Elastic anomaly of YBa2Cu3O7−x superconductors containing different oxygen contents

Four samples of YBa₂Cu₃O_7–x with different values of x were prepared and characterized, and their elastic properties from 80 K to room temperature were investigated. It was found that an anomalous softening of the elastic moduli G, K, and E temperatures slightly higher than T _c appeared only on the specimens with the superconducting orthorhombic phase. As the value of x became small and the transition width T _c became narrower, the softening became remarkable. The elastic constants also showed a thermal hysteresis at low temperatures. Relationships between this behaviour and proposed structural and domain changes are discussed.On leave from University of Mar del Plata, Argentina.     

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.     

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.     

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.     

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.     

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 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.     

Novel sponge-like structure created in GdBa2Cu3O7−δ ceramics by hot spot

A hot spot, which is a local area glowing orange, appears in a GdBa₂Cu₃O_7−δ ceramic rod when a voltage exceeding a certain value is applied to the rod at room temperature. The rod with the hot spot shows various functional characteristics that give rise to applications in devices. We found that the hot spot created a sponge-like structure in the rod. The elemental map revealed that the sponge-like structure was composed of Gd₂BaCuO₅ grains. The hot spot is considered to decompose GdBa₂Cu₃O_7−δ into Gd₂BaCuO₅ and liquid phase, and the liquid phase moves toward the periphery of the rod, leaving sponge-like structure composed of Gd₂BaCuO₅ grains. The novel sponge-like structure created in the GdBa₂Cu₃O_7−δ rod by the hot spot may bring about new applications for magnetic separations of fluids.     

Characterization of β-Ga2O3 films deposited under different growth temperature by pulsed laser deposition

Journal of Materials Science: Materials in Electronics - A series of β-Ga2O3 films were prepared on polished Al2O3 substrates by pulsed laser deposition at temperatures between 580 and 780...     

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.     

Terahertz Conductivity of Overdoped Y1−x Ca x B2Cu3O7−δ

In the current work we have investigated the terahertz response of Ca overdoped YBaCuO thin films using both time and frequency domain spectroscopy methods. For both methods a basic data analysis was performed using the two fluid and a variable dielectric function (VDF) models. The imaginary part of the conductivity was proportional to 1/ω, known from the delta-function response. The real part of conductivity showed a well known frequency and temperature dependence, where it increases below T _c and obtains maxima at about 50 K. However, a sharp decrease of the real part of the conductivity was observed at about 10 cm^?1. This decrease occurs below T _c and becomes dominant as temperature decreases. It was observed on the 5% and 10% Ca doped samples but it was more dominant on the 10% case. Moreover, this sharp decrease in σ ₂(ω)at 10 cm^?1 was not observed in optimally doped YBCO samples. We would like to stress at this point that these values are much smaller than those obtained by Microwave and Tunneling measurements, arguing for the existence of a complex order parameter in the overdoped regime with an imaginary component of about 1.8 meV.     

Suppression of superconductivity in the Er1− x Pr x Ba2Cu3O7−δ

Measurements of electrical resistivity. X-ray diffraction patterns, magnetic susceptibility and thermoelectric power of the Er_1-x Pr _x Ba₂Cu₃O_7- system have been made. The superconducting transition temperature was found to decrease monotonically with praseodymium concentration, x. From the susceptibility data, it was determined that the valence of praseodymium lies between +3 and +4. The thermoelectric power was found to increase with x, and the slopes of dS/dT were negative except for the case x= 0. The. tendency of the thermopower to change with increasing praseodymium concentration has been qualitatively explained using the theory for strongly correlated systems.     

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.     

Preparation of High-Performance SmBa2Cu3O7−x Superconducting Films by Chemical Solution Deposition Approach

Pure and Co³⁺-doped SmBa₂Cu₃O_7?z (SmBCO) superconducting films were prepared on (00l) LaAlO₃ single crystal substrate by self-developed fluorine-free chemical solution deposition approach. According to the X-ray diffraction and SEM observation, SmBCO films with biaxial texture possess dense, smooth, and microcrack-free surface microstructures. However, critical transition temperature (T _c) of a Co³⁺-doped SmBCO film is lower than that of pure SmBCO film, which may be attributed to the Co³⁺ doping decreasing the concentration of hole carriers for doped film. In addition, the Co³⁺-doped film has higher normalized critical current densities (J _c) in the whole magnetic fields, indicating better magnetic flux pinning properties. These results show that Co³⁺ doping by this chemical method is one of the promising ways to prepare high-performance SmBCO films.     

Sn diffusion coefficient and activation energy determined by way of XRD measurement and evaluation of micromechanical properties of Sn diffused YBa2Cu3O7−x superconducting ceramics

This study manifests not only the effect of Sn diffusion on physical, electrical, mechanical, structural and superconducting properties of the bulk YBa₂Cu₃O_7?x (Y123) superconductors prepared by the conventional solid-state reaction route by use of electrical resistance, X-ray diffraction analysis (XRD), electron dispersive X-ray, scanning electron microscopy, transport critical current density (J _c ) and Vickers microhardness (H _v ) measurements but also the diffusion coefficient and the activation energy of tin (Sn) in the Y123 material for the first time. The diffusion coefficient and the activation energy of Sn are investigated in the temperature range 500–945 °C using the change of the lattice parameters extracted from the XRD patterns. The resistance (at room temperature), critical (onset and offset) temperature, variation of critical temperature, hole-carrier concentration, crystallinity, lattice parameter, texturing, surface morphology, lotgering index, element distribution, critical current density, oxygen content, load dependent microhardness, elastic modulus and yield strength values are obtained for the pure and Sn diffused samples and compared with each other. One can see that all superconducting parameters given above depend sensitively on the Sn diffusion on Y123 system. The obtained results exhibit that the room temperature resistance enhances with the Sn diffusion because of the hole filling when the onset $ (T_{c}^{onset} ) $ ( T c onset ) and offset $ (T_{c}^{offset} ) $ ( T c offset ) critical temperatures are obtained to be about 93.4 and 89.6 K for the pure sample as against 92.2 and 88.1 K for the Sn diffused sample, respectively. This may be attributed to the fact that the decrement in the critical temperatures is due to the deterioration of crystallinity and descend in the grain size. As for the critical current density measurements, J _c values are obtained to be about 125.4–65.3 A/cm², respectively, for the undiffused and Sn diffused materials. This may be led to the decrease of the flux pinning mechanism stemming from the stacking faults, planar and micro-defects. At the same time, XRD measurements display that the samples produced in this work exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines. Besides, the peak intensities belonging to major phase (Y123) decrease monotonously with Sn diffusion in the system; however, new peaks belonging to the minor (BaCuO₂) phases start to appear for Sn diffused sample confirming both the reduction of the grain size and degradation of the critical temperature. Moreover, the pure sample is confirmed by both enhancement of a and b lattice constants and the decrement of the cell parameter c of the sample in comparison with that of the Sn diffused sample. According to SEM examination, the crystallinity and grain connectivity suppress with the Sn diffusion. EDX measurements illustrate that not only do the elements used for the preparation of the Y123 superconductors with and without Sn diffusion distribute homogeneously but also the level of Cu element reduces with the Sn diffusion, presenting that the Cu²⁺ ions may partly be diffused by tetravalent tin (Sn⁴⁺) ions. Further, surprising results of the Vickers microhardness values demonstrate that the pure sample visualizes Indentation Size Effect (ISE) feature; however, the Sn diffused sample reports Reverse Indentation Size Effect (RISE) nature. Additionally, the diffusion coefficient is observed to increase from 1.11 × 10^?9 to 2.82 × 10^?8 cm² s^?1 as the diffusion-annealing temperature increases, verifying that the Sn diffusion at lower temperatures is much less significant as compared to the higher ones. Temperature dependence of the Sn diffusion coefficient and activation energy in the range of 500–945 °C is defined with the aid of the following equation: $$ D = 7.78 \times 10^{ - 6} { \exp }\left[ {\left( {( - 0.590 \pm 0.005){\raise0.7ex\hbox{${\text{eV}}$} \!\mathord{\left/ {\vphantom {{\text{eV}} {k_{B} T}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${k_{B} T}$}}} \right)} \right] $$ D = 7.78 × 10 ? 6 exp [ ( ( ? 0.590 ± 0.005 ) eV / eV k B T k B T ) ] .     

Scanning/friction force microscopy study of YBa2Cu3O7−δ single crystals grown in BaZrO3 crucibles

Very pure YBa₂Cu₃O_7– (YBCO) single crystals grown in BaZrO₃ crucibles are studied in the as-grown and the oxidized state by scanning force (SFM), friction force (FFM) and scanning tunneling microscopies (STM). The images show clean terraces with step-heights of one unit cell along YBCO(001), i.e. 1.2 nm. Only close to step edges is material contrast observed by FFM indicating traces af flux. Some crystal surfaces exhibit over-layer features, such as star-like, ribbon-like and checkerboard-like structures, which exhibit friction contrast implying the presence of different materials on the surface. Tunneling spectroscopy at 4.7 K in high vacuum reveals a superconducting energy gap of 2 26 me V.     

Physical and Mechanical Properties of GdBa2Cu3O7−δ Added with Nanosized CoFe2O4

Superconductor samples of type (CoFe₂O₄) _x GdBa₂Cu₃O_7?δ , 0.0≤x≤0.1 wt.%, were synthesized by the conventional solid-state reaction technique. The prepared samples were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), electrical resistivity, AC magnetic susceptibility, and Vickers microhardness. Phase examination by XRD indicated that the orthorhombic structure of Gd-123 is not affected by nanosized ferrite CoFe₂O₄ addition, whereas the volume fraction of Gd-123 increased with x up to 0.01 wt.%. Morphological investigations by SEM revealed that the porosity among grains decreased as x increased from 0.0 to 0.1 wt.%. The superconducting transition temperature T _c was estimated from electrical resistivity and the AC magnetic susceptibility measurements. A reduction in T _c was observed as x increased from 0.0 to 0.1 wt.%, consisting with the effect of adding magnetic impurities to HTSCs. Room temperature microhardness measurements were carried out at different applied loads (0.5–10 N) for the study of the mechanical performance of the studied samples. The experimental results were analyzed using Meyer’s law, Hays–Kendall approach, elastic/plastic deformation model, and proportional specimen resistance model. The results showed that all the samples exhibited normal indentation size effect, and Vickers microhardness values were increased as nanosized CoFe₂O₄ addition increased. The analysis showed that the proportional specimen resistance model is the best model for computing the load-independent microhardness values of our samples.     

Fluctuation Conductivity of RE1−2x Ca x M x Ba2Cu3O7−δ (RE=Nd,Y and M=Pr,Th) Superconductors

Polycrystalline samples of RE_1?2x Ca _x M _x Ba₂Cu₃O_7?δ with RE=Nd, Y and M=Pr, Th (with 0.0≤x≤0.10), superconductors were prepared by the standard solid-state method. Resistivity was measured as a function of temperature and doping concentration x. Excess conductivity was analyzed using the modified Lawrence-Doniach (LD) expressions. The fluctuation regions, crossover temperatures, coherence lengths, and effective layer thickness were obtained and the values were compared for both samples. For both samples, it was found that with increasing doping, the crossover temperatures were reduced, while the coherence length decreased. The upper critical field and critical current density were increased with increasing doping concentration due to the introduction of disorder and the enhancement of flux pinning by charge neutral doping. Furthermore, the coherence lengths of the Nd-based samples are larger than that for the Y-based samples by a factor 2. It was found that the value of critical current density in Nd(CaPr)-123 is higher than Y(CaTh)-123, from which it is suggested that CaPr doping is more effective than CaTh doping.     

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.