Single step electro chemical etch process for high volt aluminum anode foil

Aluminum electrolytic capacitor foil of a high cubic texture etched by passing the foil through an electrolyte bath containing 3% hydrochloric acid and 1% aluminum as aluminum chloride under the influence of a direct current and at a temperature of 75.degree. C. The foil thus etched has a significantly higher capacitance resulting from a greater density of tunnels, an elongation of the tunnels, and a fairly linear tunnel structure in the foil.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the etching of aluminum electrolytic capacitor 
foil of a high cubic texture in an etching bath containing hydrochloric 
acid and aluminum chloride using direct current to produce a foil that has 
a significantly higher capacitance. 
2. Description of the Prior Art 
In the typical process of etching aluminum foil for use in electrolytic 
capacitors, an electrolytic bath containing sodium chloride or other salt 
and sulfate based electrolytes has been used. However, the capacitance of 
the resulting foil does not reach the high values which are achieved with 
the present invention. 
U.S. Pat. No. 4,213,835 discloses a method for electrolytically etching a 
recrystalized aluminum foil which allows manufacture of foils with 
exclusively pure cylindrical or cubical etching tunnel structures and 
tunnel densities greater than 10.sup.7 /cm.sup.2 of foil surface. This 
process uses a potentiostatic etching technique. The problem with this 
technique is that it does not lend itself to large scale mass production 
of etched foils. 
Still other processes have been used to yield a higher specific capacitance 
in aluminum foils. The present invention is directed to achieving a 
significant increase in specific capacitance for high volt foils. 
SUMMARY OF THE INVENTION 
The invention features the etching of aluminum capacitor foil having a high 
cubic texture to produce a more uniformly etched foil and etching the foil 
under the influence of DC current in an electrolyte bath containing 3% 
hydrochloric acid and 1% aluminum in the form of aluminum chloride at 
75.degree. C. 
Several factors are necessary for increasing the specific capacitance of 
aluminum electrolytic capacitor foil. One factor is a significant 
improvement in tunnel density and tunnel structure. Substantially straight 
elongated tunnels provide much of the surface area created by etching. As 
tunnel density is increased, a corresponding enlargement of the surface 
area will occur. Another major factor in controlling the specific 
capacitance is the type of aluminum which is used. It is well known that 
the etched tunnels in anode foil suitable for high volt applications are 
predominantly in the 100 direction. It is therefore reasonable to assume 
that foil with greater 100 crystal orientation, that is foil with greater 
cubic texture, would result in a higher tunnel density. The aluminum foil 
used for etching until recently has had random cubic texture. Such foils 
may be called "non-cubic", foils with less than 25% cubic texture. When 
the extent of cubic texture exceeds 50%, aluminum foils are classified as 
high cubic. In the present invention an aluminum foil having a high cubic 
texture, greater than about 50% is preferred because when etched, the 
tunnel structures are found to be uniform and elongated and the tunnel 
density can consequently be increased. Another major factor controlling 
the increase in specific capacitance is the etch electrolyte which 
controls the tunnel initiation and the depth and width of the etched 
tunnels. As shown in Table I, a combination of a suitable choice of both 
aluminum foil to be etched and an etch electrolyte will result in a more 
uniform tunnel initiation. 
TABLE I 
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CV at 260 Volts, 
Foil Type Process Type 
V-uF/cm.sup.2 
______________________________________ 
Standard Conventional 
200 
Non-Cubic 
Standard Invented 210 
Non-Cubic 
New Hi-Cubic Conventional 
210 
New Hi-Cubic Invented 305 
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The chemistry of the etch electrolyte in the present invention is a major 
factor for controlling the etch characteristics of DC etching of high 
voltage, e.g. over 200 volt, anode foils. Hydrochloric acid is used in 
preference to other chlorides in this application because it avoids the 
precipitation of aluminum hydroxide which can clog the very elongated 
tunnels which are developed in the process of the present invention. If 
such precipitation is allowed to occur, it will interfere with the high 
cubic nature of the foil, thereby not fully utilizing the benefits that 
can be derived when a suitable combination of the high cubic texture foil 
and an etch process is employed. 
The temperature is maintained at or about 75.degree. C. to minimize 
evaporation losses thus making the process far more economical and easier 
to control. 
The process of the present invention results in a very simple single step 
etch process that yields capacitance values equal to or significantly 
higher than the best available commercial foils without requiring major 
changes in existing production machinery. The advantages of the inventive 
process consist in that etched tunnel structures with a tunnel density 
greater than 10.sup.7 /cm.sup.2 of foil surface are obtained and the 
etched tunnels are uniformly distributed over the foil. The etching or 
tunnel diameter required for a planned forming voltage is primarily 
determined by the electrolytic aluminum erosion rate which can be 
carefully controlled in the process of the present invention. With the 
help of the inventive process, the highest possible surface enlargement 
and capacitance gain with the smallest possible aluminum erosion is 
obtained while maintaining sufficient foil strength.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Aluminum electrolytic capacitor foil of at least 99.99% purity, having a 
high cubic texture and having a thickness of 100 microns is etched by 
passing it through an etch electrolyte bath containing 3% hydrochloric 
acid and 1% aluminum as a chloride under the influence of DC current at 
75.degree. C. The current density is 0.155 amps/cm.sup.2 and the etch 
coulombs are 80-90 coulombs/cm.sup.2. This process provides an etched 
tunnel structure as shown by the SEM photographs in FIGS. 2 and 4. 
The figures show tunnel structures of a cross-section of etched foil. The 
linear light grey structures are the etched tunnels of the foil and the 
boundary surfaces of the foil are slightly discernable above and below the 
tunnels against the mounting mat. In contrast to the prior art, as 
illustrated in FIG. 1, the relatively straight, elongated tunnel structure 
which appears in the vertical plan in the SEM photographs of FIGS. 2 and 4 
is evident. The light grey areas which are the tunnels penetrate further 
through the foil. They appear in a somewhat regular pattern and the 
density of the tunnel structure is significantly greater than that of the 
prior art as shown in FIG. 1. Of key interest in the photographs is the 
fact that many tunnels do not intersect one another thereby removing foil 
from the structure. When FIGS. 3 and 4 are examined under a stereo viewer, 
it is clear that the invented process gives a much higher tunnel density 
as well as a more uniform distribution of tunnels throughout the foil. 
In the process of the present invention using 100 micron, high cubic 
texture foil as described above, the amount of aluminum dissolved in terms 
of milligrams/cm.sup.2 ranges from 8.46 to 9.65. The following table shows 
the specific capacitance of the foil when anodic oxides are formed at 260 
volts and 465 volts. The CV number is the capacitance times the voltage 
and the CV/mg number is the capacitance times the voltage divided by the 
amount of aluminum dissolved per cm.sup.2. This number is a measure of the 
relative efficiency of the etching process. 
TABLE II 
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Al Dissolved 
260 Volts 465 Volts 
mg/cm.sup.2 CV CV/mg CV CV/mg 
______________________________________ 
8.46 306 36 266 31 
9.65 304 31 271 28 
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Among the parameters of the present invention, the following variations are 
permissible. The aluminum foil used is a foil having a very high cubic 
texture which means that this foil has a high orientation of grains in the 
100 direction. For the purposes of the processes of the present invention, 
it has been discovered that a foil having at least 70% of its grains 
oriented in the 100 direction is sufficient to achieve the desired 
results. Such foils are commercially available from Showa Aluminum 
Company, Toyo Aluminum Company and SCAL, a Division of Pechiney Aluminum 
Company. A high cubicity foil enables the etching of very straight tunnels 
while retaining surface area created by the etching. The tunnel initiation 
tends to be more uniform and the tunnel density can then be increased. The 
higher the cubicity of the foil, the higher the specific capacitance will 
be. For production purposes, however, it has been empirically determined 
that the cubicity of 70% or better will suffice. 
A range of values for the other parameters of the present process is also 
possible. The chemistry of the electrolyte is aimed at producing an acidic 
medium having a large presence of chloride. Hydrochloric acid is preferred 
to avoid the precipitation which would occur with sodium chloride. The 
introduction of aluminum makes the electrolyte bath less expensive and 
minimizes electrolyte replacement in the process. The range of parameters 
for the electrolyte bath are as follows. Hydrochloric acid can be present 
in concentrations ranging from 1.5% to 7% and the aluminum in the form of 
chloride can be present in the range of 0% to 2%. The temperature range of 
the process can be from 70.degree. to 85.degree. C. The current density 
can range from 0.13 to 0.185 amperes/cm.sup.2. The etched coulombs can 
range from 70 to 110 coulombs/cm.sup.2. These ranges have been empirically 
determined for purposes of mass production of a high volt foil. The 
latitude in the ranges presented enables high speed mass production of 
foil without elaborate control systems for monitoring these parameters, 
thus making the installation of the process of the present invention 
relatively inexpensive. In terms of the temperature range, over 85.degree. 
C. evaporation becomes a significant factor. If the temperature is lower 
than 70.degree. C., the etching process either will not work or will work 
less efficiently. In terms of the current density, a density below the low 
point of the range will result in lower tunnel initiation and 
consequently, lower tunnel density. If the current density is higher than 
the range stated, the tunnel size will tend to be non-uniform because 
there will be competition between the deepening of the tunnels already 
initiated and the initiation of new tunnels. As in any etching process, 
non-uniform tunnels are to be avoided since many in effect will be closed 
when the anode foil is formed with an oxide at high voltages. 
The following table represents results of some statistical tests showing 
the amount of foil dissolved by varying the time of the etching process 
and the capacitance achieved in the etched foil, all using the process of 
the present invention. 
TABLE III 
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Al Dissolved Capacitance at 260 Volts 
mg/cm.sup.2 MF/cm.sup.2 
Volt-MF/cm.sup.2 
______________________________________ 
6.54 0.95 252 
7.73 1.00 266 
8.81 1.04 276 
9.97 1.15 306 
11.17 1.21 321 
______________________________________ 
In FIG. 5, a graph is presented showing the effect of etch coulombs on 
capacitance in a formed foil. 
From the examples, it is obvious that foils etched in accordance with the 
present invention can be used in high bolt electrolytic capacitors and 
yield a significantly higher specific capacitance per cm.sup.2 than 
previously obtained. Thus to obtain the given capacitance, the capacitor 
can have a smaller volume or for the same volume can have a higher 
capacitance. Although various minor modifications may be suggested by 
those versed in the art, it should be understood that I wish to embody 
within the scope of the patent warranted hereon such modifications as 
reasonably and properly come within the scope of the appended claims.