina. Such surfece structure has ultrasharp tips with 20-100 nm height distanced ixom 10 to 500 nm, respectively. It is obvious that external part of anodic alumina has tip-like morphology too. However partial tip dissolution and smoothing during anodization in acidic solutions results in increasing of tip radii [4]. Thus taking into account known experimental results and our observations it can be concluded that the proposed approach allows to form sharp tips. 

The obtained results show that the surface of anodized aluminum after anodic alumina selective dissolution contains sharp stable tips. Such structures may be used for characterization of curvature radius of SPM tips, even ultra-sharp tips. The main advantages of the developed structure are simplicity and reproducibility of its fabrication. 

Then the anodic alumina layer formed was removed chemically in the selective etchant composed of phosphoric (6 wt.%) and chromic (1.8 wt.%) acids at 60 C. Hemispheric etching pits - replica of the alumina cell bottoms - remain on the surface of the aluminum foil. The second porous anodization of aluminum was made. At this stage, the pores on the aluminum foil surface arise not in random way but at the sites of primary alumina cell Imprints to repeat the cell size. The pore diameter and spacing are dictated by the parameters of the anodization process, specifically by the electrolyte composition and the anodization voltage. The alumina film thickness is defined by the anodization time and the anodization current density. The second stage provides a continuous development of the alumina film. Total etching process takes 10-20h to get pores of approximately 100 pm lengths. 

The anodic oxide pattern of 100 nm dimension grown by SPM-based techniques can be used as a mask that can be transferred to a substrate below. An anodized alumina mask (produced by contact mode AFM on sputtered A1 film) has been used to transfer pattern on Si by using a combination of wet and reactive ion etching. Anodic oxidation creates a protruding oxide pattern that can be etched. The pattern can form a positive or negative mask depending on what is etched, the aluminum or the oxides. In Figure 21.14 we show an example of a nanopattern (an array of points) created by anodic oxidation on a surface of aluminum. 

Electrochemical Micromachining and Microstructuring of Aluminum and Anodic Alumina... 

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