Papers by Keyword: Interstitial

Abstract: K2NiF4 type oxides have received considerable attention in recent years as possible mixed conducting membranes for technologically important applications such as solid oxide fuel cells and oxygen generators. Much of this interest has focused on the oxygen interstitial containing oxides such as La2NiO4+δ. This contribution will review some of the recent work on these and related materials and will present new data on the La2MO4+δ (M= Ni, Co or Mn) systems. In particular data on the oxide ion diffusivity of these systems, as determined by oxygen isotopic exchange and SIMS analysis highlighting the fast oxide ion diffusion (1 x 10-7 cm2s-1 at 750oC) present, will be discussed and the significance of these data evaluated in relation to solid oxide fuel cell systems. A further section of the presentation will consider the oxygen defect species present and discuss the nature of the mobile oxygen ions. Finally some preliminary data on the application of these materials as cathodes will be presented and the performance of the cathodes on intermediate temperature electrolytes discussed.

Abstract: The geometrical classification of the intercrystalline general type boundaries with various misorientations of contacting crystals and orientations of boundary planes was suggested previously. An arbitrary grain boundary breaks up into three families of thermodynamically stable boundaries as a result of fragmentation process due to annealing of polycrystalline sample. Geometrical representations of linear defects (dislocation, steps) and point defects (vacancy, interstitials) in intercrystalline and interphase general type boundaries of different families are developed. Representations about thermal excitations of boundary are developed. Different kinetic processes responsible for intercrystalline sliding, migration are considered, and expressions for characteristics of the low-temperature and high-temperature internal friction grain boundary peaks are obtained. Explanations of internal friction peaks observed near to the so-called ‘grain boundary peak’ are given. Mechanisms of atomic reorganization of grain boundary responsible for the grain boundary phase transitions are also considered. Conditions for the creation of amorphous atomic structure into boundary near to the fusion temperature of a crystal are specified.

Abstract: This work is devoted to simulation of interstitial atom diffusion in fcc metals with point defects. We used the molecular static and the Monte Carlo methods. An activation barrier set for different configurations of the carbon–vacancy complexes is simulated by the method of the molecular static (MS). Then we calculate atom jump rates for these configurations. The simulation of the carbon and vacancy migration in an fcc metal is realized on the basis of obtained atom jump rates by using the Monte-Carlo (MC) method. In particular, the calculations were made for the system of the nickel-carbon. In the result of that interstitial atom diffusion coefficient has been obtained at different temperatures.

Abstract: We highlight some of the most salient recent advances in point defects studies obtained from atomic-scale simulations performed in the framework of the density functional theory. The refinement of the theory, combined with its efficient numerical implementations and the (until now) everlasting growth of computer power allowed the transition from qualitative (in the beginning of the 90’) to quantitative results. Some of the longstanding controversies in the field have been tackled, and as far as aluminum is concerned, it has been shown that the curvature in the Arrheniusplot is due to anharmonic effects rather than to a two-defect diffusion mechanism. The anomalous diffusion in the b (bcc) phase of the group-IV elements has been related to the strong structural relaxation around vacancies, which significantly reduces their formation energy. Self-interstitials have been studied in materials of technological interest, their structure and mobility have been analyzed allowing a better interpretation of experimental results and an improved understanding of processes occurring under irradiation. Dilute interstitial solid solutions have been investigated. The strong binding between C and vacancies in bcc Fe may partially explain the observed influence of low amounts of C on Fe self-diffusion; the attraction of H to stacking faults in a Zr should favor planar dislocations glide. Intermetallics involving Fe (Fe-Al, Fe-Co) behave like highly correlated systems requiring methodological improvements of the DFT for a quantitative description. However, valuable trends concerning the structural point defects (those that allow nonstoichiometric compositions at low temperature) as well as the temperature dependence of point defects concentrations have been obtained.