Stability Field of Cotunnite-Type Zirconia

The stability field of cotunnite-type ZrO₂ was determined by both syntheses and measurements of electrical conductivity under pressure using a 6–8 anvil-type device. The products synthesized below 1000°C were ortho II, whereas those above this temperature were monoclinic when quenched. Abrupt discontinuities in conductivity were observed around 1000°C at 16.5 GPa and around 1050°C at 18 GPa. It is concluded, therefore, that the phase boundary lies a little above about 1000°C at pressures from 15 to 20 GPa with slightly positive slope. The result is consistent with that obtained by synthesis experiment.     

Enhanced Phase Stability for Tetragonal Zirconia in Precipitation Synthesis

Tetragonal ZrO₂ nanocrystallites—with or without yttria (3 mol%) doping—have been synthesized via a precipitation process in which the hydrous oxide precipitate reacts with hexamethyldisilazane (HMDS) vapor before calcination. The nanocrystallites are formed and retain a tetragonal structure for hours after calcination at temperatures of 300°–1100°C. The enhanced structural metastability has been attributed to the combined effect of suppressed grain growth and reduced surface energy that results from the HMDS treatment.     

Crystal Structure and Equation of State of Cotunnite-Type Zirconia

Cotunnite-type zirconia was studied by angular-dispersive X-ray diffraction in a diamond anvil cell after laser heating at 18 and 26.7 GPa. The structure, space group Pnam, Z = 4, was refined in situ at several different pressures on decompression and at ambient using the Rietveld method. The nine polyhedral Zr-O distances range from 2.10 to 2.56 A at ambient pressure, which represents both an increase in the average and the minimum Zr-O distance relative to the monoclinic and orthorhombic lower-pressure forms. In addition, data obtained on an unheated sample indicate that the irreversible transition to the cotunnite phase began above 25 GPa and 70% conversion was achieved by 48.5 GPa. Tne compressibility of cotunnite-type zirconia was found to be slightly anisotropic and a Birch-Murnaghan equation-of-state fit of the p-V data yielded a bulk modulus of 332(8) GPa with a first derivative of 2.3(4), which is in good agreement with the value predicted by previous ab ini-tio calculations.     

Phase Stability and Mechanical Properties of Zirconia and Zirconia Composites

Monolithic zirconia materials (3Y‐TZP, 10Ce‐TZP, and 12Ce‐TZP) and their composites with 30 vol% alumina were produced. Low‐temperature aging degradation (LTAD) and mechanical properties of materials were investigated. For assessment of phase stability in the materials, aging experiments were performed in water at 90°C for 32, 64, and 128 days. The aging phenomenon was characterized and monitored using X‐Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Four‐point bending was used to determine the flexural strength of materials before and after aging treatment in water at 90°C for 2, 4, and 6 months. The aging experiments resulted in different phase transformation rates for the materials studied. The 12Ce‐TZP containing materials showed the highest resistance to low‐temperature aging and 3Y‐TZP containing materials showed the highest bending strength. When compared, no change in flexural strength was observed between the materials not exposed to aging and the aged materials.     

Characterization of the Cotunnite-Type Phases of Zirconia and Hafnia by Neutron Diffraction and Raman Spectroscopy

The crystal structures of the cotunnite-type phases (space group, Pnam, Z = 4) of pure zirconia and hafnia prepared under high-temperature, high-pressure conditions in a multianvil device were refined by time-of-flight neutron powder diffraction. The structures of both compounds are very similar and the nine polyhedral metal-oxygen distances range from 2.133(1) to 2.546(1) Å in ZrO₂ and from 2.121(1) to 2.535(2) Å in HfO₂. The Raman spectra of both phases resemble one another strongly and are consistent with the cotunnite-type structure. These results confirm that ZrO₂ and HfO₂ undergo transitions to the same phase at high pressure.     

Phase Stability in Calcia‐Doped Zirconia Nanocrystals

Calcia‐doped zirconia exhibits all of the polymorphism seen in the yttria‐doped zirconia ceramics, but can be produced at lowered costs and in greater abundance due to the accessibility of Calcium precursors in comparison to Yttrium. Although with great challenges, there exists an opportunity to replace yttria with calcia in applications such as ionic conductors where phase stability is critical. There is a dearth of surface characterization to enable design and prediction of the polymorphism in nanoparticulate calcia–zirconia. With recent advances in water adsorption microcalorimetry, one can accurately probe surface energies of the four zirconia polymorphs: monoclinic, tetragonal, cubic, and amorphous. The surface energies can then be coupled with bulk enthalpies extracted from oxide melt drop solution calorimetry to create a nanocrystalline phase stability diagram similar to its bulk counterpart. We report here the surface and bulk thermodynamic data on polymorphs of calcia–zirconia with composition ranging from 0 to 20 mol% calcia and use it to build a nanophase diagram for this system. The effect of the humidity in the phase stability diagram trends is also addressed and demonstrated to minimize the effect of the surface energies in the overall polymorphism trends.     

Phase Formation and Stability in Reactively Sputter Deposited Yttria-Stabilized Zirconia Coatings

Yttria-stabilized zirconia (YSZ) coatings were produced by reactively cosputtering metallic zirconium and yttrium targets in an argon and oxygen plasma using a system with multiple magnetron sputtering sources. Coating crystal structure and phase stability, as functions of Y₂O₃ content, substrate bias, and annealing temperature, were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results demonstrated that highly (111)-oriented tetragonal and cubic zirconia structures were formed in 2 and 4.5 mol% Y₂O₃ coatings, respectively, when the coatings were grown with an applied substrate bias. Conversely, coatings deposited with no substrate bias had random tetragonal and cubic structures. XRD analysis of annealed coatings showed that the cubic zirconia in 4.5 mol% Y₂O₃ coatings exhibited structural stability at temperatures up to 1200°C. Transformation of the tetragonal to monoclinic phase occurred in 2 mol% Y₂O₃ coating during high-temperature annealing, with the fraction of transformation dependent on bias potential and annealing temperature.     

Effect of NiO on the Phase Stability and Microstructure of Yttria-Stabilized Zirconia

Nickel oxide (NiO) was screen printed onto the surfaces of 3 and 8 mol% yttria-stabilized zirconia (YSZ) dense pre-fired substrates and then heat treated at temperatures from 1350° to 1550°C. The effect of NiO was dependent on the yttria content of the substrate. In 3 mol% YSZ, it was found to alter the phase composition from predominantly tetragonal with a small amount of cubic phase to one consisting of approximately equal amounts of cubic and monoclinic phase. The cubic grains were much larger than the monoclinic ones and contained more nickel. Furthermore, nickel was observed to migrate through the thickness of the tile, a distance of approximately 200 μm. In the 8 mol% YSZ substrates, the phase composition was unaltered, although large grains developed under the printed NiO layer and the nickel migration was confined to the extent of these large grains.     

High-Pressure Phase Transitions in Zirconia and Yttria-Doped Zirconia

Raman spectroscopy has been utilized to characterize the phase transformations and transition pressures in pure and doped zirconia containing 3, 4, and 5 wt% Y₂O₃. The pressure-induced transformations were investigated to over 6 GPa (at room temperature) using a diamond anvil pressure cell. Pure zirconia single-crystal samples transformed to a "new" tetragonal phase (different from the one obtained at high temperatures at atmospheric pressure) at about 4 GPa. The pressure transformation, like the temperature transition, was reversible and exhibited an approximately 0.45-GPa hysteresis at room temperature. The 3 and 4 wt% Y₂O₃ crystals underwent a monoclinic ( P 2_1/b) to tetragonal ( P 4_{2 nmc}) phase transition similar to that observed at high temperatures. This phase change was found to be irreversible on releasing the pressure. The 5 wt% Y₂O₃ at atmospheric pressure consists of a tetragonal modification in a disordered cubic matrix; a gradual, but reversible, disordering transformation of the tetragonal precipitate takes place with pressure.     

Morphology and Phase Stability of Nitrogen–Partially Stabilized Zirconia (N-PSZ)

The surface layer of yttria-doped tetragonal zirconia materials that have been heat-treated with zirconium nitride was observed to consist of a nitrogen-rich cubic matrix with nitrogen-poor tetragonal precipitates. The precipitates had a thin, oblate-lens shape, similar to those observed in magnesia–partially stabilized zirconia. Because of the fast diffusion of N⁴⁻ ions, the precipitates grew rather large, up to ∼5 μm in length, and remained stabilized in the tetragonal form at room temperature. Because the nitrided layer grew in the two-phase field, the size and distribution of the precipitates each was very irregular. The nitrogen content was observed to determine the proportion of cubic and tetragonal phases in the same way as in conventional cation-stabilized partially stabilized zirconia. A ternary phase diagram for the zirconium(yttrium)–nitrogen–oxygen system was suggested to explain the concentration gradient in the cubic matrix and the phase distribution of the nitrided layer.     

高热稳定性介孔ZrO2的制备及其结构稳定性研究

该文阐述了一种催化剂(载体)的制备方法,即在非水溶剂中,通过溶胶-凝胶技术将烷氧基锆和非离子型模板剂自组装,合成出具有蠕虫状结构的介孔氧化锆。发现对凝胶适当后处理,可大大提高介孔氧化锆的热稳定性。通过XRD、TEM、等温氮吸附技术表征,表明该法合成的介孔氧化锆在600～700℃仍能很好地保持介孔结构和形貌。考察了不同后处理方式对改善介孔氧化锆热稳定性的效果,发现随着处理溶液pH升高,产物的热稳定性提高,其中,NaOH等强碱性溶液,可以使介孔氧化锆的热稳定性提高至700℃。     

Stability of Monoclinic and Orthorhombic Zirconia: Studies by High-Pressure Phase Equilibria and Calorimetry

The stability fields of two high-pressure polymorphs of ZrO₂ (ortho I and ortho II) were determined by both calorimetry and phase equilibrium experiments. Enthalpies of transition were measured by transposed temperature drop calorimetry. The entropies of transition and slopes of phase boundaries were calculated using the measured enthalpies and free energies calculated from the results of phase equilibrium experiments. From the thermodynamic measurements, it is seen that the entropy increases and the volume decreases during the monoclinic–ortho I transition, whereas both the entropy and the volume decrease during the ortho I–ortho II transition. Accordingly, the gradient of phase boundaries, dP/dT , is negative in the former and positive in the latter. These trends are consistent with those of phase equilibrium experiments.     

Microstructure and Phase Stability of Yttria-Doped Tetragonal Zirconia Polycrystals Heat Treated in Nitrogen Atmosphere

The low-temperature degradation of zirconia (ZrO₂) that was doped with 3 mol% yttria (Y₂O₃) (3Y-TZP) was prevented by the heat treatment of sintered specimens in nitrogen. The heat treatment of sintered specimens resulted in a surface layer that was stabilized by nitrogen ions, whereas the interior was only slightly affected by the heat treatment. X-ray diffractometry and transmission electron microscopy analyses revealed that the stabilized surface layer consisted of cubic grains with tetragonal precipitates. Although the presence of the surface layer decreased the strength of the sintered 3Y-TZP, the strength of nitrified specimens was maintained when low-temperature annealing was applied.     

Influence of Interaction between Neighboring Oxygen Ions on Phase Stability in Cubic Zirconia

The electronic structures of undoped c - and t -ZrO₂ were calculated by a first-principles molecular orbital method. A preliminary analysis revealed that experimental energy-loss near-edge structure profiles obtained in ZrO₂–8 mol% Y₂O₃ could be satisfactorily explained from the present theoretical calculation. The calculation suggests that the stability of t -ZrO₂ could be described by the interaction between neighboring oxygen ions rather than the covalency of Zr—O bonds. The effect of dopant cations on the stability of cubic-zirconia solid solutions can be estimated semiquantitatively in terms of the repulsive Coulomb force between neighboring oxygen ions.     

Surface Enthalpy, Enthalpy of Water Adsorption, and Phase Stability in Nanocrystalline Monoclinic Zirconia

A fundamental issue that remains to be solved when approaching the nanoscale is how the size induces transformation among different polymorphic structures. Understanding the size-induced transformation among the different polymorphic structures is essential for widespread use of nanostructured materials in technological applications. Herein, we report water adsorption and high-temperature solution calorimetry experiments on a set of samples of single-phase monoclinic zirconia with different surface areas. Essential to the success of the study has been the use of a new ternary water-in-oil/water liquid solvothermal method that allows the preparation of monoclinic zirconia nanoparticles with a broad range of (BET) Brunauer–Emmett–Teller surface area values. Thus, the surface enthalpy for anhydrous monoclinic zirconia is reported for the first time, while that for the hydrous surface is a significant improvement over the previously reported value. Combining these data with previously published surface enthalpy for nanocrystalline tetragonal zirconia, we have calculated the stability crossovers between monoclinic and tetragonal phases to take place at a particle size of 28 ± 6 nm for hydrous zirconia and 34 ± 5 nm for anhydrous zirconia. Below these particle sizes, tetragonal hydrous and anhydrous phases of zirconia become thermodynamically stable. These results are within the margin of the theoretical estimation and confirm the importance of the presence of water vapor on the transformation of nanostructured materials.     

Phase Stability in Nanocrystals: A Predictive Diagram for Yttria–Zirconia

Predicting the polymorphic stability of nanocrystals entails taking into account the thermodynamic effects from solid–vapor interfaces. With recent advances in microcalorimetry, it is finally possible to obtain reliable surface energy data which allow building of predictive diagrams, such as the usual phase diagrams for bulk crystals, for nanocrystals. Yttria doped zirconia (YZ) is currently utilized in a variety of applications; making it one of the most studied oxide systems to date. However, the polymorphism of nanocrystalline YZ (nYZ) is much less understood as compared to its bulk counterpart, and the stability conditions of cubic, tetragonal, monoclinic, and amorphous polymorphs at the nanoscale is so far only empirically defined. In this work, we present a new predictive nanoscale phase diagram for nYZ using unprecedented data on surface energies and bulk enthalpies acquired by microcalorimetry. The phase diagram shows the stable zirconia polymorph as a function of the composition and size. This is a useful tool and an important and novel advancement to relatively untouched bulk phase diagrams, which holds the key for reliable materials' design.     

Phase Characterization of Precipitated Zirconia

The phase compositions of undoped and europium-doped zirconia samples, obtained by precipitation and thermal treatment from 350° to 1000°C, have been investigated by powder X-ray diffractometry, infrared spectroscopy, and cathodoluminescence spectroscopy. The low-temperature stabilization of tetragonal zirconia is mainly controlled by the presence of anion additives, such as ammonium chloride. The influence of the crystallite size is less important. Cathodoluminescence spectra show a structural similarity between tetragonal and amorphous zirconia.     

Pressure-Temperature Phase Diagram of Zirconia

The pressure-temperature phase diagram of zirconia was determined by optical microscopy and X-ray diffraction techniques using a diamond anvil pressure cell. At room temperature, monoclinic ZrO₂ transforms to a tetragonal phase ( t II) which is related to the high-temperature tetragonal structure ( t I). The transformation pressure exhibits hysteresis and is cycle dependent. At room temperature, the initial transformation pressure for the monoclinic- t II transition on a virgin monoclinic crystal can be as high as 4.4 GPa; on subsequent cycling the transition pressure ultimately lowers to 3.29 ± 0.06 GPa. The pressure for the reverse transition is essentially constant at 2.75 ± 0.06 GPa. At pressures > 16.6 GPa, the t II form transforms to the orthorhombic cotunnite (PbCl₂) structure. With increasing temperature, the t II form transforms to the high-temperature tetragonal phase. For increasing P and T , the monoclinic- t I- t II triple point is located at T = 596°± 18°C and P = 2.26 ± 0.28 GPa, whereas for decreasing P and T , the triple point is found at T = 535°± 25°C and P = 1.7 ± 0.28 GPa.     

Phase Stability of Yttria-Stabilized Zirconia with Dissolved Cerium and Neptunium Oxides Under Oxidizing and Reducing Atmospheres

Phase stability and effects of sintering atmosphere on the crystalline structure of Np-doped yttria-stabilized zirconia (YSZ) were evaluated in comparison with those same properties and conditions for Ce-doped YSZ. Different sintering atmospheres for Ce-doped YSZ led to differences in phase formation through reduction of the dopant from Ce⁴⁺ to Ce³⁺. On the other hand, YSZ specimens containing up to 40 mol% Np formed only a fluorite-structure phase regardless of sintering atmosphere. Yttria-stabilized zirconia thus seems to accommodate Np within a wide range of concentrations and to have excellent phase stability under both oxidizing and reducing atmospheres.     

Phase Analysis in Zirconia Systems

Linear calibration curves were developed for determining the content of free ZrO₂ in partially stabilized zirconia ceramics by X-ray diffraction techniques. Two methods were studied. The matrix method, in which free ZrO₂ was considered to be distributed in a matrix (the cubic phase), gave approximately equal mass absorption coefficients for the monoclinic and cubic phases. The polymorph technique, in which the cubic phase was considered to be a polymorph of ZrO₂ and in which integrated intensities were used, gave the better results.