Nanoanalysis of dopants in novel oxygen ion conductors: Electron microscopy of perovskite-type solid oxide fuel cell electrolyt

Abstract

The aim of this thesis is to predict new solid oxide fuel cell (SOFC) electrolytes with superior physical properties that have higher oxygen ion conductivities at lower operating temperatures. The fluorite structured, yttria-stabilized zirconia (YSZ) is the currently preferred SOFC electrolyte that operates at 1000ºC. In the 1990‟s, Ishihara et al., (1994) discovered a new class of fast oxygen ion conductors, based on doubly-doped lanthanum orthogallate La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM1020), that operates at temperatures lower than YSZ. LSGM1020 belongs to the ABO3 perovskite-type crystal structure and understanding the structure and bonding should elucidate the oxygen vacancy (or oxygen defect) migration pathway which is thought to be one prerequisite for predicting new electrolytes with higher oxygen ion conduction. Currently, there are discrepancies in the determined crystal structures of LSGM1020 and its parent, the undoped LaGaO3. In a recent review, Yashima (2009) pointed out that the diffusion pathway of mobile ions is dependent on the crystal structure and that the geometry of ionic conduction and ion diffusion was fundamental for understanding ion conduction mechanisms. He claimed that there was insufficient knowledge of the diffusion pathways for these mobile ions in ionic conductors and one reason was a lack of in-situ high temperature crystallographic information. Therefore, precise determination of these crystal structures is extremely important and essential. A possible reason for crystal structure discrepancy is that, the manufacture of SOFC electrolytes is by sintering the component powders, and these powders show fine scale inhomogeneities, below the spatial limits available with neutron or X-ray diffraction techniques, where data collection is from either large crystals or finely crushed homogeneous powders. These LaGaO3 and LSGM1020 crystals are very close to the cubic perovskite aristotype, and any small deviations in the structure could lead to an incorrect structure determination. In addition, these LSGM1020 electrolytes are functional structural materials with oxygen defects, which exist at the atomic scale. Thus before developing models for the migration of oxygen vacancies in this class of materials, the in-situ defect structure must be accurately determined before predicting new electrolytes. One method to solve these crystal structure discrepancies is to use a transmission electron microscope (TEM), with its inherent high spatial resolution and ability to perform in-situ electron diffraction at a fine scale. Recently, precession electron diffraction (PED) was invented by Vincent and Midgley (1994) and unlike standard electron diffraction, PED requires only small amounts of tilt around a zone axis within the microscope and is rapid and accurate. PED has a great advantage with respect to conventional electron diffraction in that the intensities of the diffraction spots are the integrated intensities, so that PED can be used to precisely determine crystal structure at ~20 nm (Morniroli et al., 2008). Based on the uniqueness of this technique, in-situ PED was employed to precisely confirm the nanoscale room temperature structure of LaGaO3 to be the orthorhombic structure (Space group: Pbnm) and the high temperature (145°C) phase to be the rhombohedral structure (Space group: R3c). Extensive TEM experiments also confirmed that the room temperature La0.9Sr0.1Ga0.8Mg0.2O2.85, was monoclinic (Space group: I2/a). The high temperature TEM experiments confirmed that the structure of La0.9Sr0.1Ga0.8Mg0.2O2.85, at its operating temperature, was rhombohedral (Space group: R3c). TEM was also able to elucidate many twinned nanodomains (~5nm) within the perovskite-type electrolytes. This highly twinned structure could be responsible for the superior oxygen ion conduction of the electrolyte, La0.9Sr0.1Ga0.8Mg0.2O2.85. However, the mechanism of how the twins play a role in ion conduction was unresolved. To solve this problem, the conventional scanning TEM (STEM) technique was used in conjunction with electron energy-loss spectroscopy (EELS) to differentiate the average bonds between the undoped and doped lanthanum orthogallate perovskite-type structures. Nevertheless, this technique in these electrolyte materials cannot provide information of different bonds, at the atomic scale, caused by the oxygen defects. For this reason, a model to predict new electrolytes, based on the current perovskite-type structure, requires derivation. In this thesis, one such model was developed, based on, what has been termed, the scalene perovskite deviation parameter, Δasp, observed from the lattice parameters of the undoped and doped lanthanum orthogallates. It has been found that knowledge of the migration pathway for oxygen ions is unnecessary in this model and perovskite-type electrolytes with high oxygen ion conduction must have a small Δasp, together with a high twin density. Based on this conclusion, substitution by elements of Group IIIB of the periodic table for lanthanum is unlikely to create superior ion conductors in the perovskite-type lanthanide orthogallates