The U─Zr─C Ternary Phase Diagram above 2473 K

The U─Zr─C tertnary phase diagram is optimized by graphical and numerical curve fitting of self-consistent thermodynamic and phase diagram data reported in the literature. The system is described from 2473 to 3693 K using isothermal ternary phase diagrams. The solidus and liquidus curves of the U_xZr_1−xC_y solid solution were estimated from combined thermodynamic and phase diagram data. The solidus and liquidus curves in the ternary system were calculated using a modified form of a free-energy minimization method developed by Rudy and Chang. The calculated curves agree with experimental data.     

Effect of Hot-Pressing on the Bi─Pb─Sr─Ca─Cu─O System

Uniaxial hot-pressing has been used to improve the relative density and preferred gain orientation of the nearly pure (Bi, Pb)₂Sr₂Ca₂Cu₃O_10-y (2223) phase in the Bi-Pb-Sr-Ca-Cu-O system. Samples were prepared by freeze-drying, calcining, and hot-pressing the powders at 650° to 840°C at 50 and 200 MPa for 2 h. The hot-pressed samples showed a high degree of grain alignment and increased density up to 99% of theoretical. At 200 MPa, the 2223 phase was partially destroyed during hot-pressing, probably because of the weak bonding between the Bi─O₂ layers. However, the 2223 phase and superconductivity were recovered with subsequent annealing. The critical current density ( J _c) was improved 10-fold over that of sintered samples through this technique, with the highest J _c being 2100 A°Cm⁻² at 77.3 K and zero field. The hot-pressed samples also showed significant improvements in the J _c as a function of applied magnetic field. The J _c of a hot-pressed and postannealed sample at 0.2 T and 77.3 K was 2 orders of magnitude greater than that of a sintered sample.     

Phase Relations in the Ternary Systems Nd─B─N, Sm─B─N, and Gd─B─N

Phase relations and phase stabilities have been derived for the ternary systems RE─B─N (RE = Nd, Sm, or Gd) at elevated temperatures (1400°C and above) by means of X-ray powder analysis. Under the experimental conditions selected, various ternary compounds are found to be stable: Nd₃B₂N₄ with the Ce₃B₂N₄ type and (Nd,Sm,Gd)BN₂ with the PrBN₂ type. Phase equilibria at 1400°C and under 10⁵ Pa of argon are mainly characterized by the incompatibility of the RE metals Nd, Sm, and Gd with BN due to the competing equilibria between the RE tetraborides and the RE mononitrides. Each of the ternary compounds, however, is found to be in a two-phase equilibrium with hex -BN. Because of the different thermodynamic stabilities within the various structure series of ternary rare-earth boron nitrides RE₃B₂N₄ and REBN₂, the compound Nd₃B₂N₄ is observed only at temperatures below 1800°C and under 10⁵ Pa of Ar, whereas GdBN₂ is found to be stable only at temperatures above 1400°C under a partial pressure of 10⁵ Pa of N₂.     

Influence of Lead in the Processing of High-Temperature 2223 Bi─Sr─Ca─Cu─O Superconductor

Difficulty in reproducibility of results has often been encountered in the Pb-doped 2223 Bi─Sr─Ca─Cu─O superconductor compound due to lead vaporization in samples prepared by standard ceramic methods. Various processing parameters such as quenching, slow cooling, or closed vs open sintering containers were found to affect the properties significantly in samples prepared by standard ceramic methods. In contrast, samples prepared by spray-drying of nitrate solutions were very homogeneous and reproducible because of the rapid and uniform reaction of Pb with other constituents. The magnetic susceptibility and X-ray diffraction data remained unchanged for spray-dried samples processed with or without quenching and sintered in open or closed containers.     

Thermodynamics and Phase Equilibria in the Sr─Cu─O System

Phase equilibria in the SrO─CuO─Cu system were determined at 1173 K from the results of X-ray diffraction measurements using specimens annealed under low oxygen partial pressure ( p o₂= 0.21–10⁻⁸ atm). Electromotive force (EMF) measurements using ZrO₂ solid electrolyte cells were carried out in the ternary phase equilibria. Gibbs free energies for five reactions in the cell electrodes were summarized by equations with linear temperature dependence. The standard free energies of formation for Sr₂CuO₃, SrCuO₂, Sr₁₄Cu₂₄O₄₁, and SrCu₂O₂ were derived. The stability conditions of all oxides are displayed in the p-T-x diagram using the new data.     

Thermodynamics and Phase Equilibria in the Ca─Cu─O System

Phase equilibria in the CaO─CuO─Cu system were determined at 1173 K from the results of X-ray diffraction measurements using specimens annealed in the oxygen partial pressure range from P O₂= 1 to 10⁻⁸ atm. Electromotive force (emf) measurements using ZrO₂ solid electrolyte cells were carried out in the ternary phase equilibria. Gibbs free energies for the chemical reactions were summarized by equations with linear temperature dependence, and the standard free energy of formation for Ca₂CuO₃ was derived. The stability conditions of the oxides are displayed in the p – T – x diagram, and the possible phase equilibria with the liquid are evaluated.     

Magnetoplumbite-Corundum Phase Equilibria in the System Sr─Fe─Al─O at 1100°C

The phase relations in the region of the Fe₂O₃- and Al₂O₃-rich sides of the quaternary system SrO─Fe₂O₃–Al₂O₃–B₂O₃ and the location of conjugation lines between magnetoplumbite solid solution SrO·(6 − x)Fe₂O₃·xAl₂O₃ and corundum (α-Fe₂O₃, α-Al₂O₃) phases were determined at 1100°C in air by using the flux-growth method based on the Ostwald ripening mechanism. Activity–composition relations and the lattice parameters along the magnetoplumbite solid solutions were also obtained.     

2223 Phase Formation in Bi(Pb)─Sr─C─a─Cu─O: II, The Role of Temperature—Reaction Mechanism

The formation mechanism, the temperature range for the growth, and the thermal stability of the 2223 phase in Bi(Pb)─Sr─Ca─Cu─O have been investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPMA, four-probe resistance and acsusceptibility measurement. The formation of the 2223 phase was found to follow a dissolution–precipitation process. A 2212 phase first reacts with the liquid phase formed via an incongruent melting of the Ca₂PbO₄ phase, and a dissolution of CaO and CuO takes place. The 2201 phase, which has the largest negative free energy, is then precipitated from the melt; the nucleation and growth of the 2223 phase are subsequently developed by the reaction between the 2201 phase precipitates and ions of Ca²⁺ and Cu²⁺ present in the liquid phase. The 2223 phase is formed at temperatures in the range 827°C T < 856°C. The optimum temperature T _f for the formation of 2223 phase is 845°± 5°C. The 2223 phase is thermodynamically unstable at temperatures above 856°C.     

Processing and Characterization of Thin Films of the Two-Layer Superconducting Phase in the Bi─Sr─Ca─Cu─O System: Evidence for Solid Solution

Thin films in the Bi─Sr─Ca─Cu─O system have been synthesized from liquid ethylhexanoate precursors by spin pyrolysis. An extensive solid-solution range was found for the two-Cu-layer phase through the study of c -axis-oriented, single-phase thin films fabricated on single-crystal, (100)-oriented, MgO substrates. All two-layer compositions were excess in Bi and deficient in Sr + Ca relative to the ideal 2212 composition and showed an overall cation deficiency. The solidus temperature and c lattice parameter were found to vary systematically with composition. Sharp superconductive transitions were obtained in the case of a number of different compositions with T _c varying between 72 and 84 K. Evidence for significant compositional heterogeneities within single-phase two-layer thin films was found and the implications for superconductivity are discussed. Compositions within the solid-solution range gave single-phase, c -axis-oriented films over a wide temperature range extending from 730°C to an upper, solidus (or peritectic) temperature (780° to 840°C) which is dependent on the initial starting composition. A model has been developed that describes the formation of the two-layer phase from a fugitive liquid.     

2223 Phase Formation in Bi(Pb)─Sr─Ca─Cu─O: III, The Role of Atmosphere

The formation of second phases during the preparation of the 2223 phase and their stability in the Bi system under various annealing temperatures and atmospheres have been studied. The 2201 precipitates developed at ∼830°C, and their conversion to the 2223 phase can be completed in the temperature range 810°C ≤ T < 830°C. A Ca₂PbO₄-like phase can precipitate from the liquid phase at ∼830°C during cooling. A (Sr,Ca)₁₄Cu₂₄O₄₁ phase is usually found accompanying the synthesis of the 2223 phase. This secondary phase is stable in an oxidizing atmosphere but can be eliminated by annealing under a low oxygen atmosphere or by choosing a suitable starting composition and set of sintering conditions. The precipitation of Ca₂PbO₄-like phase can be avoided by using a relatively fast cooling rate. Unlike the YBa₂Cu₃O _x superconductor, the 2223 phase can be stable under a wide range of atmospheres, such as argon, air, and oxygen.     

2223 Phase Formation in Bi(Pb)─Sr─Ca─Cu─O: I, The Role of Chemical Composition

The formation of superconducting phases in the Bi(Pb)─Sr─Ca─Cu─O system has been systematically investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPMA, ICP-AES, fourprobe dc resistance, and ac susceptibility. Starting compositions, firing temperature, and the duration of heat treatment, together with the atmosphere, were found to be critical in determining the preferred formation of the 2223 phase. This paper reports the effect of the initial chemical composition, emphasizing the importance of compositional control in the synthesis of the single 2223 phase. It has been shown that, with a correct starting composition and predetermined synthesis conditions, single 2223 phase can be obtained without intergrowth by the 2212 and other impurity crystalline phases. The optimum starting composition for the preferred growth of the 2223 phase was identified as being Bi_1.7Pb_{0.3+ y} Sr₂Ca₂Cu₃O _x ( y = 0.1), with excess Pb added in order to compensate for its loss at high temperatures. The effect of Pb doping and excess Cu on the phase formation in the Bi oxide based superconducting system is discussed.     

Correlation of Subsolidus Phase Relations in the Ag─Pd─O System to Oxidation Reduction Kinetics and Dilametric Behavior

The relationship between the equilibrium subsolidus phase diagram of the Ag─Pd─O₂ system, and the oxidation/reduction kinetics and microstructure development of 70Ag/30Pd coprecipitated and alloyed powders is presented. For the same Ag/Pd ratio and similar powder characteristics, the onset temperatures of Pd oxidation and PdO reduction for coprecipitated powders are lower than those of alloyed powders. For Ag/Pd alloyed powders, complete oxidation of Pd does not occur, because of kinetic limitations and the equilibrium phases present at high temperatures. Significant differences in microstructural evolution were observed and correlated to the oxidation processes and densification kinetics. The impact of these results on the use of the Ag/Pd system as conductors is discussed.     

Synthesis, Properties, and Ternary Phase Stability of M6X Compounds in the Ti─Cu─O System

A section of the Ti─Cu─O ternary phase diagram was investigated at 925°C. The system contains two M₆X-type compounds, Ti₄Cu₂O and Ti₃Cu₃O, which are isostructural with the η-carbides and are stabilized by O. The presence of two of these compounds and the size of their single-phase fields contradict earlier predictions in the literature based on the valence and location of the elements in the periodic table. It is proposed that the atomic radii of the elements may also be a factor, especially for compounds containing Ti. The two observed M₆X-type compounds occur as products of reaction at the Ag─Cu─Ti/Al₂O₃ interface. The compounds were synthesized in bulk for characterization of their properties which influence the performance of the brazed joints. The compounds are stable in inert atmospheres up to their melting points, but are easily oxidized in ambients containing even trace amounts of O and form a coherent layer of reaction products. The values of elastic coefficients, thermal expansion, and resistivity of these compounds are intermediate between those of Al₂O₃ and Cu and would therefore provide a gradual transition across the interface.     

Influence of Oxygen Partial Pressure on the Quasi-Ternary System Cr─Mn─Ti Oxide

The quasi-ternary system Cr─Mn─Ti oxide was investigated at 1000°C under oxygen partial pressures ranging from 0.21 bar to 10^-21 bar (1 bar = 10⁵ Pa). X-ray diffraction analysis was used to identify phases and determine lattice parameters. The positions of phase boundaries as a function of oxygen partial pressure were measured using the emf method. The spinel MnCr₂O₄ may be regarded as the most interesting compound in this system. Part of the chromium can be replaced by trivalent titanium at low oxygen partial pressures and by trivalent manganese at high pressures, and the formation of a limited solid solution with the spinel Mn₂TiO₄ is possible in all cases. As a result, a coherent single-phase spinel region exists over the entire oxygen partial pressure range at 1000°C.     

Thermochemistry of BaSm2O4 and thermodynamic assessment of the BaO–Sm2O3 system

The BaO–Sm₂O₃ system is of interest for the optimization of synthesis of electroceramics. The only systematic experimental study of phase equilibria in the system was performed more than 40 years ago. The reported experimental values of the enthalpy of formation of BaSm₂O₄ are in conflict, and the reported compound Ba₃Sm₄O₉ has never been confirmed. In this work we synthesized BaSm₂O₄ by solid‐state reaction and determined its heat capacity, enthalpy of formation, and phase transitions by differential scanning calorimetry, high‐temperature oxide melt solution calorimetry and ultra‐high‐temperature differential thermal analysis, respectively. We confirmed the existence of Ba₃Sm₄O₉ and its apparent stability from 1873 to 2273 K by X‐ray diffraction on quenched laser‐melted samples but were not able to obtain single‐phase material for calorimetric measurements. The CALPHAD method was used to assess phase equilibria in the BaO–Sm₂O₃ system, using both available literature data and our new measurements. A self‐consistent thermodynamic database and the calculated phase diagram of the BaO–Sm₂O₃ system are provided. This work can be used to model and thus to understand the relationships among composition, temperature, and microstructure for multicomponent systems with BaO and Sm₂O₃.     

Characterization of C─B─N Solid Solutions Deposited from a Gaseous Phase between 900° and 1050°C

The composition, the structure, and some physical properties of boron–carbon–nitrogen solid solutions prepared by chemical vapor deposition (CVD) have been investigated. Both crystalline films and amorphous granular materials resulting respectively from heterogeneous and homogeneous nucleation were characterized by X-ray diffraction, XPS, RBS, and TEM. The compositions of these single-phase materials are gathered in two main domains located in the B/N > 1 part of the C─B─N composition diagram. It is stated that the carbon-rich domain results from structural disorder of an ideal C₅B₂N composition. The thermal behavior of these films indicates that no mass loss can be detected after 1 h at 1700°C but a graphitization and a formation of small amounts of B₁₃C₂ are observed. Density and preliminary electrical conductivity measurements were also performed.     

Li2O─SiO2─Al2O3─MeIIO Glass-Ceramic Systems for Tile Glaze Applications

In order to verify the possibility of using glass-ceramic materials as tile coatings, the devitrification processes of three industrial formulations belonging to the Li₂O─Al₂O₃─SiO₂ glass-ceramic system were investigated by differential thermal analysis, X-ray diffractometry, scanning electron microscopy, and IR spectroscopy. Compositional variations were made by addition of large amounts of MgO or CaO or PbO (ZnO) oxides as well as through smaller additions of other oxides. In these systems the surface crystallization contributes appreciably to the bulk crystallization mechanism. All the systems investigated show a high tendency toward crystallization even at very high heating rates, developing a very close network of interlocked crystals of synthetic β-spodumene-silica solid solutions (LiAlSi₄O₁₀). The results of this research are expected to establish the conditions under which these glass-ceramic systems can be practically used as tile glazes.     

Morphology of Phase Separation in AIN─Al2OC and SiC─AIN Ceramics

Solid solutions in the AIN─Al₂OC and SiC─AlN systems were fabricated by hot-pressing powder mixtures in graphite dies. X-ray diffraction showed the samples to be single phases of 2H structure. The samples were annealed between 1600° and 1900°C for up to 1000 h. In the SiC─AlN system, optical microscopy and X-ray diffraction failed to reveal microstructural or phase changes. However, electron microscopy showed that samples had decomposed. Streaking of diffraction spots occurred along directions orthogonal to {012} planes (∼43° off the c axis), which is approximately the direction along which the elastic energy function is a minimum. The orientation-dependent Young's modulus was also a minimum along this direction. In AIN─Al₂OC, optical microscopy and X-ray diffraction indicated the occurrence of decomposition. The precipitates were disk-shaped with [001] orthogonal to the disks. The occurrence of decomposition along the [001] direction suggests that it is the elastically soft direction.