LUA – 03 , presented at the 1998 Applied Superconductivity Conference, Palm Desert, California This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible. Oxide Barriers and their Effect on AC losses of Bi,Pb(2223) Multifilamentary Tapes Y.B. Huang°, M. Dhallé*, F. Marti*, G. Witz° and R. Flükiger*° *Dépt. Phys. Mat. Condensée, °Groupe Appliqué de Physique,University of Geneva, 1211 Genève, Switzerland St. Clerc and K. Kwasnitza Paul Scherrer Institute, Villigen, Switzerland AbstractThe transverse electrical resistivity in multi- filamentary 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 3 layer of < 2μm 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 15'000 A/cm 2 at 77K, 0T, i.e. still below that of our tapes without barriers (35'000 A/cm 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 more recently, periodic pressing. The presently developed tapes with oxide barriers are promising in view of their use in transformers and cables. I. INTRODUCTION The origin of AC losses in superconducting wires can be subdivided into two main categories: the hysteretic Bean-like magnetization of the superconductor on one hand and the induction of Ohmic currents in the matrix material on the other. The classical solution to reduce hysteresis losses is to subdivide the superconducting material into many filaments, thus diminishing the radial distances over which flux has to penetrate during an AC cycle. Twisting the filaments limits the relevant longitudinal length-scale and thus suppresses losses even further. In Ag-sheathed Bi(2223) tapes, however, these simple solutions are only of limited use. The excellent conductivity of Ag makes that already at relatively low magnetic AC field amplitude and frequency the induced current density in the matrix becomes comparable to the filaments super-current density. The resulting coherent shielding current pattern comprises both Ag matrix and Bi(2223) filaments. At moderate amplitudes or frequencies this gives rise to considerable Ohmic losses - the coupling losses - while at higher amplitude and/or frequency the advantages of the twisted multifilamentary structure are overturned and the hysteretic losses become comparable to those found in mono- core conductors, the so called saturation regime. The obvious solution to this problem is to increase the resistivity of the matrix material. The chemical reactivity of the Bi(2223) phase at high temperature unfortunately limits the extent to which this can be achieved by uniform doping of the Ag. For this reason we developed over the last few years composite-sheathed Bi(2223) tapes, in which each filament is surrounded by an electrically insulating barrier layer which is not in direct contact with the filaments, but suppresses the coupling currents flowing between them. In this paper we describe the various methods we used to this goal and we demonstrate their effectiveness by means of different experimental methods. II. TAPE PREPARATION A. In situ-oxidation Fig. 1 schematically depicts the three routes used for the introduction of insulating barriers. The first route consisted of the insertion of a non-Ag metallic layer in the billet, which was then deformed into mono-core wires, re-stacked and further deformed into a tape using our standard PIT process [1]. The metallic layer was oxidized in-situ after deformation. Various metals were tested but the results were non too successful, mainly because of strong interdiffusion between the metallic sheaths and subsequent chemical reaction with the Bi(2223) filaments [2]. B. Double oxide barriers The second route proves to be much more feasible. Here the initial billet consists of two Ag tubes of different diameter with a layer of pre-oxidized powder in between them. Further deformation is as described above. Again, various candidate materials were tested: TaO, MgO, MnO, Bi(2212) and BaZrO 3 . From all these oxides, BaZrO 3 turned out to be the best suited in producing highly resistive layers, either from a point of view of ease of deformation or of chemical stability at high temperatures [3]. However, whilst problems of unwanted reaction are absent, the standard PIT process Manuscript received September 14, 1998. This work was supported by the European Community Brite-Euram project BE-1563 and the Swiss PPM program.