Snezana Bošković

Ce1−xY (Nd)xO2−δ nanopowders: potential materials for intermediate temperature solid oxide fuel cells

Nanopowdered solid solution Ce 1-x Y(Nd) x O 2-δ samples (0.1 ≤ x ≤ 0.25) were made by self-propagating room temperature (SPRT) synthesis. The first-order Raman spectra of Ce 1-x Y(Nd) x O 2-δ samples measured at room temperature exhibit three broad features: the main Raman active F 2g mode at about 450 cm -1 and two broad features at about 550 (545) and 600 cm -1 . The mode at ∼600 cm -1 was assigned to the intrinsic oxygen vacancies due to the nonstoichiometry of ceria nanopowders. The mode at about 550 (545) cm -1 was attributed to the oxygen vacancies introduced into the ceria lattice whenever Ce 4+ ions are replaced with trivalent cations (Y 3+ , Nd 3+ ). The intensity of this mode increases with doping in both series of samples, indicating a change of O 2- vacancy concentration. The mode frequency shifts in opposite direction in Y- and Nd-doped samples with doping level, suggesting that different types of defect space can occur in Y- and Nd-doped ceria nanopowders.

Lattice Parameters of Gd-Doped Ceria Electrolytes

This paper deals with Gd-doped ceria solid solutions: Ce1−XGdXO2−d with x ranging from 0 to 0.2. Four different powders were synthesized by modified glycine nitrate procedure with very precise stoichiometry according to tailored composition. The method was modified by decreasing glycine/nitrate ratio to 0.5. All obtained solid solutions exhibit a fluorite-type crystal structure with composition dependent lattice parameters. The variation of the lattice parameter was studied and correlated with the equation describing the ion-packing model. It has been found that the change of lattice parameter versus Gd concentration obeys Vegards rule very well. Results also show that all powders are nanometric in size. The average size of Ce1−XGdXO2−d particles is about 20 nm.

Influence of Additive Type on Densification and Phase Transformation of Seeded Si3N4

This paper deals with densification and a®b phase transformation of Si3N4, with a constant b-Si3N4 seeds concentration regarding sintering temperature and different additive types. The seeds of b-Si3N4 were obtained by sintering of silicon nitride in a-form and additive mixture consisting of Y2O3 and SiO2. Two different Si3N4/additive mixtures (Y2O3 + Al2O3 and CeO2) containing constant amount of seeds were prepared and tested. Characterization of sintered samples involved phase analysis by X−ray diffraction and density measurements. The results indicated that in the presence of yttria-alumina mixture both the phase transformation and the densification were enhanced as compared to samples containing only CeO2 as a sintering additive. The reasons for observed behavior are discussed in detail.

Reaction of Ce1-xRexO2-δ Nanopowders Synthesis

One of the methods for powder synthesis that is both cost and time effective is the selfpropagating room temperature synthesis. We applied this method to synthesize rare earth doped ceria nanopowders. Since they exhibit very high ionic conductivity at intermediate temperatures these compositions are attractive for a new generation of nanostructured ceramics applicable in solid oxide fuel cells as electrolytes. In this paper we paid our attention to the reaction based on methathetical pathway, whereby solid solution nanopowders of rare earth elements with ceria were obtained at room temperature. Compositions of Ce1-xRexO2-δ (Re = Y , Nd) were synthesized with x ranging from 0 to 0.20. The reaction course is discussed and the properties of the obtained powders are presented.

Effect of LiYO2 addition on sintering behavior and indentation properties of silicon nitride ceramics

Abstract The influence of the sintering additive LiYO2 (5–15wt.%) on sintering behavior, microstructure and mechanical properties of Si3N4 ceramics was investigated. Since LiYO2 enables densification of Si3N4 at extraordinarily low temperatures, sintering was carried out in the range from 1200–1700°C. Densification was found to be enhanced with increasing additive content due to an increasing volume fraction of the liquid. The phase transformation and grain growth occurred through a solution-reprecipitation mechanism, where the precipitation took place preferentially on pre-existing -Si3N4 nuclei (of which the starting powder already contained 20wt.%). The indentation fracture toughness increased with both sintering time and additive content as a result of the growth of elongated grains.

Sintering of Si3N4 with Li-exchanged zeolite additive

Abstract This paper deals with the densification and phase transformation of Si3N4 with additives of Li-exchanged zeolite during pressureless sintering at significantly reduced temperatures. Dilatometric shrinkage data show that the first liquid forms as low as 1080°C. Upon sintering at 1500°C the bulk density increases to more than 95% of the theoretical density without phase transformation from -S3N4 to -Si3N4, i.e. the phase transformation lags behind the densification process. Above 1500°C the secondary phase is completely converted into a glass and the -to- transformation takes place. Under these conditions the grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. The results show that a very effective low-temperature sintering additive for silicon nitride can be obtained from Li-exchanged zeolite.

Phase Evolution of Si3N4 Ceramic with Additives from Li2O-Y2O3

This paper deals with the to Si3N4 phase transformation during pressureless sintering of Si3N4 with LiYO2 as a sintering additive. The phase evolution after heat treatments in the temperature range from 1200 to1500°C shows that -Si3N4 and -Si3N4 are the main crystalline phases in all stages of annealing. At 1200°C, the initial LiYO2 sintering additive completely disappears. X-ray patterns show a remarkable broadening of diffraction lines in the regions around 27-29° and 32-34° 2 , which can be ascribed to the Y5(SiO4)3N (x93N-apatitex94) phase. This indicates that the nucleation and formation of crystalline N-apatite takes place at this temperature, which is the onset of liquid formation. At 1600°C, a complete conversion of the secondary phase into the liquid one and of to -Si3N4 takes place.