Anodic alumina films grown in aqueous electrolytes, that are not highly alkaline, have either barrier or cellular porous structures. Barrier oxide grows in near neutral electrolyte and is a layer of uniform thickness that supports a high electric field. Cellular porous oxide grows in acid solutions in which the Al cation has high solubility, and the faradaic efficiency is substantially less than 100%. The oxide consists of a close-packed array of cells, each penetrated by a central pore with a barrier oxide at the pore base. Cell density is of the order 10 10 cm Ϫ2 and pore diameter is of the order 10 nm. Porous oxide also grows in barrier film-forming electrolyte at low field (low current density) when anodizing conditions promote oxide dissolution, e.g., soluble Al-anion complexes and elevated temperature. 1-3 A more complete description of porous films is in a review by Thompson and Wood. 4 Barrier oxide films grow in nonaqueous electrolytes, too, but a variety of other structures are found that depend on the bath composition. Reports from Ue and co-workers describe films grown in electrolytes with low water content, utilizing ethylene glycol or ␥-butyrolactone as solvent, and with solutes suitable for use in electrolytic capacitors. 5-7 Films with fibrous surface texture are frequently found, as well as films with irregular porosity. A substantial amount of organic material is incorporated into these oxide coatings, particularly when grown in the lowest water content electrolytes. Investigations have been limited to films grown to no more than 100 V, and a well developed porous cellular structure is not reported.We report here on anodic alumina with ordered porous cellular structure grown in organic electrolytes with near neutral pH and low water content, and in which aluminum cations are insoluble. The particular solution compositions were formulated for use in high voltage aluminum electrolytic capacitors 8,9 as part of a program to develop capacitors for space applications. 10 In contrast to the organic grown films cited in the previous paragraph, anodic oxide can be grown to 600 V in these electrolytes, so the development of film structure over a substantial range of barrier oxide thickness is possible. The results are presented in two parts: Part I describes the cellular oxide structure, specifies the conditions under which it forms, and gives the growth kinetics, composition and properties of the films. Part II 11 gives details of the structural changes during growth as revealed by transmission electron microscopy (TEM) examination of ultrathin oxide cross sections, and using that information an explanation is presented for the structural development of these films. Considering anodization in both aqueous and organic media, essential conditions for growth of a porous cellular structure are discussed. ExperimentalFoil coupons, 1 ϫ 1 cm, were cut from 100 m thick, fully annealed 99.99% aluminum foil. The foil has high cubicity texture, so most grains have {100} orientation. The coupons were cleaned by i...

In Part I 1 a description is given of a porous cellular anodic coating on aluminum that grows in certain organic electrolytes. The solute is a dicarboxylic acid neutralized with amine or ammonia, and the solvent contains ethylene glycol and no more than a few percent water. The porous structure develops although the faradaic efficiency for oxide deposition is near 95%, and aluminum is insoluble in the electrolyte. Ejected Al ions combine with electrolyte anions to precipitate as an Al carboxylate that is extruded from the pore. This results in a field of cylindrically shaped debris over the oxide surface. Under galvanostatic conditions the barrier layer thickens, and cells increase in size until breakdown occurs at nearly 600 V. This coating is about 1 m thick, with a barrier layer that is less than half the total thickness. The cell diameter, expressed as nanometers per volt, is substantially smaller than for oxide grown in aqueous acid, and the barrier oxide at the pore base has higher field strength than conventional oxide. Growth at steady current and voltage, as with aqueous acid anodization, is not realized.Ethylene glycol (EG) in the electrolyte is necessary to obtain the cellular structure, with ␥-butyrolactone (BL) as solvent a uniform barrier film deposits unless some EG is added. With increasing water content the cell coverage becomes patchy and with more than 4% water, even with EG as the solvent, only an occasional small cluster of cells is seen on the surface. Most observations were made for films grown in electrolytes with dodecanedioic acid (DDDA, C 12 ) as solute, neutralized with an amine. The cellular structure and extruded surface debris are obtained with dicarboxylic acids in the size range C 9 to C 36 , with and without branch chains. Thermal analysis of the coating identified substantial organic matter derived from ethylene glycol and the acid anion. The glycol-derived species is likely to be a small (C 2 ) anion that is distributed throughout the outer oxide layer. At least a portion of the acid anion-derived material identified by thermal analysis is from Al carboxylate precipitate. Although these large anions cannot easily be introduced into the coating matrix, their incorporation likely plays a role in the high field strength of the organic-rich coating.To understand how this cellular porous structure is obtained in a neutral, nonaggressive electrolyte at high faradaic efficiency, we turned to transmission electron microscopy (TEM) examination of ultramicrotomed sections of coatings grown to different thickness and in different solutions. Initially, relatively thick films on chemically etched substrate were examined. An image of one of these specimens is shown in Part I. 1 The cellular porous structure is evident, but it is not clear how this structure developed. In this paper we present results of our examination of thinner oxides grown on electropolished (EP) aluminum foil which has surface texture primarily of {100} orientation.Experimental The Al foils and anodization solutions and ...