Top-seeded Melt Texture Research Articles

The Performance of the Two-Seeded GdBCO Superconductor Bulk with the Buffer by the Modified TSMG Method.

The multiseeding technique is a method to grow large-sized REBa2Cu3O7-δ (REBCO, where RE is a rare earth element) high temperature superconducting bulks. However, due to the existence of grain boundaries between seed crystals, the superconducting properties of bulks are not always better than those of single grain bulks. In order to improve the superconducting properties caused by grain boundaries, we introduced buffer layers with a diameter of 6 mm in the growth of GdBCO bulks. Using the modified top-seeded melt texture growth method (TSMG), that is, YBa2Cu3O7-δ (Y123) as the liquid phase source, two GdBCO superconducting bulks with buffer layers with a diameter of 25 mm and a thickness of 12 mm were successfully prepared. The seed crystal arrangement of two GdBCO bulks with a distance of 12 mm were (100/100) and (110/110), respectively. The trapped field of the GdBCO superconductor bulks exhibited two peaks. The maximum peaks of superconductor bulk SA (100/100) were 0.30 T and 0.23 T, and the maximum peaks of superconductor bulk SB (110/110) were 0.35 T and 0.29 T. The critical transition temperature remained between 94 K and 96 K, with superior superconducting properties. The maximum JC, self-field of SA appeared in specimen b5, which was 4.5 × 104 A/cm2. Compared with SA, the JC value of SB had obvious advantages in a low magnetic field, medium magnetic field and high magnetic field. The maximum JC, self-field value appeared in specimen b2, which was 4.65 × 104 A/cm2. At the same time, it showed an obvious second peak effect, which was attributed to Gd/Ba substitution. Liquid phase source Y123 increased the concentration of the Gd solute dissolved from Gd211 particles, reduced the size of Gd211 particles and optimized JC. For SA and SB under the joint action of the buffer and the Y123 liquid source, except for the contribution of Gd211 particles to be the magnetic flux pinning center with the improvement of JC, the pores also played a positive role in improving the local JC. More residual melts and impurity phases were observed in SA than in SB, which had a negative impact on the superconducting properties. Thus, SB exhibited a better trapped field and JC.

A Low-Cost Fabrication Technique for the Growth of Single-Domain GdBCO Bulk Superconductor from Raw Metal Oxides

A single-domain GdBCO bulk superconductor is usually fabricated using a mixture of pre-reacted precursor powders (including GdBa2Cu3O7−δ and Gd2BaCuO5) by the top-seeded melt texture growth (TSMTG) process. However, the synthesis of pure-phase precursor powder by the solid-state sintering reaction can not only consume time for weeks but also prolong the fabrication process and can lead to a low-efficiency and high-cost process. In this paper, we reported the successful growth of single-domain GdBCO bulk superconductors by the top-seeded metallic oxide melt growth (TS-MOMG) process in which the raw materials of Gd2O3, BaO, and CuO, the three different metallic oxides, are just involved to fabricate the single-domain GdBCO bulk superconductors. After detailed investigations in the morphology, levitation force, trapped magnetic field, and critical temperature of the grown samples with different ratios, it was found that the simplified precursor-phase source can be used to fabricate the single-domain GdBCO bulk superconductors more efficiently, which can combine the crystal growth procedure and greatly economize the preparation cost and the time to the most extent.

Fabrication and properties of single domain GdBCO superconducting rings by a buffer aided Gd+011 TSIG method

Single domain REBCO bulk superconductors can be fabricated by both the top-seeded melt texture growth process and the top-seeded infiltration and growth (TSIG) process. The buffer pellet technique used in these two methods has been demonstrated to act as an efficient seed and to aid the growth of the REBCO bulk superconductors. In this paper, we combined the Gd+011 TSIG method and buffer pellet technique together to fabricate single domain REBCO superconducting rings without any machining and obvious shrinkage of the final ring, which shows a satisfactory trapped field and very good magnetic shielding at the same time. It is found that (1) a single domain GdBCO ring, with inner diameter of 10 mm and outer diameter of 32 mm, has been fabricated by this method successfully. (2) The trapped field (0.32 T, 77 K) and maximum levitation force (80.1 N, 77 K) of the single domain GdBCO ring are nearly the same as the trapped field (0.33 T, 77 K) and maximum levitation force (80.2 N, 77 K) of the single domain GdBCO bulk with the same diameter and thickness, but the uniformity of trapped field is much better for the single domain GdBCO ring. (3) The single domain GdBCO ring shows very good magnetic shielding property, the magnetic flux density shielded by the superconducting ring is up to 0.40 T at 77 K, which is much higher than that of reported samples. The results indicate that the buffer aided Gd+011 TSIG method can provide an important way to fabricate high quality single domain REBCO superconducting rings directly.

The fabrication process of a high performance and pure c-axis grown GdBCO bulk superconductor with the TSMT-IG technique

It is very important to choose a suitable seed crystal in the fabrication process of the (RE)BCO bulk superconductors by the top-seeded growth method, because a defective seed would result in the failure of the entire sample. Therefore, for optimizing the seed, a new approach is proposed in this paper for the fabrication process of Gd-Ba-Cu-O (GdBCO) bulk with promising superconducting properties. We developed the top-seeded melt texture and infiltration growth (TSMT-IG) process based on the modified top-seeded infiltration and growth (TSIG) process. In this modified infiltration growth, a mini pellet of thin melting texture (TMT) GdBCO was inserted between the NdBCO seed crystal and the solid-phase precursor pellet, resulting in the successful formation of well purified c-axis-oriented GdBCO crystals with a proper-sized TMT layer. The growth morphology, microstructure, side growth boundary, and superconducting properties of the GdBCO bulk superconductors with three different sizes of TMT pellets were studied in this paper. The results proved that the success rate for the fabrication of single-domain GdBCO bulk superconductors can be improved by the TSMT-IG process, which can also effectively inhibit the random nucleation phenomenon at some extent during the process of single-domain GdBCO bulk superconductor growth. In addition, the application of this new growth technology can help us not only to improve the growth performance of GdBCO samples with a proper size of the TMT mini pellet but also reduce the fabrication cost due to the use of only one precursor powder.

Improvement of Gd123 superconductor bulks with the additions of BaFe12O19

The flux pinning performance of the superconductors is important for the application of the Gd123 bulk superconductors. The study shows that to introduce the secondary phases into the Gd123 bulk matrix can enhance the flux pinning performance. In this article, by using top-seeding melt texture growth process method, single domain GdBa2Cu3O7-δ superconductor bulks doping with the different amounts of BaFe12O19 (0.0mol% to 0.8mol%) were successfully achieved. The property and micro-structure have also been investigated. The result shows that there is an obvious improvement on JC with 0.2mol% BaFe12O19 addition. The fine distribution and smaller size of Gd211 particles appear in the micro-structure which may result in the enhancement of JC. At the same time, BaFe12O19 may also form an effective pinning center to increase JC.

Effect of BaFe12O19 nanoparticles doped on the properties of single domain GdBa2Cu3O7-δ high-temperature superconductors

The flux pinning performance of the superconductor is important for the applications of the GdBa2Cu3O7-δ superconductor bulk. To introduce the suitable secondary phase into the GdBa2Cu3O7-δ matrix is an important way to enhance the performance of flux pinning. By using top-seeded melt texture growth process, single domain GdBa2Cu3O7-δ superconductor bulks (20 mm in diameter) doping with different quantities of BaFe12O19 nano-particles (12O19(x=0, 0.2 mol%, 0.4 mol%, 0.8 mol%)+ 10 wt% Ag2O+ 0.5 wt% Pt. The effects of different quantities of BaFe12O19 nano-particles on superconducting properties and microstructure are also investigated. The result shows that the critical current density, JC, with 0.2 mol% BaFe12O19 additions reaches a maximum value in the zero field, which is about 5.5 × 104 A/cm2. And the critical current density JC, almost increases in the whole field compared with those of the undoped bulks. The microstructure and chemical composition of GdBa2Cu3O7-δ bulk with BaFe12O19 nano-particles are implemented by the SEM-EDS technique. It is found that BaFe12O19 nano-particles keeps a similar form to that of the precursor in the final superconductor bulk. The average size of Gd2BaCuO5 particles is reduced from 1.4 μm in the undoped bulk to 0.79 μm in the bulk with 0.2 mol% BaFe12O19 nano-particles. We suggest that BaFe12O19 nano-particles may form effective magnetic flux centers in the bulks, which affects the homogeneous distribution and refinement of Gd2BaCuO5 particles. Therefore, the improvements in the critical current density and the trapped field are observed in the GdBa2Cu3O7-δ bulk with low-level doped content. The superconducting transition temperature TC, can be maintained at around 92.5 K. However, with the addition of 0.4 mol% BaFe12O19 nano-particles, the critical current density and superconducting transition temperature decrease obviously. It indicates that the excessive addition of BaFe12O19 nano-particles may affect the superconductivity properties to reduce the critical current density, JC. The result indicates that the low-level content BaFe12O19 nano-particles can be an effective second phase for the improvement of the GdBa2Cu3O7-δ superconductor bulks, which is very important for the further enhancing the superconducting properties of GdBa2Cu3O7-δ bulks by introducing the flux pinning of nano-particles.

Size effects of nano-scale pinning centers on the superconducting properties of YBCO single grains

High pinning superconductors are the most promising materials for power engineering. Their superconducting properties are governed by the microstructure quality and the vortex pinning behavior. We report on a study of the vortex pinning in YBa2Cu3O7−x (YBCO) single grain with defects induced through the addition of insulating nano-particles. In order to improve the critical current density, YBCO textured bulk superconductors were elaborated using the Top Seeded Melt Texture and Growth process with different addition amounts of Al2O3 nano-particles. Serving as strong pinning centers, 0.05% excess of Al2O3 causes a significant enhancement of the critical current density Jc under self field and in magnetic fields at 77K. The enhanced flux pinning achieved with the low level of alumina nano-particles endorses the effectiveness of insulating nano-inclusions to induce effectives pinning sites within the superconducting matrix. On the other side, we focused on the effect of the size of pinning centers on the critical current density. This work was carried out using two batches of alumina nano-particles characterized by two different particle size distributions with mean diameters PSD1=20nm and PSD2=2.27μm. The matching effects of the observed pinning force density have been compared. The obtained results have shown that the flux pinning is closely dependent on the size of the artificial pinning centers. Our results suggest that the optimization of the size of the artificial pinning centers is crucial to a much better understanding of the pinning mechanisms and therefore to insure high superconducting performance for the practical application of superconducting materials.

Growth anisotropy effect of bulk high temperature superconductors on the levitation performance in the applied magnetic field

Growth anisotropies of bulk high temperature superconductors (HTSCs) fabricated by a top-seeded melt texture growth process, that is, different pinning effect in the growth sectors (GSs) and growth sector boundaries (GSBs), possess effect on the macro flux trapping and levitation performance of bulk HTSCs. Previous work (Physics Procedia, 36 (2012) 1043) has found that the bulk HTSC array with aligned GSB pattern (AGSBP) exhibits better capability for levitation and suppression of levitation force decay above a permanent magnet guideway (PMG) compared with misaligned GSB pattern (MGSBP). In this paper, we further examine this growth anisotropy effect on the maglev performance of a double-layer bulk HTSC. In contrast to reported trapped flux cases (Supercond. Sci. Technol. 19 (2006) S466), the two superposed bulk HTSCs with same AGSBP with PMG are found to show better maglev performance. These series of results are helpful and support a new way for the performance optimization of present HTS maglev systems.

The effect of Gd<sub>2</sub>Ba<sub>4</sub>CuNbO<sub>y</sub> doping on the properties of single domain GdBCO bulk superconductors

Single domain GdBCO bulk superconductors, with compositions of GdBa 2 Cu 3 O 7-δ (Gd123):Gd 2 BaCuO 5 (Gd211): Gd 2 Ba 4 CuNbO y (GdNb2411) =1:(0.4-x):x, (x=0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16), have been successfully fabricated by the top-seeding melt texture growth process (TSMTG). The effect of the different doping amounts of GdNb2411 on the growth morphology and the magnetic levitation force properties of single domain GdBCO bulk superconductors has been investigated. The results show that single domain GdBCO bulk samples can be fabricated within the scope of the doping, and the surface morphologies of the samples are closely related to the doping content x. It is of a smooth faced plane and four clear radius for the samples with x≤0.06 mol, but it is of a wrinkle surface for the samples with x≥0.08mol and the single domain area is observed to reduce with the increasing of x. The test results of the magnetic levitation force of the samples show that the levitation force of samples increases first and then decreases with the increasing of x, the largest levitation force of 25 N is obtained in the samples with x=0.06 mol. The results is very important for study in flux pinning of nanoparticles and further improvement in the properties of GdBCO bulk superconductors.

Electron Microprobe Characterization of Melt-Textured YBCO Created Using Hybrid Powder Metallurgy

An YBCO monolith was created by conventional top-seeded melt-texture processing using a mixture of Y and YBa2Cu3O7-delta (Y123) powders. The composition of the bulk monolith thus lies in the Y-Ba-Cu oxide ternary system rather than on the binary join between Y123 and Y2BaCuO5 (Y211) and the liquid line of descent, therefore, differs from the peritectic one exhibited by Y211-Y123 mixtures. In particular, Y123 crystallizes over a temperature interval that may be as large as 50degC and from a liquid with a considerable range in Cu content. Y123 in the Y-Y123 monolith exhibits a range in composition, particularly in Cu content that we ascribe to the change in crystallization conditions. This allows the spatial pattern of crystallization of the monolith to be mapped. Crystallization commenced at the seed crystal but then appears to have moved to the outer parts of the monolith before ending in the interior of the sample.

Composition analysis of Ca-doped YBa2Cu3O7−δ/Y2BaCuO5 prepared by the top-seeding melt-texture growth

The Ca-doped YBa2Cu3O7−δ/Y2BaCuO5 bulk-textured superconductors were prepared by using the top-seeding melt-textured growth process. Composition analyses were made for such prepared samples. We found that Ca concentration in the YBa2Cu3O7−δ grains decreased along the growing direction; Ca preferred to substitute Y in the YBa2Cu3O7−δ grains, rather than to stay at the grain boundary. The accumulation of extra CuO at the growth front was considered as one of the causes of the inhomogeneous distribution of Ca.

Effects of thermomechanical treatments on the critical current densityof TSMTG-processed YBa2Cu3O7-δ ceramics

TSMTG (top seeding melt texture growth)-processedYBa2Cu3O7-δ ceramics have reached such critical currentdensities (Jc) that it is now possible to envisage technical applicationssuch as magnetic bearings or fault current limiters. Most authors believe thatin this material, defects (such as Y-211 inclusions, oxygen vacancies,dislocations, stacking faults, etc) can act as pinning centres and henceincrease critical current densities. The effects on the critical currentdensity of a uniaxial pressure, applied along a direction perpendicular to thec-axis of Y-123, at 400 °C under an air atmosphere are investigated.These conditions were chosen to avoid oxygen loss in the samples, becauseoxygen content is the most critical parameter for superconducting properties.Thus this allows a sample with the same oxygen content to be compared directlybefore and after experiment. TSMTG-processed Y-123 ceramics exposed tothermomechanical treatment show a complex behaviour. For low pressures (lowerthan 2 MPa) a significant increase in the critical current density (up to 1.37times at H = 0, for a pressure of 1.38 MPa applied for 22 h at 400 °Cunder an air atmosphere) is observed and attributed to an increase in thedislocation density. For higher pressures, the critical current densitiesunder low magnetic fields are always lower but sometimes slightly higher atmoderate magnetic fields (between 1-3 T).

Top seeding melt texture growth of NdBaCuO pellets in air

Most of NdBaCuO synthesis were realized under oxygen controlled melt growth (OCMG) which allows to avoid the solid solution formation. However, for industrial applications air synthesis is preferable due to its simplicity and its cost. In this work, different seeds and thermal cycles have been investigated in order to optimize the nucleation conditions for the obtention of a single domain. Several criteria have been defined for the choice of the seed. The solid solution Nd1+xBa2−xCu3Oy formed in these air synthesised samples appears in the range of x = 0.10 to 0.15, corresponding to between 60 K and 80 K. The nominal composition and the synthesis conditions (substrate, thermal treatments.) have to be accurately defined in order to avoid neodymium-rich solid solution formation.

Routes to obtain high performance, high Tc superconducting bulk materials

This paper reviews our investigations for understanding the fundamental relations between physical properties, crystal chemistry, phases relation and processings on the compounds YBa2Cu3O7−x and Bi(Pb)2Sr2Ca2Cu3O10+x. These two systems have in common the complexity of their crystal structure and multi-components chemistry which require challenging material processing for applications. In the YBaCuO system, horizontal bridgman and melting zone processes have demonstrated the possibility of obtaining high performances in this system, with different shaping. A high reproducibility with low material losses have been achieved with a top seeding melt texturing process. For BiSSCO based compounds, sol–gel and polymer matrix methods have been studied and developed with the achievement of promising critical current densities at 77 K.

Isothermal growth of large YBa 2Cu 3O 7−x single domains up to 93 mm

The growth of YBa 2Cu 3O 7−x single domains larger than 30–40 mm by a classical Top Seeding Melt Texturing (TSMT) method appears to be difficult. Because of the very narrow solidification range between the onset of the nucleation from the seed and the nucleation of grains in the liquid, reaction with the substrate as well as uncontrolled thermal gradients introduce severe limitations to the growth. After suppressing nucleation from the substrate and improving the temperature homogeneity around the sample, the growth rate measured in situ was used to carefully adjust the process. A YBa 2Cu 3O 7−x single domain has been grown up to 93 mm without applied thermal gradient nor large overheating. The texture is confirmed on a large volume by neutron scattering and an homogeneous 211 size distribution and content are found across the sample. This suggests that steady growth conditions have been maintained and that even larger samples could be grown.

Texturing process, superconducting and mechanical properties of Ag-doped top-seeded melt-grown YBCO pellets

Pellets of YBa 2Cu 3O 7− δ doped with a mixture of ceria (0.5 wt %) and silver (5 and 10 wt %) were textured using the top-seeding melt-texturing process with a maximum temperature of 1000°C. 430°C was found to be an optimum annealing temperature for textured silver doped YBCO and Bean critical current densities of 55 kA/cm 2 were measured under self field. Both the fracture toughness and the resistance to thermal shock have been improved by the addition of silver.

Controlled growth of single-grain YBa2Cu3O7 by top-seeding melt-texturing

A Top-seeding-melt-texturing (TSMT) process is investigated by quenching, in order to understand the growth mechanism better. A SmBa2Cu3O7 melt-processed crystal has been used as a seed crystal for growing a large single-grain YBa2Cu3O7 (Y-123) pellet in a Y-123/Y-211 composite with PtO2-addition. It was found that the growth is three-dimensional with a growth ratio R100R001 to 1. By comparing the seeded and un-seeded pellets, quenched at different temperatures during growth stage, their difference in nucleation temperatures were identified. This causes the appearance of a temperature window in which the growth of a large single-grain monolith can be realized. Finally, the growth front morphology was observed and the growth mechanism is discussed in terms of a highly supersaturated melt by the dissolution of Y-211 particles.

Processing of superconductive YBCO single-grain monoliths by Top-seeding-melt-texturing

Top-seeding-melt-texturing (TSMT) process of YBa 2Cu 3O 7 was investigated. It was found that there exits a temperature-composition window for the stable growth of single-grain monoliths of diameter larger than 2cm with levitation forces of 15N/cm 2 and critical current density as high as 5 *10 4A/cm 2.

Top-seeding melt texture growth of single-domain superconducting pellets

The top-seeding melt texture growth (TSMTG) process is investigated in a small-vertical-thermal-gradient furnace in order to better understand the conditions for growing large single-grain pellets. An crystal was used as seed in a Y123 - Y211 composite with additions. It was shown that the role of the seed is to initiate the sympathetic nucleation growth mechanism. Indeed, the additional driving force induced by the seed is clearly demonstrated by a decrease in the undercooling of the system. The seeding crystal introduces a heterogeneous nucleation centre, leading to a very well controlled nucleation rate. Furthermore, we discuss the relation between intrinsic anisotropic growth rate and the processing rate, showing the importance of the thermal conditions of the texturing process to stabilize a sympathetic growth. By careful control of the texturing parameters, we succeeded in producing single-grain pellets as confirmed by a pole figure experiment and neutron diffraction measurements. The levitation force for pellets of 2 cm in diameter reaches 16 N when an NdFeB magnet of the same diameter is used.

Seeded melt texture growth processing of with coatings

In an attempt to minimize the occurrence of surface nucleation events, the effects of coatings on the surface of (Y-123) pellets processed by top-seeded melt texture growth processing were studied. During the melt processing of coated Y-123, coatings react to form and a Yb-containing liquid, which reduce the peritectic temperature below that of Y-123. As a result of this, during cooling, the coated surface regions will freeze last and allow the oriented domain nucleated at the seed to grow without any interference from domains nucleated at the periphery of the pellet.