Study on the mechanism of oxide hydration and oxide pore closure during hydrothermal treatment of porous anodic Al2O3 films

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Study on the mechanism of oxide hydration and oxide pore closure during hydrothermal treatment of porous anodic Al2O3 films (EN)

Patermarakis, G (EN)
Kerassovitou, P (EN)

N/A (EN)

Porous anodic Al2O3 films were investigated for their behaviour during hydrothermal treatment in H2O at 100-degrees-C and a mechanism for the process of oxide hydration was proposed. The films were prepared galvanostatically in a 15% w/v H2SO4 bath at 20, 25 and 30-degrees-C and at 5, 15 and 35 mA cm-2 The resulting thicknesses were between 1.5 and 120-mu-m. It was revealed that the maximum amount of H2O 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 less-than-or-equal-to 75-mu-m. Additionally, dry or hydrated Al2O3 examined by XRD and ir spectroscopy techniques failed to reveal the presence of any crystalline form of Al2O3, 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 Al2O3 hydration mechanism was proposed including a "reaction" between H2O and Al2O3 pore wall surface. It results in the addition of OH groups and chemisorbed molecular H2O on microcrystallite surfaces effecting the separation of microcrystallites and physically adsorbed molecular H2O 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 Al2O3 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. (EN)


oxide (EN)
hydration (EN)
anodizing (EN)
aluminium (EN)
porous. (EN)
mechanism (EN)

Εθνικό Μετσόβιο Πολυτεχνείο (EL)
National Technical University of Athens (EN)

Electrochimica Acta (EN)



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