Sergio Meriani

An unusual promotion of the redox behaviour of CeO2-ZrO2 solid solutions upon sintering at high temperatures

The reduction behaviour of a high surface area CeO2-ZrO2 solid solution is compared with that of a high surface area CeO2. It is shown that, upon sintering induced by repetitive reduction/oxidation processes, the temperature of reduction of the solid solution decreases from 900 to 700 K. In contrast, the reduction at low temperatures of the CeO2 sample is strongly retarded after such treatments. The role of ZrO2 in promoting the reduction at low temperatures is discussed.

NO decomposition over partially reduced metallized CeO2-ZrO2 solid solutions

Incorporation of ZrO2 into a solid solution with CeO2 strongly enhances the reducibility of the Ce4+ in the metallized samples and favours an effective NO decomposition over the reduced catalysts.

Metal-loaded CeO2-ZrO2 solid solutions as innovative catalysts for automotive catalytic converters

The redox behaviour of a Ce0.5Zr0.5O2 solid solution is investigated by means of temperature programmed reduction (TPR) and oxygen uptake measurements. It is shown that the introduction of ZrO2 into the CeO2 framework, strongly modifies the reduction behaviour in comparison to pure CeO2. Remarkably, in contrast to the CeO2, upon repetitive reduction-oxidation processes, the temperature of reduction of the solid solution decreases from 900 to 700 K. The reduction of NO by CO over metal-loaded catalysts is investigated and the role of support Ce3+ sites in the enhancing the NO conversion is discussed.

Deposition of calcium ions on rutile (110) : A first-principles investigation

Abstract The deposition of calcium ions is the first and most crucial step of apatite nucleation on ceramic supports from ionic solution. This process is believed to initiate the growth of bone-like material on the surface of biocompatible implants. We have investigated the adsorption of Ca2+ from water solution on the rutile TiO2 (110) surface by means of first principles techniques. The preferential binding site of the calcium ion on the hydrated oxide surface was determined through a series of static calculations. Molecular dynamics simulations were then performed to elucidate the deposition pathway. The driving force for adsorption is identified in the electrostatic interaction between the Ca2+ complexes and negatively charged deprotonated sites present on the hydrated TiO2 (110) surface.

Mechanical properties in the ceria-zirconia system

Abstract The room temperature mechanical properties of samples ranging through the whole zirconia-ceria system have been determined and related to the phases present in order to establish the influence of crystallographic parameters on the material properties. Hardness, toughness and strength as a function of the amount of transformable tetragonal, non-transformable tetragonal or cubic zirconia are discussed.

The effect of wear on the tetragonal-to-monoclinic transformation and the residual stress distribution in zirconia-toughened alumina cutting tools

Abstract Zirconia-toughened alumina cutting tools have been examined after machining an AISI 1040 steel. The Raman spectrum of zirconia and the fluorescence spectrum from alumina have been used to determine the extent of the tetragonal-to-monoclinic (t-m) transformation and the residual stresses in and around the worn-out crater, respectively. Although about 15% vol. m-zirconia is present even in a brand new cutting tool, it was shown that the crater region undergoes the t-m transformation up to 40% vol. In both cases, the transformed zone is concentrated near the surface. The residual stresses in alumina are slightly compressive in the new cutting tool and away from the crater region, but they become tensile inside the crater, where the t-m transformation of zirconia has occurred to a larger extent. A stochastic model has been used to predict the stresses and it has been found to reproduce very well the experimental data. Some evidence of the formation of a spinel reaction product in the crater region has been found, likely (Mg,Fe)AlO 4 .

Redistribution of Ce and La during processing of Ce(La)-TZP/Al2O3 composites

Abstract The dispersion of second-phase particles, platelets or fibres in a ceramic body offers possibilities of considerable improvements in mechanical properties. In such second-phase reinforced composites the materials properties are to a large extent controlled by the structure of the internal interfaces, i.e. the phase and grain boundaries. The microstructure/microchemistry of aluminadispersed ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP/20A) were characterized by means of analytical TEM/STEM techniques. Local variation in the concentration of Ce stabilizer as well as of La additives and Si impurities during processing of the composite ceramics were studied by EDS X-ray analysis with high spatial resolution. Emphasis was put on the segregation of Ce and La in interface regions. Evidence was found for the presence of a vitreous intergranular phase situated in small pockets at triple grain junctions. The addition of small fractions of La 2 O 3 in TZP resulted in interreaction between matrix constituents and Al 2 O 3 particles. In this reaction a stable crystalline Al-Ce-La-Zr mixed oxide phase is formed, effectively reducing the fraction of residual vitreous intergranular phase, resulting in improved fracture toughness in this material.

Raman Piezo-Spectroscopic Behavior of Aluminum Nitride

FIG. 1. Schematic representation of the jig used to perform the piezospectroscopic calibrations. The laser microprobe is moved along the dotted line from the compressive to the tensile side, and a spectrum is collected about every 200 m m. The load registered by the load recorder is then translated to stress according to the standard elastic beam bending equation. Raman Piezo-Spectroscopic Behavior of Aluminum Nitride

No decomposition over partially reduced metallized CeO2 containing catalysts

Abstract M/CeO 2 -ZrO 2 (M=Rh, Pt) solid solutions are investigated as catalysts for the reduction of NO by CO. It is shown that incorporation of CeO 2 into a ZrO 2 framework strongly promotes the reduction of the metallized support. The reduced support efficiently decomposes NO suggesting a direct participation of the support in the NO conversion.

Thermal evolution of ceria-zirconia metallorganic precursors

Abstract Ceria-zirconia tetragonal solid solutions were obtained by using gelled metallorganic precursors as starting materials. By drying these gels, intimately mixed powders of a quasi-amorphous nature were obtained. On firing, the tetragonal solid solution sintered directly without any separation of the single oxides. Poorly sintered materials were obtained from the firing of dried gels but improved sinterability could be achieved by using calcined gels to prepare compressed pellets before firing. DTA-TG evidence indicated that the organic remnants of the dried gels were responsible for their poor sintering performance. Pre-firing treatment at 550°C gave very active powders which sintered faster and better than ground crystalline mixtures.