Influence of minor alloying elements on γ′-Al2O3 crystalline anodic formation and its impact on the dielectric properties of solid conductive polymer capacitors

Abstract

High-purity aluminum with minor alloying elements (Fe, Cu, Si, Zn) at the ppm level is essential for achieving an enhanced etched surface area in capacitor manufacturing. However, their influence on the dielectric material formation and the resulting capacitor properties remains unclear. In this study, a 99.99 % pure aluminum was heat-treated at 300, 400, and 540 °C for 3 h, followed by rapid cooling in water. After forming a hydrated alumina layer, a dielectric layer was formed by anodizing at 700 V in a 0.5 mol L⁻¹ boric acid solution at 85 °C to form crystalline γ′-Al₂O₃. Fe-enriched nanoparticles were observed due to the limited solubility of Fe in Al at temperatures below 400 °C, and their presence led to the formation of flaws in the anodic layer during anodization. The conductive polymer solid capacitors, consisting of anodized aluminum coated with PEDOT:PSS, exhibited breakdown potentials averaging between 407 and 411 V (≈41 % below the anodizing voltage of 700 V). However, after the heat treatment at 540 °C, Fe completely dissolved, and the rapid cooling retained it in solid solution, resulting in a defect-free anodic layer. Consequently, the breakdown potential for the solid polymer capacitor was comparable to, or slightly higher than, the anodizing potential.