B.A. Glowacki

Relation of critical current irreversibility to trapped flux and microstructure in polycrystalline YBa2Cu3O7

Abstract A study has been made of the critical current in silver clad YBa2Cu3O7 wires. Detailed resistive transitions, in zero magnetic field and with a magnetic field applied, indicate that below 10−3 V m−1 these materials display clear power law behaviour of the form E = kln with n values up to about 16. This characteristic is similar to that seen in conventional superconducting composite wires and has been related to the spatial distribution of inhomogeneity. For higher electric fields, E, there is a deviation from simple power law behaviour as current is progressively transferred to the silver cladding. It is shown that the variation of voltage with magnetic field at constant current can be related directly to lc(B), the variation of critical current with applied field. The dependence of lc(B) on the sintering conditions and amount of SiO2 impurity addition is presented and it is shown that lcversusB can be related to the Fraunhofer diffraction structure for a distribution of Josephson junctions in an applied field. It is further observed that lc(B) for all samples shows a striking irreversibility with cycles in the applied field. The Fraunhofer related lc(B) curve is displaced in B and the maximum lc reduced. The magnitude of the shift depends on the maximum value of B in the cycle and varies with sintering temperature and impurity content. An interpretation of the complex dependence of critical current on applied field and sample microstructure is given in terms of the hysteretic properties of a Josephson junction network mediated by trapped flux which is retained by strong pinning regions.

Transport ac losses and screening properties of Bi-2223 multifilamentary tapes covered with magnetic materials

Abstract DC current–voltage characteristics and transport ac losses in Bi-2223 multifilamentary tapes covered with different magnetic materials were measured. The results are compared with those without the magnetic cover. Also the measurements of screening properties of the magnetically covered tape exposed to a magnetic field generated by another parallel Bi-2223 tape with different orientations of the current were performed. The iron powder was found to be an effective magnetic screen to decouple the tapes. Transport ac losses of the magnetically screened tape were not affected by the current in the neighbouring tape. It was found that the presence of a continuous magnetic cover decreases the critical current I c of Bi-2223 tape by about 20%. In the range of currents 0.5 I c − I c , the total losses in the Bi-2223 tape covered with iron powder were increased 2.5 times, including the losses in the iron.

Influence of external magnetic field and its orientation on transport AC losses in Bi-2223 and Tl-1223 silver sheathed tapes

In the paper we report measurements of energy losses caused by an AC transport current in silver sheathed multifilamentary Bi-2223 and mono core Tl-1223 tapes, as a function of the magnitude and orientation of the applied external magnetic field. The external magnetic field was rotated in the range of 0 to /spl plusmn/90 degrees with respect to the plane of the tapes, and was In all cases perpendicular to the transport current flow. The transport current losses in a range of DC applied fields are consistent with the Norris equation provided the DC critical current at the same applied field is used in his expression for the loss.

Pulsed laser deposition of epitaxial YBa2Cu3O7−y / oxide multilayers onto textured NiFe substrates for coated conductor applications

Pulsed laser depositions of double-buffer and triple-buffer YBa2Cu3O7−y (YBCO)/Y2O3(YSZ)/CeO2 heterostructures have been performed in situ onto commercially available biaxially textured NiFe 50%/50% tape. The deposition in the forming gas (4% H2/Ar) from a CeO2 target and the deposition in vacuum from a CeO2:Pd composite target have been explored as two possible routes for cube-on-cube growth of the first buffer layer. The influence of the critical processing parameters on the texture is investigated and some of the issues involved in the reduction of NiO (111) and the formation of cube-on-cube NiO (200) growth are discussed. X-ray diffraction has been used for texture evaluation of the substrate and subsequent deposited layers. The substrate–buffer interface region has been studied by focused ion beam cross section electron microscopy. Both the buffers and YBCO layers show biaxial alignment with ω and scans having optimum YBCO full width at half maximum (FWHM) values of 4.3° and 8.8°, respectively. The morphology has been characterized using atomic force microscopy and scanning electron microscopy. The value of Tc (onset) has been measured at 90 K (ΔTc = 10 K). The critical current density, Jc, has been measured by transport measurements and magnetic measurements performed in a dc SQUID magnetometer.

AC losses of Ag-(Bi,Pb)SrCaCuO-2223 tapes in combination of transverse external magnetic field and transport current

The authors present the results of measurements of the AC transport (self-field) and magnetic losses on silver sheathed (Bi,Pb)SrCaCuO-2223 tapes prepared by the powder in tube method. In the frequency range 30-540 Hz the transport current losses are hysteretic in nature. Losses arising from AC external magnetic field (frequency range 65-207 Hz, amplitude up to 60 mT) are compared with losses generated by AC transport currents. The measured transport loss voltage depends strongly on the position of the potential taps on the surface of the tapes. Losses due to combined AC applied magnetic fields and transport currents are also reported.<<ETX>>

A calorimetric study of oxygen intercalation and desorption in bulk superconducting Y1Ba2Cu3O7-x

The authors have used differential scanning calorimetry and X-ray diffraction to study absorption and desorption of oxygen by superconducting Y1Ba2Cu3O7-x. They show that these reactions are reversible and discuss them in terms of underlying thermodynamic and kinetic processes. In particular, they measure the heat evolved during absorption of oxygen and correlate the forms of absorption peaks on DSC traces with the rate of ingress of oxygen. This rate is presumed to be limited by the speed with which oxygen atoms can diffuse through the superconducting phase. They find values for the activation energy and pre-exponential factor for diffusion of oxygen through the superconducting material and suggest guidelines for annealing treatments intended to optimise the oxygen content and superconducting properties of Y1Ba2Cu3O7-x.

Enhanced transport currents in Cu-sheathed MgB2 wires

Copper-sheathed MgB2 wires, prepared by an in-situ process, were exposed to neutron radiation in order to introduce defects into the superconductor. The high level of disorder (4.6 × 10−2 dpa) leads to a decrease of the transition temperature by more than 4 K, but to an increase of the slope of the irreversibility line, thus resulting in higher irreversibility fields at low temperatures. The transport currents are significantly enhanced at 4.2 K for fields above 2 T.

MgB2 superconductors for applications

Abstract The paper presents up to date results on MgB 2 filamentary conductors and coated conductors from the point of view of their possible applications. Basic physical parameters of MgB 2 compound relevant to conductor design are introduced. The correlation between the slope of the irreversibility line, critical temperature and critical current density is discussed with respect to the conductor performance and applicability. The possible origin of the observed anomalous decrease of ac susceptibility at 50 K in copper clad in situ powder-in-tube MgB 2 wires is presented. Some numerical results on critical currents, thermal stability and ac losses of the MgB 2 multifilamentary and coated conductors with magnetic cladding of their filaments are given.

Composite Cu/Fe/MgB2 superconducting wires and MgB2/YSZ/Hastelloy coated conductors for ac and dc applications

We discuss the results of a study of MgB2 multifilamentary conductors and coated conductors from the point of view of their future dc and ac applications. The correlation between the slope of the irreversibility line induced by neutron irradiation defects and in situ structural imperfections and the critical temperature and critical current density is discussed with respect to the conductor performance and applicability. We debate the possible origin of the observed anomalous decrease of ac susceptibility at 50 K in copper clad in situ powder-in-tube MgB2 wires. Different conductor preparation methods and conductor architectures, and attainable critical current densities are presented. Some numerical results on critical currents, thermal stability and ac losses of future MgB2 multifilamentary and coated conductors with magnetic cladding of their filaments are also discussed.

YBa2Cu3O7−δ coated conductor deposited onto non-magnetic ternary alloy NiCrW RABiTS tape by in situ pulsed laser deposition

Abstract Pulsed laser deposition of YBa 2 Cu 3 O 7− δ (YBCO)/buffer (Y 2 O 3 , YSZ, CeO 2 ) heterostructures have been performed in situ onto recently developed non-magnetic oxygenation resistant NiCrW tape. The influence of the critical processing parameters on texture development are investigated and the issues involved in NiO formation and relation to the substrate surface quality are discussed. The roles of Ni poisoning YBCO as well as local cation disorder are considered as possible current limiting factors. X-ray diffraction has been used for macro-texture evaluation. Both buffers and YBCO layers show good biaxial alignment with ω and ϕ scans having best YBCO FWHM values of 4.0° and 6.5° respectively. A comparison is made with results achieved on industrial Ni 50 Fe 50 tape. The film morphology has been characterized using atomic force microscopy and scanning electron microscopy. The cation disorder has been studied by Raman spectroscopy. Critical temperatures of 90 K (Δ T c =5 K) have been measured. Direct transport as well as magnetic measurements shows the critical current density J c is 0.2 MA/cm 2 in self-field at liquid nitrogen temperatures.