Oxygen Controlled Melt Growth Research Articles

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.

Effect of matrix composition on the flux pinning in a(Nd,Eu,Gd)Ba2Cu3Oysuperconductor

In order to study the effect of matrix composition on flux pinning in the ${(\mathrm{N}\mathrm{d},\mathrm{}\mathrm{E}\mathrm{u},\mathrm{}\mathrm{G}\mathrm{d})\mathrm{B}\mathrm{a}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ (NEG-123) bulk superconductors, we prepared two groups of samples with various rare-earth (RE) ratios using the oxygen-controlled-melt-growth (OCMG) process. For the first group, we kept the Nd content constant and varied the Eu/Gd ratio, and for the second group the Gd content was kept constant and the Nd/Eu ratio was varied. All the samples exhibited strongly developed fishtail effects and high critical current densities. Magnetization measurements revealed that the irreversibility field and the peak position systematically varied with Nd and Gd content. Scaling analyses suggested that two different pinning mechanisms are active in the NEG-123 system depending on the RE ratio. Those are $\ensuremath{\Delta}{T}_{c}$ pinning and normal-type pinning, however, we found that field-induced pinning also behaves like normal-type pinning on scaling analyses when $\ensuremath{\Delta}{T}_{c}$ is large. In conclusion, a difference in the peak position on scaled curves can be explained in terms of a difference in $\ensuremath{\Delta}{T}_{c}$ of field-induced pinning centers.

Scanning tunneling microscope studies on atomic structures in NdBa 2Cu 3O 7−δ single crystals II

Atomic structures of cuprate NdBa 2Cu 3O 7−δ (Nd123) processed by the oxygen-controlled-melt-growth (OCMG) method, have been studied by scanning tunneling microscope (STM). A simple free energy diagram is proposed to understand the simultaneous formation of island and striped structures observed within the same single crystal sample. Surfaces observed are the as-grown surface. Its atom images have been attributed to the metallic elements of Cu, Nd or Ba.

Superconducting properties of (Nd, Eu, Gd)-123

In the aim of optimizing the matrix composition to improve the critical current density ( J c ) and to strengthen the flux pinning in the (Nd 0.33Eu 0.33Gd 0.33)Ba 2Cu 3O y (NEG) bulk superconductors, we prepared two groups of NEG samples using a oxygen controlled melt growth process. For the first group we kept the Nd content constant and varied the Eu and Gd ratio, and for the second group the Gd content was kept constant and the Nd and Eu content was varied. In both cases we found a strongly developed fishtail effect and high critical current densities. Magnetization measurements revealed that, the irreversibility field and the peak position systematically varied with Nd content. However it was not systematic for Gd. The sample with Gd = 0.58 exhibited a large critical current density ( J c ) of 90000 A cm −2 at 77 K and 1.7 T.

Effect of Eu/Gd ratio on flux pinning in (Nd, Eu, Gd)-Ba-Cu-O

We have studied the magnetic properties of (Nd0.33EuxGd(0.66-x))Ba2Cu3Oy bulk samples preparedby the oxygen-controlled melt-growth (OCMG) process, in which x ranged from0 to 0.33. The heat treatment profiles for melt processing were determinedaccording to the peritectic decomposition temperatures obtained from thermalanalysis measurements. X-ray diffraction confirmed that the main phase isNEG-123 and the unit cell size decreases monotonously with increasing Gdcontent in the NEG-123 matrix, supporting the presence of a completeNd-Eu-Gd solid solution. The sample with x = 0.08 exhibited a large criticalcurrent density (Jc) of 90 000 A cm-2 at 77 K and 1.7 Tfor H∥ c-axis. The scaling analyses of the volume pinning forceshowed that the peak position in the scaling curve was dependent on the Eu/Gdratio, which indicates that the matrix composition mainly controls theΔTc-pinning mechanism.

The effect of initial composition on the oxygen controlled melt growth of filamentary Nd123 superconductor

We have studied a fabrication of filamentary Nd123 superconductor with high superconducting properties. The filamentary Nd123 precursor was prepared by the solution-spinning method, through a homogeneous aqueous solution containing mixed acetates of Nd, Ba and Cu with various compositions, poly(vinyl alcohol) and organic acids. The filamentary superconductor was produced by the oxygen controlled melt growth (OCMG) method. The effect of initial composition on the OCMG process of the sample was studied to enhance a reproducibility of high transport J c of the sample. The compositional region for preparing the sample with high J c was narrow.

Scanning tunneling microscope studies on the atomic structures in NdBa 2Cu 3O 7−δ single crystals

Atomic and electronic structures of the cuprate NdBa 2Cu 3O 7−δ (Nd123), which is processed by the oxygen-controlled-melt-growth (OCMG) method, have been studied by scanning tunneling microscopy/spectroscopy (STM/STS). Two characteristic microstructures, namely, island and striped structures have been observed, the latter for the first time. The former is a well-known pinning center in OCMG Nd123. The islands are of a couple of tens of nanometers in size and are found to be less conductive than the embedding matrix. Three types of stripes, namely, regular, spotted and irregularly spaced ones, have been observed. The stripe consists of lines of bright contrast and of a few atom rows running parallel to and between the neighboring lines of bright contrast. The striped band is more conductive than the matrix. The striped and flat bands of a couple of tens of nanometers wide each are found to be arranged alternately. We suggest the striped structure to be a good candidate for another strong pinning center in OCMG Nd123.

(Nd, Eu, Gd)-Ba-Cu-O superconductors with combined addition ofCeO2 and Pt

A combination of CeO2 and Pt doping has been performedfor oxygen-controlled melt-growth processed (Nd, Eu, Gd)-Ba-Cu-Owith the aim of refining the (Nd,Eu,Gd)2BaCuO5 particles,which were added to the (Nd,Eu,Gd)Ba2Cu3Oy(NEG-123) precursor. Observations using transmission electron microscopydemonstrate that sub-micrometre-sized 211 particles could be embeddedsuccessfully in the NEG-123 matrix. Energy dispersive x-ray analysis (EDX)confirmed that these sub-micrometre-sized 211 particles consisted mainly ofEu on the rare earth site for the samples containing 10 mol% (Nd,Eu,Gd)2BaCuO5. As a result, the (Nd, Eu, Gd)-Ba-Cu-O samplewith a combined addition of CeO2 and Pt exhibits an enhancedfishtail effect with a critical current density, Jc, of 73 000 A cm-2 at 3 T for fields parallel to the c-axis at 77 K.

Melt processing and characterization of bulk NdBaCuO

Nd1+xBa2-xCu3O7-δ (Nd123) bulksuperconductors have been synthesized under air and by the oxygen-controlledmelt growth method (OCMG). We have studied the effects ofNd4Ba2Cu2O10 (Nd422) additions, processing rates and dopingeffects on the microstructure and the superconducting properties in theOCMG-processed samples. Unlike the YBCO system, platinum doping or Nd422additions do not refine the Nd422 secondary phase inclusions. On the otherhand, Ce doping is effective in Nd422 refinement as in the YBCO system. Thenucleation window has been determined for growth conditions in air, and wasfound to be in a very narrow temperature range. The effectiveness of the MgOsingle crystal used as seed, the occurrence of solid solution and the oxygencontent are also examined. Good superconducting properties were obtained inOCMG samples while poor properties occurred in the air-textured sample andcerium-doped samples.

Microstructural and elastic behaviour of OCMG-processed NdBaCuO materials

A systematic investigation of an HTSC material viz; Nd 1.8Ba 2.4Cu 3.4O y superconductor prepared by the oxygen controlled melt growth (OCMG) process has been undertaken by studying its microstructure and elastic behaviour. The results are compared with those obtained on samples prepared by melt powder melt growth (MPMG) processed YBa 2Cu 3O y material. The X-ray diffraction studies of the new material clearly indicate the predominant structure of NdBa 2Cu 3O y. The microstructural investigations performed by means of optical and scanning electron microscopy confirm the presence of a second phase formed due to the uniform distribution of Nd 4Ba 2Cu 2O 10 (Nd-422) particles of size 1–12 μm size in the Nd-123 matrix. Further, magnetization measurements also show that the sample exhibits a pronounced secondary peak effect along with a critical current density value of 36 000 A/cm 2 at T=77 K in a field of 1.6 T applied parallel to the c-axis. Finally, the ultrasonic velocity measurements reveal that the elastic anomalies generally observed among most of the RE-123 materials, especially in the temperature region of 100–250 K are totally absent in the case of Nd-123 of the present investigation.

Microstructure and properties of oxygen controlled melt textured NdBaCuO superconductive ceramics

Nd 1+ x Ba 2− x Cu 3O 7− δ (123) bulk superconductors have been synthesised by the oxygen controlled melt growth method. Unlike the YBaCuO system, platinum doping or Nd 4Ba 2Cu 2O 10 (422) rich compositions do not refine the peritectic ‘422’ secondary phase, but the latter improves the microstructural quality. Low oxygen partial pressure and high purity precursors are necessary to achieve in a reproducible manner high T c and J c (up to 56 000 A/cm 2 in 0 T and 30 000 A/cm 2 under 1.5 T). The fishtail effect observed at 77 K is compared with the one observed in the YBCO system, and is discussed in terms of oxygen deficiency, Nd–Ba substitution, defects and vortex lattice. Our TEM observations did not evidence any Nd–Ba substituted clusters in the Nd123 matrix; however, some diffuse streaks, observed in the [100] zone ED pattern, support the fact that the source of the peak effect in this system is mainly due to oxygen disorder and low stability of the orthorhombic phase.

Preliminary study of a superconducting bulk magnet for the Maglev train

One of the prospective applications of high critical temperature superconductors is a superconducting magnet for the magnetically levitated (Maglev) train. Development shows that RE (rare earth) BaCuO and LRE (light rare-earth) BaCuO superconductors prepared by melt processes have a high critical current density at 77 K and high magnetic fields. LRE-Ba-Cu-O bulk superconductors melt-processed in a reduced oxygen atmosphere, named oxygen-controlled-melt-growth (OCMG) process, are very promising for high field application as a superconducting permanent magnet with liquid nitrogen refrigeration. Compared to good quality melt-grown REBaCuO bulks, LREBaCuO bulks exhibit larger critical current densities in high magnetic fields and much improved irreversibility field at 77 K, implying that more effective flux pinning can be realized in a commercially feasible way. In this study, we discuss the possibility of a superconducting bulk magnet for a Maglev train. A preliminary design of the bulk magnet and also melt processing for REBaCuO and LREBaCuO bulk superconductors and their characteristic superconducting properties are presented.

Effect of platinum addition on the microstructure and critical current density in (Nd, Eu, Gd)-Ba-Cu-O

We have studied the microstructure and magnetic properties of oxygen-controlled-melt-growth (OCMG) processed (Nd, Eu, Gd)-Ba-Cu-O superconductors, which contain about 40 mol% of (NEG211) second phase by varying the amount of Pt from 0.25 to 3 mol%. Scanning electron and transmission electron microscopic observations demonstrate that 0.5 mol% Pt is the optimum quantity to refine the 211 particles, leading to a dramatic increase in the critical current density. An addition of 2-3 mol% Pt results in the formation of relatively large NEG211 particles. values increase with increasing Pt content up to 0.5 mol% and then decrease thereafter. A high critical current density of 70 000 A at T = 77 K and an applied field of 2.2 T ( axis) is achieved in the sample with 0.5 mol% Pt. Our results demonstrate that the Pt addition is critical to the refinement of the 211 particle size and thus for the enhancement of the critical current density.

Effects of Gd 2BaCuO 5 addition on critical current characteristics in melt-processed (Nd, Eu, Gd)–Ba–Cu–O

We have investigated the effects of Gd 2BaCuO 5 (Gd211) addition on the microstructure and J c– B properties of oxygen-controlled-melt-growth (OCMG)-processed (Nd, Eu, Gd)–Ba–Cu–O system with scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and magnetization measurements. Compositional analyses revealed that small RE 2BaCuO 5 (RE211; RE: rare earth elements) particles consist mainly of Gd in the RE site, while large RE211 particles contain Nd, Eu and Gd despite the fact that only Gd211 was added to the system. Both zero field and peak J c increased with increasing Gd211 content; however, J c values were decreased when the amount of Gd211 reached 40 mol%. The sample (Nd, Eu, Gd)Ba 2Cu 3O y added with 30 mol% of second phase Gd211 showed respective critical current densities of 71 000 and 52 000 A/cm 2 in 0 T and 2.6 T at 77 K for fields parallel to the c-axis. These results indicate that Gd211 addition to (Nd, Eu, Gd)–Ba–Cu–O is effective in improving J c values through fine dispersion of Gd211.

Peak effect of critical current density in stoichiometric SmBa 2Cu 3O y sample

A fully oxygenated stoichiometric SmBa 2Cu 3O y sample showed a weak but distinct peak effect in the magnetization versus applied magnetic field curve at 40 K. The peak was located at about 2 T. The oxygen content of this sample was determined at y=7.01±0.05. Thus, it is unlikely that the peak observed for this sample was caused by ordering of oxygen atoms. When the sample was pulverized and annealed in oxygen gas at 800°C, part of SmBa 2Cu 3O y apparently decomposed into BaCuO 2 and CuO in the part of thin surface layer (less that ∼30 nm thickness) of the powder grains, according to TEM observations and EDX analyses. This would mean that the concentration of Sm became higher than the stoichiometric value in the interior of the grain to form Sm-rich region. The observed weak peak effect may be caused by the Sm-rich region inside the grains, since some Sm-rich Sm 1+ x Ba 2− x Cu 3O y samples prepared by an oxygen controlled melt growth (OCMG) process were reported to show a peak effect.

RE-Ba-Cu-O for high functional superconducting permanent magnet

Among various potential applications of melt-textured RE-Ba-Cu-O (REBCO, RE: rare earth elements) superconductors, we have examined the bulk application as the superconducting permanent magnet, especially for the magnetically-levitated (MAGLEV) train. Compared with Y-Ba-Cu-0 (YBCO), oxygen-controlled melt-growth (OCMG)-processed LREBCO (LRE: light rare earth elements) bulk superconductors are more promising for this application because of larger critical current density ( J c) values in high field and higher irreversibility field ( B irr) within the range of the liquid nitrogen refrigeration (63–77 K), implying that even higher trapped fields ( B t) are achievable in principle. In this paper, material requirements of superconducting bulks for the MAGLEV train are first presented and then processing aspects for the fabrication of good LREBCO bulks are described.

Comparative study of magnetic fields trapped in YBa 2Cu 3O y and SmBa 2Cu 3O y, superconducting bulks

We have measured the trapped field distribution in plates of melt processed YBa 2Cu 3O y (Y123) and SmBa 2Cu 3O y , (Sm123) superconductors that have different field dependence of the critical current density ( J c). Sm123 materials were fabricated by the oxygen-controlled melt-growth (OCMG) process and exhibited the peak effect. The trapped fields were measured by scanning a Hall probe on the sample surface. Measurements were performed after a field of 3.5 T was applied in the field-cooled process. On the basis of the critical state model, the magnetic field distribution was calculated taking account of the field dependence of the critical current density. The calculated values were in good agreement with the measured data. We have also estimated trapped magnetic fields for Y123 and Sm123 when the grain size is enlarged, which showed that OCMG-processed Sm123 bulk superconductors have greater potential as trapped field magnets.

Magneto optical observations of flux dynamics in Nd-Ba-Cu-O bulk superconductors

The magnetic flux dynamics of Nd-Ba-Cu-O (NdBCO) bulk superconductors produced by the Oxygen-Controlled-Melt-Growth (OCMG) method has been investigated using the Magneto-Optical (M-O) technique at various low temperatures for an external magnetic field applied both parallel (H//c) and perpendicular (H⊥c) to the c-axis of the sample. While for H//c the flux motion was isotropic, for H⊥c anisotropic penetration and expulsion of the flux lines was observed directly. In the configuration H⊥c the flux lines were observed to move in and out the sample predominantly parallel to the a-b planes. The flux behaviour has also been correlated to the microstructure. Particularly, microcracks along the basal plane were found to provide a low resistance channel for flux motion.

Field dependence of Jc for melt textured filamentary Nd123 superconductor

Filamentary Nd123 superconductors were produced by solution spinning and oxygen controlled melt growth process. A transport critical current density, J c for the filamentary Nd123 sample prepared with various conditions was examined in magnetic fields up to 10 T at 77 K. Currents were passed along the fiber axis and were always normal to the applied magnetic field. The filamentary Nd123 superconductor exhibited J c values larger than the filamentary Y123 superconductor in high fields. An anomalous peak effect was not observed for the J c– H curve of the Nd123 samples and J c values were strongly dependent on the composition of the precursor and oxygenation condition. By controlling the melt growth process and post-annealing condition, high J c values exceeding 10 4 A/cm 2 at 77 K and 0 T and >10 3 A/cm 2 at 10 T were achieved for the filamentary Nd 1.18Ba 2.12Cu 3.09O x superconductor prepared in flowing 1%O 2+Ar.

OCMG process for filamentary Nd123 superconductor produced by solution spinning

Abstract The oxygen controlled melt growth (OCMG) process for filamentary Nd123 superconductors has been studied to enhance the Jc values. The filamentary Nd1.18Ba2.12Cu3.09Ox (1) and Nd1.25Ba2.05Cu3.10Ox (2) precursors were prepared by dry spinning through a homogeneous aqueous solution containing mixed acetates of Nd, Ba and Cu, poly(vinyl alcohol). The calcined filaments were partially melted in flowing 0.1% O2+Ar. The Tc values of filament (1) was 94.5 K and 88.5 K for filament (2). By controlling the heating condition, maximum Jc values of 2.4×104 A/cm2 and 1.8×104 A/cm2 at 77 K and 0 T were attained for filaments (1) and (2), respectively. A window of the optimum heating conditions for filament (2) was narrow and the reproducibility of the high Jc value was poor.