E.S. Reddy

Melt-texture joining of YBa2Cu3Oy bulks

Practical applications of high-temperature superconductors require long length and complex shapes of the textured ceramic material. The limitation of processing the REBa2Cu3Oy (RE-123, RE = Y, Sm, Nd, etc) superconductors in desired designs requires the development of joining techniques. In this paper we report on various attempts to join sections cut from single-domain YBa2Cu3Oy (Y-123) monoliths. The lower peritectic temperature rare earth Yb-123 compounds were used as solder for melt texturing at the interface for joining. The various factors influencing the formation of defect structures during the growth of the solder phase have been investigated. Transport current measurements across the joints of the samples and also of a typical configuration of a brick wall structure prepared under optimized conditions are presented.

Processing of Y2BaCuO5 foams

Foams of conventional ceramics are attractive for a wide range of industrial applications such as filters, insulators, heat exchangers and others. Advanced functional ceramics, e.g., superconductors or piezoceramics profit from a foam structure for a variety of applications. Foams of Y2BaCuO5—being the properitectic phase in the formation of YBa2Cu3O7—find applications as supports for melt processing of bulk YBa2Cu3O7 materials or as preforms for processing of superconducting YBa2Cu3O7 foams. In this paper we discuss the processing of Y2BaCuO5 foams of various porosities and dimensions either as replicas of reticulated polyurethane foams or prepared by burning out of wax spacers embedded into a 211 preform.

Improved magnetic flux pinning in melt processed (Y,Nd)Ba2Cu3O7−δ superconductor

We report the melt processing of Nd-123 precursor pellets enriched with Nd4Ba2Cu2O10 (Nd-422), Y2BaCuO5 (Y-211) and/or nano-sized Y2O3. Microstructural analysis reveals that two different second phase particles of distinct acicular and spherical morphologies form during solidification in the samples containing Y. A large fraction of the volume of the sample prepared with excess Y2O3 contains 300?500?nm sized, spherical-shaped particles. The width and length of the acicular shaped particles in the samples are <1??m and , respectively. 90% of second phase particles are of ?600?nm size. The transition temperature, Tc, is below the value expected for NdBCO due to the formation of a (Y1?xNdx)Ba2Cu3O7?? solid solution phase. The superconducting properties, Tc and Jc, are homogeneous within (Y, Nd)BCO single grains compared to that of NdBCO due to suppressed Nd substitution in the Ba site. Significantly improved critical current densities derived from magnetic moment measurements are observed at 77?K in this system compared to both YBCO and NdBCO.

Magnetic and transport properties of YBa2Cu3Oy superconductor foams

Abstract The recently reported superconducting YBa2Cu3Oy (Y123) foams are highly interesting and promising for variety of applications. In this report we present first magneto-transport measurements of the superconducting properties of these foams. The investigations reveal the superconducting properties being similar to those of bulk melt processed materials. The 123 foams reveal a Tc of 92 K and have a magnetization Jc of 40,000 A/cm2 at 77 K and 0 T. The measurements of magnetic hysteresis versus field show a high anisotropy of the critical current density up to Jcab/Jcc∼7.

Texture formation in melt-solidified YBa2Cu3Oz thick films by artificial surface reliefs

Effects of surface reliefs on texturing multi-component functional materials, of thickness in the order of 50-100 μm are reported. As an example, YBa 2 Cu 3 O z crystals were biaxially aligned on untextured polycrystalline Ag-Pd substrates. Artificial surface reliefs resulted in up to 75-85% alignment within 10° of 0.1-1 mm square crystals of YBa 2 Cu 3 O z in the case of symmetry-matching patterns. Unlike artificial epitaxy of thin (< 1 μm) films, the crystal alignment was achieved reproducibly by mechanical patterning of the substrates at a much larger, sub-millimeter length scale. The origin of texture formation - based on various factors like topographic wall effects, crystallization pressure, surface tension, solute redistribution and capillarity - is discussed. Possible applications of this approach for superconducting coated conductors or in the field of multiseeded substrates are highlighted.

Trapped field in individual and stacked rings of bulk melt processed Y-Ba-Cu-O

Single grain rare earth barium cuprate [(RE)BCO] high temperature superconductors can trap large magnetic fields. In principle, samples can be stacked to form a quasipermanent magnet with a high length to width aspect ratio for engineering applications. The flux trapping properties of a stack of ring-shaped samples of Y-Ba-Cu-O (YBCO) were measured both on the surface and through the bore. Significant discrepancies between the experimental results and the predictions of a simple theory based on the Biot-Savart model were observed. These differences can be accounted for by the presence of a high density of small cracks parallel to the ab planes, which inhibit the circulation of current around the sample.

Large single grain (RE)-Ba-Cu-O superconductors with nano-phase inclusions

Nano-phase (5-20 nm) particles of YBa/sub 2/(Cu/sub 0.5/M/sub 0.5/)O/sub 6/ [where M=Nb, Ta, Mo, W, Zr and Hf] have been introduced successfully into RE-Ba-Cu-O single grain superconductors. A study to enlarge the size of a single grain containing these particles has been carried out involving measurement of the growth rate as a function of YBa/sub 2/(Cu/sub 0.5/M/sub 0.5/)O/sub 6/ phase concentration and degree of undercooling. The influence of the change in YBa/sub 2/(Cu/sub 0.5/M/sub 0.5/)O/sub 6/ concentration on microstructural features is also investigated and the superconducting properties of these large grain superconductors are presented.

Electrical performance of single domain YBa2Cu3Oy fabrics

Abstract The electrical performance of a new form of superconducting YBa2Cu3Oy fabrics prepared in a self-supporting form is discussed. The single domain Y123 fabrics processed by an infiltration process from precursor Y2O3 cloths have dimensions of thick films and posses the structure replica of precursor Y2O3 fabrics. The electrical properties of the fabrics determined by ac-susceptibility and dc-measurements are discussed. The Tc of the fabrics from ac-susceptibility measurements are found to be around 90 K. The transport Jc measurements in self-field showed the transition between the superconducting state and flux-flow regime to occur at a critical current density (Jc) exceeding 5.5×104 A/cm2 at 77 K. Quench tests at 77 K using pulsed currents up to five times Jc with a duration of 25 ms are discussed.

Development of multiphase ribbons as substrates for biaxially textured (RE)–Ba–Cu–O thick film coatings

A robust process is presented to produce a long length multiphase substrate comprising a large number of mutually aligned seed crystals. These seed crystals are thought to evoke a biaxial texture in a superconducting layer being prepared by a melt-texture process on these substrates. Details of the multiphase-substrate manufacture are presented, the resulting microstructures are discussed and future directions of development are highlighted.

Low temperature processing of single domain YBa2Cu3Oy thick films from Y2O3 fabrics on Ag-Pd alloy substrates

Abstract Single domain YBa 2 Cu 3 O y (Y123) thick films (∼100 μm) were fabricated on untextured Ag12 wt.%Pd alloy substrates from Y 2 O 3 cloths by an infiltration and growth process. The process involves the infiltration of Y 2 O 3 cloths placed on metallic substrates by barium cuprates and copper oxide liquids at 970 °C. The infiltrated Y 2 O 3 cloth is subsequently transformed into single domain Y123 during a slow cooling schedule in the presence of a c -axis oriented Nd123 seed crystal placed at the top center of the fabric. The solidification window for single domain growth is lowered to 970–950 °C using liquid phases containing up 10 wt.% Ag and small amounts of BaF 2 .