Melt-textured YBCO Superconductors Research Articles

Interaction mediated by size differences between Y 2BaCuO 5 and CeO 2 particles in melt-textured YBCO superconductors

The Refining of Y 2BaCuO 5 (Y211) is crucial in the fabrication of YBa 2Cu 3O x (Y123) bulk superconductors with a high critical current density ( J c ) for power engineering applications. In this work, the influence of an addition of Y211 and CeO 2 of different sizes on the Y211 refinement and superconducting properties of melt-textured Y123 superconductors is investigated. Nano-size Y211 and nano-size CeO 2 were prepared by attrition-milling (250 rpm and 2 h). Using mixed powders consisting of 85 wt.% Y123, 15 wt.% Y211, and 1 wt.% CeO 2, Y123 quasi-single crystals were fabricated via a top seed melt-textured growth process. The interaction between Y211 and CeO 2 of different particle sizes was examined in terms of the size, uniformity and density of the Y211 particles that existed in the melt-textured samples. The reaction between the nano-size Y211 and the nano-size CeO 2 was more violent due to the more frequent collisions compared to the other reactions (micro-size Y211/nano-size CeO 2 or micro-size Y211/micro-size CeO 2), resulting in the most effective for refinement of Y211 particles and the best J c value of 4.2 × 10 4 (A/cm 2) (at 77 K and a zero-field).

Dynamic scaling of ac susceptibility in melt-textured YBCO superconductors

Based on vortex glass phase transition analysis, using homogeneous function analysis, it is very convenient to obtain the dynamic scaling law of ac susceptibility in high temperature superconductors. The experimental results of melt-textured YBCO system and other high temperature superconductors are consistent with the scaling function. The critical exponent γ is estimated from the scaling result. The asymptotic properties at low frequencies are discussed.

Interplay of YBCO and embedded 211 particles in melt-textured YBCO superconductors

Interplay of YBCO and embedded 211 particles in melt-textured YBCO superconductors

Interplay of YBCO and Embedded 211 Particles in Melt-Textured YBCO Superconductors

Local orientations of superconducting YBa2Cu3O x (123) and embedded Y2BaCuO5 (211) particles within bulk, melt-textured 123 samples are studied on a local scale by means of electron backscatter diffraction (EBSD). The embedded Y2BaCuO5 (211) particles do not show any preferred texture, but their presence within the 123-matrix alters the growth of the 123-phase, independent of the orientation. EBSD enables to analyse the interactions between the 123- and 211-phases in detail.

Frequency Response of AC Susceptibility in Melt Textured YBCO Superconductors

We have reviewed the scaling relations for frequency dependent AC susceptibility proposed by Lee. et al. and other contributions. Based on vortex glass analysis, we derived a scaling equation for the peak temperature of the imaginary part and ac magnetic field frequency f. The peak temperature Tp is found to obey: Tp = Cf1/(v(z−1))+Tg, where C is a constant, and Tg is the vortex glass temperature. The scaling relationship was applied to our melt textured YBCO superconductors, which was found in good agreement with the experimental data.

Application of melt-textured YBCO to electromagnetic launchers

Melt-textured YBCO superconductors are capable of carrying higher current densities thancomparable copper conductors, and can therefore be used in electromechanical devicesrequiring high power densities. The advantage of textured YBCO superconductors is mostpronounced in large systems such as electromagnetic launchers for aircraft carriers. Ingeneral, an electromagnetic launcher consists of a series of stationary pulsed electromagnets(the primary), which attract and/or repel a carriage carrying one or more magnets (thesecondary). Several possibilities exist for the incorporation of HTSs into EM launchers,with the most direct being upgrading the magnets in the secondary to melt-texturedYBCO. A system was developed to study propulsive force in a coaxial-type launcher.Results from this study are presented and their implications for launcher developmentdiscussed. A second type of launcher was also studied, with a power supply integrated intothe launcher primary, so that the primary serves as a superconducting magnetic energystorage system. A method of optimizing energy conversion in a system of this type hasbeen found. The time dependence of the magnetic field in this type of launcher ispresented.

Flux pinning by NiO-induced nano-pinning centres in melt-textured YBCO superconductor

Nano-pinning centres ( nm) have been successfully introduced into melt-textured YBCO by doping NiO. Thedoping of NiO was realized by adding NiO nanoparticles to presintered YBCO followed bymelt-texture processing. This method can reduce the substitution level of Ni in the YBCOmatrix and can create Ni-rich second-phase precipitates with a size around 10 nm. Thecritical current density of these Ni-doped samples is significantly increased and theJc–H behaviour is remarkably improved. Our results suggest that NiO-induced nano-pinningcentres may have high pinning efficiency at a temperature around 70 K and be one of themain factors for improving the performance of YBCO. Our study reveals that nano-pinningcentres are also a feasible solution to solve the pinning degradation at high temperature inYBCO.

Roles of silver doping on joins and grain boundaries of melt-textured YBCOsuperconductor

Grain boundaries of melt-texturedYBa2Cu3Oy(YBCO) are preferentially doped with silver by a solid-state diffusion method.Microstructural and local compositional analysis reveals that the Ag exists in theYBCO as individual atoms with their distribution being highly localized within athin layer near the grain boundaries. Compared with the pristine samples, theAg-doped YBCO textured polycrystals show a significant improvement in theJc–H behaviour. Thevalue of Jc at 60 Kand 7 T reaches 4.6 × 104 A cm−2, which is almost three times the value ofJc for the undoped samples. We propose a model in which Ag could partially replace the Cu inCuO2 planes near the grain boundaries of YBCO, therefore reducing the geometric distortion ofCu–O bonds and making the effective grain boundary thinner.

Microwave bulk properties of melt-texturedhigh-TC YBa2Cu3O7−δ superconductors

Bulk properties of a melt-textured YBCO superconductor have been studied using thegrazing incidence microwave reflectivity technique. A correlation of the obtainedresults in the superconducting state with the results in the normal state at roomtemperature has been found. This shows the possibility of monitoring the qualityof the melt-textured samples at room temperature without cooling to criticaltemperature. Also, the correlation with distribution of a trapped magnetic flux atT = 77 K has been shown.

Effects of the platelet structures on the melt textured growth YBCO superconductors

Melt textured growth YBCO superconductors were fabricated by the top seeding method using Sm1.8 (Sm/sub 1.8/Ba/sub 2.4/Cu/sub 3.4/O/sub 7-x/) seed. The relationship between the Y211 particles and the platelet structures was investigated by micro-structural analysis using SEM. The microstructures of melt textured YBCO superconductors have been examined by TEM and HRTEM. The results of TEM studies clarified the direction of crystal growth and a variety of micro-defects such as twin structures and stacking faults that might behave as pinning centers. Our studies were focused on the stacking faults among those micro-defects by HRTEM and formation mechanism of the stacking faults was studied. The stacking faults formed during the tetragonal to orthorhombic transition that occurred at 450/spl deg/C in oxygen annealing. The platelet structures were clearly observed by SEM due to the chemical etching effects. The lengths of stacking faults were increased as the oxygen annealing time increased from 1 hr to 50 hr. The stacking faults were considered to relate to the oxygen contents, as the platelet structures were. The results suggested an oxygen diffusion model for the formation of the stacking faults.

Texture analysis of melt-textured YBCO superconductors

We compare the results of an X-ray based pole figure texture analysis with the local texture analysis by means of electron-backscatter diffraction (EBSD) analysis. As samples, we employ two different melt-textured YBCO samples; one fully processed and one without oxygen treatment. To enable the direct comparison of the two techniques, we employ the [1 0 3] pole figures. We find a clear coincidence between the results obtained by the two measurement techniques on our 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 information not accessible to the X-ray pole figure analysis. The surface preparation procedure is essential to enable the automated EBSD mapping as high image quality Kikuchi patterns are required. The polishing procedures are discussed in detail.

Effect of thermal neutron irradiation in boron-doped melt-textured YBCO superconductors

Y1.6Ba2.3Cu3.3Ox superconductors with different amounts of boron doping have been synthesized using the MPMG technique. Undoped and boron-doped samples were irradiated with thermal neutrons to study the effect of defects produced by the fission reaction, B(n,α)Li, on the pinning and the critical current. We observed that the pinning and the critical current density were improved as a result of thermal neutron irradiation. This improvement was slightly stronger for the boron-doped samples compared to the undoped ones.

STUDY ON THE TECHNOLOGY AND PROPERTY FOR Y2BaCuO5 PARTICLE S ARTIFICIAL DOPING TO OBTAIN SINGLE-CRYSTAL DOMAIN MELT-TEXTURED YBCO BULK SUPE RCONDUCTOR

This paper describes the method of artificial doping Y-211 phase and melt-textur ed growth with a top-seeded technique to prepare the quasi-single crystal domain YBCO bulk materials with various Y-211 particle contents.The specimens obtained have high density (more than 62g/cm3) and good mechanical toughnes s.The Jc of specimens reaches 123×106A/cm2 u nder 06T at 30K and 135×104A/cm2 under 2 T at 70K.The scanning electron microscope observation of specimens indicates that Y-211 part icles can improve the quality of melt-textured YBCO superconductor and reduce th e microcrack of specimens.Combining the microstructures with Jc measu rements shows that the smaller the particle size,the more homogeneous the partic le distribution,the higher the pinning effectiveness.The best ratio of Y-123 to Y-211 for the superconductor specimens is 1∶05 in moles,in which the Y-211 pa rticles are uniformly distributed and their size is about 200nm.

Rf impedance of sintered BSCCO and melt-textured YBCO superconductors

The authors present the temperature and field dependence of radio frequency response of sintered BSCCO 110-K phase and melt-textured YBCO with Ag and 211 inclusions. The rf resistance and reactance measurements are carried out using a marginal oscillator operating at 11.9 MHz. Melt-textured samples are found to exhibit insignificant low-field losses and hence are potential candidates for applications at radio frequencies. The field induced loss in sintered BSCCO is described by invoking two kinds of weak links. The rf penetration depth of the materials measured is found to be much larger than the London penetration depth and represents the effective screen depth. More importantly, the authors observed an anomaly in the penetration depth of BSCCO which is explained in terms of an interplay between intragranular and Josephson penetration depths.

Evidence of Enhanced Flux Pinning by Dislocations in Deformed Textured Y1Ba2Cu3Ox Superconductor

As-melt and uniaxially deformed melt textured YBCO (123) superconductors have been used to examine the role of dislocations in flux pinning. These superconductors were subjected to a high temperature annealing process. After annealing, the critical current densities (J c) of both as-melt and deformed superconductors are found to have decreased dramatically both in zero field as well as in an applied field of 1.5 T. Furthermore, the distinct characteristic of deformed superconductors, i.e., J c ( H|| c-axis)≈J c ( H|| a-b), has totally disappeared after annealing the samples at 900° C for 48 h in flowing oxygen. Transmission electron microscopy revealed that the high dislocation density normally observed in deformed 123 superconductors has been significantly reduced. In addition, several other microstructural changes were also investigated. These results indicate that the large amount of dislocations generated during the deformation process are effective flux pinning centers in these superconductors. Meanwhile, it is suggested that pinning by stacking faults is magnetically sensitive where the effect is maximum at low fields.

Investigation of certain crystal growth characteristics in melt-textured YBCO superconductor using wavelength dispersive X-ray analysis

Using wavelength dispersive X-ray analysis, it is found that some non-superconducting Y 2BaCuO 5 (Y-211) grains in the melt-textured YBCO superconductor demonstrate uneven distribution of Y atoms. Such a phenomenon can be explained by diffusion of Y atoms between the YBa 2Cu 3O y (Y-123)/liquid interface, the Y-211/liquid interface and the Y-211 grain inside. Moreover, ‘gaps’-generated due to different degrees of contractions of the matrix and Y-211 grains are observed. ‘Cavass’ which is superconducting but appears like Y-211 grain is observed and reported.

Variation of secondary electron yield in the fractured surface of melt-textured growth YBCO superconductor with Y2BaCu05 grains under a scanning electron microscope in the temperature range 81?300 K

Variation of secondary electron yield in the fractured surface of melt-textured growth YBCO superconductor with Y2BaCu05 grains under a scanning electron microscope in the temperature range 81?300 K

Field sweep rate dependence of DC and AC magnetization: comparisons with flux creep and dynamic magnetization models

The effect of DC field sweep rate on DC and AC magnetizations has been studied in melt-textured YBCO superconductors. Comparisons of results are made with the flux creep and dynamic magnetization models, and it is shown that both the anisotropy in the rate dependence of DC magnetization, and the absence of history dependence in AC susceptibility (with DC sweeps) is consistent with the dynamic magnetization models. For low fields, however, the flux creep contribution to rate dependence may still be very significant, as demonstrated by comparisons of normalized relaxation rate (d ln m/d ln t) and (1/m)(dm/d ln H).

Effect of applied magnetic field on the I–V characteristics of melt-textured YBa 2Cu 3O 7−x superconductors containing grain boundaries

The current-voltage ( I–V) characteristics of sintered and melt-textured YBCO superconductors containing grain boundaries have been studied in the presence of applied magnetic fields up to 1.5 T. The melt-textured samples contain grain boundaries of angles up to 16°. The measurements were performed at 77 K using DC transport current. In applied fields, sintered and melt-textured specimens are found to exhibit power-law I– V characteristics. In both cases, the power exponent n is found to decrease with increasing field and the corresponding decrease in J c. In sintered specimens, this behavior is interpreted in terms of network models where the n value is related to the width of the critical current distribution and the overall J c. It is also found that the network models can represent the I– V behavior in melt-textured specimens including those with grain boundaries. However, if these models do not apply to melt-textured specimens, the power-law I– V characteristics can be represented by flux motion models such as collective flux pinning or a vortex-glass transition.

I-V characteristics of melt-textured YBa 2Cu 3O 7− x superconductors containing grain boundaries

The current-voltage ( I-V) characteristics of melt-textured YBCO superconductors have been studied in single-grain samples as well as samples containing grain boundaries of high current-carrying capability. The latter samples contained one or more grain boundaries of angles in excess of 20°. The measurements were performed at 77 K using both DC and pulsed currents in self-field. In samples possessing critical current densities in the range of 13 000–63 000 A cm -2, the I-V characteristics exhibit a nearly power-law behavior with a negative curvature in a double-logarithmic plot. The power exponent n is found to be correlated to the value of J c in these samples. On the other hand, the I-V characteristics for samples containing grain boundaries and possessing critical current densities of approximately 10 000 A cm -2 are predominantly of a linear type. It is concluded that a possible J c limiting mechanism for these specimens is flux flow along the grain boundaries due to reduced pinning forces in the grain boundary plane.