Nanostructuring of Refractory Metal Surfaces by Electrochemical Oxidation: Nb and the Binary Systems Ti-Ta and Nb-Ta

Metal oxide nanotubes were produced on Ti, Nb, and TiTa and TiNb alloy surfaces by anodic oxidation in fluoride containing electrolytes. Tube diameters from 7 to 110 nm and tube lengths from 50 nm to 1.6 μm were achieved, depending on the substrate material, the anodization parameters, and the composition of the electrolyte. With the application of a voltage of 20 V for 1 h to Nb substrates tubes with 124.3±8.3 nm length were formed, while the same anodization conditions resulted in a tube length of only 66.7±14.2 nm, when applied in a glycol based electrolyte. While there was a linear connection between tube diameter and voltage for Ti anodization, Nb showed limits in the achievable tube dimensions. Furthermore, significant influences on the ion transport mechanisms were found for variations of electrolyte viscosity and stirring frequency, which highly affected the morphology and the size distribution of the obtained nanotubes. Very different anodization behaviour was found for alloys of Ti with Nb resp. Ta, leading to the conclusion that the Pilling- Bedworth ratio must be kept at an average value below 2.44, if fabrication of isolated nanotubes on an alloy surfaces should be successful. Keywords: Nanotubes, anodization, self-organization, metal oxide, Pilling-Bedworth ratio