Increasing CeO2 Content Research Articles

Structure and impedance of ZrO2 doped with Sc2O3 and CeO2

Scandia and ceria doped zirconia samples, with 10 mol% Sc2O3 and different content of CeO2, were synthesized and characterized. The XRD results depict that the sintered samples have a cubic phase structure. However, Raman spectra show that besides the main cubic phase, a secondary phase is also present in the sintered samples. The addition of CeO2 can raise the content of the cubic phase, but the minor meta-stable tetragonal phase (t′-phase) exists even at the CeO2 content as high as 10 mol%. The near-UV Raman spectra indicate that the deformed tetragonal structure predominates at the grain boundary. The addition of CeO2 can reduce the impurity at grain boundary, and no impurity can be found by near-UV Raman spectroscopy at the grain boundary of the samples with high CeO2 content. The impedance measurements show that with the increase of CeO2 content, the impedance of grain boundary decreases and the bulk impedance increases. The low impedance of grain boundary can be attributed to the formation of a clean grain boundary upon CeO2 doping, and the increase of the bulk impedance is due to the blocking effect of the large Ce(IV) ions.

Metastable forms of the tetragonal phase in compositionally homogeneous, nanocrystalline zirconia–ceria powders synthesised by gel-combustion

The crystal structure of compositionally homogeneous, nanocrystalline ZrO2–10, 30, 50, 70 and 90 mol% CeO2 powders synthesised by a pH-controlled nitrate–glycine gel-combustion process has been studied by X-ray diffraction and Raman spectroscopy. All the powders with a CeO2 content up to 70 mol% exhibited the tetragonal phase (P42/nmc space group), whereas the ZrO2–90 mol% CeO2 solid solution presented the cubic phase (Fmm space group). The axial ratio c/a decreased with increasing CeO2 content and became unity for ZrO2–70 mol% CeO2 powders. By analysing X-ray diffraction data using the Rietveld method, it was found that this material exhibited the t″-form of the tetragonal phase (the oxygen atoms were displaced from their ideal sites of the cubic phase along the c axis). Raman spectroscopy study confirmed the results found by X-ray diffraction. The morphology of the nanopowders was also evaluated.

Fracture toughness and hardness of ceria- and yttria-doped tetragonal zirconia ceramics

The fracture toughness and hardness of Ce–Y–TZP ceramics with various compositions are studied by using Vickers indentation. It is found that crack mode of Ce–Y–TZP belongs to Palmqvist crack under loads ≤490N. The hardness is enhanced with increasing mol% YO1.5, while this trend is depressed with the increase of CeO2 content. Samples with compositions located in the vicinity of t→m boundary tend to possess higher toughness. The closer the compositions near t-phase boundary of t–m, the higher the fracture toughness of Ce–Y–TZPs due to the increase of transformability or operation of relevant toughening mechanisms. As temperature and tetragonality of t-phase are regarded as the ease of degree in the t→m transformation, and the relation between As temperature and tetragonality with K1c value are further explored.

Redox Property–Local Structure Relationships in the Rh-Loaded CeO2–ZrO2Mixed Oxides

The local structure of the M–O bond in CeO2–ZrO2mixed oxides is investigated with the aim of finding a correlation between structural parameters and oxygen exchange properties. It is found that insertion of ZrO2into the CeO2lattice strongly perturbs the symmetry of the M–O bond. As the content of CeO2in CeO2–ZrO2solid solution is increased from 20 to 80 mol%, tetragonal, (t,t′,t″) and cubic phases are formed. The local symmetry of the Zr–O bond is strongly perturbed by the increase in CeO2content while no significant modification of the first Ce–O coordination sphere is found, except for lengthening of the Ce–O bond consistent with lattice parameter increase. The perturbation of the Zr–O coordination sphere, which leads to highly disordered oxygen in the lattice, is indicated as responsible for the high oxygen mobility in CeO2–ZrO2mixed oxides.

Physico-chemical properties of (U,Ce)O2

The high-temperature X-ray diffraction analysis of (U,Ce)O2 with CeO2 contents ranging from 0 to 20 mol.% CeO2 was performed to obtain the variation of the linear thermal expansion coefficient with the CeO2 content. Ultrasonic pulse-echo measurements were also carried out from room temperature to 673 K to estimate the change in the mechanical properties of (U,Ce)O2 with the CeO2 content. The variation in the linear thermal expansion coefficient at the low CeO2 content region is more steep than that expected from the linear thermal expansion coefficient of UO2 and CeO2. The Young's and shear moduli of all (U,Ce)O2 were found to decrease with rising temperature. This was due to the increase of the bond length accompanied by the thermal expansion. Although the lattice parameter decreased with CeO2 content, the moduli of (U,Ce)O2 were found to decrease with increasing CeO2 content at room temperature. These results show that in the range from 0 to 20 mol.% of CeO2, as CeO2 content increases, the bottom of the potential energy in (U,Ce)O2 is shallower and broader.

Tetragonal—Monoclinic Phase Transition Enthalpy and Temperature of ZrO2‐CeO2 Solid Solutions

DSC (differential scanning calorimeter) measurements were performed to investigate the tetragonal‐monoclinic (t‐m) transition enthalpy of the compositionally homogeneous ZrO2‐X mol% CeO2 solid solution of X= 0, 4, 8, and 12. The transition enthalpy decreases linearly with an increase of CeO2 content. The m → t transition enthalpy on heating agreed well with the inverse t → m one obtained during cooling. With increasing X, the DSC peak broadens and the transition temperature distribution of each sample increases, while the thermal hysteresis is almost independent of X.

Characterization of Glasses and Ceramics by Continuous Indentation Tests

Continuous indentation tests were performed on soda lime silica glasses and tetragonal zirconia polycrystal ceramic samples containing 10-14 mol% CeO2 (Ce-TZP ceramics) to determine their Vickers hardness (HV) and fracture toughness (K-IC). The load-indentation depth curves were taken during the loading as well as the unloading period by a computer controlled MTS machine. It was found that the loading force is a quadratic function of the indentation depth during both the loading and unloading stage of the deformation. Using the HV values and the measured length of the cracks the K-IC values were calculated. The fracture toughness of Ce-TZP ceramic samples decreases with increasing CeO2 content as a consequence of the reduced transformability of the tetragonal phase.

Microstructure, microhardness, and superconductivity of CeO2-added Y–Ba–Cu–O superconductors

The CeO2-added Y–Ba–Cu–O oxides were prepared by two different processes, the conventional solid-state reaction process and the partial melt process using powders, to examine the effect of the dopant on microstructure, microhardness, and superconductivity. In the solid-state reacted sample, most of the added CeO2 was converted to a form of BaCeO3, but some might enter into the 1-2-3 phase, resulting in the orthorhombic-to-tetragonal phase transition that accompanied the disappearance of twin structure in 1-2-3 grains. In the partially melted sample, however, the phase change was not observed up to 5 wt. % of CeO2. All the added CeO2 in these samples was consumed to form only BaCeO3 which was finely dispersed in large 1-2-3 grains during the peritectic reaction stage. The zero resistance temperature (Tc) of the solid-state reacted sample gradually decreased with increasing CeO2 content due to the phase change and the formation of BaCeO3, whereas the Tc of the partially melted sample was nearly constant regardless of CeO2 content up to 5 wt. %, owing to the separation of the second phase from the 1-2-3 superconducting phase. Microhardness of the partially melted sample increased with increasing CeO2 content. The strengthening effect appears to come from the composite system where the fine BaCeO3 particles are dispersed in a 1-2-3 matrix.