JPH0690994B2 - Electrolytic solution for electrolytic capacitors - Google Patents

Description

TECHNICAL FIELD The present invention relates to improvement of an electrolytic solution for an electrolytic capacitor, and more specifically, by adding a compound having a unique structure to the electrolytic solution, it can be used in a wide operating temperature range. The present invention relates to an improvement in an electrolytic solution for an electrolytic capacitor that provides an electrolytic capacitor that exhibits totally stable characteristics.

[Prior Art] Electrolytic capacitors are small, large-capacity, inexpensive, have excellent characteristics such as smoothing of rectified output, and are one of the important components of various electric and electronic devices. The aluminum film changed to an oxide film by
This oxide film is used as a dielectric and is formed by interposing an electrolytic solution between the oxide film and a collector cathode.

Since the electrolytic capacitor is used while constantly regenerating the derivative oxide film while performing a chemical reaction during use, the steady state of the chemical reaction that takes place between the aluminum electrode whose surface is an oxide film and the electrolytic solution is maintained. It is important to stabilize the performance that the aluminum oxide film to be maintained and used as a derivative is well retained.

In the chemical reactions that proceed during the use of electrolytic capacitors,
The electrolyte forms an ionic conductor which is a medium for the movement of ions. At the interface between the electrolytic solution and the electrode, the charge moves due to the progress of the electrode reaction, the oxidation reaction proceeds on the anode surface, and the reduction reaction proceeds on the cathode surface, along with which the ions move in the electrolytic solution as the ion conductor. An electric current flows. Therefore, the specific resistance, which is the reciprocal of the electric conductivity of the electrolytic solution, reflects the characteristic of the electrolytic solution which is an ionic conductor in the chemical reaction that proceeds during the use of the electrolytic capacitor. Scintillation is a phenomenon in which the electrochemical state fluctuates as a result of the increase in the load voltage of the capacitor and the change in the physical properties of the derivative due to the high voltage load, resulting in a change in the dielectric constant over time. The scintillation voltage (spark voltage) can be used as a measure of the withstand voltage of the capacitor, and the higher the scintillation voltage (spark voltage), the greater the withstand voltage of the capacitor. The spark voltage can be simply measured by immersing an unformed aluminum foil of an appropriate size in an electrolyte solution to be measured without assembling the final capacitor product.

Since the capacitance of the capacitor (Cap) is proportional to the dielectric constant of the dielectric, it is necessary to use a dielectric with a high dielectric constant and avoid physicochemical changes in the dielectric during use to maintain a high dielectric constant.
The tangent of the loss angle, which is the difference between the phase of the charging current and the phase of the external electric field, or the dielectric loss tangent (tan δ) is used as a measure of the power consumption of the capacitor, and a smaller value indicates less power consumption. In general, it highly depends on changes in temperature and frequency.In the case of a constant frequency, although it does not change much at around 100 ° C on the high temperature side compared to the value at room temperature, on the low temperature side it decreases geometrically as the temperature decreases. For example, at -20 ℃, it is 4 to 5 times the normal temperature, at -50 ℃.
Before and after, it will be about 100 times the normal temperature. The leakage current (LC), which is the current that flows when a certain value is reached after the start of charging, is due to the constant movement of the charge carriers in the dielectric, and the ions generated by the dissociation of impurities in the dielectric are the main constituents of the charge carriers. The magnitude of the change in leakage current reflects the stability of the electrochemical state of the dielectric. The poor appearance of the electrolytic capacitor or the valve opening due to the operation of the explosion-proof valve is mainly caused by the generation of gas due to the progress of an unfavorable chemical reaction other than the predetermined chemical reaction.Since the chemical reaction rate depends on the temperature, it becomes less problematic on the low temperature side. Although it does not often occur, it may rapidly explode on the high temperature side and may be accompanied by the danger of explosion, which is one of the important indexes for evaluating the overall performance of the capacitor.

For conventional electrolytic capacitors, ethylene glycol (EG)
Although a solution of ammonium borate dissolved in was widely used, this type of electrolytic capacitor has the drawback that the characteristics of the capacitor, which are reflected in capacitance, dielectric loss tangent, impedance, etc., decrease significantly at low temperatures. there were. In order to improve the low temperature characteristics of the electrolytic solution for driving an aluminum electrolytic capacitor using ethylene glycol (EG) as a solvent,
By adding glycol ethers such as ethylene glycol monomethyl ether (MC) or ethylene glycol monoethyl ether (EC), ethers such as ethylene glycol monomethyl ether acetate, or amides such as γ-butyrolactone or dimethylformamide to EG It is known to achieve the purpose. MC
Alternatively, when glycol ethers such as EC are used as a mixed solvent with EG, the low temperature characteristics are certainly improved as compared with the case of using EG alone solvent. However, since MC, EC, etc. have a boiling point as low as 120-140 ° C as compared with EG, it becomes difficult to use them at a high temperature such as 130 ° C. Furthermore, MC, γ-butyrolactone, dimethylformamide, etc. should be used at a high temperature. There are drawbacks such as a decrease in withstand voltage and a rapid change in tan δ. As described above, there are merits and demerits in improving the temperature characteristics of the electrolytic capacitor by the conventional technology, and it is difficult to improve the low temperature characteristics while maintaining the characteristics at high temperature (130 ° C) well, and the characteristics of the capacitors can be widened. Since it cannot be maintained constant in the temperature range, these solutions have been desired.

[Problems to be Solved by the Invention] The present invention provides an electrolytic solution for an electrolytic capacitor, which shows a totally stable characteristic over a long operating temperature range for a long time by using a unique compound in the electrolytic solution. The purpose is to provide.

[Means for Solving the Problems] According to the present invention, there is provided an electrolytic solution for driving an aluminum electrolytic capacitor, which contains tetrahydrofurfuryl alcohol as a solvent. Tetrahydrofurfuryl alcohol has the structure of the formula: It is a colorless, transparent, hygroscopic liquid at room temperature and is water, alcohol, ether, acetone, and chloroform. Freely mix with many solvents such as benzene.

If the solvent is a mixed solvent of tetrahydrofurfuryl alcohol and ethylene glycol, a suitable electrolytic solution for electrolytic capacitors can be obtained. In this embodiment, ethylene glycol is used as a main solvent, tetrahydrofurfuryl alcohol is used as a secondary solvent, ethylene glycol is used at 45 to 85% by weight, and tetrahydrofurfuryl alcohol is used at 5 to 85% by weight.
It is preferable to use 45% by weight. The electrolytic solution for electrolytic capacitors of the present invention may contain a small amount of water of about 2 to 15% by weight, if necessary.

When the solute is ammonium benzoate, a suitable electrolytic solution for electrolytic capacitors can be obtained. Besides ammonium benzoate, it is also possible to use ammonium salts such as ammonium adipate, ammonium azelate, ammonium sebacate, and other salts such as boric acid, adipic acid, sebacic acid, azelaic acid. It is preferred to use these salts at 4 to 15% by weight.

[Operation] The low-temperature characteristics of the electrolytic solution for driving an aluminum electrolytic capacitor using ethylene glycol (EG) as a sole solvent have been relatively deteriorated as high-capacity foils have been developed so far. To improve low temperature characteristics, EG is mixed with glycol ethers such as ethylene glycol monomethyl ether (MC), ethers such as ethylene glycol monomethyl ether acetate, amides such as γ-butyrolactone and dimethylformamide, and mixed. By using a solvent, the low temperature characteristics are improved. Certainly, when MC or glycol ethers such as ethylene glycol monoethyl ether (EC) is used as a mixed solvent with EG, the low temperature characteristics are improved as compared with the case of EG alone solvent. However, the boiling points of MC and EC are respectively 124.4 ° C (760 mmHg) 134.
It is as low as 8 ℃ (760mmHg), and it is difficult to improve the characteristics near 130 ℃. Also, amides of ethylene glycol monomethyl ether acetates, γ-butyrolactone, dimethylformamide, etc., decrease in withstand voltage during use at high temperature,
There is a problem of causing a rapid change in tan δ.

The present invention, especially by adding tetrahydrofurfuryl alcohol to EG as a solvent for an electrolytic solution for driving an aluminum electrolytic capacitor to form a mixed solvent, maintaining the characteristics at high temperatures while maintaining the characteristics even when used for a long time. In addition, the characteristics at low temperature are also improved.

The boiling point (760 mmHg) and melting point of typical conventional compounds used as a solvent for an electrolytic solution for driving an aluminum electrolytic capacitor and tetrahydrofurfuryl alcohol used in the present invention are shown below: Tetrahydrofurfuryl alcohol used in the present invention has a high boiling point of 178 ° C., and has a wide melting temperature range of -80 ° C., so that the temperature range existing as a liquid is wide. It is presumed that at a low temperature, deterioration of characteristics due to solidification of the solvent is reduced, and as a result, an electrolytic solution for driving an electrolytic capacitor that provides stable characteristics over a wide operating temperature range for a long time can be realized.

[Effects of the Invention] An electrolytic capacitor using the electrolytic solution for an electrolytic capacitor of the present invention shows totally stable characteristics in a wide operating temperature range,
It shows a particularly large effect on the suppression of the decrease in capacitance on the low temperature side, and the capacitance hardly changes on the high temperature side, and the loss tangent, that is, the change in power consumption and leakage current is small, and the appearance is visible even when used at high temperature for a long time. No defects occur.

[Examples] The present invention is described in more detail below with reference to Examples, but the present invention is not limited to the following Examples.

Composition of Electrolytic Solution The composition of the electrolytic solution for an electrolytic capacitor prepared according to the present invention and the composition of the electrolytic solution for an electrolytic capacitor not according to the present invention, the composition of which is changed for comparison for the purpose of comparison, are shown below.

Example 1 component weight% ethylene glycol 61.4 tetrahydrofurfuryl alcohol 26.3 ammonium benzoate 10.5 pure water 1.8 Comparative example 1 component weight% ethylene glycol 87.7 ammonium benzoate 10.5 pure water 1.8 Comparative Example 2 component weight% ethylene glycol 61.4 ethylene glycol monomethyl ether 26.3 Ammonium benzoate 10.5 Pure water 1.8 Comparative Example 3 components wt% ethylene glycol 61.4 Ethylene glycol monoethyl ether 26.3 Ammonium benzoate 10.5 Pure water 1.8 Comparative example 4 components wt% Ethylene glycol 61.4 Ethylene glycol monomethyl etherate acetate 26.3 Benzoic acid Ammonium 10.5 Pure water 1.8 Comparative Example 5 component weight% ethylene glycol 61.4 γ-butyrolactone 26.3 Ammonium benzoate 10.5 Pure water 1.8 Comparative Example 6 component weight% Ethylene glycol 61.4 Dimethylform Amide 26.3 Ammonium benzoate 10.5 Pure water 1.8 Paste properties Table 1 shows the values of the specific resistance, pH and spark voltage of the electrolyte measured by the usual methods as initial paste properties. In addition, the spark voltage was formed by forming an etched foil of 20 mm × 260 mm at 430 V in a boric acid solution, winding this anode foil, paper and cathode foil, and rating 250
A device having V, 22 μF and size 16φ × 31.5 l was prepared, and the device was impregnated with each electrolyte solution and measured using a constant current power source set to 10 mA.

Results of temperature characteristic test The results of the low temperature characteristics of the electrolytic capacitors prepared using the electrolytic solution of the present invention in Example 1 and the electrolytic solutions prepared in Comparative Examples 1 to 6 were -40 ° C and 20 ° C.
The results are shown in Table 2 by comparing the characteristics with those at ° C.
The electrolytic capacitor using the electrolytic solution of the present invention has a low temperature (−40
(° C), the loss of capacitance is small, and the increase of the dielectric loss tangent (tan δ), which is a measure of power consumption, is acceptable.
It shows characteristics that are at least as good as those of conventional electrolytic capacitors. In Comparative Examples 1 and 4, the capacitance at -40 [deg.] C. falls within this range when the other examples have a variation of about 30% as a reference, but the variation is extremely large. The effect of improving the low temperature characteristics of Comparative Example 2 using MC is great.

Table 3 shows the life evaluation results of the electrolytic capacitors prepared by using the electrolytic solution of the present invention of Example 1 and the electrolytic capacitors prepared by using the electrolytic solutions of Comparative Examples 1 to 6 at the high temperature (130 ° C.). Shown in. 130 for each element
A load of 250 V was applied in a constant temperature bath at ℃, and the characteristics after 500 hours were compared with the initial characteristics. The capacitor using the electrolytic solution of the present invention shows almost no change in capacitance even when used at 130 ° C. for 500 hours, and a slight change in dielectric loss tangent (tan δ), which reflects the electrochemical state of the dielectric. There is little change in the leakage current value, and no defective appearance occurs (Example 1).
In the high temperature characteristic test, Comparative Examples 1 and 4 give relatively good results, but as described above, both of them are not suitable because the low temperature characteristics are remarkably inferior. Comparative Examples 2, 3 and 6 are 50
The explosion-proof valve is activated before the lapse of 0 hours, and it is not preferable in terms of internal pressure rise, gas generation, and the like. Further, in Comparative Example 5, tan δ
From the initial value of 0.062 to 0.120 after 500 hours, the rate of change is large, which is not preferable in terms of power consumption fluctuation.

From the above, comprehensive evaluation shows that the electrolytic solution for electrolytic capacitors of the present invention is the most excellent.

Claims (3)

[Claims] 1. An electrolytic solution for driving an aluminum electrolytic capacitor, which contains tetrahydrofurfuryl alcohol as a solvent. 2. The electrolytic solution for an electrolytic capacitor according to claim 1, wherein the solvent is a mixed solvent of tetrahydrofurfuryl alcohol and ethylene glycol. 3. The electrolytic solution for an electrolytic capacitor according to claim 1, wherein the solute is ammonium benzoate.