Nd-Ba-Cu-O Research Articles

Infiltration growth of Mg-doped Nd–Ba–Cu–O seed crystals for the fabrication of large grain RE–Ba–Cu–O bulk superconductors

By employing the infiltration growth (IG) technique, Mg-doped Nd–Ba–Cu–O (NdBCO) seed crystals were successfully prepared through 1 wt% MgO addition both to a Nd4Ba2Cu2O10 (Nd-422) precursor powder and to a liquid source with the composition Nd2O3:CuO:BaCuO2 = 1:6:10. Differential thermal analysis (DTA) results showed that the decomposition temperature of the processed Mg-doped NdBCO crystals increased by about 20 °C compared with the general, Mg-free NdBCO seeds, thus they can be used as generic seed crystals to induce the growth of any RE–Ba–Cu–O (REBCO) single grain. This result is consistent with that observed in melt growth (MG) processed Mg-doped NdBCO crystals. Finally, highly c-axis oriented Gd–Ba–Cu–O (GdBCO) and Y–Ba–Cu–O (YBCO) large grains were successfully grown using the produced seed crystals.

Phase stability of MgO-doped Nd–Ba–Cu–O seed crystals for cold-seeded, OCMG processing

The thermodynamic stability of MgO-doped Nd1+xBa2−xCu3Oy (x=0–0.5) has been investigated as a function of solid solution level (x) under various oxygen partial pressures for use as a seed crystal composition in the melt growth of (LRE)-Ba–Cu–O (where LRE=light rare earth) bulk superconductors. The gradient of decomposition temperature, Tp, against solid solution level increases significantly with decreasing oxygen partial pressure, indicating that MgO-doped Nd–Ba–Cu–O (NdBCO) seed crystals should be grown under reduced pO2 to obtain seeds with high Tp. Based on these results, the optimum cold-seeded processing conditions for the fabrication of single grain GdBCO bulk superconductors containing silver (GdBCO/Ag) under a controlled oxygen partial pressure of 0.01atm have been determined. The onset Tc of GdBCO/Ag samples grown in air with the optimized seed crystal composition is improved significantly with increasing Ag content, indicating that silver addition to the precursor powder suppresses the formation of Gd–Ba solid solution.

Properties of GdBCO bulk superconductors melt-processed in air using a Mg-dopedNd–Ba–Cu–O generic seed crystal

A Mg-doped Nd–Ba–Cu–O (NdBCO) generic seed crystal has been developed recently forthe fabrication of any type of rare earth (RE) based (RE)-Ba–Cu–O single grain bulksuperconductor. The new generic seed significantly simplifies the top seeded melt growth(TSMG) process for light RE based (Nd, Sm, Gd or mixed rare earth elements) bulksuperconductors in particular. In this paper we report that single domain, bulk GdBCOsamples have been fabricated successfully in air using the new seed in a cold seedingprocess. In this study, precursor powders were enriched with different amounts ofBaO2 to investigate the extent of substitution of Gd for Ba in theGd1+xBa2−xCu3O7−δ (Gd-123) solid solution phase. The particle size of theGd2BaCuO5 (Gd-211) phase inclusions in the Gd-123 phase matrix and the spatial variation ofTc andΔTc alongthe a/b and c-axes of single grains fabricated with varying amounts of externally addedBaO2 have been investigatedas a function of BaO2 content. Jc valuesas high as 7.2 × 104 A cm−2 at 77 K and 0 T have been observed in large GdBCO grains processed under air.

Development of the microstructure of uranium-doped Nd-Ba-Cu-O

Melt grown Nd-Ba-Cu-O (NdBCO) has been reported to exhibit higher values of critical current density, J/sub c/ and irreversibility field, H/sub irr/, than other (RE)BCO superconductors, such as YBCO. The microstructure of NdBCO typically contains 5-10 /spl mu/m sized inclusions of the Nd/sub 4/Ba/sub 2/Cu/sub 2/O/sub 10/ phase (Nd-422) in a superconducting NdBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// phase (Nd-123) matrix. The average size of these inclusions is characteristically larger than that of the Y/sub 2/BaCuO/sub 5/ (Y-211) inclusions in YBCO. As a result, there is scope to further refine the Nd-422 size to enhance J/sub c/ in NdBCO. Large grain samples of NdBCO superconductor doped with various amounts of depleted UO/sub 2/ and containing excess Nd-422 have been fabricated by top seeded melt growth under reduced oxygen partial pressure. The effect of the addition of depleted UO/sub 2/ on the NdBCO microstructure has been studied systematically in samples with and without added CeO/sub 2/. It is observed that the addition of UO/sub 2/ refines the NdBCO microstructure via the formation of uranium-containing phase particles in the superconducting matrix. These particles are of approximately spherical geometry with dimensions of around 1 /spl mu/m. The average size of the nonsuperconducting phase particles in the uranium-doped microstructure is an order of magnitude less than their size in un-doped Nd-123 prepared with excess Nd-422. The critical current density of uranium-doped NdBCO is observed to increase significantly compared to the undoped material.

Growth and characterization of NdBa/sub 2/Cu/sub 3/O/sub 7/ and Ca-doped NdBa/sub 2/Cu/sub 3/O/sub 7/ thin films

Thin films of off-stoichiometric NdBaCuO (NBCO) and Ca-doped NdBaCuO were grown on (10 mm)/sup 2/ strontium titanate substrates by pulsed laser deposition. The statistical methods of Experimental Design and regression analysis were used to correlate the electrical and morphological properties with the growth parameters. The surface morphology of the films was studied using atomic force microscopy and scanning tunnelling microscopy.

Self-seeded melt growth of Au-doped Nd-Ba-Cu-O

Nd-Ba-Cu-O (NdBCO) precursor powder composed primarily ofNdBa2Cu3O7-δ (Nd-123) and 15 mol% Nd4Ba2Cu2O10(Nd-422) containing 0.1 wt% Au has been prepared by spray drying andsubsequent calcination for the fabrication of bulk melt-processed ceramics.The addition of Au results in the lowering of key process parameters, such asthe peritectic melting point and solidification temperatures by up to30 °C. Additional doping of Au/NdBCO with 5 wt% Ag2O lowers thesevalues further and has enabled the successful nucleation and limited growth ofsingle grains under both air and reduced oxygen atmospheres using undopedNdBCO as a seed. The advantage of this self-seeded melt growth technique isthat it avoids the lattice mismatch that results from the conventional use ofa MgO seed.

Fabrication of large grain Nd-Ba-Cu-O by self-seeded melt growth

Large, single grain Nd-Ba-Cu-O (NdBCO) samples containing Nd-422 phase inclusions have been fabricated successfully using NdBCO self-seeds by two melt processing techniques under reduced oxygen partial pressure. The first technique involves hot seeding where the seed crystal is placed onto the sample surface at the peritectic temperature. The second method involves doping precursor powder with Au and Ag to reduce the peritectic decomposition temperature of the material. This enables seeding to be achieved by introducing an undoped Nd-123 seed to the sample surface at room temperature, which simplifies processing significantly. This paper compares the microstructure of large grain NdBCO samples fabricated by the two techniques including the dependence of growth rate on undercooling, processing atmosphere and the size and distribution of Nd-422 in the Nd-123 phase matrix.

Fabrication of large grain NdBaCuO by seeded melt growth

Large, single grain NdBaCuO (NdBCO) composite samples of NdBa 2Cu 3O 7−δ (Nd-123) containing 15 and 20 mol. % non-superconducting Nd 4Ba 2Cu 2O 10 (Nd-422) phase inclusions have been fabricated successfully by a variety of techniques based on top-seeded melt growth under reduced oxygen partial pressure. Specifically, individual grains up to 2 cm in diameter have been grown using (100) oriented MgO seeding, self (NdBCO) seeding at elevated temperature and self-seeding of Ag and Au doped precursor pellets. The latter exhibit a reduced peritectic decomposition temperature compared with the undoped compound. These techniques, which vary in degree of difficulty and hence reliability, yield grains with a range of microstructural homogeneity. This paper describes the general aspects of large NdBCO grain fabrication and presents the results of the different fabrication techniques.

Synthesis and superconducting properties of c-axis aligned,single-domain NdBCO/Ag, melt-processed superconductors

We prepared c-axis aligned, large single-domain Nd-Ba-Cu-O(NdBCO) bulk superconductors with the addition of Ag2O by themelt-processing technique under a flowing Ar gas atmosphere using Ag-freeNdBCO as seed crystals. The melt-processing condition has been elaborated byvarying the contents of Ag2O and CeO2, and the molar ratio of Ba to Cu.We successfully synthesized c-axis aligned, single-domain samples, free fromserious cracks or weak links, 30 mm in diameter and 13 mm in thickness in therange of 15-20 wt% Ag2O, 0-0.5 wt% CeO2 and 0.706-0.76 for the Ba toCu ratio. The largest trapped field of these NdBCO samples was 1.23 T at 77 Kand 7.0 T at 42 K, while fracturing of the sample occurred at a further lowertemperature. We found that the field trapping performance of the presentNdBCO/Ag samples is almost the same as that of our previously reportedSmBCO/Ag samples, but the critical stress of fracturing was smaller forNdBCO/Ag.

Processing, microstructure and irreversibility of large-grainNd-Ba-Cu-O

The fabrication of Nd-Ba-Cu-O (NdBCO), which exhibitssignificant potential for high-field engineering applications, is extremelysensitive to firing atmosphere, undercooling and secondary phase contentand presents considerably greater processing challenges than that of other(RE)BCO superconductors. A detailed study of NdBCO grain growth rate hasbeen performed and a schematic process phase diagram constructed in theperitectic solidification region from isothermal growth experiments atvarious temperatures for several NdBCO compositions. This phase diagram hasbeen used to fabricate large, single grains up to 2 cm in diameter of NdBa2Cu3O7 (Nd-123) containing 15 mol% ofnon-superconducting Nd4Ba2Cu2O10 (Nd-422)phase inclusions using a top-seeded melt growth technique (TSMG) in reducedoxygen partial pressure. Magnetization measurements of samples cut from alarge grain exhibit a pronounced peak effect over a wide temperature andfield region and an anisotropic irreversibility line for fields appliedparallel (≈9 T at 77 K) and perpendicular (extrapolates togreater than 40 T at 77 K) to the crystallographic c-axis. Thetemperature dependence of both the measured critical current density,Jc, and that associated with extrinsic pinning in this materialhas been observed to follow a power law above ≈70 K, suggestingthat the density of pinning sites does not vary significantly withtemperature.

Fabrication and microstructure of large grainNd-Ba-Cu-O

Large, single grain Nd-Ba-Cu-O (NdBCO) has been successfullyfabricated using a top-seeded melt growth technique under reduced oxygenpartial pressure. We report a study of the grain growth rate within thetemperature range for which heterogeneous nucleation can be achieved in thissystem and investigate the variation in size, morphology and spatialdistribution of Nd-422 phase inclusions in the Nd-123 matrix with Nd-422 phaseinclusion concentration. The influence of Nd-422 phase particles on the growthrate, pores and macrocrack formation is also studied.

Fabrication and characterization of large Nd–Ba–Cu–O grains prepared under low oxygen pressure

A large, single-grain Nd–Ba–Cu–O (NdBCO) composite consisting of superconducting NdBa2Cu3O7-δ containing nonsuperconducting Nd4Ba2Cu2O10 phase inclusions was fabricated up to 2 cm in diameter using a top-seeded melt-textured growth technique. A MgO single-crystal seed was used to provide a heterogeneous nucleation site at the center of a presintered pellet heated above its peritectic temperature and cooled continuously in a conventional tube furnace in reduced oxygen partial pressure. This process produces individual grains with the c axis oriented at ≈10° to the seed surface which, from vibrating-sample magnetization measurements, exhibit a pronounced peak effect in their magnetic moment over a wide temperature range (50–90 K) when the supercurrent flows in the a-b planes. A very high irreversibility field (>9 T at 77 K) is also observed in these grains for field applied both perpendicular and parallel to the crystallographic c axis which is significantly greater than that observed in good-quality melt-processed Y–Ba–Cu–O. These results underline the potential of NdBCO for high-field engineering applications.

The effect of undercooling and Nd422 phase content on the nucleation of large Nd–Ba–Cu–O grains fabricated by top-seeded melt processing

The nucleation and growth of NdBa2Cu3O7–δ (Nd123)–Nd4Ba2Cu2O10 (Nd422) single-grain composites in a controlled 1% O2 in N2 atmosphere were investigated in detail as a function of solidification temperature and Nd422 phase content using a top-seeded melt growth technique. A schematic process phase diagram in the peritectic solidification region of Nd–Ba–Cu–O (NdBCO) was constructed primarily from constant isothermal growth experiments at various temperatures for several compositions and used to fabricate large single-grain material by both isothermal and continuous slow cooling over a limited temperature range. The nucleation at the seed surface and subsequent growth of uniform grains was observed to depend critically on the controlled rate of grain growth and the temperature range over which solidification occurred.

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.

Magneto optical characterizations of flux dynamics in NdBaCuO bulk superconductors

We investigated the magnetic flux dynamics of NdBaCuO (NdBCO) bulk superconductors produced by the Oxygen-Controlled-Melt-Growth (OCMG) method using the Magneto-Optical (M-O) technique at various low temperatures. An external magnetic field was applied both parallel ( H t| c) and perpendicular ( H ⊥ c) to the c-axis of the sample. While for H t| c the flux motion was isotropic, for H ⊥ c anisotropic penetration and expulsion of the flux lines were directly visualized. In the configuration H ⊥ c the flux lines were observed to move in and out of the sample predominantly parallel to the a- b planes. The flux behavior was also correlated to the microstructure. Particularly, the microcracks along the basal plane were found to provide a channel for easy flux motion. The anisotropic behavior of the flux front reflects the anisotropy in the critical current density J c. The different components of J c were independently calculated from the M-O data directly analyzing the magnetic induction profiles. These J c values were consistent with those estimated interpreting the magnetization hysteresis measurements within the framework of the extended Bean model.

A comparison of the irreversibility line between melt-processed Nd-Ba-Cu-O andY-Ba-Cu-O superconductors

Abstract In the present work, we have performed a detailed comparative study of the irreversibility line (IL) among flux-grown NdBa 2 Cu 3 O 7−d (Nd123) single crystal, melt-grown Nd-Ba-Cu-O (NdBCO) and melt-grown Y-Ba-Cu-O (YBCO) bulks by d.c. magnetometry and a.c. susceptometry. ILs were determined by measuring d.c. magnetization hysteresis loops, the fundamental χ 1 and third harmonic χ 3 signals of a.c. susceptibility for the fields parallel to the c -axis of samples ( H // c ). Among these, melt processed NdBCO exhibited the highest IL. On reduced temperature scale, however, ILs from the χ 3 signals for melt-grown NdBCO and YBCO tended to merge together, suggesting that real depinning lines are insensitive to the field-induced flux pinning centers existing in NdBCO, including the flux-grown sample, but are rather sensitive to the secondary phase inclusions existing in melt-grown samples.

Low-field behavior of magnetic and transport properties of laser irradiated Nd-Ba-Cu-O ceramic superconductors

The low-field behavior of magnetic and transport properties of high-power laser-irradiated (Q-switched ruby laser, 694 nm, 30 ns) Nd-Ba-Cu-O (NBCO) ceramic superconductors prepared by the coprecipitation technique have been investigated. Laser-irradiated NBCO samples contribute significantly to the flux pinning for low applied magnetic fields. A well-defined field valueH′ c was observed for different laser-irradiated samples which is characteristic of a cross-over between regions dominated by the inter- and intragranular effects. The value ofH′ c for a laser-irradiated sample is increased with increasing number of laser pulses. Thermal modeling of laser heating reveals that on irradiation the surface melts up to a depth of 1μm and laser-induced evaporation occurs at energy density 2.5 J/cm2. SEM studies showed grain growth due to laser-induced surface sintering which improves the interconnectivity among the superconducting grain structure and thus attributed to physical densification and consequent reduction in the total number of weak links. The increase of flux trapping and transport critical current density after laser irradiation is determined by the rigidity of the flux line lattice which may be due to the presence of additional pinning centers caused by the creation of laser-induced mobile defects and their clusters.