Y-H Shi

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.

Fabrication of high performance light rare earth based single-grain superconductors in air

Large, single-grains of (LRE)-Ba–Cu–O (LRE=light rare earth: Nd, Sm, and Gd) bulk high-temperature superconductors with significantly improved properties have been fabricated in an air atmosphere using a practical processing technique based on seeded infiltration growth. This process involves the use of a generic seed crystal, developed recently at the IRC in Superconductivity, to promote epitaxial grain nucleation. The formation of a solid-solution phase within the nucleated single grain is then suppressed effectively by providing excess Ba to the growth front under an air processing atmosphere. Critical current densities in excess of 105A∕cm2 at 77K have been observed in the resulting large grain samples, with an associated and significant improvement in trapped magnetic field. This air-based seeded-infiltration-growth process offers a significant degree of freedom both in terms of the processing parameters and the reproducibility in growth of oriented single grains.

Processing and microstructure of single grain, uranium-doped Y-Ba-Cu-O superconductor

Large grain Y–Ba–Cu–O (YBCO) superconductor doped with various amounts of depleted UO2 and containing excess Y2BaCuO5 (Y-211) and Y2O3 have been fabricated by top seeded melt growth (TSMG). The effect of depleted UO2 on the large grain microstructure has been studied systematically in samples with and without added Pt. It is found that UO2 refines the size of the second phase particles in the superconducting YBa2Cu3O7−δ (Y-123) matrix to dimensions of a few hundred nanometres with an approximately spherical morphology. Addition of Y2O3 to the uranium-doped precursor powder, rather than Y-211, yields a significantly finer distribution of second phase particles and an associated higher critical current density Jc at increased magnetic field.

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).

Growth of large sized Y Ba2Cu3O7 single crystals using the top seeded melt growth process

Neutron scattering experiments are fundamental to the study of magnetic order and related phenomena in a range of superconducting and magnetic materials. Traditional methods of crystal growth, however, do not yield single crystals of sufficient size for practical neutron scattering measurements. In this paper, we demonstrate the growth of relatively pure, large Y Ba2Cu3O7 single crystals up to 30?mm in diameter using a top seeded melt growth process. The characterization of the microstructural and magnetic properties of these crystals indicates that they contain <2% of impurity phases and, hence, exhibit only weak flux pinning behaviour.

Top seeded melt growth of Gd?Ba?Cu?O single grain superconductors

Top seeded melt growth (TSMG) has been used extensively to fabricate large, single grain Y?Ba?Cu?O (YBCO) bulk superconductors that can trap large magnetic fields. The TSMG method is relatively economical and has enabled the development of batch processes for the fabrication of a large number of bulk single grain superconductors in a single furnace. In addition, the technique allows the fabrication of complex-shaped bulk samples with controlled and strongly connected grains by using a novel, multi-seeding process. A practical processing route for processing of LRE?Ba?Cu?O (where LRE represents a light rare earth element) single grain superconductors (which have superior properties to YBCO) has been developed at Cambridge over the past three years, based on the development of a generic seed of melt textured Mg-doped Nd-123 and suppression of solid solution phase formation in air by enriching the precursor composition with excess Ba. In this paper we report the successful application of a practical TSMG process in the fabrication of high performance Gd?Ba?Cu?O (GdBCO) single grain superconductor. This method has enabled the development of a batch process for GdBCO and we demonstrate for the first time the fabrication of a large number of high performance single grains of this material in a single process. Finally, we report the processing of bulk GdBCO in the form of complex geometries with controlled grain orientation for bespoke engineering applications.

The influence of Gd-2411(Nb) on the superconducting properties of GdBCO/Ag single grains

Large Ag2O doped Gd?Ba?Cu?O (GdBCO) single grains containing Gd2Ba4CuNbOy (Gd-2411(Nb), a niobium compound) have been fabricated successfully in air by a cold-seeding melt-processing technique using a generic Mg?Nd?Ba?Cu?O seed crystal. An optimum amount of BaO2 addition to the precursor composition is found to suppress Gd/Ba solid-state substitution in the superconducting phase composition. A study of the microstructures and superconducting properties of these GdBCO bulk superconductors containing such Gd-2411(Nb) phases is presented. A trapped field of over 0.9?T has been recorded finally at 77?K for the GdBCO/Ag contained Gd-2411(Nb) with 26?mm diameter.

Growth rate of YBCO single grains containing Y-2411(M)

Y-Ba-Cu-O (YBCO) single grains have the potential to generate large trapped magnetic fields for a variety of engineering applications, and research on the processing and properties of this material has attracted world-wide interest. In particular, the introduction of flux pinning centres to the large grain microstructure to improve its current density, Jc, and hence trapped field, has been investigated extensively over the past decade. Y2Ba4CuMOx [Y-2411(M)], where M = Nb, Ta, Mo, W, Ru, Zr, Bi and Ag, has been reported to form particularly effective flux pinning centres in YBCO due primarily to its ability to exist as nano-size inclusions in the superconducting phase matrix. However, the addition of the Y-2411(M) phase to the precursor composition complicates the melt-processing of single grains. We report an investigation of the growth rate of single YBCO grains containing Y-2411(Bi) phase inclusions and Y2O3. The superconducting properties of these large single grains have been measured specifically to investigate the effect of Y2O3 on broadening the growth window of these materials.

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.

Microstructure and Superconducting Properties of Single Grains of Y-Ba-Cu-O Containing Y-2411(M) and

Y-Ba-Cu-O (YBCO) single grains have the potential to generate large trapped magnetic fields for engineering applications, and research on the processing and properties of this material has attracted interest world-wide over the past 20 years. In particular, the introduction of flux pinning centers to the large grain microstructure to improve its current density Jc, and hence trapped field, has been investigated extensively. Y2Ba4CuMOx [Y-2411(M)], where M=Nb , Ta, Mo, W, Ru, Zr, Bi and Ag, has been discovered recently to form very effective flux pinning centers due primarily to its ability to form nano-size inclusions in the superconducting phase matrix. However, the addition of the Y-2411(M) phase to the precursor composition complicates the melt-processing of single grains. The addition of Y2O3 to the precursor composition, however, broadens the growth window of single YBCO grains containing Y-2411 (M). We report an investigation of the microstructures and superconducting properties of single grains of this composition grown by top seeded melt growth (TSMG).