最佳化鋁箔電解條件以改進中高壓電容器之可靠性

Abstract

為達到提升中、高壓電容器用陽極箔之靜電容量的目的，本論文分別以HCl、H2SO4及HNO3做為中高壓電蝕製程中之第二電蝕程序(E2)反應槽液，並利用改變其濃度方式進行最佳化電蝕製程參數的尋找，從實驗結果可以發現，E2槽使用HCl為反應液時，因為Cl-的反應活性極強且與Al所形成之鈍化膜不足以阻礙Cl-對鋁的溶解作用，導致鋁箔表面會有新生成之孔洞(pits)造成過度腐蝕(over etching)現象，同時Cl-亦會在隧道組織(tunnel)底端持續進行溶解作用，造成tunnel貫穿鋁芯的現象，此現象也會讓電蝕箔的機械強度大幅下降。E2槽液使用H2SO4時進行電蝕擴孔反應時，則會因為Al2(SO4)3的鈍化效果可以完整的保護tunnel組織，導致tunnel無法順利進行擴孔反應，且隨著SO42-濃度的增加，鋁溶解反應集中於鋁箔表面進行，最後造成V型tunnel出現。當E2槽使用HNO3為反應槽液時，N03-的反應活性與Al(NO3) 3的鈍化能力呈現平衡的狀況，故在適當的濃度下tunnel的形態保持較佳的柱狀結構，讓電蝕箔靜電容量可以有效提升。本研究發現E2槽之最佳化條件為使用HNO3為反應槽液，濃度在0.25~0.75N下會有最佳之靜電容量的表現。

To improve the capacitance of anodic aluminum foils in middle and high voltage capacitors, we studied the anodic etch of the capacitors using HCl, H2SO4 and HNO3 separately as the reaction solution in the second foil etch process (E2), and thereby optimized the fabrication condition of the anodic foil based on the etch study. Accord to the experiment, the passivation layer (AlCl3) formed on the Al foil was dissolved in the HCl solution, which was the reaction solution in the E2 process, resulting in pit growth on the Al foil as a result of overetch due to the high reactivity of Cl- ions. As Al dissolution continuously occurred at the bottom of pore tunnels, the Al foil might be punched through, degrading the mechanical strength of the foil. When H2SO4 was used as the E2 reaction solution, the Al2(SO4)3 passivation layer could effectively protect the tunnel structure, resulting in the difficulty in the tunnel widening. However, as the concentration of SO42- was increased, the anodic etching reaction mainly occurred on the surface of the Al foil, and thereby V-shaped tunnels were finally produced. When the E2 reaction solution was HNO3 with an optimized concentration, the reactivity of NO3- ions was counterbalanced by the Al(NO3)3 passivation layer, and thus tunnels had a relatively regular column structure leading to an enhanced capacitance of the anodically etched Al foil. Based on the study, the optimal anodic etch condition is to use HNO3 reaction solution with a concentration in the range between 0.25N and 0.75N.