F. Wellhofer

The microstructural and critical current properties of melt-processed YBCO+Ag thick films

The effect of Ag addition on the microstructure and critical current properties of YBa2Cu3O7-x thick films is reported. A comparison between isothermally processed thick films of YBCO and YBCO+Ag2O revealed considerable differences in both surface morphology and internal microstructure. Addition of Ag reduces macroscopic cracking in the films and also reduces the extent of film/substrate reaction, thereby minimizing associated deviations from correct stoichiometry in the film. Evidence of a barium-copper-silver solution at high temperatures, which is thought to play an important role in the modification of microstructures obtained, and the decomposition temperatures of the phases involved are reported. A reduction in stability of several phases present in the Ag containing thick film precursor at high temperatures was determined using DTA. This included an approximate 30 degrees C reduction of the YBa2Cu3O7-x phase formation and peritectic decomposition temperatures in an oxygen atmosphere. The addition of Ag increases the critical current densities of the films, realizing values of approximately 3000 A cm-2.

Pulsed critical current measurements on YBCO wires and screen-printed films

Abstract Measurements of critical current densities in YBCO wires and thick films have been made using a pulsed current facility to avoid problems from ohmic heating at the electrical contacts. The measurements on bulk ceramic and extruded wire samples are considered in terms of thermally activated voltage arising from the unpinning of flux lines and a steady state flux flow regime. For textured thick film samples produced by screen-printing, critical current densities in excess of 500 A cm −2 at liquid nitrogen temperature have been achieved. The properties of such films are highly dependent on processing temperature and, in contrast to bulk and wire samples, the critical currents in such films are highly anisotropic with respect to field direction relative to the current. Also, their resistance in the flux flow regime relative to their normal state resistance is almost two orders of magnitude smaller than observed for bulk samples.

Microstructural and magnetization studies of sintered and melt-textured Y-Ba-Cu-O

For bulk applications, high‐Tc superconductors generally require high critical current density, Jc. This paper reports two methods of producing high‐Tc ceramic wires. The first is an extrusion and sintering route where Jc is routinely 600–1000 A cm−2. The second is by melt texturing or directional solidification where we have achieved in excess of 7000 A cm−2 by direct measurement at 77 K in zero field. However, the ease with which wire may be extruded and sintered is attractive and functioning devices have already been manufactured using this route. Fabrication of devices is facilitated by a unique processing route that has resulted in the strongest bulk ceramic yet reported (bend strength >200 MPa). The melt‐texturing route and the problems associated with it are examined and compared with the extrusion and sintering route. The processing and the microstructures associated with the directionally solidified YBa2Cu3Ox will be examined in detail as will the resultant superconducting properties, in particul...

High critical currents in thick films of YBCO showing spherulitic growth habits

Abstract Processing of screen printed YBCO on YSZ substrates at temperatures close to the peritectic transition have led to highly textured films with critical current densities in excess of 500 Acm −2 . The texturing is shown to be associated with spherulitic growth centred on small platelet crystallites.

Measurements of the low-temperature heat capacity of YBa2Cu3O7-y in magnetic fields up to 4.5 T

The heat capacity of several sintered specimens of YBa2Cu3O7-y, the high-temperature superconductor, has been measured from 1.5 to 10 K in magnetic fields up to 4.5 T. Part of the low-temperature anomaly in the heat capacity is ascribed to very small concentrations of localised Cu++ spins. There is a dependence of heat capacity on field which is consistent with a contribution from the electrons in flux-line cores but other features of the results are not yet understood.

Modulated optical reflectance characterization of high temperature superconducting thin film microwave devices

The modulated optical reflectance (MOR) technique is shown to provide a room temperature, noncontact, nondestructive and high spatial resolution means of assessing high temperature superconducting (HTS) thin film quality. Room temperature MOR characterizations of a number of 8 GHz planar HTS resonators indicating a range of property variations and local degradations in HTS film performance are shown to be consistent with results obtained at low temperatures by the electron beam induced voltage contrast technique. The microwave performances of some of the resonators are found to exhibit nonlinear characteristics that can be explained by HTS defects revealed by the MOR technique.

A comparison of lead-doped Bi-Sr-Ca-Cu-O superconductors at the '2223' and '2234' compositions

Superconducting Bi-Pb-Sr-Ca-Cu-O ceramics have been prepared at the Bi1.2Pb0.8Sr2Ca2Cu3Ox (2223) and Bi1.6Pb0.4Sr2Ca3Cu4Oy (2234) compositions. The properties and microstructures after sintering in air at 850 degrees C for times from 2 to 156 hours are described and compared. For the 2223 composition all heat treatment times result in multiphase samples. Bulk superconductivity giving zero resistance at 106 K and a maximum Jc of 300 A cm-2 at 77 K is achieved after 50 h heat treatment. When sintered in excess of 40 h samples of the 2234 composition consist predominately of the high-Tc, n=3, phase. Jc is again maximised after 50 h heat treatment but at lower value of 150 A cm-2 and Tc(R=0)=103 K. For longer sintering times Jc of the 2234 composition falls to zero after approximately 100 h at 850 degrees C. Samples of both compositions densify rapidly during the first two hours of sintering. Further heat treatment results in a large de-densification, the reasons for which are described in detail. A decrease in lead content with increasing sintering time is observed. The implications of these observations for the production of high-quality, bulk Bi-Pb-Sr-Ca-Cu-O superconducting ceramics are discussed.

Induction of superconductivity in pulsed laser ablated La2CuO4 thin films by post-deposition fluorination

Abstract Pulsed laser ablation has been used to fabricate La2CuO4 thin films. Superconducting properties have been successfully induced in the films by an ex-situ, post-ablation annealing process in F2 gas resulting in a Tc (onset) of 36 K. The presence of two slightly different c-axis expanded phases in the X-ray diffraction data of the fluorinated films implies a degree of inhomogeneity in F2 uptake. Critical current densities (Jc) and the irreversibility line have been established from hysteresis cycles. A Jc of ∼ 106 A cm−2 for a typical film was observed at 10 K in zero field.

Novel magnetometer and the interpretation of low field magnetic hysteresis of high Tc superconductors

Abstract A novel electronic integrating magnetometer, based on a computer controlled phase sensitive detector, has been developed to investigate magnetic hysteresis in high temperature superconductors associated with intergranular and intragranular critical currents. The high sensitivity of the magnetometer (≳ 10 −6 electromagnetic units (emu)) enables accurate measurements to be made on mg superconducting samples in low fields of up to 50 mT. A computer fitting procedure based on a modified Bean model with a field dependent critical current density gives values for the magnetic filling factor, the strength of the zero-field critical current density and the scaling field for a given assumed field dependence of critical current. Magnetization measurements on various ceramic YBCO samples, including wires of various diameters, yield values for bulk critical currents in good agreement with transport measurements.

The nonlinear surface impedance of YBa2Cu3O7−δ thin films in zero and large applied fields

We present coplanar resonator measurements of the nonlinear microwave surface impedance of laser ablated and electron-beam coevaporated YBa2Cu3O7−δ thin films at 8 GHz in zero and applied magnetic fields, including the effects of irradiation with heavy ions. Correlations between the changes in surface resistance and reactance as a function of microwave current suggest that there are different contributions to the nonlinear behavior at low and high microwave currents in both zero and applied fields. In zero field, we suggest that microwave-induced flux lines are responsible for the observed nonlinear behavior, resulting in thermal runaway at large, sample dependent, microwave currents. The observed nonlinearity completely masks any possible contribution from the intrinsic pair breaking expected for a d-wave superconductor. In a magnetic field at relatively low currents, the nonlinearity can be described by the nonlinear dynamics of flux lines pinned by defects with a continuous range of pinning strengths. ...