Compositional and valent state inhomogeneities and ordering of oxygen vacancies in terbium-doped ceria

Intragranular distributions of composition and valent state in sintered Tb-doped ceria have been systematically investigated. Through detailed studies of electron energy loss spectroscopy and energy filtering transmission electron microscopy, both compositional and valent state inhomogeneities of Ce and Tb were confirmed, which are related to the existence of nanosized domains in Tb-doped ceria. Compared with their matrix, the domains have higher Tb concentration and Ce and Tb cations in the domains tend to be trivalent. Furthermore, ordering of oxygen vacancies in the domains, which increases with increasing doping concentration, has been determined by EELS. (c) 2007 American Institute of Physics

Oxygen-vacancy ordering in lanthanide-doped ceria: Dopant-type dependence and structure model

Studies of electron energy loss spectroscopy and selected area electron diffraction (SAED) were systematically performed on 15 and 25 at. % lanthanide (Ln)-doped ceria samples (Ln=Sm, Gd, Dy, and Yb), through which the local ordering of oxygen vacancies that develops with increase in doping level was confirmed in the sequence of (Gd,Sm)>Dy>Yb. Furthermore, a monotone correlation between the development of the ordering and the degradation of ionic conductivity with increasing the doping concentration from 15 to 25 at. % was observed. Based on the analysis of SAED patterns, a structural model for the ordering of oxygen vacancies has been constructed, in which the arrangement of oxygen vacancies is similar to that in C-type Ln2O3 oxides and the 110 pairs of the vacancies are preferred. Then, the factors that can influence the formation of the ordering are discussed

Physical-Mathematical modeling and numerical simulations of stress-strain state in seismic and volcanic regions

The strain-stress state generated by faulting or cracking and influenced by the strong heterogeneity of the internal earth structure precedes and accompanies volcanic and seismic activity. Particularly, volcanic eruptions are the culmination of long and complex geophysical processes and physical processes which involve the generation of magmas in the mantle or in the lower crust, its ascent to shallower levels, its storage and differentiation in shallow crustal chambers, and, finally, its eruption at the Earth’s surface. Instead, earthquakes are a frictional stick-slip instability arising along pre-existing faults within the brittle crust of the Earth. Long-term tectonic plate motion causes stress to accumulate around faults until the frictional strength of the fault is exceeded. The study of these processes has been traditionally carried out through different geological disciplines, such as petrology, structural geology, geochemistry or sedimentology. Nevertheless, during the last two decades, the development of physical of earth as well as the introduction of new powerful numerical techniques has progressively converted geophysics into a multidisciplinary science. Nowadays, scientists with very different background and expertises such as geologist, physicists, chemists, mathematicians and engineers work on geophysics. As any multidisciplinary field, it has been largely benefited from these collaborations. The different ways and procedures to face the study of volcanic and seismic phenomena do not exclude each other and should be regarded as complementary. Nowadays, numerical modeling in volcanology covers different pre-eruptive, eruptive and post-eruptive aspects of the general volcanic phenomena. Among these aspects, the pre-eruptive process, linked to the continuous monitoring, is of special interest because it contributes to evaluate the volcanic risk and it is crucial for hazard assessment, eruption prediction and risk mitigation at volcanic unrest. large faults. The knowledge of the actual activity state of these sites is not only an academic topic but it has crucial importance in terms of public security and eruption and earthquake forecast. However, numerical simulation of volcanic and seismic processes have been traditionally developed introducing several simplifications: homogeneous half-space, flat topography and elastic rheology. These simplified assumptions disregards effects caused by topography, presence of medium heterogeneity and anelastic rheology, while they could play an important role in Moreover, frictional sliding of a earthquake generates seismic waves that travel through the earth, causing major damage in places nearby to the modeling procedure This thesis presents mathematical modeling and numerical simulations of volcanic and seismic processes. The subject of major interest has been concerned on the developing of mathematical formulations to describe seismic and volcanic process. The interpretation of geophysical parameters requires numerical models and algorithms to define the optimal source parameters which justify observed variations. In this work we use the finite element method that allows the definition of real topography into the computational domain, medium heterogeneity inferred from seismic tomography study and the use of complex rheologies. Numerical forward method have been applied to obtain solutions of ground deformation expected during volcanic unrest and post-seismic phases, and an automated procedure for geodetic data inversion was proposed for evaluating slip distribution along surface rupture

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

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

Inclusions and traces studied by TEM-AEM

Transmission Electron Microscopy (TEM) is an invaluable tool for the characterization of solid (crystalline) materials at nano- and sub-nanomete scale. It allows a wide range of imaging and diffraction techniques that provide information on elemental composition and atomic structure down to a single atom either by scanning [(S)TEM] the electron probe across the specimen or pointing it directly onto defect domains. In the latter case, imaging, for instance, of point defects, grain boundaries and hetero-phase interfaces can be obtained. STEM can be used to examine specimens to acquire information, particularly on microstructures, atomic arrangements within crystal structures, and, by using a high-angle annular detector, atomic number contrast. Energy dispersive X-ray spectroscopy (EDX) exploits X-ray emission from the excited atoms to obtain submicrometre elemental identification and compositional analysis. The addition of Electron Energy-Loss spectrometer (EELS) and Energy-Filtered (EF) imaging techniquesallows for the detection of chemical elements at greater spatial resolution, phase identification, and information on valence state, coordination and bonding environment of atoms forming the phases. The brightness of the STEM probe, i.e. the number of electrons per unit area per unit time, substantially exceeds that of third-generation synchrotron sources, making the technique a powerful mean for analysing electronic structures and identifying impurity species or dopants within nanostructures. After an overview of the above-mentioned techniques, some case studies on minerals are illustrated

A thermodynamic structural model of graphene oxide

Graphene oxide is an easy-to-make material that has a similar structure with graphene. However, the real structure of graphene oxide is still controversial, and an accurate structural model is crucial for understanding its various properties. In this study, by using molecular mechanics and density functional theory, we introduce a thermodynamically favorable structural model of graphene oxide with chemical composition variable from C1.5O to C2.5O. We also calculate their theoretical Raman spectra and electronic properties. It has been found that, in the proposed graphene oxide structure, the para-substituted epoxide groups stay in close proximity to the hydroxyl, but on the opposite sides of the carbon sheet. In addition, on the edge of graphene oxide sheet, the carboxyl prefers attachment in the armchair orientation, while the carbonyl prefers the zigzag orientation

Glass-phase movement in yttria-stabilized zirconia/alumina composites

The mechanism for siliceous liquid-phase movement during sintering and thermal etching is investigated using single crystal alumina rods in a yttria-stabilized zirconia (YSZ) matrix. Bulk glass-phase extraction and intergranular movement during sintering is attributed to a chemically driven force; however, glass-phase expulsion is predominately due to thermal expansion differences in the glass phase, alumina fiber, and YSZ matrix. An increased understanding of the glass-phase mechanism will facilitate the reduction of the resistive grain-boundary phase, which consequently will decrease the operating temperature of high-temperature solid oxide fuel cells. In this paper, we demonstrated that scavengers such as alumina in combination with a suitable thermal treatment can be used to purify the grain boundaries from unwanted impurities, such as Si, through the expulsion of the unwanted liquid impurity phase. The driving forces behind the expulsion are mechanically and chemically driven capillary flow

Structural and conductivity studies of Y10-xLaxW2O21

The aim of this work was to determine structural parameters of the Y10-xLaxW2O21 (x=0-10) solid solution series and investigate their electric properties. Crystallographic data shows a gradual increase in symmetry with increasing La content, as the structure evolves from orthorhombic, Y10W2O21, towards the pseudo-cubic structure of Y5La5W2O21. The solubility limit of La2O3 was found to be 50% (x=5). Above this level two phases were observed, La6W2O15 and (La,Y)(10+x)W2-xO21-delta. The conductivity of Y rich samples was very low, with sigma of the order 2 x 10(-5)-5 x 10(-5)S cm(-1) at 1000 degrees C, whilst ionic conductivity was observed for most La rich doped samples. The highest conductivity was observed for La10W2O21 and its doped analogues, at 1 x 10(-3)-5 x 10(-3)S cm(-1) at 1000 degrees C. Unit cell parameters were determined as a function of temperature from 0 to 1000 degrees C, and thermal expansion of these materials was determined from temperature studies carried out at the Australian Synchrotron facility in Melbourne, Victoria, Australia. (C) 2010 Elsevier Inc. All rights reserved

Nanodomain formation and distribution in Gd-doped ceria

A comprehensive study, with a combination of diverse analytical techniques, was performed to investigate nanodomain formation and distribution in gadolinium-doped ceria. It is illustrated that the nanodomain formation, originating from the aggregation and segregation of dopant cations together with associated charge-compensating oxygen vacancies, is ubiquitous throughout doped ceria. The formation of nanodomains is not limited to bulk areas as previously reported but exists at grain boundaries as well. With enhanced ordering level, such nanodomains formed at grain boundaries will decrease the ionic conductivity as a result of hindered the mobility of oxygen vacancies in doped ceria. Particularly, the nanodomains formed at grain boundaries, with strong defect interactions due to enrichment of dopants and ordered oxygen vacancies, are suggested to be another possible reason for the grain-boundary resistance, other than the widely accepted space-charge layers

Direct evidence of dopant segregation in Gd-doped ceria

Microstructures and segregations of dopants and associated oxygen vacancies in gadolinium-doped ceria (GDC) have been characterized by high-resolution transmission electron microscopy (HRTEM) and scanning TEM (STEM). Diffuse scattering was detected in 25 at. % GDC (25GDC) in comparison to 10GDC, which is ascribed to nanodomain formation in 25GDC. HRTEM, dark-field, and STEM Z-contrast imaging investigations all provide direct evidence for dopant segregation in doped ceria. It is illustrated that dopant cations cannot only segregate in grain interior forming larger nanodomains but also at grain boundary forming smaller ones. Detailed analyses about nanodomain formation and related dopant segregation behaviors are then elucidated