Melt-textured YBa2Cu3O7 Research Articles

Correlations between the structure and superconducting properties of MT-YBaCuO

Comprehensive experimental results of fully oxidized (up to YBa2Cu3O6,9-7) melt-textured YBaCuO materials with different microstructures are presented. These microstructures are built respectively: (1) with a high dislocations density but almost without twins (after high temperature treatment at 2 GPa) and (2) with a high twin density, but practically free from dislocations and stacking faults (after high temperature oxygenation at 10-16 MPa). It is shown that for attaining high critical current densities and fields of irreversibility (jc(H‖c, 0 T)=9·104 A/cm2, H irr=9.7 T at 77 K), a high twin density in YBa2Cu3O6.9-7 matrix of MT-YBCO is required. The density of twins in fully oxidized materials depends on the distances between Y2BaCuO5 inclusions, larger twin densities are related to shorter distances between inclusions. The influence of phase composition of the initial powder mixtures on the distances between Y2BaCuO5 inclusions have been characterized and discussed.

High-field paramagnetic Meissner effect up to 14 T in melt-textured YBa2Cu3O7–δ

We have performed magnetization experiments in a melt-textured YBa2Cu3O7-δ (Y123) sample with Y2BaCuO5 (Y211) inclusions, under magnetic fields up to 14T applied parallel or perpendicular to the ab plane. Magnetic anisotropy and paramagnetic moments were observed in both FC (field-cooling) and FCW (field-cooled warming) procedures and these features correspond to the so-called High-Field Paramagnetic Meissner Effect (HFPME). The HFPME effect increases monotonically as the magnetic field rises and a strong paramagnetic relaxation, toward increasing paramagnetic moment was additionally observed as a function of time. Microscopy analysis revealed a complex and correlated microstructure of the Y211 particles. These correlated defects are well known to cause strong flux pinning. Our results suggest a scenario of strong flux compression within weak or non-superconducting regions of the samples, developed as a consequence of the Meissner effect and assisted by strong flux pinning by the Y211 particles. This scenario is observed up to 14T and clearly persists beyond.

Superconducting Properties of Melt-Textured Growth YBa2Cu3O7−x Materials with Y211 Addition

Effects of Y211 phase addition into YBa2Cu3 O7−x bulk superconductors were investigated in order to provide high-quality grain structure for yttrium barium cop- per oxide (YBCO) bulk material. Various molar amounts of Y211 were added into Y123, and it was expected to affect the formation of the grains and to increase the crit- ical current density (Jc) and onset transition temperature (Tc) of YBCO. The samples were fabricated by using melt- textured growth (MTG) method and characterized in terms of ac magnetic susceptibility and superconducting quantum interference device (SQUID) magnetization measurements. A simple algorithm was used to calculate the critical cur- rent densities of the samples according to Bean's critical state model. According to addition of Y211 phases into pure Y123, bulk samples with different molar amounts showed that the highest critical temperature and critical current den- sity were obtained with a molar amount of n = 0.125 Y211-added sample as 94 K and 126.000 A/cm 2 , respec- tively, under the 1-T applied magnetic field at a temperature of 10 K.

Multiple superconducting transition and phase separation in melt-textured YBa2Cu3O7−d

We report on careful measurements of the temperature derivative of the resistivity, dρ/dT, in the region encompassing the superconducting transition of melt-textured samples of the YBa2Cu3O7−d cuprate superconductor. Measurements were carried out in orientations parallel and perpendicular to the Cu−O2 atomic planes in the presence of small magnetic fields applied parallel to the current orientation. The dρ/dT results reveal the occurrence of complex multi-peak structures that were analyzed with a new method based on the assumption that a simple series association of conductivities is adequate for describing the resistive transition of multiphased superconductors. We ascribe the multi-peak structure observed in the derivative measurements to a subtle and complex phase separation phenomenon leading to the stabilization of domains having slightly different electronic properties that are related to the ordering of labile oxygen atoms in the YBa2Cu3O7−d chain sub-structure.

Pinning in $\hbox{MgB}_{2}$- and YBaCuO-Based Superconductors: Effect of Manufacturing Pressure and Temperature

Bulk MgB2- and YBaCuO-based materials are competitive candidates for applications. The properties of both compounds can be significantly improved by high temperature-high pressure preparation methods. The transformation of grain boundary pinning to point pinning in MgB2-based materials with increasing manufacturing temperature from 800 to 1050°C under pressures from 0.1 MPa to 2 GPa correlates well with an increase in critical current density in low and intermediate magnetic fields and with the redistribution of boron and oxygen in the material structure. As the manufacturing temperature increases (to 2 GPa), the discontinuous oxygen-enriched layers transform into distinct Mg-B-O inclusions, and the size and amount of inclusions of higher borides MgBX (X>;2) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC addition. The oxygenation of melt-textured YBa2Cu3O7 - δ (MT-YBaCuO) under oxygen pressure (16 MPa) allows one to increase the oxygenation temperature from 440°C to 700-800°C, which leads to an increase of the twin density in the Y123 matrix and to a decrease of dislocations, stacking faults, and the density of microcracks, and as a result, to an increase of the critical current density, Jc, and the trapped magnetic field. In MT-YBaCuO, practically free form dislocations and stacking faults and with a twin density of 22-35 μm-1, Jc of 100 kA/cm2 (at 77 K, 0 T) has been achieved, and the importance of twins in Y123 for pinning was demonstrated experimentally.

Paramagnetic Meissner effect and strong time dependence at high fields in melt-textured high-T C superconductors

In this work we report on systematic field-cooled magnetization experiments in melt-textured YBa2Cu3O7−δ samples containing Y211 precipitates. Magnetic fields up to 14 T were applied either parallel or perpendicular to the ab planes and a strong paramagnetic response related to the superconducting state was observed. This effect is known as paramagnetic Meissner effect (PME). The magnitude of the PME increases when the field is augmented. This effect shows a strong paramagnetic relaxation, such that the paramagnetic moment increases as a function of the time. The pinning by the Y211 particles plays a crucial role in the explanation of this effect and our results suggest that the pinning capacity can produce a strong flux compression into the sample, originating the PME and the strong time dependence.

Study of the in-plane and c-axis fluctuation conductivity of melt-textured YBa2Cu3O7 under hydrostatic pressure

We have experimentally studied the pressure dependence of the fluctuation conductivity in melt-processed YBa2Cu3O7 (YBCO) for currents applied parallel or perpendicular to the Cu-O2 atomic layers. Results show that the asymptotic critical regime is described by the 3D-XY universality class for both in-plane and off-plane conductivity components. The dynamical exponent for the in-plane conductivity component is z = 1.5, independently of the applied pressure. However, a pressure induced crossover in the dynamical universality class is observed in the c-axis fluctuation conductivity, where the exponent changes gradually from z = 1.5 to z = 2. The Ginzburg number increases with pressure for both crystalline orientations. This behavior suggests that the in-plane and c-axis components of the G-L coherence length decrease upon pressure application. The fluctuation spectrum for the Gaussian regime in the planar conductivity keeps the three dimensional character. However, filamentary fluctuations are observed along the c-axis above the Ginzburg temperature. Our results for the out-of-plane fluctuation conductivity in YBCO suggest that the c-axis transport is predominantly coherent in this system.

Paramagnetic Meissner effect at high fields in YCaBaCuO single crystal and melt-textured YBaCuO

We report on systematic field-cooled magnetization experiments in an Y0.98Ca0.02Ba2Cu3O7−δ single crystal and melt-textured YBa2Cu3O7−δ samples. Magnetic fields up to 50kOe were applied and a paramagnetic response related to the superconducting state was observed. We discuss our results in terms of the flux compression scenario into the sample.

Fluctuation conductivity in melt-textured YBaCuO samples under low magnetic fields

We have studied thermal fluctuations of the electrical conductivity in melt-textured YBa2Cu3O7 (YBCO) samples under low magnetic fields. Measurements were performed either for current applied parallel or perpendicular to the c-axis and fluctuation conductivity was studied in the proximity of the superconducting transition. Two melt processed samples were prepared with different concentrations of Y2Ba1Cu1O5 (Y211) phase. For the sample with lower concentration, 3D-Gaussian and genuine critical 3D-XY-E fluctuation regimes were identified in the conductivity parallel to the ab plane and a regime beyond 3D-XY was also observed. The 3D-XY-E scaling was also identified in the fluctuation conductivity along the c-axis. In the sample with higher concentration of the Y211 phase, disorder effects are relevant. The results indicate that the superconducting state in YBCO has a three-dimensional character, and suggest the presence of a sub-dominant order-parameter component that has an appreciable projection along the c-axis.

Fluctuation conductivity along the c-axis and parallel to the ab-planes in melt-textured YBa2Cu3O7− samples doped with Y211 phase

Abstract We report measurements of in-plane and out-of-plane fluctuation conductivity under low applied magnetic fields up to 500 Oe in two melt-textured YBa2Cu3O7−δ (YBCO) samples. 3D-Gaussian and genuine critical 3D-XY-E fluctuation regimes were identified in the conductivity parallel to the ab plane. In addition, a regime beyond 3D-XY was observed in the immediate vicinity of the superconducting transition of the in-plane fluctuation conductivity. The 3D-XY-E scaling was also identified in the fluctuation conductivity along the c-axis in the sample with smaller content of the Y2BaCuO5 (Y211) phase. This result indicates that the superconducting state in YBCO has a three-dimensional character, in contradiction to some studies suggesting that critical phenomenology is fundamentally distinct in orientations parallel or perpendicular to the Cu–O2 planes of the high-temperature superconductors. However, the results suggests the presence of a sub-dominant order-parameter component in YBCO that has an appreciable projection along the c-axis.

Magnetic irreversibility and zero resistance in melt-textured YBaCuO

We have investigated a possible correlation between the magnetic irreversibility limit and the zero resistance as a function of applied magnetic field up to 50 kOe in a melt-textured YBa 2Cu 3O 7− δ sample doped with Y211 phase. The main goal is to disclose the connection between the magnetic irreversibility and zero resistance and to appoint the physical grounds of the discrepancies. The electric resistivity falls steeply as a function of temperature at the pairing transition. However, just before falling to zero value it abruptly changes drastically the slope and achieves zero resistance, T c0( H), only after a long and flat tail. Along this tail grain coupling and phase coherence of the grain aggregate takes place. The magnetic irreversibility data as a function of applied field, T irr( H), follow the trends of granular superconductors. Above 2 kOe the data follow the usual giant-flux-creep regime for H parallel to the c-axis as well as for H parallel to the ab-plane. However, in low fields the T irr( H) data exhibit the signature of a frustrated superconducting grain aggregate. Besides this, the irreversibility data T irr( H) exhibit a much lowered anisotropy, which we impute to imperfect c-axis alignment of the crystallites. Within the high-field region the T c0( H) and the irreversibility T irr( H) data agree within the experimental precision just for H parallel to the ab-plane. However, in a low-field region, where the T irr( H) data display the signature of frustration, the T c0( H) data fall visibly below the irreversibility line, which we impute to persistence of resistivity within the grain junctions.

Laboratory-Scale High Temperature Superconducting Maglev Launch System

In order to design high temperature superconducting (HTS) maglev launch system, three NdFeB permanent magnetic guideways (PMGs) are investigated and compared for the better levitation forces and guidance forces. The laboratory-scale HTS maglev launch system was designed and developed successfully in the Applied Superconductivity Laboratory (ASCLab), Southwest Jiaotong University, China. The cooling process of the melt-textured YBaCuO bulk superconductors is made in a big low temperature tube, which is connected with PMG. The method merit is to eliminate fluctuation of liquid nitrogen in the operation test. The longitudinal slopes of the PM guideway are designed with 0-30 degree from the horizontal, and transverse slopes is designed with 15 degree from the horizontal. The HTS maglev launch system is carried out an experiment in the laboratory. Test results indicate design validity of the HTS maglev launch system.

Effect of the cooling rate on the pore distribution in melt-processed YBa2Cu3O7–y superconductor

To understand the pore formation and its distribution in melt-textured YBa2Cu3O7–y, the cooling rate through the peritectic temperature was changed. The top-seeded melt-processed YBa2Cu3O7–y samples were prepared at various cooling rates (1°C/h to 100°C/h) from 1020°C to 960°C. It was found that the sample dimension and bulk density were a function of the cooling rate; the higher bulk density at lower cooling rate. It is attributed to the easier oxygen diffusion into the samples during solidification at slow cooling rate. The polished microstructural investigation showed that the interior pore density of the sample was higher than that of the edge of the sample. Pore-free regions near the sample surface due to a pore filling by a liquid diffusion were observed in all samples. The size of the pore-free region of the sample prepared at a lower cooling rate was larger.

The effect of BaCeO3 nano particles on the current density of a melt-processed YBCO superconductor

The effect of BaCeO3 addition on the microstructure and the critical current density of a melt-textured YBa2Cu3O7−y sample has been investigated. The BaCeO3 powder was prepared by a conventional solid-state reaction method and the size was reduced by attrition milling. The BaCeO3 powders with various contents (3, 5, 10, 15, 20wt.%) were mixed with YBCO (Y123 composition) powder and then melt-textured. It was found that the BaCeO3 particles with a cubic shape were trapped in the Y123 grains. The distribution of BaCeO3 particles inside 123 grains was not uniform due to the formation and of the particle free regions and the segregation in local parts. The particle segregation tends to be more serious when the particle is finer. The critical current density (Jc) at 77K of the sample with much BaCeO3 addition was higher than that of the sample with a small addition.

The Low-Field Ac Impedance of YBa2Cu3O7-δSuperconducting Slab

Using results of surface impedance measurements, the penetration of ac field into a melt-textured YBa2Cu3O7−δ slab was investigated. We obtained a linear correlation between the surface impedance and ac frequency. The impedance curve shifts towards higher temperatures — 0.9 K per decade of the frequency increase, for dc magnetic field μ0Hdc ≤ 8 × 10−3 T. This is an evidence of thermally activated flux creep. A plateau in the temperature dependence of the real impedance part was observed at approximately 88 K.

Modern superconductive materials for electrical machines and devices working on the principle of levitation

The peculiarities of high-pressure synthesis of highly dense nanostructural MgB2-based superconductive materials, of thermobaric treatment of MT–YBCO (melt-textured YBa2Cu3O7−δ-based superconductor), high-pressure sintering of YBa2Cu3O7−δ, oxygenation of MT–YBCO under high isostatic pressure of oxygen and processes of formation of superconductive junctions between MT–YBCO blocks are considered. The attained level of superconductive and mechanical properties of such materials and junctions make them promising for application in cryogenic devices working on the principle of levitation: electric motors, generators, pumps for liquid–gas transfer, magnetic bearings, flywheels, fault-current limiters, maglev transport, etc. High-pressure synthesized MgB2 (with Ti additions) blocks were for the first time tried in the superconductive electric motor at 20K and demonstrated an efficiency similar to MT–YBCO (at the same working temperature).

Dislocation configurations in twin-free melt-textured YBa2Cu3O7 processed at high pressure and temperature

Melt-textured YBa2Cu3O7 samples were deformed under high pressure–high temperature (HP–HT) conditions (P = 2 GPa, T = 900 and 800°C, t = 15 min). During the HP–HT treatment, samples were in contact with annealed zirconia, preventing oxygen loss, in such a way that the orthorhombic superconductor phase was preserved at the end of the experiment. The deformed microstructure, investigated by transmitted electron microscopy, exhibited original features characterized by a very low twin density, a memory effect on the dislocations shape and a non-classical faulted loops distribution in the matrix. A mechanism, based on the interaction between twin and gliding dislocations, is proposed to explain the nucleation of these aligned loops.

Determination of the Glassy Exponent of Melt Textured YBa2Cu3O7−x by Magnetic Relaxation

By magnetic relaxation method we obtained the glassy exponent μ (T, H) of the melt textured Yba2Cu3O7−x sample. The results have a similar behavior compared with ones using the method of magnetic field sweep, and are compatible with the model of collective pinning theory.

Orientation of embedded Y 2 BaCuO 5 particles within the YBa 2 Cu 3 O x matrix in melt‐textured YBCO superconductors

Automated electron backscatter diffraction (EBSD) was employed to study the local orientations of embedded Y2BaCuO5 (Y-211) particles within the YBa2Cu3Ox (Y-123) superconducting matrix of large grain, melt-textured Y-Ba-Cu-O (YBCO) samples. High-quality Kikuchi patterns were obtained, enabling automated mapping of the individual crystal orientations and a two-phase analysis of the samples. Investigations were performed on a variety of melt-textured YBCO samples, including samples with different element additions. We observe from the maps that the embedded Y-211 particles do not have any preferred orientation in melt-textured YBCO with (001) orientation, and that the YBCO growth is not altered for certain orientations of the Y-211 particles. In samples with (100) orientation, on the other hand, we observe only a small misorientation within the YBCO matrix, and the embedded Y-211 particles do not exhibit any texture. We can conclude from the EBSD maps obtained that the formation of small Y-211 particles does not disturb the Y-123 matrix growth, whereas the presence of large Y-211 particles leads, significantly, to the formation of subgrains. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

OIM and X‐ray texture analysis of melt‐textured YBCO superconductors

The results of two independent texture measurements on melt-textured Y-Ba-Cu-O (YBCO) high-Tc superconductors are compared with each other. The texture data are acquired by means of an X-ray based pole figure texture analysis and of a local texture analysis provided by an automated electron-backscatter diffraction (EBSD) or often called orientation imaging microscopy (OIM) analysis. As samples, we employ two different melt-textured YBCO samples; one fully processed (orthorhombic) and one without oxygen treatment (tetragonal). Pole figures in [103] direction are used to enable a direct comparison. We find a clear coincidence between the results obtained by the two measurement techniques on the melt-textured YBCO samples, however, the EBSD results are much more detailed, yielding the local grain orientation distribution and quantitative results of the grain or subgrain misorientation angles. Therefore, the EBSD measurements give informations which are not accessible by the X-ray texture analysis. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)