Microstructure evolution and kinetics of B-site nanoparticle exsolution from an A-site-deficient perovskite surface: a phase-field modeling and simulation study.

Segregation-exsolution of B-site catalytic dopants as nanoparticles from A-site-deficient perovskite (A1-xBO3-δ) surfaces has been actively used in recent years to promote the activity and durability of perovskite oxides towards efficient fuel oxidation and water splitting. The mechanistic understandings are currently gained from equilibrium thermodynamics, such as atomic scale density functional theory calculations, in terms of segregation energy, interaction energy and elastic energy. Herein, we have developed a micro-scale phase-field model framework that describes the kinetics and microstructure evolutions of the B-site segregation and nanoparticle exsolution from the A1-xBO3-δ surface. The model was derived in a thermodynamically consistent manner by employing a ternary regular-solution free-energy functional and Cahn-Hilliard kinetic equations. The key hypothesis is that the B-site nanoparticle is exsolved by a spinodal decomposition once the surface region of A1-xBO3-δ is driven to the spinodal region of the free-energy functional via B-site segregation to the surface and/or via expansion of the chemical spinodal region. The effects of oxygen partial pressure (or electric polarization), B-site supersaturation (or A-site deficiency), and segregation energy have been explicitly investigated, and the results obtained agree qualitatively with the experimental observations. The proposed model can serve as a multi-scale bridge that ties the atomic-scale understandings to the micro-scale observations and has the potential to be used for the design and optimization of nano-architectures of A1-xBO3-δ materials.

Experimental Study of Diamond-like Carbon Film on Aermet100 Steel and First-Principles Calculation of Interfacial Adhesion.

Three different types of surface-modified layers of N, C, and N+C are successfully prepared on AerMet100 steel by plasma-assisted thermochemical treatment, and diamond-like carbon (DLC) films are formed on the top surfaces of the latter two. The results show that the DLC films produced by prenitriding and then carburizing (N+C) exhibit a smoother and finer morphology and higher sp3 content than that without prenitriding (C). In addition, the wear resistance of the N+C specimen with a high hardness nitrided layer as the support for the outermost DLC films is superior to that of the C specimen. In view of the catalytic effect of the Fe3C phase on the growth of DLC films, the interfacial properties of Fe3C(001)/diamond(111) are investigated using first-principles calculations. On the basis of the most preferred Fe-terminated HCP site model, the effects of alloyed cementite (Fe2MC) on interfacial adhesion of Fe2MC(001)/diamond(111) are also investigated. Furthermore, the mechanisms of interfacial adhesion for two representative dopings (Zr weakened and V enhanced) are revealed in detail. These results are expected to provide a potential promising means for future experimental works on the preparation of high-performance DLC films on alloy steel surfaces by plasma carburizing.