US2106178A

US2106178A - Electrolytic condenser

Info

Publication number: US2106178A
Application number: US132527A
Authority: US
Inventors: Keller Fred; Charles M Hastings
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1937-03-23
Filing date: 1937-03-23
Publication date: 1938-01-25
Anticipated expiration: 1955-01-25

Description

Jan. 25, 1938. F. KELLER ET AL ELECTROLYTIC CONDENSER Filed March 25, 1937 Cu 6 R W WY 2? W N L 5 E A O flm D5 55 an m C Y B Patented Jan. 25, 1938 Pennsylvania ELECTROLYTIC CONDENSER Fred Keller and Charles M. Hastings, New Kensington, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Application March 23, 1937, Serial No. 132,527

11 Claims.

This invention relates to the manufacture of electrolytic condensers and it is particularly concerned with the type wherein aluminum is employedas the metallic conductor.

Aluminum has been extensively used in making both the so-called wet and dry electrolytic condensers because of its desirable film forming characteristics. When so used, the metal, in the form of sheet or foil, is generally folded or wound in such a manner as to provide a large surface area within a small space and thus increase the capacity of 'the condenser. It has been further found that the effective surface area of the aluminum can be considerably increased by etching the metal before forming the dielectric film on its surface. This chemical attack on the metal leaves a roughened surface which in turn serves to increase the capacity of the condensers made from such foil or sheet. By means of this improvement it is possible to reduce the quantity of metal needed for a condenser of a given capacity, or to increase its capacity if the same amount of sheet or foil is to be employed. This improvement has been of particular benefit in the manufacture of the so-called paste type of condensers where an absorbent material containing an electrolyte is placed between the sheets of aluminum. The aluminum commonly used for making these condensers contains less than 0.2 per cent total of the impurities iron, silicon and copper.

It has been discovered that certain irregularities in the capacity of etched aluminum foil condensers sometimes occur which may be traced to a non-uniformity in the etched surface. This non-uniformity may be the result of variations in the foil or the method of etching. Since the foil is produced in strips of considerable length, and these strips are passed through an etching medium, it is not surprising that there should be some variation in the surface condition over the entire length of a coil. This non-uniformity, as reflected in the difference in capacity of the condensers formed from such foil, is obviously undesirable and to be avoided if possible. Our invention is directed to changing the characteristlcs of the metal .in the form of foil to render it more uniformly susceptible to attack by the etching agent and to produce a maximum increase in the effective surface for a given amount of chemical attack. The principal object of our invention is to provide a method of treating aluminum sheet which will insure a uniform etching of the metal. Anotherobject is to provide a method of treating aluminum sheet which will create a condition in the metal whereby a maximum roughening of the surface can be secured by etching with a minimum removal of metal. still another object is to provide a practical method for preparing aluminum foil that is to be etched and used in making electrolytic condensers.

Our invention is based upon the discovery that aluminum sheets and foil which have been heated at-650 to 950 F. for a period of a few minutes up to 12 hours, depending on the rapidity with which they are brought up to temperature, cooled to room temperature and then reheated at 250 to 400 F. for a period of 6 to 36 hours and finally cooled to room temperature, can be etched uniformly and deeply with a minimum removal of metal. An alternative treatment consists in slowly cooling the metal in the furnace from the high temperature heating range to below 400 F. over a period of to 25 hours at a rate of about 25 F. per hour. After reaching a temperature below 400 F. the coiled sheet or foil can be removed from the furnace and cooled to room temperature. The thermal treatment according to either method serves to first dissolve any soluble impurities and then cause at least a partial precipitation of the dissolved constituents. We have found that the distribution and character of the undissolved particles of iron, silicon and copper impurities greatly affect the etching characteristics of the aluminum. We have found, for example, that an untreated strip of foil etches unevenly, and that although the surface may be roughened by the chemical attack yet the attack is not uniform, there being relatively deep pits in some places and only shallow pits in other areas. By way of contrast, the foil which has been thermally treated is uniformly attacked, that is, the pits are of nearly the same depth, and only a small amount of the aluminum matrix is dissolved. The uniform distribution of the precipitated particles appears to materially affect the etching characteristics of the foil, and aids in securing a maximum roughening of the surface with a minimum. removal of the aluminum.

A condenser of the type wherein etched aluminum foil is employed is illustrated in the accompanying figure. This condenser is of the paste type in which an absorbent material containing the electrolyte is placed between the metal sheets. Referring to the drawing, l is a container made from a suitable material such as aluminum, and closed at the open end with insulating material 2 formed in such a manner as to provide a threaded projection for attaching the condenser to a panel or other mounting. Within the container is mounted the-condenser itself which consists of one or more pairs of aluminum foil strips 3 with an interposed absorbent separator or sheet 4 of gauze, paper, or the like. In a D. C. condenser only one strip need be etched since it serves as the anode. The absorbent separator, impregnated with a suitable viscous electrolyte, should cover the total area of the two layers of foil to obtain maximum capacity. It is convenient to assemble this structure in the form of a long strip of previously etched and electrolytically filmed foil which is rolled into a substantially cylindrical shape as indicated, with another absorbent strip 5 between the successive turns thereof. A terminal strip 6 is formed at the end of one layer of foil and another terminal strip i is formed at the end of the other layer of foil; the terminal strip 0 being the anode foil in a D. C. condenser. An electrical connection with the anode foil is provided by the rod 3 extending through the insulating cap 2. The other layer of foil merely furnishes an auxiliary conducting path and connection to the electrolyte cathode. A suitable cathode terminal 9 may be provided as shown.

The aluminum foil or sheet to be used in making the condenser may be rolled to the desired gauge according to the usual fabrication methods and wound into coils. Where the sheet is handled in the form of coils, they may be heated in an annealing furnace for the required length of time and then either cooled in the furnace to below 400 F. or removed from the furnace and cooled to room temperature. In the latter case the coils are generally placed in another furnace operating at a temperature between 250 and 400 F. and heated for the necessary length of time. The time required to heat a particular size of coil or load of coils is a matter for experimental determination, but in any case the initial heating at 650 to 950 F. should occupy from 1 to 12 hours. In ordinary commercial operations we prefer to hold the metal at 800 to 850 F. for one to two hours. The method of cooling the metal from the elevated temperature is a matter to be determined by economic considerations inasmuch as the same effect is obtained whether the metal is cooled in the furnace or removed from the furnace and later reheated. Where the cooling is to be done in the furnace, the length of time required to reach 400 F. or lower depends upon the furnace temperature and the mass of metal being heated. Generally from 5 to 25 hours is needed for this purpose, the rate of cooling varying between 25 and 50 F. per hour. After reaching a temperature of 375 to 400 F. the metal can beremoved from the furnace and allowed to cool to room temperature.

Although it is common practice to heat the foil in coiled form, yet it is possible to obtain the same result by passing the foil in strip form through a suitably designed continuous furnace. Under such conditions the desired effect of heating may be obtained in 5 minutes instead of an hour or longer as where a coil is being treated. In any event, the high temperature treatment should serve to dissolve all the soluble impurities, that is, substantiallyeliminate irregularities in the structure of the metal resulting from working as well as equalize the distribution of soluble impurities. The period required to bring about this condition may thus vary between 5 minutes and 12 hours depending upon the mass of metal being heated at one time.

If the foil is treated in strip form, it is most convenient to apply the second thermal treatment by passing the strip through a second furnace operating at a temperature of 250 to 400 F. The period of exposure should be long enough to cause at least a. partial if not a complete precipitation of the dissolved impurities.

Where the coiled sheet is removed from the furnace immediately after the annealing treatment, it is to be cooled relatively rapidly to about room temperature, at least below 250' 1"., and

arcane then reheated to 250 to 400 F, for a period of 6 to 36 hours. Upon completion of this low temperature treatment, the metal is to be removed from the furnace and cooled to room temperature.

After either of the foregoing thermal treatments the coil, now at room temperature, is unwound and passed through an etching solution. It is obvious that any dirt or adhering foreign matter should be removed from the metal prior to etching in order to insure uniformity of attack. A number of substances are known which will attack aluminum but for commercial purposes either strong acid or caustic solutions are preferred. For the treatment of aluminum for electrolytic condensers, an etching with an aqueous solution of hydrochloric acid isusually preferred. An 11 per cent by weight solution of hydrochloric acid which is commonly used for etching aluminum for other purposes is very satisfactory for treating the metal intended for the manufacture of condensers. The sheet or foil is generally etched on both sides since both sides are active in the operation of the condenser. Where foil is to be treated, it is obvious that the time of exposure to the etching solution must be brief and closely regulated in order to avoid an undue reduction in thickness of the metal sheet.

The aluminum employed for making electrolyti c condensers should be of a commercial purity having a maximum of 0.6 per cent total of the impurities, iron, silicon and copper. However, a maximum of 0.2 per cent is preferred, because in electrolytic condensers the impurities may be a source of electrical leakage and reduce the power factor. A high leakage in condensers oi the paste type referred to hereinabove tends to shorten their useful life and it is therefore to be minimized as much as possible.

An illustration of the influence of a properly prepared and etched aluminum foil upon the capacity of a condenser is to be found in the following examples. The same material in the form of foil 0.004 inch in thickness was used in each test. The aluminum had a total iron, silicon and copper impurity content of 0.19 per cent. One group of sheet samples, designated A, was annealed at 500 F. for 1 hour, and immediately cooled to room temperature. A secorYd group, B, was annealed at 800 F. for 1 hour and immediately cooled to room temperature. A third group, C, was annealed at 800 cooled to room temperature and reheated to 350 F. for 36 hours. All of the samples were etched in an 11 per cent by weight aqueous solution of hydrochloric acid at a temperature of 65 F. for 1 minute. The electrical capacity of these samples was then determined on a Wien bridge using a 60 cycle alternating current. The average capacity in microfarads of the three groups of samples is given below.

Thermal treat- Capacity Group ment microiamds 9. 36 8m F. for 1 hr. 10. 38 800F.iorlhr.+

300 1".ior36hrs. 13.30

The improvement gained through treating the foil in accordance with our invention is at once apparent from these typical results. It may be seen that it is not only necessary to heat the foil within the limits of 650 to 950 F., but that a subsequent thermal treatment is essential to de veloping the best capacity.

Having thus described our invention and the manner in which it is to be performed,

We claim:

1. In the art of making electrolytic condensers from aliuninum sheet, the method of rendering said sheet uniformly etchable comprising heating the sheet at 650 to 950 F. for 5 minutes to 12 hours, cooling the sheet to room temperature, reheating it to 250 to 400 F. for 6 to 36 hours, and finally cooling to room temperature.

2. In the art of making electrolytic condensers from'aluminum sheet, the method of rendering said sheet uniformly etchable comprising heating the sheet at 650 to 950 F. for 1 to 12 hours, cooling the sheet to room temperature, reheating it to 250 to 400 F. for 6 to 36 hours, and finally cooling to room temperature.

3. In the art of making electrolytic condensers from, aluminum sheet, the method of rendering said sheet uniformly etchable comprising heating the sheet at 650 to 950 F. for '5 minutes to 12 hours, rapidly cooling the sheet to room temperature, reheating it to 250 to 400 F. for 6 to 36 hours, and finally cooling to room temperature.

4. In the art of making electrolytic condensers from aluminum sheet, the method of rendering said sheet uniformly etchable which contains a total of not more than 0.6 per cent of the impurities iron, silicon and copper, said method comprising heating the sheet at 650 to 950 F. for 5 minutes to 12 hours,

temperature, reheating it to 250 to 400 F. for 6 to 36 hours, and finally cooling to room temperature.

5. In the art of making electrolytic condensers from aluminum sheet, the method of rendering said sheet uniformly etchable which contains a total of not more than 0.6 per cent of the impurities iron, silicon and copper, said method comprising heating the sheet at 650 to 950 F. for 1 to 12 hours, cooling the sheet to room temperature, reheating it to 250 to 400 F. for 12 to 36 hours, and finally cooling to room temperature.

6. In the art of making electrolytic condensers from aluminum sheet, the method of rendering said sheet uniformly etchable which contains a total of not more than 0.6 per cent of the impurities iron, silicon and copper, said method comprising heating the sheet at 650 to 950 F. for 5 minutes to 12 hours and thereafter subjecting said sheet to a thermal treatment between 250 and 400 F, followed by'cooling to room temperature.

cooling the sheet to room 7. In the art of making electrolytic condensers from aluminum sheet, the method of rendering said sheet uniformly etchable which contains a total of not more than 0.2 per cent of the impurities iron, silicon and copper, said method comprising heating the sheet at 650 to 950 F. for 5 minutes to 12 hours, cooling the sheet to room temperature, reheating it to 250 to 400 F. for 6 to 36 hours, and finally cooling to room temperature.

8. In the art of making electrolytic condensers from aluminum sheet, the method of rendering said sheet uniformly etchable which contains a total of not more than 0.2 per cent of the impurities iron, silicon and copper, said method comprising heating the sheet at 800 to 850 F. for 1 to 2 hours, cooling the sheet to room temperature, reheating it to 350 to 400 F. for 6 to 36 hours, and finally cooling to room temperature.

9. In the art of making electrolytic condensers from aluminum sheet, the method of rendering said sheet uniformly etchable comprising heating the sheet at 650 to 950 F. for 1 to 12 hours, slowly cooling it in the furnace to below 400 F. within a period of 5 to 25 hours at a rate of 25 to F. per hour, removing the sheet from the furnace and cooling it to room temperature.

10. In the art of making electrolytic condensers from aluminum sheet, the method of rendering said sheet uniformly etchahle which contains a total of not more than 0.6 per cent of the impurities iron, silicon and copper, said method comprising heating for l to 12 hours, slowly cooling it in the furnace to below 400 F. within a period of 5 to 25 hours at a rate of 25 to 50 F. per hour, removing the sheet from thefurnace and cooling it to room temperature.

11. In the art of making electrolytic condensers from aluminum sheet, the method of rendering said sheet uniformly etchable which contains a total of not more than 0.2 per cent of the impurities iron, silicon and copper, said method comprising heating the sheet at 650 to 950 F. for

the sheet at 650 to 950 F.

1 to 12 hours, slowly cooling it in the furnace to below 400 F.'wlthin a period of 5 to 25 hours at a rate of 25 to 50 F. per hour, removing the sheet from the furnace and cooling it to room temperature. FRED KELLER.

CHARLES M. HASTINGS.