U.S. patent application number 10/816363 was filed with the patent office on 2005-10-06 for anodizing electrolytes for high voltage capacitor anodes.
Invention is credited to Liu, Yanming, Scheuer, Christina.
Application Number | 20050218005 10/816363 |
Document ID | / |
Family ID | 34934591 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050218005 |
Kind Code |
A1 |
Liu, Yanming ; et
al. |
October 6, 2005 |
Anodizing electrolytes for high voltage capacitor anodes
Abstract
The present invention is a low temperature (below about
60.degree. C.) anodizing electrolyte composition for valve metals
including, and not limited to, aluminum, niobium, titanium,
tantalum, zirconium, and alloys thereof. The low temperature
anodizing electrolyte composition contains at least: (1) a protic
solvent selected from the group consisting of alkylene glycols,
polyalkylene glycols, and their mono ethers, and (2) a weak
inorganic or organic acid or its salt. The present invention is
capable of anodizing valve metals for high voltage capacitors of
greater than 300 Volts with: (1) little to no gray-out at high
formation voltage, (2) high formation breakdown voltage, and (3)
high quality of oxide with low DC leakage and stable long term
performance of the anode.
Inventors: |
Liu, Yanming; (Clarence
Center, NY) ; Scheuer, Christina; (Amherst,
NY) |
Correspondence
Address: |
WILSON GREATBATCH TECHNOLOGIES, INC.
10,000 WEHRLE DRIVE
CLARENCE
NY
14031
US
|
Family ID: |
34934591 |
Appl. No.: |
10/816363 |
Filed: |
April 1, 2004 |
Current U.S.
Class: |
205/261 ;
205/332 |
Current CPC
Class: |
C25D 11/06 20130101;
H01G 9/0032 20130101; C25D 11/26 20130101 |
Class at
Publication: |
205/261 ;
205/332 |
International
Class: |
C25D 011/26 |
Claims
What is claimed is:
1. A low temperature anodizing electrolyte for anodizing a valve
metal or an alloy thereof, comprising: a) a protic solvent selected
from the group consisting of alkylene glycols, polyalkylene
glycols, alkylene or polyalkylene glycol mono ethers, and
combinations thereof; and b) a weak inorganic or organic acid, or
its salt; c) wherein the protic solvent and the weak inorganic or
organic acid, or its salt, are at a predetermined ratio of volume
or weight percentage of the electrolyte; and d) wherein the
electrolyte is used at a low temperature for anodizing valve metals
that results in little to no gray-out.
2. The electrolyte of claim 1 wherein the low temperature is about
60.degree. C. and below.
3. The electrolyte of claim 1 wherein the weak inorganic or organic
acid or its salt is selected from the group consisting of
phosphoric acid, ammonium dihydrogen phosphate, boric acid,
ammonium borate, acetic acid, ammonium acetate, other organic acids
or their salts, and combinations thereof.
4. The electrolyte of claim 1 wherein the alkylene glycols are
selected from the group consisting of ethylene glycol, diethylene
glycol, triethylene glycol, tetraethylene glycol, propylene glycol,
trimethylene glycol, diepropylene glycol, glycerol,
2-methyl-1,3-propanediol, 1,4-butanediol, 2,3-butanediol,
2,2-dimethyl-1,3-propanediol, 2,4-pentanediol, 2,5-hexanediol, and
combinations thereof; the polyalkylene glycols are selected from
the group consisting of polyethylene glycols, polypropylene
glycols, polyethylenepropylene glycol copolymers, and combinations
thereof; the alkylene or polyalkylene glycol mono ethers are
selected from the group consisting of ethylene glycol methyl ether,
ethylene glycol ethyl ether, ethylene glycol butyl ether,
diethylene glycol ethyl ether, diethylene glycol methyl ether,
diethylene glycol butyl ether, dipropylene glycol methyl ether,
tripropylene glycol methyl ether, and combinations thereof.
5. The electrolyte of claim 1 wherein the protic solvent is up to
about 90 volume percentage of the electrolyte.
6. The electrolyte of claim 5 wherein the protic solvent ranges
from about 60 to about 85 volume percentage of the electrolyte.
7. The electrolyte of claim 1 wherein the protic solvent has a
molecular weight of less than about 1000.
8. The electrolyte of claim 1 wherein the weak inorganic or organic
acid, or its salt, is up to about 15 volume or weight percentage of
the electrolyte.
9. The electrolyte of claim 8 wherein the weak inorganic or organic
acid, or its salt, ranges from about 1 to about 10 volume or weight
percentage of the electrolyte.
10. The electrolyte of claim 2 wherein the low temperature is
around 60.degree. C. and below.
11. A low temperature-anodizing electrolyte for anodizing a valve
metal or an alloy thereof, comprising: a) a protic solvent selected
from the group consisting of alkylene glycols, polyalkylene
glycols, alkylene or polyalkylene glycol mono ethers, and
combinations thereof and having a molecular weight of less than
about 1000; b) a weak inorganic or organic acid or its salt; and c)
the low temperature is below about 60.degree. C.; d) wherein the
protic solvent and the weak inorganic or organic acid or its salt
are at a predetermined ratio of volume or weight percentage of the
electrolyte; and e) wherein the electrolyte is used at a low
temperature for anodizing valve metals that results in little to no
gray-out.
12. The electrolyte of claim 11 wherein the weak inorganic or
organic acid, or its salt, is selected from the group consisting of
phosphoric acid, ammonium dihydrogen phosphate, boric acid,
ammonium borate, acetic acid, ammonium acetate, other organic acids
or their salts, and combinations thereof.
13. The electrolyte of claim 11 wherein the alkylene glycols are
selected from the group consisting of ethylene glycol, diethylene
glycol, triethylene glycol, tetraethylene glycol, propylene glycol,
trimethylene glycol, diepropylene glycol, glycerol,
2-methyl-1,3-propanediol, 1,4-butanediol, 2,3-butanediol,
2,2-dimethyl-1,3-propanediol, 2,4-pentanediol, 2,5-hexanediol, and
combinations thereof; the polyalkylene glycols are selected from
the group consisting of polyethylene glycols, polypropylene
glycols, polyethylenepropylene glycol copolymers, and combinations
thereof; the alkylene or polyalkylene glycol mono ethers are
selected from the group consisting of ethylene glycol methyl ether,
ethylene glycol ethyl ether, ethylene glycol butyl ether,
diethylene glycol ethyl ether, diethylene glycol methyl ether,
diethylene glycol butyl ether, dipropylene glycol methyl ether,
tripropylene glycol methyl ether, and combinations thereof.
14. The electrolyte of claim 11 wherein the protic solvent ranges
from about 60 to about 85 volume percentage of the electrolyte.
15. The electrolyte of claim 11 wherein the weak inorganic or
organic acid or its salt ranges from about 1 to about 10 volume or
weight percentage of the electrolyte.
16. A process for anodizing a valve metal or an alloy thereof
comprising conducting the anodization at a temperature around and
below 60.degree. C. in an electrolyte, comprising the steps of: a)
a protic solvent selected from the group consisting of alkylene
glycols, polyalkylene glycols, alkylene or polyalkylene glycol mono
ethers and combinations thereof and having a molecular weight of
less than about 1000; and b) a weak inorganic or organic acid or
its salt; c) wherein the protic solvent and the weak inorganic or
organic acid or its salt are at a predetermined ratio of volume or
weight percentage of the electrolyte; and d) wherein the
electrolyte is used at a low temperature for anodizing valve metals
that results in little to no gray-out.
17. The process of claim 16 wherein the weak inorganic or organic
acid, or its salt, is selected from the group consisting of
phosphoric acid, ammonium dihydrogen phosphate, boric acid,
ammonium borate, acetic acid, ammonium acetate, other organic acids
or their salts, and combinations thereof.
18. The process of claim 16 wherein the alkylene glycols are
selected from the group consisting of ethylene glycol, diethylene
glycol, triethylene glycol, tetraethylene glycol, propylene glycol,
trimethylene glycol, diepropylene glycol, glycerol,
2-methyl-1,3-propanediol, 1,4-butanediol, 2,3-butanediol,
2,2-dimethyl-1,3-propanediol, 2,4-pentanediol, 2,5-hexanediol, and
combinations thereof; the polyalkylene glycols are selected from
the group consisting of polyethylene glycols, polypropylene
glycols, polyethylenepropylene glycol copolymers, and combinations
thereof; the alkylene or polyalkylene glycol mono ethers are
selected from the group consisting of ethylene glycol methyl ether,
ethylene glycol ethyl ether, ethylene glycol butyl ether,
diethylene glycol ethyl ether, diethylene glycol methyl ether,
diethylene glycol butyl ether, dipropylene glycol methyl ether,
tripropylene glycol methyl ether, and combinations thereof.
19. The process of claim 16 wherein the protic solvent ranges from
about 60 to about 85 volume percentage of the electrolyte.
20. The process of claim 16 wherein the weak inorganic or organic
acid or its salt ranges from about 1 to about 10 volume or weight
percentage of the electrolyte.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a low temperature-anodizing
electrolyte and to an electrochemical process for anodizing valve
metals with that electrolyte.
[0003] 2. Prior Art
[0004] Electrolytic capacitors contain anodes that are valve metal
coated with its corresponding oxide (the dielectric) by anodizing.
The anodizing electrolyte composition and the process (the
protocol) of forming the anodes are crucial for the electrical
properties and performance of the anodes or the capacitors,
especially for high voltage capacitors.
[0005] Anodizing electrolyte compositions have long been used to
anodize valve metals for use in electrolytic capacitors. Melody et
al. disclose, in U.S. Pat. No. 5,716,511 at col. 1, lines 49-60,
one such 1950 brand of anodizing electrolyte composition. In
particular, Melody et al. wrote, "After post-sintering separation
and attachment to carrier strips or bars, the anodes are suspended
in an electrolyte solution and anodized under appropriate current
density to produce the anodic oxide dielectric. The anodizing step
may be carried out at a temperature up to about 95.degree. C. in an
electrolyte which typically consists of a . . . mixed
aqueous/ethylene glycol solution of a . . . salt of a mineral acid
such as phosphoric . . . acid. Electrolytes which tend to give the
best results (i.e. highest dielectric quality) often contain 50-60
[volume percent of] ethylene glycol or polyethylene glycol and 0.5
to 2 or more [volume percent of] phosphoric acid and are maintained
at a temperature between 80.degree. and 90.degree. C."
[0006] Melody et al. further wrote, "The organic solvents
traditionally used to anodize [valve] metal anodes, ethylene glycol
and polyethylene glycols, . . . tend to have serious disadvantages.
[Some of these disadvantages include and are not limited to]
ethylene glycol is toxic, . . . [and] glycols and polyglycols tend
to be viscous at lower temperatures." (See col. 3, lines 44-50;
bracketed material added for clarity.)
[0007] Viscosity is a critical feature of Melody et al.'s anodizing
electrolyte composition in U.S. Pat. No. 5,716,511. It is critical
because Melody et al. in the '511 patent want to lower the
temperature of the electrolyte composition during the anodization
process. By lowering the temperature below 50.degree. C., Melody et
al. claim there is less chance to create flaws on the anodized
valve metal. Melody et al., however, report that only using organic
solvents that have a low viscosity can accomplish anodizing at such
low temperatures.
[0008] In the '511 patent, Melody et al. only disclose that
polyethylene glycol dimethyl ethers having from 4 to 10 repeating
ethylene groups (generically referred to as "polyglycol di-ethers")
are acceptable low viscosity solvents for anodizing electrolyte
compositions. To confirm this, Melody et al. reported that
polyethylene glycol dimethyl ethers have a viscosity at cps at
20.degree. C. of 4.1 and ethylene glycol and polyethylene glycol
300 have viscosities of 20.9 and 75, respectively. Melody et al.
asserted that the viscosity values of glycols, polyglycols, and
higher alkyl ethers of the polyethylene glycols, such as diethyl,
dipropyl or dibutyl ethers were unacceptable for use with anodizing
electrolyte solutions at temperatures below 50.degree. C. In
particular, Melody et al. stated that (1) glycols and polyglycols
become too viscous at such low temperatures, and (2) polyethylene
glycol higher dialkyl ethers, such as diethyl, dipropyl or dibutyl
ethers do not provide the requisite solubility and low viscosity.
(See col. 3, line 44 to col. 4, line 2, and col. 4, lines 62 to
66.)
[0009] In view of these negative teachings of using organic
solvents other than polyethylene glycol dimethyl ether for
anodizing electrolyte compositions, Melody et al. compared
anodizing electrolyte compositions being maintained between
80.degree. C. and 90.degree. C. and having different organic
solvents. The results were as follow:
1 Organic Solvent Breakdown Voltage Ethylene glycol 240 Volts
Polyethylene glycol 300 260 Volts Methoxypolyethylene glycol 350
285 Volts Tetraethylene glycol dimethyl ether 500 Volts
[0010] Clearly, Melody et al. in U.S. Pat. No. 5,716,511 teach that
only polyethylene glycol dimethyl ether having from 4 to 10
repeating ethylene groups can be used as anodizing electrolyte
compositions at temperatures below 50.degree. C. to obtain desired
anodization results of breakdown voltages exceeding 300 Volts.
[0011] In U.S. Pat. No. 6,261,434 (application filing date is
post-issuance of U.S. Pat. No. 5,716,511) Melody et al. discloses
another method for anodizing valve metals. In this patent, Melody
et al. set forth that "[t]he electrolyte temperature is known to
those skilled and is typically from about 80.degree. C. to about
90.degree. C." (col. 6, lines 29-30). They then revert to using a
modified anodizing electrolyte solution and simultaneously a
conventional anodizing electrolyte solution of ethylene glycol and
phosphoric acid. In other words, it appears that in a later filed
patent application Melody et al. are teaching away from using
anodizing electrolyte solutions at temperatures below 50.degree.
C.
[0012] Returning to U.S. Pat. No. 5,716,511, Melody et al. also
teach that the organic solvent of polyethylene glycol dimethyl
ether having from 4 to 10 repeating ethylene groups in the
anodizing electrolyte can range from 10 to 75 volume percent. In
the examples, however, the organic solvent's volume percentage is
never over 60. This value is interesting because Melody et al.
confirms (col. 1, lines 56-58) that to obtain the best results
(i.e. highest dielectric quality) for an anodizing electrolyte
solution, the organic solvent should range from 50 to 60 volume
percent--which his best examples follow. In other words, Melody et
al. clearly teach away from increasing the volume percentage of the
organic solvent in an anodizing electrolyte solution to exceed 60
volume percent.
[0013] The "gray-out" on an anode is a well-known phenomenon
observed in anodizing tantalum, especially for high voltage
anodizing. It is a result of the formation of crystalline tantalum
oxide having high DC leakage, and is undesirable. Electrolyte
composition and formation protocol are crucial in preventing the
"gray-out".
[0014] Anodizing breakdown voltage and the quality of the anodic
oxide depend on the electrolyte conductivity, which, in turn, is a
function of the solvent composition and the solute concentration.
Normally, increasing the electrolyte conductivity decreases the
anodizing breakdown voltage. The electrolyte must have a breakdown
voltage well above the intended anodizing voltage. On the other
hand, too low an electrolyte conductivity results in excessive
heating of the electrolyte and the anode pellet that would cause
"gray-out", breakdown, or poor quality of the oxide. The low
conductivity electrolyte also causes high voltage drop and renders
the anodizing impractical. Suitable electrolyte conductivity is
required to have sufficient breakdown voltage while still allowing
formation of a high quality oxide.
[0015] These problems are solved by the present invention.
BRIEF DESCRIPTION OF THE FIGURES
[0016] FIGS. 1 to 3 illustrate the anode electrical properties (DCL
in nA/CV, AC Capacitance) and electrolyte conductivity (mS/cm) of
the present invention.
SUMMARY OF THE INVENTION
[0017] The present invention is a low temperature (below around
60.degree. C.) electrolyte composition for anodizing valve metals
including, and not limited to, tantalum, aluminum, niobium,
titanium, zirconium, hafnium, and alloys thereof. The low
temperature anodizing electrolyte composition contains at least:
(1) a protic solvent selected from the group consisting of alkylene
glycols, polyalkylene glycols, and their mono ethers and (2) a weak
inorganic or organic acid or its salt.
[0018] The electrolyte of the present invention is capable of
anodizing valve metals for high voltage capacitors of greater than
300 Volts with: (1) little to no gray-out at high formation
voltage, (2) high formation breakdown voltage, and (3) high quality
of oxide with low DC leakage and stable long term performance of
the anode.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0019] The present invention is directed to an electrolyte for
anodizing valve metals, particularly for use in high voltage
electrolytic capacitors. The valve metals are anodized in this
electrolyte at temperatures below sixty degrees Celsius. Such an
electrolyte allows for the anodizing of valve metals with little
gray-out at high voltages and minimal breakdown. The resulting high
quality oxide has low DC leakage and provides stable long-term
anode performance.
[0020] Valve metals include, and are not limited to, tantalum,
aluminum, niobium, titanium, zirconium, hafnium, and alloys
thereof. When such valve metals are used as an anode in an
electrolytic capacitor, they can be in the form of foil (etched or
unetched), pressed and sintered powder pellet or other porous
structures. For tantalum electrolyte capacitor, the tantalum anodes
are typically in the form of a pressed/sintered tantalum powder
pellet. Beam melt, sodium reduction, or other processes produce the
tantalum powders. This invention is particularly useful for
anodizing sintered powder pellets at high voltage.
[0021] Regardless of the process by which the valve metal powder is
processed, pressed valve metal powder structures, and particularly
tantalum pellets, are anodized in formation electrolytes. The
present invention electrolyte comprises:
[0022] A. Up to about 90 volume percentage, preferably about 60 to
about 85 volume percentage, of a protic solvent selected from the
group consisting of alkylene glycols (examples include and are not
limited to ethylene glycol; diethylene glycol; triethylene glycol;
tetraethylene glycol; propylene glycol; trimethylene glycol;
dipropylene glycol; glycerol; 2-methyl-1,3-propanediol;
1,4-butanediol; 2,3-butanediol; 2,2-dimethyl-1,3-propanediol;
2,4-pentanediol; 2,5-hexanediol; and combinations thereof),
polyalkylene glycols (examples include and are not limited to
polyethylene glycols; polypropylene glycols; polyethylenepropylene
glycol copolymers; and combinations thereof), and alkylene or
polyalkylene glycol monoethers (examples include and are not
limited to ethylene glycol methyl ether; ethylene glycol ethyl
ether; ethylene glycol butyl ether; diethylene glycol ethyl ether;
diethylene glycol methyl ether; diethylene glycol butyl ether;
dipropylene glycol methyl ether; tripropylene glycol methyl ether;
and combinations thereof) and preferably having a molecular weight
of less than 1000;
[0023] B. Up to about 15 volume percentage, preferably about 1 to
about 10 volume or weight percentage, of a weak organic or
inorganic acid or its salt thereof, examples include and are not
limited to phosphoric acid, ammonium dihydrogen phosphate, boric
acid, ammonium borate, acetic acid, ammonium acetate, other organic
acids or their salts, and combinations thereof;
[0024] C. Water; and
[0025] D. Maintained at a temperature about 60.degree. C. and
below.
[0026] Such an electrolyte has a conductivity of about 0.05 mS/cm
to about 10 mS/cm at 40.degree. C., preferably 0.1 mS/cm to about 5
mS/cm. Specific examples with different electrolyte compositions
are recorded in Table 1, and FIGS. 1-3.
[0027] The formation protocol for this process is disclosed in U.S.
Pat. No. 6,231,993 to Stephenson et al., which is (a) commonly
assigned to the present assignee and (b) hereby incorporated by
reference in its entirety. After the formation process, the anode
is exposed to conventional heat treatment and reformation
procedures. Both of these procedures are well known to those of
ordinary skill in the capacitor art.
[0028] Concentrations shown in Table 1 (Tables 1A to 1C are
portions of Table 1 separated by volume percentages of the weak
acids for comparison purposes) are volume percentage. These tables
clearly illustrate that the present invention, especially in the
preferred embodiments, is capable of anodizing valve metals for
high voltage capacitors of greater than 300 Volts with:
[0029] 1) Little to no gray-out at high formation voltage,
[0030] 2) Prevented or reduced breakdown failure, and
[0031] 3) High quality of oxide with low DC leakage and stable
long-term performance of the anode.
2TABLE 1 Anode Properties As A Function Of Electrolyte Composition
PEG Conductivity 5 min DCL 120 Hz 400 H.sub.3PO.sub.4 mS/cm @
40.degree. C. nA/CV @ 360 V Capacitance 47.6 4.1 2.550 2.068 124 50
2 1.150 1.433 125.8 50 6 2.990 3.018 124.4 50 10 6.050 Breakdown @
330 V during formation 65.7 2 0.237 0.920 126.2 65.7 6 0.772 0.639
125.7 65.7 6 0.772 0.911 125.0 65.7 10 1.523 Breakdown @ 375 V
during formation 70.1 6.5 0.260 0.610 123.6 80 2.7 0.046 0.634
128.6 80 9.3 0.276 0.391 124.4 80 9.3 0.207 0.481 124.3
[0032]
3TABLE 1A Phosphoric Acid Being Below 3 Volume Percent Of The
Electrolyte PEG Conductivity 5 min DCL 120 Hz 400 H.sub.3PO.sub.4
mS/cm @ 40.degree. C. nA/CV @ 360 V Capacitance 47.6 4.1 2.550
2.068 124 50 2 1.150 1.433 125.8 65.7 2 0.237 0.920 126.2 80 2.7
0.046 0.634 128.6
[0033]
4TABLE 1B Phosphoric Acid Being Around 5 to 7 Volume Percent Of The
Electrolyte PEG Conductivity 5 min DCL 120 Hz 400 H.sub.3PO.sub.4
mS/cm @ 40.degree. C. nA/CV @ 360 V Capacitance 50 6 2.990 3.018
124.4 65.7 6 0.772 0.639 125.7 65.7 6 0.772 0.911 125.0 70.1 6.5
0.260 0.610 123.6
[0034]
5TABLE 1C Phosphoric Acid Being Above 7 And Below 10 Volume Percent
Of The Electrolyte PEG Conductivity 5 min DCL 120 Hz 400
H.sub.3PO.sub.4 mS/cm @ 40.degree. C. nA/CV @ 360 V Capacitance 50
10 6.050 Breakdown @ 330 V during formation 65.7 10 1.523 Breakdown
@ 375 V during formation 80 9.3 0.276 0.391 124.4 80 9.3 0.207
0.481 124.3
[0035] The information presented in Table 1 (and highlighted in
Tables 1A-C) confirms that a suitable ratio of solvent and solute
is needed to obtain low DC leakage (nA/CV) and prevent breakdown
during formation. A low conductivity results from a higher ratio of
solvent and solute.
[0036] FIGS. 1 to 3 show contour plots for DC leakage in nA/CV, AC
capacitance, and electrolyte conductivity (mS/cm), respectively,
for the present electrolyte. These figures provide further support
that the present invention is a superior anodizing electrolyte in
comparison to that currently known. FIG. 1 shows that a higher
particular protic solvent concentration gives lower DC leakage.
Decreasing certain protic solvent content increases DC leakage and
causes breakdown (see Table 1) and gray-out. FIGS. 1 to 3 show that
increasing protic solvent decreases electrolyte conductivity and
slightly increases AC capacitance.
[0037] Conventional practice has been to form the valve metal to a
target formation voltage at a constant current or varied current,
with or without rest steps. The formation protocol and the current
used depend on the electrolyte, the valve metal powder type and the
size of the valve metal structure. Adjusting these parameters
according to conventional practice is well within the knowledge of
those skilled in the art.
[0038] The present invention is an improved electrolyte and thus
process for providing an anodic dielectric coating on a valve metal
structure. The present invention is directed to a high solvent
content anodizing electrolyte and method of anodizing at a low
temperature. The combination of this electrolyte composition and
low temperature anodizing allows anodizing valve metals for high
voltage capacitors of greater than 300 Volts with little to no
gray-out at high anodizing voltage, reduced failure during
anodizing and high quality of oxide with low DC leakage and stable
long term performance of the anode.
[0039] This electrolyte composition is capable of anodizing valve
metals for high voltage capacitors of greater than 300 Volts with
the following characteristics:
[0040] 1) Little to no gray-out at high formation voltage,
[0041] 2) Diminished to no breakdown, and
[0042] 3) High quality of oxide with low DC leakage and stable
long-term performance of the anode.
[0043] It is appreciated that various modifications to the present
inventive concepts described herein may be apparent to those of
ordinary skill in the art without departing from the spirit and
scope of the present invention as defined by the herein appended
claims.
* * * * *