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.     

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.     

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.     

Thermal Analysis of a SiO2─Al2O3─CaO─CaF2 Glass

A SiO₂─Al₂O₃─CaO─CaF₂ ionomer glass was investigated using thermal analysis, X-ray diffraction, and scanning electron microscopy. The purpose of this investigation was to control the susceptibility of the glass to acid attack. The differential thermal analysis trace exhibited a sharp glass transition at about 645°C and two exotherms. The first exotherm corresponded to liquid–liquid phase separation followed by crystallization of fluorite. The second, much larger, exotherm was the result of crystallization of the remaining glass phase to form anorthite. Prolonged heat treatment below the glass-transition temperature demonstrated that crystallization of fluorite can occur without prior liquid–liquid phase separation.     

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.     

Spherulitic Growth of Apatite in a MgO─CaO─SiO2─P2O5 Glass

Differential thermal analysis, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and transmission electron microscopy were used to study the crystallization of a glass with a composition of 11.2 wt% MgO, 40.5 wt% CaO, 33.3 wt% SiO₂, and 15 wt% P₂O₅. A two-phase "composite," which was composed of apatite and an intermediate phase (H-phase), was formed under appropriate heat-treatment conditions. The spherulitic morphology of apatite phase transformed from "open sheaf" into ellipsoidal as samples were heated to a higher temperature. These phenomena were due to the intermediate H-phase becoming unstable at this temperature so that the retardation effect on the apatite dendritic growth disappeared.     

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.     

Color and Selected Properties of PbO─BiO1.5─GaO1.5 Glasses

The density; molar volume; thermal expansion coefficient; dissolution rate in water, HC1, and NaOH; glass transition and crystallization temperatures; and the absorption edge in the ultraviolet-visible and infrared were measured for PbO─BiO1.5─GaO1.5 glasses. The range of compositions investigated was x PbO (100 − (x + y) )BiO_1.5. yGaO_1.5 for x between 20 and 60 cat% and y of 20, 25, 30, and 35 cat%. The glass-forming tendency increased with increased GaO_1.5 and decreased with increased PbO or BiO_1.5. The compositional dependence of these properties was consistent with the weight, size, charge, and bond strength of the cations. The Ga^{3 +} ions in these glasses are believed to act primarily as network-forming cations, whereas the majority of the Bi³⁺ and Pb²⁺ ions behave as network-modifying cations. It is suggested that a small friction of the lead ions are present as Pb⁴⁺. Depending upon melting conditions, these glasses ranged in color from brown to yellow. Various attempts, including containerless melting, were made to obtain colorless glasses, but no conditions were found which totally eliminated the color. The least color (pale yellow) was obtained when the glasses were melted in an air or nitrogen atmosphere in an alumina or gold crucible.     

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.     

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.     

Thermochemistry of the U─O and Zr─O Systems

The thermodynamic constraint relating the standard free energy of formation of an oxide (MO_c) to its oxygen potentials by Δ G _f ⁰(MO_c) =½ƒ_o ^c Δ (O₂) d C ' has been used to develop the full spectrum of oxygen potentials of the U─O and Zr─O systems at high temperatures. Henry's law is applied to the metal region in the U─O system. Oxygen dissolution in liquid Zr has been shown to deviate only slightly from Henry's law, despite the high solubility. Minor modifications of the previously reported oxygen pressures in the two-phase regions of both of the U─O and Zr─O systems result from the requirement of satisfying the integral constraint. Oxygen pressures in the oxygen-containing metal regions where no data are available have also been estimated. The ensemble of the results is displayed in the form of oxygen isobars superimposed on the phase diagrams of the U─O and Zr─O systems. The oxygen pressure-temperature–composition relation of tetragonal (β) ZrO_{2± x} has also been established.     

Subsolidus Relations in the La2O3─CuO─CaO Phase Diagram and the La2O3─CuO Binary Join

The phase diagram for the CuO-rich part of the La₂O₃─CuO join was redetermined. La₂Cu₂O₅ was found to have a lower limit of stability at 1002°± 5°C and an incongruent melting temperature of ∼1035°C. LagCu₇O₁₉ had both a lower (1012°± 5°C) and an upper (1027°± 5°C) limit of stability. Subsolidus phase relations were studied in the La₂O₃─CuO─CaO system at 1000°, 1020°, and 1050°C in air. Two ternary phases, La_1.9Ca_1.1Cu₂O_5.9 and LaCa₂Cu₃O_8.6, were stable at these temperatures, with three binary phases, Ca₂CuO₃, CaCu₂O₃, and La₂CuO₄. La₂Cu₂O₅ and La₈Cu₇O₁₉ were stable only at 1020°C, and did not support solid-solution formation.     

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.     

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.     

Bioactive Bone Cement Based on CaO─SiO2─P2O5 Glass

A CaO─SiO₂─P₂O₅─CaF₂ glass powder hardened within 4 min when mixed with an ammonium phosphate solution to form CaNH₄PO₄·H₂O. After it had soaked in a simulated body fluid for 3 d, forming hydroxyapatite, the cement showed a compressive strength of 80 MPa. Implanted into a rat tibia, the mixed paste formed a tight chemical bond to the living bone within 4 weeks. Such a bioactive cement could be useful not only for fixing various kinds of implants to the surrounding bones but also, by itself, as a bone filler.     

Subsolidus Phase Relationships in Part of the System Si,Al,Y/N,O: The System Si3N4─AIN─YN─Al2O3─Y2O3

The subsolidus phase relationships in the system Si,Al,Y/N,O were determined. Thirty-nine compatibility tetrahedra were established in the region Si₃N₄─AIN─Al₂O₃─Y₂O₃. The subsolidus phase relationships in the region Si₃N₄─AIN─YN─Y₂O₃ have also been studied. Only one compound, 2YN:Si₃N₄, was confirmed in the binary system Si₃N₄─YN. The solubility limits of the α'─SiAION on the Si₃N₄─YN:3AIN join were determined to range from m = 1.3 to m = 2.4 in the formula Y _{m /3}Si_{12- m} Al _m N₁₆. No quinary compound was found. Seven compatibility tetrahedra were established in the region Si₃N₄─AIN─YN─Y₂O₃.