F. Marti

Reversible suppression of the so-called fishtail effect in ultra pure single crystals of YBa2Cu3O7−δ achieved by proper oxygenation

We report on the absence of the so-called fishtail or peak effect in the magnetisation curves of properly oxygenated high purity (5N) single crystals of YBa2Cu3O7−δ. As we recently reported [1] high pressure oxygenation procedures are suitable to produce a monotonous dependence of the critical current density with the magnetic field. In this article we report furthermore how to reversibly produce or remove this effect on the same crystal by proper oxygenation methods. Along with the high purity of the samples, — due to crystal growth in non reactive BaZrO3 crucibles —, this proves that at least in pure samples the fishtail effect is due to different oxygenation states and/or oxygen distributions. In less pure samples, however, other sources for this anomaly like metallic impurities can act as an additional source for the fishtail anomaly.

Reduced filament coupling in Bi(2223)/BaZrO3/Ag composite tapes

Multifilamentary Bi(2223)/Ag tapes often exhibit AC loss levels comparable to those measured in monofilamentary samples, which is partly due to the large coupling currents induced in the low resistive sheath material. Surrounding the individual filaments by electrically insulating barrier layers suppresses these currents and strongly reduces the coupling. We demonstrate this effect with various types of magnetic and self-field AC loss measurements on a series of Bi(2223)/BaZrO3/Ag tapes. We also discuss the influence of barrier thickness, twist pitch length and filament arrangement on the measured losses in these composite conductors.

Effects of the reaction time on various properties of tapes

The mechanisms leading to a maximum of as a function of reaction time in monofilamentary tapes are investigated. Various properties of tapes previously reacted for 40 h in air at , rolled and submitted to additional reaction during times varying from 40 to 240 h were measured. The evolution of the morphology, phase assembly, stoichiometry, weight, critical temperature, critical current and irreversibility line is described. XRD and SEM analysis reveal no decomposition of the Bi(2223) phase and no noticeable grain growth after the 40 h additional time. Pb losses up to 40% were detected by means of thermogravimetry and EDX as function of reaction time, while an increase of from 107.25 to 109 K was observed. On transport and magnetic measurements, it is found that the observed maximum of at about 180 h is the result of two competing effects: (a) the improvement of the quality of grain boundaries and (b) the decrease of the pinning efficiency in the grains.

The analysis of current distributions in filamentary conductors: the influence of intergrowths

Filament isolation is an extremely important factor in multi-filament conductors. However, filament-bridging superconducting intergrowths have often been observed. We show from magnetisation, transport and Hall probe experiments on a range of BSCCO-2223 conductors that the effects of filament bridging are substantial. We infer that the critical current densities within the intergrowths, and also across their grain boundaries, are extremely high.

Progress in critical current density of long Bi(2223) tapes deformed by periodic pressing

Critical current densities of multifilamentary Ag-sheathed Bi(2223) up to about have been achieved at 77 K and self-field for lengths of several metres using a new route: periodic pressing. This corresponds to an increase by 30-40% compared with the values obtained for conventionally rolled tapes starting with the same powders. Several pressing steps have been introduced during the anneal instead of only the standard rolling step (based on previous studies performed on both mono- and multifilamentary tapes). In contrast to earlier attempts by pressing techniques, periodic pressing is a practical and scaleable process for the fabrication of long lengths of Bi(2223) conductor as the standard intermediate rolling step. Engineering critical current densities of have successfully been obtained for tapes with high filling factor (40%). Because of the possibility of applying three or four pressing steps during the whole annealing period, the choice of the first annealing time becomes less stringent, which may constitute an advantage.

High critical current densities in long lengths of mono- and multifilamentary Ag-sheathed Bi(2223) tapes

By optimizing the tape fabrication process, high critical current densities over long length (up to 25 m) of Bi,Pb(2223) Ag-sheathed tapes were reproducibly obtained. The critical current densities of multifilamentary tapes (up to 55 filaments) are typically in the range of 25-30 kA/cm/sup 2/. At 77 K, up to 35 kA/cm/sup 2/ were recently measured for a monofilamentary tape of 0.5 m length. Detailed measurements of the transport properties of these tapes as well as of test pancakes and currents leads will be presented.

Correlation between the normal state resistivity and the critical current density of Ag sheathed Bi(2223) tapes

Abstract The normal state resistivities of Ag/Bi(2223) tapes with critical current densities between 7 and 34 kA/cm2 at 77 K were measured after chemical etching of the silver sheath surrounding the superconducting filament. A strong correlation between the critical current density and the normal state resistivity was observed, their product Jc × ρ being constant, regardless of the sample quality. This observation is in quantitative agreement with the railway-switch and the parallel pipe model which have been proposed for the explanation of the current transport mechanism in Bi(2223) tapes. Moreover, an extrapolated critical current density value exceeding 1.3 × 105 A/cm2 at 77 K has been deduced for a hypothetical Bi(2223) tape in which all grains are connected by railway switches.

Observation of the Bi,Pb(2223) reaction mechanism and alternative ways of producing tapes with new filament configurations

Investigations on the Bi,Pb(2223) phase formation confirm a mechanism based on nucleation and growth. The same mechanism was found to hold under various external conditions: a) in pressed samples, b) in Ag sheathed tapes, c) in air or in reduced oxygen partial pressure and d) with or without the presence of Pb. A high temperature neutron diffraction investigation on monofilamentary Bi,Pb(2223) tapes shows that this phase remains stable during the cooling process after reaction. On cooling, an enhancement of the Bi(2212) phase is observed, which occurs at the expense of the other phases, (Sr,Ca)/sub 14/Cu/sub 24/O/sub 41/ and Bi(2201). New tape configurations are presented, in view of a) the reduction of anisotropy (on both, tapes or wires) and b) the reduction of AC losses. These configurations require the use of nonconventional techniques, e.g. two-axis rolling and/or periodic pressing. Using periodic pressing, j/sub c/(77K,0T) values of 35,000 A/cm/sup 2/ have been obtained in Bi,Pb(2223) multifilamentary tapes of lengths >2 m.

Development of Ag-Sheathed Bi(2223) Tapes with Improved Microstructure and Homogeneity

The development of the fabrication process of Ag-sheathed Bi(2223) tapes has been carried out in order to improve their transport and mechanical properties, as required by the power applications which are so far under study. Critical current density values of 28 kA/cm2 at 77 K have been achieved on long multifilamentary Bi(2223) tapes, with a fabrication process that has been successfully employed in the fabrication of samples longer than 50 m. The microstructure and homogeneity of Ag-sheathed multifilamentary Bi(2223) tapes has been markedly improved by employing an alternative deformation technique. In a substantial part of the fabrication process, swaging, drawing, and rolling have been replaced by deformation with an active turks-head machine, which allows the deformation of rectangular shaped wires. At present, critical current densities in excess of 25 kA/cm2 at 77 K have been achieved on long samples prepared with this technique. Moreover, innovative filament configurations have been employed for the fabrication of square-shaped Bi(2223) wires with reduced anisotropy and with critical current densities exceeding 20 kA/cm2 at 77 K.

Oxide barriers and their effect on AC losses of Bi,Pb(2223) multifilamentary tapes

The transverse electrical resistivity in multifilamentary Ag/Bi, Pb(2223) tapes is considerably enhanced after introducing inert oxide barriers, a new concept in which each single filament is surrounded by a highly resistive BaZrO/sub 3/ layer of <2 /spl mu/m thickness. With these oxide barriers, we have so far obtained a shift of the AC loss maximum from 5 Hz to >100 Hz. This corresponds to a marked lowering of AC coupling losses. The highest critical current density of these tapes is actually 15000 A/cm/sup 2/ at 77 K, 0 T, i.e. still below that of our tapes without barriers (35000 A/cm/sup 2/). The fabrication processes leading to Bi,Pb(2223) tapes with oxide barriers is described, with an emphasis on new deformation processes developed in our laboratory for the fabrication of long multifilamentary Bi,Pb(2223) tapes, comprising four roll (or two-axes) rolling and periodic pressing. The developed tapes with oxide barriers are promising in view of their use in transformers and cables.