Process for the manufacture of aluminium electrodes for electrolytic capacitors
United States Patent Miihlh&#39;ausser et al.  
 PROCESS FOR THE MANUFACTURE OF ALUMINIUM ELECTRODES FOR ELECTROLYTIC CAPACITORS Inventors: Martin Miihlhausser, Singen.  
 Hohentw; Christian Roth. Gottmadingen. both of Germany Assignee: Aluminium-Walzwerke Singen GmbI-I, Singen/Hohentw. Germany Filed: Nov. 7, 1973 Appl. No.: 413.447  
 Foreign Application Priority Data Nov. 11. 1972 Germany 2255309 US. Cl. 204/l 29.75; 29/527.7; 29/570;  
  29/DIG. 10 Int. Cl C01b 13/04 Field of Search 29/527.7, 570, 592. DIG. 16; 164/82; 204/l29.75  
 [ Apr. 22, 1975 [56] References Cited UNITED STATES PATENTS 1.935.860 11/1933 Robinson 29/570 2.106.178 1/1938 Keller ct 211.. 29/570 X 2.206.050 7/1940 Robinson 29/570 3.316.164 4/1967 Welch 204/l29.75 X 3.351.442 11/1967 Hooper 204/l29.75 X  
 Primary Eraminer-Milton S. Mehr Assistant Examiner-D. C. Rciley, 111 Attorney. Agent, or Firm-Ernest F. Marmorek 1 Claim, No Drawings PROCESS FOR THE MANUFACTURE OF ALUMINIUM ELECTRODES FOR ELECTROLYTIC CAPACITORS The electrodes of an electrolytic capacitor generally consist of a roughened i.e. etched aluminium strip. By roughening the strip an increase in surface area and thereby an increase in capacity is achieved, or for a prescribed capacity a reduction in the dimension of the electrode is achieved.  
  Pure aluminium is normally used as starting material for producing etched anode strips and for the production of etched cathode strip pure aluminium or manganese-containing aluminium alloys are used. These materials are cast as ingots with rectangular cross section by the usual continuous casting and permanent mould casting processes. The ingots are usually heated to a rolling temperature between 400C and 500C and rolled down to a plate thickness of 2-10 mm on a hot rolling mill. The plates are cold rolled in several reduction passes down to strip thickness and etched either in this condition or after an additional heat treatment.  
  The etched strips produced in this way do not, in many cases fulfill todays requirements for a satisfactory surface area increase necessary in the construction of an electrolytic capacitor of the smallest possible volume. It was of value therefore to find a process by which it is possible to obtain still greater increases in the surface area than by the known processes.  
 surface area. It is characterised in that the aluminium strip to be etched is cast directly from the molten state as a strip and then rolled to its final thickness.  
  Presumably the better etching behaviour of the material produced by strip-casting results from the significantly higher rate of solidification during casting.  
 EXAMPLES An ingot of 99% pure aluminium and an ingot of AlMn alloy, both continuously cast, were annealed at 500C for 2 hours, hot rolled to a plate thickness of 6 mm and then without heat treatment cold rolled to a strip 0.05 mm thick. For comparision a strip from a melt of 99% pure aluminium and a strip from an AlMn melt were cast directly as 7 mm thick strips in a stripcasting plant and then rolled down to the same final thickness as the other two strips without an intermediate heat treatment. All strips were degreased and then etched in an aqueous solution of 20% NaCl at 90C using a current density of 100 A/dm and consuming 3000 C/dm&#34;.  
  The determination of the capacity was carried out in accordance with the DIN 41330 specification at 0 and 10 volts in a saturated ammonium pentaborate solution having a bath resistance of 60 to 70 ohm.cm at room temperature. In addition to this the characteristic data for the mechanical properties of the strips were determined before etching. The results are summarized in the following tables.  
 Example 1 (Al 99) Capacity in uF/cm? For some years in addition to the above mentioned casting of ingots, a continuous casting of strip has been practised. By this latter technique a material is cast directly from the melt in the form of a strip l-40 mm thick and then hot rolled and cold rolled or directly cold rolled to its final thickness.  
 It was found on comparison of the aluminium strip ;made from cast strips with those made from cast ingots &#39;that the former showed a significantly greater increase in the surface area after the same etching treatment.  
  The results show that the mechanical strength after solidifying is independent of the method of casting. The result of etching and the capacity measurement on the other hand show clearly an increase in capacity in the case of the metal strip produced by the strip-casting process.  
 What is claimed is:  
  1. A process for the manufacture of aluminum electrodes, for use in electrolytic capacitors, comprising the steps of continuously casting an aluminum strip having a thickness of between about 1 mm and about 40 rolling said aluminum strip to its final thickness; and  
 etching said rolled strip electrolytically in a chloride containing electrolyte.