GB790860A - Improvements in or relating to electric capacitors - Google Patents

Abstract

790,860. Capacitors; anodising aluminium. BRITISH DIELECTRIC RESEARCH, Ltd. March 19, 1956 [March 18, 1955], No. 8000/55.. Classes 37 and 41. Dielectric stress at the edge of a wound or stacked capacitor using an anodised aluminium foil electrode 1 is reduced by rounding the edge 4 by formation of porous oxide 2. Deposited on the anodised dielectric 3 is a second electrode 6 which is in contact with a metallized insulating spacer 8. The second electrode 6 and the spacer 8 are staggered relative to the foil 1 thus enabling a soldered connection to be made to the bare foil edges 5 and a sprayed metal connection to the spacer 8. If the second electrode 6 is extended over the porous edge 2, the spacer 8 may be omitted. According to the Provisional Specification magnesium may be substituted for aluminium foil. Anodising aluminium. A 25Á. foil 1 has its edge 4 rounded by formation of a mass of porous oxide 2 in an anodic bath containing a 5-10 per cent aqueous solution of oxalic acid at a current density of 20 mA. per sq. cm. of immersed foil for 30 mins., giving an edge 15 Á thick. Alternative methods of limiting area treated include : dipping one end of a roll of foil into the electrolyte; passing the foil around an inclined roller, the lower end of which is immersed in electrolyte; or by temporarily masking the greater part of the foil with lacquer or wax. The anodised dielectric film may be formed in a solution of 60 grams boric acid in 1 litre water after a possible overall formation of a thin porous layer in oxalic acid. In an alternative process, a double width foil has its edges formed in oxalic acid followed by overall anodising before splitting longitudinally to give a bare edge 5.