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.     

Experimental Investigation and Thermodynamic Modeling of the ZrO2–SmO1.5 System

Phase equilibria of the ZrO₂–SmO_1.5 system have been studied by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The compositions of phases in the tetragonal+fluorite, fluorite+pyrochlore, and fluorite+B-Sm₂O₃ two-phase fields have been determined for samples quenched from temperatures between 1400° and 1700°C. The heat content of the fluorite phase with 30 mol% SmO_1.5 and of the pyrochlore phase with 50 mol% SmO_1.5 has been measured in the temperature range 200°–1400°C using high-temperature drop calorimetry. The transition between pyrochlore and fluorite phases is clearly first order in the SmO_1.5-rich region, while no fluorite+pyrochlore two-phase region has been detected for the samples with ZrO₂ excess. Based on the obtained experimental results and literature data, the phase diagram and thermodynamic properties were optimized using the CALPHAD approach.     

Phase Equilibria in the System ZrO2─InO1.5

Phase equilibria in the system ZrO₂─InO_1.5 have been investigated in the temperature range from 800° to 1700°C Up to 4 mol%, InO_1.5 is soluble in t -ZrO₂ at 1500°C. The martensitic transformation temperature m → t of ZrO₂ containing InO_1.5 is compared with that of ZrO₂ solid solutions with various other trivalent ions with different ionic radii. The diffusionless c → t ' A phase transformation is discussed. Extended solid solubility from 12.4 ± 0.8 to 56.5 ± 3 mol% InO_1.5 is found at 1700°C in the cubic ZrO₂ phase. The eutectoid composition and temperature for the decomposition of c -ZrO₂ solid solution into t -ZrO₂+InO_1.5 solid solutions were determined. A maximum of about 1 mol% ZrO₂ is soluble in bcc InO_1.5 phase. Metastable supersaturation of ZrO₂ in bcc InO _1.5 and conditions for phase separation are discussed.     

Possibility of Spinodal Decomposition in ZrO2-Y2O3 Alloys: A Theoretical Investigation

The thermodynamics and kinetics of cubic → tetragonal phase transformations in ZrO₂-Y₂O₃ alloys were investigated by using thermodynamic stability analysis and kinetic computer simulations to explore the possibility of a spinodal mechanism during decomposition. Based on a simple free energy model, it is shown that, depending on the alloy composition, a cubic phase aged within the t + c two-phase field may result in three different sequences of phase transformations: (1) direct nucleation and growth of the equilibrium t-phase from the c-phase matrix; (2) formation of a metastable t'-phase followed by nucleation and growth of the equilibrium c- and t-phases; and (3) formation of a transient t'-phase followed by its spinodal decomposition into two tetragonal phases with one of the tetragonal phases eventually transforming to the equilibrium c-phase. The temporal microstructure evolutions for different compositions were studied by using computer simulations based on the time-dependent Ginzburg-Landau (TDGL) model which incorporates the long-range elastic interactions.     

Application of Roman Spectroscopy for the Study of Phase Transformation in Ti0.65Sn0.35O2

Polycrystalline, Al-doped samples in the TiO₂-SnO₂ ceramic system were made and annealed in the two-phase field above the [001] coherent spinodal. The samples were examined, after various annealing times, using X-ray diffraction and Raman spectroscopy. It was observed that time constants obtained from X-ray studies were in good agreement with those obtained from Raman shifts, and both showed similar effects of grain size on the decomposition kinetics. These results suggest that Raman spectroscopy may be a useful technique for the study of phase transformations in ceramics.     

Subsolidus Equilibria in the TiO2-SnO2 System

The subsolidus miscibility gap for the TiO₂-SnO₂ system was redetermined. The critical temperature, 1430°C, is intermediate between that determined by Padurow, 1330°C, and that determined by Garcia and Speidel, 1475°C. Although the phase boundary is slightly asymmetric (the critical composition occurs at 47 mol% TiO₂), it fits the regular-solution model down to 1200°C. Calculations of the coherent spinodal using the regular-solution model indicated depression of the spinodal below T_c , by 105°, 310°, and 387° for composition fluctuations along the [001], <101>, and <100> directions, respectively. These depressions of the spinodal are much greater than those calculated by Stubican and Schultz; this discrepancy is believed to result from an error in the latter workers' calculations. During the present work, positive deviations from Vegard's law were found in this system. Both the magnitude and the sign of the deviation can be predicted using a theory based on nonlinear second-order elasticity.     

Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3–Cr2O3 and V2O3–Cr2O3 Systems

The phase diagrams in the Al₂O₃–Cr₂O₃ and V₂O₃–Cr₂O₃ systems have been assessed by thermodynamic modeling with existing data from the literature. While the regular and subregular solution models were used in the Al₂O₃–Cr₂O₃ system to represent the Gibbs free energies of the liquid and solid phases, respectively, the regular solution model was applied to both phases in the V₂O₃–Cr₂O₃ system. By using the liquidus, solidus, and/or miscibility gap data, the interaction parameters of the liquid and solid phases were optimized through a multiple linear regression method. The phase diagrams calculated from these models are in good agreement with experimental data. Also, the solid miscibility gap and chemical spinodal in the V₂O₃–Cr₂O₃ system were estimated.     

Thermodynamic Assessment of the ZrO2─YO1.5 System

An optimal set of thermodynamic functions for the ZrO₂─YO_1.5 system are obtained using phase diagram and thermodynamic data. The liquid is described by a subregular solution model. Both cubic ZrO₂ and YO_1.5 solid solutions are regarded as one cubic solution, which is also treated as a subregular solution. The ordered Zr₃Y₄O₁₂ phase is treated as a stoichiometric compound. A regular solution model is applied to the other solid solutions. Tentative equilibrium boundaries between monoclinic and tetragonal ZrO₂ solid solutions are evaluated from information about the T ₀ line. The calculated phase diagram and thermodynamic functions agree well with experimental data.     

Spectrum of Volume Relaxation Times in B2O3

Time-index of refraction isotherms were measured for B₂O₃ glass starting from both a high and a low temperature in the transformation region. The equilibrium index values at each temperature, obtained from both types of approach curve, were identical. As in the case of the density values, the equilibrium refractive index curve as a function of temperature for this glass is not a straight line. The two-relaxation-times (crossover) model was applied to B₂O₃ glass and fitted the data as well as it did in previous experiments with borosilicate crown and GeO₂ glasses. The reverse crossover which was predicted by the model was experimentally confirmed with the B₂O₃ glass. The spectrum of relaxation times narrowed with decreasing temperature, indicating approach to another region of single relaxation associated with the low-temperature Arrhenius region. The relaxation times for the low-temperature crossover agreed well with those from the high-temperature curves, indicating complete linearity in the experiments. The spectrum of relaxation times was slightly asymmetrical at constant pressure and very asymmetrical at constant volume.     

Effect of Nd2O3 Concentration on the Defect Structure of CeO2–Nd2O3 Solid Solution

An extensive X-ray study of CeO₂–Nd₂O₃ solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO_1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO_1.5 were synthesized by coprecipitation from Ce(NO₃)₃ and Nd(NO₃)₃ aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO₂–NdO_1.5 solid solutions with 0–40 mol% NdO_1.5, which changed to a rare earth C-type structure at 45–75 mol% NdO_1.5. The change in the lattice parameters of CeO₂–NdO_1.5 solid solutions, when plotted with respect to the NdO_1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C-type regions. The maximum solubility limit for NdO_1.5 in CeO₂ solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO_1.5) of solid solution.     

Stable and Metastable Phase Relationships in the System ZrO2–ErO1.5

Stable and metastable phase relationships in the system ZrO₂–ErO_1.5 were investigated using homogeneous samples prepared by rapid quenching of melts and by arc melting. The rapidly quenched samples were annealed in air for 48 h at 1690°C or for 8 months at 1315°C. Two tetragonal phases ( t - and t '-phases) were observed after quenching samples heated at 1690°C to a room temperature, whereas one t -phase and cubic ( c -) phase were found in those treated at 1315°C. Since the t '-phase is obtained through a diffusionless transformation during cooling from a high-temperature c -phase, t - and c -phases can coexist at high temperature. The t - and c -phases field spans from 4 to 10 mol% ErO_1.5 at 1690°C and from 3 to 15 mol% ErO_1.5 at 1315°C. The equilibrium temperature T ^t-m ₀ between the t - and monoclinic ( m -) phases estimated from A_s and M_s temperatures decreased with increasing ErO_1.5 contents.     

The Ba(Ti,Zr)O3–(Zr,Ti)O2 Field in the Phase System BaO–TiO2–ZrO2

The two-phase field involves a ZrO₂-TiO₂ solid solution containing no more than 4 mol% TiO₂, and a BaTiO₃-BaZrO₃ solid solution containing a maximum of 75 mol% BaTiO₃. The field narrows above 1300°C, probably because of the intrusion of a liquid-phase field into the ternary, beginning near the BaO-TiO₂ edge.     

Effect of Small Additions of Al2O3 and Ga2O3 on the Immiscibility Temperature of Na2O-SiO2 Glasses

Alumina and gallia were substituted separately for Na₂O in amounts of 0.2, 0.5, 1.0, 1.5, 2.0, and 3.0 wt% in three Na₂O-SiO₂ glass compositions (82, 84, and 86 wt% SiO₂) within the immiscibility region. The immiscibility regions for each system extend to ∼1.5 mol% of the added oxide. In general, the addition reduced the immiscibility temperature ( T _m), but at the edge of the immiscibility region (82% SiO₂) the Na₂O loss effect initially increased T _m. A structural model of the miscibility of Al₂O₃ added to silicate glasses is presented.     

Subsolidus Phase Relations in the Pseudoternary System ZrO2-YO1.5-CrO1.5 in Air

The pseudoternary system ZrO₂-YO_1.5CrO_1.5 was studied between 1300° and 1600C in air by °a quenching method. No ordered phase of the type ZrY₆O₁₁ was detected, but an ordered Zr₃Y₄O₁₂ phase at 1300°C and YCrO₃ were observed as intermediate compounds. Solid solutions ofZrO₂ and YO_1.5 coexisting with CrO_1.5 and/or YCrO₃ formed; the apex occurred between 26.5 and 27.5 wt% YO_1.5 for the cubic ZrO₂+CrO_1.5+YCrO₃, three-phase region; CrO_1.5 is slightly soluble in ZrO₂(ss).     

Reinvestigation of the System Na3AlF6-Li3AlF6

A partial phase diagram for the system Na₃AlF₆-Li₃AlF₆ was constructed from DTA and X-ray diffraction measurements. A region of solid solution extends from Na₃AlF₆ to the limiting composition Na₂LiAlF₆. At compositions between Na_1.5Li_1.5AlF₆ and NaLi₂AlF₆ a cubic phase resembling the mineral cryolithionite is stable over a narrow range of temperatures, which shifts progressively to higher temperatures with increasing lithium content. A single phase can be quenched from samples annealed within this range except near the ideal cryolithionite composition (Na_1.5Li_1.5AlF₆) where the upper temperature limit is too low to permit recombination of two solid phases to cryolithionite even with prolonged annealing. Cryolithionite precipitated from aqueous solution, a sample of the mineral, and the solid solutions containing from 53 to 65 mole % Li₃AlF₆ have identical powder patterns except for differences of lattice constants. Solid solubility of Na₃AlF₆ in Li₃AlF₆ reaches 30 mole % at the eutectic temperature.     

Lithium Ion-Conducting Glass–Ceramics of Li1.5Al0.5Ge1.5(PO4)3–xLi2O (x=0.0–0.20) with Good Electrical and Electrochemical Properties

NASICON-type structured Li_1.5Al_0.5Ge_1.5(PO₄)₃– x Li₂O Li-ion-conducting glass–ceramics were successfully prepared from as-prepared glasses. The differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance, and field emission scanning electron microscope results reveal that the excess Li₂O is not only incorporated into the crystal lattice of the NASICON-type structure but also exists as a secondary phase and acts as a nucleating agent to considerably promote the crystallization of the as-prepared glasses during heat treatment, leading to an improvement in the connection between the glass–ceramic grains and hence a dense microstructure with a uniform grain size. These beneficial effects enhance both the bulk and total ionic conductivities at room temperature, which reach 1.18 × 10⁻³ and 7.25 × 10⁻⁴ S/cm, respectively. In addition, the Li_1.5Al_0.5Ge_1.5(PO₄)₃–0.05Li₂O glass–ceramics display favorable electrochemical stability against lithium metal with an electrochemical window of about 6 V. The high ionic conductivity, good electrochemical stability, and wide electrochemical window of LAGP–0.05LO glass–ceramics suggest that they are promising solid-state electrolytes for all solid-state lithium batteries with high power density.     

Transformation-Toughened ZrO2: Correlations between Grain Size Control and Composition in the System ZrO2-Y2O3

The average grain size of ZrO₂(+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y₂O₃ content. The average grain size decreased by a factor of 4 to 5 for Y₂O₃ contents between 0.8 and 1.4 mol% and increased at Y₂O₃ contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y₂O₃-rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y₂O₃. Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent.     

Phase Relations in the System CoNb2O6–CoTa2O6

The pseudobinary system CoNb₂O₅–CoTa₂O₆ was investigated. CoNb₂O₆ crystallizes in either the columbite or rutile structure, whereas CoTa₂O₆ assumes only the trirutile structure. In an argon atmosphere at about 1400°C, CoNb₂O₆ undergoes a phase transition from columbite or rutile. Between 1000° and 1400°C the solubility of CoTa₂O₆ in CoNb₂O₆ is about 10 mole %; in the same temperature region the solubility of CoNb₂O₆ in CoTa₂O₆ varies from about 40 to 70 mole %. The extensive solubility of CoNb₂O₆ in CoTa₂O₆ is explained by the ability of niobium to induce some disorder in the trirutile phase. The columbite to rutile transformation is also discussed on this basis.     

Analytical Microscopy Study of Phases and Fracture in Y2O3-La2O3 Alloys

Transmission electron microscopic analyses defined the structures and compositions in single-phase and two-phase La₂O₃-doped Y₂O₃ materials fabricated by the transient solid second-phase sintering. The composition in single-phase, 10-mol%-La₂O₃-doped, sintered and annealed samples was found to be uniform, indicating that diffusivity was sufficiently high for homogenization in the single-phase field. Two-phase, 16-mol%-La₂O₃-doped, sintered and annealed samples showed two morphologies: (1) intragranular, lath-like, monoclinic second-phase particles (twinned and untwinned) and (2) equiaxed cubic matrix. The second-phase particles were identified as the monoclinic phase derived from the high-temperature hexagonal phase through a rapid phase transition. A short, high-temperature anneal (2200°C for 1 min) of 9 mol% La₂O₃-Y₂O₃ composition was found to retain the hexagonal phase. Microchemical analyses of the phases suggested adjustments to the Y₂O₃-La₂O₃ phase diagram. Observation of the interactions of the intragranular second-phase particles with crack propagation indicated crack deflection as one of the mechanisms responsible for toughening (1.5 vs 0.9 MPa · m^1/2).