GB1105430A - Improvements in or relating to manufacture of electrical capacitors - Google Patents

Abstract

1,105,430. Electrolytic capacitors. SIEMENS A.G. 5 Aug., 1966 [6 Aug., 1965; 4 July, 1966], No. 35088/66. Heading H1M. In the manufacture of a solid electrolytic capacitor in which the electrolyte comprises a plurality of layers of manganese dioxide formed by pyrolysis of manganese nitrate, at least one layer is of manganese dioxide of reduced electrical conductivity as compared with normal manganese dioxide. The consequent increase in loss factor and impedance is compensated for by an increase in breakdown resistance during intermediate forming, so that high voltages may be employed and capacitors with high working voltages made. The low conductivity layers may be formed by incorporating in the manganese dioxide crystal lattice nickel oxide, cobalt oxide, zinc oxide or both nickel and zinc oxide, and this is achieved by including the metal carbonate or nitrate in the manganese nitrate solution or melt prior to pyrolysis. Alternatively, pyrolysis of the manganese nitrate may be carried out in the presence of a reducing agent, e.g. manganese formate, formic acid, urea, hydrazinium nitrate, acetic acid, ammonium acetate benzoic acid, certain sugars or, best, manganese acetate. Particularly advantageous results are obtained using a stock solution of manganese nitrate and manganese acetate which has been allowed to age. Preferably the first layer is formed from a solution containing less than 60% manganese nitrate, subsequent layers being formed from concentrated solutions or melts. The first layer may be of normal conductivity, and such a layer may be repeated after every two or three layers of lower conductivity, intermediate reforming being carried out after each application. Preferably, however, if a high voltage intermediate formation is required, the first layer is of low conductivity. For example this layer may be formed by pyrolysis following impregnation with a solution containing less than 20% nitrate made up of manganese and zinc nitrates in proportions of about 2 to 1. Subsequent layers are formed from manganese nitrate alone, since under the pyrolysis conditions sufficient zinc diffuses into the outer layers to give a gradient of increasing conductivity from the inner to the outer layers. It is preferred to carry out an annealing treatment in air after the first electrolyte layer has been deposited and this may be repeated between later depositions. Reforming is carried out after annealing. Advantageously, the anodized electrode, prior to deposition of the first layer, is annealed in the absence of oxygen at a temperature substantially higher than that of subsequent pyrolysis. During pyrolysis, the electrode may be rotated about its axis. The capacitor is completed by application of graphite and metal layers in the conventional way.