Therefore, the etching conditions must be adjusted to obtain properties appropriate for the intended operating voltage. For low-voltage electrolytic capacitors (6.3 to 100 V),
sponge-like structures provided very large specific capacitance (Figure 2), whereas for high-voltage capacitors (160 to 630 V),tubular structures are typical to accommodate the greater oxide thickness while leaving space for the electrolyte to penetrate(Figure 3).