U.S. patent application number 11/099917 was filed with the patent office on 2005-09-29 for capacitor containing aluminum anode foil anodized in low water content glycerine-phosphate electrolyte without a pre-anodizing hydration step.
Invention is credited to Harrington, Albert Kennedy, Kinard, John Tony, Melody, Brian John, Stenzinger, Duane Earl, Wheeler, David Alexander.
Application Number | 20050211565 11/099917 |
Document ID | / |
Family ID | 32987550 |
Filed Date | 2005-09-29 |
United States Patent
Application |
20050211565 |
Kind Code |
A1 |
Kinard, John Tony ; et
al. |
September 29, 2005 |
Capacitor containing aluminum anode foil anodized in low water
content glycerine-phosphate electrolyte without a pre-anodizing
hydration step
Abstract
A capacitor comprising an aluminum anode and a dielectric layer
comprising phosphate doped aluminum oxide and process for making
the capacitor. The capacitor has a CV Product of at least 9
.mu.F-V/cm.sup.2 at 250 volts. Furthermore, the capacitor is formed
by the process of: forming an aluminum plate; contacting the plate
with an anodizing solution comprising glycerine, 0.1 to 1.0%, by
weight, water and 0.01 to 0.5%, by weight, orthophosphate; applying
a voltage to the aluminum plate and determining an initial current;
maintaining the first voltage until a first measured current is no
more than 50% of the initial current; increasing the voltage and
redetermining the initial current; maintaining the increased
voltage until a second measured current is no more than 50% of the
redetermined initial current, and continuing the increasing of the
voltage and maintaining the increased voltage until a final voltage
is achieved.
Inventors: |
Kinard, John Tony; (Greer,
SC) ; Melody, Brian John; (Greer, SC) ;
Wheeler, David Alexander; (Williamston, SC) ;
Stenzinger, Duane Earl; (Simpsonville, SC) ;
Harrington, Albert Kennedy; (Fountain Inn, SC) |
Correspondence
Address: |
John B. Hardaway, III
NEXSEN PRUET JACOBS & POLLARD, LLC
Fed. Sta.
P.O. Box 10107
Greenville
SC
29603-0107
US
|
Family ID: |
32987550 |
Appl. No.: |
11/099917 |
Filed: |
April 6, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11099917 |
Apr 6, 2005 |
|
|
|
10390529 |
Mar 17, 2003 |
|
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|
Current U.S.
Class: |
205/318 ;
205/324 |
Current CPC
Class: |
C25D 5/18 20130101; C25D
11/06 20130101; H01G 9/045 20130101; H01G 9/0032 20130101; C25D
11/16 20130101; H01G 9/055 20130101; Y10S 428/935 20130101 |
Class at
Publication: |
205/318 ;
205/324 |
International
Class: |
C25D 009/06 |
Claims
1-4. (canceled)
5. A process for preparing a capacitor comprising: forming an
aluminum plate; without pre-hydration, contacting said plate with
an anodizing solution comprising glycerine, about 0.1 to about
2.0%, by weight, water and about 0.01 to about 0.5%, by weight,
orthophosphate; applying a voltage to said aluminum plate of at
least about 220 volts.
6. The process for preparing a capacitor of claim 5 wherein said
voltage is applied in increasing increments with an age time
between each said increment.
7. The process for preparing a capacitor of claim 6 wherein said
increments are less than about 75 volts.
8. The process for preparing a capacitor of claim 7 wherein said
increments are at least about 20 V to no more than about 50 V.
9. The process for preparing a capacitor of claim 6 wherein said
age time is sufficient for the current to decrease to from about 1
to about 50% of an initial current.
10. The process for preparing a capacitor of claim 9 wherein said
age time is sufficient for the current to decrease to from about 10
to about 30% of said initial current.
11. The process for preparing a capacitor of claim 10 wherein said
age time is sufficient for the current to decrease to about 20% of
said initial current.
12. The process for preparing a capacitor of claim 5 wherein said
anodizing solution is at a temperature of about 25.degree. C. to
about 125.degree. C.
13. The process for preparing a capacitor of claim 12 wherein said
anodizing solution is at a temperature of about 80.degree. C. to
about 105.degree. C.
14. The process for forming a capacitor of claim 5 wherein said
anodizing solution comprises about 0.01 to about 0.1% soluble
orthophosphate.
15. The process for forming a capacitor of claim 5 wherein said
soluble orthophosphate is selected from a group consisting of
ammonium phosphate, alkali metal phosphate, amine phosphate and
mixtures thereof.
16. The process for forming a capacitor of claim 5 wherein said
soluble orthophosphate is selected from a group consisting of
mono-sodium phosphate, di-potassium phosphate, and sodium potassium
phosphate.
17. The process for forming a capacitor of claim 5 wherein said
soluble orthophosphate is selected from a group consisting of
mono-ammonium phosphate and di-ammonium phosphate.
18. The process of claim 5 wherein said anodising solution
comprises about 0.1 to about 1%, by weight, water.
19-22. (canceled)
23. A process for preparing a capacitor comprising: forming an
aluminum plate; contacting said plate with an anodizing solution
comprising glycerine, about 0.1 to about 2.0%, by weight, water and
about 0.01 to about 0.5%, by weight, orthophosphate; applying a
voltage to said aluminum plate and determining an initial current;
maintaining said first voltage until a first measured current is no
more than 50% of said initial current; increasing said voltage and
redetermining said initial current; maintaining said increased
voltage until a second measured current is no more than about 50%
of said redetermined initial current, and continuing said
increasing said voltage and said maintaining said increased voltage
until a final voltage is achieved.
24. The process for preparing a capacitor of claim 23 wherein said
final voltage is above 220 volts.
25. The process for preparing a capacitor of claim 24 wherein said
voltage is increased by no more than about 75 volts.
26. The process for preparing a capacitor of claim 25 wherein said
voltage is increased by at least about 20 V to no more than about
50 V.
27. The process for preparing a capacitor of claim 23 wherein said
first measured current or said second measured current is from
about 1 to about 50% of said initial current.
28. The process for preparing a capacitor of claim 27 wherein said
first measured current or said second measured current is from
about 10 to about 30% of said initial current.
29. The process for preparing a capacitor of claim 28 wherein said
first measured current or said second measured current is about 20%
of said initial current.
30. The process for preparing a capacitor of claim 23 wherein said
anodizing solution is at a temperature of about 25.degree. C. to
about 125.degree. C.
31. The process for preparing a capacitor of claim 30 wherein said
anodizing solution is at a temperature of about 80.degree. C. to
about 105.degree. C.
32. The process for forming a capacitor of claim 23 wherein said
anodizing solution comprises about 0.01 to about 0.1% soluble
orthophosphate.
33. The process for forming a capacitor of claim 23 wherein said
soluble orthophosphate is selected from a group consisting of
ammonium phosphate, alkali metal phosphate, amine phosphate and
mixtures thereof.
34. The process for forming a capacitor of claim 23 wherein said
soluble orthophosphate is selected from a group consisting of
mono-sodium phosphate, di-potassium phosphate, and sodium potassium
phosphate.
35. The process for forming a capacitor of claim 23 wherein said
soluble orthophosphate is selected from a group consisting of
mono-ammonium phosphate and di-ammonium phosphate.
36-40. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present invention is a divisional application of U.S.
patent application Ser. No. 10/390,529 filed Mar. 17, 2003 which is
pending.
BACKGROUND OF THE INVENTION
[0002] The present invention is related to an electrolyte solution
for anodizing aluminum anode foil for use in electrolytic
capacitors and the capacitors containing this anode foil.
[0003] We have found that low water content variations of the
glycerine and orthophosphate-containing electrolytes described in
U.S. Pat. No. 6,409,905, which is incorporated herein by reference
thereto, may be used for the anodization of aluminum foil to
voltages sufficiently high to facilitate the use of the
aforementioned foil in intermediate and high voltage electrolytic
capacitors.
[0004] Previously, the maximum anodizing voltage obtainable from
the aqueous phosphate solutions traditionally used to anodize
aluminum capacitor foil for applications requiring extreme foil
stability and oxide hydration resistance was about 220 volts, as
stated in U.S. Pat. No. 3,733,291. The corrosion of the foil being
anodized in aqueous phosphate solutions increases with the
anodizing voltage and is sufficiently severe to result in
dielectric failure above about 220 volts. The corrosion by-products
formed during aluminum foil anodizing in aqueous phosphate
solutions must be removed from the solution via filtering, etc., or
they will deposit upon the foil and anodizing tank components in
amounts sufficient to interfere with the anodizing process.
[0005] The difficulties encountered with aqueous phosphate
anodizing of aluminum foil for use in relatively low voltage
capacitors are such that, in spite of the superior electrical
stability of foil anodized in phosphate solutions nearly all of the
low voltage foil produced today is anodized in non-phosphate
solutions with the exception of a relatively small amount of
phosphate which may be present to help impart hydration resistance.
Due to the voltage limitations of aqueous phosphate anodizing
solutions mentioned above, intermediate and high voltage capacitor
foils have not traditionally been anodized in aqueous phosphate
solutions.
[0006] Aluminum electrolytic capacitors for use at intermediate
voltages typically contain anode foil hydrated by passing the foil
through a hot water bath prior to anodizing, as defined in U.S.
Pat. No. 4,582,574. These capacitors are typically for use at
voltages from 150 to 250 volts and contain anode foil anodized to
about 200 to 350 volts. This pre-anodizing hydration step is
carried out in order to reduce the amount of electric current
required to form the anodic oxide dielectric layer and is normally
applied to foils to be anodized to 200 volts and above, as
described in U.S. Pat. No. 4,481,073. By carefully adjusting the
parameters of the pre-anodizing hydration process, as described in
U.S. Pat. No. 4,242,575, the hydration process may be successfully
employed with foils which are anodized to voltages significantly
less than 200 volts. The energy savings associated with the
pre-anodizing hydration process is sufficiently great that the vast
majority of aluminum foil manufactured today is processed in this
manner.
[0007] The crystallinity of the anodic oxide present on aluminum
anode foil is another factor directly determining the cost of the
foil for a given rating of capacitor. Crystalline anodic aluminum
oxide has a higher withstanding voltage per unit thickness than
does amorphous anodic aluminum oxide. As a result of the higher
withstanding voltage of crystalline oxide, only about 10 angstroms
of crystalline oxide is required to support each volt of applied
field during anodizing as compared with approximately 14 angstroms
for each volt of applied field for amorphous oxide. As a result of
the higher withstanding voltage of crystalline anodic aluminum
oxide, the capacitance of anode foil coated with crystalline oxide
may be as much as about 40% higher than anode foil anodized to the
same voltage but coated with amorphous oxide.
[0008] Crystalline anodic aluminum oxide may be readily produced by
anodizing aluminum anode foil in solutions containing salts of
dicarboxylic acids as the primary ionogen, as described in U.S.
Pat. No. 4,481,084. Anodic oxide formation in solutions of
dicarboxylic acid salts (generally at 70-95.degree. C.) may be
combined with a pre-anodizing foil hydration step to achieve a very
significant savings in both energy and foil consumed per unit
capacitance at a given anodizing voltage.
[0009] Hydration resistance, which is an important consideration
for foil used in electrolytic capacitors, may be enhanced by the
inclusion of a small amount of an alpha-hydroxy carboxylic acid
(such as tartaric acid or citric acid) in the anodizing electrolyte
solution, as described in U.S. Pat. No. 4,481,084. The tendency of
anodic aluminum oxide to absorb water, forming a variety of
hydrated species having impaired dielectric properties appears to
be, at least in part, a function of the hydration status of the
outermost portion of the anodic oxide at the end of the anodizing
process. Lilienfeld, in U.S. Pat. No. 2,826,724 states that "it is
the hydration stratum of the oxide film, adjacent the
film-electrolyte interface, which causes most of the power loss;
and that the progressive development of hydration at the interface
causes the aforesaid instability."
[0010] Alwitt, in U.S. Pat. No. 3,733,291, describes a method of
removing the residual hydration layer from the outer surface of
anodized aluminum capacitor foil which has been exposed to a
pre-anodizing hydration step (Alwitt refers to this as a "preboil")
prior to anodizing in order to conserve electrical energy during
anodizing. Alwitt employs a dilute phosphoric acid solution,
generally with a small chromate content (to inhibit corrosion), to
dissolve the outer, hydration layer.
[0011] In addition to the problems associated with the residual
hydration layer on anodized foil, which has been processed through
a pre-anodizing hydration or preboil step prior to anodizing, there
exists another potential problem with the stability of the anodic
oxide grown on preboiled aluminum foil. The formation of the anodic
oxide on preboiled foil takes place via a dehydration reaction in
which the layer of pseudoboehmite (i.e. hydration product) is
progressively dehydrated from the foil-oxide interface outward.
Apparently, the dehydration does not take place through the
ejection of water molecules but rather through the ejection of
hydrogen ions and the liberation of oxygen gas within the body of
the oxide. The liberated oxygen gas may become trapped within the
anodic oxide, rendering the oxide susceptible to cracking and
dielectric failure in service. This topic is treated well in the
article, entitled: "Trapped Oxygen in Aluminum Oxide Films and Its
Effect on Dielectric Stability", by Walter J. Bernard and Philip G.
Russell (Journal of the Electrochemical Society, Volume 127, number
6, June 1980, pages 1256-1261).
[0012] Stevens and Shaffer describe a method of determining the
concentration of oxide flaws as a function of distance from the
metal-oxide interface for trapped-oxygen flaws which are exposed
via thermal relaxation steps followed by re-anodizing under
carefully controlled and monitored conditions ("Defects in
Crystalline Anodic Aluminum", by J. L. Stevens and J. S. Shaffer,
Journal of the Electrochemical Society, volume 133, number 6, June
1986, pages 1160-1162).
[0013] Stabilization processes have been developed which tend to
expose and repair trapped oxygen flaws (in anodic oxide films on
preboiled foils) as well as impart hydration resistance to the
oxide film. Examples of these processes are described in U.S. Pat.
Nos. 4,113,579 and 4,437,946.
[0014] For maximum anodic oxide film stability on aluminum foil, it
is desirable to form the anodic film in a phosphate solution and,
again, for maximum stability (i.e., freedom from trapped oxygen
flaws) the foil should not be preboiled prior to the anodizing
process.
[0015] The skilled artisan has therefore been limited in the
ability to form oxides on the anode at high voltage, particularly
with phosphate incorporation into the oxide layer.
BRIEF SUMMARY OF THE INVENTION
[0016] It is object of the present invention to provide an improved
process for anodizing aluminium.
[0017] It is another object of the present invention to provide a
process for anodizing an aluminum surface at high voltage, over 220
volts, without pre-boil or surface hydration, while still
incorporating the advantages offered by phosphate in the oxide
layer. This has previously been unavailable to those of ordinary
skill in the art.
[0018] It is another object of the present invention to provide an
anodizing solution which can provide a capacitor with a capacitance
above 9 .mu.f-V/cm.sup.2 at 250 V which was previously not
available to the art.
[0019] A particular feature of the present invention is that one
variation of the electrolyte family described in U.S. Pat. No.
6,409,905, i.e, glycerine-based electrolytes containing
orthophosphate as the anionic portion of the ionogen may be used to
anodize aluminum foil to high voltages, for example 1000 volts. The
use of these electrolytes, then, overcomes the limitations of
traditional aqueous phosphate electrolytes in so far as the maximum
anodizing voltage achievable with aqueous electrolytes (i.e. 220
volts, as given in U.S. Pat. No. 3,733,291) may be exceeded by many
hundreds of volts. Furthermore, the use of low-water content
glycerine-based, orthophosphate-containing electrolyte solutions
for anodizing aluminum avoids the corrosion of the anode foil by
essentially eliminating the subsequent formation of aluminum
phosphate precipitates which normally occurs during the
anodization.
[0020] Another particular feature is that when the low-water
content, glycerine based electrolytes of U.S. Pat. No. 6,409,905
are used to anodize aluminum foil which has not been preboiled
(i.e. relatively hydrated-oxide free) an unanticipated high
capacitance value is obtained over prior art anodizing techniques
for the intermediate voltage anodizing range of about 250-350
volts.
[0021] A preferred embodiment is provided in a capacitor comprising
an aluminum anode and a dielectric layer comprising phosphate doped
aluminum oxide. The capacitor has a CV Product of at least 9
.mu.F-V/cm.sup.2 of surface area at 250 volts.
[0022] Yet another embodiment is provided in a process for
preparing a capacitor. The process comprises forming an aluminum
plate. Without pre-hydration the plate is contacted with an
anodizing solution comprising glycerine, 0.1 to 2.0%, by weight,
water and 0.01 to 0.5%, by weight, orthophosphate. A voltage is
applied to the aluminum plate of at least 220 volts.
[0023] Yet another embodiment is provided in process for preparing
a capacitor. The process comprises forming an aluminum plate. The
plate is contacted with an anodizing solution comprising glycerine,
0.1 to 2.0%, by weight, water and 0.01 to 0.5%, by weight,
orthophosphate. A voltage is applied to the aluminum plate and an
initial current is determined. The first voltage is maintained
until a first measured current is no more than 50% of the initial
current. The voltage is increased and initial current redetermined.
The increased voltage is maintained until a second measured current
is no more than 50% of the redetermined initial current. The
voltage increases and voltage maintaining are continued until a
final voltage is achieved.
[0024] A particularly preferred embodiment is provided in a
capacitor comprising an aluminum anode and a dielectric layer
comprising phosphate doped aluminum oxide. The capacitor has a CV
Product of at least 9 .mu.F-V/cm.sup.2 of surface area at 250
volts. Furthermore, the capacitor is formed by the process of:
forming an aluminum plate; contacting the plate with an anodizing
solution comprising glycerine, 0.1 to 2.0%, by weight, water and
0.01 to 0.5%, by weight, orthophosphate; applying a voltage to the
aluminum plate and determining an initial current; maintaining the
first voltage until a first measured current is no more than 50% of
the initial current; increasing the voltage and redetermining the
initial current; maintaining the increased voltage until a second
measured current is no more than 50% of the redetermined initial
current, and continuing the increasing of the voltage and
maintaining the increased voltage until a final voltage is
achieved.
BRIEF SUMMARY OF THE DRAWINGS
[0025] FIG. 1 is a graph of an embodiment of the present invention
illustrating the improvement of the present invention as indicated
by the graph of .mu.f-V/cm.sup.2 as a function of voltage. The area
(in cm.sup.2) is the surface area of the anode which increases with
an etched surface as known in the art.
[0026] FIG. 2 is a graph of an embodiment of the present invention
illustrating the improvement of the present invention as indicated
by the graph of .mu.f-V/cm.sup.2 as a function of voltage following
heat-treatment of the anodized coupons at 400.degree. C. for 15
minutes, followed by anodizing the original voltage in the original
solution for 1 hour.
[0027] FIG. 3 is a graph illustrating the impact of a hydrated
surface and the absence of the high capacitance observed when a
non-hydrated surface is treated in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The inventors of the present application have found that the
modification of the electrolytes described in U.S. Pat. No.
6,409,905 to be useful for the anodizing of aluminum foil to
several hundred volts. Generally speaking, glycerine solutions of
ammonium, amine, or alkali metal orthophosphate salts containing
from about 0.01 wt % to about 0.5 wt % soluble orthophosphate salt
and from about 0.1% to about 2.0% water, more preferably about 0.1%
to about 1.0% water, may be successfully used to anodize aluminum
foil to high voltages. Lower orthophosphate salt concentrations and
higher solution resistivities are preferably used for higher
anodizing voltages in accordance with the principles of aluminum
anodizing which have long been established by those familiar with
the art. For most high voltage applications, we have found it to be
advantageous to employ dibasic potassium phosphate as the ionogen,
at a preferred concentration of 0.01% to 0.1%, by weight, depending
upon the maximum desired voltage.
[0029] The electrolyte soluble orthophosphate salt may be an
ammonium phosphate, an alkali metal phosphate, an amine phosphate,
or mixtures thereof. Suitable alkali metal salts include, but are
not limited to, mono-sodium phosphate, di-potassium phosphate, and
sodium potassium phosphate. Suitable ammonium salts include, but
are not limited to, mono-ammonium phosphate or di-ammonium
phosphate.
[0030] The solution temperature employed may be varied over a wide
range, for example, from room temperature, or about 25.degree. C.,
to about 125.degree. C., but the temperature is most conveniently
maintained between about 80.degree. C. and 105.degree. C. In this
range (i.e. about 80.degree. C. and 105.degree. C.) the water
content of the electrolyte will tend to be automatically maintained
between about 0.2% and 1.0% by contact with the atmosphere through
the vapor pressure of the water present and the hygroscopicity of
the glycerine solvent.
[0031] It is preferable that the anode metal is placed into the
anodizing solution followed by sequentially increasing the voltage
stepwise with current age down prior to the next increment.
[0032] The voltage increase is preferably done in increments. The
maximum size of the increment is chosen to be less than that
necessary to create failure in the oxide. As the resistivity of the
anodizing solution increases the maximum voltage step which can be
implemented without oxide failure increases. Based on the present
invention, a voltage step of less than 75 volts is preferable.
Higher steps can be taken, particularly at higher voltages with
high resistivity anodizing solutions, yet the time required for
adequate age down increases and therefore no substantial benefit is
observed. Smaller voltage increases can be employed with the
disadvantage being loss of efficiency. It is most desired that the
voltage increase be at least 20 volts per step to optimise the
efficiency without compromising product quality. A voltage increase
of about 50 volts for each step has been determined to be optimal
for the present invention.
[0033] After each voltage increase the voltage is maintained until
a sufficient decrease in current is realized. The more the current
is allowed to decrease prior to the next voltage increase the
better for efficiency of anodization yet a decrease is observed in
productivity. It is preferred that the anode be maintained at
voltage long enough to allow the current to decrease to at least
less than 50% of the original current and more preferably at least
30% of the original current. The upper limit of hold time for
current decrease is based on efficiency. Allowing the current to
decrease to 1%, or less, of the original current is acceptable yet
the loss in efficiency exceeds the advantages obtained. It is most
preferred that the voltage be maintained at each step for a time
sufficient to allow the current to decrease to about 10-30% of the
original current. This has been determined to be an optimal
condition between suitable product and manufacturing efficiency. It
has been found that a decrease in current to at least about 20% of
the original current at each voltage step is optimum to achieve
superior product performance with reasonable manufacturing
efficiency. The current may be allowed to decrease to a low level
at the last voltage step in order to obtain a very low leakage
dielectric film.
[0034] The process for manufacturing a stacked foil conductive
polymer is known in the art. Specifically, stacked foil conductive
polymer-containing solid capacitors may be treated with the
inventive solution to produce an anodic oxide film on the edges of
the coupon, repair any cracks in the anodic oxide from handling,
and impart hydration resistance to the anodic oxide already present
on the coupon.
[0035] The stacked foil conductive polymer-containing solid
capacitors are typically prepared from anode foil coupons cut from
etched and anodized foil and mounted on carrier bars, by welding or
similar means, for processing.
[0036] In a particularly preferred embodiment coupons are cut and
welded to a process bar. Masking is applied to prevent wicking of
the materials used to produce the conductive polymer into the weld
zone of the coupons.
[0037] The coupons are then immersed in an anodizing electrolyte of
the present invention and are processed as described above.
[0038] The edge-anodized and rinsed coupons are then ready for
processing into capacitors.
EXAMPLES
[0039] A series of aluminum coupon anodizing runs was conducted
using a solution of dibasic potassium phosphate, K.sub.2HPO.sub.4,
water and glycerine, within the concentrations of the present
invention, at a temperature of 95.degree. C..+-.5.degree. C. The
maximum voltage of each anodizing run was increased by 50 volts per
run, from 50 to 1000 volts. The voltage used for each run was
applied in a series of 50-volt steps. The current was allowed to
"age-down" to below 20% of the initial value at each voltage step
before again raising the voltage.
[0040] The concentration of K.sub.2HPO.sub.4 varied with voltage
with 0.05% being used for the first half of the series of coupons
and 0.01% K.sub.2HPO.sub.4 for the higher voltages.
[0041] The coupon capacitance was measured for each anodizing
voltage and the CV product (capacitance.times.voltage) was
calculated per cm.sup.2 of surface area throughout the formation
voltage range. The results are provided in FIG. 1.
[0042] FIG. 1 shows that the CV/cm.sup.2 product for plain aluminum
foil is approximately 5 microfarad-volts per square centimeter for
the first 200 volts, then the CV product jumps to approximately 14
and decreases back to the baseline of about 5 CV/cm.sup.2 product
over the next 150 volts. This unanticipated increase is thought to
be due to a structural rearrangement within the oxide as 250
anodizing volts are approached. Anode coupons held as long as 15
hours at voltage still show this anomalously high CV product at 250
anodizing volts.
[0043] The anomalously high CV at 250 volts is apparent, though
smaller, even following a thermal relaxation step at 400.degree. C.
(15 minutes) followed by a second anodizing step in which the
coupons are held at the original anodizing voltages for an hour, as
shown in FIG. 2.
[0044] If the foil is first preboiled prior to anodizing in the
electrolytes of the present invention, the anodizing proceeds
smoothly up to very high voltages, but the anomalous CV behaviour,
at 250 volts, observed with un-preboiled foil is absent.
[0045] FIG. 3 shows the results obtained with coupons which were
exposed to water at 95.degree. C..+-.3.degree. C. for 5 minutes
prior to anodizing. These coupons were anodized in the same manner
as those of FIG. 1. In this case the CV product of approximately 7
microfarad-volts/cm.sup.2 is that commonly found for crystalline
anodic aluminum oxide and no anomaly is seen.
[0046] We have, then, found that orthophosphate salt solutions in
glycerine may be used to anodize aluminum foil to at least 1000
volts.
[0047] We have found that foil anodized in these solutions exhibits
an anomalously high capacitance at 250-350 volts. This anomaly is
probably due to an oxide structure change in these solutions at
about the 250 volt anodizing voltage.
[0048] We have demonstrated that this anomaly is not observed with
"preboiled" foil.
[0049] We have found that both very high voltages (i.e., 1,000
volts) and very high capacitance (at 250 volts) are made possible
through the use of solutions which are self maintaining from the
standpoint of water content (i.e. they stabilize at about 0.2 to
1.0% at 80-105.degree. C.).
[0050] Furthermore, analysis of the highest-voltage solution used
to prepare the coupons for FIG. 1 was found to contain only 2 ppm
aluminum after the anodizing work was completed, signifying an
almost complete elimination of the corrosion associated with prior
art aqueous phosphate anodizing solutions.
[0051] The invention has been described with particular emphasis on
the preferred embodiments. It would be realized from the teachings
herein that other embodiments, alterations, and configurations
could be employed without departing from the scope of the invention
which is more specifically set forth in the claims which are
appended hereto.
* * * * *