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.     

Phase Composition and Compatibilities at 930° to 950°C in the System Europium(III) Oxide-Barium Oxide-Copper(II) Oxide in Air

Phase composition and compatibilites at 930° to 950°C were determined for the system Eu₂O₃–BaO–CuO in air. The binary compound Eu₂CuO₄ dissolves Ba to the extent 0 x 0.02 in Eu_2-xBa_xCuO₄, whereas the other binary compounds, Eu₂BaO₄ and BaCuO₂, do not exhibit solid solubility. Three ternary compounds were obtained, Eu₂BaCuO₅ and two solid solution phases. The first contains the 90 K '123' superconductor and has solubility limits represented by Eu_1+xBa_2-xCu₃O_7±y, where 0 x 0.5. The second has a solubility limit represented as Eu_1+xBa_8-xCu₄O_y, where 0 x 0.44. The limited solid solution range of this phase provides insight concerning the probable solid solution range of the analogous phase in the Y₂O₃-BaO-CuO system.     

Solid Phase Relations at 950°C in La-Ba-Cu-O

Subsolidus phase relations in the La₂O₃–BaO–CuO system were studied at 950°C. Three previously reported binary compounds exist (La₂CuO₄, BaLa₂O₄, and BaCuO₂) and five previously reported ternary phases occur (La_2-xBa_xCuO_4-(x/2)+δ, La_4-2xBa_2+2xCu_2-xO_10-2x, La_2-xBa_1+xCu₂O_6-(x-2), La_3-xBa_3+xCu₆O_14±δ, and La₄BaCu₅O_13+δ). Of the seven phases in the diagram, all but BaLa₂O₄, BaCuO₂, and La₄BaCu₅O_13+δ were shown to exhibit significant ranges of solubility. The diagram is important in that both >30 K (La_2-xBa_xCuO_4-(x/2)+δ) and >90 K (La_3-xBa_3+xCu₆O_14+δ, x=1) superconductors occur.     

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.     

Assessment of the Thermodynamic Values for PuO1.5 and High-Temperature Determination of the Values for PuC1.5

Thermodynamic values for PUO_1.5 were assessed using an improved method for estimating fef ° 1.5 and new data for S°₂₉₈ 1.5. Based on the assessment, a value of ΔH°₂₉₈, 1.5=–828 kJ/mol is recommended. Measurements of (CO) pressure over the nominal equilibrium 1.5+ 1.5+ C were performed between 1348 and 1923 K, yielding pressures between 0.644 and 11600 Pa. Second- and third-law analyses were used to obtain a value for ΔH°₂₉₈ 1.5=–93.3°3.3 kJ/mol.     

Phase Equilibria of the La2O3-SrO-CuO System at 950°C and 10 kbar

Phase equilibria of the La₂O₃-SrO-CuO system have been determined at 950°C and 10 kbar (1 GPa). Stable phases at the apices of the ternary phase diagram are CuO, La₂O₃, and SrO. Stable intermediate phases are La₂CuO₄ in the LaO_1.5-CuO binary and Sr₂CuO₃, SrCuO₂, and Sr₁₄Cu₂₄O₄₁ in the CuO-SrO binary. The La_2-xSr _x CuO_4-δ solid solution is stable where 0.0 ≤ x ≤ 1.3, the La_2-xSr_1+xCu₂O_6+δ solid solution is stable where 0.0 ≤ x ≤ 0.2, the La_8-xSr _x Cu₈O_20-δ solid solution is stable where 1.3 ≤ x ≤ 2.7, the La _x Sr_14-x-Cu₂₄O₄₁ solid solution is stable where 0 ≤ x ≤ 6, and the La_1+xSr_2-xCu₂O_5.5+δ phase is stable where 0.04 ≤ x ≤ 0.16. The La₂O₃-SrO-CuO phase diagram at 950°C and 10 kbar is almost identical to that determined by other authors at 950°C and 1 atm, in terms of phase stability and solid-solution ranges.     

Transformation-Induced Twinning: The 90° and 180° Ferroelectric Domains in Tetragonal Barium Titanate

Ferroelectric twin domains resulting from the cubic ( c ) to tetragonal ( t ) phase transformation at the Curie point T _C≈130°C in pressureless-sintered BaTiO₃ ceramics, using TiO₂-excess powder, have been investigated using scanning and transmission electron microscopy. Both 90° and 180° domains were identified by spot splitting along characteristic crystallographic directions in the selected-area diffraction patterns and/or from the shape of domain boundaries. Lamellar domains were found predominantly with the 90° types. The 180° domain boundaries mostly appeared wavy in shape, while the 90° ones, having sharpened ends, attained a dagger shape. Failure of Friedel's law in the non-centrosymmetric t -BaTiO₃ was adopted to validate the existence of the 180° domains. The 90° domains with boundaries lying in     are reflection–inversion twins, and the 180° domains lying in {100) _t and }220) _t are inversion twins. Convergent-beam electron diffraction was performed to ensure that changing of the polar direction [001] _t across the 90° and the 180° domain boundaries was consistent with the domain type. It was also used to confirm whether the 180°-type walls are inversion domain boundaries produced by the loss of an inversion center when the cubic phase transforms into tetragonal symmetry. The formation of such ferroelectric domains is discussed with reference to the crystal symmetry reduction from     ( c -phase) to P 4 mm ( t -phase) with a loss of mirror plane (m) and roto-inversion axis     upon c → t phase transformation.     

Reaction of UC with Nitrogen from 1475° to 1700°C

The reaction of nitrogen with arc-melted UC was studied from 1475° to 1700°C at an N₂ pressure of about 400 torr. The reaction appeared to proceed toward equilibrium in distinct stages: (1) UC and N₂ reacted to form UC₂ and UC_0.8N_0.2; (2) UC₂ reacted with N₂ to produce more UC_0.3N_0.2 and free C; and (3) this U(C,N) reacted with N₂ to produce more free C and the equilibrium product, a high-N U(C,N). These results are consistent with known thermodynamic and phase behavior in the U-C-N system.     

The System Magnesia-Silica-Water Below 300°C.: I, Low-Temperature Phases from 100° to 300°C. and Their Properties

Reactions in the ternary system MgO-SiO₂-H₂O were studied over the temperature range 100° to 300° C. and were found to produce only two phases. Under conditions of 100° to 200° C. and atmospheric pressure up to 20,000 lb. per sq. in., and regardless of the initial MgO/SiO₂ ratio, the predominant magnesium silicate product was found to have a MgO/SiO₂ ratio of 1.5. In the range 200° to 300°C. at 210 to 20,000 lb. per sq. in. two stable phases, 3MgO.2SiO₂.2H₂O (I) and 3MgO.4SiO₂.H₂O (II), were observed. In this case the phase that was favored was determined by the molar ratio of MgO/SiO₂ of the reaction mixture at the start of the run. The physical and chemical properties of phases (I) and (II) resembled those of the natural minerals serpentine and talc respectively. Two different morphologies were observed in electron micrographs of phase (I). From 100° to 160°C. it occurred as crumpled foils, and at 170°C. fibrous crystallites, which resembled the natural asbestos mineral chrysotile, appeared at the expense of some of the foils.     

Zirconia-Related Phases in the Zirconia/Titanium Diffusion Couple After Annealing at 1100°–1550°C

A diffusion couple of 3 mol% Y₂O₃–ZrO₂ and titanium was isothermally annealed in argon at temperatures between 1100° and 1550°C. The phases and microstructure in the ceramic side were investigated using scanning electron microscopy and transmission electron microscopy, both attached to an energy-dispersive spectrometer. After annealing at 1100°C/6 h, zirconia grains did not grow conspicuously and evolved only traces of oxygen, resulting in t -ZrO_{2− x} but not α-Zr. At temperatures above 1300°C, a significant amount of oxygen evolved from zirconia, reducing the O/Zr ratio, such that α-Zr was excluded from t -ZrO_{2− x} during cooling, yielding a higher O/Zr ratio (≈2). When held at 1550°C/6 h, zirconia grains grew rapidly. The α-Zr was segregated on grain boundaries during cooling by the exsolution of zirconium from ZrO_{2− x} , while twinned t '-ZrO_{2− x} or lenticular t -ZrO_{2− x} , which was embedded in ordered c- ZrO_{2− x} , was found. The ordered c -ZrO_{2− x} was identified by the     {113} superlattice reflections of its electron diffraction patterns.     

Ionic Conductivity of the System Si-Al-O-N from 850° to 1400°C

Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si₃N₄-Al₂O₃, containing β-sialon, X phase, β-Si₃N₄, and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si⁴⁺ migration for the temperature range studied. Pure X phase shows ionic conduction by Si⁴⁺ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si₃N₄-Al₂O₃ containing β-sialon, β-Si₃N₄ X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively.     

Effect of Carbon Precursors on the Microstructure and Bonding State of a Boron–Carbon Compound Grown by LPCVD

Methane (CH₄) and propylene (C₃H₆) were used to fabricate a boron–carbon coating by a low-pressure chemical vapor deposition (LPCVD) technique. The effects of carbon precursors on the phase, microstructure, and bonding state of the deposits have been investigated. X-ray diffraction results show that the 2θ value of the deposit from the C₃H₆ precursor shifts to 25.78° when the coating is deposited at 1223 K, and shifts to 26.1° when deposited at 1273 K, compared with the 2θ value of the pyrocarbon (PyC) peak deposited by LPCVD, which is 25.42°, and no boron–carbon (B–C) compound peak exists. However, the phases of coating deposited from CH₄ include B₂₅C, B₁₃C₂, elemental carbon, and boron. X-ray photoelectron spectroscopy (XPS) results show that the percent contents of boron atom in the coatings from the CH₄ precursor are 61.18% and 67.78% when deposited at 1223 and 1273 K, respectively, much higher than that from the C₃H₆ precursor, 10.85% and 15.30%, respectively. Scanning electron microscopy (SEM) results show that the coatings deposited from CH₄ have a coarse crystal-like morphology; however, the coatings deposited from the C₃H₆ precursor are smooth. The formation of PyC from C₃H₆ is more facile than that from CH₄, which leads to differences in the phase, atom content, and microstructure of coatings from CH₄ and C₃H₆ precursors.     

β-Sialon Ceramics Prepared at 1700°C by Hot Isostatic Pressing

Dense, single-phase β-sialon ceramics were sintered at 1700°C and 200 MPa using the glass-encapsulated hot isostatic pressing technique. The materials were very hard, 1500 to 1700 kg / mm² (98 N load), but were fairly brittle, with an indention fracture toughness of about 3 MPa · m^1/2. The addition of 1 wt% Y₂O₃ before sintering had a positive effect on the toughness, especially at the low x compositions of Si_3-xAl_xO_xN_4-x, where K_IC∼4 MPa · m^1/2.     

Mn-Ta Oxides: Phase Relations at 1200°C

Subsolidus phase relations among oxides in the system Mn–Ta–O were experimentally determined by the quenching method. The conditions during the heating period were 1200°C, 1 atm total pressure, and various partial pressures of oxygen between 10^–17 atm and 1 atm. At the limits of this range, the stable assemblages at f -= 10^–17 atm are: MnO, Mn₆Ta₂O₁₁, Mn₄-Ta₂O₉, Mn_1.4 TaO_3.9, MnTa₂O₆, and β-Ta₂O₅; at p _o2= 1 atm they are: Mn₃O₄, Mn_1.4TaO_4.2, MnTa₂O₆, and β-Ta₂O₅. There are five univariant assemblages of three solid phases plus vapor in the phase diagram.     

1000°C Phase Relations and Superconductivity in the (Nd-Ce-Cu)-O Ternary System

The phase relations involving the 24 K n -type Nd_{2- x} Ce _x CuO₄ superconductor were investigated at 1000°C in air. The terminal solid solubility was confirmed to be x = 0.2. This solid solution is the only ternary phase in the Nd₂O₃–CeO₂–CuO diagram. A binary (1 − y )CeO₂– y NdO_1.5 solid solution exists out to y = 0.4. Phase diagrams for NdO_1.5–CeO₂–CuO (1000°C) and NdO_1.5–CeO₂ (900° to 1500°C) are presented.     

Phase Equilibria in the Quaternary System Ti-Al-C-N

The quaternary system Ti-Al-C-N and its binary and ternary boundary systems are investigated using powder methods and XRD analysis. Phase equilibria at 1375°C are presented in an isothermal network for alloys up to 50 at.% Ti. In the vertical section Ti₂AIC_1-x-Ti₂AlN_1-x a complete series of solid solutions exists at 1495°C, but a wide miscibility gap occurs at 1375°C. The vertical section Ti₃AlC_1-x-Ti₃AlN_1-x is more complex because of the occurrence of the quaternary, tetragonally distorted phase Ti₃Al(C,N)_1-x ( a = 0.41135(4) nm, c = 0.41366(5) nm) and the transformation of perovskite-type Ti₃AlN_1-x into filled Re₃B-type Ti₃AlN_1-x below 1200°C.     

Liquid–Solid Equilibria in the System NiO–TiO2–SiO2 in the Temperature Range 1430° to 1660°C

Equilibrium relations in the system NiO–TiO₂–SiO₂ in air have been investigated in the temperature range 1430° to 1660°C. The most conspicuous feature of the phase relations is the existence of a cation-excess spinel-type phase, in addition to NiO and NiTiO₃, on the liquidus surface and at subsolidus temperatures down to 1430°C. Three invariant points have been located on the liquidus. There is a peritectic at 1540°C characterized by coexisting NiO ( ss ), spinel( ss ), cristobalite, and liquid of composition 47 wt% NiO, 29 wt% TiO₂, and 24 wt% SiO₂. Two eutectics are present, one at 1480°C, with spinel( ss ), NiTiO₃, cristobalite, and liquid (42 wt% NiO, 43 wt% TiO₂, and 15 wt% SiO₂), as the coexisting phases. The other is at 1490°C with NiTiO₃, rutile, cristobalite, and liquid (32 wt% NiO, 56 wt% TiO₂, and 12 wt% SiO₂). A liquid miscibility gap extends across the diagram from the two bounding binary systems NiO–SiO₂ and TiO₂–SiO₂.     

Microstructural Analysis in Processing of the High-Tc Superconductor Ba2YCu3O7-x in Air

In the synthesis of the superconducting compound Ba₂YCu₃O_7-x from a stoichiometric mixture containing BaCO₃, Y₂O₃, and CuO In air, a low-melting liquid phase is formed at about 890°C. The liquid phase was identified as a ternary eutectic located within the compatibility triangle Ba₂YCu₃O_7-x–BaCuO₂–CuO. The implication of this finding for the processing of Ba₂YCu₃O_7-x is discussed.     

Axial Thermal Expansion of TetragonaI ZrO, Between 1150° and 1700°C

The lattice constants for three samples of tetragonal ZrO₂ were measured in a high-temperature X-ray diffractometer in the range 1150° to 1700°C in an ambient air atmosphere. The unrefined lattice constants were the same for these materials only after each had been previously heat-treated at 1550° to 1750°C in an ambient air atmosphere. The thermal expansions of the two axes are linear and are given by: a value, A = 3.588₂+ 4.50 X 10+(T) , within 0.001₃ A; and c value, A = 5.188₂+ 7.57 × 10⁻⁵(T), within 0.002₂ A, where T is in °C for the range 1150° to 1700°C. The lattice constant values are a = 3.639₉ A and c = 5.275₈ A at 1150°C and a = 3.678₂ A and c = 5.339₇ A at 1700°C.     

Fe-Nb Oxides: Phase Relations at 1180°C

Subsolidus phase relations of the oxides in the system Fe–Nb–O were experimentally determined at 1180°C, 1 atm total pressure, and variable partial pressures of oxygen. Niobium pentoxide reacts readily with either ferrous or ferric oxide at subsolidus temperatures and the following ternary compounds were synthesized: Fe₄Nb₂O₉, FeNbO₄, and FeNb₂O₆. It is shown that the rutile structure of NbO₂ will take Fe-Nb₂O₆ into solid solution and that the spinel structure of Fe₃O₄ will incorporate up to 7 at.% Nb.