To develop a model system containing regularly spaced misfit dislocations for studies of the radiation resistance of nanoscale defects, epitaxial thin films of Cr, Mo, and Cr(x)Mo(1-x) alloys were deposited on MgO(001) by molecular beam epitaxy. Film compositions were chosen to vary the lattice mismatch with MgO. The film structure was investigated by x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and scanning transmission electron microscopy (STEM). Epitaxial films with reasonably high crystalline quality and abrupt interfaces were achieved at a relatively low deposition temperature, as confirmed by STEM. However, it was found by XRD and RBS in the channeling geometry that increasing the Mo content of the CrMo alloy films degraded the crystalline quality, despite the improved lattice match with MgO. XRD rocking curve data indicated that regions of different crystalline order may be present within the films with higher Mo content. This is tentatively ascribed to spinodal decomposition into Cr-rich and Mo-rich regions, as predicted by the Cr(x)Mo(1-x) phase diagram.
Metallic iron exposed to air even at room temperature is instantly passivized by a thin layer of oxide, as what happens on the surface of metallic utensils we use daily. This layer of oxide has a thickness of 2 -3 nm, and the formation process of this thin oxide layer is normally referred to as initial oxidation. Relative to the abundant experimental observation and theoretical derivation on the growth of thick oxidation layer on metal surface at high temperature, atomic level understanding of the initial oxidation is rather limited. This is related to the fact that the initial oxide layer normally forms at an uncontrollable manner. The kinetic theory for the initial oxidation has been established by Caberra and Mott more than half a century ago [1]. In this theory, they made two assumptions: (1) tunneling of electrons from the metal to the surface absorbed oxygen, leading to the ionization of the surface absorbed oxygen, thus inducing an electrical field, and (2) the electric field drives outward transport of metal ions to combine with the surface chemically adsorbed oxygen ions. No experimental observation has ever been made to demonstrate that the cations outwards transport indeed happens. We report in this letter our observation of injection of vacancies into single crystal α-Fe nanoparticle during the initial oxidation of the iron at room temperature and condensation of those vacancies leads to the formation of void which is subsequently enclosed in the Fecore/oxide-shell structured nanoparticles. These observations provide direct evidence that, during the initial oxidation of iron at room temperature, the outward diffusion of iron is indeed the dominant process. There exists a critical size of ~ 8 nm for which the iron has been fully oxidized, leading to a hollow iron oxide nanoparticle. For particle larger than the critical size, an iron/iron oxide core-shell structure was formed and voids reside at the interface between the oxide shell and the iron core. The present observation also adds new dimensions for designing of metal/metal-oxide/quantum antidotes (vacancy cluster) nanoclusters for applications related to optical, magnetic, and electrical properties of the core-shell structured nanoparticles.Contrasted with the understanding of high temperature growth of thick oxide layer, less is known about the initial oxidation process of iron and the structural nature of their product. The initial oxidation process has two features: the formed oxide layer is normally just a few nanometer; and the growth rate is pretty fast. Depending on the type of migrating lattice defects, the reaction of oxide either can be at the metal-oxide interface (oxygen ions inward transport) or at the oxide-gas interface (metal ions outward transport). Under the frame of the Caberra-Mott theory of oxidation of metal [1], the oxidation process of iron can be described as the following: upon initial attachment of oxygen onto the surface of metal and formation of a thin layer of oxide, electron tunnels through the thin oxide layer and ...
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