METHOD OF HEATING CATHODE FOIL FOR ELECTROLYTIC CAPACITORS

Abstract: Treating cathode foil prior to etching, the treatment consisting of an annealing operation at a temperature of at least 300° C, followed by cooling at a rate of at least 50° C/­ min to a temperature below 175° C. Starting material is Al-foil having 0.15-5.7% Cu, 0-2% Mn and a total quantity of not more than 2% by weight of other minor constituents. This results in a higher impact strength of the foil after an etching treatment.

Description: "Method of heating cathode foil for electrolytic capacitors" The invention relates to a method of heating cathode foil for electrolytic capacitors and to electrolytic capacitors having cathode foils treated by such a method. An electrolytic capacitor having aluminium electrodes consists of an aluminium anode provided by means of anodic oxidation with a dielectric aluminium oxide skin, an aluminium cathode which has been etched so as to produce an enlarged effective surface, and an electrolyte solution. The most usual form of such an electrolytic capacitor is the aluminium wound capacitor, wherein the anode foil and the cathode foil are rolled with an interleaved, so-called separator consisting of an electrically insulating material into a wound body. The wound body is impregnated with an electrolytic solution and is encapsulated. It is important in an electrolytic capacitor to have the desired capacitance using as small an area of electrode foils, as possible, which means that the effective surface of the metal to be anodically oxidised (formed) must be as large as possible. Before the anodic oxidation of the anode foil, this foil is etched, usually electrolytically, so as to enlarge the effective surface of the foil. To obtain a large capacitance per unit area of the foils, it is important that the effective surface area of the cathode foil which is encapsulated in the non-formed state in normal capacitors is likewise increased by etching in such a way that roughening and capacitance respectively are comparable to the roughening and capacitance of the anode foil. In this way it is achieved that the total capacitance of the capacitor is determined to an increased extent by the anode capacitance alone, in accordance as the cathode capacitance increases. British patent application 2008324, which has been laid open to public inspection, discloses such a cathode foil, which consists of aluminium having a copper content between 0.15 and 5.7 fo by weight, and a total quantity of not more than 2p by weight of other minor constituents. Cathode foil consisting of aluminium having a manganese content between G.2 and 2 % by weight and a copper content between 0.15 and 5.3 $ by weight is described in the European patent application publication 0000965-A, which has been laid open to public inspection. After etching, these two kinds of foils have a higher capacitance value than previously known cathode foils. From an article by L. van Alphen et al in the "Zeitschrift fur Metallkunde" 70, 158-169 (1979), it appears that an annealing operation after rolling must satisfy very specific requirements in order to have a favourable influence on the capacitive properties and the strength of the foil. From this article it appears that foil having a copper content of 1.5 % by weight or more has, after etching in the non-annealed condition as well as after annealing followed by the slow cooling, customary in the technology, prior to etching, a low impact strength. A high impact strength is, however, important in view of the mechanical load to which the foil is subjected during unwinding and winding and during the etching operation. It appeared, however, that this phenomenon also applies to foil having a copper content of less than 1.5 % by weight. The invention has for its object to increase this impact strength while retaining the other properties of the foil after etching. According to the invention, the method of treating cathode foil consisting of aluminium having a copper content between 0.15 and 5.7 < 0 by weight, a manganese content from 0 to 2 Xio by weight and a total quantity of contaminations of not more than 2 % by weight, this foil teing subjected to a heat treatment prior to etching, is characterized in that the foil is heat-treated at a temperature of at least 3000C and cooled thereafter at a rate of at least 500cumin to below a temperature of 17500 in a fluid which, in this temperature range, does not react in a deleterious way with the aluminium. It was surprisingly found that working of the foil does not only give it a higher impact strength, but that, furthermore, a not inconsiderable gain in capacitance is obtained after etching of the foil. The temperature at which the heat treatment must be effected depends on the copper content of the foil. A temperature of 3000C is used for foil having a Cu-content of 0.15 - 0.5 %, 4000C for foil having a Cu-content of 1.5 % and 5500C for foil having a Cu-content of 5 fo. During the subsequent rapid cooling, phase separation must be prevented. Cooling must be effected at at least the abovementioned minimum rate, in any case to below a temperature of 17500. Below this temperature the cooling rate is no longer critical. Quenching is preferably effected in a flowing gaseous medium which does not react in a deleterious way with the aluminium in the temperature range passed through during this quenching operation. A cooling method which is attractive for practically purposes is cooling between air knives, between which the foil is unwound after the heat treatment in wound position, that is to say rapidly flowing cold air is directed to the top and bottom surfaces of the foil by two or more gaps of the same width as the foil width which are arranged at an angle with respect to the foil. Quenching in water is not suitable for this purpose as, at the temperatures suitable for the heat treatment since water reacts with the aluminium, a type of oxide then being formed which unfavourably affects the capacitance. An alternative possibility is quenching in oil; this method is, however, also less attractive in practice as it requires an additional degreasing operation. The invention will now be further described by way of non-limitative example, with reference to the following Examples. Example 1. Aluminium foil, 60/um thick and having the following composition in percent by weight: Cu 3.6 Fe 0.22 Ti 0.015 Mg 0.04 Mn 0.39 Si o.o68 Al remainder, is heated at a temperatur of 550 C for 2 hour and thereafter cooled in flowing air at a rate of 5000C/min. The foil is electrolytically etched in an aqueous solution of 250 g/l NaCl with an electric charge of 12 Coul/cm2. Thereafter the foil has a capacitance of 410/uF/cm2. If cooling is done in an oven, that is to say from 5000C to 250C in a period of 4 hours, the foil has, after etching in the same conditions, a capacitance of only 70/uF/cm2. Example 2. Aluminium foil, 30/um thick and having a Cucontent of 1.20 % by weight and a total content of other minor constituents of 0.5 % by weight, is heated at 4500C for 2 hour and cooled in an oven at a rate of 2500C/hour. At a temperature Tx, a sample is taken each time from the batch in the oven and cooled at a rate of 5000C/min to ambient temperature. The impact strength after etching of all these samples was measured. Table I shows the results. TABLE I impact strength Tx (0C) (mJ/15mm foil width) 450 100 400 80 350 80 325 90 300 20 275 10 Table Is^sws that phase separation takes place between 325 and 3000C and that for this foil the heat treatment must be effected above this temperature in order to obtain the effect of the invention by quenching. In a similar test, the entire batch is heated at 4500C for 2 hour and thereafter cooled to room temperature at a rate of 5000C/min. Thereafter a sample is heated each time for 10 minutes at a temperature Ty, cooled at the same rate to ambient temperature, the impact strength and the capacitance are measured, both values after anodic etching with a charge of 1000 Coulombs/ dm2 in a 250 g/l NaCl solution. Table II shows the results. The capacitance of the foil which was not subjected to reheating and quenching was 225/uF/cm2 and the impact strength was 100 mJ for a 15 mm wide strip of foil. TABLE II EMI5.1 <tb> T <SEP> (o <SEP> impact <SEP> strength <SEP> capacitance <SEP> ! <SEP> <tb> <SEP> y <SEP> -(mJ/15mm <SEP> <SEP> foil <SEP> width) <SEP> F/cmê <SEP> <tb> <SEP> 350 <SEP> 50 <tb> <SEP> 325 <SEP> 5 <SEP> 175 <tb> <SEP> 300 <SEP> 5 <tb> <SEP> 275 <SEP> 5 <SEP> 150 <tb> <SEP> 250 <SEP> 1 <SEP> 5 <tb> <SEP> 225 <SEP> 5 <tb> <SEP> 200 <SEP> 15 <tb> Z = not determined

Claims: CLAIMS: 1. A method of treating cathode foil for electrolytic capacitors, the foil consisting of aluminium having a copper content between 0.15 and 5.7 % by weight, a manganese content from 0 to 2 % by weight and a total quantity of not more than 2 % by weight of other minor constituents the foil being subjected to a heat treatment prior to etching, characterized in that the foil is heattreated at a temperature of at least 3000C and is thereafter cooled at a rate of at least SOOC/min to below a temperature of 1750C in a fluid which, in this temperature range, does not react in a deleterious way with the aluminium. 2. A method as claimed in Claim 1, characterized in that the foil is cooled in a flowing gaseous medium. 3. A method as claimed in Claim 2, characterized in that the foil is unwound between air knives after the heat treatment in wound position. 4. An electrolytic foil capacitor the cathode of which is made or foil obtained in accordance with the method claimed in Claim 1 and the surface area of which has subsequently been increased by etching.

Inventors: KNOL, BEREND, SLAKHORST, JUSTINUS W.H.G.
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1980
Main classification: H01G9
IPC codes: C22F1, C22C21, H01G9