US 2886432 A
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United States Patent to Alnminium-Industrie-Aktien-Gesellschaft, Chippis, Switzerland No Drawing. Application November 15, 1956 Serial No. 622,287
Claims priority, application Switzerland November 18, 1955 7 Claims. ('Cl. 75-138) Our present invention relates to an aluminum foil for electrolytic condensers, which foil has to be roughened by etching before the subsequent coating with an oxide or other dielectric film.
It is very difiicult to etch satisfactorily aluminum foils of highest purity without use of electric current. Moreover, difiiculties arise often during the electrolytic etching of high purity aluminum foils, which ditficulties are caused for instance by the coarse grain structure of the metal.
A preferred, well known chemical etching bath consists in 6 to 10 percent hydrochloride acid with an addition of ferric chloride. For electrolytic etching, one uses for instance a solution of 1 percent sodium chloride in water.
It is known to add little quantities of chemical elements to the high purity aluminum in order to improve its ability to be etched. For instance, additions of 0.001 to 0.1 percent of lead, tin, bismuth, antimony, thallium, titanium, nickel, palladium, platinum, zirconium and other elements have been proposed. Amongst them, the addition of 0.01 to 0.03 percent boron has also been proposed.
Up to now, the proposed additions were not completely satisfactory. In practice, there are often dilficulties in the etching plant, mainly because of the formation of a coarse grain in the metal. Moreover, the addition of most of the earlier proposed elements is detrimental as to the leakage current and the shelf life of the capacitor.
In the course of long and systematic researches in laboratories and in etching plants we invented an aluminum foil the composition of which not only allows an extraordinary good and uniform etching even without use of electric current, no troubles arising for instance through formation of a coarse grain, but also leads to a great increase of capacity of the electrolytical condensers made from this foil. The increase of capacity amounts to about 20 percent.
The aluminum foil according to our invention is characterized by the fact that it is made from purest aluminum of at least 99.98 percent with the usual amounts of silicon, iron and so on as impurities and contains beside these impurities 0.005 to 0.05 percent boron, 0.001 to 0.005 zinc and 0.0003 to 0.002 percent magnesium, the proportion of zinc to magnesium being greater than 2:1, preferably 3:1 to 4: 1. The content of boron, zinc and magnesium has no detrimental influence on the leakage current and the shelf life of the capacitor.
Preferably the iron content is increased, for instance up to 0.006 percent at most. Very good results were obtained with an iron content of 0.003 percent with a tolerance of 10.0005 percent. The increase of the iron content causes a substantial acceleration of the etching. In a hydrochloric acid etching bath, without use of electric current, one may for instance obtain the same roughening within 3 minutes as within 5 minutes without increased iron content.
The best results were obtained with aboronconte'nt of 001 percent with a tolerance of i0.005percent.
The boron content for itself allows an increase of capacity of the electrolytical condenser" by about 20 percent. 5 5 However, the addition of-boron alone to high purity aluminiiin does not hinder with certainty'the formation "'of the "coarse grain structure.
A content of zinc and magnesium without boron causes a fine grain structure but no appreciable increase of the capacity of the condenser. The highest effect is obtained only with the combined content of boron, zinc and magnesium.
The aluminum foil according to our invention presents for the chemical etching several advantages: one obtains a higher roughening, the etching is shortened and the metal has a fine grain structure. A fine grain structure is important for the rolling as well as for the etching. The advantages appear for instance from the following table:
Electrical values for 540 Volt formation voltage Type of high purity Remarks aluminum Capacity Surface of the conincrease denser, by etching pF/IOO cm (fold) 99.99 p.c. Al 13-14. 5 5. 2-5. 8 Partly useless because of coarse grain structures. 99.99 p.c. Al+0.0015 p.c. 14. 345 5. 7-6.0 Throughout flne Mg +0.004 p.c. Zn grain structure. 99.99 p.c. Al +0.0025 15. 4 6. 15 Partly useless bep.c. B cause of coarse grain structure. 99ig9 p.c. Al +0.011 p.c. 17. 1 6. 85 Do.
99.99 p.c. Al +0.015 p.c. 18. 5 7. 2 Throughout fine B +0.0015 p.c. Mg grain structure. +0.0035 p.c. Zn
The etching has been carried out according to the method described in Example 2 of the US Patent No. 2,699,382. The foil thickness was 0.1 mm.
What we claim is:
1. An aluminum foil for electrolytic condensers of a composition consisting essentially of aluminum, zinc, and magnesium, the aluminum being of a purity not less than 99.98%, the zinc and magnesium being present in amounts suificient to improve the fineness of the grain structure of the composition, the amount of zinc not exceeding about 0.005% and the amount of magnesium not exceeding about 0.002%.
2. The foil of claim 1 in which the composition contains iron in amount not exceeding about 0.006%.
3. The foil of claim 1 containing also boron in an amount not exceeding .05%.
4. The foil of claim 1 wherein the zinc content is between .001 and .005 and the magnesium content between .0003 and .002% with the proportion of zinc to magnesium between 2:1 and 4: 1.
5. The foil of claim 2 wherein the iron content is 0.003% with a tolerance of 10005 6. The foil of claim 2, the boron content of which is .01% with a tolerance of :.000 5%.
7. The foil of claim 3 wherein the zinc content is between .001 and .005% and the magnesium content between .003 and .002% with the proportion of zinc to magnesium between 2:1 and 4: 1.
(References on following page) UNITED STATES PATENTS Bonsack Aug. 8, 1933 Stroup Aug." 12, 1941 Whitzel et a1 Mar. 20, 1951 FOREIGN PATENTS Great Britain Mar. 12, 1943 Great Britain May 3, 1943 Great Britain Aug. 3, 1955 4 OTHER REFERENCES ASM Transactions, vol. 41, 1949, page 444. Study on Formation of Dielectric Film of Super-Purity Aluminum Anode in Electrolytic Condensers, 3rd Report, 5 by Nakamura and Nishizaka. Light Metals (Japan), No.
20, 58-67, August 1956. Digested in: Light Metals Bulletin, vol. 19, No. 3, February 1, 1957, page 104.
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,886,432 May 12, 1959 Hans Schmitt et al.3
It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters .Patent should read as corrected below.
Column 2, line 69, for "0003" read ,0003
Signed and sealed this 29th day of September 1959a SEAL) Attest:
KARL H. AXLINE ROBERT c. WATSON Attesting Ofiicer Commissioner of Patents