Microstructure Study at Different Thicknesses of Disc-Shaped YBa2Cu3O7?x Prepared by FQMG Method

The flame-quench-melt-growth (FQMG) method was used for the preparation of bulk YBa₂Cu₃O_7?x (Y123). Both the superconducting and nonsuperconducting regions changing with thickness of superconducting sample were determined by X-ray diffraction (XRD) patterns and also observed at micrographs taken by the polarized optical microscope. It was observed that superconducting forming ratio which was examined for different thicknesses of the sample ranging from 6.54 to 1.58 mm strongly enhanced. This means that a region near the top surface of the sample contains more superconducting phase than the other regions. These observations would be helpful in designing and manufacturing the practical applications of YBCO systems.     

Bulk pulsed-current magnetostriction of an YBa2Cu3O7−x ceramic superconductor

The bulk magnetostriction of an YBa₂Cu₃O_{7− x} ceramic superconductor under the action of dc current pulses was measured at T=77 K with the aid of a differential dilatometer with an error not exceeding 10⁻⁷. The magnetostriction in this material is negative and obeys a logarithmic law as function of the current density. It is demonstrated that a considerable contribution to magnetostriction of the YBa₂Cu₃O_{7− x} ceramics is due to the grain boundary layers.     

Fabrication of Micro-pipes of YBa2Cu3O7−δ Superconductor

Synthesis of micro-pipes of YBa₂Cu₃O_7−δ superconductor has been carried out for the first time by using chemical method. The shape, size and length of these micro-pipes depends upon dimensions of growth matrix and structure of geometry, i.e., circular, square, rectangular and hexagonal, etc., which could be uniformly coated with superconductor. The onset temperature of diamagnetism of the material is found to be 93 K, which is not shifted to lower values by the application of DC magnetic field; this is most likely due to large surface area provided by micro-pipes to the shielding currents. These micro-pipes behave as pinning centers, hence they can be used for the fabrication of devices that are capable of working in high magnetic fields.      

Intermittent Flux Penetration at Different Temperatures in?YBa2Cu3O7?x on NdGaO3 Substrates

In type-II superconductors, increasing applied magnetic field penetrates gradually in the form of magnetic vortices. It is of great interest to understand the dynamics of magnetic flux in different superconducting materials, as this phenomenon can severely limit the performance of superconductors in applications. YBa₂Cu₃O_7?x (YBCO) is an important high-temperature superconductor, but until recently, it has been hard to make wires from it due to misalignment of superconducting grains. A solution to this problem is to deposit YBCO on vicinal substrates to better align the grains. Some of these samples show a strongly intermittent flux penetration at low temperatures. In this work, we have studied flux penetration in YBCO deposited on a 14° vicinal substrate of NdGaO₃ (NGO) at different temperatures.     

Dependence of the Structural, Electrical and Magnetic Properties of YBa2Cu3O7?δ Bulk Superconductor on the Ag Doping

The effects of Ag dopants on the superconducting properties of YBCO bulk samples, prepared under solid-state reaction method, have been studied by resistivity?Ctemperature (???CT), X-ray diffraction (XRD), scanning electron microscope (SEM), bulk density, AC susceptibility, DC magnetization measurements, and theoretical analysis. Small Ag substitutions (x??0.10) do not effect T _c of pure YBa₂Cu_3?x Ag _x O_7??? , while we observed small decreases of 1.5?K for larger doping levels (x??0.15). AC susceptibility measurements for sintered YBCO pellets have been performed as a function of temperatures at constant frequency and AC field amplitude in the absence of DC bias field. The critical current densities (J _c ) have been estimated as a function of magnetic field from the magnetization data employing Bean??s critical state model. The increase in Ag amount (x) in YBa₂Cu_3?x Ag _x O_7??? (x??0.10) system effectively decreases the intragrain critical current density, which is attributed to the reduced connectivity between the grain boundaries. In the case of small additions (x=0.05), Ag atoms fill partly the pores, leading better crystallization of the grains.     

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.     

Enhanced Critical Current Density of FeF2 Added YBa2Cu3O7?δ

The effect of nanosized FeF₂ (??120?nm) addition on the critical current density, J _c, of YBa₂Cu₃O_7??? was studied. YBa₂Cu₃O_7??? superconductor was prepared by the solid-state reaction method with addition of 0 to 0.10 wt% FeF₂ additions. The magnetization critical current density, J _cm (at 77?K), was estimated from the extended Bean model by using vibrating sample magnetometer (VSM) measurements. The transport critical current density, J _ct, was measured using the four-point probe method with the 1???V/cm criterion. The onset transition temperature, $T_{\mathrm{c}\mbox{-}\mathrm{onset}}$ showed a maximal value at 92?K for 0 to 0.03?wt% additions. J _c increased with FeF₂ addition up to 0.03?wt%. Addition of FeF₂ of more than 0.03?wt% suppressed $T_{\mathrm{c}\mbox{-}\mathrm{onset}}$ and J _c. The inter-grain current density was enhanced by two orders of magnitude on 0.03?wt% FeF₂ addition. In addition to the condensation energy associated with their core, the full vortex magnetic energy by addition of nanosized FeF₂ enhanced J _c in the system.     

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.     

Revisiting Heat Capacity of Bulk Polycrystalline YBa2Cu3O7−δ

In this letter, we present the superconducting property characterization of a phase pure, reasonably good quality YBa₂Cu₃O_7?δ sample. Studied compound is crystallized in orthorhombic Pmmm space group with lattice parameters a, b, and c being 3.829(2), 3.887(1) and 11.666(3) Å, respectively. Bulk superconductivity is observed below 90 K as evidenced by resistivity and dc/ac magnetization measurements. The resistivity under magnetic field (ρTH) measurements showed clearly both the intra-grain and inter-grain transitions, which are supplemented by detailed (of varying frequency and amplitude) ac susceptibility studies as well. The upper critical field at 0 K, i.e. H _c2(0) being determined from ρTH measurements with 50 % criteria of resistivity drop, is ～70 T. Studied polycrystalline YBa₂Cu₃O_7?δ is subjected to detailed heat capacity (C _P ) studies. C _P exhibited well-defined anomaly at below 90 K, which decreases with applied field. Although the C _P anomaly/peak at T _c reduces with applied field, the same is not completely suppressed in high applied fields of up to 12 T. The Sommerfeld constant (γ) and Debye temperature (Θ _D), as determined from low temperature fitting of C _P (T) data to Sommerfeld–Debye model, are 10.65 mJ/mole-K² and 312.3 K, respectively. The results are compared with existing literature on bulk polycrystalline superconducting YBa₂Cu₃O_7?δ sample.     

Growth of YBa2Cu3O7−x thin films by ion beam sputtering

Polycrystalline, superconducting YBa₂Cu₃O_7−x films have been grown by ion beam sputtering (IBS) of bulk stoichiometric ceramic target. We found that the superconducting properties of the films are very sensitive to the choice of the substrates and post-deposition annealing conditions. Films deposited on SrTiO₃ substrates were found to be the best with a critical onset transition temperature, T_c to be as high as 95 K with a maximum current carrying capacity in superconducting state of 1.5 × 10⁵ A/cm² which is close to the best reported value for single-crystal YBa₂Cu₃O_7−x films.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

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.     

YBa2Cu3O7−x Thin Films by Citrate-Based Non-fluorine Precursor

A new fluorine-free chemical solution deposition route has been developed for YBa₂Cu₃O_7−x (YBCO) thin films. The precursor was formed by dissolving metal nitrates and citric acid in ethylene glycol. Polyethyleneimine was used to modify the solution. The preparation of YBCO thin films consisted of repeated dip coating, drying, heat-treating in air and annealing in flowing argon with a small addition of oxygen and with final treatment in pure oxygen. The films were characterized by XRD and SEM and show dominating and well-textured YBCO phase. The AC susceptibility and magnetization measurements reveal a critical temperature about 90 K and a high critical current density, comparable with that of pulse laser deposited films. The simplicity and high performance of the method may be attractive for large-scale superconducting applications.     

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.     

Shadowed Off-Axis Pulsed Laser Deposition of YBa2Cu3O7?x Thin Films

Using a shadowed pulsed laser deposition allows us to reduce the number of droplets on the YBa₂Cu₃O_7–x film surface obtained by pulsed laser deposition method. In this study, we present a new modification of pulsed laser deposition method: a shadowed off-axis pulsed laser deposition. The substrate surface lies in the plain defined by the laser beam. A rectangular shadow mask located between target and substrate reduce the quantity of large particles deposited from the diffusion flow. Such geometry allows us to carry out the laser plume scanning on the target surface for simultaneous one-step double-sided deposition films with a droplet-free surface. Both films had good superconducting properties.     

Superconducting Properties of Ag and Sb Substitution on Low-Density YBa2Cu3O δ Superconductor

The effects of silver (Ag) and antimony (Sb) substitution on low-density YBa₂Cu₃O _δ (YBCO) superconductor were investigated. Two series of sample with a nominal composition of YBa_2?x Ag _x Cu₃O _δ and YBa_2?x Sb _x Cu₃O _δ where x=0.05, 0.10, 0.15, 0.20, 0.30, 0.40 and 0.50 were synthesized and characterized. All Ag-doped samples showed metallic behavior at the normal state and T _C?onset was found at 90 K. T _C?zero decreased as the Ag concentration increased. Optimum Ag concentration was achieved at x=0.20 where T _C?zero has the highest value of 87 K and J _C at 70 K is 16.50 A/cm². For Sb-doped case with x≤0.30, the samples showed metallic behavior above T _C?onset while semiconducting behavior was shown for x≥0.40. The optimum Sb concentration was achieved at x=0.15 where T _C?zero is 85 K and J _C value measured at 70 K is 2.75 A/cm². T _C?onset and T _C?zero were found to decrease toward higher Sb concentration. The crystallographic structure transformed to tetragonal in Sb-doped samples of x≥0.30 while other samples remain orthorhombic.     

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     

Significant Change in Mechanical Properties of YBa2Cu3O7−x Bulk Superconductors Diffused with Sn Nanoparticles

Mechanical behavior of YBa₂Cu₃O_7?x (Y123) superconductors exposed to Sn nanoparticles diffusion is determined by the way of Vickers microhardness (H _v ) conducted at different applied loads (0.245N≤F≤2.940 N). Load dependent microhardness, load independent microhardness, elastic modulus, and yield strength values are estimated from the microhardness curves. Unpredictably, the findings of the H _v values reveal that the undiffused sample and Sn diffused sample prepared at 500 ^°C exhibit reverse indentation size effect (RISE) behavior while the other samples obey indentation size effect (ISE) nature. Further, we extract the load independent (true) microhardness using the Meyer’s law, proportional specimen resistance (PSR), elastic/plastic deformation (EPD), Hays–Kendall (HK) approach and indentation-induced cracking (IIC) model, and compare the true hardness with the apparent hardness.     

Buffer layers for the growth of YBa2Cu3O7−x films on silicon

Experimental structural characteristics of the SrTiO₃, NdGaO₃, and CeO₂ buffer films obtained by ion-plasma sputter deposition on silicon substrates are presented. It is demonstrated that the phase composition and the level of internal stresses in the films strongly depend on the deposition temperature. Optimum technological conditions for the growth of oriented SrTiO₃, NdGaO₃, and CeO₂ films on silicon are determined. The structural perfection of the buffer layers thus obtained is sufficient to provide for the subsequent growth of high-quality YBa₂Cu₃O_7−x films on silicon substrates.