T. Rajasekharan

Fabrication of textured (RE = Y, Gd) composites by infiltration and growth of preforms by liquid phases

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.

Microstructural and magnetisation study in melt-grown YBaCuO samples

Abstract The effect of Y 2 BaCuO 5 (211) and silver on the microstructure of melt-grown YBa 2 Cu 3 O 7− x (123) has been systematically investigated. An optimal amount (28 mol%) of 211 in the starting composition of 123 with 5 wt.% of silver gave critical current densities >10 4 A/cm 2 . Magnetisation measurements using a SQUID magnetometer have been utilised to characterise the superconducting properties of the material at various stages of optimisation. Beans and Kims critical-state models are used to analyse the magnetisation of the optimised composition (123 + 28 mol% + 5 wt.% Ag). An anticlockwise tilt observed in the magnetisation curve above 70 K in the optimised sample has been explained as being due to the presence of an additional paramagnetic contribution arising from 211 particles present in the 123 matrix.

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

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.

Shape Forming Simultaneous with Jc Enhancement in REBa2Cu3O7 Superconductors

An infiltration and growth (IG) process which enables the fabrication of three-dimensional (3D) components of REBa 2 Cu 3 O 7 (RE = Y, Gd, Sm, Nd, etc.) (RE-123) superconductors with a highly textured microstructure is described. The advantages of the process in comparison with conventional melt processing are discussed. The process has been demonstrated to yield highly favorable microstructures in the case Y-123 processed in air, as well as in the case of Gd-123 processed in reduced oxygen partial pressure.

Infiltration-growth processing of superconductor

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.

Nucleation and growth of the orthorhombic phase in melt textured YBa2Cu3O6+δ

Abstract The nucleation and growth of the orthorhombic phase by transformation through oxygen ordering from the tetragonal phase in melt textured YBa2Cu3O6+δ (123) are investgated by polarized light microscopy. The samples for observation were collected from the interior of partially oxygenated bulk melt textured material. The orthorhombic phase is observed to nucleate basically at three different sites, viz. along domain boundaries, at platelet boundaries and around macroscopic cracks. The nuclei at domain boundaries are observed to grow along the platelet boundary gaps with a cellular morphology with sharp tips, whereas the growth front of the phase growing from the boundaries of platelets into their interior is planar. The growth of the phase from the domain boundaries into the domains differs depending on the orientation of the platelets at the point of nucleation. The origin of inter-platelet gaps in the melt processed material is discussed in the light of the observations made during this work. The ratio of growth rates of the orthorhombic phase along the gaos and into the platelets is estimated.

Effect of content on the macro-defect structure of melt processed superconductor

Applications of superconducting material demand materials with minimized macro-structural defects like cracks, voids and pores. Such defects affect the current density values attainable and also preclude shaping the material into useful components. In order to study and minimize the above defects, we have melt processed with systematically increasing amounts of (211) as reinforcement. It is observed that practically all the shrinkage associated with melt processing occurs at the melting stage and not at the texturing stage. The variation of shrinkage as well as the size of pores residual in the samples with the increasing 211 content can be correlated to the size of the 211 particles in the pro-peritectic liquid at . Macro-cracks, parallel to the intrinsic micro-cracks between the 123 platelets, are observed in all the domains. The number and nature of these cracks are independent of the 211 content. The origin of these cracks is discussed.

On the characteristics of peritectic Gd2BaCuO5 at the pro-peritectic stage with increasing amounts of external Gd2BaCuO5

Abstract The effect of different amounts of Gd 2 BaCuO 5 (211) in the starting composition of GdBa 2 Cu 3 O 7− y (123) on the formation and refining behaviour of the 211 phase particles produced during the peritectic decomposition of 123 has been investigated. In the absence of any extra 211 in the liquid at the pro-peritectic stage, the morphology of the peritectically formed 211 particles is acicular with high aspect ratios; whereas, even small amounts of already existing 211, by acting as nucleating sites for the 211 generated by the decomposition of 123, change the morphology from acicular to circular. The average size of the 211 is found to decrease according to an inverse power law with respect to the number of nucleating centres. The change in morphology is discussed and a quantitative analysis of the average sizes of the 211 is presented.

Microstructural investigations of melt grown YBa2Cu3O7

The results of microstructural investigations on bulk YBa2Cu3O7 prepared by melt growth process are reported.

Microstructural and magnetic properties of textured GdBa2Cu3Oy/Gd2BaCuO5 composites fabricated from Gd2BaCuO5 preforms

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.