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Ivas T.,ETH Zurich | Grundy A.N.,Concast AG | Povoden-Karadeniz E.,Christian Doppler Laboratory | Gauckler L.J.,ETH Zurich
Calphad: Computer Coupling of Phase Diagrams and Thermochemistry | Year: 2012

The CALPHAD (Calculation of Phase Diagrams) framework to calculate the thermodynamic properties of multicomponent systems is extended to account for the change in thermodynamic properties by the size effect in small particle systems. We have applied this framework to the CeO 2CoO binary in order to elucidate the "liquid phase sintering like" fast densification at unusually low temperatures of cobalt oxide doped ceria. The phase diagram for 5 nm particles shows a significant drop of the eutectic from 1645 °C to 1327 °C. The size-dependent thermodynamic stability of intergranular films is examined using CALPHAD approach and melting temperature of 1180 °C is found for 1 mol% CoO doped CeO 2 corresponding to an intergranular film thickness between 1 and 4 nm. This result comes close to the unusual low sintering temperatures of 1100 °C, with a "liquid- sintering"-like characteristic that was previously experimentally observed for CeO 2 with small amounts of transition metal oxides such as CoO. © 2011 Elsevier Ltd. All rights reserved.

Ivas T.,ETH Zurich | Povoden-Karadeniz E.,Christian Doppler Laboratory | Grundy N.,Concast AG | Jud-Sierra E.,Drexel University | And 2 more authors.
Journal of the American Ceramic Society | Year: 2013

New phase diagram data and a thermodynamic assessment of the CeO-Gd 2O3-CoO system using the CALPHAD approach are presented. This information is needed to understand the surprisingly low sintering temperature (950°C-1050°C) of CeO2-based materials doped with small amounts of transition metal oxide (e.g., CoO). Experimental phase equilibria between 1100°C and 1300°C are reported based on the analysis of annealed and molten samples. No isolated compound exists in the ternary. At 1300°C the Co solubility in the ternary compounds Ce1-x-yGd xCoyO2-x/2-y (fluorite) is 2.7 mol% and is less than 1 mol% in the Gd2-xCexO3+x/2 (bixbyite). The Ce solubility in the perovskite GdCoO3-δ was found to be 1 mol%. The lowest temperature eutectic melt in the ternary has a composition of 57.2 mol% Co and 41.1 mol% Gd melting at an onset temperature of 1303 ± 5°C, which is close to the binary eutectic in the Gd2O 3-CoO system at 60 ± 2 mol% Co and 1348 ± 1°C. © 2012 The American Ceramic Society.

Povoden-Karadeniz E.,ETH Zurich | Povoden-Karadeniz E.,Christian Doppler Laboratory | Chen M.,Technical University of Denmark | Ivas T.,ETH Zurich | And 3 more authors.
Journal of Materials Research | Year: 2012

The thermodynamic La-Sr-Mn-Cr-O oxide database is obtained as an extension of thermodynamic descriptions of oxide subsystems using the calculation of phase diagrams approach. Concepts of the thermodynamic modeling of solid oxide phases are discussed. Gibbs energy functions of SrCrO 4, Sr 2.67Cr 2O 8, Sr 2CrO 4, and SrCr 2O 4 are presented, and thermodynamic model parameters of La-Sr-Mn-Chromite perovskite are given. Experimental solid solubilities and nonstoichiometries in La 1-xSr xCrO 3-δ and LaMn 1-xCr xO 3-δ are reproduced by the model. The presented oxide database can be used for applied computational thermodynamics of traditional lanthanum manganite cathode with Cr-impurities. It represents the fundament for extensions to higher orders, aiming on thermodynamic calculations in noble symmetric solid oxide fuel cells. © 2012 Materials Research Society.

Scherrer B.,ETH Zurich | Harvey A.S.,ETH Zurich | Tanasescu S.,Institute of Physical Chemistry Ilie Murgulescu | Teodorescu F.,Institute of Physical Chemistry Ilie Murgulescu | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

For perovskites with the general formula ACoO3-δ (A = La, Pr, Nd, Sm, and Gd) the influence of the A-site cation on the electrical conductivity, electronic structure, thermodynamic stability, and oxygen stoichiometry was studied. The perovskite oxide powders were produced by a combined citric acid and ethylenediaminetetraacetic acid complexing method. Ceramic specimens sintered at 1100°C in air were single-phase perovskites. With increasing temperature, the electrical conductivity shows three discrete regimes. All compositions show semiconductivity up to a transition temperature of ∼300°C-450°C and then behave like metallic conductors. The activation energies for the semiconductivity, as well as the transition temperatures to the metallic-like conduction, decrease monotonically with increasing pseudocubic lattice parameters, i.e., with increasing ionic radii of the A cation. This behavior correlates with decreasing oxygen nonstoichiometry and increased thermodynamic stability. The highest conductivity and the lowest activation energy of 0.66 eV were found for LaCoO3-δ, which also had the lowest semiconductor-metal transition temperature at 269°C, the lowest oxygen nonstoichiometry of δ = 0.008, and the highest Gibbs free energy change for the decomposition reaction of 42.37 kJ/mol at 850°C. GdCoO3-δ had the highest oxygen nonstoichiometry with δ = 0.032, a high activation energy of 1.19 eV for the semiconductivity with a high transition temperature at 452°C, and the lowest Gibbs free energy change of 26.54 kJ/mol at 850°C. X-ray absorption spectroscopy data imply an increasing Co low-spin character with decreasing cation radius from La to Gd, while an increase in temperature increases the number of holes or Co 3d bandwidth. This correlates well with the electrical conductivity data. © 2011 American Physical Society.

Ivas T.,ETH Zurich | Grundy A.N.,Concast AG | Povoden E.,ETH Zurich | Zeljkovic S.,University of Banja Luka | Gauckler L.J.,ETH Zurich
Acta Materialia | Year: 2010

New phase diagram data and a thermodynamic assessment of the Gd 2O3-CoO system using the CALPHAD approach are presented,giving liquidus data and mutual solid solubilities of Co in Gd 2O3 and Gd in CoO. The thermodynamic model parameters for the ternary Gd-Co-perovskite phase and for the mutual solid solubilities of Co in Gd2O3 and Gd in CoO are optimized to reproduce these new experimental data, as well as phase diagram data from literature. The Gd2O3-CoO phase diagram is refined based on the results of experiments using combined differential thermal analysis and thermogravimetry, scanning electron microscopy and X-ray diffraction techniques. 2010 © Acta Materialia Inc.

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