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

Abstract

Aging hydrous aluminum oxide films in boric acid solutions accelerated the surface area decrease and markedly changed the dielectric properties of the films. The effect on dielectric properties could be removed by immersion in a borate-free electrolyte. The observations were accounted for by a model in which adsorbed borate anions increased the relaxation time of a MaxwellWagner process occurring within a compact inner region of the films. It was postulated that borate anions penetrated to the interior of the oxide by diffusion along platelet surfaces. Borate was the only anion found to exert this particular effect on the hydrous oxide films.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. In this paper we present the results of an investigation of the effect of borate species on some physical and dielectric properties of pseudoboehmite films. We think these results are of intrinsic interest. In addition, they are of practical importance because borate electrolytes are widely used for forming barrier anodic oxides on aluminum and as electrical contact in the measurement of dielectric properties. Experimental ProcedureThe general procedures and materials were the same as used previously (1). All hydrous oxide films were produced by reacting aluminum foil with boiling water for 3 min. Initial work was done in solutions of 50 g/liter HaBO3 with NH4OH added to give a resistivity of 230 ohm-cm and a pH about 6.5. Subsequently, solutions of H3BOs were used containing 1N NaNO3 as supporting electrolyte and adjusted to pH 6.1 _ 0.1 with NaOH and HNO3.

Electrochemical Current Noise on Aluminum Microelectrodes

“…8,9 In the present work, current bursts suggesting metastable pitting were also found after NaCl injection, but only at potentials Ϫ0.64 V vs. SCE or higher. 24 Another possibility is suggested by the work of Alwitt 34 who found that removal of borate ions from solution produced significant decreases in the resistance of the barrier layers of pseudoboehmite films on aluminum.…”