Effects of organic acid and phosphoric acid at the low-voltage stage of the high-voltage forming process for the anode dielectric film of an aluminum electrolytic capacitor

The dielectric film formed on the anode foil surface of aluminum electrolytic capacitors through multistage formation is the primary working medium of aluminum electrolytic capacitors, and its quality directly determines the overall performance of aluminum electrolytic capacitors. Industrial production shows that replacing boric acid with organic or phosphoric acid in the low-voltage formation stage can improve the specific capacitance, voltage resistance, and other electrical properties of the dielectric film. However, the effects and mechanisms of using organic or phosphoric acid at the low-voltage stage on the dielectric film structure on the anode foil and the electrical properties of the formation are unclear. In this study, organic acids and ammonium dihydrogen phosphate were used in the low-voltage stage of the formation, and their effects on the structure, as well as the electrical properties of the dielectric film on the high-voltage anode foil, were revealed by comparing them with boric acid. At the low-voltage stage, the organic acid removes the poorer-quality dielectric film precursor (hydrated film). By contrast, the phosphoric acid inhibits the ion transportation in the dielectric film, which reduces the thickness of the hydrated film by about 20% and 23.7% for the organic and phosphoric acids, respectively, compared with that of the boric acid. Selection of the hydrated film by organic acid enhances the crystallinity of the dielectric film in the high-voltage stage but increases its grain size. Comparing the electrical properties of the dielectric films on the high-voltage anode foil under the three formation routes of organic, phosphoric, and boric acids shows that organic acid could enhance the withstand voltage and specific capacitance of the anode foils compared with boric acid. On the contrary, phosphoric acid reduces the ramp-up time. This study can provide guidance for the optimization of the formation liquid system in the industrial production process of high-voltage anode foils for aluminum electrolytic capacitors and promote the development of aluminum electrolytic capacitors toward high specific capacitance and miniaturization.