J. Cochrane

Melt-textured growth and characterisation of a (Nd/Y)Ba2Cu3O7-δ composite superconductor with very high critical current density

Abstract A (Nd/Y)Ba 2 Cu 3 O 7−δ composite high- T c superconducting material has been fabricated in air using a melt-textured growth (MTG) technique. The material has a very high critical current density, J c ∼ 8.77 × 10 4 A cm −2 at 77 K, B = 0 T, and J c ∼ 7.59 × 10 4 A cm −2 at 77 K, B = 0.5 T, with B || c (using the modified Bean model formula) which is attributed, in part, to the partial substitution of the Nd 3+ in the Ba 2+ site. The presence of Y211 and/or Nd422 precipitates were not detected within the 123 grains indicating that these phases are not important in order to obtain samples with a high J c in this composite material.

Thermopower of ultra-high purity YBa2Cu3O7−δ single crystals

Abstract We have measured the thermopower of Y 2 O 3 crucible grown, ultra-high quality twinned YBa 2 Cu 3 O 7−δ single crystal in both the a - b (in-plane) and c axis (out-of-plane) orientations. We report negative values for the in-plane thermopower and positive values for the out-of-plane thermopower of overdoped YBCO single crystals. The in-plane measurements correlate with results obtained from sintered polycrystalline YBCO [1] where the a - b plane dominates the electronic conduction process. We discuss the implications of these intrinsic results in terms of the high temperature metallic thermopower relation.

Effects of helium absorption on the superconducting mechanism of YBa2Cu3Oy

Abstract During the course of a series of measurements of the electrical conductivity and Hall effect in the YBa2Cu3Oy superconductors, it was observed that the presence of an helium atmosphere during the measurements led to reversible changes in the electrical properties including increases in the critical temperature. The results of a series of measurements to investigate this effect are described and possible mechanisms considered. It is suggested that helium enters the lattice and leads to different internal pressure effects in different parts of the unit cell.

Seebeck coefficient as an indicator of oxygen content in YBCO

Abstract Measurement of the Seebeck coefficient at room temperature provides a simple method for the determination of the oxygen content and carrier sign for sintered, polycrystalline, high- T c YBa 2 Cu 3 O 7− δ samples. From our measurements and those reported previously, a universal curve of oxygen content (7- δ ) versus Seebeck coefficient has been developed for these materials. This technique is also useful for the determination of both spatial sample homogeneity, and quality control of batch powder processing conditions in large-scale manufacturing operations, as well as grain orientation in melt-processed materials.

Electron microprobe analysis and synchrotron diffraction study of crystalline (Nd0.5Y0.5)Ba2Cu3O7−δ

Abstract A melt-textured growth process, starting with the composite material (Nd 0.5 Y 0.5 )Ba 2 Cu 3 O 7−δ as feedstock, was used to grow large intergrowth crystals containing 97% (Nd/Y)Ba 2 Cu 3 Of 7−δ [(Nd/Y)123] and 3% (Nd/Y) 2 BaCuO 5 [(Nd/Y)211]. Electron microprobe analysis and synchrotron diffraction studies showed the detailed compositions to be Nd 0.55 Y 0.43 Ba 1.95 Cu 3.00 Sr 0.10 O 7−δ and Nd 0.60 Y 0.40 Ba 2.00 Cu 2.95 O 6.76 for the (Nd/Y)123 phase and Nd 0.57 Y 1.37 Ba 0.99 -Cu 1.00 Sr 0.01 O 5− x and Nd 0.65 Y 1.35 Ba 1.00 Cu 1.00 O 4.95 for the (Nd/Y)211 phase, respectively. The small strontium impurity detected arose from contamination of the high purity barium carbonate used to prepare the feedstock. The (Nd/Y)211 structure showed a significant variation in the occupancy of one of the rare-earth sites, while no evidence was found for the substitution of Nd 3+ or Y 3+ on the Ba 2+ site in either phase. These intergrowth crystals showed a significantly lower critical current density when compared with the unusually large value for the pure melt processed material which has been found to contain Nd 3+ substituting on the Ba 2+ sites.

Electronic Structure of Nd0.5Y0.5Ba2Cu3O7‐δ Single Crystals

The new bulk, melt-textured grown composite Nd 0.5 Y 0.5 Ba 2 Cu 3 O 7-δ , which also forms large single crystals, has the standard high-T c 123 orthorhomhic structure when oxygenated and a very high critical current density in a magnetic field of 0.5 T. Both of these results were not anticipated, as the Nd and Y ions are significantly different in size, while the absence of Y211 and/or Nd422 precipitates within the 123 grains indicates that a new pinning mechanism is responsible for the high J c . A possible mechanism has been attributed in part to the partial substitution of Nd 3+ in the Ba 2+ site, but to further understand this material and to further optimize its processing route for application purposes, we have carried out an XPS study of its electronic structure using cleaved surfaces of single crystals The data obtained are analysed and discussed in terms of that found for high-quality Re 123 single crystals where Re is either Nd or Y.

Densification of YBa2Cu3O7−δ by uniaxial pressure sintering

Abstract Preliminary studies have been made of the preparation of high density YBCO by hot pressing techniques. Materials prepared at 1000°C under an axial load of 1 tonne on a 2.5 cm diameter die have been characterized and shown to exhibit superconducting behaviour after oxygenation for extended periods.

Growth and properties of large single crystals of YBa2Cu3O7−δ

Abstract The use of a molten eutectic flux based on mixed alkali halides has allowed us to grow single crystals of YBCO with thickness of up to 4mm and total volumes approaching 0.3 cm 3 . The crystals are superconducting as grown but are improved by oxygen annealing. Both neutron scattering and optical microscopy confirm the single crystal nature.

Systematics of the YxBal−xCuOy superconductors

A detailed study of a series of compositions in the Y 2 O 3 -BaO-CuO phase diagram has shown that the changes in structural and electrical properties may be associated with an intrinsic increase in the oxygen concentration as the Y:Ba ratio is increased. The origin of the change is thought to be associated with structural faults which appear as changes in the planar stacking of this orthorhombic material.

Superconductivity in the Bi-Sr-Ca-Cu oxide compositional diagram

Abstract Careful study of the distribution of room temperature resistance in the wuaternary compositional diagram of the Bi-Sr-Ca-Cu oxides has demonstrated a wide range of materials which exhibit superconducting transitions in the temperature range 60–110 k. Energy dispersive analysis has shown that these are multiphase materials with a number of common phases which appear to be responsible for the observed superconductivity. Among these is the 2212 composition which has been reported previously and two new phases with stoichiometries 2213 and 2215.