Critical Transition Temperature Values Research Articles

Critical current density and mechanical performance of MgB2 superconductors prepared with different magnesium sources

In this study, the critical current density and mechanical performance of MgB2 superconductors prepared using different magnesium sources (Mg and MgH2 with 99.9% and 96.5% purities, respectively) were analyzed. When the samples were characterized structurally and electrically, we found that the grain sizes were about 40 nm and transition temperature was around 30 K being lower than the literature. Critical current densities of all samples were calculated using Bean's model and our calculations yielded critical current density values higher than 106 A cm−2 in self-field. The highest critical transition temperature value belongs to M800 (Mg with 99.9% purity) sample and when the critical current density and transition temperature values of this sample are considered, this sample appears to have the potential for practical use. Vicker's microhardness measurements were performed and yield stress (Y), elastic modulus (E), brittleness (Bi), ductility (D) and fracture toughness (KIC) were calculated. All samples exhibited Indentation Size Effect (ISE) and microhardness measurements were compared with some microhardness models with were compatible with ISE behavior.

Role of Bi/Tm substitution in Bi-2212 system on crystal structure quality, pair wave function and polaronic states

This comprehensive study finds strongly out the crucial variations in the dc electrical resistivity, superconducting, crystal structural and flux pinning mechanisms with the partial replacement of homovalent Tm+3 inclusions by Bi+3 impurity in the active layers of Bi-2212 superconducting material. Materials of type Bi2.1-xTmxSr2.0Ca1.1Cu2.0Oy with molar ratio changes of 0.00 ≤ x ≤ 0.30 are prepared by conventional solid-state reaction route in atmospheric pressure and the characterizations are exerted by the bulk density, dc electrical resistivity (ρ-T), powder X-ray diffraction (XRD), critical current density (Jc), scanning electron microscopy (SEM) and electron dispersive X-ray (EDX) experimental measurements. The combination of experimental results evaluated from the bulk density, dc electrical resistivity, XRD and EDX measurements points out that the Tm foreign impurities mostly incorporate successfully into the Bi-2212 crystal lattice. In fact, the EDX investigations verify that the thulium impurities may mostly be substituted for the bismuth sites in the crystal structure. All the experimental results declare that the characteristic features improve regularly with the increment in the Tm impurity level up to x = 0.07 beyond which the properties degrade dramatically. In this respect, the sample with x = 0.07 exhibits highest electrical conductive/metallic characteristics as a consequence of the refinement in the crystal structure quality and connectivity between the superconducting grains, being favored by bulk density and related relative degrees of granularity surveys. Likewise, the material presents the maximum offset-onset critical transition temperature values of 85.61 K–85.85 K due to the increment in the formation of effective and strong electron-phonon coupling probabilities and optimization of mobile hole carrier concentrations in the Cu-O2 consecutively stacked layers. In other words, the optimum content level leading to transit inherently over-doped nature into optimally doped state strengthens the amplitude of pair wave function for the Bi-2212 material. In more sophisticated interpretations, the presence of optimum dopant in the crystal structure changes the vibrational mode intensities of O (1)CuA1g, B1g phonons and O (2)SrA1g phonon so that the formation possibility of bipolaron out of two polarons increases strongly in a polarizable lattice (polaronic effect). Additionally, the highest self-field Jc of 95 A/cm2 confirms the fact that the optimum dopant augments the effective nucleation centers along the intragrain and inter-grain boundaries in the crystal system. Similarly, the material prepared with x = 0.07 presents the smoothest, densest, largest average crystalline distribution, lowest porosity and most uniform surface appearance with the finest connection between the superconducting grains. The XRD results (the increased high phase, c-axis length and average grain size but decreased a-axis length) also show the optimum dopant level of x = 0.07 for Bi-2212 crystal system. All in all, the paper developing a strong methodology about why the characteristic properties improve with the presence of Tm impurity in the Bi-2212 system may be a pioneering research to construct newly, novel and feasible market areas for the Bi-2212 superconducting ceramics in the universe economy.

Effect of MgB2 addition on thermal fluxon motions of two-dimensional pancake vortices in Bi-2223 superconducting ceramics

This study reports the variations of magnetoresistivity performance and pinning ability for the bulk Bi1.8Pb0.4Sr2(MgB2)xCa2.2Cu3.0Oy ceramic materials with the MgB2 addition (0 ≤ x ≤ 0.5) in the crystal structure by magnetotransport measurements performed in the applied magnetic fields (0–7T). According to the results, the maximum critical transition temperature values (Tcoffset and Tconset) are about 114.1 K and 121.3 K for the best sample exposed to x = 0.05 content level of MgB2 addition while the minimum Tcoffset and Tconset are obtained to be 109.4 K and 118.6 K for the pure sample at zero applied magnetic field, respectively. Besides, the highest Tcoffset value of 69.6 K is observed for the former sample at 7 T external field strength. This is attributed to the fact that the existence of optimum MgB2 in the Bi-2223 crystal structure results in the increased nucleation center formations so that the binding between the dopants and adjacent layers can pin the fluxon motions of two-dimensional (2D) pancake vortices at higher magnetic field such a value of 7 T. On the other hand, the main reason of decrement in the Tcoffset parameters stems from the pair-breaking mechanism based on the changed spin states of cooper-pair electrons. Moreover, the flux pinning energy parameters estimated from thermally activated flux creep model tend to augment considerably with the increment of MgB2 content level up to x = 0.05. Additionally, the coherence length and penetration depth parameters as well as the irreversibility and upper critical fields confirm the positive effect of optimum MgB2 addition on the elastic moduli of vortex lattice, Josephson coupling length, magnetic performance and especially pinning ability throughout the Bi-2223 crystal structure.

Formation of nucleation centers for vortices in Bi-2223 superconducting core by dispersed Sn nanoparticles

Abstract In this study, we investigate the effect of the Sn diffusion at different annealing temperature in the range of 650–850 °C on the microstructural, electrical, mechanical and superconducting properties of the Bi 1.8 Pb 0.4 Sr 2.0 Ca 2.1 Cu 3.0 O y materials prepared by the standard solid state reaction method with the aid of bulk density, dc resistivity (ρ-T), transport critical current density ( J c ), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Vickers microhardness ( H v ) measurements. It is found that all the properties given above are significantly dependent upon both the Sn nanoparticles inserted in the Bi-2223 superconducting matrix and diffusion annealing temperature. Namely, the dc resistivity measurements indicate that all the samples exhibit the metallic behavior as a result of the electron–phonon interaction in the Bi-2223 crystal structure or logarithmic divergence in density of states at the Fermi level. Further, the normal state resistivity decreases with the enhancement of the annealing temperature owing to the increment of the metallic connection between the superconducting grains due to the optimization of the hole density and possible changes in the lattice vibration. Likewise, onset and offset critical transition temperature values tend to enhance with the diffusion annealing temperature, confirming not only the improvement of crystallinity and especially connectivity between the superconducting grains but the increment in the average grain size and mobile hole concentration in the Cu–O 2 slabs of the Bi-2223 system, as well. The decrement in the degree of the broadening stems from the degradation of the porosity, grain boundary resistivity and grain boundary weak-links in the system. Moreover, the results of J c show that the Sn inclusions (highly dispersed nanoparticles) form the effective flux pinning centers for the vortices in the crystal structure, and tightly bound to these nucleation centers. Thus, the J c value steeply increases from 852 A cm −2 to 4307 A cm −2 with the diffusion annealing temperature. Similar findings are valid for the microstructural characteristics obtained from the SEM images belonging to the samples. The annealing temperature of 850 °C develops considerably the surface morphology and interaction between the superconducting grains. Hence, the sample annealed at 850 °C for 24 h exhibits the smoothest and densest surface morphology. Similarly, the bulk density analysis illustrates that the densities regularly rise from 5.582 to 6.102 g/cm 3 with increasing the annealing temperature, confirming that more and more Sn inclusions penetrate into the superconducting grains or over grain boundaries in the crystal structure. This is associated with the enhancement of the connectivity (interaction) between the superconducting grains. At the same time, the results of the XRD experiments display that all the samples composed of two superconducting phases (Bi-2223 and Bi-2212) present the polycrystalline superconducting phases but in different intensity of diffraction lines. With increasing the annealing temperature up to 850 °C, the intensity of diffraction lines belonging to the high- T c phase (Bi-2223) becomes stronger and so the best superconducting properties are observed for the sample prepared at the annealing ambient of 850 °C for 24 h, being even favored by the largest (lowest) cell parameter c ( a ). Furthermore, Vickers microhardness measurements conducted at different applied indentation test load (0.245 N ⩽ F ⩽ 2.940 N) demonstrate that all the Bi-2223 ceramics obey the typical Indentation Size Effect (ISE) behavior, and the mechanical properties increase with the diffusion annealing temperature. The long and short of it is that the Sn inclusions and especially diffusion annealing temperature up to 850 °C are favorable for the formation velocity of Bi-2223 phase.

Effect of zirconium diffusion on the microstructural and superconducting properties of YBa2Cu3O7−δ superconductors

This study reports not only the role of zirconium (Zr) diffusion on the superconducting, physical and microstructural properties of YBa2Cu3O7−δ (Y123) bulk superconductors prepared by the conventional solid-state reaction method with the aid of scanning electron microscopy (SEM), electron dispersive X-ray (EDX), X-ray diffraction (XRD), electrical resistivity (ρ–T) and transport critical current density (Jc) measurements but also the diffusion coefficient and the activation energy of zirconium in Y123 system for the first time. Zr diffusion is examined in the temperature range 500–945°C via the variation of the lattice parameters evaluated from the XRD patterns. It is found that while the room-temperature resistivity reduces in the presence of the Zr ions in the system, the critical transition temperature values measured enhance. The onset (Tconset) and offset (Tcoffset) critical temperatures are found to be about 94.4K and 91.5K for the Zr-diffused sample as against 93.7K and 89.4K, respectively, for the pure sample. The increase in the critical temperatures is due to the improvement of crystallinity and the enhancement in the grain size. Likewise, the presence of the Zr atoms in the system, the Jc value enhances from 128Acm−2 to 160Acm−2 at 77K owing to the increase of the flux pinning mechanism stemming from the stacking faults, planar and micro-defects. Further, XRD measurements display that the samples produced in this work exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain the Y123 phase only, illustrating the incorporation of Zr atoms into the crystalline structure of the Zr-diffused sample. Based on the refinement of cell parameters done by considering the structural modulation, the Zr diffusion is confirmed by both a reduction of a and b lattice parameters and an enhancement of the cell parameter c (about 0.24%) of the sample in comparison with that of the pure sample. As for the SEM investigations, the crystallinity and grain connectivity improve with the Zr diffusion. According to the results obtained, the Tconset, Tcoffset and Jc values of the samples studied are found to depend strongly on the microstructure. Moreover, EDX measurements indicate that the elements used for the preparation of samples distribute homogeneously. Y3+ and Ba2+ cations may partly be substituted by Zr4+ ions and the substitution at the Cu site is less likely, presenting that the superconducting properties of the Zr-diffused sample are more superior to the pure sample. Further, the diffusion coefficient of the Zr in Y123 system in a range of 500–945°C is determined to change from 4.12×10−10 to 9.77×10−8cm2s−1 with activation energy of about 1.010eV, confirming that the Zr diffusion at lower temperatures is much less significant and the Zr ions primarily proceed through defects in the polycrystalline samples.

Crystal growth and phase relation of Sm 1+ xBa 2− xCu 3O z

Using the top-seeded solution-growth method, Sm 1+ x Ba 2− x Cu 3O z (SmBCOss) single crystals and thick films were grown in different solvents. Inductively coupled plasma compositional analysis was performed to both Ba–Cu–O solvents and SmBCOss solid solutions. The results indicate that a nearly stoichiometric SmBa 2Cu 3O z phase with high critical transition temperature value, is controllable in a wide range of the thermodynamic parameters. The phase relation of the Sm–Ba–Cu–O system was discussed in comparison with other RE–Ba–Cu–O systems (RE=Y and Nd).