Síntese e caracterização de filmes anódicos de NB-AL2O3 sobre al por oxidação eletrolítica à plasma

Abstract

In this study, the anodization under high electric field, also known as Plasma Electrolytic Oxidation (PEO), was used for the synthesis of oxide coatings containing Nb2O5 and Al2O3 on Al substrate. The mechanisms of the anodic oxide growth was investigated in different experimental conditions using a factorial design 22 to optimize the experiments and to analyze the influence of variables on the properties of the films formed. The anodic films of Nb2O5 / Al2O3 were prepared by galvanostatic anodization of Al in an electrolytic solution containing different quantities of ammonium oxaloniobate complex (NH4H2(NbO(C2O4)3).3H2O) in aqueous solution of ammonium hydroxide (NH4OH). The anodization’s were carried out in a glass reactor with external jacket insulated to control the temperature of the electrolyte. The samples were cut in 1.3 cm x 0.5 cm x 0.2 cm dimensions from a commercial aluminum plate (98,7%). The synthesis variables studied were temperature, composition and electrolyte concentration. The anodic films were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-rays Diffraction (XRD) and reflectance diffuse spectroscopy. The influence of the variables on electrochemical, morphological and compositional responses were evaluated using the chronopotentiometric curves, pore density from SEM images analysis and Niobium content in the anodic coatings from EDS spectra. The increase of the electrolyte concentration decreased approximately 20 V the potential oscillation as observed in chronopotentiograms, reduced the pore density of the films and increased the Nb content into oxide. The electrolyte temperature presented a more significant effect on the morphological responses. The increase of the electrolyte temperature promoted a decrease of the pore density of the oxide coatings. The electronic and microstructure properties were also investigated. The bandgap values of the oxide coatings were estimated from diffuse reflectance spectra in the UV-Vis region. X-ray diffraction patterns revealed a crystalline coating containing polymorphs of Niobium pentoxide and gama-Al2O3.