Structure of Anodic Aluminas

Anodic aluminas are reported in the literature to have both an amorphous and a crystalline structure. The majority of anodic 

As yet, no explanation has been advanced for such specific anion or pH effects. 

According to El-Mashri et al.,190 the A106 A104 ratio determines the hydration capacity of anodic oxides. Tetrahedral sites are hydrated easily to form a boehmite-like structure, which is known to be composed of double layers of Al-centered octahedra, weakly linked by water molecules to other layers.184 As the oxide formed in H3P04 contains about 70% tetrahedral aluminum bonds, its hydration ability should be higher than that of the oxide formed in tartrate solution. However, this has not been found in practice, which is interpreted by El-Mashri et al. as being due to some reduction of A104 by incorporated phosphate species. 

Besides the amorphous alumina films formed in the majority of acidic electrolytes (except those formed in chromic acid and exhibiting traces of y-Al203194), there are possibilities of forming oxides with a more or less pronounced crystallinity. These oxides are formed in alkaline solutions195 and especially in sodium carbonate baths.196 According to the data provided by Hiroshi and Yoshimura,197 these oxides contain a y- A1203 phase easily hydrated and converted into a bayerite-like substance. 

Specific structural features are observed in the formation of composite oxides. Kobayashi, Shimizu, and their co-workers have, in a series of papers, reported studies of the structure of barrier alumina films, anodically formed on aluminum covered by a thin (5 nm) layer of thermal oxide.198,199 Their experiments have shown that the thermally oxidized thin layer generally contains y- alumina crystals of about 0.2 nm size. This layer does not have a pronounced effect on ionic transport in the oxide during anodization. Also, islands of y -alumina are formed around the middle of anodic barrier oxides. They are nucleated and developed from tiny crystals of y -Al203 and grow rapidly in the lateral direction under prolonged anodization.198,199 

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H. Masuda and K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina, Science 268 1466-1468 (1995). 

Anisotropic optical properties of free nanopotous anodic alumina films transparent in the visible spectrum for the restrided range of pore diameters and pore intervals are discussed. The basic experimental procedure is presented for the production of these films. Light scattered along pores was experimentally found to have partially a polarization perpendicular to the polarization of the incident light. The results obtained show that the nanoporous structure of anodic alumina films can be purposeful used in LCD to control a light propagation. 

Cherenda, N. N., Uglov, V. V., Litvinovich, G. V., and A. L. Danilyuk. 2003. The effect of Ti ions implantation on the structure of anodic alumina films. Nuclear Instruments and Methods in Physics Research B 211 219-226. 

Martin CR (1994) Nanomaterials a membrane-based synthetic approach. Science 266 1961-1966 Martin CR (1996) Membrane-based synthesis of nanomaterials. Chem Mater 8(8) 1739-1746 Masuda H, Fukuda K (1995) Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina. Science 268 1466-1468 Mishra JK, Bhunia S, Baneqee S, Baneqi P (2008) Photoluminescence studies on porous silicon/ polymer heterostructure. J Lumin 128 1169-1174 Moller K, Bein T (1998) Inclusion chemistry in periodic mesoporous hosts. Chem Mater 10(10) 2950-2963... 

Wang, Y.C., Leu, I.C. Hon, M.H. (2004). Dielectric property and structure of anodic alumina template and their effects on the electrophoretic deposition characteristics of ZnO nanowire arrays, /. Appl. Phys. Vol. 95, ppl444-1449. 

The true role of incorporation of anions in the formation of anodic alumina is being intensively discussed. Baker and Pearson183 have considered the anion effect in modifying the structure of anodic oxides to be due to the coordinative ability of anions to replace alumina tetrahedra in the body of the oxides. Dorsey184,185 has postulated that in porous oxides, anions stabilize the network of alumina tetrahedra and octahedra. 

It is interesting to note that, due to its uniform and nearly straight pore structure, planar anodic alumina membranes have been used as the probes for monitoring catalyst deactivation at the single pore level [Nourbaksh et al, 1989a]. The nearly idealized structure is suitable for various surface analysis techniques such as scanning and transmission election microscopy and associated EDX and XPS to be applied to fresh as well as spent catalysts. In the case of hydrotreating of crude-derived heavy oils, catalyst... 

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. 

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. 

Figure 2. SEM (a,b) and TEM (c) micrographs of the structure xerogel/anodic alumina (a) - as anodized anodic alumina film of 5 pm thick fabricated on Si, (b) - after one spin-on deposition of Eu-doped titania xerogel (c) - ultramicrotomed sections of the terbium-doped alumina xerogel/PAA structure of 30 pm thick. Bottom of the pore was filled with terbium-doped alumina xerogel after five spin-on depositions.

An increase of lanthanides PL could be observed due to (i) an increase of the number of xerogel layers within the volume of anodic alumina pores [15, 16], (ii) an increase of the concentration of lanthanide ions with respect to the concentration of the host oxide in the same sol [14, 15, 17], (iii) an increase of the thickness of the film [15, 18] and (iv) tailoring of the parameters of the structure depending on the emission range of the incorporated lanthanides. 

Atsumi and Miyagi [319] proposed a two-step EMT to predict the optical properties of anodized alumina films based on the assumption that spherical aluminum particles are distributed in columnar structures within an anodized alumina film. Using this model, theoretical parameters such as the volume ratio and distribution range of alumina particles were determined, their dependencies on the anodization conditions were evaluated, and the optical constants of the... 

Pilatos, G., Vermisoglou, E. C., Romanos, G. E., Karanikolos, G. N., Boukos, N., Eikodimos, V., and Kanellopoulos, N. K. (2010). A closer look inside nanotubes Pore structure evaluation of anodized alumina templated carbon nanotube membranes through adsorption and permeability studies. Adv. Fimt. Mater. 20(15), 2500-2510. 

Another potential vertical medium is not a continuous thin film, but rather an assembly of metal particles deposited in well defined pores in an alumina film on Al [109-117]. The shape anisotropy of the particles gives the desired vertical anisotropy. In producing such structures, the proper conditions for the anodization of the Al disk and for the subsequent control of pore size are as important as the conditions of metal deposition. The structures of such disks are discussed in detail below. 

Figure 28. Distribution of sulfur and carbon in anodic aluminas corresponding to the different stages of porous structure growth, as determined by Auger spectroscopy.160...

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