D.N. Matthews

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.

Two classes of growth spirals on single crystals of YBCO

Abstract A detailed examination of the (001) surfaces of CuO-flux grown single crystals of YBCO (YBa 2 Cu 3 O 7− x ) has shown the presence of two types of growth feature. The first of these consists of relatively large growth steps which can be in the form of spirals, arcs, instabilities or complex intergrowths which are similar to those reported by a number of workers. The second type involves much more conventional growth spirals with a step height close to the limit of optical DIC detection and which only become readily visible after decoration. The nature of these two classes of growth feature is discussed.

Flux trapping energies in YBCO in the presence of a transport current

Abstract The time dependence of the residual voltage in specimens of YBCO driven into the mixed state by the simultaneous presence of an external magnetic field and a transport current, has been used to determine the flux pinning energies for various sample types in the presence of a transport current. The time decays vary strongly from specimen and there is a good correlation with the critical current density. However, the derived values of the pinning energies are remarkably similar and only for the highest critical current density smaple was a significant current dependence observed.

Superconductivity at 108 K in sulphur-substituted YBa2Cu3Ox

Abstract In an intensive series of investigations of the effects of elemental substitutional changes on the superconducting properties of YBa 2 Cu 3 O x , we have recently studied the behaviour induced by sulphur substitution. All samples retained full superconducting transitions to 20% replacement of CuO by CuS and the 10% specimen showed a transition at 108 K. With repeated cycling, however, this degraded to 89 K at which it was stable.

The effect of silver on the voltage-current characteristics of silver-sheathed PBSCCO superconducting tapes

Abstract The I-V characteristics of silver-sheathed PBSCCO tapes have been measured at 4.2 K and 77 K in magnetic inductions up to 1.7 T applied parallel to the c -axis of the tape. The silver has dramatic effects on the transport current which leads to a misrepresentation of the V-I characteristics of the superconductor. On correcting the data, linear log ( V )-log ( I ) plots are obtained which are typical of a logarithmic pinning potential. The effect of the silver on the estimate of I c has also been investigated and we have found that provided a small voltage criteria (

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.

Surface morphology of flux-grown single crystals of YBa2Cu3O7−δ

Abstract Single crystals of YBa 2 Cu 3 O 7−δ have been grown in a CuO-BaCuO 2 flux at a temperature of 930°C and examined using SEM, XRD and electron diffraction techniques. The observed crystal habit is predominantly square planar with the c -axis normal to the plane. Surface growth spirals are observed in approximately 25% of the crystallites and in some cases extensive step decoration is exhibited. The growth surface usually involves two such spirals with Burgers vectors of the same sign. Opposed spirals have been observed in only two cases.

Magnetoresistance relaxation and flux creep in high-Tc superconductors

Abstract The appearance of hysteretic voltages on ceramic high temperature superconductors during the measurement of the mixed state resistance is now well known., While these voltages can be attributed to a field dependence of the critical current density, the primary phonomenon is the V—H—i characteristic which is representative of a magnetic field induced, dissipative process in the granular material. Two mechanisms may be responsible for these effects; one is the well known flux-flow or flux-creep phenomenon where the electrical energy is used in changing the vortex system while the other is a more conventional thermal loss as the intergranular contacts are driven normal by the applied field. The observation of a corresponding time dependent residual voltage which is associated with flux trapped in the superconductor is examined in terms of the two models, both of which are found to predict the observed time decay and also justify the derivation of the flux-trapping energies from the decay law. The differences between the models appear in the prefactors to the logarithmic decay term.

The percolation limit and weak link effects on the superconducting phase transition in high temperature superconductors

Abstract The parameters controlling the intergranular percolating network of the new class of high temperature superconductors have been studied using a novel method to control specimen composition as the percolation limit is approached. The form of the resistive transition in two-phase composites formed from Y2BaCuO5 and YBa2Cu3O7-δ is shown to be complex and that processes occuring in each of a number of temperature ranges are discussed. Microbridges formed from these materials show characteristics which are determined by the weak-link structure and display features related to the resistive transition of the bulk material.