XPS study on the oxidation-induced segregation behavior of U–Nb alloy

Abstract

In this work, the surface structure and chemical composition of U–Nb alloy after one year of oxidation under ambient conditions were investigated using X-ray photoelectron spectroscopy (XPS) depth profile methods. Furthermore, the in-situ oxidation of U–Nb was carried out under UHV conditions, providing insights into the initial oxidation behavior of U-Nb. Studies show that, an thin (1.5 nm) layer of UO₂ intermixed with enriched Nb₂O₅ is formed on the top of the naturally oxidized U–Nb, with an inner oxide layer consisting of UO_2-x intermixed with depleted NbO₂ and NbO at below, followed by an inner layer of UO_2-x intermixed with metallic U and Nb adjacent to the U-Nb substrate. During the initial oxidation stage, U oxidizes rapidly to form UO_2-x, while the leaving Nb in the UO_2-x lattice migrates inward and enriched at the metal side of the oxide/metal interface. As oxidation continues, oxidation-induced segregation of the leaving Nb in the UO_2-x lattice toward the surface occurs and forms NbO and NbO₂ at the gas-oxide interface. The affinities of U and Nb to OH and O anions may act as the driving force of the segregation.