Certified reference materials for quantification of vanadate species in anodic alumina Rf-GDOES depth profiles

Abstract

Certified reference materials (CRM) for quantification of Radiofrequency Glow Discharge Optical Emission Spectrometry (Rf-GDOES) depth profiles of anodic oxide films are very limited. This paper demonstrates that CRM's for quantification of electrolyte-derived species can be more reliably obtained for inwardly mobile species (phosphorus) than for outwardly mobile species (vanadium) when anodising superpure aluminium in their respective solutions at several current densities. Superpure aluminium specimens were anodised to 150 V, in sodium metavanadate solution, at current densities 5, 10, 15, 20, 25 and 50 mA cm⁻². Sufficiently different concentrations were obtained by RBS for the outwardly mobile vanadate species (0.68, 0.75, 0.13, 0.06, 0.09 and 0.13 at%) compared with 1.19, 1.42, 1.48, 1.49, 2.08 and 2.37 at% for the inwardly mobile phosphorus species. Evidently greater concentrations of phosphorus species are incorporated into the oxide film. Significant reduction in vanadium concentrations with current density correspond to a large destabilisation of the V₂O₇⁻ ion by the anodising electric field. Quantitative depth profiles are determined by means of calibration plots for vanadium and phosphorus. Excellent quantification accuracy of <±0.06 at% is obtained for inwardly mobile phosphorus species by comparing the concentration determined by Rutherford Backscattering Spectroscopy (RBS) with concentration obtained directly from the quantified Rf-GDOES depth profile whereas significantly less accurate quantification accuracies are obtained for the outwardly mobile vanadium species. Quantification accuracy for depth is excellent at ±1 nm as validated by Transmission Electron Microscopy (TEM). CRM's of inwardly mobile impurity species obtained by anodising at different current densities are therefore more reliable and accurate than those of outwardly mobile species.