CeO2 Y2O3 Research Articles

Microstructural Design of Bioinert Composites in the ZrO2–Y2O3–CeO2–Al2O3–CoO System

It is shown that a scientifically sound approach to each stage of producing ZrO2-based bioinert implants (from the synthesis of starting powders to their sintering) is a necessary condition for promoting the optimum structure and high mechanical properties. Conditions for producing bioinert implants with regular, laminar, and highly porous microstructures are found. The research results serve as a scientific basis for microstructural design of various bioinert implants in the ZrO2–Y2O3–CeO2–Al2O3-CoO system.

Influence of CeO2 addition on crystal growth behavior of CeO2–Y2O3–ZrO2 solid solution

Nanopowders with cubic fluorite-type structure as well as uniform distribution in particle size were synthesized by hydrothermal method in the ternary oxide zirconia–yttria–ceria system with ceria content of 0–25mol%. X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy (Raman), specific surface area (SBET) and high resolution transmission electron microscopy (HRTEM) were applied to characterize the structure, thermal decomposition, morphological characteristic and crystal growth of the produced powders. Qualitative analyses indicate that the as-synthesized nanoparticles are single-phase crystallites with an average particle size of 4–9nm. The specific surface area, lattice parameter and microstrain are closely related to Ce4+ concentration. Moreover, activation energy of crystal growth is significantly dependent on the dopant (CeO2) concentration. It firstly increased and then decreased with increasing dopant concentration, and the maximum value was observed at the dopant concentration of 5mol%.

Ni cermet solid oxide fuel cell anodes prepared from nanoparticle Y2O3―CeO2―ZrO2 solid solutions

Ni cermet anodes were prepared from nanoparticle solid solutions of Y2O3―CeO2―ZrO2 with various CeO2 contents, (Y2O3)0.08(CeO2)x(ZrO2)0.92-x (x=0–0.08). The performance of cells with anodes prepared from solid solutions was better than that of the cells with anodes prepared by physical mixing. The performance of cells with anodes prepared from solid solutions increased with an increase in CeO2 content. Results of impedance analysis of the anode suggested that conduction of O2− is enhanced by CeO2 particles. The rate of coke deposition decreased with an increase in CeO2 content of the solid solution. The rate of C removal increased with an increase in conduction of O2−, suggesting that C reacted with O2− ions of CeO2 particles for steam reforming of methane.

Variation in properties of ZrO2–Y2O3–CeO2–Al2O3 powders during thermal treatment at 400 to 1300°C

The paper examines the processes that occur during thermal treatment (sintering) of heterophase nanocrystalline ZrO 2 -Y 2 O 3 -CeO 2 -Al 2 O 3 powders. It is shown that the disperse system is selforganized in various evolutionary ways. The common behaviors of ZrO 2 -Y 2 O 3 -CeO 2 -Al 2 O 3 nanocrystalline powders are determined. The results offer possibilities for selecting initial nanocrystalline powders to develop fine-grained high-performance ceramics.

Sintering of self-reinforced ceramics in the ZrO2–Y2O3–CeO2–Al2O3 system

The formation of self-reinforced composites in the ZrO2–Y2O3–CeO2–Al2O3 system is investigated. Depending on different sintering conditions, plates of CeAlO3, T-ZrO2, and α-Al2O3 are formed in the matrix based on a solid solution of ZrO2. The self-reinforced composite with fracture toughness 17–20 MPa ⋅ m0.5 is obtained in optimum sintering conditions. In this composite, α-Al2O3 plates as well as individual Ce2O3 ⋅ 11Al2O3 plates are arranged in the matrix based on T-ZrO2. Selfreinforced composites in the ZrO2–Y2O3–CeO2–Al2O3 system have a considerable potential for various engineering ceramic materials with tailored combinations of properties, e.g., for medical applications (surgical tools and bioinert implants).

The influence of high-temperature sintering on microstructure and mechanical properties of free-standing APS CeO2–Y2O3–ZrO2 coatings

In this study, ceria–yttria co-stabilized zirconia (CYSZ) free-standing coatings, deposited by air plasma spraying (APS), were isothermally annealed at 1315 °C in order to explore the effect of sintering on the microstructure and the mechanical properties (i.e., hardness and Young’s modulus). To this aim, coating microstructure, before and after heat treatment, was analyzed using scanning electron microscopy, and image analysis was carried out in order to estimate porosity fraction. Moreover, Vickers microindentation and depth-sensing nanoindentation tests were performed in order to study the evolution of hardness and Young’s modulus as a function of annealing time. The results showed that thermal aging of CYSZ coatings leads to noticeable microstructural modifications. Indeed, the healing of finer pores, interlamellar, and intralamellar microcracks was observed. In particular, the porosity fraction decreased from ~10 to ~5% after 50 h at 1315 °C. However, the X-ray diffraction analyses revealed that high phase stability was achieved, as no phase decomposition occurred after thermal aging. In turn, both the hardness and Young’s modulus increased, in particular, the increase in stiffness (with respect to “as produced” samples) was equal to ~25%, whereas the hardness increased to up to ~60%.

Physical characterization of Y2O3–CeO2–TiO2 (YCT) mixed oxides and Ni/YCT cermets as anodes in solid oxide fuel cells

Mixed oxides in the binary Y2O3–CeO2 (YC) and ternary Y2O3–CeO2–TiO2 (YCT) systems as well as the corresponding Ni cermets were evaluated in terms of application as anodes in solid oxide fuel cells (SOFCs) between 650 and 900 °C. X-ray diffraction (XRD) analysis of the YCT powders calcined up to 1,400 °C showed the cubic fluorite structure of YC and also the formation of an additional phase with pyrochlore structure. The thermal expansion of the ceramics measured in air and Ar/4% H2 showed no significant differences in the temperature range of 25–800 °C. The absolute values of the total electrical conductivity of the ceramics measured between 450 and 900 °C in Ar/4% H2 increased by about 1–2 orders of magnitude compared to those measured in air. Ni/Y0.20Ce0.80O1.9 and Ni/Y0.20C0.75Ti0.05O1.9 cermets with 40 vol% Ni exhibited improved long-term stability regarding their electrical conductivity after annealing at 1,000 °C. The diffusion coefficient of Ce in the 8YSZ electrolyte was measured by compatibility tests. Electrochemical measurements on single SOFCs showed high polarization resistance at the anode/electrolyte interface.

Catalysts for the thermo-catalytic cracking (TCC) process: Interactions between the yttria in yttria-doped alumina aerogel and the mono-oxide MoO3, CeO2, and bi-oxide MoO3–CeO2 species

The influence of yttria used as a doping agent for catalyst support, on the dispersion and the resistance to sintering of MoO3, CeO2 and bi-oxide MoO3–CeO2 species in the TCC catalysts has been investigated. Characterization techniques used included N2 adsorption, X-ray diffraction, Raman, thermogravimetric analysis, and iso-electric point (IEP). It was explicitly found that yttria-doped alumina aerogel is by far more capable of homogenously dispersing the active molybdenum species and significantly retarding their sintering at quite high-calcination temperature than conventional alumina and alumina aerogel. Characterization results of supported mono-oxide MoO3 catalysts have clearly indicated that the fraction of (surface) tetrahedral monomolybdate species increases in the detriment of crystalline MoO3 as we varied the support from conventional alumina to yttria-doped alumina aerogel with increasing amounts of the yttria loading. This can be attributed to the incorporation of yttria into alumina aerogel network, which led to a change in the support surface charge (IEP) and subsequently the stability of surface molybdate species. The dispersion degree of ceria (substituted ceria) on supported mono-oxide CeO2 was improved as well, upon using yttria-doped alumina aerogel. Such improvement was attributed to the formation of stronger Ce–Y–O bond, and/or the formation of CeO2–Y2O3 solid solution. In the bi-oxide MoO3–CeO2, characterization results have indicated the presence of surface interaction between Mo and Ce, probably through the formation of surface “Mo–O–Ce” type phase between the dispersed ceria and the molybdate monolayer. This interaction, which was highly favourable over yttria-doped alumina surface, contributed significantly to the overall surface stability of Mo–Ce catalysts.

Toluene oxidation on Pd catalysts supported by CeO2–Y2O3 washcoated cordierite honeycomb

A novel CeO2–Y2O3 (CY) washcoat on cordierite honeycomb was prepared by an impregnation method, which was used as a support to prepare a Pd catalyst. A model reaction of the complete combustion of toluene was conducted to evaluate the performance of the developed Pd/CY catalyst. The CY washcoat support and the Pd/CY catalyst were characterized by XRD, Raman spectroscopy, H2-TPR and SEM techniques. The results show that compared with conventional washcoat the CY washcoat has better adhesion and higher vibration- and heat-resistance. The CY washcoat can anchor well Pd onto the cordierite honeycomb substrate. The formation of a CeO2–Y2O3 solid solution and the steady present of PdO occur at high calcination temperatures, resulting in a better thermal stability. On a Pd/CY catalyst calcined at 500°C, a 99% of toluene conversion was obtained at 210°C, and it was stable for reaction time up to 30h.

Comparative studies on the growth conditions of CeO2 and Y2O3 buffer layers on NiW tapes

We comparatively studied the growth conditions of Y2O3 films and CeO2 films on biaxially textured Ni and NiW tapes. We found that the windows of both substrate temperatures and oxygen partial pressures for the proper growth of CeO2 films were wider than those of Y2O3 films. XRD data showed that the qualities of in-plane and out-of-plane textures of the two types of films were almost similar. The SEM observations indicated that the surface of the CeO2 films was smoother than those of Y2O3 films. The inferiority of CeO2 films to Y2O3 films was crack formation. However, we found that if YSZ over-layers were grown on the CeO2 films using dc-reactive sputtering, the cracks were not observed on the surface of the YSZ films anymore.

Blocking Grain Boundaries in Yttria‐Doped and Undoped Ceria Ceramics of High Purity

CeO2 samples doped with 10, 1.0, and 0.1 mol% Y2O3 and undoped CeO2 samples of high purity were studied by impedance spectroscopy at temperatures <800°C and under various oxygen partial pressures. According to microstructural investigations by SEM and analytical STEM (equipped with EDXS), the grain boundaries were free of any second phase, providing direct grain‐to‐grain contacts. An amorphous siliceous phase was detected at only a few triple junctions, if at all; as a result, its contribution to the grain‐boundary resistance was negligible. Nevertheless, the specific grain‐boundary conductivities were still 2–7 orders of magnitude lower than the bulk conductivities, depending on dopant concentration, temperature, and oxygen partial pressure. The charge carrier transport across the grain boundaries occurred only through the grain‐to‐grain contacts, whose properties were then determined by the space‐charge layer. The space‐charge potential in acceptor‐doped CeO2 was positive, causing the simultaneous depletion of oxygen vacancies and accumulation of electrons in the space‐charge layer. The very low grain‐boundary conductivities can be accounted for by the oxygen‐vacancy depletion; the accumulation of electrons became evident in weakly doped and undoped CeO2 at high temperatures and under low oxygen partial pressures.

Effect of adhesive composition on bonding of silicon nitride ceramic

Si3N4–Si3N4 ceramics were bonded by using Y2O3–Al2O3–SiO2, Y2O3–La2O3–Al2O3–SiO2, Y2O3–CeO2–Al2O3–SiO2 solders. Joining was carried out at 1823 K for 15 min at atmospheric pressure. The effects of adhesive composition on interfacial wetting and joint strength were investigated. The microstructure and chemistry of joints were characterized by scanning electron microscopy, electron probe microanalyses and X-ray diffraction. The results show that some elements of the solder diffuse into silicon nitride to form a diffusion layer. As compared with the joint strength of silicon nitride bonded with Y2O3–Al2O3–SiO2 solder, the joint strength with Y2O3–La2O3–Al2O3–SiO2 solder increased, while the one with Y2O3–CeO2–Al2O3–SiO2 solder decreased. The results indicate that the effect of adhesive composition on joint strength is related to interfacial wetting and joint thickness.

Correlation of mechanical properties and composition in tetragonal CeO2–Y2O3–ZrO2 ceramic system

Vickers hardness, fracture toughness, bending strength, resistance to hydrothermal aging of Ce–Y–TZP ceramics with respect to ceria and yttria contents are revealed on the basis of results in the present study as well as earlier data in our group. Through corresponding contour maps, the relationships between compositions and mechanical properties can be explored. It appears that there exists an optimum composition region of 6.5–7mol% CeO2 and 2.25–3mol% YO1.5, exhibiting high strength of 750–900MPa, high toughness of 10–17MPam1/2, and sufficient resistance to low temperature degradation.

Thermodynamic calculation of Ms in ZrO2–CeO2–Y2O3 system

The difference of Gibbs free energy between tetragonal and monoclinic phases in ZrO2–CeO2–Y2O3 as a function of composition and temperature is thermodynamically calculated from the three related binary systems. In 8 mol% CeO2–0.5 mol% Y2O3–ZrO2, the equilibrium temperature between tetragonal and monoclinic phases, T0, is obtained as 832.5 K and the Ms temperature of this alloy with a mean grain size of 0.90 μm is calculated as 249.9 K using the approach derived by Hsu et al. [J. Mater. Sci., 18(1983)3206; 20(1985)23; Acta Metall., 37(1989)3091; Acta Metall. Mater., 39(1991)1045; Mater. Trans. JIM, 37(1996)1284], which is in good agreement with the experimental one of 253 K.

Chemical and electrical features of the interphase: electrode–electrolyte in the system (Y,Ca)MnO3–CeO2–Y2O3

The scope of this paper is to report the results obtained about the solid state phase relations in the interfacial region for a system consisting of Y2O3-modified CeO2 solid electrolyte and Y0.7Ca0.3MnO3-based ceramic electrode. The ceria–yttria solid electrolyte has been doped with CaO for improving the grain boundary conductivity, according to the results described elsewhere. The electrode was fixed on the electrolyte by powder pressing and the sandwich was cofired at several temperatures. The microstructure was observed with Scanning Electron Microscopy (SEM), and the reaction products were established by X-Ray Diffraction (XRD) of the cofired materials. Complex Impedance Spectroscopy was used for determining the total electrical conductivity and the electrical behaviour of the interphase electrode–electrolyte. The boundary region between the electrode and the electrolyte showed no reaction phases. The complex impedance measurements indicate that the (Y,Ca) manganites have the conductivity comparable to that of the Ag electrodes.

Red–ox properties and phase composition of CeO2–ZrO2 and Y2O3–CeO2–ZrO2 solid solutions

The crystalline structure and red–ox behaviour of CeO2–ZrO2 and Y2O3–CeO2–ZrO2 solid solutions were studied by means of XRD, TPR and EPR techniques. It has been found that Y2O3 doping stabilizes a cubic structure at a low content of both Y2O3 (10.0mol%) and CeO2 (7.0mol%) and favours the reduction of the sample. The formation of O−2 anion radicals on the surface of CeO2–ZrO2 and Y2O3–CeO2–ZrO2 in comparison with pure CeO2 was studied. A higher activity of O−2 in the reoxidation of the samples and towards CO was observed for yttria doped CeO2–ZrO2.

Improvement of Electrical Conductivity in Fluorite Related Y2O3 and Fluorite CeO2 Systems Based on A Unique Effective Index

Y2O3 has a crystal structure of c-type rare-earth oxide. Y2O3 does not show an oxide ionic conductivity. On the other hand, CeO2 based oxide is one of the most interesting of the fluorite oxides since the ionic conductivity of it is higher than that of yttria-stabilized zirconia. However, CeO2 based oxides are partially reduced and develop electronic conductivity under reduced atmosphere.

Thermal barrier coatings with two ZrO2 based ceramic layers

A new thermal barrier coating (TBC) structure, composed of two ZrO2 based ceramic layers with different chemical composition on an MCrAlY bondcoating is proposed. In the double ceramic coatings, the lower layer consists of high strength ZrO2–Y2O3 and the upper layer of ZrO2–CeO2–Y2O3 with high heat shielding capacity. The new bilayer structure TBC was prepared by plasma spraying, and its properties, including thermal conductivity and thermal shock resistance, were evaluated. The thermal conductivity of the coatings showed good agreement with values calculated by the parallel plates model. The coatings also exhibited improved thermal shock resistance. This result is supported by FEM analysis, which revealed that tensile stress accompanying thermal shock is reduced and, in fact, applied only to the strong lower layer.

ZrO2CeO2Y2O3: electrical and mechanical property relationships with the microstructure

ZrO2CeO2Y2O3 ternary ceramic alloys were studied to investigate their electrical and mechanical behaviour. The electrical conductivity, fracture strength and toughness of one composition from each tetragonal (t), cubic (c) and two-phase (t + c) region were measured. These properties were correlated with the microstructure. The highest conductivity was found in the c-phase samples, while the t-phase samples exhibited the best fracture properties. The two-phase specimens showed intermediate behaviour for both the electrical and mechanical properties.

Pressureless sintering of Y2O3-CeO2-doped sialons

Various sialon materials have been prepared by pressureless sintering at 1775 and 1825 °C using Y2O3 and/or Ce02 as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y2O3/CeO2: 100/0, 75/25, 50/50, 25/75, 0/100 corresponding to 6.0 and 9.25 wt% for the pure Y2O3 and pure CeO2, respectively. Only one of the compositional series reached full density at 1775 °C with cerium replacing yttrium, whereas at 1825 °C all compositional series except one became dense. The samples sintered showed that yttrium but not cerium stabilizes the α sialon phase in these ceramics. The dense cerium-sialon ceramics sintered at 1825 °C have as good hardness and indentation fracture toughness as the corresponding yttrium-sialon ceramics, or even higher for the β sialon type of materials. For the mixed α-β sialon materials the hardness decreased as the amount of a sialon phase decreased by increasing cerium-doping.