Sol-gel coating of ZrO₂ film in aluminium etch pit and anodizing properties

ZrO₂ films were coated on aluminum etching foil by the sol-gel method to apply ZrO₂ as a dielectric material in an aluminum(Al) electrolytic capacitor. ZrO₂ films annealed above 450 °C appeared to have a tetragonal structure. The withdrawal speed during dip-coating, and the annealing temperature, influenced crack-growth in the films. The ZrO₂ films annealed at 500 °C exhibited a dielectric constant of 33 at 1 kHz. Also, uniform ZrO₂ tunnels formed in Al etch-pits 1 μm in diameter. However, ZrO₂ film of 100-200 nm thickness showed the withstanding voltage of 15 V, which was unsuitable for a high-voltage capacitor. In order to improve the withstanding voltage, ZrO₂-coated Al etching foils were anodized at 300 V. After being anodized, the Al₂O₃ film grew in the directions of both the Al-metal matrix and the ZrO₂ film, and the ZrO₂-coated Al foil showed a withstanding voltage of 300 V. However, the capacitance of the ZrO₂-coated Al foil exhibited only a small increase because the thickness of the Al₂O₃ film was 4-5 times thicker than that of ZrO₂ film.