Search

The effect of a galia (Ga2O3) addition on the crystallographic phase evolution and the crystal growth behavior of zirconia (ZrO2) rich powder were investigated. Binary compositions of galia-zirconia powder (on the zirconia-rich side of the quasi-binary phase diagram) were obtained by the sol-gel method and calcined at different temperatures for 2 h. Rietveld refinement (FullProf program) was used to determine the crystallographic phase composition. Crystal size was approximated based on X-ray diffractions analysis combined with estimates of average crystal size obtained by processing high-resolution scanning electron microscopy images. The surface area of the powders obtained was measured via the Brunauer-Emmett-Teller method. Zirconia-galia sol-gel powders were found to form tetragonal or mixed tetragonal-monoclinic solid solution phases depending on both composition and calcination temperature. It was shown that galia suppressed crystal growth process in zirconia, producing finer nano-powders with higher surface area compared to pure zirconia. [ABSTRACT FROM AUTHOR]

The effect of a galia (Ga2O3) addition on the crystallographic phase evolution and the crystal growth behavior of zirconia (ZrO2) rich powder were investigated. Binary compositions of galia-zirconia powder (on the zirconia-rich side of the quasi-binary phase diagram) were obtained by the sol-gel method and calcined at different temperatures for 2 h. Rietveld refinement (FullProf program) was used to determine the crystallographic phase composition. Crystal size was approximated based on X-ray diffractions analysis combined with estimates of average crystal size obtained by processing high-resolution scanning electron microscopy images. The surface area of the powders obtained was measured via the Brunauer-Emmett-Teller method. Zirconia-galia sol-gel powders were found to form tetragonal or mixed tetragonal-monoclinic solid solution phases depending on both composition and calcination temperature. It was shown that galia suppressed crystal growth process in zirconia, producing finer nano-powders with higher surface area compared to pure zirconia. [ABSTRACT FROM AUTHOR]

Abstract: The dissolution enthalpy, ΔHDS, and the formation enthalpy, ΔHf,ox, of bulk lutetium‐doped cerium oxide (LuDC) were studied at 701°C in molten sodium molybdate. For the composition range of Ce1−XLuXO2−X/2, studied 0 ≤ X ≤ 0.3, the ΔHDS decreases linearly and smoothly with lutetium content according to ΔHDS, kJ/mol = 73.5(1.0)−165.1(5.5)·x). The enthalpy of formation, ΔHf,ox, becomes more exothermic linearly with lutetium content. No anomaly in ΔHf,ox is observed at low Lu2O3 concentration as reported previously for several other rare‐earth‐doped ceria systems, suggesting possible differences in clustering and microstructure, which may also be related to difference in processing conditions. [ABSTRACT FROM AUTHOR]

CaTiO3 perovskite has been proposed as a ceramic waste form for immobilization of 90 Sr. Nonradioactive coprecipitated xerogel powders with nominal atomic ratios of Ca: Zr: Ti = 0.75:0.25:1.00 were synthesized to mimic the fate of ( Ca0.7590 Sr0.25) TiO3 solid solution after complete decay of the Sr and its intermediate product Y to stable Zr when an excess B4+ ( Ti and 90 Zr) cations will present. Ca: Ti = 1.00:1.00 samples were used as a reference. The powders were heated to various conditions to explore the thermodynamic stability of its oxides. The heated Ca: Zr: Ti = 0.75:0.25:1.00 samples formed a major orthorhombic Ca( Zr1− x Ti x) O3 perovskite phase. The Ti/( Ti + Zr) ratio of the perovskite preserves its nominal ratio at 600°C. The Zr rejects from the Ca( Zr1− x Ti x) O3 with further increasing the temperature, following the formation of Ca- Ti- Zr- O secondary phases. This study indicates a tendency of the Zr to segregate from an original ( Ca, Sr) TiO3 waste form when the stoichiometry is controlled by the conversion of Sr to Zr (in normal oxidation states). [ABSTRACT FROM AUTHOR]

High-temperature solution calorimetry in molten 3Na2O• 4MoO3 solvent at 975 K is used to measure the enthalpy of mixing of ( Ca xSr1- x) TiO3 and ( Ba xSr1- x) TiO3 solid solutions. Both solid solutions show positive enthalpy of mixing, however, the ( Ba xSr1- x) TiO3 shows less deviation from ideality, with interaction parameter λ = 8.5 ± 3.8 kJ/mol, than ( Ca xSr1- x) TiO3 with interaction parameter λ = 25.3 ± 2.0 kJ/mol. The smaller interaction parameter of the ( Ba xSr1- x) TiO3 system is attributed to the volume mismatch of the end-members. The larger interaction parameter of ( Ca xSr1- x) TiO3 may be related to larger ionic radius difference of Ca and Sr and may also reflect the additional energy needed to overcome the lattice distortion of the orthorhombic CaTiO3 to incorporate it into the cubic perovskite SrTiO3. ( Ba xCa1- x) TiO3 has a large positive heat of mixing and limited solid solubility as derived from its phase diagram. This study indicates the tendency of SrTiO3 and BaTiO3 to separate from a CaTiO3 host matrix, which may be significant in perovskite-bearing nuclear waste ceramics incorporating 90 Sr and 137 Ba. [ABSTRACT FROM AUTHOR]

CaTiO3 has been proposed as a ceramic waste form for immobilization of radioactive waste that might include short-lived fission products 137Cs and 90Sr and consequently their decay products 137Ba (stable), 90Y (intermediate), and 90Zr (stable). In order to characterize substitution relations of Y3+ for M2+ (M=Ca, Sr, Ba singly or in combination) in MTiO3, precursor mixtures with nominal atomic ratios of M:Y:Ti=0.75:0.25:1 were synthesized by solid-state reactions. To ensure homogeneous starting material and to explore low temperature behavior of the Ca-Y-Ti-O system, coprecipitated xerogel powder was synthesized and heated to various temperatures for different periods of time. All M-Y-Ti-O systems formed two major phase mixtures of perovskite MTiO3 and pyrochlore Y2Ti2O7 with low substitution of M2+ by Y3+ in MTiO3 and Y3+ by M2+ in Y2Ti2O7. The study of xerogel confirmed the tendency of the formation of mixtures of CaTiO3 and Y2Ti2O7 phases, even at lower temperature. The temperature dependence of the substitution of Ca2+ by Y3+ in CaTiO3 appears to be small. Despite the minor substitution of M by Y in the perovskite no significant 90Y3+ build-up is expected to occur in the waste form due to the rapid decay of 90Y3+ relative to that of 90Sr2+. [ABSTRACT FROM AUTHOR]

Rare-earth (RE) oxides are important in myriad fields, including metallurgy, catalysis, and ceramics. However, the phase diagram of RE oxides in the nanoscale might differ from the phase diagrams for bulk, thus attracting attention nowadays. We suggest that grain size in the nanoscale also determines the obtained crystallographic phase along with temperature and pressure. For this purpose, nanoparticles of Sm 2 O 3 and Eu 2 O 3 were mixed in an inert MgO matrix via the sol-gel method. This preparation method allowed better isolation of the oxide particles, thus hindering the grain growth process associated with increasing the temperature. The mixed oxides were compared to pure oxides, which were heat-treated using two methods: gradual heating versus direct heating to the phase transition temperature. The cubic phase in pure oxides was preserved to a higher extent in the gradual heating treatment compared to the direct heating treatment. Additionally, in MgO, even a higher extent of the cubic phase was preserved at higher temperatures compared to the pure oxide, which transformed into the monoclinic phase at the same temperature in accordance with the phase diagram for bulk. This indicates that the cubic phase is the equilibrium phase for nanosized particles and is determined also by size.

A novel method to obtain Al₂O₃⁻ZrO₂ nanocomposites is presented. It consists of the co-precipitation step of boehmite (AlO(OH)) and ZrO₂, followed by microwave hydrothermal treatment at 270 °C and 60 MPa, and by calcination at 600 °C. Using this method, we obtained two nanocomposites: Al₂O₃⁻20 wt % ZrO₂ and Al₂O₃⁻40 wt % ZrO₂. Nanocomposites were characterized by Fourier transformed infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. Sintering behavior and thermal expansion coefficients were investigated during dilatometric tests. The sintering temperatures of the nanocomposites were 1209 °C and 1231 °C, respectively-approximately 100 °C lower than reported for such composites. We attribute the decrease of the sintering temperature to the specific nanostructure obtained using microwave hydrothermal treatment instead of conventional calcination. Microwave hydrothermal treatment resulted in a fine distribution of intermixed highly crystalline nanoparticles of boehmite and zirconia. Such intermixing prevented particle growth, which is a factor reducing sintering temperature. Further, due to reduced grain growth, stability of the θ-Al₂O₃ phase was extended up to 1200 °C, which enhances the sintering process as well. For the Al₂O₃⁻20 wt % ZrO₂ composition, we observed stability of the zirconia tetragonal phase up to 1400 °C. We associate this stability with the mutual separation of zirconia nanoparticles in the alumina matrix.