T.D. Withnell

YBa2Cu3O7−δ/Y2Ba4CuMOy single grain nanocomposite superconductors with high critical current densities

Nanocomposite Y?Ba?Cu?O superconductors containing a wide range of Y2Ba4CuMOy (Y-2411), where M = Nb, Ta, Mo, W, Zr, Hf, Ru, Ag, Sb, Sn?and?Bi, embedded in the YBa2Cu3O7?? matrix, have been fabricated successfully. The Y-2411 phase typically has dimensions in the range 20?50?nm and it is observed to form effective flux pinning sites even at higher magnetic fields. A direct correlation between the Y2Ba4CuMOy phase inclusion content in the YBa2Cu3O7?? matrix and increased magnetic flux pinning has been observed. The critical current density increases by an order of magnitude for a nanocomposite compared to that of Y-123 without inclusions.

The effect of size, morphology and crystallinity of seed crystals on the nucleation and growth of Y–Ba–Cu–O single-grain superconductors

The effect of size, morphology and crystallinity of seed crystals on the nucleation and growth of melt processed Y–Ba–Cu–O (YBCO) during top seeded melt growth (TSMG) has been investigated. Conventional seeding of bulk YBCO with a small seed crystal of square planar or irregular geometry leads to point nucleation of the YBa2Cu3O7−δ (Y-123) phase and a subsequent square planar c-sector growth habit. The use of seeds of triangular and round geometries, however, results in a deviation from this growth habit symmetry. The use of large surface area seed crystals of dimensions 20 mm × 10 mm, on the other hand, influences the growth of bulk YBCO in two significant ways. Firstly, rapid epitaxial nucleation of the Y-123 phase occurs beneath the seed crystal, followed by grain growth via the usual peritectic solidification in the incongruent melt. Secondly, a large area seed reduces the processing time of large single grains compared to that achieved by TSMG with small area seeds. The present study is particularly relevant to the fabrication of single-grain samples with controlled growth sectors, reduced processing times and the multiple seeding of large area bulk materials containing good grain boundaries.

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.

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.

Characterization of nano-composite M-2411/Y-123 thin films by electron backscatter diffraction and in-field critical current measurements

Thin films of nano-composite Y-Ba-Cu-O (YBCO) superconductors containing nano-sized, non-superconducting particles of Y2Ba4CuMOx (M-2411 with M = Ag and Nb) have been prepared by the PLD technique. Electron backscatter diffraction (EBSD) has been used to analyze the crystallographic orientation of nano-particles embedded in the film microstructure. The superconducting YBa2Cu3O7 (Y-123) phase matrix is textured with a dominant (001) orientation for all samples, whereas the M-2411 phase exhibits a random orientation. Angular critical current measurements at various temperature (T) and applied magnetic field (B) have been performed on thin films containing different concentration of the M-2411 second phase. An increase in critical current density Jc at T < 77 K and B < 6 T is observed for samples with low concentration of the second phase (2 mol % M-2411). Films containing 5 mol % Ag-2411 exhibit lower Jc than pure Y-123 thin films at all fields and temperatures. Samples with 5 mol % Nb-2411 show higher Jc(B) than phase pure Y-123 thin films for T < 77 K.

Effects of Irradiation on Vicinal YBCO Thin Films

The achievable transport critical current densities in coated conductors are limited in part by low angle grain boundaries. Effects like vortex channeling found in these low angle boundaries at low fields can also be studied in vicinal thin films. The influence of neutron irradiation, a proven method for increasing magnetic and current carrying properties, on these effects was investigated in this work. An YBCO vicinal sample, grown on a SrTiO3 substrate with the ab plane oriented at 6 degrees off parallel, was fabricated through pulsed layer deposition (PLD) and both transition temperature and transport current measurements were performed using a 2-axis probe. The sample was rotated at varying temperature in an applied magnetic field of up to 6 T. The angular dependence of the critical density was determined before and after an irradiation to a neutron fluence of 1 times 1021 m-2 in the TRIGA Mark II reactor in Vienna.