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<p>Porous anodic aluminum oxide (AAO) films with thicknesses exceeding 500&#xa0;μm present significant potential for advanced X-ray optical applications. However, the stable, long-duration fabrication of such thick films requires overcoming process limitations, such as voltage rise and thickness non-uniformity, which are governed by the complex role of electrolyte additives like ethylene glycol (EG). This study systematically elucidates the time-dependent trade-off of EG by decoupling its competing positive and negative mechanisms. We demonstrate that the primary benefit of EG is its improved wettability, which promotes O₂ gas bubble detachment at the pore bottoms. This mechanism—not enhanced ion circulation—is the true cause of the observed improved thickness uniformity, as it prevents local reaction passivation. Conversely, the primary cost of EG is its high viscosity, which hinders ion transport (diffusion). This leads to concentration polarization at the pore bottoms, causing a significant (non-ohmic) voltage rise that becomes rate-limiting during long-duration growth. We further show that this “cost” (polarization) can be mitigated by increasing the oxalic acid concentration (e.g., to 0.5&#xa0;M), which enhances the diffusive flux of reactants. This mechanistic framework enables a practical operating window (0.5&#xa0;M oxalic acid, 20 vol.% EG) for the stable, burn-free fabrication of thick (&gt; 500&#xa0;μm), uniform porous AAO films, paving the way for their application in X-ray optics and other fields.</p>

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Stable fabrication methods for thick porous anodic aluminum oxide films

  • 期刊论文

  • 发表日期：

    2025年12月5日

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  • Masaki Yamanashi

摘要

Porous anodic aluminum oxide (AAO) films with thicknesses exceeding 500 μm present significant potential for advanced X-ray optical applications. However, the stable, long-duration fabrication of such thick films requires overcoming process limitations, such as voltage rise and thickness non-uniformity, which are governed by the complex role of electrolyte additives like ethylene glycol (EG). This study systematically elucidates the time-dependent trade-off of EG by decoupling its competing positive and negative mechanisms. We demonstrate that the primary benefit of EG is its improved wettability, which promotes O₂ gas bubble detachment at the pore bottoms. This mechanism—not enhanced ion circulation—is the true cause of the observed improved thickness uniformity, as it prevents local reaction passivation. Conversely, the primary cost of EG is its high viscosity, which hinders ion transport (diffusion). This leads to concentration polarization at the pore bottoms, causing a significant (non-ohmic) voltage rise that becomes rate-limiting during long-duration growth. We further show that this “cost” (polarization) can be mitigated by increasing the oxalic acid concentration (e.g., to 0.5 M), which enhances the diffusive flux of reactants. This mechanistic framework enables a practical operating window (0.5 M oxalic acid, 20 vol.% EG) for the stable, burn-free fabrication of thick (> 500 μm), uniform porous AAO films, paving the way for their application in X-ray optics and other fields.