Study of the spontaneous oxidation of sodium in air by EPMA

OLIVA, F.Y.; EZEQUIEL LEIVA; GERMÁN LENER; DANIEL BARRACO; J. C. TRINCAVELLI

Córdoba

Simposio; 14th International Symposium on Radiation Physics; 2018

The growth of the corrosion layer occurring when a sodium surface is exposed to air under room conditions was studied. To this purpose, the ratio between O-Kα and Na-Kα characteristic peak intensities was determined from the x-ray emission spectrum of a metallic sodium sample exposed to air at different times. The sodium sample was irradiated by 25 keV electrons in a SEM-FEG Zeiss instrument model Sigma. x-ray spectra were obtained using an energy dispersive spectrometer attached to the SEM, which consists of a silicon drift Oxford detector with an Aztec characterization system. During the spectra acquisition, controlled amounts of air were allowed to enter into the SEM chamber through a manual valve.It is reasonable to assume that, for very thin sodium oxide layers, the oxygen peak intensity IO-Kα linearly increases with the film mass thickness ρx, for layers of mass density ρ and linear thickness x, because self-absorption and electron scattering can be neglected [1]. For larger thicknesses, the slope of the IO-Kα vs. ρx curve decreases until the oxide thickness equals the electron range, i. e., the mean electron penetration depth, where saturation takes place. This saturation effect occurs because the whole region of the material reached by the electron beam is within the oxide layer. On the other hand, ρ.x is expected to increase with the exposition time to air. The evolution of IO-Kα with time can be seen in Fig. 1, along with an opposite trend of INa-Kα.Figure 2 shows the intensity ratio R=IO-Kα/INa-Kα as a function of exposure time to air t along with Monte Carlo simulations [2] performed for both bulk Na 2 O and Na 2 O 2 samples. As can beseen, two regimes seem to occur: in a first stage, Na 2 O would be formed, and in a second stage, after the first 9 seconds of air exposure, Na 2 O 2 would grow on top of the previous layer.Additional simulations were carried out for different oxide layer thicknesses x on metallic sodium to find a relationship between R and x. From this relationship and the one shown in Fig. 2 between R and t, a law to describe the oxide growth with time was found for each regime.