US2616799A - Electrolytic capacitor - Google Patents

Description

NOV. 4, R. D. BUGEL ELECTROLYTIC CAPACITOR Filed March 1, 1951 ALUMINUM PLUS IMPURITIES CU, FE, SI

ELECTROLYTE DIELECTRIC AL O INVENTOR ROELOF DIRK BUGEL AGENT Patented Nov. 4, 1952 1 UNITED STATES PATENT OFFICE ELECTROLYTIC CAPACITOR Application March 1, 1951, Serial No. 213,367 In the Netherlands March 22, 1950 3 Claims. 1

This invention relates to electrolytic capacitors in which at least one electrode consists of aluminium coated with an oxide layer which fulfils the function of a dielectric. If the capacitor is intended for use with alternating current, it must comprise at least two such electrodes. The counter-electrode used is an electrolyte, for example on the basis of boric acid and glycol, which may be provided in the liquid state or in the form of a paste.

It is known to utilise aluminium of great purity for the said electrodes, for example aluminium containing less than some hundredths per cent of impurities. With the use of such material capacitors may be made having a low leakage current which increases slightly during operation, such capacitors consequently having a sufficient life. Impurities in the aluminium have been found detrimentally to affect the dielectric properties of the aluminium-oxide layer due to the formation of oxides of other metals in this layer. This view has led to the use of aluminium of high purity for the manufacture of electrolytic capacitors.

It is common practice to subject the electrode material to an etching operation in order to ensure a high capacitance per unit-volume. It has been found, however, that according as the aluminium has a greater purity, the etching operation involves greater difficulties, it being in practice impossible for aluminium of 99.99% purity to be etched to a sufficient extent.

It follows therefore that concerning the life of the capacitor and the etching treatment of the electrode material, the requirements for the purity of the aluminium conflict with one another. In addition, the consumption of energy in forming the oxide layer decreases with higher purity of the kinds of aluminium commercially sold.

The figure of the drawing is a cross section of a part of an electrolytic capacitor.

During the investigation which has led to the present invention it has been found that the in fluence exerted by various impurities occurring in commercial aluminium upon the etching and forming treatments is not the same. For the sake of completeness, it is mentioned that some elements such as magnesium, manganese and titanium in the form of impurities in or small additions to the aluminium have no detrimental effect on capacitor life, as is, however, the case with the impurities normally occurring in practice, which are substantially iron and silicon and, to a smaller extent, copper. Probably as a result 2 of the purificafion methods utilised, the content of iron is higher than the content of silicon even in the very pure kinds of aluminium which are commercially sold.

It has been found that the iron contained in aluminium is advantageous for the etching treatment, but detrimental to the process of forming the oxide layer, whereas silicon is harmless to the forming process and, although to a smaller extent than iron, facilitates the etching treatment. When disregarding copper as an impurity, since the content of copper is only some thousandths per cent even in commercial kinds of aluminium, which contain in total about 0.2% impurities, it appears that in regard to the composition a compromise is possible in which the above-mentioned disadvantages occurring in commercial kinds of aluminium may be decreased by a suitable choice .of the ratio between iron and silicon.

According to the invention, an electrolytic capacitor comprising at least one aluminium electrode provided with an oxide layer, is characterized in that this electrode contains less than 0.01% copper, the content of iron being comprised between 0.03% and 0.08% and the content of silicon being from 0.01% to 0.05% higher, preferably at least 0.02% higher, than the iron content.

Highly satisfactory results are obtained with aluminium having an iron content between 0.03 and 0.06% and a silicon content between 0.05% and 0.09%, and being the greater the higher the iron content.

In view of the fact that the conventional methods of purifying aluminium lead to products, the iron content of which is higher than the silicon content, it will. as a rule, be necessary to add a small amount of silicon to the commercial purified aluminium.

Electrolytic capacitors comprising electrodes of aluminium containing iron and silicon in the quantities indicated above, are substantially identical as regards life and quality with capacitors in which aluminium of 99.99% purity is utilised as the electrode material, whilst in regard to the etching treatment during manufacture a considerable advantage is obtained with respect to the last-mentioned capacitors. This will be made clear with reference to some examples in the table following hereinafter.

In this table, the etching period, the forming period, the sum of these periods and the forming energy are indicated for some kinds of aluminium foil of a thickness of 250 microns in which the ratios between the iron and silicon contents are different and which, as is usually the case, have been heated at 300 C. for about 10 hours. The etching period chosen is the time in minutes required to ensure etching up to 50% of the initial weight at a temperature of 0 C. in a 10% solution of hydrochloric acid in which 30 gmsfiof aluminum'aredissolvedzper litre. The forming period indicated is the time in minutes required for forming an oxide layer by anodic oxidation with the use of a conventional solution of boric acid and borate (for example a solution containing 4% of boric acid and-0.14% of borax) until the current de'n'sityfiat a voltage.

of 400 volts has decreased to aboutramamps/dmfii- The forming energy indicated in the table'isto -be understood to mean the energy kilowatthours consumed during the forming period 'for 25 foils having a surface area of 50 cm. each.

The table shows that the total time required fforthe etching and forming treatmentsiis lowest 'in 'Example 2. Furthermore, it appears that the aluminium of greater purity in Example 3v as compared with that in Example '2 .ryields .only little decrease in consumption of energy, whilst. as comparedwithiExample.1-which does not satisfy the conditions of "the invention"in' this. case a decrease lin consumption of energy .is achieved,

tween 003% and-0.08% iron, and an amount of silicon "which 'is 0.02% greater than the amount of iron.

=3.Anaelectrode material for an electrolytic capacitorwhich consists of aluminum of greater than 99% purity, less than .01% copper, between 0.03% and 0.06% iron, and between 0.05% and 0.09% silicon.

RO-EBOF DIRKBUGEL.

REFERENCES CITED 'ihe'fdllowing'meferences are of record in the "file of this ,patentz UNITED STATES PATENTS Number Name -'Date 1578,85? iSiegmund Mar. 30, "1926 1,710,073 Ruben Apr. 1-3, 1'929 2,024,210 Edelman Dec. 17, 1935 OTHER REFERENCES "Electrolytic Gondensers 'by 'PhilipiR-zfloursey,

published by Chapman -'& HallLtd., London, .1937,

page-.52.

Claims (1)

1. AN ELECTRODE MATERIAL FOR AN ELECTROLYTIC CAPACITOR WHICH CONSISTIS OF ALUMINUM OF GRETER THAN 99% PURITY, LESS THAN 0.01% COOPER, BETWEEN 0.03% AND 0.08% IRON, AND AN AMOUNT OF

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