Papers by Author: Gianguido Baldinozzi

Abstract: Several samples of (U,Nd)O₂ with different Nd content (ranging from 2 to 50 in Nd₂O₃ mass. %) were prepared. For composition ranging from 6 to 20 in Nd₂O₃ mass %, two crystalline phases are unambiguously evidenced by X-ray and neutron diffraction, by optical microscopy, and by SEM-EDX and EPMA. The influence of the preparation route on the distribution of the domains made by these two phases is evidenced and discussed.

Abstract: The present study is concerned with the influence of sputter-coatings CaO on the oxidation behavior of Ni polycrystals. The experiments were performed in air, in the temperature range 800°-1200°C. Below 1200°C, CaO coatings reduce the oxidation rate, while this beneficial effect disappears at 1200 °C. The oxidized specimens were examined by SEM and X-Ray diffraction, but also by EPMA depth profiling to evaluate the scale composition. Furthermore, electrical conductivity measurements and kinetic demixing studies were carried out on Ca-doped NiO single crystals, to get a better insight regarding the transport processes involved during oxidation. These last results show that the key features allowing to explain the effect of CaOcoatings on the oxidation rate of Ni are the influence of calcium on the increase of the dissociation pressure of NiO, which delays the oxidation of nickel, the kinetic demixing of the cations, which controls the distribution of CaO precipitates in the scale responsible for blocking effects, and the increase of the diffusion coefficient of both the cations and the cationic vacancies, which play a decisive role at high temperature, when the scale growth is dominated by lattice diffusion.

Abstract: This work was directed at a comprehensive study of the role of the nanostructure and nanochemistry on the transport properties of yttria-stabilized zirconia. Alumina additions lead to a decrease of sgb when the samples have clean grain boundaries, while sgb goes through a maximum in samples having glassy grain boundaries. The differences were attributed to the strong interaction between Al2O3 and SiO2 impurities leading to a glassy phase depletion at the grain-boundaries, due to a change in wettability. Moreover, XPS analyses show that Si and Y segregate near these interfaces according to a kinetic demixing process, explaining why a faster cooling rate after sintering has a beneficial effect on sgb.

Abstract: Zirconia can be considered to be one of the most important ceramic materials because of its large range of industrial applications (catalysis, coatings, spacecraft shielding, paint additives, oxygen sensors, fuel cells, nuclear fuel matrices, an alternative high permittivity material to replace silicon oxide as a gate dielectric in MOS devices). Many of these applications require the use of zirconia in a nanocrystalline form. It is now well established that a monoclinc to tetragonal phase transition is trigged by the grain size of zirconia. The mechanism of this phase transition in zirconia is not yet clearly understood. Several experiments point out that the thermodynamic properties of nanocrystalline solids are particle-size dependent. Size-related effects like the reduction of the melting temperature and displacement of the phase boundaries can be predicted. Zirconia can be considered a textbook example for describing these effects. In this ceramic several polymorphic transformations occur with the change of external parameters (Temperature, pressure, …). In this paper, the behaviour of the tetragonal to monoclinic martensitic phase transition within Landau theory framework in particular will be discussed, pointing out the peculiar effects related the small grain size of the nanoparticles. Neutron diffraction experiments will illustrate the of these arguments and provide some insight to the understanding of the behaviour of nanocrystals in severe environments, such as in nuclear reactors or in space applications.