STUDY ON THE FORMATION OF DIELECTRIC FILM OF HIGH-PURITY ALUMINUM ANODE OF ELECTROLYTIC CONDENSER

Abstract

We can think of a number of factors that affect the dielectric property of an electolytic condenser, but by far the most important one is the purity of aluminum used as its anode. In Japan today aluminum of 99.9% up purity is usually used for the anode of an electrolytic condenser, but as to which one of its main impurities such as Fe, Si, Cu, etc. is the biggest factor of affecting the dielectric property, the question has been left urclarified. In this connection, the authors conducted, back in 1950, an experimental investigation with a view to ascertaining which one of those fine impurities had the most unfavorable influence on the formation of dielectric film, when it was found that Cu, Si, Mg, Mn, etc. dissolved in small qnantity in solid solution had little or nothing to do with the question, but that Fe or Ti that exists in precipitated form, no matter how little, exereises an unfavorable influence. On the other hand, it was also ascertained then that if 0.1% or less Zn and Mg or 0.05% or less Mn was added to the aluminum of 99.99% purty, its dielectric property was rather improved.In their latest examination the authors made further scrutiny into the similar influence of those impurities contained in aluminum ranging in purity from 99.9 to 99.99%. Consequently, it was found that even the 99.99% grade aluminum sometimes proves inferior to 99.9% one in point of its dielectric property and that the dielectric property solely depends on its iron content. The conclusion thus obtained is that in specifying the purity of aluminum to be used for an electrolytic condenser, it is more important to specify the limit of allowing the iron to be contained in it than its synthetic purity.Besides, since the annealing operation is usually made in an electric air furnace, which causes an oxide film to appear on the surface of aluminum foil, it is natural that the higher the annealing temperature goes up, the poorer its dielectric property should become. This time we have made an anuealing test by means of vacuum heating so as to prevent the oxide film coming out on the surface of the testing material, for which we have used 99.99% aluminum. Nevertheless, we have found that the higher the annealing temperature rises, the poorer, though slightly, its dielectric property becomes, and that the size of recrystallized grains seems to have something to do with it. t follows therefore that the annealing temperature for aluminum of high purity (99.9 to 99.99%) should be held as low as it is required for the recrystallization temperature (300°C), and in this connection, it is commendable from practical viewpoint that the annealing should be conducted at around 350°C so that the aluminum foil may be prevented from getting smeared with oil on the surface.