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.     

Volatility of PuO2 in Nonreducing Atmospheres

The vapor pressure of plutonium dioxide (PuO₂) was investigated in the range 1450° to 1775°C in air, argon, and oxygen atmospheres by a transpiration technique. There were strong indications that PuO₂ can vaporize congruently or as a suboxide species, depending on the atmosphere. The δH°₂₉₈ for vaporization in 1 atm of oxygen is approximately 154,000 cal per mole. The estimated standard free energy of formation (δG°_f) of gaseous PuO₂ is −121,000 + 10.7 T from 1227° to 1827°C.     

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.     

Melting Behavior in the System UO2–PuO2

The melting points of UO₂ and PuO₂ in a helium atmosphere were found to be 2730°× 30° C and 2280°× 30° C, respectively. With the exception of a melting maximum at the composition 90 UO₂–10 wt% PuO₂, the liquidus exhibits good continuity and agrees well with that calculated from thermodynamic data. X-ray diffraction data on melted PuO₂ and UO₂-PuO₂ solid solutions indicate that oxygen is evolved during melting but that no reduction to a second-phase plutonium suboxide occurs. The oxygen-plutonium atomic ratio of melted PuO₂ is 1.62, so that the 2280° C temperature reported here is the result of a dissociation reaction and is considered to be a pseudo melting point. Lattice parameters of melted UO₂–PuO₂ samples vary linearly with composition but are approximately 0.2% greater than anticipated because of an oxygen deficiency.     

High-Temperature Behavior of MoSi2 and Mo5Si3

The high-temperature stability and behavior of MoSi₂ was studied by heating dense sintered specimens under a vacuum of 10⁻⁵ mm Hg in the temperature range 1700° to 2000°C. The resulting material was examined using physical measurements, X-ray analysis, and metallographic techniques. The decomposition of MoSi₂ into Mo₅Si₃ is described. The Mo₅Si₃-MoSi₂ eutectic temperature was determined as 1900° C, and the melting points of MoSi₅ and Mo₅Si₃ were determined as 1980° and 2085° C, respectively.     

Effect of YO1.5 Dopant on Unit-Cell Parameters of ZrO2 at Low Contents of YO1.5

The effect of YO_1.5 dopant on unit-cell parameters of ZrO₂ (YO_1.5=0 to 14.6 mol%) were examined by the X-ray whole-powder-pattern decom-position technique. The unit cell of monoclinic ZrO₂ has the largest expansion along the direction perpen-dicular to (100). The rate of increase of the unit-cell volume of monoclinic ZrO₂ with YO_1.5 content is greater than that of tetragonal ZrO₂ and comparable to that of cubic ZrO₂.     

Syntheses and Unit-Cell Determination of Ba3V4O13 and Low-and High-Temperature Ba3P4O13

The syntheses and the results of unit-cell determinations ofBa₃V₄O₁₃ and the two forms (low- and high-temperature) of Ba₃P₄O₁₃ are presented. Ba₃V₄O₁₃ crystallizes in the monoclinic system, space group Cc or C2/c with unit-cell dimensions a=16.087, b=8.948, c=10.159 (x10nm), β=114.52° Low-Ba₃P₄O₁₃ crystallizes in the triclinic system, space group P1 or P1 with unit-cell dimensions a=5.757, b=7.243, c=8.104 (x10 nm) α=82.75°, β=73.94°, γ=70.71°. Low-Ba₃P₄O₁₃ transforms at 870°C into high-Ba₃P₄O₁₃ which crystallizes in the orthorhombic system, space group Pbcm (No. 57) (or Pbc2, No. 29) with unit-cell dimensions a =7.107, b=13.883, c=19.219 (x10 nm). No relations have been found between the structures of the tribarium tetravanadate and the tribarium tetraphosphate.     

Thermodynamic evaluation of the BaO-CaO-YO1.5 system

A series of experiments were performed to study the solid solubility of CaO in BaY₂O₄, and the observed results were then adopted to the present thermodynamic evaluation to derive a set of thermodynamic database for the BaO-CaO-YO_1.5 system. The database was constructed by the CALPHAD method where the binary parameters from the BaO-CaO and CaO-YO_1.5 systems were presently optimized, those from the BaO-YO_1.5 system were simulated by our previous assessments, and only limited amount of ternary parameters were introduced. All the model parameters were emanated from the Bragg-Williams approximation where the liquid and terminal solid-solution phases were treated by the one-sublattice model, and two ternary intermediate phases, named BCY (BaCa₂Y₆O₁₂) and BaY₂O₄, were described by the three-sublattice and two-sublattice models, respectively. Good agreement between the experimental data and the calculated results demonstrates that the present thermodynamic database is self-consistent and credible and able to be used to design novel refractory.     

Thermodynamic Assessment of the CaO–MgO–Al2O3 System

The system CaO–MgO–Al₃O₃ has been assessed with the Calphad technique using a computerized optimization procedure called parrot . The rather meager experimental information, mainly on liquidus relations, is described reasonably well, but the lack of data, especially on solid-phase relations, implies that the present assessment should be regarded as provisional. The system contains one stable ternary phase with the stoichiometry 3CaO·2Al₂O₃·MgO.     

Characteristics of PbO–BiO1.5–GaO1.5 Glasses Melted in SnO2 Crucibles

PbO-BiO_1.5-GaO_1.5-based glasses are good candidates for optical applications, because of some of their interesting characteristics, such as high refraction indices and high transmission in the ultraviolet (UV), visible (VIS), and infrared (IR) regions. A limited stage in the processing of these glasses is the corrosion that is caused by the melt in all currently used conventional crucibles, such as noble metals (platinum or gold) and Al₂O₃. The absorption of crucible material by the glass composition may reduce the transmission level, the cutoff in the UV-VIS, and IR regions, and the thermal stability. In this study, a SnO₂ crucible has been tested for PbO-BiO_1.5-GaO_1.5 molten glass. Optical and thermal analyses show, in some cases, advantages over the use of platinum and Al₂O₃ crucibles. A visible cutoff value of 474 nm has been measured, and a longer melting time (850°C for 4 h) results in a significant reduction of the O-H absorption band at 3.2 μm.     

Room-Temperature KIc Values for Single-Crystal and Polycrystalline MgAl2O4

The K_lc values for (100), (110), and (111) single-crystal MgAl₂O₄ as well as those for polycrystalline MgAl₂O₄ with transgranular and intergranular fractures are presented and discussed on the basis of their elastic moduli. It is observed that the single-crystal toughnesses are directly related to the elastic modulus. Polycrystalline and single-crystal toughnesses are comparable; however, the intergranular fracture has the lowest toughness.     

High-Temperature Deformation of Stoichiometric 239PuO2

The deformation behavior of Stoichiometric ²³⁹PuO₂ was examined at 800° to 1500°C, using direct and diametral compression. Maximum ductility was observed at 1000°C, but above this temperature both strength and ductility decreased and the fracture mode changed from transgranular to inter-granular. The deformation activation energy measured at 1000°C was 598 kJ/mol. Comparison to the deformation behavior of hypostoichiometric ²³⁹PuO_2-x suggests that high-temperature dislocation motion becomes more difficult with increasing O/Pu ratio due to effects of stoichiometry on diffusion rates. Deformation mechanisms in ²³⁹PuO₂ appear to be a combination of dislocation motion and grain-boundary sliding.     

Thermodynamic Analysis of the Cubic–Tetragonal Phase Equilibria in the System ZrO2–YO1.5

The cubic-tetragonal (c-t) phase equilibria in the system ZrO₂-YO_1.5 are thermodynamically analyzed from Landau's phenomenological theory. The calculated c-t two-phase field is depicted as a miscibility gap with a sharp maximum and the spinodal region as originally predicted by Hillert and Sakuma. However, the observed c-t two-phase field and the spinodal region are better described by the present model. In addition, this model can be used to discuss the nature of the c-t diffusionless transformation from the order parameter in contrast with the original model. The predicted change in the tetragonality of t-ZrO₂ with YO_1.5 content is slightly different from that in the c/a axial ratio estimated from X-ray diffraction analysis. The displacement of cations and anions may not take place simultaneously during the c-t transformation.     

High-Temperature Deformation of Hypostoichiometric 239PuO2

The high-temperature deformation of hypostoichiometric ²³⁹PuO₂ was investigated using constant-displacement-rate compression tests. Considerable ductility (10 to 25% true strain) was observed at 800° to 1500°C. Yield stresses decreased with increasing temperature and decreasing strain rate. Strain-rate sensitivity values increased from 0.011 at 800° to 0.205 at 1500°C, whereas deformation activation energies were 121 to 157 kcal/mol. Deformed microstructures showed no evidence of grain-boundary cracking, suggesting that dislocation motion is the dominant high-temperature deformation mechanism.