Ceria-doped Zirconia Research Articles

Application of internal gelation to sol–gel synthesis of ceria-doped zirconia microspheres as nuclear fuel analogous materials

Optimizing of internal gelation based on the sol–gel synthesis was carried out for manufacturing of plutonium-doped zirconia microspheres used as nuclear fuels. In the present study, cerium was used as a surrogate for plutonium. So far, the Er 0.05Y 0.1Ce x Zr 0.85− x O 1.925 ( x = 0.0–0.2) microspheres were successfully sintered to suppress the rapid reaction such as the thermal decomposition of by-products such as ammonium nitrate and formaldehyde at the calcination process. However, some cracks and pores were still observed in the sintered microspheres. As a result, these sintered densities reached 71–86% TD. From the crystallographic point of view, the sintered zirconia microspheres had the cubic fluorite structure and well formed the solid-solution oxide. The lattice constant of cubic zirconia increased with the amount of doped ceria.

Fabrication of Anode Supported Electrolyte with CeScSZ Electrolyte and NiO- CeScSZ Anode by EPD Technique

We investigated fabrication of anode-supported electrolyte by EPD technique and subsequent co-firing process. Ceria-doped scandia-stabilized zirconia (CeScSZ) thin electrolyte films, around 10 μm thick, were successfully fabricated on the NiO-CeScSZ anode substrates. Results of SEM observation and OCV measurements indicated that all anode supported CeScSZ films co-fired at various temperatures from 1473 K to 1548 K were fully dense and showed good chemical stability under a large oxygen potential gradient typical of SOFCs. The power generation test with platinum cathode indicated a poor performance due to the low activity of platinum electrode for the cathode reaction. On the other hand, with a lanthanum strontium manganite (LSM) cathode, the cell performance improved and the power density reached 480 mWcm -2 at a current density of 0.8 Acm -2 at 1073 K.

Studies of zirconia-doped ceria nanomaterials for CO and C3H8 oxidation

Results of structural refinements and catalytic evaluation of a series of palladium supported ceria-zirconia synthesized through a surfactant-assisted technique were reported. It was found that zirconium addition to ceria leads to a crystalline structure distortion and inhibits the crystal growth at high temperature. Rietveld refinements confirm that all the ceria-zirconia samples have nanocrystals with cationic defects in their structures. The Pd-supported catalysts were very active for both CO and propane complete oxidation. The best catalyst 3wt%Pd/Ce0.8Zr0.2O2 has a light-off temperature (T 50) as low as 120°C for CO oxidation and 180°C for C3H8 combustion.

Sintering Behaviour of Zirconia and Ceria-Doped Zirconia Powders Cristallized under Hydrothermal Conditions

Sintering Behaviour of Zirconia and Ceria-Doped Zirconia Powders Cristallized under Hydrothermal Conditions

Comparison of R‐Curves from Single‐Edge V‐Notched‐Beam (SEVNB) and Surface‐Crack‐in‐Flexure (SCF) Fracture‐Toughness Test Methods on Multilayered Alumina–Zirconia Composites

The single‐edge V‐notched‐beam fracture testing method was used to measure the short‐crack R‐curve behavior of a 90‐vol%‐alumina–10‐vol%‐ceria‐doped zirconia multilayered composite. The initiation and extension of short cracks from the V‐notch tip were observed in situ via optical microscopy. Initial crack lengths of 15–50 μm were obtained repeatedly. The resulting R‐curve was similar to the R‐curve that was measured using the surface‐crack‐in‐flexure testing method. The ability to control the position of the V‐notch tip within a material permits study of the influence of specific microscopic features on the fracture behavior.

Experimental and theoretical aspects of the stabilization of zirconia

Using the Rietveld method, phases of ceria-doped zirconia, calcined at temperatures of 600 and 900 °C, were quantitatively analysed for different concentrations of ceria. The results show that the stabilization of zirconia depends on the dopant concentration and calcination temperature. Moreover, the theoretical calculation using the ab initio Hartree–Fock–Roothaan method indicates that the most stable phases for ceria-stabilized zirconia are cubic or tetragonal, in accordance with experimental results.

The effect of dopants and organic washing on crystallization temperature of yttria- and ceria-doped zirconia gels

The effect of dopants and organic washing on crystallization temperature of yttria- and ceria-doped zirconia gels

Ce3+↔Ce4+ conversion in ceria-doped zirconia single crystals induced by oxido-reduction treatments

Optical absorption and resonant Raman techniques have been used to study the Ce3+↔Ce4+ conversion in cerium-doped yttria-stabilized zirconia single crystals (YSZ). The reduced material presents an intense orange coloration associated with a broad absorption band at 460 nm that we assign to a 4f→5d transition. Its experimental oscillator strength is ≈1×10−2. The Raman spectra consist of the lattice phonon bands at 600 cm−1 and below together with an intense electronic Raman peak at 2086 cm−1. The anomalous intensity of this peak is due to a resonance effect by the excitation in the 4f→5d transition. The peak is associated with the 2F72→2F52 electronic transition within the 4f1 configuration of Ce3+. Heating in ai r at 1000°C produces a complete discoloration of the sample. The effect of the oxidation can be fully reversed by reducing the sample at the same temperature. Optical spectroscopy has been used to determine the kinetics of the oxido-reduction process. Isothermal annealing in the temperature range 750–950°C were performed in both oxidizing and reducing atmospheres. The results indicate that the oxidation process is diffusion-limited with an activation energy of ≈1.7 eV. For the reduction two alternative limiting processes are proposed: electron diffusion and ionization reaction.

X‐ray Diffraction Study of the t‐to‐m Phase Transformation in 12‐mol%‐Ceria‐Doped Zirconia at Low Temperatures

The t‐to‐m phase transformation in a 12‐mol%‐ceria‐doped zirconia at subzero temperatures was studied in situ by XRD with a cryogenic sample stage. It was found that the t‐to‐m transformation continuously proceeded to 30 vol% as the temperature decreased to 125 K, when a burstlike transformation suddenly occurred with a transformation to 75 vol%. A preferred orientation of the (111)m plane parallel to the surface during the preburst transformation and XRD line broadening of both m and t phases after the burst were observed. The preburst transformation, which has not been revealed by previous dilatometry studies, is explained as surface martensitic transformation. This surface transformation occurs at higher temperatures than the burstlike transformation due to less matrix constraint and the higher probability of nucleation.

Electrical conductivity and point defect behavior in ceria-stabilized zirconia

Ceria-stabilized zirconia has tremendous potential as a thermal barrier coating in gas turbines and as a fuel cell material. Unlike most stabilized-zirconia systems which are ionic conductors, ceria-doped zirconia has exhibited large electronic contributions at moderate temperatures and oxygen activities. The focus of this research has been to characterize this mixed conduction behavior. Electrical conductivity measurements have been performed as a function of composition, temperature and oxygen partial pressures. At high oxygen activities, oxygen vacancies are the dominant charge carriers while at lower oxygen partial pressures an electron hopping mechanism dominates. Reduced cerium ions and Ce 3+-oxygen vacancy associates generate these conducting electrons. Kröger-Vink diagrams have been constructed which detail these mechanisms and describe the point defect behavior for ceria-stabilized zirconia.

Ceria-doped zirconia-graphite as possible refractory for tundish nozzles in steelmaking

In zirconia-graphite refractories, calcia-stabilized zirconia is sensitive to silica coming from oxidized additives (Si, SiC). Accordingly, ceria is considered here as an alternative cheap stabilizer. Ceria-doped zirconia has been prepared and the monoclinic/tetragonal distribution has been analysed. At 1500°C in oxygen, the solution of ceria in zirconia is sluggish; to obtain a pure tetragonal phase, a 16 mol% ceria concentration (equal to the solubility limit) is necessary. Such samples are stable below 1050°C, in spite of previous claims to the contrary. At 1300°C, silica does not destabilize these samples, even in reducing conditions. However, at low oxygen pressure, the pyrochlore Ce2Zr2O7 is formed, and the remaining zirconia transforms into the monoclinic modification. Nevertheless, below 1175°C, a metastable, purely tetragonal modification is obtained, because the activation energy needed for the formation of Ce2Zr2O7 is too high. In any case, the reduction does not destroy the polycrystalline texture. This fact, combined with the resistance to silica, favours the use of these materials in steelmaking.

Coloration in ceria-doped zirconia induced by reduction heat treatments

Coloration is an important effect in zirconia ceramics. We have observed a series of colors in a 12mol% ceria-doped zirconia after reduction heat treatments in H 2+ Ar, vacuum and charcoal. The coloration has been particularly attributed to the reduction of ceria, which acts as the color dye. The reduction of zirconia proceeds at higher temperature and contributes only to the blackening. XPS and TG analyses have provided evidences for the reductions of ceria and zirconia in the reduction heat treatments.

Phase Separation in 12 mol% Ceria‐Doped Zirconia Induced by Heat Treatment in H2 and Ar

The phase separation in 12 mol% CeO2─ZrO2 ceramic heattreated in a mixture of H2 and Ar was investigated by X‐ray diffractometry (XRD), X‐ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and Raman scattering. After heat treatment at temperatures above 1200°C, the tetragonal solid‐solution phase separated into Zr2Ce2O7 and the monoclinic phase. Raman scattering measurements also provided supplementary evidence for the phase separation. XPS showed that the valence change from Ce4+ to Ce3+ predominantly occurred, whereas the reduction from Zr4+ to Zr3+ took place above 1200°C. It is concluded, that in the highly reduced sample, where the valence changes from Ce4+ (Zr4+) to Ce3+ (Zr3+), the phase separation is noticeably promoted. Below 1000°C the phase separation was suppressed because of no appreciable valence change to trigger the phase separation, and the single tetragonal phase was retained.

Mechanical properties of yttria- and ceria-doped zirconia ceramics

The authors report the mechanical properties of ZrO 2 doped with 3 mol% Y 2 O 3 and 5, 10, 15 and 20 mol% CeO 2 , respectively, and the effects of annealing in moist air at 150°C.