Patent GB-1046238-A (GB)
Title: Improvements in or relating to electrolytic capacitors
IPC: H01G9/02
Filed: 1963-10-16
Abstract: 1,046,238. Capacitors. TELEGRAPH CONDENSER CO. Ltd. Oct. 16, 1963, No. 40832/63. Heading H1M. An electrolytic capacitor comprises valve metal electrodes, one or both of which may be covered by a dielectric oxide layer immersed in an electrolyte, and a surface layer of a chelate compound of the valve metal overlying the oxide layer; a low constant water concentration being maintained by a chemical equilibrium between the constituents of the electrolyte in which the chelate layer participates. Aluminium foil electrodes which may be etched are separated by a non-conductive porous spacer and spirally wound; the anode being anodically oxidized e.g. in aqueous boric acid, and the assembly is inserted in an insulatedly lined metal container containing electrolyte which is then sealed by a disc of synthetic resin impregnated fabric carrying riveted terminal tags, crimped between a lug and an internal rib of the container; the disc being surfaced with resilient material. A space is left above the electrolyte, and the container may incorporate a bellows permtiting variations of the electrolyte volume without change of internal pressure. The container may be vented. In a non-polar capacitor, both foils may be anodized with or without an intervening unoxidized electrode. The foils may be of tantalum, niobium, zirconium or titanium. The electrolyte comprises an ionisable solute dissolved in a liquid aliphatic hydroxy acid, which acid may be α, # or γ hydroxybutyric acid α or # hydroxypropionic acid hydroxy acetic acid 2 methyl, 2 hydroxypropionic acid or an isomeric monohydroxy monocarboxylic aliphatic acid with up to 16 carbon atoms per molecule, e.g. α hydroxy phenyl acetic acid 3 hydroxy 3 pyridino propionic acid α hydroxy # cyclohexyl propionic acid glycolic acid α hydroxy valeric acid α hydroxy palmitic acid α hydroxy myristic acid γ hydroxy caproic acid Ionisable solutes may be tri n-butylamine picrate di-ethanolamine picrate tetra-methyl ammonium hydroxide sodium or ammonium hydroxides and 2.4 dinitrophenol or 2 nitrophenol mono-isoamylamine salt of p-nitrophenol sodium picramate triethylamine salt of 2.6 dinitro-4-methyl phenol tripropanolamine salt of styphnic acid tetraethyl ammonium picrate tetramethyl ammonium nitrate sodium acetate lithium acetate sodium nitrate calcium nitrate ammonium borate or other amino salts of the hydroxy acids or of picric and acetic acids having simple alkyl groups of 1 to 10 carbon atoms which may be hydroxylated with not more than one hydroxyl per group. Inert diluents such as butyronitrile or other nitriles, triethylphosphate or other trialkyl phosphates, dimethyl formamide, dibutyl orthophthalate may be added. The electrolyte must be free from chlorides. The formed aluminium oxide layer enters an equilibrium reaction with the hydroxy acid and water to form aluminium hydroxide which reacts with the hydroxy acid electrolyte to produce an aluminium chelate plus water, and an equilibrium reaction also occurs between the hydroxy acid and the appropriate lactone plus water, so that the water content is maintained constant and a stable chelate surface is maintained on the oxidized film; while the hydroxy acid esterifies to produce the lactone plus water. Further lactone may be added. The electrodes may be of tantalum, niobium, zirconium or tantalum, and the capacitor may be nonpolarized with two formed electrodes with or without an intervening unformed electrode, and the electrodes may be rolled with a porous separator and inserted in a metal container of electrolyte which is sealed by swaging the edges over an insulating disc covered with resilient material and carrying terminal lugs connected internally to the electrodes.