Advances in Science and Technology Vol. 46

Abstract: The solid microstructure built in the solid governs the properties of materials elaborated from the melt. In order to clarify the dynamical mechanisms controlling solidification processing, we use in situ and real-time synchrotron X-ray radiography at ESRF (European Synchrotron Radiation Facility) to analyze microstructure formation in thin aluminum alloys solidified in the Bridgman facility installed at the ID19 beamline. During directional solidification of Al - 3.5 wt% Ni alloys, global mechanical constraints induced by the shape are found to act on the solid microstructure. In particular, radiography videos of dendritic growth show disorientations of sidebranches induced by mechanical stresses. In the solidification of AlPdMn quasicrystals, live imaging reveals that facetted growth proceeds by the lateral motion of ledges at the solid-melt interface. When the solidification rate is increased, the kinetic undercooling becomes sufficient for grain nucleation and growth in the liquid. These grains develop specific features that can be attributed to grain competition and concomitant poisoning of growth caused by the rejection of aluminum in the melt.

Abstract: Unmixing of a semiconducting ferrite spinel, (Co0.203Fe0.797)3O4 in an electric field was examined at 1200oC in its disorder regimes of prevailing (cation) vacancies and interstitials, respectively. It has been found that Co becomes enriched at the cathode in the interstitial-prevailing regime and at the anode in the vacancy-prevailing regime, but to much smaller a degree than expected. This confirms that the direction and degree of unmixing is governed by the difference in electrochemical mobility of the competing cations, and the effective charges of the mobile cations are appreciably reduced from their formal ones by the cross effect between the mobile ions and electrons.

Abstract: Praseodymium based dielectric thin films have been deposited by Metal-Organic Chemical Vapour Deposition (MOCVD). Special emphasis has been placed upon deposition parameters crucial to obtain Pr2O3 phase and upon interfacial characterization. In addition, dielectric properties have been correlated to structural and compositional characteristics of praseodymium containing films. The breakdown (BD) characteristics of Pr2O3 films have been investigated by an innovative and handling approach based on C-AFM. Moreover, the BD kinetics have been elucidated considering the role of defects in the conduction mechanisms.

Abstract: As the result of oxidation of Cr-Mn steels in SO2 the three layer scale is formed. The intermediate layer of this scale is composed mainly of MnCr2O4 spinel whereas FeCr2O4 spinel is present in small amount. MnO dominates in the outer layer. The inner, very thin scale layer contains oxides/sulfides mixture. The aim of this study was to examine self-diffusion processes in both spinels by multitracer method of diffusion measurements to know which of the transport processes during oxidation is the smallest one and deciding on the corrosion rate. In diffusion experiments the radioisotopes 54Mn, 51Cr and 59Fe were used. The serial sectioning method was applied to simultaneous evaluation of diffusion rates of chromium, manganese and iron in both spinels at 1073 K and 1173 K under the pressure of 105 Pa in SO2 containing 10 Pa O2. These spinels were obtained by modified sol-gel method from nitrates. Structures of the spinels were examined by X-ray spectrometry. It was found, that the diffusion rates of metals are higher in MnCr2O4 spinel. Moreover the dominant mechanism of manganese transport (the highest one) in studied samples is the volume diffusion while chromium and iron are transported mainly through the high diffusivity paths.

Abstract: α-Iron foils were exposed to various gas atmospheres containing all or a number of the components NH3, CO, H2 and N2 for different periods of time at 550°C. In this way surficial compound layers were generated which contain different iron nitrides (ε, γ’), iron carbonitride (ε) and/or iron carbide (cementite, Fe3C). These compound layers were used to study phase transformations associated with N- and/or Cdiffusion processes in the corresponding phases. These studies involved (a) the layer-growth kinetics of cementite and (b) various solid-state phase transformations occurring in compound layers upon annealing in vacuum.

Abstract: In this paper, the nature of steady state demixing of yttria-stabilized zirconia in an electric field is examined for the case of open system conditions (very fine grain material) wherein the defects are everywhere close to internal surfaces which allow the Schottky defect reaction to occur. It is shown that under such conditions the applied force needs to be some 20 -25 times larger than for the case of closed system conditions (Schottky defect reaction occurs only at the external surfaces) in order to achieve the same degree of demixing of the cation components. The effect of the constant electric field assumption as a substitute for the constant current condition is also analyzed for a wide range of applied electric fields. It is shown that in most cases, the assumption of a constant electric field provides only a moderate correction to the shape of the concentration profile.

Abstract: The aim of this paper is to describe the recent progress in using atomistic simulation techniques to develop simulation models of ceria nanotubes and to model oxygen ion transport at ceria surfaces. The basis of these atomistic techniques is to use the Born Model of Solids where parameterized analytical equations are employed to describe the interactions between atoms. Once these interatomic forces are specified energy minimization and molecular dynamics techniques can be applied to the models.

Abstract: In this paper, the current understanding about the relationships between tracer diffusion coefficients, interdiffusion coefficients and d.c. ionic conductivities in metallic alloy and ionic systems is reviewed with emphasis on recent progress. Quasione component systems are analyzed in terms of the Haven Ratio (and tracer and collective correlation factors). Two and three component alloy systems are analyzed in terms of the Darken-Manning equation and vacancy-wind factors (and tracer and collective correlation factors). Ionic systems with two mobile components are analyzed in terms of a corrected Nernst-Planck Equation.