Processing and properties of superconducting YBa2Cu3O7−x powder by single-step calcining in air

The preparation of orthorhombic YBa₂Cu₃O_7–x powder using only one calcination step is described. Yttrium and copper nitrates and barium carbonate were used as precursor materials. The use of an appropriate precipitant agent leading to a complete coprecipitation of the Y³⁺ and Cu²⁺ cations, promoted the rapid low temperature formation of YBa₂Cu₃O_7–x (900C for 4 h) with no secondary phases. X-ray diffraction patterns of the obtained powder showed the presence of the only one phase with the orthorhombic structure, and lattice parameters ofa=0.3824 nm,b=0.3893 nm,c=1.1676 nm and =1.775 which correspond to a superconducting YBa₂Cu₃O_6.95 phase. Sintering compacted samples at 940C several times led to highly dense ( 96% theoretical density) superconductor bodies, and a value ofT _c as high as 94 K was measured. Although AES depth profiles in fracture surfaces evidenced good grain boundaries of the superconducting YBa₂Cu₃O_6.95 samples, however, theJ _c measured was never greater than 15 A cm^–2, which could be due to a weak-link region between highJ _c grains. Slow degradation of the YBa₂Cu₃O_6.95 ceramics seems to take place by the formation of Ba(OH)₂ and YO(OH) at the superconductor surface in contact with the moisture of the air.     

Preparation and characterization of high-T c superconducting YBa2 Cu3 O7−x filaments

High-T _c superconducting filaments of the Y-Ba-Cu-O system were prepared using a low-cost suspension spinning method, where the solvent was removed by a phase inversion technique. The Y-Ba-Cu oxide precursor, containing polysulphone (PSF), was spun as a filament into a precipitating medium, removing the solvent by phase inversion and using water as a non-solvent. The green product filament was washed, dried and subjected to a heat treatment to remove the binding material and generate the oxide in, the appropriate superconducting phase. Stoichiometry, porosity and grain size were investigated by scanning electron microscopy, energy-dispersive X-ray analysis and electron probe micro analysis, while crystal structure was checked by X-ray diffraction analysis. Resistivity measurements as a function of temperature were performed by the four-point method and typicalT _c values of 88 K were observed, whileJ _c in the range of 125 A cm^–2 at 77 K and zero field.     

The superconducting state of YBa2Cu3O7−δ

Many low temperature properties of high T_c superconductors deviate significantly from the detailed predictions of BCS theory. Here we discuss whether these effects could be caused by either: (a) an unconventional pairing state, or (b) local randomness in the gap function due to the intrinsic disorder. We review recent experiments pertinent to these questions: Josephson effects in (001) oriented planar junctions between YBa₂Cu₃O₇- and classic superconductors and the temperature dependence of the a-b plane electro-magnetic penetration depth at low temperatures. We also calculate the density of states of s-wave superconductors with local quenched disorder in the gap function so as to determine whether s-wave pairing could be consistent with the low energy quasiparticle excitations seen in many experiments.     

Normal state properties of lithiated YBa2Cu3O7−x samples

Measurements of the resistivity in the normal state of lithiated YBa₂Cu₃O_7−x are reported. At low x_Li-values (<0.06), lithium substitutes copper of the CuO₂ planes. The samples are metallic; a progressive localization of the charge carriers is observed. In the intermediate concentration range, lithium occupies both copper sites planes and chains which leads to a two-phased system; the slope dϱ/dT is nearly independent of temperature. In the concentrated range, lithium is in the chain site. The resistivity behaviour is well described by the simple relation with ρ₀T₀=10Ω.cm.K, a result which is consistent with a single YBCO₆-type phase.     

Sintering of YBa2Cu3O7−x compacts

Heating of YBa₂Cu₃O_7−x compacts above about 930°C is shown to induce liquid formation. Presence of the liquid phase results in excellent densification, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities (≈ 500 A/cm²) have been obtained in these solid-state sintered samples.     

A study on the stability of superconducting YBa2Cu3O7−x phase

The thermal stability of the superconducting phase of nominal composition YBa₂Cu₃O_7–x -sintered pellets has been studied with respect to different temperatures (ranging from 300 to 950° C), time (ranging from 1 to 72 h), oxygen partial pressure (from 4 Pa to 1 atm) and carbon dioxide partial pressure (from 10^–4 Pa to 1 atm). Annealed samples were characterized by X-ray diffraction analysis, optical microscopy, and resistive measurements of the superconductive transition temperature. A stability field of the orthorhombic and tetragonal phases was obtained, showing a region of coexistance. The decomposition of the 1 2 3 phase is found to be strongly influenced by the presence of a small amount of CO₂ (1 p.p.m.) in the sintering atmosphere. A sintering process is proposed to avoid the formation of by-products.     

Preparation of superconducting YBa2Cu3O7−x powders by a solution technique

Bulk superconducting YBa₂Cu₃O_7–x powder has been synthesized by a solution technique using a mixture of Ba-ethylenediaminetetra-acetic acid (EDTA) and [Y, Cu]-citric acid complexes. A light-blue, molecular-level, homogeneously mixed precursor was prepared, and transferred to powder form through vacuum drying. The vacuum-dried powder was decomposed at 800 °C for 4 h under flowing oxygen, then heat treated at high temperature from 850 to 950 °C for 6–12 h. The results ofT _c measurements and X-ray analysis show that the orthorhombic, superconducting phase can be formed at temperatures above 850 °C following low-temperature annealing. A sharp transition (T2 K) and high density can be achieved after 930 °C heat treatment. The 930 °C heat treated sample shows aJ _c value of 510 A cm^–2. It is concluded that this solution technique provides better stoichiometric control and lower reaction temperature than the conventional solid-state sintering process.     

Growth and processing of single crystal superconducting YBa2Cu3O7−δ

A method for growing millimeter-sized crystals of YBa₂Cu₃O_7– is described. Processing conditions were developed to improve the superconducting transition temperature and sharpness, as observed by measurements of the dc magnetic susceptibility. Some of our crystals have been used in observations of the Raman Effect, specific heat, nuclear magnetic resonance, and nuclear quadrupole resonance, with results indicating that the samples are excellent.     

High-temperature deformation behaviour of YBa2Cu3O7−x

High-temperature compression tests were performed in air for YBa₂Cu₃O_7–x polycrystals with grain sizes of 3 and 7 m at various strain rates between 1.3×10^–5 and 4×10^–4s^–1 and at temperatures between 1136 and 1253 K. Steady state deformation appeared above 1203 K for both samples. A stress exponent of 1.3 and an activation energy of 150 kJ mol^–1 were evaluated. The compression tests and microstructural observations revealed that there was a difference in deformation mechanism above and below 1203 K. The dominant mechanism was diffusional creep associated with grain-boundary sliding above 1203 K, and dislocation glide accompanied with grain-boundary sliding below 1203 K. The growth of anisotropic grains and their preferred arrangement were enhanced by deformation.     

Laser patterning of superconducting YBa2Cu3O7–x thin films

Abstract

Epitaxial YBa₂Cu₃O_7–x thin films have been patterned using aKrF excimer laser. Contact masks and mask imaging have been used to obtain YBa₂ Cu₃O_7–x bridges 8–20 μm in width. The process has been carried out under different environmental and laser fluence conditions. The best morphology is obtained in vacuum and with the highest laser fluences. The material composition in the bridges depends strongly on the fluence but not on the environment. The composition edges become steeper as the fluence is increased over the ablation threshold. Mask imaging allows retention of the electrical properties of the original film in the bridges, with a critical temperature greater than 91 K, a transition width of 1·5 K, and a critical current density greater than 10⁶ A cm^?2 at 77 K. The bridges defined by contact masks have slightly inferior superconducting properties but they can be recovered by annealing in flowing oxygen.

MST/3224     

Physical properties of YBa2Cu3O7−x superconductor sintered at different temperatures

YBa₂Cu₃O_7–x ceramics prepared by sintering at 920 and 1030° C were studied in some detail. Although samples prepared at 920° C had lower density, they had larger transport critical current at 77 K. A correlation between the transport critical current and the magnetic field dependence of the second maximum in the imaginary part of the initial susceptibility was found and discussed. According to our results samples prepared at higher temperature had larger but (in spite of almost ideal density) poorly connected grains.     

Electromagnetic properties of electrophoretic YBa2Cu3O7−δ films

In order to apply highT _c superconductivity to general microwave cavities and to shielding devices for quasistationary magnetic fields, we have developed an electrophoretic coating technique. This work reports about the continuation of our experiments. More than 60 samples of electrophoretic layers deposited on silver substrates were fabricated by systematically varying the processing parameters. The r.f. surface resistance measured at 21.5 GHz and 77 K at low excitation fields is found to be a sensitive measure of the quality of the samples and falls rapidly with increasing average grain size. Textured electrophoretic layers of large grain size show the smallest sensitivity ofR _s against the r.f. surface magnetic fieldH _s. The magnetic sensitivitydR _s /dH _s is found to be in direct proportion toR _s (77 K). After optimizing the sintering procedure the electrodes of the prototype cavity for a compact hydrogen maser were coated. The cavity was successfully operated at 1.42 GHz, and a surface resistance of 1 mΩ was achieved at 77 K. This compares to 4 mΩ for copper at the same temperature.     

Investigation of Lu effect on YBa2Cu3O7−δ superconducting compounds

This study reports the effect of Lu addition on the microstructural and superconducting properties of YBa₂Lu_xCu₃O_7?δ (Y123) superconducting samples with x = 0, 0.1, 0.3, 0.5 and 0.7 by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistivity and transport critical current density (J_c) measurements. The samples prepared by the liquid ammonium nitrate and derivatives are exposed to various annealing time (20, 40 and 60 h) and temperature (950, 960 and 970 °C), and the best ambient for the sample fabrication is determined to be 970 °C for 20 h. Zero resistivity transition temperatures (T_c), critical current densities (J_c), variation of transition temperatures, hole-carrier concentration, grain size, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the bulk superconducting samples prepared at 970 °C for 20 h are compared with each other. T_c and J_c values of the samples are inferred from the dc resistivity and the critical current measurements, respectively. The results show that the T_c value of the pure sample is about 90.6 K while the sample doped with 0.1 wt% Lu has the maximum T_c value (92.5 K). However, beyond x = 0.1, the T_c value is observed to decrease toward to 83.5 K with increment in the Lu addition. Similarly, the J_c values measured are found to reduce from 142 to 76 A/cm² with the addition. Moreover, XRD measurements show that both pure and Lu-doped samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain Y123 and Y211 phase, confirming the incorporation of Lu atoms into the crystalline structure of the samples studied. At the same time, comparing of the XRD patterns of samples, the intensity ratio of the characteristic (110) and (013) peaks on the sample doped with 0.1 wt% Lu is more than that on the other samples prepared. Additionally, SEM images display that the sample doped with 0.1 wt% Lu obtains the best crystallinity, grain connectivity and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.7). Further, EDX results demonstrate that the Lu atoms doped are successfully introduced into the microstructure of the Y123 samples studied and the maximum Cu element level is observed for the sample doped with 0.1 wt%, explaining that why this sample obtains the best superconducting properties compared to others. According to all the results obtained, it is concluded that the 0.1 wt% Lu addition into the Y123 system improves the microstructural and superconducting properties of the samples studied.     

Fabrication and microstructure of the d.c.-magnetron-sputtered YBa2Cu3O7 − x superconducting thin films

Systematic investigations were conducted to determine the effect of deposition conditions on the microstructure and superconducting properties of YBa₂Cu₃O_{7 – x} thin films produced by d.c. magnetron sputtering on (001) MgO substrate. The films were c-axis preferentially oriented with respect to the (001) MgO surface at substrate temperatures of 680–700 C. X-ray diffraction patterns clearly indicated the existence of the c-axis alignment normal to the substrate surface and some second phases. The second phases, including a Cu-rich phase and Y₂O₃, were identified by energy dispersive X-ray spectrometer and the microstructures were analysed by electron and atomic force microscopes.     

Nonlinear Power Handling of YBa2Cu3O7−x Films and Microwave Devices

Physical mechanisms which limit the power handling of YBa₂Cu₃O_7?x films and devices are discussed in terms of a quantitative classification scheme. The possible limitations are devided into magnetic or thermal, and global or local in nature. Analytical estimations are compared with measurements of YBa₂Cu₃O_7?x films (Ø = 1″–2″) using a niobium-shielded sapphire resonator at 19 GHz, and disk resonators at 2 GHz. Magnetic effects are found to play an essential role in nonoptimized films in terms of weak-links, and in high-quality films if the lower critical field Bcl is reached. The majority of films and disk resonators appear to suffer from microwave heating. Global heating appears predominantly at CW operation. Local heating results mainly from defects in films of medium quality. Defect-induced quenches are observed at moderate field levels, sometimes resulting in an irreversible degradation of the power handling.     

Preparation,crystallization and properties of rapidly solidified YBa2Cu3O7−δ

YBa₂Cu₃O_7−δ was prepared in the glassy state by rapid solidification from the melt. The quenched material was paramagnetic down to liquid-He temperature. Diamagnetic behavior developed after crystallization of the glass at high temperatures (> 800°C). A well-defined superconducting transition occurred at 90 K for material heat treated and crystallized above 900 °C.     

Fabrication and characterization of YBa2Cu3O7−x /Sn composite superconductors

YBa₂Cu₃O_7–x /Sn (YBCO/Sn) composites sintered at 230 °C exhibited a percolation threshold for electrical conductivity at 20 vol% Sn, with semiconducting and metallic behaviour below and above it, respectively. The simultaneous decrease in =d/dT and ₀ with increase in Sn content was related to formation of defect-free interfaces. The diamagnetic shielding property of the composites weakened with Sn content, as deduced from magnetic levitation experiments.     

Analysis of deformed YBa2Cu3O7−δ

In this study, polycrystalline YBa₂Cu₃O_7– (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10^–4 sec^–1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa₂Cu₃O_7– and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.     

Microstructure and dielectric properties of BaZr x Ti1−x O3 ceramics

The crystalline structure and dielectric properties of BaZr _x Ti_1−x O₃ ceramics with x = 0.05, 0.10, 0.15, and 0.20 were investigated. As zirconium increased, the a-axis lattice constant gradually increased, however, the c-axis lattice constant and c/a ratio gradually decreased. When x = 0.20, the crystal structures of the BZT ceramics are very close to cubic, different from the tetragonal structure when x < 0.20. The temperature dependence of the dielectric constant was studied and an enhanced diffuse phase transition behavior is found to be caused by the increased Zr content. The decreases of coercive electric field and remanent polarization were the result of increase of Zr/Ti ratio in BaZr _x Ti_1−x O₃.