Single domain Y?Ba?Cu?O (YBCO) composed of a YBa2Cu3Oy (Y-123) superconducting bulk matrix with discrete, non-superconducting Y2BaCuO5 (Y-211) phase inclusions has been fabricated by a seeded infiltration and growth (IG) technique in the form of cylindrical pellets up to 32?mm in diameter. Sample shrinkage in the radial direction for single domains prepared by this technique is relatively low at 5% and independent of sample size, in contrast to the shrinkage observed in samples grown by conventional melt processing, which increases significantly with increasing sample diameter. Furthermore, samples grown by the IG technique exhibit low porosity of typically 0.9% of the bulk volume fraction, compared with a corresponding value of around 4.9% observed in samples fabricated by conventional melt processing. Fine Y-211 particles were observed to be embedded within the Y-123 superconducting matrix for the IG processed samples, leading to a high critical current density, Jc, of over 100?000?A?cm?2 at 77.3?K in self-field. The distribution of Y-211 particles in the IG sample microstructure, however, was inhomogeneous (unlike in previous reports), which leads to a variation in the spatial distribution of Jc. The volume fraction of Y-211 in the vicinity of the seed crystal (i.e.?corresponding to the initial c-sector growth stage), in particular, is typically around 5%, compared with a value of up to 30% in the a growth sectors more distant from the seed crystal (which corresponds well to the theoretical value for the sample composition studied here). The volume fraction of Y-211 inclusions in the c growth sector more distant from the seed was around 22%. Finally, a trend of the variation in the distribution of Y-211 particles in the Y-123 matrix grown by the IG technique was similar to that in sample grown by conventional melt processing.

https://iopscience.iop.org/article/10.1088/0953-2048/18/11/002/pdf

Seeded infiltration and growth of large, single domain Y–Ba–Cu–O bulk superconductors with very high critical current densities

An infiltration and top seed growth method (ITSG), has been successfully used to grow a single grain of YBa2Cu3O7−x (Y123) bulk material. The nucleation of Y123 grain is controlled by seeding the Y2BaCuO5 (Y211) preform with a Sm123 or MgO seed crystal. Various configurations influencing the liquid infiltration such as that due to capillarity or gravitational force have been studied. The resulting mono-domain Y123 samples are found to be free of macrocracks, a negligible shrinkage and a microstructure with uniformly distributed fine Y211 particles. The superconducting properties of the final Y123/Y211 samples are discussed.

http://iopscience.iop.org/article/10.1088/0953-2048/17/11/014/pdf

Infiltration and top seeded grown mono-domain YBa2Cu3O7−x bulk superconductor

Large, single grains of Y?Ba?Cu?O (YBCO) bulk superconductor composed of a YBa2Cu3O7?? (Y-123) matrix with Y2BaCuO5 (Y-211) inclusions have been fabricated using a seeded infiltration and growth (IG) process. Y-211 precursor pellets prepared from powders containing different Y-211 particle sizes were processed by the seeded IG technique to investigate the effect of Y-211 size on the single grain microstructure. The superconducting properties of the single grains fabricated in this study are observed to correlate well with the variation in Y-211 particle size in the fully textured Y-123 phase matrix. The critical current density, Jc, in particular, has been observed to increase from 68?000?A?cm?2 for a reference sample to 94?000?A?cm?2 at 77?K in self-field for a sample prepared using refined Y-211 precursor powder with added platinum oxide.

https://iopscience.iop.org/article/10.1088/0953-2048/19/8/005/pdf

The effect of Y-211 precursor particle size on the microstructure and properties of Y–Ba–Cu–O bulk superconductors fabricated by seeded infiltration and growth

Most of the potential applications of the high-temperature superconductors require large critical currents, much higher than those obtained using a classical sintering process. This paper reviews different processes and corresponding recent results in the field of texturing high-Tc superconductors in order to align and develop the planes where the current flows easily. Different melt-textured growth methods are discussed from both theoretical and practical viewpoints, and the limitations associated with the diffusion and interface phenomena are introduced. Influence of the process parameters, as well as the microstructure of the precursors and the use of additives in order to enlarge the single domain size and to optimise the superconducting characteristics is discussed. Mechanical texturing and texturing under a magnetic field, as well as combination of both processes, are also considered. Recent results, which appear to be an important breakthrough for the industrial development because of the large crystallization rates are also mentioned.

https://iopscience.iop.org/article/10.1088/0953-2048/12/7/202/pdf

Texturing of high-Tc superconductors

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.

https://iopscience.iop.org/article/10.1088/0953-2048/23/10/105020/pdf

Unprecedented current density to high fields in YBa2Cu3O7 − δ superconductor through nano-defects generated by preform optimization in infiltration growth process

Melt processing of (Sm-123) is done by infiltrating liquid phases into an Sm-211 preform and allowing the growth of Sm-123 to take place. The melt-textured samples are found to be free of macrodefects, and the microstructure contains uniformly distributed fine-sized Sm-211 particles. The behaviour of the critical current density of the samples in high fields is reported here.

Infiltration-growth processing of superconductor

Refinement of the insulating Sm-211 precipitates in the microstructure of melt processed Sm-123 with 30 mol% Sm-211, is done by adding small amounts of CeO 2 , ZrO 2 , PrO 2 and Ag. The microstructural features and the J c dependence on field is discussed. It is observed that the sample with PrO 2 has a large concentration of sub-micron sized 211 particles in the microstructure. A J c increase has been observed in the samples with the additions, but the increase is not directly correlated to the size of 211 particles.

Refinement of insulating Sm2BaCuO5 phase and its effect on Jc in melt processed SmBa2Cu3Oy system

Melt-processed Sm-123 samples with an excess Sm2BaCuOy (Sm-211) phase is studied by transmission electron microscope (TEM) and superconducting quantum interference device magnetometer, to explain the origin of the peak effect exhibited in the magnetization hysteresis loops of light RE-123 samples in intermediate fields at 77 K. TEM images of the sample showing peak effect reveal dilute concentrations of areas with ortho-II structure distributed on a nanometer scale within the twin regions of the sample. The ortho-II structure is an oxygen deficient 123 with a lower Tc than the bulk of the material and will be a source of flux pinning by turning normal in high fields, thus contributing to the peak effect. With an increase in Sm-211 concentration, the peak effect disappears from the hysteresis loop, as does the ortho-II structure in the TEM images.

Origin of peak effect in the magnetization hysteresis loop of melt-processed REBa2Cu3Oy superconductors

Melt processing of SmBa 2 Cu 3 O y (Sm-123) is done in argon atmosphere by the solid liquid melt growth (SLMG) route, with increasing concentration of insulating Sm 2 BaCuO 5 (Sm-211) content. The effects of Sm-211 on the microstructure and on the behaviour of current density (J c ) in high magnetic fields, are discussed. It is observed that as the Sm-211 concentration is increased, the morphology of the trapped 211s change from acicular having a high aspect ratio to nearly spherical. The J c increases for low additions of 211, but for high concentrations, the J c drops drastically. The above data is explained on the basis of pinning from the 211 particles and the matrix.

N.V.N. Viswanath

Papers

Infiltration-growth processing of superconductor

Refinement of insulating Sm2BaCuO5 phase and its effect on Jc in melt processed SmBa2Cu3Oy system

Origin of peak effect in the magnetization hysteresis loop of melt-processed REBa2Cu3Oy superconductors

Effect of Sm2BaCuO5 addition on the properties of melt processed SmBa2Cu3Oy