M.-A. Einarsrud

TEM observations of rhombohedral and monoclinic domains in LaCoO3-based ceramics

Twin structures in rhombohedrally distorted polycrystalline LaCoO3, La0.8Ca0.2CoO3-δ and La0.5Sr0.5Co0.5Fe0.5O3-δ have been studied by transmission electron microscopy. Normal {100} and {110} twinning was predominant in LaCoO3 and La0.8Ca0.2CoO3-δ. In addition, ferroelastic domains with a one-dimensional superstructure corresponding to a tripling of the pseudocubic lattice parameter, a c, were found in some grains. The monoclinic superstructure vanished over time and was sensitive to external mechanical stress. In La0.5Sr0.5Co0.5Fe0.5O3-δ no normal twinning was observed, but domains with a one-dimensional superstructure of two times a c were observed. The superstructures have been interpreted as atomic scale twinning, which reduce the crystal symmetry from rhombohedral to monoclinic.

Characterization and sintering of gels in the system MgO–Al2O3–SiO2

Abstract Cordierite aerogels, made by supercritical drying, and xerogels, formed by ambient pressure drying, have been prepared by combining two different recipes. The chemical composition of the gels varied from stoichiometric cordierite 2MgO·Al 2 O 3 ·5SiO 2 to 0·5MgO·1·4Al 2 O 3 ·5SiO 2 due to different procedures for washing of the gels. The crystallization of nearly stoichiometric cordierite gels was shown to be relatively complex involving the formation of several metastable phases such as μ-cordierite (Mg 2 Al 4 Si 5 O 18 ), spinel (Al 6 Si 2 O 13 ) and sapphirine (Mg 4 Al 8 Si 2 O 20 ) before the equilibrium phase composition was obtained at around 1350°C. On the other hand, during crystallization of gels with stoichiometry close to 0·5MgO·1·4Al 2 O 3 ·5SiO 2 the equilibrium phases mullite, cristobalite and α-cordierite were the major phases formed during heat treatment. A lower densification rate was observed for aerogels compared to xerogels due to a larger pore size. A lower crystallization temperature in aerogels probably due to heterogeneous nucleation reduced the densification. For gels with a composition near 0·5MgO·1·4Al 2 O 3 ·5SiO 2 nucleation and densification occur simultaneously and large differences in the densification behavior was observed. ©

Reactions between La1−x CaxMnO3 and CaO-stabilized ZrO2 part I Powder mixtures

The chemistry and microstructure of the interface between calcium substituted lanthanum manganite and cubic calcia stabilized zirconia have been studied. The aim was to investigate the chemical stability of these materials as a model system for, respectively, the cathode and the electrolyte in solid oxide fuel cells. The relative amounts and time dependence of the formation of secondary phases (La2Zr2O7 and CaZrO3) and inter-diffusion between the primary phases were observed to depend on temperature, partial pressure of oxygen, and composition of the manganite. 30 mole % Ca on La-site and A-site deficiency of the manganite were shown to stabilize the heterophase interface in air. Reducing conditions were shown to destabilize the primary phases and increase the rate of formation of secondary phases. Pore-coarsening with increasing amount of Ca in the manganite was the most striking feature in the time dependence of the microstructure. The present findings are discussed in relation to the thermodynamic and kinetic stability of the cathode/electrolyte interface of conventional solid oxide fuel cells consisting of yttria stabilized zirconia and strontium substituted lanthanum manganite.

Reactions between La1−x CaxMnO3 and CaO-stabilized ZrO2 Part II Diffusion couples

The chemical stability of diffusion couples and coarse grain powder mixtures of calcium substituted lanthanum manganite and cubic calcia stabilized zirconia have been studied. The aim was to investigate the chemical stability of these materials as a model system for respectively the cathode and the electrolyte in solid oxide fuel cells. With increasing amount of Ca in lanthanum manganite, the major secondary phase was shifted from La2Zr2O7 to CaZrO3, and the thickness of the reaction layers of secondary phases was increasing with increasing heat treatment time. Precipitation of La2O3 had taken place in the perovskite containing low amounts of Ca (0 and 20 mol %). The transport mechanisms of the cations were strongly dependent on the interface geometry. La0.7Ca0.3MnO3 was observed to give the most stable interface to zirconia both in air and in reducing atmosphere (pO ∼ 10−6 atm). A-site deficiency of LaMnO3 was also observed to increase the stability. However, we conclude that a thin film of an electrode material consisting of lanthanum manganite on a zirconia substrate is unstable, regardless of A-site deficiency, because the solubility limit of Mn in the zirconia is not reached. From the experimental data, a reaction mechanism has been proposed, based on observations of relative diffusion rates.

Detailed TEM characterization of PbTiO3 nanorods

1D functional oxides at nm-scale are interesting for fundamental reasons and promising for future applications. Here, ferroelectric PbTiO3 nanorods, produced through a hydrothermal process, have been studied in detail by transmission electron microscopy. The length (up to one μm) and the diameter (30–100 nm) as well as the growth direction ([001]) of the nanorods could easily be determined using conventional imaging and electron diffraction techniques. However, variations along the length of the rods were clearly visible in the bright field images. Steps on the outer surfaces of the rods could be identified using energy filtered transmission electron microscopy and spectrum imaging thickness maps. The thickness variation parallel to the electron beam affected the bright field contrast and energy dispersive spectroscopy of the nanorods. From cross-sectional specimens, it was determined that the outer surfaces of the rods were dominantly {110} type, leading to a rectangular cross-section. The cross section diameter of the rods was reduced by the introduction of {100} surfaces. In addition, the cross-sectioned specimen revealed the presence of internal channels in the growth direction, especially in the bottom part of the rods. Such a detailed structural description of the nanorods was necessary to study the possible ferroelectric domain structure and to reveal the growth mechanism of the rods.

Reactions between La1−xCaxMnO3 (x=0.3 and 0.6) and CaO-stabilized ZrO2 powder mixtures studied on a nano-meter scale

Solid oxide fuel cells and oxygen permeable membranes have received considerable attention during the last decade due to the increasing demand for electrical energy and easily transportable fuels combined with the requirement of low emission of CO2. This work concentrates on the stability of ceramic interfaces in general, and more specifically to heterophase solid state interfaces related to solid oxide fuel cells and oxygen permeable membranes. Reaction mechanisms are discussed and requirements and properties of suitable materials are determined. This thesis consists of three parts: 1) Structure of Ca-substituted lanthanum manganite (Paper I), 2) Reactions between cathode and electrolyte for SOFC applications (Papers II-IV) and 3) Chemical and mechanical aspects of sealing dense oxygen permeable membranes (Papers V and VI).