High temperature X-ray diffraction and thermo-gravimetrical analysis of the cubic perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ under different atmospheres

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) with the cubic perovskite structure is known to be metastable at low temperature under an oxidizing atmosphere. Here, the thermal and chemical expansion of BSCF were studied by in situ high temperature powder X-ray diffraction and thermo-gravimetrical analysis (TGA) in partial pressure of oxygen ranging from an inert atmosphere (∼10−4 bar) to 10 bar O2. The BSCF powder, heat treated at 1000 °C and quenched to ambient temperature prior to the analysis, was shown to oxidize under an oxidizing atmosphere before thermal reduction took place. With decreasing partial pressure of oxygen the initial oxidation was suppressed and only reduction of Co/Fe and loss of oxygen were observed under an inert atmosphere. The thermal expansion of BSCF under different atmospheres was determined from the thermal evolution of the cubic unit cell parameter, demonstrating that the thermal expansion of BSCF depends on the atmosphere. Chemical expansion of BSCF was also estimated based on the diffraction data and thermo-gravimetrical analysis. A hexagonal perovskite phase, coexisting with the cubic BSCF polymorph, was observed to be formed above 600 °C during heating. The phase separation leading to the formation of the hexagonal polymorph was driven by oxidation, and the unit cell of the cubic BSCF was shown to decrease with increasing amounts of the hexagonal phase. The hexagonal phase disappeared upon further heating, accompanied with an expansion of the unit cell of the cubic BSCFAuthor preprin

Surface Diffusion of Oxygen Transport Membrane Materials Studied by Grain-Boundary Grooving

Mass transport mechanism responsible for grain-boundary grooving during thermal annealing of polished ceramics of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La2NiO4+δ (LN) was revealed by atomic force microscopy. Surface diffusion mechanism was confirmed for both materials by the evolution of the grain-boundary width (w) with annealing time (t), and the surface diffusion coefficient was determined from the slope of w versus t1/4 following the theory by Mullins. An Arrhenius temperature dependence of the surface diffusion was observed, and the activation energy was determined to be 220 ± 30 and 450 ± 30 kJ/mol, respectively, for BSCF and LN. The surface diffusion data are discussed with respect to similar data for other oxide materials and cation and oxygen anion diffusion in BSCF and LN. Finally, the dihedral angle for both LN and BSCF was determined, and these are typical in the range reported for other oxide materials

Mechanism of hydrothermal growth of ferroelectric PZT nanowires

The formation mechanism of hydrothermally grown monocrystalline ferroelectric PZT nanowires is investigated. It is shown that the growth proceeds via a two-step process. Particles of the centrosymmetric PX-phase grow initially, having a fibrous morphology which is compatible with the highly anisotropic crystalline structure of this material. In the second stage, the PX-phase transforms into the ferroelectric perovskite phase, retaining the initial fibrous morphology. The solubility limit of Zr ions in the PX phase is maintained into the perovskite phase. While the PX and the perovskite phases have a similar composition, the reconstructive transformation process, in which edge-sharing octahedra chains of the PX-phase transform into a 3D network of corner sharing octahedra in the perovskite, involves incorporation and then release of oxygen (or hydroxyl). (C) 2012 Elsevier B.V. All rights reserved