On the dynamically formed oxide films in molten Mg

The so-called“Oxide/Metal/Oxide sandwich”method is one of the technique used to investigate the dynamic oxidation of metals which happens during the casting process.In this study,characteristics of the oxide films formed on the molten magnesium in dynamic conditions have been investigated using the aforementioned method.The air bubbles were released into the cast sample at the pressure of 0.2 atm through a quartz tube of 1 mm internal diameter.The interface of two adjacent entrapped bubbles is considered as the Oxide/Metal/Oxide(OMO)sandwich.The sandwiches were characterized by the aid of the optical and scanning electron microscopy and also X-Ray diffraction analyses.Two different approaches,including measuring the width of the folds formed on the oxide films and the edge of the sandwiches,were used to estimate the thickness of the films.The thicknesses were estimated to be in the range of 200 to 800 nm.The features such as fold,wrinkle,and crack were observed on the OMO sandwiches.On the microscopic scale,the oxide films were rough and porous.This is attributed to the non-protective behavior of oxide films.The XRD patterns indicated that the oxide films formed on the pure molten magnesium in dynamic conditions are crystallized MgO.

Surface oxidation study of molten Mg–Al alloys by oxide/metal/oxide sandwich method

The dynamic oxidation of molten Mg–Al alloy was investigated via the oxide/metal/oxide(OMO)sandwich method.The characteristics of sandwiches were explored using optical microscopy,scanning electron microscopy,X-ray energy dispersive spectroscopy,and X-ray diffraction analyses.The results showed the formation of porous oxide films with varying thicknesses from 0.43 to 16.7 mm.Both the measurements and calculations confirmed the literature findings that the oxidation product consists mainly of MgO and Mg Al_(2)O_(4)compounds.The increase in thickness and amount of folds formed on the oxide films implies the significant effect of aluminum in reducing the oxidation resistance of magnesium.

Design and control of a p-xylene oxidation process

The p-xylene(PX) oxidation process is of great industrial importance because of the strong demand of the global polyester fiber.A steady-state model of the PX oxidation has been studied by many researchers.In our previous work,a novel industrial p-xylene oxidation reactor model using the free radical mechanism based kinetics has been developed.However,the disturbances such as production rate change,feed composition variability and reactor temperature changes widely exist in the industry process.In this paper,dynamic simulation of the PX oxidation reactor was designed by Aspen Dynamics and used to develop an effective plantwide control structure,which was capable of effectively handling the disturbances in the load and the temperature of the reactor.Step responses of the control structure to the disturbances were shown and served as the foundation of the smooth operation and advanced control strategy of this process in our future work.

Discrete dislocation plasticity analysis of dispersion strengthening in oxide dispersion strengthened(ODS) steels

A discrete dislocation plasticity analysis of dispersion strengthening in oxide dispersion strengthened(ODS) steels was described. Parametric dislocation dynamics(PDD) simulation of the interaction between an edge dislocation and randomly distributed spherical dispersoids(Y2O3) in bcc iron was performed for measuring the influence of the dispersoid distribution on the critical resolved shear stress(CRSS). The dispersoid distribution was made using a method mimicking the Ostwald growth mechanism. Then, an edge dislocation was introduced, and was moved under a constant shear stress condition. The CRSS was extracted from the result of dislocation velocity under constant shear stress using the mobility(linear) relationship between the shear stress and the dislocation velocity. The results suggest that the dispersoid distribution gives a significant influence to the CRSS, and the influence of dislocation dipole, which forms just before finishing up the Orowan looping mechanism, is substantial in determining the CRSS, especially for the interaction with small dispersoids. Therefore, the well-known Orowan equation for determining the CRSS cannot give an accurate estimation, because the influence of the dislocation dipole in the process of the Orowan looping mechanism is not accounted for in the equation.

Oxidation behavior of cobalt nanoparticles studied by in situ environmental transmission electron microscopy

The dynamics of oxidation of cobalt nanoparticles were directly revealed by in situ environmental transmission electron microscopy.Firstly,cobalt nanoparticles were oxidized to polycrystalline cobalt monoxide,then to polycrystalline tricobalt tetroxide,in the presence of oxygen with a low partial pressure.Numerous cavities(or voids) were formed during the oxidation,owing to the Kirkendall effect.Analysis of the oxides growth suggested that the oxidation of cobalt nanoparticles followed a parabolic rate law,which was consistent with diffusion-limited kinetics.In situ transmission electron microscopy allowed potential atomic oxidation pathways to be considered.The outward diffusion of cobalt atoms inside the oxide layer controlled the oxidation,and formed the hollow structure.Irradiation by the electron beam,which destroyed the sealing effect of graphite layer coated on the cobalt surface and resulted in fast oxidation rate,played an important role in activating and promoting the oxidations.These findings further our understanding on the microscopic kinetics of metal nanocrystal oxidation and knowledge of energetic electrons promoting oxidation reaction.