Microstructure and properties of K2O doped superconducting YBa2Cu3O7−x

K doped YBa₂Cu₂O _x powders have been obtained by incorporating K₂CO₃ to previously coprecipitated amorphous hydroxide mixtures. Synthesis was performed at 900C and an orthorhombic 1 2 3 phase was the final product. Sintering of isopressed bars was carried out from 920 to 940C. K doping up to 5 at % led to the development of dense bodies with good superconducting behaviour. Oxygen pick-up during the cooling step following sintering was well accomplished despite the high density of the samples. Transition temperatures 90 K were measured. Doping above that percentage led to samples with lower density values. The presence of non-superconducting phases, such as CuO and a green phase, was very significant, and suppressed the superconducting transition.     

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.     

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.     

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.     

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.     

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     

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.     

A photoemission study of the YBa2Cu3O7–Au composite

Characterization of the YBa2Cu3O7–Au composite at room temperature has been performed using X-ray photoelectron spectroscopy. Measured spectra on the chemically etched surfaces have been found to be representative of the bulk material. Analysis of the core-level spectra of copper indicated the contribution of the Cu(3+) valence state in the samples. The core-level spectra of gold showed the Au valence state to be between 1+ and 2+. The incorporated gold atoms had no influence on the level positions of the other constituents of the material.     

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.     

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.     

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.     

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.     

Influence of preparation conditions on characteristics of the YBa2Cu3O7−δ compound

Synthesis of ceramic YBa₂Cu₃O_7– by solid-state reaction was performed under different conditions. Different values of cooling rate and oxygen flow were used, and no significant influence on superconducting characteristics of the samples was observed. A major influence on their mechanical properties was found.     

The phase conversion of precursor powders for powder melting process YBa2Cu3O7−x superconductors

The effects of heating rate and holding time on the formation of YBa₂Cu₃O_7−x phase in precursor powders for YBa₂Cu₃O_7−x superconducting bulks prepared by powder melting process have been investigated. The phase conversion of the precursor powders is studied by X-ray diffraction and found to be different for different heating rates during heating. The YBa₂Cu₃O_7−x phase is formed during heating to peritectic temperature at 100 and 400 °C/h, but not at 6,000 °C/h. The longer the holding time, the more the amount of YBa₂Cu₃O_7−x phase between 880 °C and about 950 °C. The results are useful for understanding the mechanism of powder melting process and controlling the process conditions.     

Superconductivity of YBa2−x Nd x Cu3O y Solid Solution

The solubility of Nd at the Ba sites and the superconductivity of YBa_2?x Nd _x Cu₃O _y were investigated by X-ray powder diffraction and measurements of the electrical resistance and ac susceptibility. The single Re123 phase was obtained for x≤0.30. The onset transition temperature $T_{\text{c}}^{{\text{on}}}$ is insensitive to the Nd content x in the region of x≤0.40. All are higher than 95 K. The zero resistance transition temperatures $T_{\text{c}}^{{\text{zero}}}$ , however, exhibits two-step variation with the increase of x. For x≤0.25, $T_{\text{c}}^{{\text{zero}}}$ are all above 92 K. The highest $T_{\text{c}}^{{\text{zero}}}$ of 94 K was obtained for x=0.25. For x≥0.3 $T_{\text{c}}^{{\text{zero}}}$ drops sharply to about 84 K. Finally $T_{\text{c}}^{{\text{zero}}}$ falls to 30 K and $T_{\text{c}}^{{\text{zero}}}$ is below 10 K for x=0.5. The two-step variation of T _c might be an indication of the existence of two trap levels for holes.     

An Approach of the Sintering YBa2Cu3O7−δ System

The abnormal glow discharge applied to the sintering process is a recent technique used for processing both metallic and ceramic materials. In this paper, we use the abnormal glow discharge as an alternative method for the sintering step of the YBa₂Cu₃O_7?δ superconductor material. The physical properties of the sintered samples by glow discharge were compared with the properties of sintered samples in resistive furnace, which is commonly used for production of high temperature superconductor ceramics. The structural analysis of the YBa₂Cu₃O_7?δ samples was carried out by the X-ray diffraction technique, microstructural analysis by Scanning Electron Microscopy and electrical resistivity by the four probes method (resistance measurements as a function of temperature). The experimental results permitted to establish the similar structure and morphology for all samples: produced by plasma and resistive furnace sintering. The superconducting behavior was corroborated for both sintering processes.     

Novel approaches in the melt-texturing of YBa2Cu3O7−y

Electrochemically pre-textured samples have been subjected to melt-processing in order to produce dense and highly textured bulk samples of YBa₂Cu₃O_7–y (YBCO). Full oxygenation of these samples has been achieved at high processing temperatures of 720 °C by electrochemical titration, in order to increase the superconducting transition temperature to > 89 K. These samples show a large magnetic hysteresis, and the value of J _c calculated using the Bean model is in the range 4000–6000 A cm^–2 at 77 K and 1 tesla magnetic field, and is independent of the applied field in that range. In another variation of the melt-processing technique — referred to as isothermal melt-textured growth — highly textured samples have been produced by the movement of the solidification front at a constant temperature in an oxygen activity gradient.