LOCAL DISORDER IN DOPED CERIA: A CRYSTALLOGRAPHIC STUDY

In recent years, doped ceria compounds have attracted increasing attention as electrolytes for application in Solid Oxide Fuel Cell devices, thanks to their high performance at intermediate temperature. In cerium oxide, ionic conductivity is driven by the presence of oxygen vacancies, introduced for charge balance after doping with a lower valent cation. With increasing doping concentration the oxygen diffusion is impeded, probably because of the formation of aggregates that trap oxygen vacancies. Their nature is though still under debate. Owing to the close interplay between transport properties and local structure, an accurate description of the defects on the atomic scale is of the utmost importance for understanding the mechanisms at work in doped ceria. In this study we propose to unveil the complex disorder in doped ceria with a combined powder diffraction study. On the one hand, X-ray and Neutron powder diffraction are joint to couple their potentialities; on the other hand, two different crystallographic approaches are exploited. The conventional crystallographic methods allow to study the long range structure modifications that occur with doping. The Pair Distribution Function is instead employed to study the local structure, in terms of deviations from the long range atomic ordering. The goal of the thesis is to provide a crystallographic description of the atomic rearrangements induced by doping, looking for a relationship between the local structure and the transport properties

A Travelling Tale:Shakespeare on the Italian Stage

This thesis considers the transposition from page to stage of some of Shakespeare’s plays in Italy. In particular it concentrates on different approaches to Shakespeare’s texts and different ways to transform them into theatrical action.The first chapter has an introductory function, and lays the groundwork for subsequent discussion. It illustrates the encounter between the work of the English playwright and the Italian people through an overall view of the reception of Shakespeare in Italy from the first mention of his name in 1667 to Francesco De Sanctis’s critical writings in the mid-nineteenth century.The following chapters discuss how Shakespeare’s plays have been adapted for the stage by some prominent Italian actors and directors. The focus is on three periods of the history of Italian theatre. The Great Actors of the mid-nineteenth century offered stagings of Shakespeare’s plays that focused on the main character, thus depriving them of anything that did not enhance the role of the lead actor. The generation of the directors, that flourished in Italy in the mid-twentieth century, advocated a philological reading of the playtexts, after they had been so severely altered by the generation of the actors. Finally, all through the 1960s and 1970s, the experimentalists of the ‘Nuovo Teatro’ (‘New Theatre’) returned to what can be defined ‘the actor’s theatre’, and their approach veered towards very personal productions considered by many irreverent to and irrespectful of the playtexts.Combining general theoretical discussion with the close reading of some adaptations, the thesis offers an analysis of different movements in Italian productions of Shakespeare, and an argument about different versions of Shakespearian ‘authenticity’ in Italy

Disorder in La1-xBa1+xGaO4-x/2 ionic conductor: resolving Pair Distribution Function through inside from first principles modeling

Ionic conduction in dry LaBaGaO4 occurs through the vacant oxygen sites formed by the substitution of Ba for La. The resulting La1 12xBa1+xGaO4 12x/2 solid solution shows significant disorder characteristics. The local structure of compositions x = 0, 0.20 and 0.30 was studied using the pair distribution function (PDF). Unfortunately, increasing peak overlap and the number of independent structural parameters make PDF modeling challenging when dealing with low-symmetry phases. To overcome this problem, density functional theory (DFT) was employed to create different structural models, each one with a different relative position for the substitutional Ba ion with respect to the oxygen vacancy. The atomic distributions generated by DFT were used as a starting point to refine experimental PDF data. All models result in the formation of Ga2O7 dimers, with their major axis oriented along the c axis. At the local scale, the most stable DFT model also provides the best fit of the PDF. This accounts for the dopant as first and second neighbors of the vacancy and of the O bridge in the dimer, suggesting that substitutional barium ions act as pinning centers for oxygen vacancies. Above 6 \uc5 the average orthorhombic structure fits the PDF better than the DFT models, thus indicating that Ga2O7 dimers are not correlated with each other to form extended ordered structures. The combination of DFT simulations and X-ray diffraction/PDF refinements was used successfully to model the local atomic structure in La1 12xBa1+xGaO4 12x/2, thus suggesting that this approach could be positively applied in general to disordered systems

Hydrogen Embrittlement Behavior of 18Ni 300 Maraging Steel Produced by Selective Laser Melting

A study was performed to investigate the hydrogen embrittlement behavior of 18-Ni 300 maraging steel produced by selective laser melting and subjected to different heat treatment strategies. Hydrogen was pre-charged into the tensile samples by an electro-chemical method at the constant current density of 1 A m(-2) and 50 A m(-2) for 48 h at room temperature. Charged and uncharged specimens were subjected to tensile tests and the hydrogen concentration was eventually analysed using quadrupole mass spectroscopy. After tensile tests, uncharged maraging samples showed fracture surfaces with dimples. Conversely, in H-charged alloys, quasi-cleavage mode fractures occurred. A lower concentration of trapped hydrogen atoms and higher elongation at fracture were measured in the H-charged samples that were subjected to solution treatment prior to hydrogen charging, compared to the as-built counterparts. Isothermal aging treatment performed at 460 degrees C for 8 h before hydrogen charging increased the concentration of trapped hydrogen, giving rise to higher hydrogen embrittlement susceptibility.11Ysciescopu

Phase Transformations in the CeO2-Sm2O3System : A Multiscale Powder Diffraction Investigation

The structure evolution in the CeO2-Sm2O3system is revisited by combining high resolution synchrotron powder diffraction with pair distribution function (PDF) to inquire about local, mesoscopic, and average structure. The CeO2fluorite structure undergoes two phase transformations by Sm doping, first to a cubic (C-type) and then to a monoclinic (B-type) phase. Whereas the C to B-phase separation occurs completely and on a long-range scale, no miscibility gap is detected between fluorite and C-type phases. The transformation rather occurs by growth of C-type nanodomains embedded in the fluorite matrix, without any long-range phase separation. A side effect of this mechanism is the ordering of the oxygen vacancies, which is detrimental for the application of doped ceria as an electrolyte in fuel cells. The results are discussed in the framework of other Y and Gd dopants, and the relationship between nanostructuring and the above equilibria is also investigated

Investigation of TiCr Hydrogen Storage Alloy

A new reversible hydrogen storage material, based on TiCr metal alloy, is proposed. Cr and Ti were mixed and melted in a final atomic ratio of 1,78. Chemical-physical characterisations, in terms of XRD and SEM-EDX, were performed. The quantification of Laves phases was performed through Rietveld refinements. The atomic Cr/Ti ratio was determined by EDX analysis and 1,71 was obtained. The H2 sorption/desorption measurements by Sievert apparatus were carried out. After different tests varying temperature and pressure, a protocol measurement was established; and a H2 sorption value of 0,4 wt% at 200 °C/10 bar with a fast kinetic at 5 bar (Dwt% of about 0,3 wt%) were obtained. Hydrogen desorption measurements performed in the same conditions of T confirmed a totally reversible trend. A confirm of metal hydride formation was recorded by XRD, in fact, comparing X-Ray patterns before and after volumetric tests a notable difference was recorded

Relaxor ferroeletric behavior inSr1−xPrxTiO3 : Cooperation between polar and antiferrodistortive instabilities

Chemical doping at the Sr and Ti sites is a feasible way to alter the quantum paraelectric state of SrTiO3 perovskite. Doping with Pr is known to induce relaxor ferroelectricity at room temperature in the Sr1 12xPrxTiO3 solid solution. The relationship between its dielectric properties and structural phase transition has been debated, but no definitive structural argument has been proposed. Here we present a systematic structural study of Sr1 12xPrxTiO3 (0.020 _ x _ 0.150).We establish the structural phase diagram using high-resolution x-ray powder diffraction by finding the antiferrodistortive structural phase transitions for all the compositions studied. By using pair distribution function analysis, we show the mismatch between local and long-range structures in terms of increased local order parameters. Finally, we propose a correlation between the local structural order parameters and the emergence of hard polar modes as found by Raman spectroscopy. Our results are quantitatively consistent with recent theoretical calculations showing that the increase of local tetragonality and local octahedral tilting above a critical value in fact underlie the polar instability. This confirms that structural orders involving both polar and antiferrodistortive characters compete and cooperate at different levels, promoting ferroelectricity in Sr1 12xPrxTiO3

Differential Pair Distribution Function applied to Ce1-xGdxO2-x/2 system

The Pair Distribution Function technique based upon X-ray diffraction data is a powerful tool to unveil disorder on the nanometric scale, which is however element insensi-tive. To overcome this problem, Differential Pair Distribution Functions (DPDF) can be obtained by exploiting the anomalous dispersion of X-rays near the absorption edge of a certain element. In this paper the DPDF method is briefly reviewed and applied to the case of gadolinium doped ceria electrolytes. XRPD data have been collected at the Ce-K edge on the ID31 beamline of the European Synchrotron Radiation Facility (ESRF). The validity of this approach to extract chemical specific information is also briefly discussed

Defect clustering in Ce1-xGdxO2-x/2 using the PDF Analysis

Aim of this work is to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in Ce1-xGdxO2-x/2 (CGO) electrolytes for solid oxide fuel cells by means of the Pair Distribution Function (PDF) analysis. The whole range of Gd concentration xGd (0 ~10 \uc5. The same models applied at lower r values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < r < ~6 \uc5 range was obtained for all the CGO samples applying a biphasic model encompassing both a fluorite CeO2-like and a C-type Gd2O3-like phases. This provides evidence that extended defects at local scale exist in the CGO. Gd-rich and Ce-rich droplets coexist in the sub-nanometric range

Charge ordering transition in GdBaCo2O5: evidence of reentrant behavior

We present a detailed study on the charge ordering (CO) transition in GdBaCo2O5 system by combining high resolution synchrotron powder/single crystal diffraction with electron paramagnetic resonance (EPR) experiments as a function of temperature. We found a second order structural phase transition at TCO=247 K (Pmmm to Pmma) associated with the onset of long range CO. At Tmin = 1.2TCO, the EPR linewidth rapidly broadens providing evidence of spin fluctuations due to magnetic interactions between Gd3+ ions and antiferromagnetic couplings of Co2+/Co3+ sublattices. This likely indicates that, analogously to manganites, the long-range antiferromagnetic order in GdBaCo2O5 sets in at TCO. Pair distribution function (PDF) analysis of diffraction data revealed signatures of structural inhomogeneities at low temperature. By comparing the average and local bond valences, we found that above TCO the local structure is consistent with a fully random occupation of Co2+ and Co3+ in a 1:1 ratio and with a complete charge ordering below TCO. Below T = 100 K the charge localization is partially melted at the local scale, suggesting a reentrant behavior of CO. This result is supported by the weakening of superstructure reflections and the temperature evolution of EPR linewidth that is consistent with paramagnetic (PM) reentrant behavior reported in the GdBaCo2O5.5 parent compound.Comment: 30 pages, 10 figure