Estudo comparativo das membranas cerâmicas de alumina anódica preparadas com ácido oxálico, sulfúrico e fosfórico.

Abstract

The anodic alumina membranes are obtained from the anodisacao of an aluminum plate with high purity (99,997%). The microstructure of these layers of anodic alumina consists of hexagonal cells, with a pore placed in the center of each cell, in a perpendicular direction to the surface of the metal (aluminum). There is a model that suggests an explanation for the anodic alumina film growth, based on the migration of the ions 0 2 + and A l 3 + by a change mechanism and forming alumina (AI2O3). During the anodisation process, some ions, are incorporated in the membrane as impurity. However, there is a necessity of new researches to observer the influence of this ions in the pore and cell diameter. These membranes were obtained by using the phosphoric, oxalic and sulfuric acids as electrolytes. All the membranes were obtained with success presenting a specific coloration related to the used electrolyte and proportional the applied voltage, due to incorporation of anions as impurity. The results from the scanning electron microscopy revealed a very organized structure, with pores evenly well distributed on the surface and parallel to each other. The membrane obtained with acid phosphoric and 90 Volts presented a pore diameter and cell diameter of 128 nm and 217 nm, respectively, and a porosity of 32%. The membrane obtained with acid oxalic and 100 Volts presented a pore diameter and cell diameter of 77 nm and 218 nm, respectively, and a porosity of 9,5%. It was not possible to estimate the values for membrane obtained with sulfuric acid due to the irregularities in its structure. The energy dispersive spectroscopy showed that the membranes were constituted essentially by the Al and O elements, also it was found the presence of P, C and S elements for the membranes obtained with acid phosphoric, oxalic and sulfuric, respectively. The infra-red spectroscopy confirmed the presence of the anions that come from the electrolytes, showing absorbance peaks with wavelengths attributed to the bond vibrations related to phosphates, oxalates and sulfates, also the intensity of the peaks are proportional the applied voltage. The thermal differential analysis showed exothermic peaks with a maximum at 850 °C, for membrane obtained with phosphoric acid, a maximum of 854 °C for the membrane obtained with oxalic acid and a maximum of 936 °C for the membrane obtained with sulfuric acid. This suggesting a change from amorphous microstructure to a polycrystalline one. The thermogravimetric analysis showed a loss of mass of 4,8%, 8,2% and 14,2% for the membranes obtained with phosphoric, oxalic and sulfuric acids, respectively. This loss is due to dehydration, elimination of the hydroxyl groups and also due to the impurities incorporated in the membrane from the electrolytes. The data obtained showed that the membrane obtained with acid phosphoric presented the greater facility during of the process and obtaining the best results.