Nadendla Hari Babu

Seeded infiltration and growth of single-domain Gd–Ba–Cu–O bulk superconductors using a generic seed crystal

We report the seeded infiltration and growth (IG) of Gd–Ba–Cu–O (GdBCO) bulk superconductors for varying molten Ba–Cu–O liquid compositions using an Mg-doped Nd–Ba–Cu–O (Mg-NdBCO) melt-textured seed crystal developed recently at the IRC. All the samples were grown in the form of single grains, although the size of the grain was observed to decrease for an increasingly Ba-rich initial composition. The size of Gd2BaCuO5 (Gd-211) particles in the GdBa2Cu3Oy (Gd-123) phase matrix is almost constant for samples with different BaO2 content in the liquid source pellet. The volume fraction of Gd-211 in the Gd-123 matrix (Vf211), however, decreases significantly with increasing BaO2 content. The superconducting transition temperature, Tc, and transition width within the entire seeded IG bulk GdBCO grain are also extremely sensitive to BaO2 concentration. A maximum critical current density, Jc, of 88 000 A cm−2 at 77.3 K in self-field is observed in this study in the sample containing the greatest concentration of BaO2 in the liquid source pellet grown in air and without further Ar and/or low oxygen heat treatment (which would give further improvements in the superconducting properties of the samples). The highest value of Jc observed in this study of 115 000 A cm−2 at 77.3 K in self-field was obtained in the sample grown in a 1% O2+N2 atmosphere. These results indicate that the seeded infiltration and growth technique produces effective pinning centres in the bulk microstructure, at least in the self-field limit.

Synthesis of YBa2Cu3O(7-δ) and Y2BaCuO5 nanocrystalline powders for YBCO superconductors using carbon nanotube templates.

We fabricate nanosized superconducting YBa(2)Cu(3)O(7-δ) (Y-123) and nonsuperconducting Y(2)BaCuO(5) (Y-211) powders using carbon nanotubes as template. The mean particle size of Y-123 and Y-211 is 12 and 30 nm, respectively. The superconducting transition temperature of the Y-123 nanopowder is 90.9 K, similar to that of commercial, micrometer-scale powders fabricated by conventional processing. The elimination of carbon and the formation of a high purity superconducting phase both on the micro- and macroscale is confirmed by Raman spectroscopy and X-ray diffraction. We also demonstrate improvement in the superconducting properties of YBCO single grain bulk samples fabricated using the nanosize Y-211 powder, both in terms of trapped field and critical current density. The former reaches 553 mT at 77 K, with a ∼20% improvement compared to samples fabricated from commercial powders. Thus, our processing method is an effective source of pinning centers in single grain superconductors.

Gd?Ba?Cu?O bulk superconductors fabricated by a seeded infiltration growth technique under reduced oxygen partial pressure

Single-grain Gd?Ba?Cu?O (GdBCO) bulk superconductors have been grown by a seeded infiltration and growth (SIG) technique under a 1% O2+N2 atmosphere using a generic MgO-doped Nd?Ba?Cu?O (MgO?NdBCO) seed placed on the sample surface at room temperature (the so-called the cold-seeding method). Partial melting of the MgO?NdBCO seeds fabricated in air under notionally identical thermal processing conditions, however, limited the reliability of this bulk GdBCO single-grain process. The observed seed decomposition is attributed to the dependence of the peritectic temperature Tp of MgO-doped Nd1+xBa2?xCu3Oy solid solution (MgO-doped Nd-123ss, where ss indicates solid solution) compounds on both oxygen partial pressure during the melt process and the level of solid solution (x). The peritectic decomposition temperature of MgO-doped Nd-123ss, with x ranging from 0 to 0.5 under p(O2) = 1.00 atm, was observed to remain constant at 1120??C. Tp was observed to decrease linearly as a function of solid solution level, on the other hand, under oxygen partial pressures of both p(O2) = 0.21 and 0.01 atm. Based on these results, MgO-doped NdBCO seed crystals should be grown under reduced oxygen partial pressure in order to obtain a stable MgO-doped NdBCO seed crystal suitable for cold-seeding processes of large-grain (RE)BCO bulk superconductors (where RE is a rare earth element).

Solidification behaviour of an AA5754 Al alloy ingot cast with high impurity content

Abstract In view of the recycling of aluminium scrap for automotive sheet application, we have investigated the solidification behaviour of AA5754 alloy containing additional amounts of impurity elements such as Si, Fe, and Cu. Ingot casts with a high impurity content resulted in coarse α-Al dendrites and complex-shaped secondary phases. A large volume of coarse Chinese script and needle-type Fe-bearing intermetallic phases were observed to form at the centre of an ingot. In addition to the grain-boundary eutectic, spherically shaped rosette-type eutectic phases were observed within the Al grain in the high-impurity alloy. The more uniform size distribution of the Fe-bearing intermetallics observed in the Al–Ti–B grain refined alloy is attributed to the presence of a large fraction of α-Al grain boundaries which distributes the eutectic liquid where the Fe-bearing intermetallic forced to nucleate and grow.

Growth Rate and Superconducting Properties of Gd-Ba-Cu-O Bulk Superconductors Melt Processed in Air

A generic Mg-doped Nd-Ba-Cu-O seed crystal has been developed recently for the fabrication of any type of rare earth (RE) based (RE)-Ba-Cu-O single grain bulk superconductor in air. The new generic seed simplifies significantly the top seeded melt growth (TSMG) process for light rare earth based (Nd, Sm, Gd, or mixed rare earth elements) bulk superconductors, in particular. GdBCO single grains have been fabricated successfully in air using the new seed in a cold-seeding process. In this study, precursor powders were enriched with different amounts of BaO2 to investigate the extent of substitution of Gd for Ba in the Gd1+xBa2-xCu3O7-delta solid solution phase. The growth process of large single grains in air was investigated at various growth temperatures under isothermal processing conditions. Crystal growth rate as a function of under-cooling and BaO2 content has been determined from these experiments. The spatial variation of Tc and transition temperature width for applied field aligned along the a/b and c-axis of grains fabricated with different BaO2 content has also been investigated in order to understand the extent of the formation of Gd/Ba solid solution with varying growth temperature and precursor composition. These results have been used to establish the optimum conditions for fabricating solid solution-free, large single grains of GdBCO in air.

Processing, microstructure and characterization of artificial joins in top seeded melt grown Y–Ba–Cu–O

A technique based on the presence of an unreacted barium-cuprate phase within the melt-processed microstructure (predominantly at the platelet boundaries), which melts below the peritectic temperature of the YBa2Cu3O7–δ (Y123) phase has been developed to join large Y–Ba–Cu–O (YBCO) grains. On heating under an applied pressure of 0.1 MPa, the Ba–Cu–O liquid phase is released to the grain interface. This technique has been investigated for YBCO a–b-plane-aligned grain boundaries with misorientation angles of up to 45° and an aligned c-axis grain boundary. Optical and transmission electron microscopy studies indicate that the low-angle a–b plane grain boundaries are epitaxial in nature and are generally of high microstructural quality. The electrical and magnetic properties of the grain boundaries fabricated by this technique have been characterized by I–V and transport current measurements as a function of magnitude and orientation of the applied field. The temperature dependence of the irreversibility field of low-angle a–b grain boundaries is similar to that of the grains, providing direct evidence for the quality of the join. A join of nominally zero misorientation between the a–b planes was found to support a critical current density, Jc, exceeding 2000 A cm−2 at 77 K in fields of up to 4 T.

Fabrication of Ca-doped large grain Y-Ba-Cu-O superconductors

Doping the rare earth site in superconducting YBa/sub 2/Cu/sub 3/O/sub x/, (Y-123) with Ca generates additional positive charge carrier density in the Cu-O planes and, at a certain hole concentration, enhances the critical current density and irreversibility field of the material. 3 cm sized large, single grain samples of YBCO doped with various amounts of Ca have been fabricated by a top seeded melt growth technique. Very fine, sub-micron sized secondary phase particles of Y-211, which enhance flux pinning, have been observed in the (Y,Ca)Ba/sub 2/Cu/sub 3/O/sub y/ matrix without the addition of Pt. It is considered that Ca doping within the Y-211 phase is responsible for the refinement of these particles. In this paper we discuss the effect of Ca doping on growth rate, microstructural features and superconducting properties of large YBCO pseudo-crystals.

Synthesis of dense bulk MgB2 by an infiltration and growth process

We report the processing of dense, superconducting MgB2 (????2.4 g cm?3) by an infiltration and growth technique. The process, which involves infiltration of liquid magnesium at 750 ?C into a pre-defined boron precursor pellet, is relatively simple, results in the formation of a hard, dense structure and has the potential to fabricate large bulk samples of complex geometries. X-ray diffraction has been used to confirm the presence of the MgB2 primary phase with only residual magnesium content in the fully processed samples. The samples exhibit sharp superconducting transitions at 38.4 K and have critical current densities of up to 260 kA cm?2 in self-field at 5 K. Modest measured values of Hc2(0) of 17 T suggest that superconductivity in bulk MgB2 fabricated by this technique is in the clean pairing limit.

Strongly Coupled Artificial Bulk HTS Grain Boundaries With High Critical Current Densities

A multi-seeding process has been developed to fabricate single Y-Ba-Cu-O (YBCO) grains containing strong artificial grain boundaries. Multi-seeding of heterogeneous YBCO grains with controlled orientation was achieved using large Sm-Ba-Cu-O (SmBCO) single crystal seeds of rod-like geometry with slots of various widths (up to 13 mm) cut into their bottom surface (i.e. parallel to the c-axis of the seed) to produce a bridge-like structure. Several YBCO grains with artificial grain boundaries were fabricated from these seed crystals and used to investigate the effect of varying the distance between the individual grain nucleation sites and the grain orientation (in-plane and out of plane) on the nature of grain boundaries. The measured local magnetic critical current density (Jc) and the magnitude of the trapped field across these artificial grain boundaries indicate that seed alignment is a key parameter in achieving strongly-coupled grain boundaries in multi-seeded grains.

The effect of the addition of zirconium-containing compounds on the microstructure and superconducting properties of mono-domain Y–Ba–Cu–O bulk superconductors

The effect of the addition of BaZrO3 and Y Ba2ZrO5.5 zirconium-containing compounds on the microstructure and superconducting properties of single-domain Y–Ba–Cu–O (YBCO) bulk superconductors has been investigated for various levels of Zr content. The distribution of Y2BaCuO5 (Y-211) particles along both the crystallographic a and c axes of the Y Ba2Cu3Oy (Y-123) superconducting matrix is observed to be rather discontinuous in Zr-containing samples, even with a small amount of secondary phase addition. The YBCO microstructure is observed to separate into regions of high and low concentrations of Y-211 particles when Zr-containing compounds are added to the precursor powder. Furthermore, Y-211 particles are observed to accumulate along the growth sector boundaries in the Y-211 free region of the Y-123 matrix. The extent of the Y-211 free region increases with increasing zirconium content. Increased concentrations of zirconium-containing compounds in the precursor powder trigger an undesirable segregation within the melt-processed microstructure of a band consisting of a liquid channel and a high concentration of Y-211 particles in both a and c axis growth sectors. The addition of BaZrO3 and Y Ba2ZrO5.5 to the precursor powders does not affect the superconducting transition temperature, Tc, of the bulk material. Critical current densities, Jc, of around 60 000 and 47 000 A cm−2 in self-field at 77.3 K were recorded for the samples containing 0.329 mol% BaZrO3 and 0.658 mol% Y Ba2ZrO5.5, respectively.