JP4088849B2 - Manufacturing method of solid electrolytic capacitor - Google Patents

Description

【０００１】
【発明の属する技術分野】
本発明は、固体電解コンデンサ、特に多孔質の焼結体の陽極を有するタンタル電解コンデンサの製造方法に関するものである。
【０００２】
【発明が解決しようとする課題】
タンタル電解コンデンサは、タンタル金属粉末に有機質バインダを混ぜて所定形状に成形してから焼成して多孔質の焼結体を得、その表面に化成処理によって陽極酸化皮膜を形成させ、その上に二酸化マンガン半導体層を形成させ、更にその上に黒鉛等による陰極層を形成することによって得られる。
【０００３】
このようなコンデンサは、容量の増大を計るためにタンタル金属粉末を極力微細化して焼結体の表面積を増大させているが、これに伴って酸化皮膜の欠陥が増し、コンデンサの漏れ電流が増大し、その原因は次のように考えられる。タンタル金属粉末の微細化により焼結体の内部の空隙も微細になり、焼成時にバインダ等に起因する不純物が外界へ排出され難くなり、そのために微量の不純物が空隙内に残るようになる。この残留する不純物は、化成処理によって焼結体の表面に酸化皮膜を形成する際に皮膜内に欠陥部として取込まれ、更に二酸化マンガン半導体層を形成する際には硝酸マンガン溶液への浸漬及び焼成のサイクルの反復による熱的及び機械的な繰返し応力によってこの欠陥が拡大するためと考えられる。従って本発明はこのような酸化皮膜の欠陥の発生を抑制しようとするものである。
【０００４】
【課題を解決するための手段】
本発明は、タンタル等の弁作用を有する金属の粉末の焼結、焼結体の化成処理による陽極皮膜の形成及び金属塩溶液を含浸させて焼成する酸化物半導体層の形成を含む固体電解コンデンサの製造工程において、金属粉末の焼結後、化成処理に先立って、焼結体を過酸化水素液に浸漬処理することを特徴とする。この浸漬処理によって、焼結体の外表面及び空隙内に残留する不純物は、過酸化水素の強い酸化力によって分解され、外界へ排出される。
【０００５】
図１は焼結体の寸法、重量を異にする３通りのロットＡ、Ｂ、Ｃよりそれぞれ試料を抽出し、これを３５ｗｔ％の過酸化水素液に浸漬した場合の浸漬時間と残留炭素濃度との関係を示す。なお、ロットＡは後述の実施例と同一のロットである。各ロットとも、試料を過酸化水素液に数１０〜数１００時間浸漬することによって、残留炭素濃度が初期値から数〜数１０％減少するが、０％にはならない。これは、焼結体中の空隙の形状が極めて複雑で、微細な空隙の最奥部まで不純物を除去できないためと思われる。しかし、そのような微細な空隙の最奥部は、化成処理に際して酸化皮膜によって閉鎖され、そこに存在する不純物は無害になるので強いて除去する必要はない。従って残留炭素濃度を初期値より２０〜３０％程度を引下げれば、前述の不純物による酸化皮膜の欠陥を事実上除きうることが判明した。
【０００６】
【発明の実施の形態】
タンタル金属の可及的に微細な粉末に、溶剤に溶解した樟脳、ニトロセルローズ等の有機質バインダーを加えて陽極リードと共に所定形状に成形し、乾燥して固結させる。この固結物を１４００℃〜１６００℃で焼成すると焼結体が得られ、このときバインダーの大部分が揮散する。次に焼結体を１０ｗｔ％以上の過酸化水素液に数１０〜数１００時間浸漬し、引上げて常温〜２００℃で乾燥する。
【０００７】
過酸化水素液処理を終えた焼結体は、常法により酸化皮膜形成のための化成処理を行なう。例えば、焼結体を硝酸の稀水溶液中でコンデンサの定格の約３倍の電圧及び数１０ｍＡ／ｇ以下の電流で２時間以上化成処理し、タンタル金属の表面に酸化皮膜を形成させる。
【０００８】
次に、焼結体に硝酸マンガンまたはこれに硝酸アンモニウムを添加した水溶液を含浸させ、約２５０℃で硝酸マンガンを熱分解させ、酸化皮膜上に二酸化マンガン層を形成させる。そして、この含浸及び熱分解の工程を数回乃至１０数回反覆して、二酸化マンガン層を十分な厚さに成長させる。この工程の反覆の間に、初回の化成処理の際より低い電圧による再化成を２〜３回行なえば、酸化皮膜に出来た欠陥を修復して製品歩留を向上できる。
【０００９】
二酸化マンガン層が十分成長したら、焼結体の表面に黒鉛層を形成し、更にその上に銀層を形成してこれに陰極リードを半田付けし、樹脂外装を施して完成品を得る。
【００１０】
【実施例】
重量１１０ｍｇのタンタル焼結体を、常温で過酸化水素液（３５ｗｔ％）に２９１時間浸漬して常温で乾燥した後、０．００１ｍｏｌ％の硝酸水溶液中で４８Ｖの電圧及び３０ｍＡ／ｇの電流で８時間化成処理し、焼結体表面に酸化皮膜を形成させる。
【００１１】
次に、第１回の含浸−熱分解処理として、焼結体に比重１．３８の硝酸マンガン溶液を含浸させ、２５０℃で熱分解させて酸化皮膜上に二酸化マンガン層を形成させる。続いて、焼結体を電導度６４．５±３．３μＳ／ｃｍ（２５℃）の酢酸溶液中で、４０Ｖの電圧で５分間再化成処理する。
【００１２】
次に、第２回及び第３回の含浸−熱分解処理として、焼結体に比重１．３８の硝酸マンガン溶液を含浸させてこれを２５０℃で熱分解させる工程を２回繰返し、更に第４回及び第５回の含浸−熱分解処理として、焼結体に比重１．５８の硝酸マンガン溶液を含浸させてこれを２５０℃で熱分解させる工程を２回繰返す。第５回の含浸−熱分解処理の後、初回の再化成と同じ電導度６４．５±３．３μＳ／ｃｍ（２５℃）の酢酸溶液中で、３０Ｖの電圧によって５分間再化成処理する。
【００１３】
更に、第６回乃至第９回の含浸−熱分解処理として、焼結体に比重１．７３の硝酸マンガン溶液を含浸させてこれを２５０℃で熱分解させる工程を４回繰返し、第１０回の含浸−熱分解処理として、焼結体に比重１．４６の硝酸マンガン溶液を含浸させてこれを２５０℃で熱分解させる工程を１回行なう。そして、初回及び前回と同じ電導度６４．５±３．３μＳ／ｃｍ（２５℃）の酢酸溶液中で２５Ｖの電圧によって第３回目の再化成を５分間行なう。
【００１４】
第３回目の再化成処理を終えた焼結体の二酸化マンガン層の上に、黒鉛層及び銀層を設け、これにリード線を取付け、樹脂外装を施して、定格電圧が１６Ｖである固体電解コンデンサの製品を得る。
【００１５】
比較例として、上記実施例における過酸化水素液への浸漬処理を実施しないほかは上記実施例と全く同様な製法により、定格電圧が１６Ｖである固体電解コンデンサを製造した。
【００１６】
上記実施例製品２０個及び比較例製品２０個について、タンタル焼結体中に残留炭素濃度の平均値、定格電圧１６Ｖ（ＤＣ）における容量、ｔａｎδ値及び漏れ電流値の平均値、過負荷試験（８５℃、２０Ｖ、１時間）における故障コンデンサ数を試験した結果を表１に示し、漏れ電流の累積度数（％）を図２に示す。
【００１７】
【表１】

【００１８】
表１から明らかなように、本発明の実施例では、過酸化水素液への浸漬処理を行なったことにより、これを行なわない比較例と較べて、残留炭素量が約３分の２に減少し、ｔａｎδ値が改善され、漏れ電流が約５分の１に減少し、過負荷試験では比較例は全数が故障したのに対し実施例では故障が皆無であった。図２から明らかなように、上記比較例Ｂでは漏れ電流が１μＡ以下のものが全体の２０％にすぎないのに対し、上記実施例Ａでは漏れ電流が１μＡ以下のものが全体の９５％と非常に高い比率を占めていることが判った。
【００１９】
【発明の効果】
上記実施例から明らかなように、本発明によるときは、固体電解コンデンサの漏れ電流を大幅に減少させると共にｔａｎδ特性も改善し、過負荷電圧に対する耐性を向上させ、高品質の固体電解コンデンサを製造することができる。
【図面の簡単な説明】
【図１】焼結体の過酸化水素液への浸漬時間と残留炭素の関係を示す線図である。
【図２】本発明実施例及び比較例のコンデンサの漏れ電流値と累積度数との関係を示す線図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a solid electrolytic capacitor, particularly a tantalum electrolytic capacitor having a porous sintered anode.
[0002]
[Problems to be solved by the invention]
A tantalum electrolytic capacitor is obtained by mixing a tantalum metal powder with an organic binder and forming it into a predetermined shape, followed by firing to obtain a porous sintered body. A anodic oxide film is formed on the surface by a chemical conversion treatment. It is obtained by forming a manganese semiconductor layer and further forming a cathode layer made of graphite or the like thereon.
[0003]
In such a capacitor, the surface area of the sintered body is increased by miniaturizing the tantalum metal powder as much as possible in order to increase the capacity, but with this, defects in the oxide film increase and the leakage current of the capacitor increases. The cause is considered as follows. Due to the refinement of the tantalum metal powder, the internal voids of the sintered body also become fine, and impurities due to the binder and the like are difficult to be discharged to the outside during firing, so that a trace amount of impurities remains in the voids. The remaining impurities are taken in as defects in the film when the oxide film is formed on the surface of the sintered body by chemical conversion treatment, and when the manganese dioxide semiconductor layer is further formed, it is immersed in a manganese nitrate solution. This is thought to be due to the expansion of this defect by thermal and mechanical cyclic stress due to repeated firing cycles. Therefore, the present invention intends to suppress the occurrence of such defects in the oxide film.
[0004]
[Means for Solving the Problems]
The present invention relates to a solid electrolytic capacitor including sintering of a metal powder having a valve action such as tantalum, formation of an anode film by chemical conversion treatment of the sintered body, and formation of an oxide semiconductor layer impregnated with a metal salt solution and firing. In the manufacturing process, after the metal powder is sintered, the sintered body is immersed in a hydrogen peroxide solution prior to the chemical conversion treatment. By this immersion treatment, impurities remaining in the outer surface and voids of the sintered body are decomposed by the strong oxidizing power of hydrogen peroxide and discharged to the outside.
[0005]
FIG. 1 shows samples extracted from three types of lots A, B, and C with different sizes and weights of sintered bodies, and immersed in a 35 wt% hydrogen peroxide solution and the residual carbon concentration. Shows the relationship. Lot A is the same lot as in the examples described later. In each lot, when the sample is immersed in a hydrogen peroxide solution for several tens to several hundred hours, the residual carbon concentration is reduced by several to several tens of percent from the initial value, but does not become 0%. This is presumably because the shape of the voids in the sintered body is extremely complicated and impurities cannot be removed to the innermost part of the fine voids. However, the innermost part of such fine voids is closed by an oxide film during the chemical conversion treatment, and impurities existing there are harmless, so it is unnecessary to remove them. Therefore, it has been found that if the residual carbon concentration is lowered by about 20 to 30% from the initial value, defects in the oxide film due to the aforementioned impurities can be virtually eliminated.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
An organic binder such as camphor or nitrocellulose dissolved in a solvent is added to the finest possible powder of tantalum metal, molded into a predetermined shape together with the anode lead, dried and solidified. When this solidified product is fired at 1400 ° C. to 1600 ° C., a sintered body is obtained, and at this time, most of the binder is volatilized. Next, the sintered body is immersed in a hydrogen peroxide solution of 10 wt% or more for several 10 to several 100 hours, pulled up, and dried at room temperature to 200 ° C.
[0007]
The sintered body that has been treated with the hydrogen peroxide solution is subjected to a chemical conversion treatment for forming an oxide film by a conventional method. For example, the sintered body is subjected to chemical conversion treatment for 2 hours or more in a dilute aqueous solution of nitric acid at a voltage about three times the rating of the capacitor and a current of several tens of mA / g or less to form an oxide film on the surface of the tantalum metal.
[0008]
Next, the sintered body is impregnated with an aqueous solution containing manganese nitrate or ammonium nitrate added thereto, and manganese nitrate is thermally decomposed at about 250 ° C. to form a manganese dioxide layer on the oxide film. The impregnation and pyrolysis steps are repeated several to ten times to grow a manganese dioxide layer to a sufficient thickness. If re-chemical conversion with a lower voltage is performed 2 to 3 times during the reversal of this process during the first chemical conversion treatment, defects produced in the oxide film can be repaired and the product yield can be improved.
[0009]
When the manganese dioxide layer is sufficiently grown, a graphite layer is formed on the surface of the sintered body, a silver layer is further formed thereon, a cathode lead is soldered thereto, and a resin sheath is applied to obtain a finished product.
[0010]
【Example】
A tantalum sintered body having a weight of 110 mg was immersed in a hydrogen peroxide solution (35 wt%) at room temperature for 291 hours and dried at room temperature, and then in a 0.001 mol% nitric acid aqueous solution at a voltage of 48 V and a current of 30 mA / g. Chemical conversion treatment is performed for 8 hours to form an oxide film on the surface of the sintered body.
[0011]
Next, as the first impregnation-pyrolysis treatment, the sintered body is impregnated with a manganese nitrate solution having a specific gravity of 1.38 and thermally decomposed at 250 ° C. to form a manganese dioxide layer on the oxide film. Subsequently, the sintered body is subjected to re-chemical conversion treatment in an acetic acid solution having an electric conductivity of 64.5 ± 3.3 μS / cm (25 ° C.) at a voltage of 40 V for 5 minutes.
[0012]
Next, as the second and third impregnation-pyrolysis treatments, the step of impregnating the sintered body with a manganese nitrate solution having a specific gravity of 1.38 and thermally decomposing it at 250 ° C. is repeated twice. As the fourth and fifth impregnation-pyrolysis treatments, the step of impregnating the sintered body with a manganese nitrate solution having a specific gravity of 1.58 and thermally decomposing it at 250 ° C. is repeated twice. After the fifth impregnation-pyrolysis treatment, the reconstitution treatment is carried out at a voltage of 30 V for 5 minutes in an acetic acid solution having the same conductivity as that of the first re-formation of 64.5 ± 3.3 μS / cm (25 ° C.).
[0013]
Further, as the 6th to 9th impregnation-pyrolysis treatments, the step of impregnating the sintered body with a manganese nitrate solution having a specific gravity of 1.73 and thermally decomposing it at 250 ° C. is repeated 4 times. As the impregnation-pyrolysis treatment, a step of impregnating the sintered body with a manganese nitrate solution having a specific gravity of 1.46 and thermally decomposing it at 250 ° C. is performed once. Then, a third re-formation is performed for 5 minutes with a voltage of 25 V in an acetic acid solution having the same conductivity of 64.5 ± 3.3 μS / cm (25 ° C.) as the first time and the previous time.
[0014]
Solid electrolysis with a rated voltage of 16 V is provided on the manganese dioxide layer of the sintered body after the third re-chemical conversion treatment, by providing a graphite layer and a silver layer, attaching a lead wire to this, and applying a resin sheath. Get a capacitor product.
[0015]
As a comparative example, a solid electrolytic capacitor having a rated voltage of 16 V was manufactured by the same manufacturing method as in the above example except that the immersion treatment in the hydrogen peroxide solution in the above example was not performed.
[0016]
About 20 examples products and 20 comparative products, the average value of residual carbon concentration in the tantalum sintered body, the capacity at rated voltage 16V (DC), the average value of tan δ value and leakage current value, overload test ( The results of testing the number of failed capacitors at 85 ° C., 20 V, 1 hour) are shown in Table 1, and the cumulative frequency (%) of leakage current is shown in FIG.
[0017]
[Table 1]

[0018]
As is apparent from Table 1, in the examples of the present invention, the amount of residual carbon was reduced to about two-thirds as compared to the comparative example in which the immersion treatment in the hydrogen peroxide solution was performed. However, the tan δ value was improved and the leakage current was reduced to about 1/5. In the overload test, all of the comparative examples failed, whereas the examples had no failure. As is clear from FIG. 2, in Comparative Example B, the leakage current of 1 μA or less is only 20% of the total, whereas in Example A, the leakage current of 1 μA or less is 95% of the total. It turns out that it occupies a very high ratio.
[0019]
【The invention's effect】
As is clear from the above embodiments, according to the present invention, the leakage current of the solid electrolytic capacitor is greatly reduced and the tan δ characteristic is improved, the resistance against overload voltage is improved, and a high quality solid electrolytic capacitor is manufactured. can do.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the immersion time of a sintered body in a hydrogen peroxide solution and residual carbon.
FIG. 2 is a diagram showing the relationship between the leakage current value and the cumulative frequency of the capacitors according to the embodiment of the present invention and the comparative example.

Claims (4)

弁作用を有する金属の粉末の焼結体を過酸化水素液に浸漬した後、焼結体表面に陽極酸化皮膜を形成し、更にその上に二酸化マンガン層を形成することを特徴とする固体電解コンデンサの製造方法。Solid electrolytic process characterized in that a sintered metal powder having a valve action is immersed in a hydrogen peroxide solution, then an anodized film is formed on the surface of the sintered body, and a manganese dioxide layer is further formed thereon. Capacitor manufacturing method. 上記の弁作用を有する金属はタンタルであることを特徴とする請求項１記載の固体電解コンデンサの製造方法。2. The method of manufacturing a solid electrolytic capacitor according to claim 1, wherein the metal having a valve action is tantalum. 上記焼結体は、上記金属粉末を有機質バインダを用いて成形した後に焼成したものであることを特徴とする請求項１記載の固体電解コンデンサの製造方法。2. The method for manufacturing a solid electrolytic capacitor according to claim 1, wherein the sintered body is obtained by forming the metal powder using an organic binder and then firing the metal powder. 上記過酸化水素液は１０ｗｔ％以上の濃度であることを特徴とする請求項１記載の固体電解コンデンサの製造方法。2. The method for manufacturing a solid electrolytic capacitor according to claim 1, wherein the hydrogen peroxide solution has a concentration of 10 wt% or more.

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