The decomposition of the cubic perovskite-type oxide Ba(x)Sr(1-x)Co(0.8)Fe(0.2)O(3-delta) (BSCF) into hexagonal and cubic perovskite-type phases has been examined by means of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED) and X-Ray Diffraction (XRD). SEM and TEM measurements reveal that the new hexagonal phase grows predominantly at the grain boundaries of BSCF ceramics and that the cation composition of the newly formed hexagonal phase differs from that of the starting material. An orientational relationship between the hexagonal and the parent cubic phase was also observed. By means of ex situ XRD the phase fraction of the hexagonal phase was determined as a function of annealing time. A kinetic analysis of the data, based on Avrami-type kinetics, indicates that the decomposition is independent of the initial A-site composition, and the obtained reaction order supports the conclusion that the hexagonal phase grows at the grain boundaries in dense ceramic samples.
Benefiting from a strong cooperation with Forschungszentrum Jülich, Karlsruhe Institute of Technology (KIT), and AVL List GmbH respectively, Plansee has been focusing on the development and industrialization of metal-supported SOFC and components for mobile applications. In the scope of some challenging development projects a novel MSC configuration and a first pilot fabrication route could be demonstrated successfully. Currently, the work is ongoing towards a continuous and reliable manufacturing of standard cells as well as the demonstration of system-relevant stack tests. This paper gives an overview about the latest results in cell and stack development as well as about the manufacturing route for cost-effective metal-supported cells.
In situ XAS Nucleation Non-stoichiometry Amorphous oxide Insulator-metal transition a b s t r a c t Within the ternary system Ga-O-N we performed experimental and theoretical investigations on the thermodynamics, structure and kinetics of new stable and metastable compounds.We studied the ammonolysis of b-Ga 2 O 3 at elevated temperatures by means of ex situ X-ray diffraction, ex situ neutron diffraction, and in situ X-ray absorption spectroscopy (XAS). From total diffraction pattern refinement with the Rietveld method we analyzed the anionic occupancy factors and the lattice parameters of b-Ga 2 O 3 during the reaction. Within the detection limits of these methods, we can rule out the existence of a crystalline oxynitride phase that is not derived from wurtzite-type GaN. The nitrogen solubility in b-Ga 2 O 3 was found to be below the detection limit of about 2-3 at.% in the anionic sublattice. The kinetics of the ammonolysis of b-Ga 2 O 3 to a-GaN and of the oxidation of a-GaN to b-Ga 2 O 3 was studied by means of in situ X-ray absorption spectroscopy. In both cases the reaction kinetics could be described well by fitting linear combinations of b-Ga 2 O 3 and a-GaN spectra only, excluding that other crystalline or amorphous phases appear during these reactions. The kinetics of the ammonolysis can be described well by an extended Johnson-Mehl-Avrami-Kolmogorow model with nucleation and growth of GaN nuclei, while the oxidation kinetics can be modeled by a shrinking core model where Ga 2 O 3 grows as a layer. Investigations by means of TEM and SEM support the assumptions in both models.To investigate the structure and energetics of spinel-type gallium oxynitrides (g-galons) we performed first-principles calculations using density-functional theory. In addition to the ideal cubic g-Ga 3 O 3 N we studied gallium deficient g-galons within the Constant-Anion-Model.In highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO 1.2 we found at a temperature around 670 K an insulator-metal transition, with a conductivity jump of seven orders of magnitude. We demonstrate through experimental studies and density-functional theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric b-Ga 2 O 3 within the metastable oxide matrix. By doping with nitrogen the critical temperature and the conductivity in the highly conducting state can be tuned.
La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) cathodes on metal-supported solid oxide fuel cells (MSCs) were fabricated by a novel sintering approach and electrochemically tested in single-cell measurements. The sintering of cathodes on complete cells was performed under argon atmosphere at 950 • C in order to prevent strong oxidation of the metallic support. During this sintering process, a phase decomposition of LSCF occurred, which was found to be reversible upon heating in ambient air. The observed performance increase of MSCs with cathodes sintered ex situ, compared to cells processed under standard conditions, revealed a beneficial effect of the increased sintering temperature on cell performance. At 750 • C and 0.7 V a current density of 0.96 A/cm 2 was achieved. A stronger adherence of the cathodes sintered ex situ was observed after single-cell measurements. In additional experiments, La 0.58 Sr 0.4 CoO 3-δ (LSC) was applied as an alternative cathode for MSCs. These cells were activated in situ at 850 • C due to the lower thermochemical stability of LSC and indicated potential for further improvement of the cell performance. The successful electrochemical characterization of the cells with LSCF cathodes sintered ex situ confirmed the applicability of the novel sintering procedure as well as the improved adherence achieved by the optimized processing.
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