Oxidation kinetics of uranium treated by pulsed laser nitriding in air

Abstract

The oxidation kinetics of uranium surfaces modified by pulsed laser nitriding in air at temperatures of 420, 440, 450 and 460 K has been investigated. X-ray diffraction (XRD), Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) were used to characterize the structure, morphology and composition of the samples. The nitrided uranium was consisted of three parts: an outer thin uranium oxide (UO2) layer in contact with air, an inner oxygen diffusion layer at the interface, which was formed by the diffusion of oxygen into the uranium mononitride (UN) and the quantity of oxygen descended gradually as the modified layer thickening, and an underlying UN layer mixed with metallic uranium. Oxide thickness measurements via in situ XRD indicate that a linear oxidation kinetics for the nitrided uranium was observed at 460 K. At temperature range 420–450 K, the oxidation kinetics can clearly be subdivided into two stages: a non-linear curve, initial region of slow oxide-film growth, which is followed by a much faster growth stage of a linear curve. The transformation critical thickness was about 300 nm. The average activation energy of oxidation was estimated to be 118.8 kJ/mol and 89.5 kJ/mol, when the oxide-film for new growth was below and above the critical thickness, respectively.