Thermodynamic prediction of the nonstoichiometric phase Zr1–zCezO2–x in the ZrO2–CeO1.5–CeO2 system

A thermodynamic estimation of the ZrO₂–CeO₂ and ZrO₂–CeO_1.5 systems, as well as the cubic phase in the CeO_1.5–CeO₂ system has been developed and the complex relation between the nonstoichiometry, y, in Ce_zO_2–y and the oxygen partial pressure at different temperatures is evaluated. The behavior of the nonstoichiometry phase Zr_1–zCe_zO_2–x is described based on the thermodynamic estimation in the ZrO₂–CeO₂, CeO_1.5–CeO₂ and ZrO₂–CeO_1.5 systems. Additionally, the interdependence among miscellaneous factors, which can be used to describe the change in oxidation states of cerium such as the oxygen partial pressure, the CeO_1.5 fraction in CeO_1.5–CeO₂ in the quasi-ternary system, the nonstoichiometry y and the difference between the activity of CeO₂ and CeO_1.5 are predicted. The calculated results are found to be very useful to explain the influence of pressureless sintering at different O₂ partial pressures on the mechanical properties of CeO₂-stabilised ZrO₂ ceramics     

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 0–25 mol%. X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy (Raman), specific surface area (S_BET) 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 4–9 nm. 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 5 mol%.     

Thermal conductivity of single- and multi-phase compositions in the ZrO2–Y2O3–Ta2O5 system

Compositions in the ZrO₂–Y₂O₃–Ta₂O₅ system are of interest as low thermal conductivity, fracture resistant oxides for the next generation thermal barrier coatings (TBC). Multiple phases occur in the system offering the opportunity to compare the thermal properties of single, two-phase, and three-phase materials. Despite rather large variations in compositions almost all the solid solution compounds had rather similar thermal conductivities and, furthermore, exhibited only relatively small variations with temperature up to 1000 °C. These characteristics are attributed to the extensive mass disorder in all the compounds and, in turn, small interfacial Kapitza (thermal) resistances. More complicated behavior, associated with the transformation from the tetragonal to monoclinic phase, occurs on long-term annealing in air of some of the compositions. However, the phases in two of the compositional regions do not change with annealing in air and their thermal conductivities remain unchanged suggesting they may be suitable for further exploration as thermally stable TBC overcoats.     

Structural studies of a ZrO2–CeO2 doped system

The local structure around Zr, Ce and dopant atoms (Fe and Ni) in the ZrO₂–CeO₂ system investigated by X-ray absorption spectroscopy (XAS) is reported to better understand the tetragonal phase stabilization process of zirconia. The first coordination shell around Zr atoms is not sensitive to the introduction of dopants or to an increase in the ceria content (from 12 to 20 mol%). Ce ions maintain the eight-fold coordination as in CeO₂, but with an altered bond distance. The formation of vacancies resulting from reduction of Ce atoms can be discarded, because XANES spectra clearly show that Ce ions are preferentially in a tetravalent state. XANES and EXAFS experiments at the Fe K-edge evidence that the local order around Fe is quite different from that of the Fe₂O₃ oxide. On the one hand, ab initio EXAFS calculations show that iron atoms form a solid solution with tetragonal ZrO₂. The EXAFS simulation of the first coordination shell around iron evidences that the substitution of zirconium atoms by iron ones generates oxygen vacancies into the tetragonal network. This is a driven force for the tetragonal phase stabilization process. For Ni doped samples, EXAFS results show that Ni–O mean bond length is similar to the distance found in the oxide material, i.e., NiO compound. Besides this result, no evidence of similar solid solution formation for Ni-doped systems has emerged from the EXAFS analysis.     

Liquid phase formation in the system Al2O3–Y2O3–AlN: Part II. Thermodynamic assessment

The mixing parameters of liquid phase in the Al₂O₃–Y₂O₃–AlN system were assessed based on differential thermal analysis (DTA) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX) investigations of selected compositions. Phase diagram of the Y₂O₃–AlN system was calculated. Liquidus surface of the Al₂O₃–Y₂O₃–AlN system was constructed and compared with experimental results on primary crystallisation fields. Calculated temperatures of invariant reactions were in agreement with DTA results. Vertical sections of the Al₂O₃–Y₂O₃–AlN system were calculated and compared with experimental data     

Phase relations in ZrO2–La2 Zr2O7–Y2 Zr2O7system

Abstract

The phase relations in the system ZrO₂–La₂Zr₂O₇– Y₂Zr₂O₇ have been investigated using X-ray diffraction. Mixed oxide phase assemblages were prepared by hydrolysing zirconium butoxide with solutions of Y and La nitrates, followed by drying, calcining, and sintering. The cubic zirconia phase can accept into solid solution the larger, non-cubic stabilising lanthanum ion in the presence of a suitable proportion of the cubic stabilising oxide of yttrium. As the amount of the larger rare earth element ion is increased formation of pyrochlore and tetragonal type compounds is favoured.     

Effect of solidification path on the microstructure of Al2O3–Y2O3–ZrO2 ternary oxide eutectic ceramic system

The solidification path of the Al₂O₃–Y₂O₃–ZrO₂ ternary oxide eutectic composite ceramic is determined by a high temperature DTA and laser floating zone (LFZ) directional solidification method to investigate the effect of solidification path on the microstructure of the ternary oxide. The DTA and microstructure analyses show that the YAG or Al₂O₃ tends to form as primary phase under the unconstrained solidification conditions, and then the system enters ternary eutectic solidification during cooling from 1950 °C at rate of 20 °C/min. The as-solidified composite ceramic shows a divorced irregular eutectic structure consisting of Al₂O₃, YAG and ZrO₂ phases with a random distribution. The primary phases are however completely restrained at the directional solidification conditions with high temperature gradient, and the ternary composite by LFZ presents well coupled eutectic growth with ultra-fine microstructure and directional array. Furthermore, the eutectic transformation and growth mechanism of the composite ceramic under different solidification conditions are discussed.     

Thermodynamic calculation and microstructure characterization of spinel formation in MgO–Al2O3–C refractories

In this paper, by simulating the gas phase conditions inside the MgO–Al₂O₃–C refractories during continuous casting process and combining with thermodynamic analysis, as well as SEM analysis, the gas-gas and gas-solid formation of MA spinel were clarified in carbon containing refractories. Thermodynamic calculations showed that gas partial pressure of CO, O₂ and Mg could meet the formation and stable existence conditions of MA spinel in MgO–Al₂O₃–C refractories under service environment, and nitrogen could not affect the formation of MA spinel at 1550 °C in the thermodynamic condition. The formation processes of MA spinel were analyzed experimentally under embedding carbon atmosphere. The carbon-coated alumina powders in MgO–Al₂O₃–C refractories prevented the direct contact between magnesia and alumina. Mg gas was formed by carbon thermal reaction, then reacted with alumina (gas-solid) and gas containing aluminum (gas-gas) to generate MA spinel. Through gas-gas or gas-solid reaction, the formation of MA spinel was effectively controlled. By means of SEM analysis, a two-layer structure with dense outer spinel layer and loose inner layer was formed in MgO–Al₂O₃–C refractories.     

Thermochemical stability of the Y2O3–SiO2 system

A thermodynamic study was carried out in order to quantify the stability of yttrium silicates under different atmospheres (argon, air, moisture), and to predict their behaviour as an EBC. For this purpose, the partial pressures of gaseous species formed during the reactive vaporization in equilibrium with yttrium silicates were evaluated and compared to those over each oxide taken separately. The partial pressures of silicon and rare earth hydroxides at the equilibrium over rare earth silicates are lower than those over each single oxide. Thus yttrium silicates appear to be sufficiently stable to be promising materials to be used as environmental barrier coatings.     

Insights into the ternary eutectic microstructure formed in intercolony regions in Al2O3–ZrO2(Y2O3) system

Al₂O₃/ZrO₂(Y₂O₃) pseudo-binary eutectic melt may solidify in the dendritic form at the low G/v ratio (thermal gradient G divided by growth rate v) associated with the Y₂O₃ microsegregation along the monovariant line. Especially, in the system with high Y₂O₃ content (≥4.5 mol%), an Al₂O₃/ZrO₂/YAG ternary eutectic in situ microstructure growing with a “plane front” will appear at the boundaries of dendritic structures. The volume fraction of the ternary eutectic increases with the increase in Y₂O₃ content. It was found that the amount of ternary eutectic can be well predicted by the Scheil equation (or nonequilibrium lever rule).     

The role of residual stresses in layered composites of Y–ZrO2 and Al2O3

Laminar composites, containing layers of Y–TZP and either Al₂O₃ or a mixture of Al₂O₃ and Y–ZrO₂ have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection in alumina layers was observed. The increase of crack deflection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack deflection did not occur independently on layer thickness. The observed changes have been correlated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stresses found were the result of the difference in the thermal expansion and sintering shrinkage of alumina and zirconia and the crystallographically anisotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical fluorescence of Cr⁺³ dopants in alumina.     

Autothermal Reforming of Methane Over CeO2–ZrO2–La2O3 Supported Rh Catalyst

Autothermal reforming of methane was studied over La-doped ceria–zirconia-supported Rh catalysts. The CH₄ conversion increased from 49 to 60% on increasing the content of La³⁺ from 5 to 15%, while further increase in the La³⁺ content led to a slight decrease on both CH₄ conversion and H₂ selectivity. H₂-TPR and UV–vis DRS spectrum showed that the interaction between Rh and the support was enhanced by increasing the content of La. We speculated that a so-called “Rh–La interfacial species” was formed on the surface of the support, which played an important role in catalytic activity. The balance between exposed Rh and the “Rh–La interfacial species” was necessary to improve the catalytic activity. Upon increasing the Rh loading on 15% La-doped ceria–zirconia support, the balance was built, i.e., CH₄ conversion increased from ~60 to 69% by increasing Rh loading from 0.1 to 0.5 wt% and only 2% conversion was elevated by doubling the Rh loading from 0.5 to 1.0 wt%.     

Phase relation studies in the CeO2–La2O3–Er2O3 system at 1500°C

Materials based on CeO₂–La₂O₃–Er₂O₃ system are promising candidates for a wide of applications, but the phase relationship has not been studied systematically previously. To address this challenge, the isothermal section of the phase diagram for 1500 °C was investigated. The phase relations in the CeO₂–La₂O₃–Er₂O₃ ternary system at 1500 °C were studied by X-ray diffraction and scanning electron microscopy in the overall concentration range. To study phase relationships at 1500 °C the as-repared samples were thermally treated in two stages: at 1100 °C (for 300 in air) and then at 1500 °C (for 70 h in air) in the furnaces with heating elements based on Fecral (H23U5T) and Superkanthal (MoSi2), respectively. The solid solutions based on various polymorphous forms of constituent phases and with perovskite-type structure of LaErO₃ (R) with orthorhombic distortions were revealed in the system. No new phases were found. The isothermal section of the phase diagram for the CeO₂–La₂O₃–Er₂O₃ system has been constructed. It was established that in the ternary CeO₂–La₂O₃–Er₂O₃ system there exist fields of solid solutions based on hexagonal (A) modification of La₂O₃, cubic modification of CeO₂ with fluorite-type structure (F), cubic modification Er₂O₃ and with perovskite-type structure of LaErO₃ (R) with orthorhombic distortions. The maximal solubility of ceria in LaErO₃ was found to be around ∼ 2 mol% CeO₂ along the section CeO₂–(50 mol % La₂O₃ –50 mol% Er₂O₃).     

Solid solutions in ternary phase diagram CeO2–ZrO2–Er2O3 using pyrolitic precursors

Abstract

ZrO₂ based ceramics have important technological properties with interesting industrial applications. In the present paper erbia is investigated as a potential replacement for yttria in zirconia based ceramics. Several compositions in the ternary CeO₂–ZrO₂–Er₂O₃ system and to the binary system CeO₂–Er₂O₃, not yet reported, were prepared. Owing to the low reactivity of cerium and erbium oxides, all the powders were prepared from highly reactive powder precursors which could attain equilibrium in short processing times. The method was based on the pyrolysis of soluble salts, according to a technique already previously applied to obtain shorter firing times and lower firing temperatures. The presence of two solid solutions in CeO₂–Er₂O₃ and in CeO₂–ZrO₂–Er₂O₃ systems was stated and its limits were reported.     

Effect of adding Y2O3 on structural and mechanical properties of Al2O3–ZrO2 ceramics

The effects of adding 1–8 wt% Y₂O₃ on phase formation and fracture toughness of Al₂O₃–xZrO₂–Y₂O₃(AZY) ceramics were studied. Phase formations of the samples were characterized by the X-ray diffraction (XRD) technique. It was found that the major phase was rhombohedral-Al₂O₃, while the minor phase consisted of the monoclinic-ZrO₂, tetragonal-ZrO₂ and monoclinic-Y₂O₃. It was found that Y₂O₃ contents did not clearly influence grain shape of AZY ceramics. The results obtained from the microhardness test could be used to evaluate the fracture toughness. It was found that the smaller grains had high fracture toughness. The maximum fracture toughness of 4.827 MPa m^1/2 was obtained from 4 wt% Y₂O₃. Refinement of lattice parameters using Rietveld analysis revealed the quantitative phases of AZY ceramics. This shows that under adding Y₂O₃ conditions the proportion of tetragonal-ZrO₂ phase plays an important role for the mechanical properties of AZY ceramics.     

Fabrication and microstructure of Al2O3–ZrO2(3Y)–SiC nanocomposites

Al₂O₃–ZrO₂(3Y)–SiC composite powder was prepared by the heterogeneous precipitation method. Calcinating temperature of the powder was important to obtain dense sintered body. The nanocomposites were got by hot-pressing, and addition of ZrO₂ did not raise the sintering temperature. Some Al₂O₃ grain shape was elongated, and Al₂O₃ grain size was about μm. Nano SiC particles were observed uniformly distributing throughout the composites, and most of them were located within the matrix grains. Because SiC particles located within ZrO₂ grains influenced the phase transformation of ZrO₂, the sintering of nanocomposites, which controlled grain size and transformable ZrO₂ amount, become important to get high performance. The strength of 80 wt% Al₂O₃–15 wt% ZrO₂–5 wt% SiC nanocomposites was 555 MPa, and toughness was 3·8 MPa m^1/2, which were higher than those of monolithic Al₂O₃ ceramics. ©     

Phase diagram study and thermodynamic modeling of the Na2O–Al2O3–ZrO2 system

The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was experimentally studied at 1500°C–1650°C by a classical equilibration/quenching method and differential thermal analysis followed by X-ray diffraction phase analysis and electron probe micro-analysis. A sealed Pt crucible was utilized to prevent the volatile loss of Na₂O during high-temperature phase equilibrium experiments and the hydration upon quenching. The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was revealed for the first time. Based on the present experimental data and available binary modeling results in literature, the thermodynamic modeling of the ternary system was performed using the Calculation of Phase Diagram method and the phase diagram of the entire the Na₂O–Al₂O₃–ZrO₂ system was constructed and the optimized thermodynamic properties for all solids and liquid phase within the ternary system were obtained.