Review and Chemical Thermodynamic Representation of 〈U1–zCezO2±x〉 and 〈U1–zLnzO2±x); Ln = Y, La; Nd, Gd

The entire data base for the dependence of the nonstoichiometry, x , on temperature and chemical potential of oxygen (oxygen potential) in 〈U_{1– z} Ce _z O_{2+ x} 〉 and 〈U_{1– z} Ln _z O_{2+ x} 〉 was retrieved from the literature and represented by a thermodynamic method. The method reproduces the behavior of the experimental data and also results in partial molal Gibbs free energy quantities that are useful for any thermodynamic calculation involving these nonstoichiometric phases. The behavior of these systems is also compared with that for 〈U_{1– z} Pu _z O_{2± x} 〉.     

Volatility of UO2±x and Phase Relations in the System Uranium—Oxygen

The volatility of UO_2±x and the phase relations in the system uranium-oxygen were studied using thermogravimetric techniques. Chemical reactions describing the loss of uranium from UO_2±x at temperatures between 1100° and 2200°C in oxygen pressures between approximately 10² and 10⁻⁶ torr are proposed. Results were obtained requiring the consideration of UO₄(g) as the uranium-bearing vapor species above UO_2±x. Evidence supporting the existence of UO₄(g) included the volatilization of material with an oxygen-to-uranium ratio of 4 during the decomposition of UO_2+x(0.Z > × > 0) to near-stoichiometric UO₂ in vacuum above 1500°C and the dependence of the evaporation rate of the uranium dioxide on the oxygen pressure between 1200° and 1500°C. The equilibrium oxygen pressures over compositions between UO_2.02 and UO_2.63 in the UO_2+x and U₃O_8-y regions and over the boundary between these phases were measured between 1000° and 1600°C. The equilibrium oxygen-to-uranium ratio of UO_2±x was less than 2 above 1700°C in vacuum.     

Phase Relations in the System MgO-V2O3-VO2 at 1200°C

Phase relations in the quasi-ternary system MgO-V₂O₃-VO₂ at 1200°C were studied using the quenching technique under controlled O₂ atmospheres. A new phase of a type z VO _y Mg_{2− x} V_{1+ x} O₄ (0< x <1, y ≥1.5, z >0) was found with a compositional region along the MgV₂O₄-Mg₂VO₄ join. Equilibrium P _{O 2} observed for Mg_{2− x} V_{1+ x} O₄ is quite different from that for V _n O_{2 n -1} with an equal ratio of V³⁺/V⁴⁺, corresponding to the V³⁺ stabilities in two types of compounds. Thus, the phase relations in the ternary system were constructed on a conventional triaxial diagram as a function of P _O2.     

The Effect of Microstructure and Microcracking on the Thermal Conductivity of UO2-U4O9

Several microstructures were produced in dense polycrystalline UO₂-U₄O₉ specimens by cooling UO_2+x, solid solutions at different rates. Void formation (intragranular pores, grain-boundary microcracks) was observed in specimens which had a nonuniform distribution of the U₄O₉ precipitate; such formation was probably caused by the relief of stresses generated by differential thermal contraction. The microstructures were assessed quantitatively and used to calculate thermal conductivities from theoretical models. The effect of microcracking was calculated. Thermal conductivities were also determined from thermal diffusivity values measured by the laser-flash method. The thermal conductivity was primarily influenced by the amount of U₄O₉, present and the occurrence of voids, whereas the effect of U₄O₉, distribution was minor. Good agreement was obtained between theoretical predictions of thermal conductivity based on microstructure and experimental values.     

Phase Relations in Crystalline Ceramic Nuclear Waste Forms: The System UO2+x-CeO2-ZrO2-ThO2 at 1200°C in Air

Steady-state phase relations in the system UO_2+x-CeO_z-ZrO₂-ThO₂ were determined for application to phase relations in high-level crystalline ceramic nuclear waste forms. Samples were treated at 1200°C at an oxygen partial pressure of 21.3 kPa and a total pressure of 101.3 kPa. Phase assemblages were found to be comprised of cubic solid solutions of the fluorite-structure type, solid solutions based on ZrO₂, and orthorhombic solid solutions based on U₃O₈.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

Composition Limits and Magnetic Properties of Sintered Y2.66Gd0.34FexAl0.677Mn0.09O12 Garnets

Single-phase garnet solid solutions can be synthesized between the composition limits of x =4.18 and x =4.22 in Y_2.66Gd_0.34Fe _x Al_0.677Mn_0.09O₁₂ at temperatures between 1340° and 1500°C in O₂. Solid solutions occur only on the Y₂O₃-excess side of the stoichiometric garnet composition. Electromagnetic properties and microstructural features of sintered garnets depend critically on small changes in Fe content in the vicinity of the garnet solid-solution region. An intergranular spinel-type second phase exists for compositions when x >4.22 and has a deleterious effect on remanent induction and magnetic loss at 3 GHz. The relative density of powder compacts sintered for 16 h at 1500°C in O₂ increases with increasing Fe content (i.e. as x increases) in the garnet solid solution.     

Thermodynamic Stability of K+-β-Alumina

K₂O activity in K⁺-(α+β)-alumina was determined from emf data of the galvanic cell Pt,O₂,(α+β)-alumina/K⁺-β-alumina/K₂SO₄,SO₂+SO₃+O₂,Pt. K₂O activity in the K⁺-(α+β)-alumina was expressed by the equation log a_k2o (±0.038)=(−18295±120)(K/T)+(0.998±0.110), where 961 K+-β-alumina are discussed: the standard Gibbs energy of formation of K₂O·11Al₂O₃ from K₂O and α-Al₂O₃ and from K, O, and Al; the thermodynamic stabilities of K⁺-β-alumina in the atmospheres of SO_x (x=2, 3) and CO₂; the equilibrium vapor pressure of potassium over K⁺-β-alumina under a constant O₂ pressure; and the stability of K⁺-β-alumina in the molten Na-K alloy.     

Phase Equilibrium Relationships at Liquidus Temperatures in the System FeO–Fe2O3–Al2O3–SiO2

Phase equilibrium data at liquidus temperatures are presented for mixtures in the system FeO–Fe₂O₃–Al₂O₃–SiO₂. The volume located between the 1 and 0.2 atm. O₂ isobaric surfaces of the tetrahedron representing this system was studied in detail. Scattered data were obtained at lower O₂ pressures. Results obtained in the present investigation were combined with data in the literature to construct a phase equilibrium diagram, at liquidus temperatures, for the entire system FeO–Fe₂O₃–Al 2 O₃–SiO₂. Methods for interpretation of the diagram are explained.     

A- and B Site Cosubstituted Ba6−3xSm8+2xTi18O54Microwave Dielectric Ceramics

Tin (Sn) substitution into the B-site and Nd/Sn cosubstitution into the A- and B-sites were investigated in a Ba _{6−3 x} Sm_{8+2 x} Ti₁₈O₅₄solid solution ( x = 2/3). A small amount of tin substitution for titanium improved the temperature coefficient of resonant frequency (τ_f) but led to a decrease of the relative dielectric constant (ɛ) and the quality factor ( Qf ). The Ba_{6−3 x} Sm_{8+2 x} (Ti_{1− z} Sn_z)₁₈O₅₄-based tungsten-bronze phase became unstable for compositions with a tin content of ≥10 mol%, where BaSm₂O₄and Sm₂(Sn,Ti)₂O₇appeared, and finally, these phases became the major phases. On the other hand, Nd/Sn cosubstitution led to a good combination of high ɛ, high Qf , and near-zero τ_f. Excellent microwave dielectric properties were achieved in Ba_{6−3 x} (Sm_{1− y} Nd _y )_{8+2 x} (Ti_{1− z} Sn _z )₁₈O₅₄ceramics with y = 0.8 and z = 0.05 sintered at 1360°C for 3 h: ɛ= 82, Qf = 10 000 GHz, and calculated τ_f=+17 ppm/°C. The tolerance factor and electronegativity difference exhibited important effects on the microwave dielectric properties, especially the Qf value. A large tolerance factor and high electronegativity difference generally led to a higher Qf value.     

Preparation and Electrical Properties of New Oxide Ion Conductors Ce6−xGdxMoO15−δ (0.0≤x≤1.8)

A series of oxide ion conductors Ce_{6− x} Gd _x MoO_15−δ (0.0≤ x ≤1.8) have been prepared by the sol–gel method. Their properties were characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), Raman, IR, X-ray photoelectron spectroscopy (XPS), and AC impedance spectroscopy. The XRD patterns showed that the materials were single phase with a cubic fluorite structure. The conductivity of Ce_{6− x} Gd _x MoO_15−δ increases as x increases and reaches the maximum at x =0.15. The conductivity of Ce_4.5Gd_1.5MoO_15−δ is σ_t=3.6 × 10⁻³ S/cm at 700°C, which is higher than that of Ce_4.5/6Gd_1.5/6O_2−δ (σ_t=2.6 × 10⁻³ S/cm), and the corresponding activation energy of Ce_4.5Gd_1.5MoO_15−δ (0.92 eV) is lower than that of Ce_4.5/6Gd_1.5/6O_2−δ (1.18 eV).     

Oxygen Potential of U0.85Zr0.15O2+x Solid Solutions at 15000°C

Oxygen potentials for U_0.85Zr_0.15O_2+x solid solutions with the fluorite structure (prepared at 1500°C) were measured at 1500°C by a thermogravimetric technique. The oxygen potentials were 12 to 20 kJ/mol lower than those of pure UO_2+x at a given oxygen-to-metal ratio; they cannot be represented as a function of mean uranium valence .     

Thermodynamic Evaluation for the Y2O3–BaO–CuOx System

The thermodynamic data for the Y₂O₃–BaO–Cu₂O–CuO quaternary system were optimized from measured thermodynamic data. A two-sublattice model for ionic solution was used to express the Gibbs free energy of the liquid phase, and a two-sublattice regular solution model was used for the nonstoichiometric YBa₂Cu₃O_6+δ superconducting compound. The optimized thermodynamic data were used to calculate the phase diagrams of the Cu₂O–CuO binary system and the CuO _x –Y₂Cu₂O₅ and CuO _x –BaCuO₂ quasi-binary systems. The results were in good agreement with reported measured data. The liquidus projection and isothermal and vertical sections of the Y₂O₃–BaO-CuO _x quasi-ternary system were calculated. The effect of oxygen pressure on some reaction temperatures was predicted by calculating them at various oxygen pressures, and the oxygen contents (6 +δ) in YBa₂Cu₃O_6+δ were calculated at various temperatures and oxygen pressures. The results were compared with experimental data.     

Thermal Expansion of Solid Solutions in the ZrTiO4—In2O3—M2O5 (M = Sb, Ta) System

Axial and dilatometric thermal expansions and phase transformations were studied for solid solutions having the α-PbO₂ structure in the ZrTiO₄—In₂O₃—M₂O₅ (M = Sb, Ta) system with nominal formulas of Zr _x Ti _y In _z Sb _z O₄ and Zr _x Ti _y In _z Ta _z O₄ where x + y + 2 z = 2. With increased substitution of z , the cell volume increased, the difference in the b parameters at room temperature between those quenched from 1400° and 1000°C decreased, and the thermal expansion decreased. The axial thermal expansion of ZrTi _y In _z · Ta _z O₄ with z = 0.3 was almost identical with that of HfTiO₄, and those with z = 0.4 and z = 0.45 were smaller than that of HfTiO₄. Unit-cell volumes of these compound were compared with those of single oxides to make it clear that the unit-cell volume of ZrTiO₄ was small anomalously and to distinguish the normal and abnormal substitution systems. These results were explained by the working hypothesis proposed for these compounds.     

Reactions During the Processing of U3O8—Al Cermet Fuels

The cermet fuel (U₃O₈ dispersed in Al) being considered for use in the Savannah River Site Reactors is thermodynamically unstable because of the potential for an exothermic metallothermic reduction reaction. This paper describes work performed to quantify the extent of reaction during powder metallurgy (P/M) processing of the U₃O₈—Al cermet fuel, and to determine the effect of partial reduction to U₄O₉ on the metallothermic reduction reaction. During the fabrication of the U₃O₈—Al cermet fuel by the P/M technique, a significant portion of the U₃O₈ is reduced to U₄O₉. The reaction between U₄O₉ and Al is also exothermic; however, the maximum heat released by the reaction is substantially less than that released for the U₃O₈—Al reaction, approximately 335 J (80 cal) per gram of oxide reacted compared to 940 J (225 cal). Metallothermic reduction reactions for U₃O₈/U₄O₉/Al mixtures do not occur at the normal reactor operating temperature, ∼ 370 K (∼ 100°C) or at temperatures below the melting point of aluminum, 930 K (660°C).     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

Thermodynamic Stability of UMoO6 by the Transpiration Method

The equilibrium vaporization of UMoO₆(s) in dry air was studied by the transpiration method in the temperature interval 1110°≤ T (K) ≤ 1250°. Apparent pressure of the trimer measured over the two-phase mixture UMoO₆+ U₃O₈, with dry air as the carrier gas, was used to calculate the partial pressure of the trimer, (MoO₃)₃(g). In accordance with the vaporization reaction (UMoO₆) = 1/3(U₃O₈) + 1/6(MoO₃)₃ (g) + 1/6O₂ (g), the free energy of formation of UMoO₆)(s) is given by (δ G° in kJ-mol⁻¹) δ G° (UMoO₆) = (−1962 ± 10) + (0.463 ± 0.008) T     

Phase Relations In The Pseudo-Ternary System La2O3–SrO–Fe2O3

The phase relations in the pseudo-ternary system La₂O₃–SrO–Fe₂O₃ have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr _{n +1− v} La _v Fe _n O_{3 n +1−δ} ( n =1, 2, 3, and ∞) and Sr_{1− x} La _x Fe₁₂O₁₉, while only limited solubility of La in Sr_{4− z} La _z Fe₆O_13±δ was observed. At high Fe₂O₃ content, a liquid with low La₂O₃ content was stable at 1300°C.     

Phase Equilibrium Studies in the System Iron Oxide- A12O3 in Air and at 1 Atm. O2 Pressure

Phase equilibrium data, obtained by using the quenching technique, are presented for the system iron oxide-A1₂O₃ in air and at 1 atm. O₂ pressure in the temperature interval 1085° to 1725°C. Stability ranges of the various phases are delineated, and approximate compositions of crystalline and liquid phases are determined. Special attention is focused on the phase Fe₂O₃·A1₂O₃(ss) and its stability relationships.