Electrolyte containing long chain dicarboxylic acids for very high voltage aluminum electrolytic capacitors

Abstract: An improved electrolytic capacitor is parti­ cularly useful for operation at voltages above 500 volts is produced by employing an electrolyte containing a straight chain saturated aliphatic dicarboxylic acid in which the carboxylic moieties are separated by at least 14 carbon atoms.

Description: BACKGROUND OF THE INVENTION The invention in the instant application relates to electrolytic capacitors and to an improved electrolyte therefor. In particular, the invention in the instant application relates to electrolytic capacitors having aluminum cathode and anode members which are suitable for use in high operating voltages, in particular voltages above 500 volts and electrolyte solutions for electrolytic capacitors to be used at such high voltages particularly voltages above 500V. These capacitors must be capable of maintaining the dielectric oxide film on the anode by forming new oxide at the applied voltage. Thus, if the forming voltage is 700V no dielectric breakdown should occur at 550V. In addition, it is necessary that the resistivity of the electrolyte solution should be as low as possible because this resistivity contributes to the (ESR) equivalent series resistance of the capacitor. Further the capacitor should be capable of operating over a wide temperature range. Although attempts have been made to produce an electrolytic capacitor employing aluminum anode and cathode members capable of operating at a voltage of greater than 500 volts in a wide temperature range while exhibiting a low ESR such attempts have not provided to be successful. Thus, Japanese Kokai 60,132,313 as abstracted in 103 Chem Abs 674 shows the use of an electrolyte comprising a solution of a salt of an amine with an organic carboxylic acid where the arboxylic acid is a polybasic side chain substituted acid of 16-22 carbons. The resulting capacitors have a voltage rating up to as high as 480V and exhibit relatively low ESR values. However, there is no suggestion of producing capacitors with voltage ratings of above 500 volts. Japanese Kokai 57-27013 shows electrolytes for electrolytic capacitors comprising solutions of dibasic acids having akyl side chains and 16-22 carbons dissolved in a solvent of which the main component is ethylene glycol. This electrolyte may include small amounts of ammonia and boric acid. As shown in the specification of this patent, capacitors having a voltage rating as high as 500 volts may be produced. However, here too there is no suggestion of producing electrolytic capacitors having a voltage rating greater than 500 volts. European Patent Application 0227433 shows an electrolytic capacitor employing as an electrolyte fill an electrolyte solution comprising a quaternary ammonium salt of a branched chain aliphatic dicarboxylic acids having 11-30 carbon atoms dissolved in a polar organic solvent. Employing such electrolytes, this reference shows the production of capacitors voltage having ratings of up to 475 volts. However, there is no suggestion that by the use of the electrolytes disclosed therein, capacitors having voltage ratings above 500 volts and exhibiting low ESR values may be produced. Additionally, it should be noted that while electrolytic capacitors having voltage ratings of 500 volts and slightly higher may be produced with use of conventional boric electrolytes, it is necessary in such cases to employ unusually thick paper pads in order to minimize the voltage across the dielectric aluminum oxide since the boric acid limits the maximum voltage across the oxide. As a result, a high resistance is introduced. This high resistance is a significant factor in producing a voltage drop across the capacitor and permits the manufacture and use of high voltage photo-flash capacitors. These capacitors require a high leakage current at the rated voltage. Since photoflash capacitors are relatively small, any heat dissipated does not increase significantly the operating temperature of the capacitors, however such a situation is not tolerable in large computer capacitors rated at 500 volts or above. In such case, a high leakage current may lead to a thermal run away resulting in the destruction of the capacitor. US-A-4 747 021 discloses an electrolyte consisting essentially of ammonium benzoate and an aliphatic dicarboxylic acid having an odd number of 3-21 aliphatic C-atoms, dissolved in ethylene glycol of a water content of up to 5 % and set at a pH of 4-8 by addition of ammonia. SUMMARY OF THE INVENTION A principal object of this invention is to provide electrolytic capacitors comprising aluminum anode and cathode members which may be satisfactorily employed at voltages above 500 volts in a wide temperature range while exhibiting relatively low equivalent series resistances at the operating voltages. According to the invention, there is provided an electrolyte with the features of claim 1, and an electrolytic capacitor with the features of claim 6. The latter comprises aluminum anode and cathode members separated by insulating spacers impregnated with an electrolyte consisting essentially of a solution of a straight chain saturated aliphatic dicarboxylic acid in which the carboxylic moieties are separated by at least 14 carbon atoms in a mixture of at least one polar organic solvent and water in an amount of from 4-30% by weight of the organic solvent, the acid being present in an amount of 2-9.5% by weight in the electrolyte. It has been found that the novel capacitors of the invention exhibit voltage ratings of above 500 volts together with low equivalent series resistances and are capable of operating over a temperature range from -40°C to 105°C. BRIEF DESCRIPTION OF THE DRAWING The sole figure in the drawing is a graph showing the variation of properties of a representative electrolytic capacitor of the invention with temperature. DETAILED DESCRIPTION OF THE INVENTION While any suitable polar organic solvent or mixture of polar organic solvents may be employed, it has been found that very satisfactory results may be achieved by us of a mixture of polar organic solvents which consists of at least one of the solvents of the group of ethylene glycol, dimethylacetamide, dimethylformamide, and butyolactone and at least one solvent selected from the group consisting of tetrahydrofurfuryl alcohol, dimethylsulfoxide, N-methylacetamide, and N-methyl formamide. For complete solubility of the dicarboxylic acid it is necessary that a sufficient ammonia or an amine be added to the solution to provide an amount of pH of 6.0 - 10.0 and preferably from 7.0 - 10.0. The pH of the electrolyte solution is adjusted by the addition of ammonia or an amine, preferably selected from the group consisting of dimethylamine, dimethylamine, and piperidine. While the polar organic solvents as recited may readily be employed in the electrolyte solution of the invention to produce good results, it has been found that particularly good results are achieved by use of a mixture of polar organic solvents consisting of a major amount of ethylene glycol, a minor amount of tetrahydrofurfuryl alcohol and a minor amount of dimethylacetamide. This is especially so when the dicarboxylic acid employed is a saturated straight chain acid having a molecular weight of about 565 where the carboxylic acid groups are separated by 34 -CH₂ (methylene) groups. This acid is hereinafter termed "Dicarboxylic Acid". However other straight chain saturated aliphatic dicarboxylic acids the carboxylics of which are separated by at least 14 carbons such as thapsic, or docosanedioic acids may also employed. Up to 5% by weight of borate may be present to prevent corrosion. The borate is preferably employed in the form of boric acid. In some cases it is found that the addition of phosphoric acid in the amount of up to 10% by weight provides improved results. Particularly improved results have been found by use of an electrolyte consisting essentially of a solution of about 2-5% of weight of the dicarboxylic acid. The invention will now be described in greater detail with reference to the following examples: Capacitor sections were made using 700EFV aluminum anode foil, aluminum cathode foil and paper separators. The three compositions listed above were evaluated in regard to the aging or "burn in" of capacitor sections treated with them. The aging procedure involved applying 4ma per unit in a constant current mode, the maximum voltage applied to these capacitors was 570 using a 70°C oven. The capacitors containing the 609D composition required over 6.5 hours to reach the age voltage of 570. These capacitors were allowed to remain at this voltage for about 16 hours in an 85°C oven. The unit were allowed to cool and the leakage current, capacitance and ESR were determined. The leakage current behaviour was found to be unsatisfactory. The leakage current was at first satisfactory, in that it was within the limit required by the design. However, on standing at room temperature for several days or at elevated temperatures for a few hours, the leakage current increased to unsatisfactory levels. The two compositions 749T and 749U behaved in a much better manner. The capacitors made with these fill electrolytes reached age voltage of 570 in 2.700hr. These were also burned in by leaving them in an oven at 85C with the 570V applied for a period of 16hr. The initial electrical properties of the capacitors made from the compositions, 749T and 749U are shown in the following table: From these results it was concluded that the water level in the composition was critical to the aging behavior of the fill electrolyte. The trace level of phosphate in the composition, 749U, likewise appears to show a positive benefit as compared to the 749T in that the leakage current values are much lower. OPEN CUP SCINTILLATION AND AGE BEHAVIOR Variations of the 749U formulations were investigated in regard to the open cup scintillation behavior and resistivities of the mixtures. The compositions were as follows: These formulations were used in treating capacitor sections of the same design as was described above. The sections were assembled and aged in the same manner. The final voltage was 570. The initial properties of these capacitors were measured and the averages are as follows: The above capacitors were further aged in a 70°C oven at 610 volts for 16 hours. Again, all units aged up to this voltage without any electrical shorts forming. Thus the ultimate use voltage of these compositions is greater than 610V. The completed units were measured as a 600V design. The average values for each of the fill electrolytes are tabulated below: The following composition was also prepared. This mixture was heated to between 65°C and 90C after the final addition of the amine. The initial properties measured for this composition are; 1) resistivity at 30C, 1280 ohm.cm 2) pH = 9.68 3) Scintillation voltage 500. There were no age losses up to and including a voltage of 610, for capacitors made by using the 700 formation voltage anode foil and composition B. It was thus concluded that the high water concentrations improve the age yields of the capacitors in spite of the fact that the higher levels of water cause a lower open cup scintillation voltage. Samples of these capacitors were life tested with full rated voltage of 550 volts at 85C with the following results: The low temperature characteristics of a capacitor at 550 volts DC, employing the 749V electrolyte and having a capacitance of 240 uF, an impedance (IMPED) at 120 Mz of 0.517 ohms and an equivalent series resistance (ESR) of 0.217 ohms at room temperature (25°C) is shown in the sole figure of the drawing. As will be apparent from this figure, which is a graph in which the temperature is shown along the abscissa and (as a ratio to their values at room temperature) the impedance IMP, equivalent series resistance ESR and capacitance CAP are shown along the ordinate, this capacitor is useful even at a temperature as low as -40°C. Normal glycol fill electrolyte capacitors, operating at 450 volts, are useful only at much higher temperature, generally not less than about -20°C.

Claims: An electrolyte for use in an electrolytic capacitor, said electrolyte consisting essentially of a solution of a saturated aliphatic dicarboxylic acid in a mixture of at least one polar organic solvent and water, wherein the aliphatic dicarboxylic acid is a straight chain dicarboxylic acid in which the carboxylic moieties are separated by at least 14 carbon atoms, which acid is present in an amount of 2 - 9.5 wt. % in the electrolyte, and wherein the electrolyte further comprises water in an amount of 4 - 30 wt% relative to the organic solvent and an amount of ammonia or an amine so that the pH value of the electrolyte is in the range 6.00 - 10.00. An electrolyte according to claim 1, characterized in that said polar organic solvents consist of a mixture of at least one solvent selected from the group consisting of ethylene glycol, dimethylacetamide, dimethylformamide and butyrolactone, and at least one solvent selected from the group consisting of tetrahydrofurfuryl alcohol, dimethylsulfoxide, N-methylacetamide and N-methylformamide. An electrolyte according to claim 1 or 2, characterized in that the solution contains a borate in an amount of up to 5 wt. % relative to the organic solvent. An electrolyte according to claim 1, 2 or 3, characterized in that the solution contains an amine selected from the group consisting of dimethylamine, and piperidine. An electrolyte according to claim 1, 2, 3 or 4, characterized in that the electrolyte contains phosphoric acid up to an amount of 10 wt. %. An electrolytic capacitor comprising aluminium anode and cathode members, which members are separated by an insulating spacer, which spacer is impregnated with an electrolyte, wherein the composition of electrolyte used is according to one of the preceding claims.

Inventors: DAPO ROLAND FRANCIS, DAPO, ROLAND FRANCIS
Applicants: PHILIPS ELECTRONICS NA, PHILIPS ELECTRONICS NORTH AMERICA CORPORATION
Application year: 1989
Main classification: H01G9
IPC codes: H01G9