Effects of polymer corrosion inhibitor on widening etch tunnels of aluminum foil for capacitor
Introduction
Aluminum electrolytic capacitor is smaller than other capacitors but has a greater capacity per volume and has been widely used as important electronic component. Its capacitance is determined by both the electrode surface area and the thickness of the dielectric layer. Since working voltage determines the oxide layer thickness (1.4 nm/V), the surface area becomes the most important parameter to be controlled. Electrochemical DC etching in hot acidic solution is an important industrial process used for increasing the surface area of the Al foil electrode by formation of etch tunnels for high-voltage application [1], [2], [3]. In general, the electrochemical etching process consists of three continual steps: pretreatment, etch tunnel formation, and widening of the etch tunnels. Pretreatment is usually applied to obtain an even distribution of high-density tunnels, including various chemical or physical methods. Etch tunnel formation is obtained by DC etching high-purity aluminum foils with {0 0 1} texture in acid solutions with chloride ions, using crystallographical dissolution. Widening of the etch tunnels is usually accomplished in dilute nitric acid or hydrochloric acid solutions by chemical etching without DC current or electrochemical etching with applying an electric current [4], [5].
In order to enlarge the specific internal area by increasing tunnels density, improving tunnels distribution and reducing the rate of dissolution concentrated on local area, many corrosion researchers have studied the effects of the grain size, cubic texture, elemental impurities of pure aluminum foils [6], [7], [8], [9], acid solution concentration, temperature and composition, current parameters [10], [11], [12], [13], [14], [15] and physical masks [16], [17], [18] on pretreatment and etch tunnel formation. However, few works have been focused on the etching characteristics during widening of etch tunnels. Additionally, almost no research has been reported on the effects of polymer inhibitor on widening etch tunnels of aluminum foil for capacitor.
In this study, we investigated the effect of the additive of polymeric corrosion inhibitor, polystyrene sulfonic acid (PSSA) to dilute nitric acid solution on the etching behavior during widening etch tunnels. The molecular formula of PSSA is (C8H8O3S)n. Its appearance is light yellow liquid. Its average molecular weight, solid content (wt.%) and density are 30,000, 30% and 1.21 g/mL, respectively. The relationship among etched morphology, specific capacitance and polarization curve is interpreted. The results can help to clarify the function of PSSA during the widening etch tunnels.
Section snippets
Specimen
A commercial(from HEC) tunnel-etched aluminum foil (without widening etch tunnels) for high voltage usage with high purity (>99.99%) and cubic texture (>95%) was used as specimen and its thickness is about 116 μm, as illustrated by Fig. 1. This foil is rich in numerous tapered tunnels and much of its area was orientated with the (1 0 0) plane parallel to the surface. The tunnels followed a 〈1 0 0〉 direction, and the metal texture caused the tunnels to be mostly aligned normal to the surface.
Electrochemical etching etch tunnels
Widening
Effect of PSSA on the property of foils
Fig. 3 shows the effect of different amounts of PSSA additive to the 3% HNO3 solution at 70 °C on the specific capacitance, weight loss and thickness of pre-etched foils after tunnel widening, respectively. It can be found that the specific capacitance sharply increases with PSSA concentration up to 0.15 mL L−1 and then gradually decrease for additional concentration of PSSA. Trace amount of PSSA can significantly enhance specific capacitance. Meanwhile, after tunnel widening, the samples change
Discussion
In the conventional tunnel etching and widening etchant for Al foil, the dissolution of the Al foil surface tends to proceed preferentially over the growth of the etching holes. Therefore, it is difficult to widen the tunnels effectively and increase the surface area and specific capacitance to a desired extent. As a corrosion inhibitor, PSSA is a kind of polymer electrolyte having cation exchange and dispersible or soluble linear structure. When the pre-etched Al foil is subjected to the
Conclusions
During DC widening tunnels of etched aluminum foil with {1 0 0} texture for capacitor in HNO3 solution, the addition of trace amount of PSSA to the solution has a significant influence on the shape of etch pits and tunnels. With PSSA, the dissolution of exterior surface of etch tunnels of Al foil is suppressed and the dissolution of interior surface of etch tunnels of Al foil is facilitated. The tunnels transform from circular cane to circular column the pits-merging is weaken and the tunnels are
Acknowledgments
The work is financially supported by the Chinese National Nature Science Foundation (Grant. 51172102/E020801).
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