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.     

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.     

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.     

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.     

Processing and Characterization of Thin Films of the One-Layer Phase in the Bi-Sr-Ca-Cu-O System: Ca Solubility

Thin films in the Bi-Sr-Ca-Cu-O system have been synthesized from liquid ethyl hexanoate precursors by spin pyrolysis. The extension of the ternary one-layer phase field into the quaternary system has been determined through the replacement of Sr by Ca in single-phase, c -axis oriented thin films. A significant Ca solubility in the one-layer phase was found, the maximum being around 0.4 per 1 Cu. All onelayer compositions were excess in Bi and deficient in (Sr + Ca) relative to the ideal composition and showed over all cation deficiency. The c lattice parameter and solidus temperature were found to vary systematically with composition. The interpretation of the Ca solubility results in terms of Ca doping of the cation-deficient "Sr"sites and implications regarding the high- T_c Cu-layer phases in the system are discussed.     

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.     

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.     

Phase Equilibria in the System ZrO2─InO1.5

Phase equilibria in the system ZrO₂─InO_1.5 have been investigated in the temperature range from 800° to 1700°C Up to 4 mol%, InO_1.5 is soluble in t -ZrO₂ at 1500°C. The martensitic transformation temperature m → t of ZrO₂ containing InO_1.5 is compared with that of ZrO₂ solid solutions with various other trivalent ions with different ionic radii. The diffusionless c → t ' A phase transformation is discussed. Extended solid solubility from 12.4 ± 0.8 to 56.5 ± 3 mol% InO_1.5 is found at 1700°C in the cubic ZrO₂ phase. The eutectoid composition and temperature for the decomposition of c -ZrO₂ solid solution into t -ZrO₂+InO_1.5 solid solutions were determined. A maximum of about 1 mol% ZrO₂ is soluble in bcc InO_1.5 phase. Metastable supersaturation of ZrO₂ in bcc InO _1.5 and conditions for phase separation are discussed.     

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.     

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.     

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.     

Properties of Sm2O3─Al2O3─SiO2 Glasses for In Vivo Applications

The compositional range for glass formation below 1600°C in the Sm₂O₃─Al₂O₃─SiO₂ system is (9–25)Sm₂O₃─(10–35)Al₂O₃─(40–75)SiO₂ (mol%). Selected properties of the Sm₂O₃─Al₂O₃─SiO₂ (SmAS) glasses were evaluated as a function of composition. The density, refractive index, microhardness, and thermal expansion coefficient increased as the Sm₂O₃ content increased from 9 to 25 mol%, the values exceeding those for fused silica. The dissolution rate in 1 N HCl and in deionized water increased with increasing Sm₂O₃ content and with increasing temperature to 70°C. The transformation temperature ( T _g ) and dilatometric softening temperature ( T _d ) of the SmAS glasses exceeded 800° and 850°C, respectively.     

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.     

Effect of Deposition Temperature on the Growth of Yttria-Stabilized Zirconia Thin Films on Si(111) by Chemical Vapor Deposition

An experimental study was made on the effect of deposition temperature on the growth of yttria-stabilized zirconia (YSZ) thin films in the chemical vapor deposition (CVD) process. The YSZ thin films were obtained in a temperature range of 650°–850°C, using β-diketone chelates and a Si(111) substrate. Dense and mirrorlike YSZ films with uniform thickness were prepared; the deposition rate was 12–20 nm/min at those temperatures. An examination of the crystalline structure of the YSZ films was made, and the appropriate temperature for the growth of c -axis-oriented YSZ thin films using a Si(111) substrate was determined. The quality of the YSZ films was strongly dependent on the deposition temperature. As the temperature increased, the film growth mechanism changed from being controlled by surface reaction to being controlled by gas-phase diffusion.     

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.     

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.     

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.     

Preparation of Barium Titanate Films at 55°C by an Electrochemical Method

Work by previous investigators has shown that BaTiO₃ films can be synthesized from solution over temperature ranges from 80°C to greater than 200°C. In the present work, electrically insulating crystalline films of BaTiO₃ have been electrochemically deposited on titanium substrates at temperatures as low as 55°C. Auger spectroscopic analyses with depth profiling indicate that a titanium oxide layer whose thickness is governed by current density acts as a precursor to BaTiO₃. Formation of BaTiO₃ is found to be favored only in highly alkaline solutions. This is consistent with the phase stability reported for the Ba─Ti─CO₂─H₂O system at 25°C. Lower processing temperatures (55°C) favor the formation of thick, electrically resistive, and wellcrystallized BaTiO₃ films, apparently due to increased oxygen solubility in the electrolyte solution. Films produced at 100°C are much thinner and are electrically conductive due to fissures and pores in their microstructure. Initial studies on the effect of current density indicate the formation of thinner and porous films with thicker titanium oxide intermediate layers.     

Phase Equilibria in the Hafnia–Yttria–Lanthana System

The HfO₂–Y₂O₃–La₂O₃ system was studied in the wide range of temperatures (1250°–2800°C) and concentrations by methods of X-ray analysis at 20°C, petrography, differential thermal analysis in helium at temperatures to 2500°C, thermal analysis in air using a solar furnace at temperatures to 3000°C, and electron microprobe X-ray analysis. The complete phase diagram was constructed. The liquidus and solidus projections, crystallization paths for the alloys, isothermal (1250°, 1600°, and 1900°C) and polythermal sections are presented. The structure of the boundary binary systems defines the phase equilibria in the ternary system. No ternary compounds were found. Ternary solid-solution regions were determined based on constituent oxides and intermediate phases.