Some Physical and Dielectric Properties of Hydrous Alumina Films

Abstract

Properties of pseudoboehmite films, produced by the reaction of aluminum foil with boiling water, were studied by a variety of techniques. For reaction times of 1-30 rain the average film porosity decreased linearly with increasing film weight. The surface area of freshly prepared films was similar to that of alumina gel, but decreased on aging in air. Aging in 1N NaNO~ inhibited the area decrease. The series capacitance decreased on aging in air or nitrate electrolyte. It was determined that with the sample immersed in electrolyte the measured dielectric properties were those of an inner region impermeable to electrolyte. This layer was not the same as a typical barrier oxide. The films exhibited a low frequency dielectric dispersion. From the temperature dependence of the loss maxima the activation energy and relaxation time for the process were found to be 13.4 kcal/mole and 6.2 x 10 -15 sec. These are in the range found for other systems containing hydroxyl chains or bound water.Films produced by the reaction of aluminum or aluminum oxide with water are of importance in corrosion (1), the sealing of porous anodic oxides (2), and the manufacture of electrolytic capacitors (3). A recent paper by Vedder and Vermilyea (4) has greatly extended our knowledge of the formation and physical properties of these porous hydrous films. They identified the film produced by reaction with boiling water as pseudoboehmite, a poorly crystallized oxide with a structure close to that of boehmite but containing more hydroxyl than corresponds to the formula A1OOH. The composition of the film will vary, but reaction at 100~ has always been found to result in an average water content of 27% (3, 5, 6).Some new data on film thickness and surface area are presented here, but the primary purpose of this paper is to report on the dielectric properties of films produced by the reaction of aluminum with boiling water. This was of interest to us because these films are often incorporated into the dielectric layer of electrolytic capacitors (3). There may be more general interest in the comparison of the dielectric properties of this film with those of other materials such as boehmite, ice, and silica gel. ProcedureThe hydrous films were grown on 99.99% A1 foil specimens that had been cleaned by chemical polishing in a mixture of 15 parts 70% HNO3 + 85 parts 85% H~PO4 for 2 min at 85~ followed by a 10 min etch in 1N NaOH at room temperature. The reaction with boiling, distilled deionized water was in Pyrex beakers for times up to 30 min. For reaction times of 3 min or less, two samples were prepared consecutively and then the water discarded. For longer reaction times fresh water was used with each sample.Series capacitance and RC were measured with a resistance ratio arm bridge. The maximum amplitude of the a-c signal was 0.2V rms. A large capacitance (1300 ~f) was in series with the signal generator to block the passage of d-c current through the measuring cell. Two measuring cell designs were used. For measurements at a frequency of 120 Hz...

A Duplex Mechanism-Based Model for the Interaction Between Chromate Ions and the Hydrated Oxide Film on Aluminum Alloys

“…However, chloride ion induces the deprotonation of aluminum hydroxide to form aluminum oxyhydroxide or pseudoboehmite. Pseudoboehmite has a lamellar structure 80 and a substantial void fraction and a very high specific area 81 and would not be expected to be protective. Bernard and Randall attribute the loss in electrical and mechanical properties undergone by oxide films on aluminum when exposed to aqueous solutions to the hydration of the aluminum oxide to form its hydrated counterparts.…”

The influence of porosity and morphology of hydrated oxide films on epoxy-aluminium bond durability

“…Studies of the properties of the lms formed on aluminium immersed in boiling water have the potential to reveal information about the factors that in uence the formation of strong and durable adhesive bonds. Alwitt [10] has reviewed several authors' [11][12][13] research into the properties of lms grown on pure aluminium immersed in boiling water. The hydrated aluminium oxide lm formed in boiling water was reported to have a porosity that increased with the immersion time [11].…”

“…Alwitt [10] has reviewed several authors' [11][12][13] research into the properties of lms grown on pure aluminium immersed in boiling water. The hydrated aluminium oxide lm formed in boiling water was reported to have a porosity that increased with the immersion time [11]. In addition, lm mass increased as a function of time, according to a power law [10].…”

The Effect of Boric Acid on the Dielectric Properties of Hydrous Alumina Films

“…A sample aged in air for 11 days had a specific area of 78 m~/g (1). This is about the same as the specific area of films aged in borate for 40 hr.…”

“…In a previous paper we presented some information on the properties of pseudoboehmite films grown on aluminum in boiling water (1). It was noted that these hydrous oxide films attained a much lower capacitance when immersed in a borate electrolyte than when immersed in other salt solutions.…”