Oxygen Controlled Melt Growth Research Articles

(Gd,Y,Er)Ba2Cu3Oy bulk superconducting system processed via infiltration growth

Ternary bulk REBa2Cu3Oy superconductors processed via oxygen-controlled melt growth (OCMG) have exhibited superior superconducting properties with respect to binary and single systems. However, the OCMG process is rather complicated, expensive, and difficult to scale-up into industrial level. To overcome these issues, we have studied the yet unexplored ternary bulk (Gd,Y,Er)Ba2Cu3Oy “(Gd,Y,Er)-123″ system, fabricated via trendy infiltration growth (IG) process. We studied the effect of varying Y-211 and Er-211 ratio (Gd: Y: Er = 0.33: 0.33-x: 0.33+x, where x = 0, 0.05, 0.1, 0.15 and 0.2) in the preform on the final superconducting properties and microstructure. We succeeded to fabricate (Gd,Y,Er)-123 single-grain bulks (diameter – 20 mm). The critical current density (Jc) and trapped field increased proportionally with the increased amount of Er-211 in the preform. Hall-probe measurements showed that the best bulk (x = 0.2) exhibited trapped field of 0.53 T at 77 K (applied field of 1 T) and self-field Jc values of 238 and 63 kA/cm2 at 50 and 77 K, respectively. FE-SEM micrographs revealed formation of two different types of secondary phases in the matrix (small and large). EDX analysis showed that the smaller 211 particles were Er-rich, while the larger ones were an irregular mixture of three RE-211 phases.

The effect of Ca doping on the properties of melt-processed Gd-based bulk superconductors

The effect of Ca doping to melt-processed Gd-based bulk superconductors fabricated by the oxygen-controlled melt growth (OCMG) method was studied. First, we simply added CaCO 3 to the starting materials to dope Ca and observed an increase in the trapped field and the critical current density ( J c) up to, at least, 0.31 wt.% CaCO 3. However, an additional oxygen annealing resulted in a decrease of J c in magnetic fields and in a monotonous exponential-like field dependence of J c. This suggests that oxygen deficiency had caused the enhancement of J c, although the same annealing process as the non-doped sample was applied. We also prepared samples by adding not only CaCO 3 but also BaCO 3 and CuO in a molar ratio of Ca:Ba:Cu = 1:2:3, and observed a large peak effect of J c up to at least 0.31 wt.% CaCO 3. It is likely that this large peak effect is also caused by oxygen defects, although the superconducting transition temperature was significantly large. These results indicate that J c at 77 K can be significantly enhanced by introducing a proper amount of oxygen deficiency and compensating at the same time the decrease of carrier density by Ca doping.

Flux pinning and orientation control of large grain LREBa2Cu3Oy

The ternary high-Tc superconductor (Nd0.33, Eu0.39, Gd0.27)Ba2Cu3Oy ‘‘NEG-123’’ doped by 5 mol% of the nonsuperconducting secondary phase (Nd0.33, Eu0.33, Gd0.33)2BaCuO5 ‘‘NEG-211’’ was prepared by oxygen controlled melt growth (OCMG) process using MgO seeds. Various contents of ZnO up to 0.07 mol% were added. At the ZnO concentration around 0.035 mol%, a substantial reduction of liquid phase loss was observed, accompanied with a perfect facet line up to the bottom of the sample, with c-axis reflection plane. Magnetization measurements showed that at this optimum ZnO addition the superconducting transition temperature was slightly depressed. On the other hand, flux pinning performance around the secondary peak, observed at liquid nitrogen temperature, was continually improved with increasing ZnO content up to 0.035 mol%. It was found that the optimum content of ZnO is crucial for flux pining enhancement and orientation control of MgO seeded melt-textured NEG-123 system.

ELASTIC BEHAVIOR OF OXYGEN CONTROLLED MELT GROWTH PROCESSED NEG-123 HTSC MATERIALS

A series of superconducting materials having compositional formula, ( Nd - Eu - Gd )- Ba - Cu - O (123) + x%( Nd - Eu - Gd ) Ba - Cu - O (211) or NEG-123 + NEG-211 (where x ranges from 0–50%) were prepared by the Oxygen Controlled Melt Growth (OCMG) process. After characterizing the materials by studying their microstructure, measuring Jc and Tc values, the ultrasonic longitudinal velocity (Vl) measurements were carried out both as a function of composition (x% of 211 phase) as well as the temperature, over a temperature range 80–300 K. It has been found that the variation of ultrasonic longitudinal velocity is similar to that of Jc with increasing x values (concentration of NEG-211). These results indicate that by dispersing 40% fine NEG-211 particles in the superconducting matrix of NEG-123, both the critical current densities as well as the ultrasonic velocity values are found to exhibit highest values. It is known that the lattice defects of ceramic superconductors are manifested in the form of elastic anomalies, especially in the normal temperature region (100–250 K). In order to understand the complete scenario of elastic anomalies of HTSC materials, a systematic investigation of the ultrasonic velocities of HTSC materials prepared by the OCMG technique has been undertaken and using the results an attempt has been made to generalize the scenario of the elastic anomalies in HTSC materials.

Synthesis and transport properties of oxygen controlled melt growth textured(MixedRE)Ba2Cu3Oy (RE = Nd,Sm, Gd, Eu)

((RE)0.33Eu0.33Gd0.33)Ba2Cu3Oy bulksuperconductors (RE = Sm, Nd) were textured by the top seeded melt textured growth method in a reducedoxygen atmosphere, so-called oxygen controlled melt growth (OCMG). Largesingle domains of diameter 16 mm were successfully grown. Their microstructureand their magnetic and transport properties were investigated and compared.The resistive transition was found around 88 K, which is not the optimalTc of 95 K that could be obtained on a small re-oxygenated sample cutout from the bulk. Transport measurements were also performed onthese samples at various temperature from 77 to 88 K. In the case ofNd0.33Eu0.33Gd0.33Ba2Cu3Oy (NEG) composition, the angular dependence of the critical current densities along the(a,b) planes as a function of magnetic field was also measured.Transport critical current densities higher than50 000 A cm−2 arereached at 77 K and up to 2 T, corresponding to the nominal critical currents of 90 A injected through sectionsless than 0.25 mm2 reproducibly. This confirms the high quality of the single domain obtained with a wellcontrolled process, evidencing that mixing of rare earth is a very promising composition fortransport and high-field applications.

Melt processing and superconducting properties of single-domain GdBa 2Cu 3O y, (Sm 0.5Gd 0.5)Ba 2Cu 3O y and (Sm 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y superconductors fabricated in air

Single-domains of GdBa 2Cu 3O y , (Sm 0.5Gd 0.5)Ba 2Cu 3O y and (Sm 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y with and without BaCuO 2− δ doping were successfully prepared under the ambient atmosphere via a two-step cooling method. At 77 K, all samples show a well-developed secondary peak effect. The combination of two-step cooling method and BaCuO 2− δ addition is very effective to suppress the LRE–Ba substitution and hence improve superconducting properties. The Gd–Ba–Cu–O samples exhibit very high J c values, which are comparable to those of the oxygen-controlled melt-growth (OCMG) Gd–Ba–Cu–O samples.

Position dependence of superconducting properties of large single domain Sm–Ba–Cu–O bulk superconductors prepared under various conditions

We investigated the position dependence of superconducting properties of c-axis oriented large single domain Sm–Ba–Cu–O (SmBCO) bulk superconductors, which have been successfully fabricated with the oxygen controlled melt growth method using Nd–Ba–Cu–O as a seed crystal under various conditions. The superconducting properties of small specimens cut from the large single domain SmBCO strongly depend on their location and T c is always lower under the seed crystal than that near the edge of the sample. To enhance the homogeneity of the bulk superconductor, we have studied the effect of the maximum temperature ( T max) and the holding time ( t h) during partial melting. It was found that the variation of the superconducting properties with position can be greatly reduced with higher T max and/or longer t h.

Thermoelectric power studies of (Nd–Eu–Gd)Ba–Cu–O superconductors

Thermoelectric power measurements of a series of (Nd–Eu–Gd)Ba–Cu–O(1 2 3) + x% (Nd–Eu–Gd)Ba–Cu–O(2 1 1), where x ranges from 0 to 50, high temperature superconducting materials prepared by the oxygen controlled melt growth (OCMG) process, were carried out over a temperature range 80–225 K. It has been observed that thermoelectric power values are very low but positive and are found to increase almost linearly with decreasing temperature, reaching a maximum value at temperatures close to the superconducting transition temperatures of the materials. On further cooling the materials, the thermopower is found to fall suddenly to zero at their respective T c values. It has been reported that a number of physical properties and their variation with temperature especially in the normal temperature region (above T c) influence the behavior of HTSC materials in their superconducting state and as such an attempt has been made to understand the variation of thermopower (in the normal temperature region) with temperature. The experimental thermopower data has been fitted to different theoretical models and it has been observed that the narrow band phenomenological model alone is in a position to explain the variation of thermopower with temperature especially in the normal temperature region. The narrow band model has also been used to understand how the band spectrum changes with varying dopent’s concentration. The model has also been used to explain the variation of T c with increasing concentration of NEG-211.

Scanning tunneling microscopy studies of high Jc NEG123 crystals

We have performed ultrahigh vacuum scanning tunneling microscopy measurements at room temperature on (Nd0.33Eu0.38Gd0.28)Ba2Cu3Oy (NEG123) crystals grown by an oxygen controlled melt growth process. Two characteristic nanostructures, namely, modulation and island structures have been observed on a cleaved surface of the NEG123 with high Jc and high irreversible field Hirr higher than 10 T at 77 K. These structures may be associated with an element substitution and the good Jc–B property.

Fabrication of multi-filamentary LRE123 superconductors

The fabrication of multi-filamentary LRE123 (LRE=Nd, Sm, Eu and Gd) superconductors was examined. The precursor LRE123 filaments were prepared by solution spinning through aqueous poly(vinyl alcohol) solution containing mixed Nd, Sm, Eu, Gd, Ba and Cu acetates. The as-drawn filaments were heated to remove volatile components and were oxygen controlled melt growth processed in flowing 1%O 2+Ar and 0.1%O 2+Ar and then oxygenated. The filamentary superconductors were passed through Ag paste and sintered. By controlling the heating condition, a composite with excellent connection between the filaments and metallic Ag matrix was obtained. An overall transport critical current ( J e) of more than 1900 A/cm 2 at 77 K and 0 T was achieved for the composite with LRE123 filaments partially melted in flowing 0.1%O 2+Ar. The superconductivity at 77 K was maintained by applying a field more than 10 T. The field dependence of J e for the composite with LRE123 filaments was superior for the composite with Y123 filaments.

In situ monitoring of the eutectic and peritectic reactions in NdBCO prepared by oxygen controlled melt growth process by measuring the change in p(O 2)

A new, simple method for in situ monitoring of peritectic decomposition and recombination temperatures of NdBCO superconductor by measuring the change in the oxygen partial pressure, p(O 2) is described. The NdBCO system is usually prepared by the oxygen controlled melt growth process to achieve high transition temperature, T c and sharp superconducting transition. The present technique is based on monitoring the release or uptake of oxygen accompanying the decomposition or recombination of the NdBCO peritectic reactions in the processing furnace. The eutectic temperature of BaCuO 2–CuO was also detected by this technique. DTA and TGA were used simultaneously with the change in the oxygen partial pressure, p(O 2), to confirm the change in p(O 2) at the eutectic and peritectic reaction temperatures.

Single-domain NEG-123 bulk superconductors for high magnetic field applications

The single-domain (Nd 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y “NEG-123” bulk superconductors of 22 mm diameter were prepared with the oxygen-controlled melt-growth process. 40 mol% of (Nd 0.33Eu 0.33Gd 0.33) 2BaCuO 5 “NEG-211” sub-micron particles was dispersed in the NEG-123 matrix to improve the flux pinning capability at 77 K. For the controlled crystal growth, we used the melt-textured Nd-123 as a seed. The trapped fields were measured at 77 K using a Hall probe scanned at 2 mm above the pellet surface. In the single-domain NEG bulk 22 mm in diameter and 9.5 mm in thickness, a trapped magnetic field of 0.7 T was recorded at 77 K and 2 T, which corresponds to the total 2.7 T including the external field.

Improvement in Field Trapping Capability of (Nd,Eu,Gd) Ba2Cu3Oy with Ag2O Addition

We prepared (Nd,Eu,Gd) Ba2Cu3O7-δ (NEG-123) pellets of 22 mm diameter and 9 mm thickness with 10 mole% NEG-211, 0.5 mole% Pt, and various amounts of Ag2O ranging from 0 to 50 wt% using the oxygen-controlled melt-growth (OCMG) process. The irreversibility field increased with Ag2O content up to 20 wt%, and even with 50 wt% the magnetization loop was open at 77 K and 7 T. In the sample with 20 wt% Ag2O, a large critical current density of 10 kA cm-2 was recorded even at 7 T and 77 K for fields applied parallel to the c-axis. The trapped field in the NEG-123 bulk was measured using a Hall probe at 77 K after field-cooling in 5 T. A single-peak profile observed in the sample with silver addition proved that silver dispersion is effective in improving mechanical properties without deterioration of electromagnetic properties.

Microstructure and flux pinning in oxygen-controlled melt-growth processed (Nd 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y

We fabricated single-domain (Nd 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y (NEG123) bulk superconductors with a top-seeded melt-growth process in Ar–0.1%O 2 atmosphere using melt-textured Nd-123 as a seed crystal. We added a variety of secondary phase particles ((Nd,Eu,Gd)211, (Eu,Gd)211, Eu211, Gd211) in different amounts to the NEG123. Scanning electron microscopy observations demonstrated that the morphology, the particle size, and the distribution were strongly dependent on the concentration and the type of secondary phase. In addition, the chemical analyses revealed that the matrix composition also changes with the secondary phase particles. They also strongly affected the critical current density ( J c) and the irreversibility fields. The self-field and peak J c values will be discussed with relation to the type of pinning mechanisms.

Oxygen controlled melt growth process of filamentary Gd123 superconductors

The precursor filament of Gd123 superconductors was prepared by solution spinning through a homogeneous aqueous solution of the metallic acetates. The precursor sample was partially melted in flowing 1% O 2+Ar under various heating conditions. The maximum J c value of 2.1×10 4 A/cm 2 at 77 K and zero field was attained for filamentary Gd 1.18Ba 2.12Cu 3.09O y superconductors. The sample had well aligned texture. The field dependence of J c for the sample was investigated. An angle dependence of J c along the filament diameter was detected. The J c value above 1×10 3 A/cm 2 was obtained at 77 K and 10 T.

Improved superconducting properties due to Ag 2O addition in NEG-123

We prepared NEG+10 mol% NEG-211+0.5 mol% Pt with 0–50 wt.% Ag 2O, which are subjected to the oxygen-controlled melt-growth process. Microstructural observation showed that fine Ag particles are dispersed at lower Ag 2O contents. At higher contents, Ag particles exhibited various sizes and shapes and segregated mostly in the vicinity of microcracks and pores. The irreversibility field increased with Ag 2O content up to 20 wt.%, and even with 50 wt.% the magnetization loop was open at 77 K and 7 T. In the sample with 20 wt.% of Ag 2O, a large critical current density of 10 kA cm −2 was recorded even at 7 T and 77 K with the field applied parallel to the c-axis of the sample.

Single-domain NEG bulk superconductors for high magnetic field applications

Single-domain (Nd, Eu, Gd)–Ba–Cu–O bulk superconductors, silver free and with 10 wt.% of silver, were prepared with the oxygen-controlled melt-growth process. Seeding techniques were used to promote single-grain growth with controlled orientation using a melt-textured Nd-123 as a seed. The trapped field in the (Nd, Eu, Gd)–Ba–Cu–O bulk of 22 mm diameter and 9 mm thickness was measured at 77 K after field cooled in 7 T. The trapped-field profile was measured by means of a Hall probe at the distance of 5 mm from the sample surface. A single and multiple peaks were observed on (Nd, Eu, Gd)–Ba–Cu–O samples with and without silver addition, respectively. In the sample with silver, a maximum trapped field of 0.5 T was achieved at 3 T after the field reduction from 7 T.

Effect of ZrO 2 and BaZrO 3 addition on the microstructure and superconducting properties of melt processed NdBa 2Cu 3O 7− δ

We investigated the microstructure and superconducting properties of the Nd–Ba–Cu–O with 0–1.0 wt.% ZrO 2 and 0–2.24 wt.% BaZrO 3 synthesized by the oxygen-controlled melt-growth process in 1% O 2–Ar atmosphere. The microstructure of Zr-added NdBCO was not homogeneous, and one could observe the segregation of Nd422 particles, the residual liquid phase including the agglomeration of BaZrO 3 particles and the Nd422-free regions. Such inhomogeneity is caused by the strong influence of BaZrO 3 on the solid/liquid interfacial energy during the melt growth.

Effect of the composition and process on the Nd 1Ba 2Cu 3O y bulk properties in oxygen-controlled melt growth synthesis

Nd 1+ x Ba 2− x Cu 3O y bulk superconductors have been synthesised by the oxygen-controlled melt growth method (OCMG). The texturing thermal cycle of the pellets has been optimised and the influence of the support determined. Then, the effects of Nd 4Ba 2Cu 2O 10 and Nd 3.6 Ba 2.4Cu 1.8O 9.6 additions and Ce, Sn and Ag dopants on the microstructure and the superconducting properties have been tested. Under low field, good properties are obtained due to the refinement of the secondary phases. An oxygen-annealing with 600°C steps has been investigated. To improve superconducting properties under high field, a cooling rate of 1°C h −1 during 20°C after the nucleation window has a benefit effect.

Angular dependence of Jc for filamentary (Nd,Eu,Gd)123 superconductors prepared by a solution spinning method and oxygen controlled melt growth process

The filamentary (Nd 0.333Eu 0.333Gd 0.333) 1.18Ba 2.12Cu 3.09O x superconductors were prepared by solution spinning method and partially melted in flowing 1% O 2+Ar and then oxygenated by a two-step process. The filamentary sample had a dense and well-aligned texture along the diameter as well as the length. The J c of the sample was measured at 77 K in fields up to 10 T. Anisotropic behavior of the J c– B curve along the filament diameter was detected. High J c value of more than 3500 A/cm 2 at 77 K and 10 T was achieved by applying the field in a direction normal to the c-axis of the sample.