Formation and Transformation of Cubic ZrO2 Solid Solutions in the System ZrO2—Al2O3

In the system ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminum alkoxides. The values of the lattice parameter, a, increase linearly from 0.5095 to 0.5129 nm with increasing Al₂O₃ content. At higher temperatures, the solid solutions transform into tetragonal ZrO₂ and α-Al₂O₃. Pure ZrO₂ crystallizes in the tetragonal form at 415° to 440°C.     

Phase Equilibria and Ionic Conductivity in the System ZrO2−Yb2O3−Y2O3

The phase equilibria in the zirconia-rich part of the system ZrO₂−Yb₂O₃−Y₂O₃ were determined at 1200°, 1400°, and 1650°C. The stabilizing effects of Yb₂O₃ and Y₂O₃ were found to be quite similar with <10 mol% of either being necessary to fully stabilize the cubic fluorite-structure phase at 1200°C. The two binary ordered phases, Zr₃Yb₄O₁₂ and Zr₃Y₄O₁₂, are completely miscible at 1200°C. These were the only binary or ternary phases detected. The ionic conductivities of ternary specimens in this system were measured using the complex impedance analysis technique. For a given level of total dopant, the substitution of Yb₂O₃ for Y₂O₃ gives only minor increases in specimen conductivity.     

Electrical Conductivity in the System ZrO2—CaZrO3

The electrical conductivity of compositions in the system ZrO₂-CaZrO₃ was examined. The composition of maximum conductivity in this system is within the cubic solid-solution region close to the low-calcia cubic solid-solution phase boundary. The conductivity decreases from the single-phase cubic solid solution to both of the adjacent two-phase regions; the decrease is more rapid on the ZrO₂-rich two-phase side. This effect is discussed on the basis of differing microstructures.     

Ionic Conductivity of the Fluorite-Type Hafnia–R2O3 Solid Solutions

The ionic conductivity of the hafnia-scandia, hafnia-yttria, and hafnia-rare earth solid solutions with high dopant concentrations of 8, 10, and 14 mol% was measured in air at 600° to 1050°C. Impedance spectroscopy was used to obtain lattice conductivity. A majority of the investigated samples exhibited linear Arrhenius plots of the lattice conductivity as a function of temperature. For all investigated dopant concentrations the ionic conductivity was shown to decrease as the dopant radius increased. The activation enthalpy for conduction was found to increase with dopant ionic radius. The fact that the highest ionic conductivity among 14-mol%-doped systems was obtained with HfO₂─Sc₂O₃ suggested that the radius ratio approach should be used to predict the electrical conductivity behavior of HfO₂─R₂O₃ systems. A qualitative model based on the Kilner's lattice parameter map does not seem to apply to these systems. For the three systems HfO₂─Yb₂O₃, HfO₂─Y₂O₃, and Hf₂O₃─Sm₂O₃ a conductivity maximum was observed near the dopant concentration of 10 mol%. Deep vacancy trapping is responsible for the decrease in the ionic conductivity at high dopant concentrations. Formation of microdomains of an ordered compound cannot explain the obtained results. A comparison between the ionic conductivities of doped HfO₂ and ZrO₂ systems indicated that the ionic conductivities of HfO₂ systems are 1.5 to 2.2 times lower than the ionic conductivities of ZrO₂ systems.     

Mechanism of Decomposition of the Cubic Solid Solutions in the System ZrO2—MgO

The phase diagram for the system ZrO₂-MgO was reinvestigated by using coprecipitated gels. The thermal stability of the defective cubic solid solutions was investigated by a study of kinetics. The decomposition curves were sigmoidal, indicating that the formation of the tetragonal solid solutions proceeded by a process of nucleation and growth. Decomposition comprised: (1) formation of the nuclei of the tetragonal solid solution on the grain surface, (2) growth of the nuclei leading to the formation of an interface, and (3) the irregular growth of that interface into the interior of the grain. During the decomposition magnesium ions diffused out of the regions of the growth of the tetragonal phase and partly precipitated as magnesium oxide at the grain boundaries. The decomposition rate increased with the degree of ordering or clustering of defects and impurities and with the number of the anion defects; it showed a maximum at about 1200°C.     

Phase Relationships in the System CaO—AI2O3—P2O5

Primary fields of crystallization in the system CaO-Al₂O₃-P₂O₅ at temperatures from 900° to 1600°C. were determined by the method of quenching. Three ternary eutectics were established: CaO·P₂O₅-Al₂O₃·3P₂O₅-Al₂O₃·P₂O₅, 2CaO·P₂O₆-CaO·P₂O₅-Al₂O₃·P₂O₅, and 3CaO·P₂O₆-2CaO·P₂O₅-Al₂O₃·P₂O₅. The rate of decomposition of Al₂O₃·3P₂O₅ was determined at several temperatures. The boundary was established between the field A1₂O₃·P₂O₅, which covers about 35% of the ternary diagram, and the fields Al₂O₃·3P₂O₅, 2CaO·P₂O₅, and CaO·P₂O₅. A portion of the Al₂O₃·P₂O₅-3CaO·P₂O₅ boundary also was established. A compound with the composition 2Al₂O₃·3P₂O₅ did not appear in the system. No calcium aluminum phosphates were found.     

Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

Electrical Conductivity in the ZrO2-Rich Region of Several M2O3—ZrO2 Systems

The electrical conductivity of M₂O₃-ZrO₂ compositions containing 6 to 24 mole % M₂O₃, where M represents La, Sm, Y, Yb, or Sc, was examined. Only Sm₂O₃, Y₂O₃, and Yb₂O₃ formed cubic solid solutions with ZrO₂ over most of this substitutional range. Scandia forms a wide cubic solid solution region with ZrO₂ at temperatures above 130°C whereas the cubic solid solution region at room temperature is narrow (6 to 8 mole % Sc₂O₃). Lanthana additions to ZrO₂produced no fluorite-type cubic solid solutions within the compositional range investigated. Generally, the electrical conductivity of these cubic solid solutions increased as the size of the substituted cation decreased and the electrical conductivity for each binary system attained a maximum at about 10 to 12 mole % M₂O₃.     

Phase Stability and Physical Properties of Cubic and Tetragonal ZrO2 in the System ZrO2–Y2O3–Ta2O5

The cubic ( c -ZrO₂) and tetragonal zirconia ( t -ZrO₂) phase stability regions in the system ZrO₂–Y₂O₃–Ta₂O₅ were delineated. The c -ZrO₂ solid solutions are formed with the fluorite structure. The t -ZrO₂ solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m^1/2), and their instability/transformability to monoclinic zirconia ( m -ZrO₂) increases with increasing tetragonality. On the other hand, the t -ZrO₂ solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m^1/2), and their transformability is not related to the tetragonality.     

Crystallization and Properties of Fe2O3—CaO—SiO2 Glasses

Nucleation and crystal growth rates and properties were studied in a two-stage heat treatment process for Fe₂O₃-CaO-SiO₂ glasses. Glass transition (T_g) and crystallization temperatures (T _c ) for the glasses lay between about 612.0° and 710.0°C, and 858.5° and 905.0°C, respectively, and magnetite was the main crystal phase. For a glass of 40Fe₂O₃. 20CaO·40SiO₂ (in wt%) the maximum nucleation rate was (68.6 ± 7) × 10⁶/mm³·s at 700°C, and the maximum crystal growth rate was 9.0 nm/min^1/2 at 1000°C. The mean crystal size of the magnetite increased from 30 to 140 nm with variation of nucleation and crystal growth conditions. The glass showed the maxima in saturation magnetization and coercive force, 212.1 × Wb/m² and 30.8 × 10³ A/m, when heat-treated for 4 h at 1000°C and 1050°C, respectively. The variation of the saturation magnetization could be quantitatively interpreted well in terms of the volume fraction of the magnetite, whereas that of the coercive forces could be explained only qualitatively in terms of the particle size of the magnetite. Hysteresis losses showed the maximum value of 1493 W/m³ when heat-treated at 1000°C for 4 h prenucleated at 700°C for 60 min, and increased linearly with increasing heat treatment time under a magnetic field up to 800 × 10³ A/m.     

Phase Stability, Phase Transformation Kinetics, and Conductivity of Y2O3—Bi2O3 Solid Electrolytes Containing Aliovalent Dopants

Single-phase, cubic solid solutions of baseline composition 25% Y₂O₃—75% Bi₂O₃ with and without aliovalent dopants were fabricated by pressureless sintering of powder compacts. CaO, SrO, ZrO₂, or ThO₂ was added as an aliovalent dopant. Sintered samples were annealed between 600° and 650°C for up to 4000 h. Samples doped with ZrO₂ or ThO₂ remained cubic, depending upon the dopant concentration, even after long-term annealing. By contrast, undoped, CaO-doped, and SrO-doped samples transformed to the low-temperature, rhombohedral phase within ∼ 200 h. Conductivity measurements showed no degradation of conductivity in samples that did not undergo the transformation. In samples that underwent the transformation, a substantial decrease in conductivity occurred. The enhanced stability of the ZrO₂- and ThO₂-doped samples is rationalized on the basis of suppressed interdiffusion on the cation sublattice.     

Rhombohedral Phase in Y2O3-Partially-Stabilized ZrO2

Tetragonal-to-rhombohedral stress-induced phase transformation was studied by X-ray diffraction on the ground surfaces of tetragonal zirconia polycrystals and partially stabilized zirconia containing 2.0 to 5.0 mol% Y₂O₃ prepared by hot isostatic pressing. The rhombohedral phase increased with Y₂O₃ content and also with hot isostatic pressing temperature. The stability of the rhombohedral phase was studied with regard to surface finish and thermal annealing. The subsequent heat treatment of the specimens was found to cause the reverse rhombohedral-to-tetragonal transformation.     

High-Temperature Creep of Y2O3-Stabilized ZrO2

The compression creep behavior of Y₂O₃-stabilized ZrO₂ (YSZ) was studied at temperatures to 2000 ° C. The function of Y₂O₃ content and grain size was tested in specimens with various impurity concentrations and porosity distributions. For relatively fine-grained specimens, creep rates increased with the 1.5 power of the applied stress at low stresses and with the third power at high stresses. The results for coarse-grained specimens can, in general, be fit by the cube dependence. The 1.5 power can be reduced to a linear dependence by correcting for an apparent threshold stress, which decreases with increasing temperature. Creep activation energies for YSZ are 128 ± 10 kcal/mol, independent of Y₂O₃ content, impurity level, grain size, and porosity distribution. In addition, over a broad range of temperatures and stresses the absolute values of the steady-state creep rates are influenced only by grain size and O₂ partial pressure.     

Influence of Annealing on the Electrical Conductivity of Polycrystalline ZrO2+8 Wt% Y2O3

The electrical conductivity of partially stabilized zirconia was investigated as a function of frequency (2 Hz to 100 kHz) and temperature (400° to 1000°C) by measuring ac admittance on a 2-probe cell using Lissajous figures. The dependence of the conductivity on annealing time was investigated with in situ conductivity measurements for prolonged annealing at 800°, 900°, and 1000°C. The aging behavior of two commercial zirconias was studied. At higher temperatures (7<750°C) the decrease in total conductivity arises mainly from changes within the grains, whereas at lower temperatures the decrease arises principally from the grain boundaries. Differences in initial conductivity and aging rates between samples from the two sources were related to grain size and impurity effects.     

Microstructural Evolution in a ZrO2 Wt% Y2O3 Ceramic

Several unusual microstructural features, i.e., 90° tetragonal ZrO₂ twins containing antiphase domain boundaries, tetragonal ZrO₂ precipitates in a colony morphology, and precipitate-free zones at the perimeter of cubic ZrO₂ grains containing fine tetragonal ZrO₂ precipitates, were observed in a single ZrO₂-12 wt% Y₂O₂ ceramic annealed at 1550°, 1400°, and 1250°C, respectively. The type of phase transformation responsible for each microstructural feature is described.     

Internal Friction, Dielectric Loss, and Ionic Conductivity of Tetragonal ZrO2-3% Y2O3 (Y-TZP)

The defect structure in 3 mol% Y-TZP was studied by correlated internal friction, dielectric loss, and ionic conductivity experiments. A prominent mechanical and dielectric loss peak occurs in the temperature range between 380 and 550 K that depends on the frequency of measurement. The relaxation parameters were determined as H_m = 90 ± 3 kJ·mol⁻¹, τ_∞= (1.0_+1.5^−0.6) × 10₋₁₄ s for the mechanical relaxation and H_d = 84 ± 3 kJ·mol⁻¹, τ_∞= (1.6_+1.7^−0.9) × 10^–13 s for the dielectric relaxation. The ionic conductivity below 790 K is controlled by an activation enthalpy of H _σ= 89 ± 3 kJ·mol⁻¹; at higher temperatures H _σ= 60 ± 3 kJ·mol^–1. An atomistic model is presented which assumes that oxygen vacancies are trapped by yttrium ions forming anisotropic complexes which—by reorientation—cause anelastic and dielectric relaxation. At higher temperatures (>790 K) these complexes are dissociated, which leads to the reduced activation enthalpy for ionic conductivity.     

Phase Relations in the System ZrO2-Y2O3 at Low Y2O3 Contents

Subsolidus phase relations in the low-Y₂O₃ portion of the system ZrO_2-Y₂O₃ were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y₂O₃ is soluble in monoclinic ZrO₂, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y₂O₃ below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y₂O₃ from ∼500° to ∼1600°C. At higher Y₂O₃ compositions, cubic ZrO₂ solid solution occurs.     

Metastable Phase Formation in Plasma-Sprayed ZrO2 (Y2O3)–Al2O3

Rapidly solidified ZrO₂ (Y₂O₃)–Al₂O₃ powders were prepared by melting fine-particle aggregates in a high-enthalpy plasma flame and then rapidly quenching them in cold water or on a copper chill plate. To ensure complete melting and homogenization of all the particles before quenching, the water-quenching treatment was often repeated two or even three times. The resulting melt-quenched powders and splats displayed a variety of metastable structures, depending on composition and cooling rate. ZrO₂-rich material developed an extended solid solution phase, whereas eutectic material formed a nanofibrous or amorphous structure. Under high cooling rate conditions, the ZrO₂-rich material developed a nanocomposite structure ( t -ZrO₂+α-Al₂O) directly by melt-quenching, whereas, more typically, such a structure was developed only after postannealing of the as-quenched metastable material.