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

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

Bulk superconducting nano-composites with high critical currents

Flux pinning sites are most effective if their size is comparable to the superconducting coherence length, which is on the nano-meter scale for RE-Ba-Cu-O superconductors [RE = rare earth element]. Introducing nano-phase inclusions directly into the bulk superconducting material has only been partially successful to date, however, due primarily to the absence of chemically stable phases that can co-exist with RE-Ba-Cu-O without suppressing its key superconducting properties. We have identified novel isostructural phases based on (RE)2Ba4CuMOy (where M = W, Zr, Nb, Ag and Bi) and have fabricated successfully superconducting bulk nano-composites with a high current carrying capability. The average size of the nano-inclusions is observed to vary from 20 nm to 300 nm depending on element M. An observed improvement in Jc under low and high external magnetic fields at 77 K correlates directly with an increased density of nano-inclusions in the superconducting matrix

Characterization of bulk MgB2 synthesized by infiltration and growth

Superconducting MgB2 has been synthesized successfully by a modified infiltration and growth (IG) technique. The ambient pressure technique is relatively simple and scalable to complex shaped bulks. The extent of MgB 2 phase formation has been found to be influenced strongly by the IG process time and/or temperature, and this is found to reflect in the X-ray diffraction patterns, magnetization measurements, and microhardness. Scanning electron microscopy images show a bimodal particle size distribution with 20-50 nm sized fine precipitates in the inter particle region. A critical current density of 400 kA cm-2 was measured at 5 K.KACST-Cambridge Research Centre, Cambridge, U.K

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

Numerical analysis of non-uniformities and anisotropy in high-temperature superconducting coils

—High-temperature superconducting (HTS) coils play an important role in a number of large-scale engineering applications, such as electric machines employing HTS coated conductors. Non-uniformities and anisotropy in the properties of the coated conductor along its length and width can have a large impact on the performance of the tape, which directly influences the performance of an HTS electric machine. In this paper, the specific influences of non-uniformity and anisotropy on the dc properties of coils, such as the maximum allowable dc current, and the ac properties, such as ac loss, are analyzed using a numerical model based on the H formulation. It is found that non-uniformity along the conductor width has a large effect on the ac properties (i.e., ac loss) of a coil, but a relatively small effect on the dc properties (i.e., critical current). Conversely, non-uniformity along the length has a small effect on the ac coil properties, but has a large effect on the dc properties. Index Terms—AC loss, critical current density (superconductivity), high-temperature superconductors, numerical analysis, superconducting coils, transport ac loss.This work was supported in part by a Henan International Cooperation Grant, China: 144300510014. The work of D. Hu and J. Zou was supported in part by Churchill College, by the China Scholarship Council, and by the Cambridge Commonwealth, European, and International Trust. The work of M. D. Ainslie was supported by a Royal Academy of Engineering Research Fellowship.This is the accepted manuscript. The final version is available from IEEE at http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6983573

Mitigation of Demagnetization of Bulk Superconductors by Time-Varying External Magnetic Fields

Large, single-grain high-temperature superconducting (HTS) bulks have significant potential to replace permanent magnets in various engineering applications. However, based on our previous research, the trapped field in a bulk superconductor can be attenuated or even erased when a bulk is subjected to a time-varying, external magnetic field. Therefore, it is important to develop a method to protect bulks from demagnetization by (a) improving the thermal conduction of the bulk and/or (b) reducing AC losses. Improvement in the thermal conduction of bulks involves modification of the material fabrication process, which may have a detrimental effect on its superconducting properties. Employing shielding materials around a bulk helps to decrease the AC losses, but also provides a durable way to maintain the original material properties. In this paper, two shielding cases are proposed and evaluated numerically: ring-shaped shielding with a copper coil, and surface shielding with a ferromagnetic material. Based on the numerical modelling results, the ring-shaped coil works well for externally applied AC fields of larger magnitude and higher frequency. However, the ferromagnetic material was preferable for surface shielding for relatively lower fields. Finally, an optimal shield design is presented.M. D. Ainslie would like to acknowledge financial support from a Royal Academy of Engineering Research Fellowship. D. Hu and J. Zou would like to acknowledge financial support from Churchill College, the China Scholarship Council and the Cambridge Commonwealth, European and International Trust. This work was supported in part by a Henan International Cooperation Grant, China: 144300510014.This is the author accepted manuscript. The final version is available from IEEE via http://dx.doi.org/10.1109/TASC.2016.252582