Diego G. Lamas

Synchrotron X‐ray diffraction study of the tetragonal–cubic phase boundary of nanocrystalline ZrO2–CeO2 synthesized by a gel‐combustion process

The crystal structures of a number of nanocrystalline ZrO2–CeO2 solid solutions, synthesized by a pH-controlled nitrate-glycine gel-combustion process, were studied. By using a synchrotron X-ray diffractometer, small peaks of the tetragonal phase, which correspond to forbidden reflections in the case of a perfect cubic fluorite structure, were clearly detected. By monitoring the most intense of these reflections, 112, as a function of the CeO2 content, the tetragonal–cubic phase boundary was found to be at 85 (5) mol% CeO2. For a CeO2 content up to 68 mol%, a tetragonal phase with c/a > 1 (known as the t′ form) was detected, whereas, between 68 and 85 mol% CeO2, the existence of a tetragonal phase with c/a = 1 and oxygen anions displaced from their ideal positions in the cubic phase (the t′′ form) was verified. Finally, solid solutions with higher CeO2 contents exhibit the cubic fluorite-type phase.

Metastable forms of the tetragonal phase in compositionally homogeneous, nanocrystalline zirconia–ceria powders synthesised by gel-combustion

The crystal structure of compositionally homogeneous, nanocrystalline ZrO2–10, 30, 50, 70 and 90 mol% CeO2 powders synthesised by a pH-controlled nitrate–glycine gel-combustion process has been studied by X-ray diffraction and Raman spectroscopy. All the powders with a CeO2 content up to 70 mol% exhibited the tetragonal phase (P42/nmc space group), whereas the ZrO2–90 mol% CeO2 solid solution presented the cubic phase (Fmm space group). The axial ratio c/a decreased with increasing CeO2 content and became unity for ZrO2–70 mol% CeO2 powders. By analysing X-ray diffraction data using the Rietveld method, it was found that this material exhibited the t″-form of the tetragonal phase (the oxygen atoms were displaced from their ideal sites of the cubic phase along the c axis). Raman spectroscopy study confirmed the results found by X-ray diffraction. The morphology of the nanopowders was also evaluated.

Synthesis of nanocrystalline CeO2–Y2O3 powders by a nitrate–glycine gel-combustion process

In this work, the synthesis of CeO 2 -10 mol% Y 2 O 3 powders by a nitrate-glycine gel-combustion route was investigated. Special attention was given to the influence of the glycine/metal ratio and calcination temperature on powder morphology. In contrast to the usual reported behavior, the best powder properties (crystallite size, 4.5-7 nm; specific surface area; 25-40 m 2 /g) were obtained for slow combustion processes with glycine/metal ratios of 1.5-2, whereas energetic reactions resulted in large crystallite and particle sizes. Furthermore, it was found that the crystallite size increases considerably even at moderate calcination temperatures (350-550 °C), showing the high reactivity of these nanopowders.

Preparation and ionic transport properties of YDC–YSZ nanocomposites

The ionic transport properties of dense nanocomposites of yttria-doped ceria (YDC) and yttria-stabilized zirconia (YSZ), in a volume ratio of 1 : 1, have been investigated by electrochemical impedance spectroscopy. These materials were prepared by means of a fast-firing process starting from nanopowders with high specific surface area, avoiding the formation of secondary phases and minimizing the opportunity for grain growth to occur during sintering. An important enhancement of the total ionic conductivity with decreasing crystallite size was observed, as reported in the literature for other nanostructured ionic conductors. This result can be attributed to an almost ideal parallel connection between both nanophases, coupled to an increase of their grain boundary ionic diffusivity. In addition, it was proved that the presence of the YSZ phase significantly improves the mechanical properties of ceria-based nanoceramics.

Local structure of the metal-oxygen bond in compositionally homogeneous, nanocrystalline zirconia-ceria solid solutions synthesized by a gel-combustion process

Compositionally homogeneous ZrO2–CeO2 nanopowders have been characterized by Raman and extended x-ray absorption fine structure (EXAFS) spectroscopies. These techniques revealed a tetragonal-to-cubic phase transition as a function of CeO2 content, as observed in a previous synchrotron x-ray diffraction study. The tetragonal–cubic phase boundary was found to be at (85 ± 5) mol% CeO2. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the Zr–O first neighbour coordination sphere, while the Ce–O coordination sphere preserves its cubic symmetry over the whole composition range.

Local atomic structure in tetragonal pure ZrO2 nanopowders

The local atomic structures around the Zr atom of pure (undoped) ZrO2 nanopowders with different average crystallite sizes, ranging from 7 to 40 nm, have been investigated. The nanopowders were synthesized by different wet-chemical routes, but all exhibit the high-temperature tetragonal phase stabilized at room temperature, as established by synchrotron radiation X-ray diffraction. The extended X-ray absorption fine structure (EXAFS) technique was applied to analyze the local structure around the Zr atoms. Several authors have studied this system using the EXAFS technique without obtaining a good agreement between crystallographic and EXAFS data. In this work, it is shown that the local structure of ZrO2 nanopowders can be described by a model consisting of two oxygen subshells (4 + 4 atoms) with different Zr—O distances, in agreement with those independently determined by X-ray diffraction. However, the EXAFS study shows that the second oxygen subshell exhibits a Debye–Waller (DW) parameter much higher than that of the first oxygen subshell, a result that cannot be explained by the crystallographic model accepted for the tetragonal phase of zirconia-based materials. However, as proposed by other authors, the difference in the DW parameters between the two oxygen subshells around the Zr atoms can be explained by the existence of oxygen displacements perpendicular to the z direction; these mainly affect the second oxygen subshell because of the directional character of the EXAFS DW parameter, in contradiction to the crystallographic value. It is also established that this model is similar to another model having three oxygen subshells, with a 4 + 2 + 2 distribution of atoms, with only one DW parameter for all oxygen subshells. Both models are in good agreement with the crystal structure determined by X-ray diffraction experiments.

Grain size distribution of nanocrystalline systems

The diffraction pattern of nanocrystalline Ce 0.9 Zr 0.1 O 2 was analyzed by whole powder pattern modeling, a recently proposed method for the study of line broadening. The main result in this typical case of study—the crystalline domain size distribution—matches closely the corresponding information obtained by transmission electron microscopy. Further information on nature and content of lattice defects is also discussed.

Synchrotron X-ray powder diffraction and extended X-ray absorption fine structure spectroscopy studies on nanocrystalline ZrO2–CaO solid solutions

The crystal structure and the local atomic order of a series of nanocrystalline ZrO2–CaO solid solutions with varying CaO content were studied by synchrotron radiation X-ray powder diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy. These samples were synthesized by a pH-controlled nitrate–glycine gel-combustion process. For CaO contents up to 8 mol%, the t′ form of the tetragonal phase (c/a > 1) was identified, whereas for 10 and 12 mol% CaO, the t′′ form (c/a = 1; oxygen anions displaced from their ideal positions in the cubic phase) was detected. Finally, the cubic phase was observed for solid solutions with CaO content of 14 mol% CaO or higher. The t′/t′′ and t′′/cubic compositional boundaries were determined to be at 9 (1) and 13 (1) mol% CaO, respectively. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the first oxygen coordination shell around the Zr atoms.

Catalytic Behavior of PdO ∕ NiO ∕ SDC Composites for Partial Oxidation of Methane: Application as Anodes of Single-Chamber IT-SOFCs

PdO/NiO/SDC composites (SDC: Sm 2 O 3 -doped CeO 2 ) have been proposed to be excellent anodes for single-chamber, intermediate-temperature solid oxide fuel cells (SC-IT-SOFCs) operated under mixtures of CH 4 and air, reaching high power densities at relatively low working temperatures. In this work we evaluated the catalytic activity of these composites in the partial oxidation of CH 4 , finding that their high performance is due to the excellent catalytic properties of these materials. The effect of the addition of PdO was investigated by analyzing NiO/SDC and PdO/NiO/SDC composites under different reaction temperatures and CH 4 :O 2 ratios. Finally, PdO/NiO/SDC composites were evaluated under operation in SC-IT-SOFCs similar to those reported in the literature using SDC electrolytes and Sm 0.5 Sr 0.5 CoO 3 cathodes. Our results confirmed the feasibility of SC-IT-SOFCs.

High-temperature X-ray powder diffraction study of the tetragonal-cubic phase transition in nanocrystalline, compositionally homogeneous ZrO2-CeO2 solid solutions

, CO and hydrocarbons from automotiveexhausts , as anodes in fuel cell technologies, and others. Inparticular, the metastable forms of the tetragonal phase havebeen investigated widely since they are the most suitable forapplications Trovarelli, 2002; Di Monte and Kaspar, 2005 .The crystal structure of compositionally homogeneousZrO