Journal ArticleDOI

# Microstructural investigations on melt grown RE-Ba-Cu-O (RE = Y, Gd and Nd) systems

01 May 1996-Journal of Materials Science (Kluwer Academic Publishers-Plenum Publishers)-Vol. 31, Iss: 10, pp 2557-2561

Abstract: A melt growth process has been carried out on RE-Ba-Cu-O (RE = Y, Gd and Nd) superconductors in order to fabricate grain oriented samples that support a high critical current density (J c) Preliminary results on the microstructural data obtained on these samples are reported in this paper
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Abstract: A process for the fabrication of high-quality and near-net-shaped superconducting specimens of (Y-123) with uniformly distributed fine (Y-211) is discussed. The process involves the fabrication of 211 preforms by conventional ceramic routes such as uniaxial and isostatic pressing, injection moulding and slip casting, and pressureless infiltration basically from a reservoir containing liquid phases. A compact of 123 or 123 rich in liquid phases(s), acting as a source of liquid phases, is placed in contact with the 211 preform, and heated above the peritectic temperature of 123. The liquid from the source compact infiltrates the 211 filler and the peritectic reaction occurring between the preform material (211) and the matrix (liquid phases) during slow cooling from the peritectic temperature results in the growth of 123 with uniformly distributed fine 211 particles. We have also demonstrated that the present process can be extended to the fabrication of other rare earth (RE) superconductors where a solid solution of the kind occurs due to comparable size of the RE and Ba atoms, by choosing the RE = Gd system as an example. Herein the advantages and bounds of the process are reviewed; the effect of various cooling rates on the growth kinetics of 123 and the resulting macro- and microstructures are discussed. Fabrication of composites containing fine and very uniformly distributed Ag is also demonstrated by the process. Comparisons are made between the characteristics of the samples produced in the present process and the existing melt texturing process. The utility of the process is demonstrated by the fabrication of a three-dimensional component - a hollow cylinder.

108 citations

Journal ArticleDOI
Abstract: We report record high current densities of 230 kA cm − 2 at zero field, and in excess of 10 kA cm − 2 up to 7 T at 77 K, in YBa2Cu3O7 − δ (Y-123) superconductors fabricated by a modified infiltration growth (IG) process. This was accomplished by optimizing the Y2BaCuO5 (Y-211) preform, into which liquid phases were infiltrated, through a combination of high pressure compaction and limiting the sintering temperature. The optimized sample yielded a Y-123 superconductor with a uniform distribution of fine-grained Y-211. Strong and almost invariant flux pinning observed to high fields up to 7 T, suggest a temperature independent flux pinning mechanism originating from defects in the size range 15–50 nm. Since the present sample has no added grain refiners, nano-sized dopants or mixed rare earths leading to low Tc solid solutions, a unique opportunity presents itself to investigate the cause of the enhanced flux pinning to high fields. We have therefore investigated our samples by transmission electron microscopy, and the studies revealed the presence of domains in the sample with nano-sized defects starting from the domain boundaries, as a possible source of enhanced flux pinning.

39 citations

Journal ArticleDOI
Abstract: The effect of compaction pressure applied to the Y 2 BaCuO 5 (Y-211) preform during its fabrication, on the final microstructures and current densities ( J c ) is investigated. The necessity for optimizing the sintering conditions in order to provide mechanical stability to the preform prior to the infiltration of liquid phases is demonstrated. The sample fabricated under optimized conditions showed J c values better than 10 3 A cm −2 up to applied magnetic fields of 6.5 Tesla at 77 K. The microstructural factors influencing the field dependence of J c ‘ J c ( H )’ at low and high fields are investigated. Extensive nano-twinning with large number of crossing twins observed in the optimized sample and the associated high defect densities are correlated to be the source of flux pinning to high fields. The fact that YBa 2 Cu 3 O 7− δ (YBCO, Y-123) superconductor obtained employing the present Preform Optimized Infiltration and Growth Process (POIGP) yields a homogenous and dense distribution of fine Y-211 particles, that creates suitable twin/defect densities to provide flux pinning at high fields, is of technical importance.

29 citations

Journal ArticleDOI
E. Sudhakar Reddy1, T. Rajasekharan1Institutions (1)
Abstract: Bulk GdBa 2 Cu 3 O y superconductors with uniformly distributed inclusions of micron-sized Gd 2 BaCuO 5 (211) have been prepared by the infiltration and texturing of pre-shaped porous 211 preforms with liquid phases in a high purity Ar atmosphere. The microstructural features at various stages of processing with particular emphasis on the nature of the 211 inclusions are studied. The superconducting properties like the transition temperature and critical current densities of the resulting composites are discussed.

11 citations

Journal ArticleDOI
Abstract: Recently, the significant peak effect was observed in the CeO2 doped Y–Ba–Cu–O (YBCO) bulk materials grown using the infiltration growth (IG) method. The peak effect was attributed to the compositional fluctuations in the matrix, (Y,Sm)BCO. However, it was found that the peak effect was strongly spatially dependent. The inductively coupled plasma mass spectrometry (ICP-MS) results show that the concentration of Sm ranges from 5.8 × 10−2 to 7.0 × 10−3 wt% within the bulk. Notably, the peak effect is only observed in specimens where the concentration of Sm is higher than 1.5 × 10−2 wt%. The spatial distribution of Sm dissolved from the SmBCO seed is the result of the way in which the liquid (BaCuO2 and CuO) flows during the melting process of the IG technique. To suppress the variation in the composition, nano-sized Sm2O3 particles are added to the precursor powders in this study. Microstructural analysis demonstrates that with the addition of nano-sized Sm2O3, the concentration of Sm can be enhanced to be higher than 1.5 × 10−2 wt% within the bulk. Therefore, the peak effect can be observed throughout the bulk material. Besides, more symmetrical trapped magnetic field profiles can be obtained.

10 citations

##### References
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Journal ArticleDOI
Abstract: A liquid phase processing method for the fabrication of bulk YBa2Cu3Ox superconductors with large current carrying capacity has been developed. Slow cooling through the peritectic transformation (1030–980 °C) has been shown to control the microstructure of these superconductors. A cooling rate of 1 °C/h in this temperature range has yielded a microstructure with long plate type, thick grains oriented over a wide area. Current density up to 18 500 A/cm2 has been obtained by continuous direct current measurements and in excess of 62 000 A/cm2 with pulse current of 10 ms duration and 75 000 A/cm2 using 1 ms pulse. The strong magnetic field dependence observed in sintered bulk 1‐2‐3 superconductors is also minimized to a large extent where a current density in excess of 37 000 A/cm2 is obtained in a field of 6000 G.

697 citations

Journal ArticleDOI
Abstract: We report a new process which promises high critical current density in oxide superconductors. The process consists of three stages. Firstly a YBa2Cu3Ox sample is rapidly heated and quenched from the Y2O3 plus liquid region. Subsequently the quenched sample is reheated to the Y2BaCuO5 plus liquid region, and then slowly cooled with a temperature gradient in flowing oxygen. The process enables us to grow a superconducting phase unidirectionally and to suppress the second phase intrusion, leading to the production of well textured YBa2Cu3Ox which yields a high Jc value in the presence of magnetic fields. It is also found that Bean's critical state is realized in such high Jc samples.

687 citations

Journal ArticleDOI
TL;DR: The present work demonstrates that problems caused by low critical current densities and significant deterioration in weak magnetic fields can be overcome through proper microstructural control using molten oxide processing.
Abstract: The progress toward major applications of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$-type high-${T}_{c}$ superconductors has been hindered by low critical current densities (${J}_{c}$) and their significant deterioration in weak magnetic fields. The present work demonstrates that these problems can successfully be overcome through proper microstructural control using molten oxide processing. Melt-textured growth of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ from a supercooled melt created an essentially 100% dense structure consisting of locally aligned, long, needle-shaped grains (typically 40--600 \ensuremath{\mu}m in length). The needles appear to have their long axes parallel to the conduction plane (basal plane) of the orthorhombic structure, with a low-angle orientation change between adjacent grains. This new microstructure, which completely replaces the previous granular and random structure of the sintered precursor, exhibits a dramatically higher transport ${J}_{c}$ (7400 A/${\mathrm{cm}}^{2}$ at 77 K) than the typical sintered materials (${J}_{c}$=150--600 A/${\mathrm{cm}}^{2}$). Even more significant is the much reduced field dependence of ${J}_{c}$(\ensuremath{\approxeq}1000 A/${\mathrm{cm}}^{2}$ at H=1 T as compared to \ensuremath{\approxeq}1 A/${\mathrm{cm}}^{2}$ in the sintered structure), indicating that the coupling between grains is much stronger in the new structure. The mechanism responsible for the suppressed weak-link behavior in the melt-textured material is inferred to be the combined effects of the densification, alignment of crystals, and formation of cleaner grain boundaries.

491 citations

Journal ArticleDOI
Abstract: REBa2Cu3Oy superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T c due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa2Cu3Oy with fine Y2BaCuO5 inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba1-x , REx )2Cu3Oy phase in a good superconductive matrix.

298 citations

Journal ArticleDOI
Abstract: Experiments are reported on two techniques for melt‐texture processing Ba2YCu3O6.5 by directional solidification from a semisolid melt containing particles of BaY2CuO5 and a copper‐rich liquid. One of these employs an electric resistance furnace with ambient or oxygen enriched atmosphere; the other is a laser‐heated furnace operating at 1.3 atm oxygen. Solidification interface morphologies and other structural features were examined in quenched specimens. Depending on growth rate and temperature gradient, three different types of growth morphologies of the growing 123 phase were observed: ‘‘faceted plane front,’’ ‘‘cellular dendritic’’ or ‘‘equiaxed blocky.’’ The interface temperature decreased markedly with increasing growth rate for the faceted plane front specimens. In the remaining specimens, solidification took place over a range of temperatures. The temperature of the ‘‘root’’ of the solidification front dropped, but temperature of the solidification front ‘‘tip’’ did not. A solidification model is ...

247 citations

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