Study On The Technology And Mechanism Of Manufacturing Aluminum Electrolytic Capacitor Foils With High Capacitance

Recent development of small electronic devices requires increase in capacitance of aluminum electrolytic capacitors. Higher capacitance can be obtained with enlarging surface area of aluminum foils and increasing the dielectric constant of dielectric films on aluminum foils. The main goal of this paper is to investigate the mechanism of corresponding processes for increasing capacitance of aluminum foils used for low-voltage aluminum electrolytic capacitors. The important results and innovations are as follows:1. The pit corrosion initiation and growth mechanism of high-purity aluminum in the corrosive media was studied.The galvanostat electrochemical current noises of high-purity aluminum in different corrosive media were studied. It was found that pit corrosion mostly initiates from the active spots in and on the passive film. These active spots mostly correlate with the surface structure and electrochemical environment of high-purity aluminum, not directly with the composition of solution. Pit initiation is the result of the interaction between active spots and corrosive ions such as Cl- and so on. Whether active spots become corrosion nuclei is influenced by impact from corrosive ions and protection from inhibitive ions, so, by the composition of solution. Increasing the concentration of Cl- enhances pits initiation probability, and addition of SO42- greatly decreases pits initiation probability of high-purity aluminum in the corrosive media.2. AC corrosion mechanism of high-purity aluminum was studied. The mechanism of Cl- penetrating through the passive film by oxygen-vacancy-chain and the inhibition mechanism of SO42- were first explained.AC corrosion mechanism of high-purity aluminum was studied by means of cyclic triangular wave potentiodynamic method. The mechanism was proposed that Cl- penetrates through the passive film by oxygen-vacancy-chain in the crystalline boundary, and impacts aluminum substrate. According to this mechanism, the corresponding experimental phenomena were explained. The inhibition mechanism of SO42- in the AC corrosion of high-purity aluminum was studied. Under the effect of local electric field, O2- that dissociated from SO42- captures the oxygen vacancy, decreases the concentration of oxygen vacancies, and modifies the network of Cl- penetrating ways in the passive film. So, a certain amount of SO42- addition is advantageous to the generation of the sponge structure on aluminum foils.3. The YF model was proposed, and the theoretical surface roughness factor of aluminum foils was calculated.A model for calculating the theoretical surface roughness factor K of aluminum foils was developed. The simulation results showed that the morphology of etched aluminum foils is similar to the actual one, and the surface roughness factor K obtained by calculating units trace is approximate to the practical one of aluminum foils for high-voltage use. However, the K value of aluminum foils for low-voltage use is larger than the practical one. This suggests that it is possible to further increase the capacitance of low-voltage aluminum capacitors. 4. The technologies of pits initiation process were studied. How to control the pits initiation is a very important problem in the surface enlarging processes of aluminum foils by electrochemical etching. This paper contrasted AC pits initiation process with DC pits initiation process. The technical parameters of AC pits initiation process were optimized. As the result, the capacitance of etched aluminum foils can be obtained up to 18.8 at the rated withstanding voltage of 51V by normal pit growth processes.5. The fabricating technologies of high dielectric constant oxide films were studied.To increasing the capacitance of Aluminum foils, increase in the dielectric constant of dielectric films is as important as increase in surface area. This paper studied a series technology, which are compatible with the current etching and formation linkage line, for fabricating high dielectric constant oxide films on aluminum...