Doping Effect of Y2Ba4CuMoO y Particles on Microstructure and Superconducting Properties in Polycrystalline YBa2Cu3O7−δ

Synthesis of polycrystalline YBa₂Cu₃O_7?δ (YBCO) + x mol.% Y₂Ba₄CuMoO _y (Y-24Mo1) (x=0.0, 0.1, 0.2, 1.0, 2.0 and 20.0) superconductors have been done in air via a standard solid state reaction technique. The effects of Y-24Mo1 particles on the microstructure and the superconducting transition temperature were systematically investigated through XRD and SEM techniques. When Y-24Mo1 was added to the YBCO, the orthorhombic structure maintained even at the highest concentration. Tiny peaks of Y-24Mo1 having (022) orientation and some other impurity were detected in the XRD spectrum when x=20.0. SEM results reveal the presence of some entities, and these entities changed with increase of Y-24Mo1 content. The Y-24Mo1 addition modifies the normal state resistivity of samples from metallic to semiconducting, and is accompanied by a two-step transition. Moreover, low Y-24Mo1 concentration leads to a relatively sharp superconducting transition and a broad transition take place for Y-24Mo1 addition greater than 0.2 mol.%. These results can be induced by the inhomogeneities, by the change of the effective coupling strength, and by the variation of the coupling between grains.     

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.      

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.     

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.     

Formation temperature of bosons in high-T c YBa2Cu3O7−y systems

At different equilibrium temperatures, a precise measurement of the electrical resistivity (R) is performed for high T _c polycrystalline YBa₂Cu₃O_7–y systems. T ² law is applied in the part of the R-T relationship far above the superconducting transition temperature. Since Landau and Pomeranchuk predicted the T ² law arising from electron-electron scattering, a higher critical temperature of T ² law is defined as a start point (T _es) of electron-electron scattering. On the other hand, we defined a lower critical temperature of T ² law as the formation temperature (T _ee) of an electron-electron pair, i.e. the formation temperature of boson. T _ee relates to the offset temperature of the superconducting transition .     

Influence of Boron Doping on Transport Properties of?YBa2Cu3O7?y HTS

The nominally pure and boron-doped YBa₂Cu₃B _x O_7?y samples with B-doping level x varying between 0 and 0.15 were prepared by the solid state reaction. X-ray diffraction analysis shows that all the obtained compounds are single YBa₂Cu₃O_7?y phase. The small additives of boron in YBa₂Cu₃B _x O_7?y (x=0.025 and 0.05) do not essentially affect the critical temperature T _c and it remains near 90 K. The higher-level boron doping causes degradation of T _c and tail remains on the ??(T) curve for x=0.15 at 65 K. Lowest-level boron doping applied leads to a significant improvement in J _c compared to the undoped sample (from 100 A?cm^?2 for a control sample to 147 A?cm^?2 for a B-doped sample with x=0.025). Grain boundary critical current density and superconducting volume fraction decrease with increasing amount of added B₂O₃. The obtained results indicate possibility of boron dopant being entered into the lattice-sites.     

The Effects of Chemical Doping and Hydrostatic Pressure on T c of Y1−y Ca y Ba2Cu3O x and NdBa2Cu3O x Single Crystals

Westudied T _c of Y Ba ₂ Cu ₃ O _x (Y123), Y _0.89 Ca _0.11 Ba ₂ Cu ₃ O _x (YCa123) and NdBa ₂ Cu ₃ O _x (Nd123) single crystals with various oxygen contents x. Compared to T _c (x) of Y123 the T _c (x) curves of YCa123 are shifted to lower oxygen contents and the maximum transition temperature T _c,max decreases with increasing Ca content whereas in Nd123 T _c (x) is shifted to higher oxygen contents and T _c,max is increased. According to the universal parabolic T _c (n _h ) behavior the differences in T _c (x) of Y123, YCa123 and Nd123 can be ascribed to different hole concentrations n _h in the CuO ₂ planes caused by doping via changes in chemistry or structure. In order to study the influence of structural changes on T _c we examined the hydrostatic pressure effect dT _c /dp (p 0.6GPa). In the underdoped region, at n _h 0.11, the examined compounds show a peak in dT _c /dp which is very pronounced for systems with well ordered CuO chains. As this peak occurs at the same n _h in all investigated systems it is not caused by oxygen ordering, but its origins might be found in a strong influence of lattice deformations on the electronic structure.     

Local Structure and Superconducting Properties of Bi 2 Te 3 -Doped YBa 2 Cu 3 O 7 − δ

The samples of (Bi₂Te₃) _x (YBa₂Cu₃ O _7?δ) _1?x (x = 0, 2.5 %) have been synthesized at normal pressure, and the effect of the doping of topological insulator Bi₂Te₃ on the superconductor YBa₂Cu₃ O _7?δ is studied. The diamagnetism properties are investigated by zero-field cooling DC magnetization measurement. In the Bi₂Te₃-doped sample, an enhancement of superconducting fractions is observed, and the superconducting transition temperature (T _c ) increases. For the sample with x= 2.5 %, the phase transition of the tetragonal phase to the orthorhombic phase is observed by X-ray diffraction, and the increase of the degree of local structure disorder is confirmed by extended X-ray absorption fine structure, so it can be concluded that the superconducting fraction enhancement is attributed to the phase transition induced by the addition of topological insulator Bi₂Te₃. Scanning electron microscopy images show that the added Bi₂Te₃ is found to adhere to the surface of YBa₂Cu₃ O _7?δ grains, and it is considered that the strain induced by grains’ interaction leads to the phase transition which enhances the degree of local structure disorder at the same time. An absorption edge shift to the high-energy side is observed by the X-ray absorption near-edge structure. The edge shift can be attributed to the electron transfers from YBa₂Cu₃ O _7?δ to the additives. It is widely accepted that the hole intensity is closely related with T _c , so in YBa₂Cu₃ O _7?δ the added holes by the transition may lead to the enhancement of T _c . Thus, it can be concluded that the topological insulators can produce structural and electronic effect on the YBCO superconductors in the composite materials.     

Improvement of the Superconducting Transport Properties of YBa2Cu3O7?δ by BaZrO3 Doping

We have doped the YBa₂Cu₃O_7– superconducting ceramics with BaZrO₃ up to 75wt.% and studied the changes of some physical properties. The most important finding is the enhancement of the critical current density, which has a maximum at around 5 wt.% doping level. Compared with the undoped samples, the critical current density is four times higher if the doping compound is introduced prior to the calcination treatment, or two times higher if BaZrO₃ is introduced prior to the sintering treatment. The dependence of the critical current density on the doping level is consistent with the dependence of the Seebeck coefficient. We also observed a very small decrease of the critical temperature. The real density of the samples decreases with increasing doping level as expected from the ratio between the theoretical densities of YBa₂Cu₃O_7– and BaZrO₃, the effect being important for doping levels higher than 25 wt.%.     

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.     

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.     

Effect of correlated and point-like disorder on transport properties of YBa2Cu3O7−δ single crystals

Pinning by point-like defects and twin boundaries in single-crystalline YBa₂Cu₃O_7– has been investigated by measuring resistance and current-voltage characteristics in fields up to 5 T. A number of untwinned and twinned samples with different densities of point-like defects were studied. The experimental results are compared within the framework of vortex-entanglement.     

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.     

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.     

Magnetic and Electrical Properties of Superconducting Ceramic YBa2Cu3O7−δ Co-doped with Ca and Zn

Superconducting Y_1?x Ca _x Ba₂(Cu_0.98Zn_0.02)₃O_7?δ ceramics with 0≤x≤0.5 have been prepared by the solid state reaction method. Structural, magnetic and electrical properties of the components have been studied. To characterize the samples, X-rays diffraction (XRD), scanning electron microscopy (SEM) and magnetic measurements techniques are used. Obtained results show that increasing the Ca content leads to a significant decrease of the Y123 phase. While, the parasite phase BaCuO₂ increases. For x>0.3, the coexistence of metallic and semiconducting character of samples is observed. DC susceptibility measurements reveal a reduction of the Meissner fraction with Ca doping, suggesting the existence of the flux pining effect.     

Zn and Ni Doping Effect on Anomalous Suppression of T c in an Over Doped Region of TlBa2Ca2Cu3O9−δ

Variation of T _c in the over doped region have been investigated for Zn and Ni doped samples of TlBa₂Ca₂Cu₃O_9– to obtain a clue to understand the anomalous suppression of T _c. We expected that if the anomaly in Tl-1223 originates from the stripe order like La-214 system, the suppression of T _c is enhanced by the doping. The results showed that the anomaly disappeared by both of the doping contrary to the expectation so that the possibility of the stripe order was ruled out. Two possible explanations were proposed based on a self-doping mechanism and inhomogeneous charge distribution to crystallography different CuO₂ layers.     

Effect of Doping of Zn and Ca into ErBa2Cu3O7?δ Superconductor Prepared via Co-precipitation Method

Zn- and Ca-doped ErBa₂Cu₃O_7−δ were successfully prepared via coprecipitation method using metal acetates as the starting salts. The precipitated samples were calcined for 20 h at 900 °C and sintered at 920 °C for 24 h. All heat treatments were carried out under oxygen environment. Results show that there is a selectivity of the doping site depending on the ionic radius of the dopant. Furthermore, increase in the critical temperature, T _c , was observed in 0.05 mole of calcium and zinc doped samples. The difference in ionic radius of the dopant led to the increase in porosity as the ionic radius decreases. On the other hand, structural distortion increased as the difference of ionic radius became larger.