TRANSLUCENT FILMS OF ALUMINUM OXIDE RICHARD E. VOLLRATH' Norman Bridge Laboratory, California Institute of Technology, Pasadena, California Received August 7, 1989
It is well known that a layer of aluminum oxide can be formed on a surface of aluminum by electrolytic oxidation (1). The aluminum is made the anode in an electrolytic cell with a cathode of some non-polarizing material, such as carbon, and an electrolyte consisting of an aqueous acid, such as oxalic acid. Layers of aluminum oxide of considerable thickness can be built up by continuing the electrolysis sufficiently long. The author during a search for a thin film to satisfy certain requirements attempted to make thin sheets of aluminum oxide by oxidizing aluminum foil electrolytically. It turned out to be possible to convert thin aluminum sheets into sheets of aluminum oxide (2).2 The method of doing this is illustrated in the following description of a procedure by means of which sheets of aluminum oxide were prepared. A piece of aluminum foil 0.002 cm. thick, 2 cm. wide, and 10 cm. long is fastened in a clamp constructed as shown in figure 1 (a). It consists of two strips, A and B, of &in. aluminum between which the foil is clamped by means of a screw holding the two strips together. The foil and a section of the clamp are immersed, as shown, in a 3 per cent aqueous solution of oxalic acid contained in a 1-liter battery jar or beaker. The clamp holding the foil is connected to the positive terminal of a 120-volt D.C. supply, while a carbon rod serves as the cathode. A rheostat or lamp bank is connected in series with the cell and the current adjusted to about 30 milliamperes. The actual voltage is not important but the current should not be too high, otherwise the foil may become overheated during the electrolysis. It turned out after a number of trials that alternating current could be used without making any difference in the results. Upon allowing the above current to flow for about 2 hr. the foil will become almost completely transparent and acquire a peculiar silky appearance. It is necessary that the foil be completely immersed in the electrolyte, that is, the clamp itself should be partly immersed as shown in figure 1. If the foil projects from the solution the portion at the air-electrolyte interface is 1
On leave from the University of Southern California, Los Angeles, California.
* During
a discussion on electrolytic condensers Mott showed an audience aluminum oxide films, but gave no details of how they were made.
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RICHARD E. VOLLRATH
soon converted into aluminum oxide, which extends aa a band across the foil and, because it is an insulator, cuts off the current and stops the oxidation of the remaining parts of the foil. The aluminum foil treated in this manner when examined under the microscope appears to consist of a translucent glassy iilm in which a large number of minute irregular fragments of aluminum are dispersed. The silky appearance mentioned above is due to these minute fragments of aluminum. Apparently the progress of the oxidation causes small areas of aluminum in the foil to become completely surrounded by aluminum oxide, which, being an insulator, isolates these areas electrically so that they cannot be further oxidized. To overcome this difficulty the arrangement shown in figure 2 was adopted. In figure 2 the foil is held between 2 strips of aluminum, A and
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FIQ. 1. Apparatus for oxidizing aluminum foil
B, one of which is made long enough to permit placing it across the top of a battery jar or beaker as shown. The foil hangs down from this clamp and dips into the electrolyte which, again, is 3 per cent aqueous oxalic acid. At the beginning of a run the lower end of the foil is permitted to dip a few millimeters below the surface of the solution, which a t the start is a few centimeters deep. The foil and the carbon rod serving as the cathode are connected in series with enough resistance to limit the current to about 2 milliamperes per square centimeter of foil surface. The ammeter A is in the circuit to permit this adjustment to be made. A short time after the current is started the section of the foil dipping below the surface of the electrolyte is converted into the oxide, as the transparency indicates. At
TRANSLUCENT FILMS O F ALUMINUM OXIDE
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this point a hissing noise due to heating develops in the region where the foil emerges from the electrolyte, and the current finally ceases because the oxide is a non-conductor. At this stage a 3 per cent solution of oxalic acid is allowed to flow into the cell from a reservoir whose liquid level is about 50 cm. above that in the cell. In figure 2 the reservoir is shown much lower in order to conserve space in the drawing. The rate of flow of the oxalic acid solution is adjusted by means of a stopcock until the electrolyte
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OXALIC ACJD
I
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FIG.2. Apparatus for olridiaing aluminum foil
level in the cell rises just fast enough ta permit the immersed portion of the foil to be completely oxidized. In thie manner a piece of aluminum foil 0.002 cm. thick and 2 cm. wide was oxidized at the rate of about 2 cm. per hour. It was foand possible to oxidize foils as thin as 0.0005 cm. Foils thinner than thrs e m be oxidized, but it is very difficult to remove them from the electrolyte without tearing them. However, suitable mechanical means for lifting out the oxidized foils could be devised.
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WALTER D. BONNER AND MAX B. WILLIAMS
The aluminum oxide foils prepared in this manner have considerable flexibility but break if bent too sharply. It is to be suspected that the lack of flexibility may be due to a survival of the crystalline structure in the original aluminum foil. I n transparency these films resemble thin tissue paper or tracing paper. It is rather remarkable that every scratch or rolling mark on the aluminum foil is faithfully reproduced on the oxide film. The author has attempted to convert thin aluminum tubing into an equivalent oxide tube with the idea of using it as a dsusion thimble or semipermeable membrane, but so far without success. Further work is being carried out to determine the optical and electrical properties of the material. REFERENCES (1) G~NTHER-SCHULLE: 2. Physik 9, 349 (1920). This does not relate directly to the above work but contains many pertinent references. (2) MOTT:Trans. Am. Electrochem. SOC.6, 162 (1904).
T H E AZEOTROPIC SYSTEM ALCOHOL-WATER-BENZENE' WALTER D. BONNER A N D MAX B. WILLIAMS* Department of Chemist~y,University of Utah, Salt Lake City, Utah Received July 7, 1939
Since ethanol, although miscible with water in all proportions, forms an azeotropic system of lower boiling point than either component, it is not possible to separate water and alcohol completely by fractionation. If, however, a sufficient quantity of benzene is added to aqueous alcohol, a complete separation of the water and alcohol can be effected by fractionation. The mechanism of this separation is interesting. If sufficient benzene has been added, the system separates into two liquid phases. The lower, denser phase contains about equal amounts of water and alcohol with little benzene, while the upper phase contains benzene, alcohol, and little water. This heterogeneous system of three components and three phases has two degrees of freedom. If one fixes the pressure and the composition, the boiling point is thereby fixed, and the mixture boils at a constant temperature so long as two liquid phases are present. When one of the two liquid phases disappears, the boiling temperature will suddenly rise. If Contribution No, 55 from the Chemical Laboratories of the University of Utah. Present address: Department of Chemistry, Cornel1 University, Ithacs, New York. 1
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