M. Zinkevich

Ce1−xY (Nd)xO2−δ nanopowders: potential materials for intermediate temperature solid oxide fuel cells

Nanopowdered solid solution Ce 1-x Y(Nd) x O 2-δ samples (0.1 ≤ x ≤ 0.25) were made by self-propagating room temperature (SPRT) synthesis. The first-order Raman spectra of Ce 1-x Y(Nd) x O 2-δ samples measured at room temperature exhibit three broad features: the main Raman active F 2g mode at about 450 cm -1 and two broad features at about 550 (545) and 600 cm -1 . The mode at ∼600 cm -1 was assigned to the intrinsic oxygen vacancies due to the nonstoichiometry of ceria nanopowders. The mode at about 550 (545) cm -1 was attributed to the oxygen vacancies introduced into the ceria lattice whenever Ce 4+ ions are replaced with trivalent cations (Y 3+ , Nd 3+ ). The intensity of this mode increases with doping in both series of samples, indicating a change of O 2- vacancy concentration. The mode frequency shifts in opposite direction in Y- and Nd-doped samples with doping level, suggesting that different types of defect space can occur in Y- and Nd-doped ceria nanopowders.

Thermodynamics of Fe–Sm, Fe–H, and H–Sm systems and its application to the hydrogen–disproportionation–desorption–recombination (HDDR) process for the system Fe17Sm2–H2

Abstract Thermodynamic properties of the Fe–Sm, Fe–H, and H–Sm systems have been analyzed by means of the CALPHAD method. Thermodynamic models have been defined to describe the Gibbs energy of the individual phases and the model parameters have been optimized using different experimental information: phase diagram data, calorimetric data, and equilibrium partial pressures of hydrogen. The entropy of Fe17Sm2 has been derived from experimental data on low-temperature heat capacity. The thermodynamic parameters for Fe17Sm2 have been evaluated using binary (Fe–Sm) and ternary (Fe17Sm2–H2) data simultaneously. The calculated phase diagrams and thermodynamic properties are in agreement with experiments. The thermodynamics of hydrogen in the interstitial solid solution Fe17Sm2Hδ have been described by compound energy formalism. Combining the obtained thermodynamic descriptions of the individual phases, the equilibrium temperatures of the recombination reaction: Fe+SmH2±x⇒Fe17Sm2+H2, which is an important part of the HDDR process, have been calculated as a function of hydrogen pressure.

Phase diagrams and thermodynamics of rare-earth-doped zirconia ceramics*

Results of the comprehensive experimental and computational phase studies of the systems ZrO2-REO1.5 (RE = La, Nd, Sm, Gd, Dy, Yb) are summarized. Various experimental techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), differential thermal analysis (DTA), and high-temperature calorimetry are employed to study the phase transformation, phase equilibria between 1400 and 1700 °C, heat content and heat capacity of the materials. A lot of contradictions in the literature are resolved, and the phase diagrams are reconstructed. Based on the experimental data obtained in this work and literature, the systems ZrO2-REO1.5 are thermodynamically optimized using the CALPHAD (CALculation of PHase Diagram) approach. Most of the experimental data are well reproduced. Based on the present experiments and calculations, some clear characteristic evolutions with the change of the ionic radius of doping element RE+3 can be concluded.

Thermodynamic modelling in the ZrO2–La2O3–Y2O3–Al2O3 system

Abstract The thermodynamic database for the ZrO2 – La2O3 – Y2O3 – Al2O3 system has been derived using previous descriptions for the six binary systems and the ternary ZrO2 – La2O3 – Al2O3 system. The parameters of the Y2O3 – Al2O3 system were adjusted due to changes in the Y2O3 thermody-namic parameters. The thermodynamic parameters of the La2O3 – Y2O3 system were slightly changed to get consistency between calculations and new experimental results for the ZrO2 – La2O3 – Y2O3 ternary system. Diverse kinds of phase diagrams of the ZrO2 – La2O3 – Y2O3 and La2O3 – Y2O3 – Al2O3 ternary systems have been calculated. The present thermodynamic description of the quaternary system is consistent with available experimental results for lower-order systems. It was used to calculate isoplethal sections for compositions related to thermal barrier coating (TBC) and its interaction with thermally grown oxide Al2O3. The T0-lines have been calculated for diffusionless transformations between fluorite, tetragonal, monoclinic and pyrochlore phases in the ZrO2 – LaO1.5 system as well as driving forces for partitioning of non-equilibrium phase into equilibrium phase assemblages. These data could be applied for determination of desirable ranges of material composition for TBC.

Thermodynamic assessment of the Mo-Zr system

The Mo-Zr system was critically assessed using the calculation of phase diagrams (CALPHAD) technique. The solution phases (liquid, bcc, and cph) were modeled with the Redlich-Kister expression for the excess Gibbs energy. The intermetallic Mo2Zr Laves phase, which has a measurable homogeneity range, was treated by a two-sublattice model with Mo and Zr on both sublattices. A set of self-consistent thermodynamic parameters for the Mo-Zr system was obtained. With the optimized functions for the Gibbs energy of the individual phases, most of the experimental information can be well reproduced.

Reassessment of the Fe-Gd (Iron-Gadolinium) system

The Fe-Gd binary system was reassessed in order to obtain a consistent thermodynamic description. Compared to previous work, some new experimental information was used. It was established that not all the experimental data are interconsistent. The assessment was done by means of the software package THERMO-CALC, using models for the Gibbs energy of individual phases.

Thermodynamic assessment of Gd-Zr and Gd-Mo systems

Two binary systems, Gd-Zr and Gd-Mo, were assessed for the first time. Despite the very limited experimental data, consistent thermodynamic descriptions for all phases were obtained. The assessment was done using the computer program BINGSS.

Thermodynamic modeling of the Fe-Mo-Zr system

Abstract Thermodynamic properties of the ternary Fe–Mo–Zr system have been analyzed by means of the calculation of phase diagrams method. Thermodynamic models have been defined to describe the Gibbs energy of the individual phases, and the model parameters have been optimized using constitutional data available from the literature. A two-sublattice model was used to describe wide homogeneity ranges of the ternary C14 and C15 Laves phases. A self-consistent thermodynamic description of the Fe–Mo–Zr system was obtained. The calculated phase equilibria are compared with experiments.

Thermodynamic assessment of the systems La2O3-Al2O3 and La2O3-Y2O3

Abstract The thermodynamic parameters of the systems La2O3–Al2O3 and La2O3–Y2O3 are assessed using the Calphad technique. The available phase equilibrium data and calorimetric measurements for the LaAlO3, perovskite phase have been used. The calculated phase diagrams are in reasonable agreement with experimental results. However, experimental data for the La2O3–Y2O3 system demonstrated large uncertainty and new experimental study of solid state reactions would be useful to verify the calculated diagram. Experimental investigations of thermodynamic properties are necessary for both systems to verify the calculated phase diagrams and thermodynamic properties.

Crystal growth of MgB2 from Mg-Cu-B melt flux and superconducting properties

A new method for the preparation of single crystals of the superconducting intermetallic MgB2 compound from a Mg–Cu–B melt flux is presented. The high vapour pressure of Mg at elevated temperature is a serious challenge of the preparation process. The approximate thermodynamic calculations of the ternary Mg–Cu–B phase diagram show a beneficial effect of Cu, which extends the range of formation of MgB2 to lower temperatures. Within the as-solidified Mg–Cu–B melt flux the MgB2 compound forms plate-like single crystals up to a size of 0.2×0.2×0.05 mm3 or alternatively rims peritectically grown around MgB4 particles. AC-susceptibility measurements were conducted with specimens selected from different parts of the as-solidified flux containing MgB2 particles. Peritectically formed MgB2-particles display the highest transition temperature of Tc=39.2 K and a relatively narrow transition width of ΔTc=1.3 K. Other sections of the sample exhibit various superconducting transitions from Tc=39 K to 7.2 K. This variation of Tc is attributed to a finite homogeneity range of the MgB2 compound whereas significant Cu solid solubility in MgB2 can be excluded.