E. Sudhakar Reddy

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.

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.

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 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.

Melt processing and microstructural analysis of GdBa2Cu3Oy with varying amounts of Gd2BaCuO5 phase

Bulk samples of GdBa2Cu3Oy (Gd-123) with varying amount of Gd2BaCuO5 (Gd-211) phase are melt processed. The processing conditions and the resulting macro and microstructures were discussed. The Tc of the material is found to be sensitive to the processing atmosphere, and the percentage of 211 in the precursor powder. Pure Gd-123 samples behaved differently as compared to the compositions with excess 211 phase. The amount of porosity and the average size of pores in the samples decreases with the increasing amount of Gd-211 in the precursor powder. The densification of the melt textured material is ascribed to the densification process of the precursors prior to the texturing stage. Irrespective of the Gd-211 content macro-cracks within domains were observed in parallel to the intrinsic micro-cracks. At the melting stage refinement of the Gd-211 particles originating from the decomposition of Gd-123 was observed with the increasing amounts of external Gd-211 in the liquid. The quantitative analysis of the microstructural features, i.e., the residual Gd-211, platelet width, domain sizes of the resulting melt textured materials were presented. Addition of Ag resulted in mild refining of 211 particles.

Microstructural studies of PrBa 2 Cu 3 O y during melt processing

The microstructures of PrBa 2 Cu 3 O y during melt processing have been studied for samples quenched at different stages of the thermal cycle. The absence of the Pr 2 BaCuO 5 phase in the Pr–Ba–Cu–O phase diagram, analogous to the Y 2 BaCuO 5 (211) phase in the Y–Ba–Cu–O system, makes it interesting to study the microstructure of PrBa 2 Cu 3 O y (Pr-123) during melt processing. The nature and morphology of the properitectic particles, the nucleation of Pr-123, and the microstructures resulting from melt processing are investigated and discussed as compared with those in YBa 2 Cu 3 O y . At the properitectic stage the morphology of PrBaO 3 (Pr-110) particles is angular. Pr-123 is found to nucleate on all Pr-110 particles with a needle-like morphology, whereas in other 123 systems no preferred morphology for the 123 during nucleation is observed. The melt textured microstructure of Pr-123 revealed similar features like domain formation with the trapped properitectic phase. The only difference is the absence of platelet gaps within domains as observed in other 123 systems. All these observations suggest that as compared to other 123 systems the microstructure of the Pr–Ba–Cu–O system behaves differently due to the absence of the analogous 211 phase.

MICROSTRUCTURAL EVOLUTION OF THE ETA -PHASE IN THE CU-SN SYSTEM

The formation of η-phase by a peritectic reaction in the Cu–Sn metallic system has been cited in the literature while discussing the solidification of the high temperature superconductor Yba 2 Cu 3 O 7 through the peritectic temperature, from Y 2 BaCuO 5 and liquid phases. Similar schematic phase diagrams and driving forces have been invoked to discuss the reactions in both cases. In this paper, we have studied the microstructural evolution of the η-phase from є + liquid in the Cu–Sn system, by observing quenched microstructures at various stages of processing, when subjected to a thermal schedule similar to the one used for the melt-processing of YBa 2 Cu 3 O 7 . A comprehensive picture of the mechanism by which the η phase nucleates and grows has evolved.

Effect of Ag on the melt processed GdBa2Cu3Oy/Gd2BaCuO5 composites

Abstract The effect of Ag addition on the microstructural and critical current densities of melt processed GdBa 2 Cu 3 O y /Gd 2 BaCuO 5 (Gd-123/211) composites as compared to the sample without Ag are studied. Ag is found to refine Gd-211 particles to some extent. The addition of Ag resulted in a non-uniform distribution of Gd-211 in the Gd-123 matrix. The width of the platelets and gap width was found to decrease with Ag addition. Coarse Ag particles were observed to associate with Gd-211 free regions. The role of Ag was found to be different compared to that of Gd-211 particles in modifying the microstructure of the composites. The presence of Ag reduced the normal state resistivity, and marginally increased the critical current density of the material.