Effects of a Liquid Phase in the Y-Ba-Cu-O Superconductor

Effects of a liquid phase in the YBaCuO ceramic high-temperature superconductor were studied with respect to densification, grain growth, and critical current density. In addition to the normal 1:2:3 ratio of Y₂O₃, BaCO₃ and CuO, two other ratios of 0.8:2:3 and 1.1:2:2.8 were adopted to prepare the specimen with and without a liquid phase, respectively. The liquid phase enhanced densification and grain growth but penetrated into the grain boundary of the superconducting grains, decreasing the critical current density.     

Thermal Neutron Irradiation of Y-Ba-Cu-O Superconductor

A Y-Ba-Cu-O superconductor was irradiated at 360 K in the NRX reactor, with a well-thermalized flux of 1.82 × 10¹⁷ n/(m²·s) (E < 1 MeV). Superconducting characteristics at 90 K disappeared by a fluence of 6.6 × 10²¹ n/m² (10 h) but were recovered after a postirradiation heat treatment at 760 K in flowing oxygen. The results suggest that suitable shielding will be required for any application which exposes the superconductor to neutron irradiation.     

Preparation of Monodispersed Y-Ba-Cu-O Superconductor Particles via Sol—Gel Methods

The high-T _c superconducting material YBa₂Cu₃O_{7- x} was synthesized using a modified amorphous citrate or oxalate process. Phase-pure superconducting powders were obtained by firing the citrate precursor to 900°C and the oxalate precursor to 950°C inflowing O₂. The preparation is highly reproducible and leads to powders with excellent homogeneity and sinterability. The resulting material was examined by transmission electron microscopy, thermogravimetry, and X-ray diffraction. Particle sizes for the superconductors obtained varied from 75 to 3300 Å (7.5 to 330 nm).     

Low-Temperature Phase Equilibria in the Y-Ba-Cu-O System

Ultrasonically prepared freeze-dried nitrate precursors and high-precision solution calorimetry were used to investigate the low-temperature thermodynamic stabilities of compounds in the Y-Cu-O, Ba-Cu-O, Y-Ba-O, and Y-Ba-Cu-O pseudobinary and pseudoternary systems at 1 atm of oxygen. Y₂Cu₂O₅, Y₂BaCuO_s, and BaCuO₂ were found to be metastable below 682°, 728°, and 710°± 5°C, respectively. The only stable phases in the Y-Ba-Cu-O system at 298 K and 1 atm of oxygen are Ba₂Cu₃O₆, CuO, BaO₂, and Y₂O₃. By compiling the calorimetric and phase equilibria data, a series of Y-Ba-Cu-O isothermal phase diagrams were constructed between 25° and 900°C at 1 atm of oxygen.     

Effect of Annealing Conditions on Microstructural Development and Phase Transformation in Silicon Carbide

The effect of annealing with and without applied pressure on the microstructural development and phase transformation was investigated in fine-grained β-SiC ceramics containing α-SiC seeds. Materials annealed without pressure had a microstructure consisting of elongated grains, while materials annealed with pressure showed a duplex microstructure consisting of small matrix grains and some of elongated grains. However, annealing with pressure (25 MPa) was found to greatly retard phase transformation from β→α polytypes and inhibit grain growth. This change in lattice parameter suggests that the retardation of phase transformation and grain growth might be attributed to a reduced mass transport rate, which is the result of Al being introduced into the SiC by the annealing pressure.     

Transition Temperatures for the Tetragonal-Orthorhombic Phase Transition in the 123 Superconductor

Transition temperatures for the reversible orthorhombic-tetragonal phase transition in the "123'superconductor have been measured using dynamic X-ray diffraction. Results are reported for heating/cooling rates up to 180°C/min and in oxygen partial pressures from 3.2 to 21 kPa and are consistent with a rapid second-order (order-disorder) transition. In air and at cooling rates ≤12°C/min, the transition is perfectly reversible with no detectable hysteresis. At higher heating or cooling rates and at lower oxygen pressures the transition has a definite hysteresis and a measurable dependence on time.     

Oxygen-Ion Diffusion in a 110 K Phase BiPbSrCaCuO Superconductor

The oxygen-ion conductivity of a hot-pressed 110 K phase (2223) superconductor was determined by using an O₂,Pt/ZrO₂/110 K phase/ZrO₂/Pt,O₂ cell and impedance spectroscopy at temperature of 600°-800°C and oxygen partial pressures ( P _O2) of 0.21-0.001 atm (∼21-0.1 kPa). The oxygen-ion diffusivities were calculated via the Nerst-Einstein equation and were greater in air than at P _O2= 0.001 atm. Decomposition of the 110 K phase at 800°C and P _O2= 0.001 atm increased the bulk diffusivity but had a lesser effect on the grain-boundary diffusivities. The activation energies for oxygen diffusion, at different P _O2 values, have been determined.     

铋系超导体的成相过程与机制

本文考察了铋系超导体的成相过程,解释了成相机制及高价离子对成相的影响.     

Phase Relationships and Related Microstructural Observations in the Ca-Si-Al-O-N System

A systematic set of samples with compositions in the Ca alpha-SiAlON (α') system has been fabricated using pressureless sintering to evaluate the phase relationships on, and near, the Ca α' plane. A phase behavioral diagram for the Ca α' plane within the Jänecke prism has been determined and found to be similar to those for the Y and Sm α' systems. However, distinct differences were also observed; two additional regions (α'+ 27R + liquid) and (α'+ 33R + liquid) have been identified. Investigation of the microstructures revealed elongated Ca α' grains in the samples prepared by pressureless sintering. The aspect ratio of the elongated grains of α' increased as the concentration of stabilizing cation was increased. Single crystalline phase Ca α' samples of comparable density to hot-pressed material were fabricated successfully using pressureless sintering.     

Direct Amorphous-to-Cubic Perovskite Phase Transformation for Lead Titanate

The phase evolution of lead titanate processed by the polymeric precursor method was investigated by thermal analysis, X-ray diffraction, and high-resolution transmission electron microscopy. The results showed that the cubic perovskite PbTiO₃ (PT) phase is formed from an inorganic amorphous precursor at a temperature of 444°C. A gradual transition from cubic to tetragonal perovskite PT was observed with the increase of calcination time at this temperature. HRTEM results showed that the cubic PT particles have a size of around 5 nm. The identification of cubic PT as an intermediate phase supports the hypothesis that the chemical homogeneity was kept at the molecular level during the synthesis process, with no cation segregation.     

Phase and Microstructural Stability of Electrolyte Matrix Materials for Molten Carbonate Fuel Cells

LiAlO₂ powder is used as a material for molten‐carbonate fuel cell (MCFC) matrices. The physical and chemical stabilities of LiAlO₂ powder during MCFC operation determine the performance and lifetimes of the cells. Change to the phase and particle size in the allotropic phase of LiAlO₂ was examined with long‐term stability tests on pure α‐LiAlO₂ matrix, Al‐reinforced α‐LiAlO₂ matrix, Al‐reinforced γ‐LiAlO₂ matrix, aqueous γ‐LiAlO₂ matrix and an α‐/β‐LiAlO₂ mixture powder in molten carbonate at 650 °C in air. In the γ‐LiAlO₂ and α‐/β‐LiAlO₂ mixture, the particle growth was continuous from the early stages of heat‐treatment to 20,000 h. Crystalline phase transformation (γ‐LiAlO₂ and β‐LiAlO₂ to α‐LiAlO₂ and γ‐LiAlO₂, respectively) of these powders and matrices also occurred, and γ‐LiAlO₂ made the third phase like LiAl₅O₈. By contrast, the sizes of these particles and the crystalline phase of α‐LiAlO₂ did not change during immersion tests. These results show that, among α‐/β‐ and γ‐LiAlO₂, α‐LiAlO₂ is the most stable phase in molten carbonate.     

Cathodoluminescence: A Microstructural Technique for Exploring Phase Distributions and Deformation Structures in Zirconia Ceramics

The scanning electron microscope cathodoluminescence behavior of two partially stabilized zirconias is presented. Monoclinic material, either occurring on grain boundaries or produced by deformation, appears strongly luminescent, whereas in cubic-plus-tetragonal material grain boundaries appear dark, as do deformed regions associated with hardness indentations and wear tracks. These preliminary observations suggest that cathodoluminescence may be a valuable microstructural technique for characterizing the microstructure and deformation behavior of zirconia-based ceramics.     

Recrystallization and Phase Transformation in Reaction-Sintered Sic

Reaction-sintered Sic specimens prepared at ∼1500°C and heat-treated at ∼1850°C in the presence of molten silicon were examined by transmission electron microscopy. The β-Sic grains in as-prepared specimens grew to a large size when heat-treated in the presence of molten Si; the number of growth twins in such °-Sic crystals was very small. The boundary areas of α- and β-Sic crystals were microsyntactic, consisting of thin strips of α- and β-Sic after heat-treating. These changes were due mostly to thin lamellar growth of α-Sic into β-Sic grains along the basal plane. There is also a high density of similar growth of transformation twins in β-Sic crystals. The results provide clear evidence of in situ solid-state transformations in sic.     

Metal Nitrate-Urea Decomposition Route for Y-Ba-Cu-O Powder

Y-Ba-Cu-O powder, having desirable composition, particle size, compaction, and sintering properties, has been prepared by a novel combustion process involving metal nitrate-urea decomposition. Single-phase Y-Ba-Cu-O is obtained by reacting the mixture of yttrium, barium, and copper nitrates in 123 stoichiometry with urea at 900°C for a period of 1 h. Following grinding in acetone the powder possessed an average particle size of 1 μm, a surface area of 42.2 m²/g, a bulk density of 2.7 g/cm³, and could be sintered to 92% theoretical density at 930°C with the resulting material having a T _c of 90 K.     

Effect of β-to-α Phase Transformation on the Microstructural Development and Mechanical Properties of Fine-Grained Silicon Carbide Ceramics

Ultrafine β-SiC powders mixed with 7 wt% Al₂O_3, 2 wt% Y₂O₃, and 1.785 wt% CaCO₃ were hot-pressed and subsequently annealed in either the absence or the presence of applied pressure. Because the β-SiC to α-SiC phase transformation is dependent on annealing conditions, the novel processing technique of annealing under pressure can control this phase transformation, and, hence, the microstructures and mechanical properties of fine-grained liquid-phase-sintered SiC ceramics. In comparison to annealing without pressure, the application of pressure during annealing greatly suppressed the phase transformation from β-SiC to α-SiC. Materials annealed with pressure exhibited a fine microstructure with equiaxed grains when the phase transformation from β-SiC to α-SiC was <30 vol%, whereas materials annealed without pressure developed microstructures with elongated grains when phase transformation was >30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mechanical properties could be achieved through annealing with or without pressure.     

Phase Equilibria and Phase Transformation in the Aluminum Nitride-Aluminum Oxycarbide Pseudobinary System

Samples in the pseudobinary AIN–AI₂OC were fabricated by hot-pressing mixtures of ANI, AI₄C₃, and AI₂O₃ powders in graphite dies in an atmosphere of nitrogen. The resulting dense samples were subjected to thermal treatments over a range of temperatures from 1550° to 1950°C. The hot-pressed as well as annealed samples were examined using optical microscopy, scanning transmission electron microscopy, and X-ray diffraction. An electron microprobe was used to determine composition. X-ray diffraction showed that AI₂OC dissolved in AIN up to 44 mol% at 1800°C. Thermal treatment at lower temperatures led to the decomposition of the solid solution into two isostructural phases. In samples containing ∼14 to 60 mol% AIN, the morphology of the precipitates was lenticular. Diffraction contrast analysis showed that the precipitates were rich in ANI. Lattice images showed that the (001) planes of the 2H structure were continuous between the matrix and the precipitates. These precipitates appear to be similar to Guinier-Preston zones observed in metallic alloys. When annealed for long periods of time, interface dislocations formed, signifying partial loss of coherency. In some compositions (∼61 to 64 mol% AIN), a lamellar microstructure developed similar to that observed in cellular phase separation. Also, in some of the compositions, an additional phase was observed whose composition and structure were not determined.     

Phase Transformation of Diphasic Aluminosilicate Gels

Aluminosilicate gels with compositions Al₂O₂/SiO₂ and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC₂H₅)₄. Upon heating the pseudo-boehmite transforms to γ-Al₂O₃ around 400°C, then to δ-Al₂O₃ at 1050°C, and at 1200°C reacts with amorphous SiO₂ to form mullite. Some twinned θ-Al₂O₃ forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al₂O₃ or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape.     

Microstructural Evidence of Reconstituted Limestone Blocks in the Great Pyramids of Egypt

How the Great Pyramids of Giza were built has remained an enduring mystery. In the mid-1980s, Davidovits proposed that the pyramids were cast in situ using granular limestone aggregate and an alkali alumino-silicate-based binder. Hard evidence for this idea, however, remained elusive. Using primarily scanning and transmission electron microscopy, we compared a number of pyramid limestone samples with six different limestone samples from their vicinity. The pyramid samples contained microconstituents (μc's) with appreciable amounts of Si in combination with elements, such as Ca and Mg, in ratios that do not exist in any of the potential limestone sources. The intimate proximity of the μc's suggests that at some time these elements had been together in a solution. Furthermore, between the natural limestone aggregates, the μc's with chemistries reminiscent of calcite and dolomite—not known to hydrate in nature—were hydrated. The ubiquity of Si and the presence of submicron silica-based spheres in some of the micrographs strongly suggest that the solution was basic. Transmission electron microscope confirmed that some of these Si-containing μc's were either amorphous or nanocrystalline, which is consistent with a relatively rapid precipitation reaction. The sophistication and endurance of this ancient concrete technology is simply astounding.     

The Intergranular Phase in Hot-Pressed Silicon Nitride: II, Evidence for Phase Separation and Crystallization

Compositional and morphological evidence obtained using transmission electron microscopy is presented indicating that the noncrystalline phase in a hot-pressed silicon nitride can undergo phase separation. The compositional range determined for the phase separation is close to the known miscibility gap in the pseudobinary system SiO₂-MgO. Crystallization of Si2N₂O and MgSiO₃ from the noncrystalline phase is also evident, suggesting that it can contain some dissolved nitrogen and that the intergranular phase may be crystallizable.