Polycrystalline Superconductors Research Articles

Deep Learning Segmentation of Polycrystalline Superconductors with Different Compositions

Deep Learning Segmentation of Polycrystalline Superconductors with Different Compositions

Larger grains in high-Tc superconductors synthesized by the solid-state reaction route

Solid-state synthesis is widely used in exploratory research to study various structural modifications that affect the properties (critical temperature, critical current density, irreversibility field, etc.) of superconductors. The popularity of this method is due to its relative simplicity and availability of the necessary equipment. Combining solid-state synthesis and top-seeded melt growth allows us to increase the grain size in a Tm- and Nd-based 1-2-3 superconductor. Samples with a grain size up to 0.1 mm have been obtained. X-ray diffraction, scanning electron microscopy and magnetization measurements have been used for investigating this superconducting material. The magnetization width ΔM has increased significantly in the synthesized samples. However the temperature dependence of the intragrain critical current density and the pinning force scaling give evidences that the pinning mechanism in the obtained superconductor is essentially the same as in polycrystalline superconductors synthesized by standard solid-state technology. The increase in grain size in the synthesized samples is the main reason for the high values of ΔM.

Role of grain boundary networks in vortex motion in superconducting films

We study the vortex dynamics of the polycrystalline superconductors in the presence of both random point defects and the generated grain boundary (GB) networks with Voronoi diagram. The synergistic effect of adjacent GBs on restricting the vortex motion in intragranular region is proposed and the corresponding intensity factor of the synergistic effect which characterizes the strength of the synergistic restriction of adjacent grain boundaries is also determined in the present work. The interconnected GBs offer easy-flow channels for vortices in addition to pinning effects on the vortices. The combined channels and the vortex flow patterns in the superconducting film are analyzed in detail from molecular dynamics simulations. Furthermore, it is discovered that the critical current increases with the decrease of magnetic field intensity, temperature, and the average grain size. The large number of vortices results in the enhanced repulsive interaction forcing the vortices to move out from the GBs. The thermal depinning from GBs leads to the lower Lorentz force range. The increase of the grain size causes the number of GBs to decrease. In summary, these effects leads the critical current to become a decreasing function of magnetic field, temperature, and grain size.

Enhancements of critical current density in Bi1.6Pb0.4Sr2Ca2Cu3O10+δ superconductors by additions of SnO2 nanoparticles

The impact of adding SnO2 nanoparticles on the enhancements of critical current density (Jc) of the Bi1.6Pb0.4Sr2Ca2Cu3O10+δ superconductors was studied. Polycrystalline superconductors with stoichiometry of (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1−x(SnO2)x where was x was ranged from 0.000, 0.002, 0.004, 0.006, 0.008 to 0.010 were made through traditional solid state reaction technique. X-ray diffraction data revealed that all fabricated samples consisted of Bi-2223 and Bi-2212 superconducting phases. The volume fraction of the Bi-2223 phase decreased with SnO2 addition. Analyses of textural structure using scanning electron microscopy data evidenced a clear decrease in grain orientation and porosity with increased doping levels. SnO2 presence in connection to Bi-2223 deceleration reduced the critical temperature. Values of Jc deduced from the magnetization hysteresis loops measured at different temperatures of 35 K, 45 K, 55 K, 65 K were found to enhance for the SnO2 added samples. In the x = 0.002 samples, the field dependent Jc achieved its maximum values. In order to have a deeper understanding in the improvements of flux pinning properties, small and large bundle fields were determined by using the collective pinning theory. The obtained values indicate the expansion of the two corresponding regimes with appropriate. By analyzing the dependence of normalized Jc versus normalized critical temperature, the dominant flux pinning mechanisms in all samples were consistent with δl pinning. Besides, the addition of SnO2 nanoparticles was likely to produce pinning centers in form of core point as evidenced by using the Dew–Hughes model.

Anisotropy and Crystallite Misalignment in Textured Superconductors

A misalignment of anisotropic crystallites causes small values of anisotropy and decreases the critical current density of textured polycrystalline superconductors. To relate the crystallite misalignment and out-plane anisotropy, the magnetic properties of the textured Bi2223 polycrystalline superconductor were investigated. A distribution of orientation angles of crystallites was determined using different data: scanning electron microscopy images and hysteresis magnetization loops when an external magnetic field was applied at different angles with respect to the texturing plane of the sample. It was demonstrated that the standard deviation of the distribution and the magnetic disorder angle of crystallites in textured samples can be determined from the magnetization data in perpendicular directions. These data may be either the irreversible magnetization measured for two different orientations of the sample or the simultaneously measured magnetization projections parallel and perpendicular to the magnetic field.

Critical Current Densities Through Josephson Junctions in Low Magnetic Fields

Understanding the properties of grain boundaries in polycrystalline superconductors is essential for optimizing their critical current density. Here, we provide computational simulations of 2D Josephson junctions (JJs) in low magnetic fields using time–dependent Ginzburg–Landau theory, since they can be considered a proxy for a grain boundary between two grains. We present data for junctions with a wide range of superconducting electrodes of different Ginzburg–Landau parameter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\kappa$</tex-math></inline-formula> ) values and geometries, as well as normal barriers with different strengths of pair–breaking — characterized by the thickness of the junction and the junction condensation parameter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\tilde{\alpha }_{\text{n}}}$</tex-math></inline-formula> ). We describe our results using analytic solutions, and hence provide a detailed description of Josephson junctions in low fields up to that required for a single fluxon to penetrate the junction.

Critical current density of superconducting-normal-superconducting Josephson junctions and polycrystalline superconductors in high magnetic fields

We investigate the in-field critical current density ${J}_{\text{c}}(B)$ of superconducting-normal-superconducting (SNS) Josephson junctions (JJs) and polycrystalline superconducting systems with grain boundaries modeled as Josephson-type planar defects, both analytically and through computational time-dependent Ginzburg-Landau (TDGL) simulations in two and three dimensions. For very narrow SNS JJs, we derive analytic expressions for ${J}_{\text{c}}(B)$ that are high-field solutions for ${J}_{\text{c}}(B)$ for JJs across the entire applied field range up to the effective upper critical field ${B}_{\text{c2}}^{*}$. They generalize the well-known (low-field) exponential junction thickness dependence for ${J}_{\text{c}}$ from de Gennes, often used in the Josephson relation. We then extend our analytic expressions to describe wider junctions using physical arguments, and we confirm their agreement with TDGL simulations. These results are then compared with the current densities found in superconductors optimized for high-field applications. They provide an explanation for the Kramer field dependence and inverse power-law grain size dependence widely found in many low-temperature superconductors, and the power-law field dependence ${J}_{\text{c}}(B)\ensuremath{\sim}{B}^{\ensuremath{-}0.6}$ found at intermediate fields in some high-temperature superconductors including powder-in-tube ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ and $R{\text{Ba}}_{2}{\text{Cu}}_{3}{\text{O}}_{7}$ tapes ($R=\text{rare}\phantom{\rule{4.pt}{0ex}}\text{earth}$). By reanalyzing critical current density data using the mathematical framework derived here and confirmed using TDGL, we enable an analysis of ${J}_{\text{c}}$ data that provides the local properties of grain boundaries in high-field superconductors and hence a deeper understanding of how grain boundaries influence ${J}_{\text{c}}$ in high magnetic fields.

第一原理計算とフェーズフィールド法を用いた鉄系多結晶型超伝導材料の固相焼結シミュレーション

To improve performance of a high-temperature polycrystalline superconductor, it is effective to predict microstructural evolutions during a solid-state sintering using the phase-field method. However, interfacial and surface properties required for the phase-field simulation of the sintering process are largely unknown. In this study, the surface energies of iron-based polycrystalline superconductor BaFe2As2 are calculated using the first-principles calculation. The results show that the stable suraface termination depends on the chemical potentials of Ba and As atoms. On the basis of the calculated surface energies, the anisotropy function of surface energy for BaFe2As2 that can be incorporated into the phase-field model of the solid-state sintering is calibrated.

Low-field high-harmonic generation in Mo6S6I2 Chevrel-phase superconductor

This research carried out experiments on the Chevrel-phase Mo6S6I2 polycrystalline superconductor. The behavior of harmonic generation in the dc magnetic fields, the dependence of the high harmonics on the ac magnetic field amplitude, the dc magnetic field, and the temperature are investigated. The experimental results are well described by the classical Bean critical state model, when the current does not depend on the dc field and even harmonics are not observed.

Tin-oxide nanoparticles doping impact in the polycrystalline superconducting $$\mathrm{Y}_3\mathrm{Ba}_5\mathrm{Cu}_8\mathrm{O}_{18\pm \delta }$$ and $$\mathrm{Y}_1\mathrm{Ba}_2\mathrm{Cu}_3\mathrm{O}_{7-\delta }$$ composites

The well-known yttrium founded polycrystalline superconductors named cuprates with high critical temperature ( $$T_\mathrm{C}$$ ) and arrangement of pure $$Y_3\mathrm{Ba}_5\mathrm{Cu}_8\mathrm{O}_{18\pm \delta }$$ (shortened-Y358) and $$Y_1\mathrm{Ba}_2\mathrm{Cu}_3\mathrm{O}_{7-\delta }$$ (shortened-Y123) accompany with various (0.00, 0.40, and 0.50 wt%) nanoparticles Tin-doped (SnO2) composites were manufactured by solid-state response method. The X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDX) apparatuses were used to analyse the micro-features and morphological structures of the synthetic mixtures. The XRD associated with the pure Y358 compares to the Y123 specimen shown alike structures for both samples with approximately three times longer lattice parameter c. The SEM images related to the pure compounds exhibited different intergrowth crystals from the doped ones. The EDX diagrams display all the compositional components in appropriate quantities. Electrical resistivity $$\rho (T)$$ measurements were performed to evaluate not only superconducting transition temperatures $$(T_{\text {c zero}})$$ , ( $$T_\mathrm{C}^\mathrm{Peak}$$ ) and ( $$T_{\text {c onset}}$$ ) but also critical parameters such as critical magnetic field $$B_{\mathrm{c}2}(0)$$ and critical current density $$J_\mathrm{c}(0)$$ in pure and SnO2 added YBCO specimens. The data correlated to Y123 and Y358 compounds shown that rising SnO2 nanoparticle insertion into these samples triggered the enhancement of critical parameters which describes better flux pinning. Comparing these data regarding application and superconducting properties point of view confirmed that the sample Y358 with 0.50 wt% nano-sized (SnO2) inclusion is superior among them.

Phase Formation and Conductivity Fluctuation Investigation in Nanoparticle SnO2-Added Y3Ba5Cu8O18 ± δ Polycrystalline Superconductor

The influence of 0, 0.20, 0.40, 0.50, and 0.60 wt % nano-sized tin oxide (SnO2) particles on electrical conductivity fluctuation in normal and superconducting state of the Y3Ba5Cu8O18 ± δ (denoted as Y‑358) polycrystalline samples is studied. Phase formation and microstructures have been systematically examined. By increasing the content of SnO2 in YBCO matrix, X-ray diffraction technique showed slight variation in lattice parameters and overall reduction in the orthorhombicity. Scanning electron microscopy observations and the crystallite size calculation also revealed that the grain size and the average crystallite size decreased compared to the SnO2-free sample. Aslamazov–Larkin and Lawrence–Doniach prototypes were used to analyze conductivity fluctuations based on the electrical resistivity ρ(T) measurements. Superconducting transition temperatures TC and TLD have been reported. The influence of SnO2 addition on the superconducting properties indicates that with the addition of SnO2 nanoparticles into Y-358 compound, some parameters values such as zero-resistance critical temperature TC zero, coherence distance alongside the c axis at 0 K ξc(0), and super-layer length d decrease in total, while anisotropy γ, critical magnetic fields Bc1(0), Bc2(0), and critical current density Jc(0) increase in SnO2-added Y-358 specimens compared to the pure one. The reasons corresponding to these scenarios are discussed in details.

31P NMR studies of an iron-based superconductor Ba0.5Sr0.5Fe2(As1−x P x )2 with T c = 29 K

We report 31P NMR studies of an oriented polycrystalline superconductor of Ba0.5Sr0.5Fe2(As1−x P x )2 with x ∼ 0.4 (T c = 29 K) at T = 14−325 K and B 0 = 1 and 5 T. The 31P Knight shift Kab at B 0 ⊥ c shows a nearly T-independent uniform spin susceptibility above T c and a spin singlet decrease below T c. The 31P nuclear spin-lattice relaxation rate 1/T 1 shows an asymptotic behavior of a + bT (a and b are constants) at T > 100 K and the minimum at 40 K with an upturn toward T c. The a term in 1/T 1 indicates the presence of two-dimensional antiferromagnetic spin fluctuations. The negative θ = −15 K of the Curie-Weiss-type antiferromagnetic spin susceptibility in the analysis of 1/T 1 T suggests antiferromagnetic instability in the superconducting state. Discussions are made from the self-consistent renormalization (SCR) theory for the spin fluctuations with interlayer correlation.

Enhancement of Flux Pinning by Artificial Point Defects in K-substituted (Bi, Pb)-2223 Polycrystalline Superconductors

Enhancement of Flux Pinning by Artificial Point Defects in K-substituted (Bi, Pb)-2223 Polycrystalline Superconductors

Temperature dependence of the in-plane and grains resistivities in Bi-2223 polycrystalline superconductors

Based on transport measurements performed on Bi-2223 polycrystalline superconductors, we have determined the in-plane resistivity, $$\rho _{ab}(T)$$ , the effective grain resistivity, $$\rho _g(T)$$ , and the effective intergranular resistivity, $$\rho _j(T)$$ , by applying a model recently reported by our group. The obtained dependencies allowed us to determine the critical temperatures of ab planes, grains as a whole, and intergranular junctions for four different polycrystalline samples. In addition, we were able to propose a very simple and general method to determine the first two critical temperatures previously mentioned by using measurements performed in polycrystalline superconducting samples comprised by highly anisotropic crystallites. Finally, our results may explain the two-step superconducting transition, observed in the samples set, as a feature of this compound linked to its high electrical anisotropy.

Activation energy of oxygen diffusion: A possible indicator of supercurrents through YBa2Cu3O7 grain boundaries

Oxygen diffusion is the main transportation form during the preparation process of polycrystalline superconductor YBa2Cu3O7 (YBCO) thin films. Especially the diffusion of oxygen ions in grain boundaries (GBs) can lead to the formation of oxygen vacancies, and finally have an impact on the critical current density (Jc) of YBCO conductors. To deeply understand the supercurrent-limiting mechanism of GBs, for an YBCO bicrystal (containing a single GB), in this article we intensively studied the diffusion of oxygen ions through GB region by molecular dynamics simulations. The calculated results show that the oxygen diffusion in the YBCO bicrystals can be enhanced in GBs, and specifically which is closely related to the grain boundary angle (θ). The most interesting finding is that the activation energy of oxygen diffusion shows an exponential fall with the grain boundary angle, which is exactly consistent with the relation of the critical current density versus the grain boundary angle (Jc ~ θ) in GBs. We firmly believe that the activation energy of oxygen diffusion should have an essential relevance with the Jc especially in YBCO GBs, and can be as an indicator for predicting supercurrents. For these results we further gave an in-depth discussion.

Direct Microwave Synthesis of 11-Type Fe(Te,Se) Polycrystalline Superconductors with Enhanced Critical Current Density**Supported by the National Natural Science Foundation of China under Grant Nos 11474339 and 11774402, the National Basic Research Program of China under Grant No 2016YFA0300301, the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No XDB25000000, and the Youth Innovation Promotion Association of Chinese

We report a direct microwave synthesis method for the preparation of 11-type high quality Fe(Te,Se) polycrystalline superconductors. The bulk samples are rapidly synthesized under the microwave irradiation during several minutes, with a subsequent annealing process at 400°C. The samples exhibit a nearly single phase of the tetragonal PbO-type crystal structure with minor impurities. Morphology characterization shows high density, tight grain connectivity and large grain sizes around with small cavities inside the sample. Resistivity and magnetization measurements both show similar superconducting transitions above 14K. The magnetic hysteresis measurements display broad and symmetric loops without magnetic background, and a high critical current density Jc about 1.2 × 104 A/cm2 at 2K and 7T is estimated by the Bean model. Compared with the solid-state reaction synthesized samples, these superconducting bulks from microwave-assisted synthesis are possibly free of the interstitial Fe due to smaller c-axis, higher Tc in magnetic transitions, better M–H loops without magnetic background and greatly enhanced Jc, and are promising as raw materials for the non-toxic Fe-based superconducting wires for large currents and high field applications.

Optimization of High-Energy Ball-Milling Time for Fe1.1Se0.5Te0.5 Polycrystalline Superconductors

The Fe 1.1 Se 0.5 Te 0.5 polycrystalline superconductors were prepared by a high-energy ball-milling (HEBM) technique. Ball-milling times of 0, 2, 4, 6, and 8 h were adopted to optimize this technique. The influences of ball-milling time on the phase distributions of milled powders and sintered bulks, the morphology of ball-milled powders, and superconducting properties of final sintered bulks were systematically studied. It was found that with increasing ball-milling time, the ball-milled powders changed from a crystal to a noncrystalline and then to an alloy. At the same time, the main phases in ball-milled powders changed from original mixed powders of Fe, Se, and Te to (Se, Te) solid solution, and then to the two phases of β-Fe(Se, Te) and δ -Fe(Se, Te), accordingly. Due to the reduction in diffusion length, it was more likely for HEBM powders to form FeSeTe ternary alloys. Thus, the obtained Fe(Se, Te) bulk with HEBM time of 4 h exhibited the highest T c of 14.3 K and the largest content of superconducting phase. However, in the samples achieved by longer HEBM time, phase segregation and oxidation were observed. Therefore, the HEBM time of 4 h is the most suitable for the fabrication of high-quality precursor powders according to these experiments.

The Circulation Radius and Critical Current Density in Type II Superconductors

A method is proposed for estimating the length scale of currents circulating in superconductors. The estimated circulation radius is used to determine the critical current density on the basis of magnetic measurements. The obtained formulas are applicable to samples with negligibly small demagnetizing factors and to polycrystalline superconductors. The proposed method has been verified using experimental magnetization loops measured for polycrystalline YBa$_2$Cu$_3$O$_{7-d}$ and Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$ superconductors.

Циркуляционный радиус и плотность критического тока в сверхпроводниках второго рода

AbstractA method is proposed for estimating the length scale of currents circulating in superconductors. The estimated circulation radius is used for determining the critical current density on the basis of magnetic measurements. The obtained formulas are applicable to samples with negligibly small demagnetizing factors and to polycrystalline superconductors. The proposed method has been verified using experimental magnetization loops measured for polycrystalline YBa_2Cu_3O_7 – δ and Bi_1.8Pb_0.3Sr_1.9Ca_2Cu_3O_ x superconductors.

Modeling the trapped field distribution in a polycrystalline bulk superconductor based on a critical state model

The trapped field distribution of a polycrystalline bulk superconductor induced by perpendicular magnetic field is theoretically analyzed based on the critical state model. A rational model is presented to describe the current pattern changes near a low-angle grain boundary (GB) that is observed in others’ experiment. The current pattern that varies with the ratio between GB critical current density and grain critical current density is analyzed based on the constant critical current density assumption. In addition, the magnetic field that trapped in the prism both on magnetization ascent and descent stage have been discussed in detail on the basis of the current distribution that we deduce above. The results of this work can also be useful for AC loss analysis and stress calculating of polycrystalline superconductors.