Study on the mechanism of oxide hydration and oxide pore closure during hydrothermal treatment of porous anodic Al 2O 3 films

Abstract

Porous anodic Al 2O 3 films were investigated for their behaviour during hydrothermal treatment in H 2O at 100°C and a mechanism for the process of oxide hydration was proposed. The films were prepared galvanostatically in a 15% w/v H 2SO 4 bath at 20, 25 and 30°C and at 5, 15 and 35 mA cm −2. The resulting thicknesses were between 1.5 and 120 μm. It was revealed that the maximum amount of H 2O uptake, on prolonged hydration and retained by the oxides, is directly related to the pore void volume of initially produced dry films. Their ratio, m w,m/ v, is always close to 1 g cm −3 for all films having a mean thickness of ⪕ 75 μm. Additionally, dry or hydrated Al 2O 3 examined by XRD and ir spectroscopy techniques failed to reveal the presence of any crystalline form of Al 2O 3, AlOOH and Al(OH) 3. The absence of any crystalline hydrated forms is consistent with the dependence of m w,m on v supporting the hypothesis of a microcrystalline nature of films. An Al 2O 3 hydration mechanism was proposed including a “reaction” between H 2O and Al 2O 3 pore wall surface. It results in the addition of OH groups and chemisorbed molecular H 2O on microcrystallite surfaces effecting the separation of microcrystallites and physically adsorbed molecular H 2O on the hydroxylated/hydrated surfaces in the intercrystallite spaces causing the swelling of the moderately coherent hydrated oxide layer on pore walls. Ultimately, the process results in the depletion of Al 2O 3 at points where it has become emaciated, ie at the mouths of conical pores in the cases of the maximum limiting film thicknesses attained and in the production of a hydrated layer, on pore walls, of variable thickness sufficient to fill up pores along a distance from the pore base, depending on structural film features.