Surface oxidation behavior of MgNd alloys

The oxidation kinetics of MgNd alloys oxidized in pure O₂ at high temperatures has been investigated. The results revealed two stages of the reaction: A fast initial oxidation was followed by a slow oxide growth with a parabolic kinetics. For MgNd alloys (Nd = 25 wt.%), the oxidation rate increased with the enhancement of the oxidation temperature. A sudden ignition was found for this alloys oxidized at 873 K up to about 80 min. Moreover, the increase of the Nd content would harm the oxidation resistance of the MgNd alloys. By Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, it was found that a triplex structure of oxide film formed. The outer layer was composed of MgO, Nd₂O₃ and Nd(OH)₃, the middle layer mainly consisted of MgO and Nd₂O₃, and the inner layer was the transitional layer made of MgO, Nd₂O₃ and the content of the substrate. The protective oxidation was associated with the formation of the dense Nd₂O₃/MgO layer during isothermal oxidation process. The oxidation mechanisms for the formation of oxide film are discussed.     

Density functional study of oxygen adsorption on the Mg3Nd (0 0 1) surface

The adsorption of oxygen atoms on Mg₃Nd (0 0 1) surface was studied based on density function theory (DFT), in which the exchange-correlation potential was chosen as the generalized gradient approximation (GGA) in the Perdew and Wang (PW91). The most preferred adsorption position was at the top-hollow site. Upon the optimization on top-hollow site with different coverage, it was found that the adsorption energy decreased with oxygen coverage. The density of states analysis showed that obvious charge transfer took place between O atom and the nearest Nd atom and chemical bond formed between O atom and the nearest Nd atom after O adsorption. The result of surface energy as a function of chemical potential change of oxygen indicated the clean Mg₃Nd (0 0 1) surface was easy to adsorb oxygen and form 1.00 ML surface.