B. Hensel

A model for the critical current in (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes: The role of small-angle c-axis grain boundaries and of the texture

Detailed analyses of the microstructure of silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes by scanning electron microscopy and X-ray diffraction, as well as measurements of the critical current density and the electrical resistivity of the isolated superconductor cores, have been performed on a series of short samples with filament thicknesses of approx. 40–50 μm and jc (77 K, B=0) ≈ 15 000 A/cm2. The experimental data cannot be satisfactorily explained within the framework of the “brick wall” model for the current transport in highly textured polycrystalline high-temperature superconductors. We therefore propose a new model for Bi/Pb(2223) tapes, based on the frequent small-angle c-axis grain boundaries that serve as highly conducting connections for the current between neighbouring grains, without the necessity for the supercurrent to flow perpendicularly to the CuO planes. The model provides a semi-quantitative description of the field dependence of the critical current density and its anisotropy, and correlates them with characteristics microstructural features of the Bi/Pb(2223) tapes.

Distribution of the transport critical current density in Ag sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes produced by rolling

Abstract By using a strip-cutting technique, we directly measured the lateral distribution of the transport critical current density of Ag sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes. For various long tapes prepared by the rolling technique with a critical current density of 23 000 A/cm 2 at 77 K and 0 T, the local critical current density of longitudinal 0.2 mm wide strips was reproducibly found to vary from 18 000 A/cm 2 at the center to 46 000 A/cm 2 at the sides of the filament. In all cases, a symmetrical behavior of j c at both sides of the central axis was observed. The value of the transport j c of 46 000 A/cm 2 at 77 K and 0 T in the external strips of Bi(2223) tapes produced by rolling processes comes quite close to the highest reproducible values reported so far on short pressed tapes. The field dependence of the critical current densities of the individual strips does not depend on their position inside the tape. Thus the observed difference in j c between central and external strips is not primarily due to a variation of the degree of texturing. A higher degree of compression and a lower amount of secondary phases were found for the external strips with the highest critical current density.

Pressed and cold rolled Ag-sheathed Bi(2223) tapes: A comparison

Abstract High critical current densities in monofilamentary Ag-sheathed tapes of Bi(2223) have been obtained on tapes produced by different deformation techniques. For cold rolled tapes of > 1 m in length, promising critical currents of j c (77 K, 0 T)=18500 A cm -2 have been measured. These tapes have a relatively weak dependence of j c vs. B their corresponding value j c (77 K, 1 T) being 3550 A cm -2 . The present results on cold rolled tapes are compared to those of typical short uniaxially pressed tapes of j c (77 K, 0 T)=26000 A cm -2 and j c (77 K, 1 T)=5200 A cm -2 . This comparison reveals both for 4.2 and 77 K a weaker field dependence of j c for the rolled tapes with respect to the pressed ones. At zero field, the rolled tapes exhibit lower j c values, but reach higher j c values at high fields, the crossover occuring at 3 T for 77 K and at 14 T for 4.2 K. The j c (4.2 K, 15 T) value for rolled Bi(2223) tapes is 4.7×10 4 A cm -2 . The rolled tapes also exhibit a stronger anisotropy at 4.2 K, where the ratio j c⊥ / j c‖ at 10 T is 2.2 compared to 1.5 for pressed tapes. Scanning electron microscopy and X-ray diffraction analysis are extensively used to investigate the microstructure of the Bi(2223) phase in the two samples.

Hot rolling of silver-sheated Bi (2223) tapes

Abstract Silver-sheated Bi(2223) tapes have been prepared using a newly developed hot-roll apparatus where the rolls can be heated. up to 850°C. At 77 K and 0 T, a critical current density of 18 000 A/cm 2 has been measured on tapes after hot rolling at 800°C. Microstructural analyses and X-ray diffraction have been performed. The field dependence and the anisotropy of the critical current were investigated at 77 K. The hot-rolled tapes show a higher density and better grain alignment compared to cold-rolled tapes. The critical current density at 77 K and 1 T is 3500 A/cm 2 , i.e., it decreases by a factor of 5 will respect to the value in zero field. This is a considerable improvement compared to the published data on cold-rolled tapes where this factor is almost 7 for our reference tape and between 8 and 12 in the published data.

Mechanically induced flux pinning in (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Abstract It is shown that mechanical deformation enhances intragrain flux pinning in silver-sheathed tapes of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x (2223) material. Tapes with zero transport critical current density ( J ct ) - but nonzero magnetic hysteresis - were produced with prereacted 2223 powders. Comparisons of the hysteresis in tapes directly after a heat treatment and after a subsequent pressing step show a clear increase of the hysteresis, almost a factor of three at 20 K and 3 T, corresponding to a five-fold increase in J c . The J c values can be lowered again by an additional heat treatment. Bent platelets can be observed in the microstructure after, but not before, pressing, suggesting that dislocations that could have been introduced in the pressing step may act as pinning centers.

Preparation by in-situ reaction and physical characterization of Ag(Au) and Ag(Pd) sheathed (Tl, Pb, Bi) (Sr, Ba) 2Ca2Cu3O9−δ tapes

Superconducting tapes with high-purity substituted Tl(1223) powders (> 95 wt.%, Tc ∼120 K) were produced by a new method based on an in-situ reaction under high pressure of helium (25 bar He/0.5 bar O2). A Ag(20 wt.% Au) alloy showed to be a good sheath material for an in-tape reaction up to 990°C, even when involving partial melting of the oxide which favors the grain growth. Due to an interaction between Pd and the oxide, a Ag(6 wt.% Pd) alloy is much less suitable for tape preparation. Reproducible transport critical current densities of 9 kA/cm2 (77 K, 0 T) were obtained for Ag(Au) sheathed Tl(1223) tapes with in-tape-reacted powder, the highest value being 10 kA/cm2. The magnetic-field dependence of jc of such tapes was improved with respect to Ag sheathed tapes with pre-reacted Tl(1223) powders. Locally textured samples with up to 50 μm large plate-like grains were obtained after partial melting in Ag(Au) sheathed tapes.

TEM study of twist boundaries and colony boundaries in (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes

Abstract The grain boundaries in high-jc (Bi,Pb)2Sr2Ca2Cu3Ox silver-sheathed tapes have been investigated using transmission electron microscopy. The typical microstructural feature in these tapes are the colonies, which consist of grains with a common c-axis. The grains within a colony are piled up along the common c-axis and are separated by [001] twist boundaries. It was found that the rotation angle of these twist boundaries takes several definite values. Colony boundaries are formed between colonies having slightly tilted c-axes with respect to each other and are virtually free from amorphous material and intergranular phases. Two basic types of colony boundaries have been observed: the small-angle c-axis tilt boundary and the edge-on c-axis tilt boundary. The weak bonds between the BiO double layers are responsible for the mica-like crystalline structure of Bi2Sr2Can−1CunOx and their importance concerning the formation of colony boundaries is discussed.

Critical current density normal to the tape plane and current-transfer length of (Bi,Pb) 2Sr2Ca2Cu3O10 silver-sheathed tapes

Abstract We have measured the critical current density parallel (jc) and normal (j c n ) to the plane of monofilamentary (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes prepared on the laboratory scale (l ≤ 10 m) by the powder-in-tube technique with rolling as the only tape-forming process. For the range of critical current densities along the tape jc(T = 77 K, B = 0 T) = 20…30 kA/cm2 we obtain j c n ) = 2…3 kA/cm 2 normal to the tape plane. We have a directly measured the current-transfer length Lc, i.e. the length along the tape that is necessary for the distribution of the current over the filament cross-section. As an upper limit we find Lc ≤ 2 mm at T = 77 K for tapes with jc(T = 77 K, B = 0 T) > 20 kA/cm2. Transmission electron microscopy of the polycrystalline (Bi,Pb)2Sr2Ca2Cu3O10 filament confirms the dominant role of the frequent small-angle c-axis tilt grain boundaries (“railway switches”) for the current transport, and the observed low anisotropy ratio j c j c n ≈ 10 as well as the magnetic-field dependence j c n (B) agree well with the “railway-switch” model.

Aspects of the current understanding of the supercurrent transport in (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 silver-sheathed tapes—the “railway-switchr model

The “railway-switch” model describes the superconducting current transport in (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes under the basic assumption that small-angle c axis tilt grain boundaries (“railway-switch”) constitute strong intergrain links for the supercurrent in the textured filament [B. Hensel, J.-C. Grivel, A. Jeremie, A. Perin, A. Pollini, and R. Flükiger,Physica C 205, 329 (1993)]. We give an overview of the model and some recent experimental results with the objective to identify the mechanisms that limit the critical current density. The measurements have been performed on monofilamentary “powderin-tube” samples [Jc(T < 77K, B < 0 T) < 20..30 kA/cm2] that were prepared in long lengths by rolling as the only tape-forming process. We conclude that the low intragrain critical current density jcc along the c axis (or the even lower critical current density jct across twist boundaries or intergrowths) is the dominant limitation for the transport critical current in high-quality tapes. We discuss possible starting points for a performance improvement of the (Bi,Pb)2Sr2Ca2Cu3O10 silver-sheathed tapes for applications.

The effect of splayed columnar defects on the magnetisation of Bi2Sr2Ca2Cu3O10/Ag tapes

Abstract Columnar defects are known to provide excellent flux pinning sites in high Tc superconductors. The changes in pinning properties of Bi2Sr2Ca2Cu3O10/Ag tapes due to perfectly splayed amorphous tracks were examined measuring the DC magnetisation versus temperature T and magnetic field H. We compare the hysteretic magnetisation ΔM(T,H) and the magnetic relaxation rate S(T,H) before and after irradiation. The data show how played columnar defects indeed increase the pinning strength at high temperature, but enhance vortex motion at low temperature. We attribute the cross-over between these two regimes to a change in vortex dimensionality.