High temperature electrolytic capacitor

A high-temperature aluminum electrolytic capacitor contains an electrolyte of mono(di-n-propylammonium) adipate or mono(diisopropylammonium) adipate, a phosphate salt, and water dissolved in ethylene glycol to provide a capacitor capable of operating at least at 200 VDC and 130.degree. C.

BACKGROUND OF THE INVENTION 
This invention relates to an aluminum electrolytic capacitor capable of 
operation at 200 VDC or higher at an ambient temperature of 130.degree. C. 
through the use of an electrolyte containing mono(di-n-propylammonium) 
adipate or mono(diisopropylammonium) adipate as solute, and a phosphate 
salt and water dissolved in ethylene glycol as solvent. 
Heretofore, electrolytes for aluminum electrolytic capacitors operating at 
200 V or higher most commonly contained salts of boric acid or boric acid 
derivatives as the solute in ethylene glycol as solvent. The maximum 
operating temperature for such an electrolyte system is less than 
100.degree. C. and normally 65.degree.-85.degree. C. The temperature 
limitation is due to the facile reaction of glycol with boric acid and 
other borate species to form polymeric glycol-borates and water. The 
minimum operating temperatures are above -20.degree. C. inasmuch as glycol 
freezes at -17.4.degree. C. 
The effective temperature operating range of aluminum electrolytic 
capacitors has been expanded in both directions by replacing the glycol 
solvent with N,N-dimethylformamide (DMF) which has a boiling point of 
153.degree. C. and a freezing point of -61.degree. C. There are known 
prior art DMF electrolytes that can be effectively used over the 
temperature range -55.degree. C. to 125.degree. C. However, DMF is a very 
aggressive solvent and attacks most materials of construction. The most 
resistant material for sealing gaskets and O-rings is Butyl rubber. 
Unfortunately, DMF will be transmitted through a Butyl rubber closure at a 
rate that increases with increasing temperature, thus limiting the life of 
the capacitor since the capacitors will not function adequately when 
approximately one-half the solvent has been lost. 
DMF also has a flash point of 67.degree. C. making it undesirable for use 
as solvent in capacitors that are to be used in confined spaces. In 
contrast, glycol has a boiling point of 197.2.degree. C. and a flash point 
of 116.degree. C. and is much easier to contain. Rates of transmission of 
glycol through both Butyl rubber and ethylene-propylene rubber (EPR) are 
almost negligible. 
For current power supply operations, it is desirable to provide an aluminum 
electrolytic capacitor capable of operating continuously at 200 VDC or 
higher at an ambient temperature of 130.degree. C. with modest low 
temperature properties. 
It would be desirable to use ethylene glycol as solvent for the reasons 
given above. If glycol is used, then the solute can not be boric acid or a 
borate because of its reaction with glycol as described above. The solute 
should be one that will not react with glycol or any other cosolvent that 
might be used. The solute must also be stable at operating temperatures of 
130.degree. C., and at somewhat higher temperatures. 
The major cause of resistivity increase in an electrolyte is amide 
formation, particularly where the solute is an ammonium or substituted 
ammonium salt of a dicarboxylic acid. For example, diammonium adipate, a 
known solute for electrolytic capacitor electrolytes, will rapidly form 
adipamide, a non-conducting species, at 125.degree. C. when used in an 
ethylene glycol solvent. Since adipamide is insoluble in glycol, this 
reaction is readily detected. For other salts that undergo amide formation 
but form soluble amides, the reaction can be detected by increases in 
resistivity. Amide formation proceeds most readily with ammonium salts, 
and more readily with salts of primary amines than with salts of secondary 
amines. Amide formation is most difficult with salts of tertiary amines, 
as a carbon-nitrogen bond must be broken for it to proceed. 
SUMMARY OF THE INVENTION 
An aluminum electrolytic capacitor capable of continuous operation at 200 
VDC or higher and 130.degree. C. is provided by the use of an electrolyte 
system of mono(di-n-propylammonium) adipate or mono(diisopropylammonium) 
adipate as solute, a phosphate salt, and water dissolved in ethylene 
glycol as solvent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, wound capacitor section 10 consists of anode foil 11 
of aluminum having on its surface an insulating oxide barrier layer. 
Cathode foil 13 is also aluminum. Electrolyte absorbent layers 12 and 14, 
preferably paper, are positioned between the anode foil 11 and cathode 
foil 13 and interwound therewith. Tabs 15 and 16 are connected to 
electrodes 11 and 13, respectively, to provide for connection of the 
electrodes to leads. When completely wound, section 10 is impregnated with 
an electrolyte (not shown) of this invention. 
FIG. 2 shows a cross-section of an axial capacitor in which the cathode tab 
16 of capacitor section 10 is welded at 23 to cathode lead 24. Anode tab 
15 is welded to portion 17 of insert 18 positioned in bushing 19 and 
welded at 20 to anode lead 21. Electrolyte (not shown) impregnates section 
10. 
The electrolyte is a solution in ethylene glycol of 
mono(di-n-propylammonium) adipate or mono(diisopropylammonium) adipate, a 
phosphate salt, and water. 
As pointed out earlier, for operation at 130.degree. C., the electrolyte 
must be stable at 130.degree. C. and preferably stable at somewhat higher 
temperatures. For this reason, 150.degree. C. was chosen for the stability 
screening test. 
The desired room-temperature resistivity of the electrolyte depends on the 
voltage rating of the capacitor and the operating temperatures to which 
the capacitor will be subjected. For 200 VDC capacitors operating at 
130.degree. C., the room-temperature resistivity should be at least 700 
.OMEGA.-cm and approximately 700-800 .OMEGA.-cm; preferably, this 
resistivity should increase to no more than 1200 .OMEGA.-cm after 1000 hr 
at 150.degree. C. For higher voltage capacitors, electrolytes with higher 
resistivities should be employed, and for lower voltage capacitors, 
electrolytes with lower resistivities. 
The electrolyte must have a maximum anodization voltage at least equal to 
the rated voltage of the capacitor at the operating temperature, i.e. at 
least 200 V at 130.degree. C. for a 200 V capacitor, to be able to reform 
damaged barrier oxide layer on the anode foil during operation. For 
operating voltages of 200 V and higher, it has been found that a phosphate 
salt must be included in these electrolytes to insure continuous 
operation. 
Thirteen different salts, ammonium or substituted-ammonium, were evaluated 
in a thermal stability screening test conducted at various temperatures 
ranging from 105.degree. C. to 150.degree. C. The test involves measuring 
the room-temperature resistivity of the electrolytes, sealing samples of 
the electrolytes in glass tubes, and heating the sealed tubes to the test 
temperature. At approximately 500 hr intervals, samples are withdrawn and 
cooled to room temperature; the resistivity is measured at room 
temperature. Of the 13 salts tested, 7 gave electrolytes that were 
satisfactory at 150.degree. C. The seven satisfactory salts included 
bis(tert.butylammonium) adipate, a di-salt, two piperidinium salts that 
had fairly complicated syntheses, diethyl- and trimethylammonium salts 
that subsequently proved unsatisfactory at the desired voltages, and the 
di-n-propylammonium and diisopropylammonium salts of the present 
invention. 
Since the monodipropyl and monodiisopropyl salts were easier to make than 
the piperidinium salts, these two were selected for further study. 
EXAMPLE 1 
Resistivity data, maximum formation voltages, and 150.degree. C. stability 
data are presented below for mono(di-n-propylammonium) adipate and 
mono(diisopropylammonium) adipate in ethylene glycol-water mixtures. 
Resistivity is given in ohm-cm, and stability is measured by resistivity 
at 25.degree. C. after heating at 150.degree. C. for the indicated time. 
Electrolyte A was prepared in-situ from 11.1 g of dipropylamine, 16.1 g of 
adipic acid, 68.9 g of ethylene glycol and 4.0 g of water and corresponds 
to a solution of mono(di-n-propylammonium) adipate in ethylene 
glycol-water. 
Electrolyte B was also prepared in-situ from 8.1 g of diisopropylamine, 
11.7 g of adipic acid, 76.2 g of ethylene glycol, and 4.0 g of water and 
corresponds to a solution of mono(diisopropylammonium) adipate in ethylene 
glycol-water. 
TABLE 1 
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Resistivity Max. formation 
Stability 
Elec- 
at Voltage at 
at hours 
trolyte 
25.degree. C. 
-20.degree. C. 
150.degree. C. 
25.degree. C. 
150.degree. C. 
500 1000 
2000 
__________________________________________________________________________ 
A 728 9370 51.2 450 415 999 1097 
1119 
B 933 12790 67.7 460 430 969 969 
957 
__________________________________________________________________________ 
Electrolyte B has the better stability characteristics, but its 
room-temperature resistivity of about 930 .OMEGA.-cm makes it more 
desirable for capacitors operating at higher than 200 VDC. 
EXAMPLE 2 
Twenty-five capacitors rated at 50 .mu.F and 200 VDC were life-tested at 
200 VDC and 130.degree. C. Table 2 gives average results for these 
capacitors containing an electrolyte containing mono(di-n-propylammonium) 
adipate and made from 65.3 g of ethylene glycol, 18.8 g of adipic acid, 
13.0 g of di-n-propylamine, 2.4 g of water, and 0.5 g of ammonium 
dihydrogen phosphate as phosphate ion source. The room-temperature 
resistivity of this electrolyte is about 785 ohm-cm. Capacitance is in 
microfarads, leakage current is microamperes, and weight loss, a measure 
of stability, in milligrams. 
TABLE 2 
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120 Hz Leakage Current 
Hours Capacitance ESR 2 min 5 min Wt. Loss 
______________________________________ 
0 55.2 0.85 5.86 3.39 -- 
424 54.3 0.67 2.52 1.29 25.3 
1000 54.3 0.75 2.18 1.06 56.0 
1430 53.9 0.73 3.00 1.30 77.5 
1572 54.2 0.75 2.33 1.01 85.4 
2000 54.1 0.75 2.39 1.08 107.2 
2500 53.9 0.80 2.14 0.97 132.9 
3000 53.5 0.80 2.46 1.17 155.7 
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The rate of weight loss is a useful predictor of the ultimate life of a 
capacitor. A rule-of-thumb is that when a capacitor loses 40-50% of its 
electrolyte, it starts to deteriorate electrically and becomes a risk. For 
example, for a capacitor containing 2000 mg of electrolyte, a weight loss 
rate of 100 mg/1000 hrs would predict the capacitor would lose 40-50% of 
its electrolyte, or 800-1000 mg, in 8000-10,000 hrs. At this point it 
would be predicted to start to deteriorate and go off-specification 
electrically. The capacitors shown above do contain 2000 mg of 
electrolyte, and, from the data above, a life of 15,000-20,000 hrs is 
predicted before reaching the 40-50% (or 800-1000 mg) wt loss. Since these 
values are for 130.degree. C. operation, these are extremely stable 
capacitors. 
EXAMPLE 3 
In this example, the effect of a phosphate salt on electrolyte performance 
is shown. The electrolyte of Example 2 (electrolyte 1a) was used in 
capacitors rated at 50 .mu.F-200 VDC which were aged at 275 V for 2.5 hrs 
at 105.degree. C. and compared with the results for the same electrolyte 
without phosphate (electrolyte 1b). 
A different formulation (electrolyte 2a) made from 44.5 g adipic acid, 30.8 
g dipropylamine, 67 g of water, and 1000 ml of ethylene glycol with and 
without 3.8 g of ammonium dihydrogen phosphate was tested in 10 .mu.F-450 
V capacitors at 85.degree. C. for 1 hr at 400 V, 1 hr at 450 V and 2 hr at 
475 V. For these higher voltage capacitors, electrolytes with higher 
resistivities were needed; the version without phosphate (electrolyte 2b) 
had a room-temperature resistivity of about 1596 .OMEGA.-cm while that 
with phosphate (electrolyte 2a), about 1540 .OMEGA.-cm. Both had a maximum 
formation voltage of 490 V. 
TABLE 3 
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Electrolyte 
No. shorts/No. capacitors 
Leakage current, .mu.A 
______________________________________ 
1a 0/25 3.4 
1b 40/40 -- 
2a 3/10 275.9 
2b 11/11 -- 
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In both sets of capacitors, the non-phosphate version failed completely. 
However, when the same phosphate was added to a conventional low voltage 
electrolyte containing ammonium adipate, water, and ethylene glycol 
formulated for 6 V service, the presence of the phosphate had a 
detrimental effect on electrical properties of the tested capacitors. 
Leakage currents for the non-phosphate vs phosphate version were 58 and 65 
.mu.A, respectively, initially. At 250 hrs the leakage current values were 
230 vs 727 .mu.A, and at 500 hrs the values were 362 vs 762 .mu.A. 
Thus, while the presence of phosphate is necessary for the electrolytes of 
the present invention, it is not beneficial to all adipate electrolytes. 
While ammonium dihydrogen phosphate has been used in the examples, other 
phosphate salts may be used providing they have sufficient solubility.