Abstract
The effect of the water content of several electrolytic solutions on their ability to form dielectric oxide films on aluminum was examined. Anhydrous electrolytes were found to be incapable of forming such films, but the addition of small amounts of water made it possible for these solutions to perform as normal oxide-film formers. The anodizing behavior of an anhydrous electrolyte composed of methyl cellosolve and ammonium picrate was investigated in detail in both experimental aluminum anodizations and in finished capacitors, in an attempt to correlate the behavior of the electrolyte as an anodizer of bare foil with the aging and life test characteristics of capacitors containing that electrolyte. The results tend to support the conclusion that water is essential to the proper performance of electrolytic capacitors.
Highlights
It is generally believed that water is a necessary component in electrolytes used for the formation of oxide trims on aluminum and other valve metals
Probably essential that water be present in the operating electrolyte of an aluminum electrolytic capacitor where it functions as the source of oxygen for the formation of oxide during the anodization of bare foil edges and oxide defects after capacitor construction
The intent of this work is to show that the presence of water is critical to the formation of a normal dielectric oxide on aluminum, and to show the effect of an electrolyte of known water content on long term, high temperature capacitor performance
Summary
It is generally believed that water is a necessary component in electrolytes used for the formation of oxide trims on aluminum and other valve metals. The most interesting solutesthose with the best anodizing characteristics are borates and phosphates Both solutes may be subject to either esterification or condensation in some nonaqueous solvents, giving rise to uncertain amounts ofwater, and the results obtained by the anodic oxidation of aluminum in such solutions are necessarily obscured to some extent by these reactions.[4] In those cases in which the solute and solvent cannot react to form water the solutions can presumably be made "anhydrous" in the conventional sense, but the problem in this instance is rendered difficult by consideration of the extremely minute amounts of water required for the electrochemical reaction needed to bring about the formation of anodic oxide films. It is not intended to imply here that such a low concentration of water could be used with a very high electrochemical efficiency, but the calculation points out that extreme care should be taken to exclude water, and even the view that the solution may be considered to be strictly "anhydrous" may still be questioned
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