Uranium Oxide Phase Equilibrium Systems: I, UO2–Al2O3

The UO₂–Al₂O₃ phase equilibrium system was found to contain no new compounds or solid solutions. Uranium dioxide melted at 2878°± 22°C. and Al₂O₃ melted at 2034°± 16°C. The eutectic temperature was approximately 1930°C. There is an indication that two immiscible liquids formed above the eutectic temperature between 53 and 74 mole % Al₂O₃.     

Oxidation of ZrB2–SiC: Influence of SiC Content on Solid and Liquid Oxide Phase Formation

The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB₂–SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB₂–SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB₂–SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB₂–SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO₂–SiO₂–B₂O₃ (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO₂ precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO₂ because they have less B₂O₃, which results in a smaller amount of precipitated ZrO₂ and consequently fewer convection cells.     

A Study of the Phase Relations of ZrO2–TiO2 and ZrO2–TiO2–SiO2

The phase relations of the systems ZrO₂–TiO₂ and ZrO₂–TiO₂–SiO₂ were investigated. X-ray diffraction techniques served as the principal means of analysis. The binary system ZrO₂–TiO₂ was found to be one of partial solid solutions with no intermediate compounds. A eutectic point was found to exist at 50 to 55 weight % ZrO₂ and 1600°C. A preliminary investigation of the ternary system ZrO₂–TiO₂–SiO₂, although not extensive, resulted in a better understanding of this system, with a fairly accurate location of some of its boundary lines. A eutectic point was located at 2% ZrO₂, 10% TiO₂, and 88% SiO₂ at approximately 1500°C.     

Systematics of Phase Transition and Mixing Energetics in Rare Earth, Yttrium, and Scandium Stabilized Zirconia and Hafnia

Energetics of rare earth, yttrium, and scandium stabilized zirconia and hafnia have been systematically investigated by oxide melt solution calorimetry. The enthalpies of formation with respect to the oxide end members were simultaneously fit to a quadratic function to extract interaction parameters and enthalpies of transition of the oxide end members to the fluorite structure. ZrO₂–SmO_1.5 and HfO₂–SmO_1.5 show the most exothermic enthalpies of formation and interaction parameters, whereas ZrO₂–ScO_1.5 has the least exothermic enthalpy of formation and interaction parameter. This suggests that the ZrO₂–ScO_1.5 system shows the least short range order among all investigated systems, consistent with its high ionic conductivity. The extrapolated enthalpy of transition of the rare earth oxide end members to the cubic fluorite structure increase to more endothermic values with decreasing cation size. The γ-cubic fluorite phase transition in ZrO₂–ScO_1.5 was investigated by differential scanning calorimetry (DSC). The phase transition is reversible, occurs at 1000°–1200°C and shows hysteresis (∼100°C). The enthalpy of transition is endothermic on heating and increases from 1.7±0.1 kJ/mol (22 mol% ScO_1.5) to 2.9±0.2 kJ/mol (30 mol% ScO_1.5).     

Phase Equilibrium in the System PbO-TiO2–ZrO2

Phase equilibrium relations in the system PbO–TiO₂–ZrO₂ were studied by quenching in the range where the PbO content is 50 mole % and more. Isotherms were examined at 1100°, 1200°, and 1300°C and tie lines were determined between the liquid and solid solution in equilibrium. The incongruent melting point of PbZrO₃ was 1570°C and the equilibrium between liquid, PbO-type solid, and PbZrO₃ is peritectic. Pb(Zr,Ti)O₃ solid solutions containing more than 14 mole % PbZrO₃ decomposed to liquid, ZrO₂, and Pb(Zr,Ti)O₃ and the decomposition temperature rises from 1340° to 1570°C with increasing PbZrO₃ content. The system PbTiO₃–PbZrO₃ should not be treated as a binary, but as a section of the ternary system.     

Preparation and Properties of Calcium Zirconate

The preparation of CaZrO₃ by solid-state reactions at 1450° to 2000°C. is described. A very stable material was formed by the reaction of CaCO₃ and ZrO₂, in equimolar proportions, at a final temperature of 1850°C. Lattice constant values for this material agreed with data reported by previous investigators. The linear coefficient of thermal expansion for CaZrO₈ over the temperature range 25° to 1300°C. was found to be 11.5 × 10⁻⁶ per °C. Specimens of CaZrO₃ were compatible with ZrO₂, MgO, Al₂O₃, and BeO but reacted with SiC and mullite.     

Immiscibility Area in the System TiO2–ZrO2–SiO2

A furnace for use in conjunction with the X-ray spectrometer was developed which was capable of heating small powdered specimens in air to temperatures as high as 1850°C. This furnace was also used for the heating and quenching of specimens in air from temperatures as high as 1850°C. An area of two liquids coexisting between 20 and 93 weight % TiO₂ above 1765°± 10°C. was found to exist in the system TiO₂–SiO₂, which is in substantial agreement with the previous work of other investigators. The area of immiscibility in the system TiO₂–SiO₂ was found to extend well into the system TiO₂–ZrO₂–SiO₂. The two liquids were found to coexist over a major portion of the TiO₂ (rutile) primary-phase area with TiO₂ (rutile) being the primary crystal beneath both liquids. The temperature of two-liquid formation in the ternary was found to fall about 80°C. with the first additions of ZrO₂ up to 3%. With larger amounts of ZrO₂ the change in the temperature of the boundary of the two-liquid area was so slight as to be within the limits of error of the temperature measurement. Primary-phase fields for TiO₂ (rutile), tetragonal ZrO₂, and ZrTiO₄ were found to exist in the system TiO₂–ZrO₂–SiO₂. SiO₂ as high cristobalite is known to exist in the system TiO₂–ZrO₂–SiO₂.     

Phase Diagram of the Alumina–Zirconia–Samaria System

The phase diagram of the Al₂O₃–ZrO₂–Sm₂O₃ system was constructed in the temperature range 1250°–2800°C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this ternary system. Two new ternary and one new binary eutectics were found. The minimum melting temperature is 1680°C and it corresponds to the ternary eutectic Al₂O₃+F-ZrO₂+SmAlO₃. The solidus surface projection, the schematic of the alloy crystallization path, and the vertical sections present the complete phase diagram of the Al₂O₃–ZrO₂–Sm₂O₃ system.     

Phase Equilibrium Relationships in a Portion of the System MgO–Al2O3–2CaO·SiO2

The system MgO–Al₂O₃–2CaO·SiO₂ comprises a plane through the tetrahedron CaO–MgO–Al₂O₃–SiO₂. A total of 108 compositions were prepared having an alumina content below the line joining 2CaO·Al₂O₃SiO₂ (gehlenite) and MgO·Al₂O₃ (spinel). Quenching experiments were carried out on 96 of these compositions at temperatures up to 1590°C. Three binary eutectic systems and two ternary eutectic systems are described. Compositions on this plane are of significance in an investigation of the constitution of basic refractory clinkers made from Canadian dolomitic magnesites. They also concern the compositions of certain blast furnace slags.     

Phase Equilibria and Structural Species in NdCl3–NaCl, NdCl3–CaCl2, PrCl3–NaCl, and PrCl3–CaCl2 Systems

Equilibrium phase diagrams for the systems NdCl₃–CaCl₂ and NdCl₃–NaCl were determined by differential thermal analysis. A simple eutectic was observed at 59 ± 1 mol% CaCl₂ and 600°± 2°C in the NdCl₃–CaCl₂ system. A compound NaCl.3NdCl₃ which melts incongruently at 545°± 5°C to NdCl₃ and a liquid containing approximately 47 mol% NaCl, and a eutectic at 68 mol% NaCl and 439°± 2°C were found in the NdCl₃–NaCl system. On the basis of agreements between the activities calculated by the Clausius–Clapeyron equation and Temkin's model using the present data for the NdCl₃–CaCl₂ system and the literature data for the PrCl₃–CaCl₂ system, the melts in the former system consist of Nd³⁺, Ca²⁺, and Cl⁻ ions and in the latter system of Pr³⁺, Ca²⁺, and Cl⁻ ions. The above approach indicates the presence of Na⁺, Cl⁻, and NdCl^2-₅ ions in the NaCl-rich melts and Nd³⁺, Cl⁻, and NdCl⁻₄ in the NdCl₃-rich melts in the NdCl₃–NaCl system. Analogous ions were indicated in the melts of the PrCl₃–NaCl system.     

Zirconia Transport by Liquid Convection during Oxidation of Zirconium Diboride–Silicon Carbide

During high-temperature oxidation of ZrB₂–SiC composites, a multi-layer oxide scale forms with a silica-rich borosilicate liquid as the surface oxide layer. Here, a recently proposed novel mechanism for the high-temperature oxidation of ZrB₂–SiC composites is further investigated and verified. This mechanism involves the formation of convection cells in the oxide surface layer during high-temperature oxidation of the composite. The formation of zirconia deposits found in the center of the convection cells is proposed here to be the consequence of liquid transport. The nature and deposition mechanism of the zirconia is reported in detail, using calculated phase equilibrium diagrams and microstructure observations of a ZrB₂-15 vol% SiC composite tested at 1550° and 1700°C in ambient air for various times. The calculated phase equilibrium diagrams for the binary ZrO₂–B₂O₃ system as well as the ternary B₂O₃–SiO₂–ZrO₂ system at 1500°C are reported here to interpret these results.     

Alkaline-Earth Tungstates: Equilibrium and Stability in the M-W-O Systems

Phase relations in the systems alkaline earth oxide-tungsten trioxide and their stability with metallic tungsten were investigated by the quenching technique using sealed capsules. In the system BeO-WO₂, no intermediate compounds were formed. Binary mixtures resulted in a eutectic at 1185°C and 37 mole % BeO. In the system MgO-WO₃, the 1:1 compound was stable and melted congruently at 1358°C. There are two crystalline modifications of this compound. The well-known wolframite-type form is stable below 1165°C. Two eutectics were found: 1120°C and 28.5 mole % MgO and 1318°C and 55.0 mole % MgO. In the systems CaO-WO₃, SrO-WO₃, and BaO-WO₃, two binary compounds are stable. The 1:1 compounds with the scheelite-type structure melt, respectively, at 1580°, 1535°, and 1475°C, and form eutectics with WO₃ at 1135°, 1073°, and 935°C, all with a eutectic composition near 75 mole % WO₃. The 3:1 compounds having the distorted (NH,)₃FeF₆-type structure melt at ∼2250°, 2225°, and 1795°C. The eutectics between these two compounds are at 1490°C and 56.5 mole % CaO, 1410°C and 57.0 mole % SrO, and 1320°C and 58.2 mole % BaO. Phase transformations to an ideal (NH₄)₃FeF₆-type structure in Sr₃WO₆ and Ba₃WO₆, were observed at 1100° and 805° C, respectively, by application of both high-temperature X-ray diffraction and DTA. At 1700°C, metallic tungsten exists in equilibrium with liquid, the alkaline earth oxides, W₁₈O₄₉, the 3:1 ternary oxides, and with combinations thereof.     

Phase Diagram of Wollastonite-Zirconia

A reexamination of the system CaO·SiO₂–ZrO₂ has been conducted in order to use this system to obtain ZrO₂-toughened wollastonite materials. The results have shown that CaO·SiO₂–ZrO₂ is a pseudobinary system with an invariant peritectic point at 1467°± 2°C, which is in disagreement with previously reported results.     

Experimental Establishment of the CaAl2O4–MgO and CaAl4O7–MgO Isoplethal Sections within the Al2O3–MgO–CaO Ternary System

The isoplethal sections CaAl₂O₄–MgO and CaAl₄O₇–MgO of the Al₂O₃–MgO–CaO ternary system have been experimentally established at 1 bar total pressure and air of normal humidity. The sections obtained provide new data and information that are in disagreement with thermodynamic evaluations and optimizations of the Al₂O₃–MgO–CaO ternary system published to date. These differences arise mainly from the inclusion, or exclusion, of the binary compound Ca₁₂Al₁₄O₃₃, mayenite, as a stable phase in the reported studies of the system. The presence or absence of this compound within the system has an important impact on the solid state and melting relationships of the whole ternary system. The present study confirms the solid-state compatibility CaAl₂O₄–MgO and CaAl₂O₄–MgO–MgAl₂O₄ up to 1372°± 2°C, the peritectic melting point of the later mentioned subsystem.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Sintering of Ceria-Doped Tetragonal Zirconia Crystallized in Organic Solvents, Water, and Air

Amorphous CeO₂–ZrO₂ gels were prepared by coprecipitation in ammonia solutions. The onset of crystallization of the gels, from calcining in air, was 420°C, while 200° to 250°C in the presence of water and organic solvents such as methanol and ethanol. The sintering behaviors of CeO₂–ZrO₂ powders were sensitive to the crystallizing conditions, since hard agglomerates formed when the precipitated gels were crystallized by normal calcination in air, whereas soft agglomerates formed when they were crystallized in water or organic solvents. CeO₂–ZrO₂ powders crystallized in methanol and water at 250°C were sintered to full theoretical density at 1150° and 1400°C, respectively, whereas that crystallized by calcination in air at 450°C was sintered to only 95.2% of theoretical density, even at 1500°C.     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

Some Properties of the Oxides of Vanadium and Their Compounds

Vanadium tetroxide and vanadium pentoxide were prepared and some of their physical properties were measured. A brief survey was then made of some of their binary oxide compounds. Various mixtures of V₂O₄ or V₂O₆ and BeO, MgO, CaO, SrO, BaO, Al₂O₃, SiO₂, TiO₂, CeO₂, ZrO₂, Nb₂O₆, and U₃O₈ were heated. When compounds were formed, some of their properties were determined. Refractoriness, thermal expansion, and optical properties were considered of special interest. Vanadium pentoxide was found to have a linear thermal expansion of only 0.63 × 10⁻⁶ per °C. from 30° to 450°C.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

High-Temperature Phase Diagram for the System Zr.

The melting, eutectic, peritectic, solidus, and liquidus temperatures in the system Zr-O have been measured directly by a simple optical pyrometric technique requiring only a few hundred milligrams of sample. The saturation solubility of oxygen in α-Zr( s ) between 1270° and 1980°C and the lower phase boundary of the ZrO₂–_α phase between 1900° and 2400°C have been measured by an isopiestic equilibration method. The oxygen solubility limit in α-Zr( s ) agrees well with previous low-temperature studies and reaches a maximum solubility of 35°1 at.% O at the eutectic temperature, 2065°°5°C. The maximum melting temperature of α-Zr( ss ) is 2130°°10°C and corresponds to a composition of 25°1 at.% O. Both of these temperatures are approximately 150° higher than previously reported. Liquidus compositions above the eutectic temperature were obtained via mass spectrometry from the kinetic behavior of the liquid solution-ZrO_2–x( s ) mixture as it approached equilibrium at 2125°°5°C. The lower phase boundary or solidus of the ZrO_2–x phase departs appreciably from ideal stoichiometry above 1900°C and smoothly reaches its most reduced composition, 61 at.% (ZrO_1.56), near 2300°C. The solidus is retrograde at higher temperatures. The melting temperature of the stoichiometric dioxide is 2710°°15°C.