Nanocrystalline cerium oxide based powders (CeO2, (GdO1.5)0.2(CeO2)0.8, and (SmO1.5)0.2(CeO2)0.8) have been produced using combustion synthesis with glycine as fuel and nitrate as oxidizer. The pure CeO2 powders prepared by using different glycine-to-nitrate ratios have been characterized by X-ray diffraction (crystallite size), thermogravimetry, infrared spectroscopy, surface area analysis, and transmission electron microscopy. The influence of calcination temperature on crystallite size, surface area, and carbonate species remaining from the combustion reaction has been studied especially for the near stoichiometric glycine/nitrate ratio (G/N = 0.55) to reveal the optimal synthesis conditions for all three compositions. A G/N ratio of 0.55 and calcination at 550 °C in oxygen flow gave high quality powder with a crystallite size of ∼10 nm. The powders had excellent sintering properties with an onset of densification at ∼600 °C.

No abstract

The sintering properties and microstructure of La1−xAxNbO4 powders (x=0, 0.005, and 0.02 and A=Ca, Sr, and Ba), prepared by spray pyrolysis have been investigated. Dense materials (>97%) were obtained by conventional sintering at 1200°C and by hot pressing (25 MPa) at 1050°C, respectively. Homogeneous materials were obtained and the average grain size obtained by the two densification methods was ∼2.0 and ∼0.4 μm, respectively, for the 2% doped materials. Pure lanthanum ortho‐niobate (LaNbO4) showed a higher degree of grain growth. In the acceptor‐doped materials, secondary phases were observed to inhibit grain growth at 1200°C. At 1400°C or higher, molten secondary phases in the Ba‐doped materials resulted in severe grain growth, causing microcracking during cooling due to crystallographic anisotropy. A low solubility of AO (A=Ca, Sr, and Ba) in LaNbO4 is inferred from the presence of secondary phases, and 1 mol% solubility of SrO in LaNbO4 was found by electron microprobe analysis. The electrical conductivity in wet hydrogen of the materials demonstrated that the main charge carrier was protons up to 1000°C and reached a maximum value of ∼8·10−4 S/cm at 900°C.

No abstract