Papers by Author: R. Muccillo

Abstract: Zirconia-8 mol% yttria porous solid electrolytes were obtained by mixing with different amounts (0 to 5 wt.%) of KI prior to sintering. Potassium iodide acts as sacrificial pore former, being removed from the ceramic pellet upon sintering at 1400 °C/2 h. The alkali halide content was evaluated by X-ray fluorescence analysis, the density of the pellets by the Archimedes method, and the pore content by observation in scanning probe (SPM) and scanning electron (SEM) microscopes of polished and etched surfaces. The oxide ion resistivity was determined by impedance spectroscopy analysis in the 5 Hz-13 MHz frequency range from 300 to 500 °C. Porous specimens with high skeletal density were obtained. The higher is the alkali halide content, the higher is the pore volume and the total electrical resistivity. A correlation is found between the pore content, evaluated by SEM and SPM, and the electrical behavior analysis from the impedance plots of the porous specimens.

Abstract: Investigation on polycrystalline electroceramics involves the synthesis, the consolidation and the analysis of the electrical behavior, along with careful evaluation of the final microstructure. The synthesis of ceramic powders with controlled characteristics is crucial in the study of materials with optimized properties. Distinct properties may be found in ceramic materials prepared by the several existing methods, due to chemical and phase homogeneities, and to the particle size distribution or medium particle size. In this work, yttrium-doped barium zirconate proton conductor was synthesized by spray pyrolysis, and characterized by several techniques aiming identifying the influence of some parameters of this method of synthesis with particle characteristics. Nanocrystalline powders synthesized at 600-700oC were found to be cubic and single phase. Moreover, depending on the gas flow and furnace temperature, spheroid and porous or cubic and solid particles may be obtained.

Abstract: Yttria-stabilized zirconia is a singular polycrystaline ceramic with a range of technological applications. The combination of its physical properties is responsible for application as solid electrolytes in solid oxide fuel cells. High densification is required for this and other applications. The sintering of this solid electrolyte is still a matter of investigation. To reduce the sintering temperature, the introduction of additives is an effective approach. In this work, the effects of lithium addition on microstructure and electrical conductivity of 8 mol% yttria-stabilized zirconia was studied by scanning electron microscopy and impedance spectroscopy, respectively. Cylindrical pellets were prepared by pressing, followed by sintering at 1200oC without and with 1 and 2 mol% lithium (metal basis). As precursor materials both lithium carbonate and lithium fluoride were used. The main microstructure features were correlated with the results of electrical conductivity.

Abstract: Ceria-based materials have been extensively studied due to their wide range of technological application. In this work, nanostructured powders of 20 mol% gadolinia-doped ceria pure and containing 1 mol% manganese were synthesized by the cation complexation technique. Powder materials were calcined at 600°C, uniaxially pressed and sintered in the 1200-1500°C range for soaking times of 1, 2 and 4 h. X-ray diffraction patterns evidenced a single-phase fluorite-like structure in all studied specimens. The evolution of grain sizes was evaluated by scanning electron microscopy on polished and thermally etched surface of sintered pellets. The relative density decreases for soaking times above 1300°C (with Mn) and 1400°C (without Mn). The grain size increases with manganese addition. The role of the additive on the electrical conductivity of gadolinia-doped ceria was evaluated by impedance spectroscopy measurements.

Abstract: SrTi_0,65Fe_0,35O_3-δ, Ca_0,5Sr_0,5Ti_0,65Fe_0,35O_3-δ, CaTi_0,65Fe_0,35O_3-δ ceramic powders were synthesized by the polymeric precursor technique using CaCO₃, SrCO₃, C₁₂H₂₈O₄Ti and Fe (NO₃)₃.₉H₂O. After calcination, each powder was heat treated at temperatures chosen according to data collected on thermogravimetric-differential thermal analysis experiments. The compositions were analyzed by X-ray diffraction for structural phase evaluation (either perovskite cubic or orthorhombic), laser scattering for determination of particle size distribution and average particle size, transmission electron microscopy (TEM) for observation of particle shape and average true size. Pressed powders sintered at 1250°C were analyzed by X-ray diffraction and X-ray fluorescence; their surfaces were observed by scanning probe microscopy (SPM) for topographical analysis of grains and grain boundaries. TEM results show that the powders consist of agglomerated nanoparticles. Sr-based compounds have cubic perovskite phases whereas Ca-based compounds are orthorhombic. SPM images show intergranular features which might be responsible for reported blocking of charge carriers observed in impedance spectroscopy diagrams.

Abstract: Closed-end tubes of yttria-stabilized zirconia (YSZ) are required in oxygen sensors for automotive applications to control fuel consumption and pollutant emission. High temperature oxygen sensors were prepared by electrophoretic deposition (EPD) of submicron YSZ particles after suitably adjusting the rheological characteristics of the suspension of the ceramic particles in alcoholic medium. The ceramic tubes were analyzed by X-ray diffraction and impedance spectroscopy. The morphology of sintered specimens was studied by scanning electron microscopy. The processing by EPD of tubular ceramic solid electrolytes with electrical conductivity similar to the ones found in commercial lambda sensors has been successfully achieved.

Abstract: The influence of Mn addition (from 1 up to 10 mol%) on densification and electrical conductivity of CeO2 was studied in detail. Bulk specimens were prepared by the conventional technique of mixing suitable amounts of cerium dioxide and manganese carbonate, followed by pressing and sintering. The apparent density of sintered specimens was determined by the immersion method and electrical characterization was carried out by impedance spectroscopy measurements in the 300 to 550 °C range. High densification (> 95% of the theoretical density) was obtained for small addition of Mn (1 mol%) to CeO2 sintered at 1300 °C for only 0.1 h. Electrical conductivity changes in sintered pellets depend on the sintering procedure, but not on the manganese content.

Abstract: Lanthanum beta alumina powders were obtained by the polymeric precursor technique using lanthanum nitrate, aluminum nitrate, ethylene glycol and citric acid. The transformations that occur during thermal treatment of the precursor solution were evaluated by thermogravimetric and differential thermal analysis. Fourier Transform Infrared analysis for residual carbon qualitative detection and gas adsorption analysis for evaluating specific surface area, BET method, were carried out in powder specimens heat treated at different temperatures. High calcination temperature leads to the formation of hard agglomerates. The powders calcined at 800°C for 4 h have high specific surface area, ~ 120 m2/g. All processed powders and green pellets sintered at different temperatures were analyzed by X-ray diffraction for structural phase determination. Single phase LaAl11O18 pellets have application as solid electrolytes in disposable electrochemical devices for monitoring dissolved oxygen species in molten steel at very high temperatures, > 1500 °C, during steel production.

Abstract: Ceramic ZrTiO4 powders were prepared by a modified sol-gel method using zirconium oxychloride and titanium tetraisopropoxide. In situ high temperature X-ray diffraction results show that crystallization of the amorphous gel starts at 400 °C. Singlephase ZrTiO4 nanoparticles were obtained after heat treatment at 450 oC for 1 h. An average particle size of 46 nm has been determined by nitrogen adsorption analysis. After pressing these sinteractive powders, pellets with controlled pore size distribution were obtained by sintering at temperatures as low as 400 oC. The analysis of pores by mercury porosimetry shows an average porosity of 45 %. Pressing and sintering the nanosized powders prepared by that modified sol-gel technique produced pellets that are good candidates to be used in humidity sensing devices.