Papers by Author: E.N.S. Muccillo

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: Polycrystalline CaCu₃Ti₄O₁₂ ceramics were prepared by solid state reactions by spark plasma sintering (SPS) technique. In this study, the effects of the dwell temperature on structural, microstructural and dielectric properties of CaCu₃Ti₄O₁₂ ceramics have been investigated. Powder mixtures were calcined at 900°C for 18 h before SPS consolidation. The dwell temperatures were 850, 900, 915 and 930°C. Sintered pellets were characterized by X-ray diffraction, scanning electron microscopy and impedance spectroscopy. X-ray diffraction patterns show evidences of a single-phase perovskite-type structure. The calculated lattice parameter is 7.40 Å. The hydrostatic density increases slightly with increasing dwell temperature. Scanning electron microscopy observations revealed a heterogeneous microstructure for all samples. The dielectric loss remains constant over a wide temperature range. The obtained permittivity is approximately 10³ at 1 kHz. The increase of the dwell temperature is found to produce a brittle ceramic.

Abstract: Oxygen-ion conductors based on strontium-and magnesium-doped lanthanum gallate have been proposed to be used as solid electrolyte in solid oxide fuel cells operating at intermediate temperatures (500-700 °C), due to their high ionic conductivity and stability over a wide range of oxygen partial pressures. In this work, the effect of attrition milling on phase composition of powder and consolidated specimens prepared by solid state synthesis has been investigated. The results show that both the attrition milling and the calcination temperature play a major role in the phase composition. Powders with negligible amount of secondary phases were obtained after two steps of calcination at high temperature followed by attrition milling.

Abstract: Samaria-doped ceria is a candidate solid electrolyte for applications in intermediate-temperature solid oxide fuel cells. One of the main concerns related to this solid electrolyte is its low sinterability, even when prepared by chemical methods. In this work, an alternative method of sintering was used aiming obtaining high densification with negligible grain growth in the late stage of sintering. Commercial samaria-doped ceria powder was pressed into pellets and sintered following the two-step sintering process. The first (peak) temperature was varied from 1100 to 1500 °C and the dwell temperature between 1050 and 1400 °C with soaking time of 5 h. The sintered ceramics were characterized by several techniques to correlate the processing-property characteristics. Relatively high densification has been obtained. Electrical conductivity data obtained by impedance spectroscopy could account for the observed improved densification.

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: Yttria-stabilized zirconia is the most developed solid electrolyte for use in hightemperature solid oxide fuel cell (SOFC). Commercial yttria-stabilized zirconia powders reach high densification at temperatures around 1400 °C. The use of additives may increase the densification rate by means of a liquid phase formation during sintering. However, these additives should not cause any degradation on the ionic conductivity of the electrolyte. The main purpose of this work was to study the effect of Co addition on the densification and electrical conductivity of yttriastabilized zirconia. Green compacts were prepared by pressureless sintering a mixture of commercial 8 mol% yttria-stabilized zirconia with cobalt carbonate. Linear shrinkage results show that the temperature at which the shrinkage starts decreases with increasing Co content. Impedance spectroscopy measurements reveal that Co additions to stabilized zirconia decrease the total electrolyte conductivity even for Co contents as low as 0.05 mol%.

Abstract: High ionic conductivity ceramics have potential technological applications in chemical sensors, ceramic permeable membranes, oxygen pumps, and solid oxide fuel cells. Recently ionic conductivity values as high as those of doped zirconia solid solutions have been found in a lanthanum molybdate compound. The high ionic conductivity of this compound, La2Mo2O9, is obtained at temperatures above the structural phase transition temperature (~580 °C). In this work the La2Mo2O9 ceramic material was prepared by the polymeric precursor technique and sintered at several dwell temperatures and soaking times to study the effect of sintering conditions on phase transition. It was found that there is a strong dependence of phase transition on the sintering profile. At 950 °C the phase transition is suppressed for short soaking times, whereas it is observed to occur for longer times. Moreover, the relative magnitude of conductivity is also dependent on the sintering conditions. The main conclusion is that the phase transition in La2Mo2O9 is particle sizedependent.

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.

Abstract: Ceramic ZrTiO4 powders were prepared by a 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. Single-phase ZrTiO4 nanoparticles of 46 nm average particle size, determined by nitrogen adsorption analysis, were obtained after heat treatment at 450°C for 1 h. After pressing these sinteractive powders, pellets with controlled pore size distribution were obtained by sintering at temperatures as low as 400°C. The analysis of pores by mercury porosimetry gives an average porosity of 45%. The electrical resistivity, determined by impedance spectroscopy measurements at 24°C under different humidity environments, shows the ability of these pellets to adsorb water vapor in the porous surfaces.