Anodization, of aluminum

Knoevenagel condensation of malonic acid with heptaldehyde [111-71-7] followed by ring closure, gives the fragrance y-nonanoic lactone [104-61-0] (6) (14). Beside organic synthesis, malonic acid can also be used as electrolyte additive for anodization of aluminum [7429-90-5] (15), or as additive in adhesive compositions (16). 

Oxide, hydroxide and basic salts of aluminum are less soluble at pH values of about 7 than those of zinc [17], which explains the easy passivatability. Galvanic anodes of aluminum alloys are primarily employed in the area of offshore technology. The anodes work in relatively pure seawater flowing with a high velocity so that by using suitable alloys, passivation phenomena are rare. Their low weight is particularly favorable in view of a service time of 20 to 30 years. 

Boeing Process Specification, BAC 5555 Issue M. Phosphoric acid anodizing of aluminum for structural bonding. Boeing Airplane Company, 1995. 

In a number of works, a potentiostatic regime has been used for the experimental and theoretical study of the anodization of aluminum and other valve metals.80 Upon the application of a constant potential step, Va, barrier-forming electrolytes are characterized by a sharp increase in the anodic current to a certain maximum. Both the slope and the maximum are determined by the impedance of the cell circuit. Subsequently, there is a continuous decrease in the anodic current, which is due to oxide growth. The decay of the anodic current can be described by the expression81... 

The catalyst is not necessary either for the electrocarboxylation of aryl halides or various benzylic compounds when conducted in undivided cells and in the presence of a sacrificial anode of aluminum [105] or magnesium [8,106], Nevertheless both methods, i.e., catalysis and sacrificial anode, can be eventually associated in order to perform the electrocarboxylation of organic halides having functional groups which are not compatible with a direct electroreductive process. 

The precise reactions involving the decomposition of ammonium ions and nitrate ions at the anodic surface to form N-doped titania are currently unclear, and the subject of ongoing studies. However, the anodization of aluminum in nitric acid has been studied previously and is known to be relatively complex... 

Monoliths that were anodized extensively (72) had an anodization thickness of up to 25 pm with a BET surface area of 40 m /g, which is sufficient for many applications. However, because this layer contained only mesopores (pore diameters up to 20 nm) and no macropores, internal diffusion limitations can easily be a problem. An extensive report on the anodization of aluminum monoliths, with the aim of using the anodization layer as catalyst support, was provided by Burgos et al. (73). 

The AAO substrates with uniform and parallel nanoporous structure were prepared in a two-step electrochemical anodization of aluminum foils in 4 % oxalic acid, at the constant current density of 3 A/dm, during 60 min. The pore diameter and average interpore spacing were 40 5 nm and 120 20 nm, correspondingly. The arrays of Ag nanoparticles on AAO were formed by thermal evaporation of silver onto a AAO substrate at room temperature. The mass thickness of silver was varied by varying the deposition time. Two series of samples with thickness 30, 60, 90, 120, 150 and 180 nm were prepared. 

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. 

III. TECHNOLOGICAL ASPECTS OF LOCALIZED ANODIZATION OF ALUMINUM SUBSTRATES... 

The electrochemical conversions of solid compounds and materials that are in direct contact with electrolyte solutions or liquid electrolytes (ionic liquids), belong to the most widespread reactions in electrochemistry. Such conversions take place in a wide variety of circumstances, including the majority of primary and secondary batteries, in corrosion, in electrochemical machining, in electrochemical mineral leaching, in electrochemical refining (e.g., copper refining), and in electrochemical surface treatments (e.g., the anodization of aluminum). 

Figure 17.1.14 Ellipsometric results for anodization of aluminum in 3% tartaric acid (pH 5.5).

The samples of PAA were fabricated by anodizing of aluminum layer deposited by magnetron sputtering onto silicon substrates. The anodizing was carried out in... 

Anodizing of aluminum under certain conditions produces self-organized regular periodic stmctures with hexagonal cells and controlled sizes of the pores from 10 through 500 nm [6, 7]. Unique porous films with thickness up to 200 pm with an insignificant variance of the pore diameter oriented perpendicularly to the planar surface were fabricated [8, 9]. Porous anodic alumina (PAA) is a relatively transparent material and could be fabricated on the substrates transparent in the wide spectral range (Fig. 1 [ 10]) that makes it attractive for optical excitation of diverse luminescent inclusions. 

Plasmochemical synthesis of carbon nanotubes was carried out at 500-700 °C in the barrier-discharge reactor with two coaxial electrodes between which the pulsed voltage (10 kV, 25 kHz) was applied. As the synthetic reaction, the disproportion of carbon monoxide was used. All experiments were performed at atmospheric pressure. Nickel-modified hexagonal close-packed nanochannel alumina templates were fabricated by anodization of aluminum with subsequent... 

As stated above, a variety of deposition techniques have been developed in recent years to produce amorphous alumina coatings (112—115), among them the anodization of aluminum. Amorphous aluminas, denoted as a-Al203 or sometimes as P-AI2O3, can also be prepared by conventional liquid-phase reactions (134), by the decomposition of some hydroxides, or by sol-gel procedures aimed at the preparation of mesoporous materials. In several instances, IR (110) and Raman (135) spectra have demonstrated that amorphous aluminas may contain impurities originating from the... 

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