Distribution Of Y-211 Particles Research Articles

Complex microstructural analysis of YBCO single-grain bulks with artificial holes: Effect on superconducting properties

This work presents an investigation of the microstructural and superconducting properties of standard single-grain bulk superconductor Y–Ba–Cu–O (YBCO) and samples fabricated with artificial holes. It was shown that the presence of artificial holes reduces the absolute porosity of the sample. The presence of artificial holes did not improve the homogeneity of oxygen distribution in the samples after oxygenation. The distribution of Y211 particles in the samples has a negligible effect on the macroscopic superconducting properties. In the process of crystal growth, the ellipticity of the introduced holes and their displacement in the direction of crystal growth in the growth sectors arise. Macroscopic superconducting properties, such as levitation force and trapped magnetic field at 77 K, were supplemented by magnetization measurements of superconducting transition temperature and critical current density. The levitation force and the trapped magnetic field were higher for the sample with holes. For the measured trapped magnetic field, the decrease of the porosity in the a-growth sectors, which have a higher volume fraction of Y2BaCuO5 (Y211) particles, and consequently stronger pinning, is significant.

Pulsed Field Magnetization of Single-Grain Bulk YBCO Processed From Graded Precursor Powders

© 2016 IEEE.Large single-grain bulk higherature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems, and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y-Ba-Cu-O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials. In this paper, experimental results on the pulsed field magnetization of a graded single-grain YBCO bulk sample are presented. Pulsed fields of magnitude up to 5.5 T were applied to the sample, at temperatures of 40 and 65 K. The trapped field profiles indicate that magnetic flux enters the sample in a more uniform manner than standard YBCO samples, resulting in a higher overall trapped field and indicating the success of this processing technique to produce a more homogeneous sample.

Numerical simulation and analysis of single grain YBCO processed from graded precursor powders

Large single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems and electric machines. However, macro-segregation of Y-211 inclusions in melt processed Y–Ba–Cu–O (YBCO) limits the macroscopic critical current density Jc of such bulk superconductors, and hence, the potential trapped field. Recently, a new fabrication technique with graded precursor powders has been developed, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties of such materials. In order to develop this graded fabrication technique further, a 3D finite-element numerical simulation based on the H-formulation is performed in this paper. The trapped field characteristics of a graded YBCO sample magnetized by the field cooling method are simulated to validate the model, and the simulation results are consistent with the experimental measurements. In addition, the influence of the graded technique and various graded Jc distributions for pulsed field magnetization, recognized widely as a practical route for magnetizing samples in bulk superconductor applications, is also investigated, with respect to the trapped field and temperature profiles of graded samples. This modelling framework provides a new technique for assessing the performance of various sizes and geometries of graded bulk superconductors, and by adjusting the Y-211, and hence Jc, distribution, samples can be fabricated based on this concept to provide application-specific trapped field profiles, such as the generation of either a high magnetic field gradient or a high level of uniformity for the traditionally conical, trapped field profile.

The processing and properties of single grain Y–Ba–Cu–O fabricated from graded precursor powders

The preparation of single grain, Y–Ba–Cu–O (YBCO) bulk superconductors by top-seeded melt-growth (TSMG) usually involves precursor powders that contain a uniform distribution of the constituent YBa2Cu3O7−δ (Y-123) and Y2BaCuO5 (Y-211) phase compounds. However, it has been observed that the concentration of Y-211 particles in the fully melt processed superconducting bulk increases significantly with distance from the seed, which results in a degradation of superconducting properties towards the edge and bottom of the sample. Here we investigate the effect of preparing bulk YBCO superconductors by TSMG using spatially graded Y-211/Y-123 precursor powder. The graded precursor bulks were prepared with a maximum composition of 40 wt% Y-211 in the vicinity of the seed, which decreased to 30 wt% and then 20 wt% towards the bottom and edge of the green body. Standard samples were melt processed from precursor powders containing 30 wt% Y-211 to enable comparison. The field trapping ability, Tc and Jc, of three graded and two standard samples were investigated and compared statistically. The distribution of Y-211 particles along different growth directions of the samples was analysed, and any crystallographic misorientation was investigated. The observed distribution of Y-211 particles in YBCO is explained qualitatively by trapping/pushing theory, and its correlation with the superconducting properties of the melt processed bulk samples has been analysed. Finally, the practical feasibility of the graded technique is evaluated.

Enhanced Growth Rate of a YBCO Bulk in the Melt-Textured Process Under 1 atm Oxygen Pressure

By the employment of 1 atm oxygen pressure as the growth atmosphere, we report a success in the enhanced growth rate of the melt-textured (MT) YBCO bulk. A well-oriented c -axis single-domain bulk with a size of 42 mm in diameter was fully grown at a high average growth rate of about 0.32 mm/h, which is 1.5 times larger than that in air. It was found that the increased oxygen partial pressure has a profound influence on the MT growth (MTG) for YBCO. The enhanced growth rate can be interpreted by both increased solubility and diffusion coefficient of yttrium in the Ba-Cu-O liquid under high oxygen partial pressure. After oxygenation, single-domain samples were determined to have a sharp superconducting transition temperature at 91.7 K, indicating that the superconductivity of crystals grown under high oxygen pressure does not degrade. Additionally, due to the rapid growth, the trapping effect of Y211 particles becomes predominant. As a result, a relatively more homogeneous distribution of Y211 particles was observed in the Y123 matrix. Finally, the most important advantage of this technique is that the enhanced growth rate of Y123 gives less opportunity from self-nucleation in the liquid, which takes place beyond the growth window and the embryonic period. Thus, this oxygen-increased MTG process is very promising for producing large-sized YBCO bulks with good superconducting properties.

Study on Microstructure Characterization of YBCO Bulk Prepared by Directional Seeded Infiltration-Growth

Abstract The seeded infiltration-growth process is an alternative to replace conventional melt processing techniques for the preparation of bulk YBa2Cu3O7-s (Y-123) with finely dispersed small size of Y2BaCuO5 (Y-211) particles. The directional solidification is also a quite effective technique to produce well-oriented cylinder grains. Well-oriented single domain Y-123 bulks can be fabricated by combining seeded infiltration-growth and directional solidification technique (DSIG). The infiltration-growth process is able to control the size and distribution of Y-211 particles in the final Y-123 superconductor and can fabricate net-shape bulk. Y-211 particles and a liquid phase precursor were used as starting materials in the present investigation. The main parameters (temperature gradient, configuration and maximum processing temperature) of the directional seeded infiltration-growth process were discussed in the present paper. The result shows that Y-211 particle sizes upon using 035 liquid phase composition are smaller than those upon using other two liquid phase compositions.

High-performance single grain Y–Ba–Cu–O bulk superconductor fabricated by seeded infiltration and growth

Large single grains of Y–Ba–Cu–O (YBCO) bulk superconductor of up to 32mm in diameter have been fabricated by a seeded infiltration and growth (IG) technique. Small Y2BaCuO5 (Y-211) particles are observed in the seeded IG sample microstructure, albeit with a rather inhomogeneous distribution in the superconducting YBa2Cu3Oy (Y-123) phase matrix. Values of magnetic critical current density, Jc, measured in self-field along the a and c growth sectors of the bulk samples prepared by seeded IG appear higher than the corresponding values for samples fabricated by conventional top seeded melt growth (TSMG), despite the inhomogeneous distribution of Y-211 particles in the former. A maximum trapped field of 0.42T at 0.2mm above the sample surface was observed at liquid nitrogen temperature (the actual mean temperature during measurement was around 78K) in samples fabricated by seeded IG due to the increased Jc, despite the relatively small YBCO grain size (25mm diameter×7.4mm thickness). This value of trapped field is almost two times greater than that of samples fabricated by TSMG and of a similar size.

Seeded infiltration and growth of large, single domain Y–Ba–Cu–O bulk superconductorswith very high critical current densities

Single domain Y–Ba–Cu–O (YBCO) composed of aYBa2Cu3Oy (Y-123) superconducting bulk matrix with discrete, non-superconductingY2BaCuO5 (Y-211) phase inclusions has been fabricated by a seeded infiltration and growth (IG)technique in the form of cylindrical pellets up to 32 mm in diameter. Sample shrinkage inthe radial direction for single domains prepared by this technique is relatively low at 5%and independent of sample size, in contrast to the shrinkage observed in samples grown byconventional melt processing, which increases significantly with increasing samplediameter. Furthermore, samples grown by the IG technique exhibit low porosity oftypically 0.9% of the bulk volume fraction, compared with a corresponding value ofaround 4.9% observed in samples fabricated by conventional melt processing. FineY-211 particles were observed to be embedded within the Y-123 superconductingmatrix for the IG processed samples, leading to a high critical current density,Jc, ofover 100 000 A cm−2 at 77.3 K in self-field. The distribution of Y-211 particles in the IG sample microstructure, however,was inhomogeneous (unlike in previous reports), which leads to a variation in the spatial distributionof Jc. The volume fraction of Y-211 in the vicinity of the seed crystal (i.e. corresponding to the initialc-sector growth stage), in particular, is typically around 5%, compared with a value of up to 30% inthe a growth sectors more distant from the seed crystal (which corresponds well to the theoreticalvalue for the sample composition studied here). The volume fraction of Y-211 inclusions in thec growth sector more distant from the seed was around 22%. Finally, a trend of thevariation in the distribution of Y-211 particles in the Y-123 matrix grown by the IGtechnique was similar to that in sample grown by conventional melt processing.

High critical current density in RE–Ba–Cu–O bulk superconductors with very fine RE 2BaCuO 5 particles

We have studied the microstructure and critical current density ( J c) of Gd–Ba–Cu–O and Y–Ba–Cu–O bulk superconductors. The size of RE 2BaCuO 5 (RE211) particles trapped in REBa 2Cu 3O y (RE123) matrix could be refined by employing ultra-fine RE211 powder as a precursor, which led to high J c values exceeding 1 × 10 5 A/cm 2 at 77 K in self-field for Gd–Ba–Cu–O. In the case of Y–Ba–Cu–O, however, the size reduction of Y211 led to inhomogeneous distribution of Y211 particles, in that the volume fraction of Y211 phase largely decreased near a seed crystal. We also studied superconducting properties of (Gd,Y)–Ba–Cu–O bulk material fabricated from precursor of Gd123-Y211. The employment of finely-milled Y211 powders resulted in a dramatic enhancement of J c values in this system. The maximum J c value for (Gd,Y)–Ba–Cu–O reached 2.5 × 10 5 A/cm 2 at 77 K.

High critical current density in Y–Ba–Cu–O bulk superconductors with very fine Y211particles

We have succeeded in enhancing the critical current density(Jc) of melt-textured Y–Ba–Cu–O superconductors by employing ultra-fine Y211powder as precursor. The Y211 starting powders were reduced down to 110 nmin diameter with ball-milling. Single-grain Y–Ba–Cu–O samples up to 33 mm indiameter were fabricated from the precursors containing finely milled Y211powders. The Y211 inclusions in the melt-textured samples could be refined bydecreasing the particle size of the Y211 starting powders. The maximum value ofJc at 77 Kwas 1.1 × 105 A cm−2 in a self-field. However, the size reduction of Y211 led to an inhomogeneous distribution ofY211 particles, in which the volume fraction of the Y211 phase greatly decreased near aseed crystal. An Ag-added bulk sample 33 mm in diameter exhibited a high trappedmagnetic field of 1.6 T at 77 K.

Mechanical properties of TSMG-YBCO

The fracture toughness, K/sub c/, of YBCO prepared by the top-seeded-melt-growth (TSMG) method can be improved by increasing the yttrium content. K/sub c/ increases from 1.01 MPa/spl radic/m for samples with 30 mol % Y/sub 2/Ba/sub 1/Cu/sub 1/O/sub 5/ (Y-211) addition to a value of 1.44(22) MPa/spl radic/m for samples with 60 mol % Y-211 addition. Substituting CeO/sub 2/ for PtO/sub 2/ results in a further enhancement of K/sub c/. In both cases the improvement can be explained by a refinement and homogenization of the distribution of Y-211 particles. The incorporation of ductile particles also contributes to an enhanced fracture toughness of the melt-textured monoliths.

Effects of Pt doping on the size distribution and uniformity of particles in large-grain YBCO

Accurate and detailed image analysis of the (Y-211) particle distribution in large-grain Pt-doped and undoped fabricated by seeded peritectic solidification has been performed using scanning electron microscopy. The size distribution of Y-211 particles along the a or b and c directions of doped and undoped grains has been measured and observed to approximate well to a log-normal distribution. The mean Y-211 particle size, which is independent of growth direction, is approximately and in the doped and undoped grains, respectively. The particle size distribution is significantly narrower for the Pt-containing material, however, corresponding to a much greater particle size uniformity. Such a uniform dispersion of fine Y-211 particles is desirable since it both enhances the critical current density of melt-processed YBCO and reduces liquid loss during peritectic solidification.