Vapor Pressure of Barium and Copper Components in YBa2Cu3O7−x Superconductor Ceramics

Purified air is passed over a specimen of YBa₂Cu₃O_{7– x} at 890°C; the vaporized substances are condensed in a pure alumina tube, then subjected to inductively controlled plasma analysis. Vapor pressure values of 2.5 × 10⁻⁵ Pa for BaO( g ), 1.2 × 10⁻⁴ Pa for Cu( g ), and 2.2 × 10⁻⁵ Pa for CuO( g ) are obtained under 2.1 × 10⁴ Pa (0.21 bar) of oxygen pressure. No Y vapor is detected at this temperature.     

Simultaneous Thermogravimetry and Evolved-Gas Analysis of YBa2Cu3O7 and LaBa2Cu3O7 Superconductors

Concurrent thermogravimetry (TG) and evolved-gas analysis (EGA) were done for YBa₂Cu₃O_7-z and LaBa₂Cu₃-O_7-z superconductors. The sample weights were monitored by thermobalance and the evolved O₂ and CO₂ species were monitored by quadruple mass spectrometer (QMS). No diffraction peak for the impurity phase containing a carbonate group was observed in the X-ray diffraction patterns for these samples, but the release of CO₂ was detected by EGA. CO₂ gas began to evolve from YBa₂Cu₃O_7-z at 543°C and from LaBa₂Cu₃O_7-z at 692°C. Preparation of high-quality YBa₂Cu₃O_7-z and LaBa₂Cu₃O_7-z superconductors is discussed on the basis of results of these thermal analyses.     

Water Corrosion of YBa2Cu3O7−δ Superconductors

Solid-state sintering was used to make YBa₂Cu₃O_7−δ superconducting bulk materials. Corrosion of the YBa₂Cu₃O_7−δ superconductor material was investigated in a humid environment. The superconducting materials exhibited significant corrosion after 4 h at 80° and 100% relative humidity. A grain-boundary phase was formed, and the percent superconducting phase in the material decreased by approximately 60%. The transition temperature (T_c) decreased with corrosion time. After 2 h of corrosion, T_c decreased from 87 to 81 K.     

Effect of Rigid Inclusions on the Densification and Constitutive Parameters of Liquid-Phase-Sintered YBa2Cu3O6+x Powder Compacts

The presence of rigid inclusions in a powder compact leads to a reduction in the densification rate of the compact and may also lead to processing defects. In this paper, the densification rate and the constitutive parameters of both homogeneous YBa₂Cu₃O_{6+ x} and composite powder compacts (YBa₂Cu₃O_{6+ x} powder with 10 vol% dense inclusions of YBa₂Cu₃O_{6+ x} ) are reported. A small amount of liquid phase, which formed during sintering, was present in the samples. However, even with the presence of a liquid phase, the addition of inclusions still reduces the densification rate of the composite and increases its viscosity. The results have been compared with a published analysis of the problem using measured values of the constitutive parameters. Both the viscosity and viscous Poisson's ratio of the porous body have been measured.     

Ultrasonic Evaluation of Oxygen Content, Modulus, and Microstructure Changes in YBa2Cu3O7–x Occuring During Oxidation and Reduction

Ultrasonic velocity measurements were used to evaluate the effects of oxidation and reduction on the oxygen content, elastic properties, and global microstructure of the YBa₂Cu₃O_{7– x} ceramic superconductor for samples ranging from 70% to 90% of theoretical density. Bulk density, velocity, and Young's modulus generally increased with increasing oxygen content upon oxidation, and this behavior was reversible. Velocity image patterns obtained from ultrasonic scans were similar after oxidation and reduction treatments for a 90% dense sample, although the velocity value at any given point on the sample was changed following the treatments. The unchanging pattern correlated with destructive measurements showing that the spatial pore distribution (fraction and size) was not measurably altered after the treatments. Changes in superconducting behavior, crystal structure, and grain structure were observed consistent with changes in oxygn content.     

Novel Solution-Soaking Method for Adding Dopants to YBa2Cu3O7−x to Achieve Higher Sintered Densities

YBa₂Cu₃O_7−x was doped with various metal ions by a new technique in which a pellet (after binder burnout) was soaked in a solution containing the appropriate ions and then dried. The sintered density of the treated pellets depended on the dopant in the solution, and in many cases it was much higher than that obtained for pure YBa₂Cu₃O_7−x (93% to 96% as compared with 85% to 90%). A study of the microstructure revealed that, in those cases where higher sintered densities were obtained, the grain size was much smaller. The grain morphology, however, did not change. It is likely that the high concentration of dopant adsorbed on the grain surface during solution soaking enhanced the rate of sintering.     

Mechanism for the Peritectic Reaction and Growth of Aligned Grains in YBa2Cu3O6+x

It is experimentally observed that the peritectic reaction, 211 + liquid → 123, can be driven essentially to completion in 1 h at an undercooling of only ∽30°C. The kinetic data, together with the observed microstructures, are inconsistent with the normal mechanism of the peritectic reaction. It is proposed that the mechanism of the reaction involves dissolution of 211 particles into the liquid and precipitation of solid 123. The aligned grain structure is explained through sympathetic nucleation of new 123 grains on existing grains.     

Effects of Temperature and Strain Rate on the Plastic Deformation of Fully Dense Polycrystalline Y1Ba2Cu3O7−x Superconductor

The knowledge of the steady-state stress for plastic deformation as a function of temperature and strain rate is essential for hot-forming superconducting material into commercially useful shapes. In this paper, results are presented on the experimental determination of the rheology of fully dense polycrystalline Y₁Ba₂Cu₃O_7−x superconducting material at temperatures ranging from 750° to 950°C and strain rates of 10⁻⁴, 10⁻⁵, and 10⁻⁶ s⁻¹. The data are best fitted by a power law: ε(s⁻¹)=8.9 × 10⁻¹⁷. (s⁻¹) σ^2.5 (Pa) exp [−2.01 × 10⁵(J·mol⁻¹)|RT]. X-ray analysis shows that the superconducting material retains its phase composition after nearly 70% total strain of the sample. A strong anisotropy in the resistivity of the deformed samples is observed because of the development of a preferred orientation of the a or b axis of Y₁Ba₂Cu₃O_7−x orthorhombic perovskite single crystals perpendicular to the principal maximum compressive stress.     

Effect of Oxygen Loss on Densification When Hot Isostatic Pressing YBa2Cu3O7−δ

Hot isostatic pressing of the high-T_c superconductor YBa₂Cu₃O_7−δ can lead to loss of oxygen and transformation of the material from the high-T_c orthorhombic phase to the nonsuperconducting tetragonal phase. It is shown that glass encapsulation helps retain the orthorhombic structure, whereas steel encapsulation resulted in formation of the tetragonal phase. Reasons for this phenomenon are discussed. The equilibrium oxygen gas pressure for the oxygen decomposition reaction in YBa₂Cu₃O₇, however, prevents full densification of this material in glass when employing hot isostatic pressing conditions of 200 MPa and 845°C.     

Formation of Grain-Boundary Carbon-Containing Phase During Annealing of YBa2Cu3O6+x

In situ annealing studies of YBa₂Cu₃O_6+x performed in an optical hot stage revealed that, at temperatures ∧450°, localized melting occurred. On subsequent cooling, a discrete second phase was observed at the YBa₂Cu₃O_6+x grain boundaries. Quantitative chemical analysis using X-ray wavelength dispersive spectroscopy indicated that the second phase was composed of a barium oxycarbonate. The source of the carbon in the second phase was identified to be CO₂ in the atmosphere.     

Transmission Electron Microscopy Study on the Annealing of YBa2Cu3O7-δ

Annealing in oxygen-rich atmosphere at temperatures between 400° and 600°C is an important step in the manufacture of superconducting YBa₂Cu₃O_7-δ. The symmetry of the orthorhombic phase requires that if more than one type of twin plane is present within a grain, a distorted region should exist inside the multiple twinned grain. This distorted region hinders the tetragonal-to-orthorhombic transformation and may account for some retained tetragonal phase inside an otherwise orthorhombic grain. A physical model is presented describing the formation of such regions and their eventual transformation into low-angle grain boundaries after long annealing. Extended annealing at intermediate temperatures apparently leads to the formation of planar faults in off-stoichiometric samples. Transmission electron microscope image contrast and energy dispersive X-ray analyses of highly defective regions suggest these defects are CuO _x ( x = 1, 2) extra layers. These extra layers tend to form near grain boundaries or free surfaces, where oxygen is readily available.     

Defect Equilibria and Transport in YBa2Cu3O7-x at Elevated Temperatures: II, Nonstoichiometry

Aspects of nonstoichiometry for the Y-Ba-Cu (1: 2: 3) system are considered. The general formula YBa₂Cu₃O_7-x has been assumed for considerations of nonstoichiometry in 1: 2:3 oxide cuprates. Assuming that copper ions may occupy different lattice positions (independently of their valency), the equilibrium constants for oxygen intercalation were determined:     

Experimental and Thermodynamic Study of Nonstoichiometry in 〈YBa2Cu3O7-x〉

The dependence of the nonstoichiometry of 〈YBa₂Cu₃O_7−x〉 (solid) has been studied over 5 orders of magnitude in oxygen pressure and from 573 to 1173 K. Hydrogen-reduction methods for determining the absolute oxygen to-metal ratio were developed. The resulting data were used to derive a chemical thermodynamic representation of the experimental variables. The data were used to derive a chemical thermodynamic representaltion of the experimental variables. The data were also compared with several other investigations to indentify the selfconsistent sets of data. The present data and thermodynamic data from the literature were correlated on an Ellingham diagram.     

Melt Processing of YBa2Cu3O7-x

Sintering YBa₂Cu₃O_{7- x} bulk forms at 1050°C followed by annealing at 980°C causes the development of a thick oriented surface layer (Lotgering factor = 0.7). The thickness of the layer depends on the thermal treatment, which is a two-step sintering process. Firing at 1050°C for 2.5 h followed by 30 h at 980°C leads to the development of a 0.1-mm-thick surface layer, with clear indication that longer annealing would result in a thicker film. Some orientation develops during un-axial compaction of the powders. Lotgering orientation factor calculation from X-ray diffraction analysis. SEM, and TEM were used to characterize the microstructure of these samples. T _c was similar to that of conventionally processed high-density samples, between 83 and 87 K. Some thermal treatments resulted in samples that displayed high resistivity above T _c , possibly caused by segregation of Cu to the grain boundaries.     

Defect Equilibria and Transport in YBa2Cu3O7-x at Elevated Temperatures: III, Defect Model and Conductivity Mechanism

Based on the measurements of electrical conductivity and thermopower (Seebeck coefficient) as a function of oxygen partial pressure, the defect structure and corresponding conduction mechanisms at elevated temperatures are considered for the Y-Ba-Cu-O (1:2:3) system. It has been postulated that the simple hopping model is not applicable to YBa₂Cu₃O_7-x. A modified conduction hopping mechanism has been proposed and equations describing the mobility of charge carriers in the studied system are derived. The most important advantage of the present model, in comparison to previous models, involves considering interactions between both electrons and electron holes and the resulting effect on their mobility terms. The observed departure of experimental data from the Heikes formula is explained by a very high concentration of defects and resulting substantial interactions between the defects which must be taken into account. The proposed transport model exhibits good agreement with experimental data of nonstoichiometry and the presently determined electrical conductivity and thermopower. It has been argued that the charge transfer mechanism depends substantially on the oxygen partial pressure and resulting oxygen content in the oxide lattice.     

Solution-Condensed YBa2Cu3O7 −x Superconductor Thin Films from Thermosetting Metal-Organic Precursors

Two different multimetal organic compounds were synthesized and used to deposit thin Y:Ba:Cu oxide films on selected metal and ceramic substrates by the dip-coating method. The rheology of the precursors is strongly influenced by the organic ligand, types of solvent, solvent–water molar ratio, and processing method. The precursor compounds were converted to suitable viscosity to achieve uniform film thickness processing on complex geometry. Superconducting transition temperatures T _c in the range of 89 to 93 K have been measured, depending on processing parameters used. The critical current density, J _c of the solution-coated films had values comparable to those for poly-crystalline samples. Y123 films exhibit c -axis alignment on Ag substrates. A prototype high- Q cavity was coated with Y123 and its performance was evaluated.     

Influence of Post Hot Isostatic Pressing on the Microstructural Development and Critical Current Density of YBa2Cu3O7-x

Samples of nearly theoretical density are produced by post hot isostatic pressing (post-HIP) of presintered YBa₂Cu₃O_7-x using argon as the pressure medium. The hot isostatically pressed samples show improved values for critical current density at 77 K. Experimental aspects of post-HIP are described, and microstructural development of YBa₂Cu₃O_7-x during sintering and post-HIP is discussed.     

Type II Magnetic Levitation on Sinter-Forged YBa2Cu3Ox Superconductor

Submicrometer powder of singlephase YBa₂Cu₃O_x (YBC), prepared from nitrate salts, were free-sintered and sinter-forged at 1223 K. The levitation force on magnets exerted by the superconductor was found to depend on the thickness of the superconductor, the shear strain imposed during sinter-forging, and the time and temperature of the high-pressure oxygen anneal after sinter-forging. The first result is direct proof of the type II nature of the YBC superconductor. In the second experiment a shear strain of 300% increased the levitation force by a factor of 3. The third result confirmed that YBC transforms to the tetragonal structure at 1223 K, which must be restored to the orthorhombic structure by annealing at lower temperature. Optimum results were obtained when the specimens were annealed in pressurized oxygen (3 MPa) for 50 h at 673 K.