GB2113916A

GB2113916A - Solid electrolyte capacitor

Info

Publication number: GB2113916A
Application number: GB08226300A
Authority: GB
Inventors: Shinichi Niwa; Hirofumi Inoue
Original assignee: Sanyo Electric Co Ltd
Current assignee: Sanyo Electric Co Ltd
Priority date: 1982-01-18
Filing date: 1982-09-15
Publication date: 1983-08-10
Also published as: GB8504864D0; NL189936C; NL8203608A; GB2153148A; GB2153148B; FR2520155B1; NL189936B; FR2520155A1; GB2113916B

Abstract

In a solid electrolyte capacitor an improvement wherein the solid electrolyte is solidified by cooling a 7,7,8,8-tetracyanoquinodimethane salt from a liquid state, the 7,7,8,8- tetracyanoquinodimethane salt remaining so stable in its liquid state as not to substantially decompose before solidification has occurred. <IMAGE>

Description

SPECIFICATION Solid electrolyte capacitor The present invention relates to a solid electrolyte capacitor and a solid electrolyte therefor.

In general, a solid electrolyte capacitor comprises a film forming metal (e.g. aluminum, tantalum, niobium) having an anodized surface and a solid electrolyte attached thereto. The solid electrolyte in the capacitorof this kind is usually made of manganese dioxide. However, manganese dioxide has such drawbacks as causing damage to the anodized surface of the metal on the thermal decomposition for its formation and having inferior repairability for the anodized surface. In order to overcome these drawbacks, the use of organic semiconductors, particularly 7,7,8,8-tetracyanoquinodimethane (hereinafter referred to as "TCNQ") salts, instead of manganese dioxide has been proposed. TCNQ salts show themselves high electroconductivity and good repairability.Since, however, they are normally formed in powdery crystals, their attachment to film forming metals is not easy. While film forming metals used in the manufacture of solid electrolyte capacitors are porous in most cases, the uniform impregnation of such porous metals with TCNQ salts is extremely difficult. In addition, TCNQ salts have a possibility of decomposition and deterioration during the operations for impregnating and drying.

Conventional procedures for adhesion of TCNQ salts onto film forming metals can be classified into the following three: (1) the procedure wherein a solution of a TCNQ salt in an appropriate solvent (e.g. dimethylformamide) is applied onto a film forming metal, followed by drying to evaporate the solvent; (2) the procedure wherein a dispersion of finely divided crystals of a TCNQ salt in an appropriate solvent (e.g. ethanol) is applied onto a film forming metal, followed by drying to evaporate the solvent; and (3) the procedure wherein a TCNQ salt is vacuum evaporated onto a film forming metal.

Among them, the procedures(1 ) is disadvantageous in not achieving a high impregnation rate, because the solubility of a TCNQ salt is usually small. Even when, for instance, dimethylformamide having a high solubility to a TCNQ salt is used as the solvent and a high temperature (e.g. 1 000C) is adopted for preparation of the solution, the solubility is at the most 1 0%. Because of this reason, repetitive application of the solution is necessary for formation of a layer of the TCNQ salt having a sufficient thickness on a film forming metal or impregnation of the metal with a sufficient amount of the TCNQ salt.When, for instance, a porous metal for a solid electrolyte capacitor rated to 1 ,uF is used, the repetitive application of the solution 5 to 1 0 times can attain an impregnation rate of only 30% or less in case of the impregnation with manganese dioxide being taken as 10056. Thus, the metal is porous, yet a solid electrolyte capacitor having a large capacity is not obtainable. Further, the metal applied with the solution is allowed to stand at a high temperature repeatedly for drying, during which, however, the TCNQ salt is more or less decomposed to cause the deterioration of the electroconductivity. Moreover, the TCNQ salt thus attached onto the metal comprises fine crystals, and therefore a binding resin such as polyvinylpyrrolidone is frequently incorporated into the solution so as to enhance the adhesive strength of such crystals.Since the binding resin is an electrically insulating material, it causes also the deterioration of the electroconductivity.

In case of the procedure (2), the micronization of powdery crystals of a TCNQ salt has a certain limitation. Further, the adhesive strength of the crystals onto a film forming metal is small. Thus, elimination of the solid electrolyte made of the TCNQ salt from the metal occurs frequently. Also, the deterioration of various characteristics such as increase of the loss and decrease of the capacity is often observed. While the adhesive strength can be improved to a certain extent by incorporation of a binding resin, such binding resin causes the depression of the electroconductivity. Since a dispersion of the crystals of the TCNQ salt is used, the impregnation rate into a porous metal is particularly inferior. Even if a ultrasonic diffusion impregnation method is applied, the impregnation rate is not much improved.In case of the procedure (3), vacuum evaporation is troublesome in operation and further not suitable for attachment of a TCNQ salt onto a porous metal.

As a result of the extensive study, it has now been found that the attachment of a TCNQ salt onto a film forming metal having an anodized surface by contacting the metal with the TCNQ salt in a liquid state and then solidifying the TCNQ salt provides a solid electrolyte capacitor having excellent performances.

A TCNQ salt in a liquid state is obtainable by simply heating the TCNQ salt in a powder state to melt. However, the sole heating usually results in decomposition of the TCNQ salt to make it electricaliy insulated. Thus, the resultant product loses any useful function as the solid electrolyte for a solid electrolyte capacitor. Some certain kinds of TCNQ salts have, however, a certain period of time until they are decomposed after their melting. Such period is relatively short but sufficient to complete the operation for attachment of them onto film forming metals. In other words, a solid electrolyte having a high electroconductivity can be obtained from such TCNQ salts, provided that the heating for liquefaction and the cooling for solidification are accomplished prior to the decomposition.Because of this reason, TCNO salts having a sufficient period of time until the decomposition after their melting for accomplishment of the necessary operations must be chosen specifically from various TCNQ salts.

Besides, it may be noted that prior to this invention, it was entirely unknown whether the TCNQ salt once melted and then solidified has any comparable performances to the one before melting. Thus, the exertion of excellent performances by the TCNQ salt once melted and then solidified could not be predicted previously.

TCNQ and its various salts as well as methods of their preparation are disclosed, for instance, in J. Am. Chem. Soc., 84, 3374-3387 (1962). TCNQ salts can be either simple salts of TNCQ represented by the formula: Mn+ (TCNQ)n- wherein M is an organic cation and n is the valence of the cation, or complex salts represented by the formula: Mn+(TCNQ-)n(TNCQ)m wherein m is a positive number and corresponds to a mole number of neutral TCNQ contrained in one mole of the complex salt, and M and n are each as defined above. In this invention, however, the use of the complex salts is much preferred due to their better electric characteristics. Further, the complex salts of the said formula wherein m is around 1 are usually employed, although it may be somewhat smaller (e.g. 0.5) or larger (e.g. 1.5).

Examples of the TCNQ salts include TCNQ salts of N-substituted quinolines, isoquinolines and pyridines, of which the substituent at the N-position is any hydrocarbon group such as C2-C18 alkyi (e.g. ethyl, propyl, butyl, pentyl, octyl, decyl, octadecyl), C5-C8 cycloalkyl (e.g. cyclopentyl, cyclohexyl), C3-C18 alkenyl (e.g. allyl), phenyl or phenyl(C1-C8)alkyl (e.g. phenethyl). Preferred examples of the TCNQ salts are N-n-propylquinoline TCNQ salt, N-isopropylquinoline TCNQ salt, N-n-hexylquinoline TCNQ salt, N-ethylisoquinoline TCNQ salt, N-n-propylisoquinoline TCNO salt, N-isopropylisoquinoline TCNQ salt, N-n-butylisoquinoline TCNQ salt, N-n-butylpyridine TCNQ salt, etc.

Illustrating this invention more in detail taking as the examples N-n-propylquinolinium+ (TCNQ-) (TCNQ) (hereinafter referred to as "P-1 salt"), N-isopropylquinolinium+ (TCNQ-) (TCNQ) (hereinafter referred to as "P-2 salt"), N-n-propylisoquinolinium+ (TCNQ-) (TCNQ) (hereinafter referred to as "P-3 salt"), N-isopropylisoquinolinium+ (TCNQ-) (TCNQ) (hereinafter referred to as "P-4 salt"), N-nbutylisoquinolinium+ (TCNQ-) (TCNQ) (hereinafter referred to as "P-5 salt") and N-n-butylpyridinium+ (TCNQ-) (TCNQ) (hereinafter referred to as "P-6 salt"), their melting points are between about 210 and 2350C: i.e.P-1 salt, 225-2350C; P-2 salt, 225-2350C; P-3 salt, 21 2200C; P 4 salt, 225-2350C; P--5 salt, 21 2200C; P-6 salt, 21 2200C. They can be prepared, for instance, by reacting the corresponding quinolinium, isoquinolinium or pyridinium iodide with TCNQ in an appropriate molar ratio (e.g. 3:4) in an appropriate solvent (e.g. acetonitrile), followed by purification.

With variation of the solvent used in the reaction of the purification, the molar ratio of the quinolinium, isoquinolinium or pyridinium portion and the TCNQ portion may be somewhat changed. For instance, in case of the solvent for the purification in the above process being acetonitrile, there is usually obtained the TCNQ salt in the complex salt form wherein m is 1. In case of the solvent being methanol, the obtained TCNQ salt is the compllex salt wherein m is less than 1. The molar ratio of the quinolinium, isoquinolinium or pyridinium portion and the TCNQ portion may be also varied, for instance, by incorporating TCNQ in the TCNQ salt in a liquid state and allowing the resultant uniform mixture to cool for solidification. For instance, the solidified product obtained by incorporation of TCNQ into the TCNQ salt in the complex form wherein m is 1 in an amount of 5% by weight to the TCNQ salt shows the increase of the electroconductivity to a certain extent (e.g. 20%). However, the incorporation of TCNQ in a large amount such as 20% by weight or more will rather decrease the electroconductivity. In general, the TCNQ salts thus produced are formed in powdery needles or rods. When they are heated to melt and kept in a melt state for a long period of time, their sudden decomposition takes place to afford electrically insulating products.The times after completion of the liquefaction and before formation of the insulating products are as follows: (seconds

\ Temperature | \ 290OC 260OC TCNQ salt P-i 26 80 P-2 3 10 30 30 120 P-4 30 125 P-5 23 70 P-6 L 19 L 60 Note: Heating was carried out by charging the crystalline powder of the TCNQ salt in a case made of aluminum and contacting the case onto a metal plate maintained at the designed temperature.

Therefore, it is necessary that the TCNQ salt in a liquid state is allowed to cool for solidification prior to its decomposition, whereby a solid electrolyte having a high electroconductivity is obtained. In case of P-i salt, P-2 salt, P--5 salt and P-6 salt, for instance, they may be heated to a temperature above their melting points but below about 3000C and, within about 1 minute (preferably within about 20 seconds) after completion of the liquefaction, started to cool at room temperature.In case of P-3 salt and P-4 salt, they may be heated to a temperature above their melting points but below about 3200C and, within about 4 minutes (preferably with about 1 minute) after completion of the liquefaction, started to cool at room temperature. As the reuslts of solidification, there are obtainable solid electrolytes having a high electroconductivity.When the cooling was carried out within several seconds after completion of the liquefaction, the resulting solidified product showed the following electroconductivity (at250C): P-i salt, 23 Qcm; P-2 salt, 35 Qcm; P-3 salt, 36 Qcm; P 4 salt, 31 Qcm; P--5 salt, 3.4 Qcm; P-6 salt, 330 Qcm; N-n-hexylquinolinium+ (TCNQ-) (TCNQ), 49 Qcm; Nethylisoquinolinium+ (TCNQ-) (TCNQ), 220 Qcm.

The solid electrolyte formed by this invention is not the collective product of fine crystals as obtained in the procedure (1) or (2) but in a polycrystalline block state. Further, it retains an excellent repairability for the anodized surface of a film forming metal.

According to this invention, the TCNO salt itself in a liquid state is applied onto a film forming metal. Thus, different from the procedure (1), the attachment of the TCNQ salt onto a film forming metal to make a layer of the TCNO salt having a sufficient thickness on the metal or impregnate the metal with a sufficient amount of the TCNQ salt can be accomplished buy a single operation. The attachment is satisfactory irrespective of the metal being foil-like or porous. Thus, the procedure of the invention is quite suitable for mass-production of solid electrolytes without any deterioration of the TCNQ salt as seen on drying in the procedure (1 ) or (2).Since the attaching power of the solid electrolyte onto the metal is sufficiently great, the use of any binding resin is not needed, and any depression of the electroconductivity due to the binding resin can be avoided.

Practical and presently preferred embodiments of the invention are illustratively shown in the following Examples.

EXAMPLE 1 N-n-Propylisoquinolinium+ (TCNQ-) (TCNQ) (i.e. P-3 sait) was prepared by reacting N-npropylisoquinolinium iodide with TCNQ in a molar ratio of 3:4 in acetonitrile. Separately, a sintered product of powder alumina was subjected to anodic oxidation to make a porous capacitor element having an anodized surface.

The above prepared P-3 salt in powder crystals was charged in an aluminum made vessel, if necessary, under pressure, and the vessel was heated to liquefy the P-3 salt. Into the P-3 salt in a liquid state kept at a temperature of 250 to 2600C, the capacitor element pre-heated to a temperature of 250 to 2600C was immersed, and immediately thereafter, the vessel was allowed to cool to room temperature, whereby the P-3 salt impregnated in the micropores of the capacitor element was solidified to give a solid electrolyte.The time required for the operation from the liquefaction of the P-3 salt to the start of the cooling was about 5 seconds, and the time required for the depression of the temperature from the start of the cooling to a temperature below the melting point was several seconds such as 4 seconds.

The vessel was eliminated from the capacitor element impregnated with the solid electrolyte. Onto the surface of the capacitor element, a graphite layer and a silver painting composition layer were formed in order by a conventional procedure. The resulting capacitor element was accommodated in an aluminum made vessel together with a negative electrode lead wire and fixed therein by the use of a solder and an epoxy resin to give a solid electrolyte capacitor as shown in Fig. 1 of the accompanying drawing. In this drawing showing a section view of the capacitor, (1) is the capacitor element immersed with the solid electrolyte, (2) is the graphite layer, (3) is the silver paint coating composition layer, (4) is the positive lead wire, (4') is the negative lead wire (5) is the aluminum made case, (6) is the solder and (7) is the epoxy resin.

When the capacitor element used in the above Example is the one giving a conventional capacitor having a capacity of 1,uF with manganese dioxide as the solid electrolyte, the resultant capacitor shows a capacity of about 1 if. This means that the solid electrolyte in this Example has an impregnation rate of nearly 100% in case of that of manganese dioxide being taken as 1 00%.

The temperature characteristics and high temperature load characteristics of the capacitor obtained in the above Example are respectively shown in Tables 1 and 2 wherein C is the capacity (,us), AC is the capacity variation rate (%), tan ô is the loss (%) and LC/30" is the leakage current (uA) 30 seconds after impressed with a rated voltage. In Tables 1 and 2, there are also shown the characteristics of the capacitor obtained in the same manner as above but using N-n-propylisoquinolinium+ (TCNQ-) (TCNQ) (i.e. P-1 salt) instead of P-3 salt.

TABLE 1 Temperature characteristics

Temperature sC -25"C +200C +85"C \ Capacity Solid \ AC tan AC tana C tan AC tan a electrolyte P-3 salt 5.0 2.4 -4.2 2.2 1074 2.6 +13.7 4.0 P-l salt -5.3 ~ 1.6 -4.5 1.5 793 2.8 +5.7 6.2 TABLE 2 High temperature load characteristics (impressed with a standardized voltage of 10 V)

Time Initial After 1000 hrs at 85"C Solid Item el ectrolyte C tan 6 LC/30" C tan 6 LC /30" electrolyte \ P-3 salt 1074 2.6 0.02 1039 1.6 0.02 .~ ~.

P-i salt 793 2.8 0.02 740 2.0 0.02 In the above Example, there may be used a rolled element prepared by rolling up separating sheets wherein an etched aluminum foil as the negative electrode and a film formed aluminum foil as the positive electrode are sandwiched instead of the above used element of powder sintered type. Namely, the rolled element is impregnated with a TCNQ salt in a liquid state, followed by solidifying, and sealed with a resin. The resulting capacitor can afford similar temperature characteristics and high temperature load characteristics to those of the capacitor of powder sintered type. In this case, the cut portion of the film formed foil, the positive electrode lead wire, etc. are required to be previously subjected to film forming treatment.However, a vessel used as the bath for the TCNQ salt in a liquid state can be as such employed as the outer shell case, and a graphite layer and a silver paint coating composition layer can be omitted. The capacitor with the rolled element (equal to the one used in a conventional dry type electrolyte capacitor rated 50 V and 2.2 4F) showed the following characteristics: capacity = 1.45 F; tan ot=1.8%; LC/30"=0.04 yA.

In connection with the said rolled element, the separating sheet(s) are preferred to be carbonized for increasing the capacity and decreasing tan oa and ESR (equivalent series resistance). For this purpose, the rolled element prepared as above may be heated at a temperature of not higher than 4000C (usually from 200 to 3000 C, preferably around 2 500C) in air for carbonization until the weight of the separating sheet is decreased to 90 40% of its initial weight.

EXAMPLE 2 In the same manner as in Example 1 but using N-isoproylquinolinium+ (TCNQ-) (TCNQ) (i.e. P-2 salt) instead of P-3 salt, there was prepared a solid electrolyte capacitor. The capacity was 0.75 F, and the impregnation rate was 75%.The high temperature load characteristics (impressed with a standardized voltage of 10 Vat 850C) were as follows: TABLE 3

Initial value Value after 500 hrs Leakage Leakage Capacity tan 8 current Capacity tan 8 current 752 nF 2.0% 0.03 C1A/10" 740 nF 2.8% 0.02 yA/10" EXAMPLE 3 A capacitor element prepared by rolling up a separating sheet (Manila paper) with a film formed foil as the positive electrode and an etched aluminium foil was subjected to carbonization of the separating sheet at 24O0C for 4 hours.The resultant capacitor element was immersed in an aluminum case charged with a TCNQ salt as liquefied by heating on a metal plate of 2900C and cooled with water rapidly for solidification. After sealing with a resin and aging, the obtained capacitor was subjected to testing for temperature characteristics and life characteristics, from which the results are shown respectively in Tables 4 and 5 wherein Cap and tan a represent respectively the capacity and the loss at 120 Hz, ESR represents an equivalent series resistance of 100 kHz, ACap represents the capacity variation to Cap at +250C and LC is the value after 1 5" from impressing with 25 V.

TABLE 4 Temperature characteristics

rr6 +8500 +2500 "wS ACap tan = ESR Cap tan 0 LO ESR ACap tan ESR TCNQ salt (%) (%) , (F) (%) (NA) 9 (%) (%) (m c\i +15.4 5.4 t m salt +6.2 3.2 320 2.65 1.8 0.10 460 -5.1 2.4 1310 ~ E t s T Ul (TCNQ) +9.4 3.2 G 2.69 0.95 0.08 O -3.4 0.83 O O C)ty O ~ X o a t) ~ XY N G 6 ~ o a) < e t cq e t 1 E 2/ e ss e e > r TABLE 5 (25 V impressed) Life characteristics

I I o a ro w5 Q ah ORy O.

s o Initial Atter 500 hours at 950C Cap tan 8 LO ESFI Cap tan a LO ESR TONO salt (NF) N (sA) 8 (F) (%) (A) B P-I salt 2.31 1.8 0.11 690 2.19 2.1 0.07 690 ~ salt 2.65 E 0.10 460 2.50 1.9 o ot 2.69 0.95 0.08 o 2.50 1.1 0.05 o, .U- .

~ ecR O o O t Ni C\i 7 E c / ao As understood from the above, the present invention can achieve the attachment of a solid electrolyte onto a film forming metal by a simple operation without deterioration of the solid electrolyte and provide a capacitor having a large capacity and excellent temperature characteristics.

Claims

1. A solid electrolyte capacitor having a solid electrolyte, wherein the solid electrolyte is obtained by colling a 7,7,8,8-tetracyanoquinodimethane salt in a liquid state for solidification, the 7,7,8,8tetracyanoquinodimethane salt being so stable in a liquid state as not substantially decomposing for a sufficient period of time from completion of its liquefaction.

2. A capacitor according to Claim 1, wherein the sufficient period of time is not more than 4 minutes from completion of the liquefaction when the 7,7,8,8-tetracyanoquinodimethane salt is kept at a temperature higher than the melting point but not higher than 3200C.

3. A capacitor according to Claim 1, wherein the 7,7,8,8-tetracyanoquinodimethane salt is retained at a temperature not higher than 3000C for a period of completion of the liquefaction to the start of the solidification and is solidified within about 1 minute after the liquefaction.

4. A capacitor according to Claim 4, wherein the substituent at the N-position is a C2-C16 alkyl group.

5. A capacitor according to Claim 4, wherein the substituent at the N-position is a C2-C18 aikyl group.

6. A capacitor according to Claim 1, wherein the 7,7,8,8-tetracyanoquinodimethane salt is the Nn-propylquinolinium salt.

7. A capacitor according to Claim 1, wherein the 7,7,8,8-tetracyanoquinodimethane salt is the Nisopropylquinolinium salt.

8. A capacitor according to Claim 1, wherein the 7,7,8,8-tetracyanoquinodimethane salt is the Nn-hexylquinolinium salt.

9. A capacitor according to Claim 1 , wherein the 7,7,8,8-tetracyanoquinodimethane salt is the Nethylisoquinolinium salt.

10. A capacitor according to Claim 1 wherein the 7,7,8,8-tetracyanoquinodimethane salt is the N-n-propylisoquinolinium salt.

11. A capacitor according to Claim 1 , wherein the 7,7,8,8-tetracyanoquinodimethane salt is the N-isopropyli soq ui noli ni u m salt.

1 2. A capacitor according to Claim 1 , wherein the 7,7,8,8-tetracyanoquinodimethane salt is the N-n-butylisoquinolinium salt.

13. A capacitor according to Claim 1, wherein the 7,7,8,8-tetracyanoquinodimethane salt is the N-n-butylpyridinium salt.

14. A capacitor according to Claim 1, wherein the solid electrolyte capacitor comprises a rolled element constructed with a metal foil as the positive electrode and another metal foil as the negative electrode and at least one separating sheet placed between those foils and impregnated with the 7,7,8,8-tetracyanoquinodimethane salt as the solid electrolyte, the said separating sheet or sheets being carbonized.

1 5. A solid electrolyte capacitor comprising a positive film forming metal having an anodized surface, a solid electrolyte and a negative collector, wherein the solid electrolyte is a complex salt between 7 ,7,8,8-tetracyanoqui nodi methane and an N-(C2-C18)alkylisoqui noli ne.

1 6. A capacitor according to Claim 1 5, wherein the molar proportion between the 7,7,8,8 tetracya noqui nodimetha ne and the N-(C2-C18)alkyl-isoqui noli ne is about 2:1.

1 7. A solid electrolyte capacitor constructed substantially as herein described with reference to any one of the three examples given and/or the accompanying drawing.