雜質及前處理對電解電容器用鋁箔直流電蝕之效應

Abstract

本論文在研究電解電容器用鋁箔之特性及原箔雜質與 前處理對其直流電蝕行為之影響。結果顯示中高壓用之鋁 箔具有高純度,高立方度,晶粒大於箔厚等特徵。低壓用鋁 箔純度略低,但立方度遠低於高壓用箔。為研究雜質鉛， 銦對鋁箔電蝕行為之影響，在諸多高壓用鋁箔中，選用兩 種電蝕形貌差異大者為研究箔材。結果顯示,鋁箔電蝕行 為與存於鋁箔內之鉛、銦等雜質含量及分佈有密切關連; 雜質在壓軋線的聚集會導致電蝕點沿壓軋線分佈,造成電 蝕分佈不均,同時不利於隧道蝕孔的發展。 經由置換還原法引入之沉澱鉛,銦等雜質可克服電蝕常發 生的壓軋線效應,提高電蝕效能。另外銦雜質對鋁電蝕點 之調質能力又強於鉛。這使得在引進銦的製程中,可在較 低溫度及較稀濃度情況下進行。 為研究雜質調質之機理，本論文亦以磷酸前處理取代雜質, 研究磷酸對鋁箔電蝕行為之影響。結果顯示,在高濃度磷 酸處理下可得到較密之隧道蝕孔分佈。由磷酸物種-pH值 之關係及ATR 分析之結果顯示：在低濃度時(<20 vol. %), 因其具較高pH值，有較多解離之單價磷酸根離子(由前處 理中引入)與氯離子競爭吸附於鋁箔表面，造成孔蝕電位 朝安定方向移動而得到較低的隧道蝕孔密度。另一方面， 在高濃度磷酸(>20 vol. %)中,其pH值較低，此時幾乎以 不解離之磷酸存在，大量的Al//AlH3PO4吸附局部電池，造成 孔蝕電位朝較活潑方向移動，而得到較高之隧道蝕孔密 度。但在非常高濃度之磷酸前處理時，由於非常大量的 AlH3PO4吸附位置互相接合而形成鈍態膜。此時會得到較安定孔蝕電位及 較低的隧道蝕孔密度。 綜觀本研究，三種表面 調質(或三種前處理)之方法(鉛，銦，磷酸引入鋁箔表面)均具 一共通原理。即:他們均以創 造眾多電化學局部電池來調整電 蝕效能，而優良之前處理 必須能提供適當且均勻分佈之局部電池。 The characteristics of aluminum foils for capacitor usage and the effects of foil impurities and pretreatment on the DC-etching behavior of foils were investigated in the present work. The experiment shows that foils for high voltage usage have high purity, high cube texture with grain sizes larger than foil thickness. The foils for low voltage usage have lower purity and cubicity. Two of the high voltage foils with different etching behaviors were chosen to study the effects of lead and indium impurities on the etching behavior. Results showed that the difference in etched morphology of the two foils are related closely to the lead and indium content and their distributions on foil surface. The impurities, such as lead and indium, segregated along the rolling lines are considered responsible for the resulted nonuniform etching pits and inhibited etchability. Through introducing the lead or indium impurities on foil surface by immersion reduction method, the rolling line effect can be improved or even overcome, and thus enhance etchability. The modification of etchability by indium appears stronger than that by lead. This facilitates employing lower temperature and dilute concentration in the indium-introducing process to optimize the etchability. The phosphoric acid pretreatment was performed in order to study its role as compared with that of the impurity introducing. The composition-pH relationship of phosphoric acid and ATR analyses showed that: in dilute (<20 vol. %) phosphoric acid with higher pH value, the pitting potential would shift toward noble direction, this is caused by more H2PO4- anions (introduced in pretreatment process) being in competition with Cl- to adsorb on aluminum surface and resulting in lower tunnel density. On the contrary, in concentrated (>20 vol. %) phosphoric acid with lower pH value, H3PO4 species dominate almost exclusively, the numerous Al//Al H3PO4 adsorbed local cells would drive pitting potential toward active direction and result in higher tunnel density. In case that extremely concentrated phosphoric acid being employed, the very high population of AlH3PO4 adsorbed sites would connect together to form a passive film and result in a noble pitting potential with much less tunnel density. Overhauling the present work, it is apparent that the three modification (or pretreatment) methods, including Pb, In and H3PO4 introducing onto aluminum surface, possess a common theme. That is, they all tends to modified the etchability by creating electrochemical local cells on foil surface, and a useful pretreatment must be capable of producing proper amount and uniformly distributed local cells.