Porous structure obtained by anodizing niobium in NaOH

Porous structures can be produced as nanotubes, nanowires and membranes, and they can be used in gas sensors, magnetic and electronic devices, biotechnology, etc. These structures can be obtained by the anodizing process, in which the electrolyte must etch the oxide to occur the pore formation. In the case of niobium, porous oxides have been obtained successfully in electrolytes containing hydrofluoric acid (HF). However, studies have shown that niobium dissolves in electrolytes of NaOH with pH 14, and porous oxides of niobium have already been obtained by the anodizing process with the occurrence of sparking. Thus, the objective of this work is to obtain porous oxides on Nb without HF addition by studying the sparking phenomenon. Therefore, niobium was anodized at different current densities (10, 25, 50, 75 and 100 mA.cm-2). The samples were evaluated concerning morphology by SEM and chemical microanalysis by EDS. It was observed that sparking depends of the oxide barrier thickness and occurs at current densities higher than 50mA.cm-2. However, the increase of the current density, in spite of not increasing the thickness of the barrier layer, caused a greater incidence of sparking. According to the assignment proposed it was possible to obtain a porous structure in niobium with NaOH electrolyte, but this structure got an irregular aspect.