R. Flukiger

Optimization of the preparation parameters of monofilamentary Bi(2223) tapes and the effect of the rolling pressure on jc

The transport critical currents of Ag-sheathed Bi(2223) tapes produced by rolling have been improved by an optimization of some crucial parameters of the tape fabrication process. New results about the correlations between the critical current density and the powder heat treatment temperature and time, the cold deformation steps, as well as the tape heat treatment temperature and time are presented. Moreover, among all the previously cited parameters, we have found that the pressure exerted on the tapes during the rolling deformation between the heat treatments has a strong influence on the transport properties: a maximum jc(77 K, 0 T) value of 30 kA cm-2 (averaged over the whole oxide cross section) on rolled Bi(2223) tapes was found for a rolling pressure of 0.6 GPa. At higher pressures, the formation of transverse cracks in the filament limits the critical current. A comparison between tapes of the same type, prepared either by rolling or by passing procedures, shows that the latter reach higher jc(77 K, 0 T) values (between 37 kA cm-2 and 43 kA cm-2) due to the possibility of applying higher pressures (2 GPa) without the formation of cracks.

Effects of Pb in the first stages of the Bi(2223) phase formation

The reaction of the precursor powders necessary for the formation of the final Bi(2223) phase was investigated with and without the presence of an Ag sheath by means of differential thermogravimetry (DTA/DTG). The nature of the endothermic peaks observed by DTA as well as the influence of Ag on their positions are discussed. By means of X-ray diffraction and energy dispersive X-ray analysis (EDX) on oil-quenched Ag-sheathed tapes it is shown that the Bi(2212) phase originally present in the precursor powders undergoes a structural modification which can be associated with the dissolution of Pb. This transformation occurs at a temperature near 836 degrees C in Ag-sheathed tapes. This work has been essentially focused on the role of Pb and its effects on the Bi(2212) phase in the first stages of transformation.

Bi,Pb(2212) and Bi(2223) formation in the Bi-Pb-Sr-Ca-Cu-O system

A new route has been found for producing the Bi2Sr2CaCu2Oy, phase with Pb substituted for part of the Bi (denoted Bi,Pb(2212)). By this novel route, Bi,Pb(2212) has been prepared in air at 860 degrees C. For comparison, this phase has also been prepared in argon at 740 degrees C by the previously known route. Bi,Pb(2212) transforms from a tetragonal to an orthorhombic structure as the Pb content grows. Once this phase was obtained with a nominal composition of Bi2-xPbxSr2CaCu2Oy, with Pb contents x=0, 0.2, 0.4 and 0.6, the Bi(2223) phase was formed starting from Bi(2212) and Bi,Pb(2212). Single-phase samples were obtained after two heat treatments at 849 degrees C with Bi,Pb(2212) and a proportion of Pb of x=0.4. This proportion corresponds to the one used previously by the authors to obtain high critical current carrying tapes in Ag-clad Bi(2223) tapes.

High pressure thermodynamic investigations on the Bi-Pb-Sr-Ca-Cu-O system

The lack of thermodynamic information concerning the Bi,Pb(2223)-liquid equilibrium has not made it possible, so far, to find any equilibrium formation route. The formation of Bi,Pb(2223) occurs via a non-equilibrium route, the grains growing from the local transient liquid. One of the main difficulties in solving the phase diagram at high temperature arises from an uncontrolled loss of some volatile elements and oxides. We have undertaken the investigation of the phase diagram at high pressure. A new prototype of High-Pressure DTA was built, operating up to 150 bar, and a dedicated High Isostatic Pressure furnace (up to 1 kbar, p(O/sub 2/)=0.2 bar) was used to anneal the samples without mass losses. We report on the effect of pressure on the Bi,Pb(2223) formation and decomposition, and on the influence of oxygen partial pressure under these conditions.

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.

Improvement of electromechanical properties of an ITER internal tin Nb3Sn wire

The critical current of an internal tin Nb3Sn wire developed by Oxford Instruments, Superconducting Technology for International Thermonuclear Experimental Reactor (ITER) (OST type-I, billet No. 7567) has been studied under axial strain at fields between 12 and 19 T at 4.2 K. Simulating the situation in a cable in conduit, where thermally induced compressive strain is important, a single wire (strand) was jacketed with AISI 316L stainless steel. The reinforced wire shows an important increase in em, the applied strain where Ic reaches its maximum, from 0.25% to 0.57%. In addition the irreversibility limit, eirr, is improved from 0.50% applied strain to >1.10%. It could also be shown that the Ic at zero intrinsic strain is almost identical. This demonstrates that jacketing does not influence the physical parameters of the original wire. Experimental data of the bare wire has been well fitted by different strain functions. However, it was not possible to model the data of the jacketed wire. There are indica...

Electrodeposition of biaxially aligned Tl-based superconductors on Ag tapes

An alternative preparation method of Tl(1212) and Tl(1223) biaxially aligned tapes on Ag was developed: electrodeposition. This method is based on the co-deposition of the metals by an electrochemical process followed by a high temperature heat treatment in order to form the superconducting phase. Nitrates of the metals are dissolved in DMSO and the deposition on conducting substrates is performed in a standard three electrodes cell. The thermal treatment is performed in a high isostatic pressure furnace (1 bar O/sub 2//50 bar He) in order to prevent Tl evaporation. The main advantage of this low cost technique is that it is very fast, (/spl sim/1 /spl mu/m/min) and can be easily scaled up to produce long length tapes in a continuous process. The desired stoichiometry is obtained by varying iteratively the concentration of nitrates in the starting solution. Moreover, the reaction temperature and time of the thermal treatment are drastically reduced with respect to the powder synthesis due to the amorphous form of the precursor. Excellent biaxial alignment is observed on both {110} Ag single crystal and textured Ag tapes, while epitaxial growth is observed on SrTiO/sub 3/ single crystal. T/sub c/ of 118 K and 60 K were measured resistively for Tl(1223) and Tl(1212) samples, respectively.

Effects of MnO additions on the formation of the (Bi,Pb)2Sr2Ca2Cu3O10-y phase

The effect of MnO additions on the formation of the (Bi,Pb)2Sr2Ca2Cu3O10-y phase has been studied in view of the mechanical reinforcement of the Ag sheath of (Bi,Pb)2Sr2Ca2Cu3O10-y superconducting tapes by alloying and/or dispersion hardening. DTA measurements showed that the onset temperature of partial melting occurring in the precursor powders is slightly lowered when MnO is added. From lattice parameter calculations as well as EDX measurements it follows that Mn substitution in the (Bi,Pb)2Sr2Ca2Cu3O10-y phase, if any, is limited to below 0.1 at.%. The results are compared to those obtained in a previous work on TiO2 added to (Bi,Pb)2Sr2Ca2Cu3O10-y.

The influence of TiO2 additions on the formation and the superconducting properties of the (Bi,Pb)2Sr2Ca2Cu3O10-y phase

In view of the mechanical reinforcement of the Ag sheath of (Bi,Pb)2Sr2Ca2Cu3O10-y tapes by dispersion hardening, the effect of TiO2 additions on the formation of the (Bi,Pb)2Sr2Ca2Cu3O10-y phase is studied. By means of DTA measurements, it is found that the onset temperature of partial melting in the precursor powders is lowered when TiO2 is added. XRD and EDX analysis showed that TiO2 additions result in the formation of SrTiO3 and a decrease of the amount of(Bi,Pb)2Sr2Ca2Cu3O10-y. Lattice parameter calculations as well as Tc determinations also show that Ti substitution in the (Bi,Pb)2Sr2Ca2Cu3O10-y phase, if any, is limited to low values. The interaction occurring between TiO2 and the precursor powders during the formation of the (Bi,Pb)2Sr2Ca2Cu3O10-y phase is described.