Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Ionic Mobilities and Association Energies from an Analysis of Electrical Impedance of ZrO2–Y2O3 Alloys

The dc conductivities of ZrO₂–Y₂O₃ ceramic alloys (in the range 3–12 mol% of Y₂O₃) have been obtained from ac impedance measurements at temperatures between 250° and 370°C. The Almond–West ac conductivity model has been applied to evaluate hopping rates in this system. The migration enthalpies were evaluated and shown to increase with yttria concentration, but all values determined were shown to be lower than the corresponding activation enthalpies for conductivity. The association enthalpies thus calculated were shown to be very small in 3 mol% Y₂O₃–ZrO₃ and to increase with yttria concentration until the yttria contents were high enough to form fully stabilized cubic zirconia. For these samples the association enthalpies are about 0.19 eV, and no longer sensitive to yttria content. The low hopping rate at high yttria concentration might be attributed to low entropy in the system, which might be attributed to the formation of vacancy clusters and/or an ordering of the structure.     

Analytical Electron Microscopy of Yttria Partitioning in the Yttria-Partially-Stabilized Zirconia Crystal

Yttria-partiaUy-stabilized zirconia was grown frcm the melt by the arc-image floating zone technique and annealed at 1700'C. The yttria concentration of the crystal was measured by analytical electron microscopy. The crystal, which contained 8.6 mol% YO_{1. 5}, consists of tetragonal and cubic phases with yttria concentrations of 3.9 and 9.7 mol% YO_1.5, respectively. There is a small difference between this result and the composition expected from the ZrO₂-Y₂O₃ phase diagram.     

Phase Distribution and Residual Stresses in Melt-Grown Al2O3-ZrO2(Y2O3) Eutectics

Al₂O₃-ZrO₂ eutectics containing 0 to 12.2 mol% Y₂O₃ (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. Processing variables were chosen to obtain homogeneous, colony-free, interpenetrating microstructure for all of the compositional range, optimum from the viewpoint of mechanical properties. The amount of cubic, tetragonal, or monoclinic zirconia phases was determined using a combination of Raman and X-ray diffraction techniques. Monoclinic zirconia was present up to concentrations of 3 mol% Y₂O₃, while the amount of tetragonal zirconia gradually increased with yttria content up to 3 mol%. Cubic zirconia was the only phase detected when the yttria content reached 12 mol%. The residual stresses in alumina were measured using the shift of the ruby R lines. Compressive stresses were isotropic when measured in the samples containing tetragonal and cubic zirconia, while higher tensile, anisotropic stresses were found when monoclinic zirconia was present. They were partially relieved in the eutectic sample without yttria. These results were compared with a thermoelastic analysis based on the self-consistent model.     

Transmission Electron Microscope Investigation of the Interface between Titanium and Zirconia

The interfaces between 3-mol%-yttria-partially-stabilized zirconia and commercially pure titanium after reaction at 1750°C were analyzed with a scanning electron microscope and an analytical transmission microscope. Zirconia was reduced to oxygen-deficient zirconia (ZrO_{2- x} ) with an O/Zr ratio as low as 1.53, causing the evolution of oxygen. Part of the oxygen could accumulate at grain boundaries, the remainder being dissolved in titanium as alpha-Ti(O). An ordered titanium suboxide (Ti₃O) could be formed from a solid solution of alpha-Ti(O) during cooling. A fine crystalline ZrO_{2- x} phase (O/Zr similar/congruent 2) was also found along with alpha-Zr near the interface on the zirconia side. The alpha-Zr was twinned with one of the twin planes being indexed as {1012}. The yttria stabilizer was excluded from zirconia as the reaction was progressing, existing as oxygen-deficient yttria. Extensive dissolution of zirconia in titanium gave rise to the formation of alpha-Ti(Zr,O) solid solution. On cooling, lamellae of Ti₂ZrO precipitated from alpha-Ti(Zr,O) with an orientation relationship of {110}_Ti2ZrO//{100}_alpha-Ti and <111>_Ti2ZrO//<011>_alpha-Ti.     

Direct Calorimetric Measurement of Enthalpies of Phase Transitions at 2000°–2400°C in Yttria and Zirconia

High-temperature differential thermal analysis provided data on phase transitions in zirconia and yttria. The tetragonal form of ZrO₂ transforms to the cubic fluorite structure at 2311°±15°C with an enthalpy of 3.4±3.1 kJ/mol. Cubic C-type Y₂O₃ transforms, probably to the fluorite structure, at 2308°±15°C with Δ H =47.7±3.0 kJ/mol. This high-temperature polymorph melts at 2382°±15°C with an enthalpy of fusion of 35.6±3.0 kJ/mol.     

Electrical Conductivity of Zirconia Stabilized with Scandia and Yttria

Electrical conductivity of zirconia stabilized with scandia and yttria (Sc₂O₃+Y₂O₃= 8 mol%) has been measured by the complex impedance method in the temperature range 573 to 1173 K. With increasing Sc₂O₃ concentration, electrical conductivity increases at temperatures above 640 K, but it decreases below this temperature. Electrical conductivity in the electrolytes examined is a result of two processes: an activation energy of 59 to 79 kJ·mol⁻¹ predominant at high temperatures and an activation energy of 109 to 125 kJ·mol⁻¹ predominant at low temperatures.     

Optical Properties and London Dispersion Forces of Amorphous Silica Determined by Vacuum Ultraviolet Spectroscopy and Spectroscopic Ellipsometry

Precise and accurate knowledge of the optical properties of amorphous silica is important because of the increasing application of SiO₂ in optical and electrooptical devices, including photolithography masks for semiconductor fabrication, recently as a potential 157 nm mask substrate. The optical properties in the vacuum ultraviolet (VUV) region have been investigated, because they convey detailed information on the electronic structure and interatomic bonding of the material. In this work, we have combined spectroscopic ellipsometry and VUV spectroscopy to directly determine the optical functions of SiO₂ in this range, thereby reducing the uncertainty in the low-energy extrapolation of the data, essential for Kramers–Kronig analysis of VUV reflectance. We report the complex optical properties of SiO₂, over the range of 1.5 to 42 eV, showing improved agreement with theory when contrasted with earlier results. In addition to the features previously reported at 10.4, 11.6, 14.03, and 17.10 eV, new interband transitions have been observed at 21.3 eV along with O 2 s transitions at 32 eV. We found the bulk plasma peak to be 23.7 eV in the energy loss function spectrum. Based on the magnitude of these new results, the Hamaker constant for SiO₂|Vacuum|SiO₂ is 71.6 zJ, which is larger than the previously reported value of 66 zJ.     

Thermodynamic Properties of Nonstoichiometric Yttria-Stabilized Zirconia at Low Oxygen Pressures

Thermodynamic properties of 8-mol%-yttria-stabilized zirconia have been determined in the 810° to 1040°C temperature range at low Po₂. A high-temperature solid-state coulometric titration method was used. The mass action constant, K_ma, can be represented at low Po₂ as K_ma=0.677 exp [(–3.98 ±0.03 eV)/kT].     

Microstructure and Fracture Toughness of Hot-Pressed Zirconia-Toughened Sialon

Zirconia-toughened sialon composites have been fabricated using conventional hot-pressing techniques. The fracture toughness and microstructure were determined for CeO₂-and Y₂O₃-stabilized ZrO₂ additives and also as a function of volume percent ZrO₂. The yttria system showed a linear increase in fracture toughness with increasing volume fraction zirconia content while the ceria-stabilized system exhibited a peak in fracture toughness at 20 vol % ZrO₂ content. The fracture toughness at 800°C was measured and correlated with the microstructure. High-temperature stability was determined and it was found that the deleterious nitride phases of zirconium could be precluded from the microstructure.     

Chemical Interactions between La-Sr-Mn-Fe-O-Based Perovskites and Yttria-Stabilized Zirconia

Orthoferrite-based perovskites are of interest as materials for the cathode in solid oxide fuel cells (SOFCs). Therefore, the chemical compatibility between perovskites of the composition (La_1−xSr_x)_zFe_1−yMn_yO_3−δ (0 # x # 0.3; 0.2 # y # 1; z = 0.90, 0.95, 1.00) and the solid electrolyte zirconia (ZrO₂) doped with 8 mol% yttria (Y₂O₃) (8YSZ) has been investigated. Powder mixtures of the two materials have been annealed at different temperatures. The formation of monoclinic ZrO₂ at 1000°C, as well as of La₂Zr₂O₇ and SrZrO₃ at 1400°C, has been determined in some samples. The reactions that are observed are discussed, with respect to the thermodynamic activities, tolerance factor, and oxygen-ion migration energies. Some perovskite compositions seem to be compatible with Y₂O₃-stabilized ZrO₂ (YSZ), thereby offering the possibility to use orthoferrite-based perovskites in SOFCs with a solid electrolyte made of YSZ.     

Qualitative X-ray Diffraction Analysis of Metastable Tetragonal (t') Zirconia

The identification and discrimination of cubic ( c ) and metastable t '-zirconia phases by conventional X-ray diffraction has proved to be a difficult problem because of the crystallographic similarities of the two phases. In this study, the c -phase and t '-phase in biphasic metastable zirconia samples have been successfully distinguished by X-ray diffraction; data were collected using a high-resolution powder diffractometer with monochromatic radiation and the peak shapes were fitted using a peak-fitting program to distinguish the different phases. From this study, the minimum temperature, T ₀, at which the t '-phase can be obtained on cooling rapidly compositions in the range 3–7 mol% Y₂O₃–ZrO₂ to room temperature was estimated as ∼1425°C. For a constant temperature above 1425°C, the lattice parameters of the t '-phase of 3–7 mol% Y₂O₃–ZrO₂ compositions within the two-phase region of the phase diagram were unaffected by the yttria content; only the amount of t '-phase formed was affected by the yttria content.     

Electronic Structure of High-Temperature Superconductors: Comparison of Theory and Experiment

The ab initio pseudofunction method has been used to compute electronic properties of superconductors in good agreement with experiment. La₂CuO₄ and CuO are computed to be antiferromagnetic and semiconducting in agreement with experiment. The 0.4-eV peak in the optical conductivity spectrum has been shown to be an interband transition for YBa₂Cu₃O₇. The bands for Bi₂Sr₂CaCu₂O₈ as well as YBa₂Cu₃O₇ agree well with angle-resolved photoemission near E _f. The shake-up peaks near 9 and 12 eV observed in photoemissio can be understood in terms of two-electron processes envolving the local densities of states of Cu and O. A mechanism for superconductivity based on these electronic properties will be advanced. All calculations use local density potentials with no parameters.     

Zirconia–Mullite Composites Consolidated by Spark Plasma Reaction Sintering from Zircon and Alumina

Mullite–ZrO₂ composites have been fabricated by attrition milling a powder mixture of zircon, alumina, and aluminum metal with MgO or TiO₂ as sintering additives, heating at 1100°C to oxidize the aluminum metal, and consolidation by spark plasma sintering (SPS). The influence of the SPS temperature on the formation of mullite, and the density and the mechanical properties of the resulting composites have been studied. For the mullite–zirconia composites without sintering additives, the mullite formation was accomplished at 1540°C. In contrast, for the composites having MgO and TiO₂, the formation temperature dropped to 1460°C. The composites without sintering additives were almost fully dense (99.9% relative density) and retained a larger amount of tetragonal zirconia. Those materials attained the best mechanical properties ( E =214 GPa and K _{I C} =6 MPa·m^1/2). To highlight the advantages of using the SPS technique, the obtained results have been compared with the characteristics of a mullite–zirconia composite prepared by the conventional reaction-sintering process.     

High-Temperature Optical Absorption Measurement of Single-Crystal, Yttria-Stabilized Zirconia

High-temperature optical absorption measurements were performed on single-crystal yttria-stabilized zirconia. The optical band gap (E_g) can be represented in the temperature range 15° to 1010°C as E_g(T)=3.40-5.16x10⁻⁴T(K)(eV).     

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.     

High-Temperature Hydroxylation of Mullite

A plane-parallel, polished, 0.9 mm thick, single-crystal (001) plate of 2:1 mullite was treated for 6 h at 1600°C in an Ar/H₂O (90/10) gas mixture at 100 kPa. Optical microscopy studies and infrared (IR) reflection spectroscopy studies of the lattice vibrations yielded no evidence for change with respect to the untreated reference crystal. However, IR absorption spectroscopy showed that structurally bound OH groups were formed by the heat treatment in the Ar/H₂O gas mixture. IR absorption depth profile analysis showed a rather homogeneous OH distribution through the crystal. Five different hydroxyl groups were separated according to dipole orientations and peak positions: E ‖ a , ω _{a 1}= 3447 cm⁻¹, ω _{a 2}= 3579 cm⁻¹; E ‖ b , ω _{b 1}= 3456 cm⁻¹, ω _{b 2}= 3544 cm⁻¹; and E ‖ c , ω _{c 1}= 3498 cm⁻¹. All IR peaks were strongly broadened (between 90 and 150 cm⁻¹) because of a distribution in O-H binding distances caused by the real structure of mullite.     

Kinetics and Mechanism of the Reduction of Nickel Oxide in Aligned Nickel Oxide-Zirconia and Nickel Oxide-Yttria Eutectic Structures

Unidirectional solidification experiments were carried out at the eutectic composition of the systems NiO-ZrO₂ and NiO-Y₂O₃, yielding materials made of regularly aligned lamellar structures. Eutectic samples were submitted to a chemical reduction treatment at 1075°C under CO/CO₂ mix tures corresponding to Po ₂ values for which only NiO should be reduced to the metal. The rapid formation of nickel be tween the zirconia or yttria platelets and the morphology of the nickel layer resulting from reduction establish that the reduction kinetics were controlled predominantly by the dif fusion of CO to the Ni/NiO interface through the porous Ni product. Experimental data of the variation of reduction depth of NiO as a function of time are qualitatively inter preted using a simple model of interdiffusion of CO in N₂/ CO/CO₂ mixtures. This model also explains the observation of an influence of the dilution of CO on the reduction depth.     

Application of an Ion-Packing Model Based on Defect Clusters to Zirconia Solid Solutions: I, Modeling and Local Structure of Solid Solutions

Oxygen vacancies can be introduced into zirconia solid solution ZrO₂–MO _u ( u = 1 and 1.5) to maintain electroneutrality. Recently, the local structures around Zr⁴⁺ and M^{2 u +} ions in ZrO₂–MO _u solid solutions have been studied through EXAFS spectroscopy, diffuse scattering analysis, and single-crystal structure analysis. The present study constructs an ion-packing model for zirconia solid solutions based on some defect cluster models. The decrease of cell volume with the occurrence of vacancies is assumed to be expressed by decreasing the coordination number (CN) of cations around the vacancy. The distribution of CNs in a solid solution was calculated from a certain defect cluster model. The average interatomic distances, the average CN, and the short-range order parameters were calculated using this distribution of CNs. The local structures calculated from the model were compared with experimental data in the systems ZrO₂–MO_1.5 (M = Y, Gd, Yb, and Ca). In the ZrO₂–YO_1.5 system, the r (s–O) interatomic distance, where s represents Zr⁴⁺ or Y³⁺ and O represents O²⁻, decreased with Y content and therefore vacancy content. The probability of finding Y³⁺ around a vacancy increases with increasing yttria content from a comparison of the calculated results with the ones from recent EXAFS studies. The present model can qualitatively explain compositional and size dependences of the dopant on various local structures.