[en] Critical currents J_c(T,B) for virgin and four ²³⁵U doped Bi2223/Ag tapes irradiated with thermal neutrons (U/n treated) were measured in the temperature range T≥50 K and magnetic field B≤1 T. Below 97 K vortex pinning in U/n treated tapes increases with matching field B_phi, as evidenced by a weaker B dependence of J_c on increasing B_phi. Hence, the field B_max (F_pmax=J_cB_max) and the irreversibility field B_irr(F_p → 0) of U/n treated tapes are enhanced compared to those for virgin tape. Since the rate of the enhancement of B_max and B_irr with B_phi depends on T, one can tune the maximum enhancement at desired T

[en] The magnetoresistance R(T,B) of virgin and four ²³⁵U doped Bi2223/Ag tapes irradiated with thermal neutrons (U/n treated) has been measured in the temperature range T≥50 K and magnetic field B≤1 T. For B≥20 mT the pinning potentials U₀ of U/n treated tapes were enhanced compared to that of virgin tape. The relative enhancement U₀(B,U/n)/U₀(B, virg.) increased with matching field B_phi and persisted to higher B at higher B_phi. For U/n treated samples, U₀ showed universal variation with B/B_phi: U₀(B≤B_phi)∝(B_phi/B)^α with α∼0.5 and faster variation at higher B. The resistive irreversibility fields B_irr(T) of all U/n treated samples for T≤100 K were also enhanced: B_irr(T,U/n)/B_irr(T, virg.) increased with B_phi and the enhancement persisted to lower T for higher B_phi. The maximum enhancements of B_irr scale well with the corresponding enhancements of U₀(B)

[en] The critical current densities J_c and the activation energies U(T,B) for the thermally activated motion of vortices in four well-characterized Ag-clad Bi2223 and Bi2212 tapes have been determined. The observed J_c(77 K, B=0)≤39000 A/cm² belong to the upper class of the results obtained so far for these materials. Like in other BSCCO samples U shows a (T_c-T) and ∼B^-α variations with T and B respectively. The comparison of pinning potentials U₀(B)=U(T=0,B) for tapes with those for the other types (single crystal, thin film) of Bi2212 and Bi2223 samples shows that at lower field U₀ depends more on the type of the sample than on the particular compound. For the same B, U₀ systematically increase in order of single crystal, thin film and tape which indicates that in BSCCO compounds, in addition to anisotropy, the specific pinning centres (introduced during processing of the sample) affect the flux motion at lower B. This conclusion is supported with a weaker field dependence of J_c in tapes exhibiting higher U₀. At elevated B (∼10 T) all U₀ data seem to converge reaching values (U₀∼300 K) typical for pinning by oxygen vacancies. At lower B, the variations of U₀, J_c and the volume pinning force F_p with B for tapes seem consistent with the collective pinning of vortices. The lack of correlation between the magnitudes of J_c, U₀ and F_pmax is explained in terms of the percolative current path in these inhomogeneous materials

[en] Full text: Significant advances in development of the Ag-clad Bi-based superconducting tapes have been recently achieved by the Wollongong group. These include understanding of the effect of initial phase composition on sintering temperature, formation mechanism of high T_c phase, control of sausaging formation during rolling and effect of atmosphere on microstructure of the tapes. Techniques such as 'sandwich rolling' process have been developed to prevent the formation of sausaging and cracks in longitudinal direction of the Ag-clad Bi-based superconducting tapes. High J_c for Ag/Bi-2212 tapes achieved through MTG in the alternating oxygen and nitrogen atmosphere. While N₂ annealing on cooling ensures high T_c, O₂ treatment during melting period enhances the stability of 2212, allowing for grain growth with ample liquid formation. Direct observation of the interface achieved by removing Ag sheath with Hg alloy shows highly dense, textured and thin-film like structure. Ca₂PbO₄ was found to have significant effect of sintering temperature. Incorporation of Ca₂PbO₄ into 2212 phase can reduce the duration of 2223 tape process to below 100 h, resulting high J_c with a record performance in magnetic field. 23.4% of zero field J_c value was retained in 1T. Grain growth in 2223 tape must be well controlled in order to optimise J_c. Features of 2212 tape and 2223 tape are compared in terms of their processing, microstructure and electromagnetic properties. Potential strong links in the tape are proposed to be the low angle tilt boundaries where the Cu-O planes are well connected. The pinning potential, U_o, for 2223 tape, determined by using magnetoresistance measurements, is larger than that for the best 2212 tape and epitaxial thin films while U_o for the latter is, in turn, higher than 2212 single crystal. This may be attributed to the difference in dislocation density in these materials

[en] The effect of the phase compositions after the final stage of heat treatment on the critical current density (J_c) in Bi:2223/Ag tapes has been investigated. A high volume fraction of Bi:2223 has been considered the most critical factor in achieving a high J_c; however, it is extremely difficult to attain pure Bi:2223 phase via routine thermomechanical processing. Although a higher volume fraction of Bi:2223 phase can be attained using a higher processing temperature, it is always found to co-exist with Bi:2201 which resides at Bi:2223 colony boundaries. As these possess a plate-like morphology, the Bi:2201 joins weak links in a conductible chain. Bi:2201 can be eliminated using an annealing step (800-825 deg. C) at the end of the last thermal cycle, resulting in a significant improvement in J_c. At annealing temperatures below 800 deg. C, Bi:2223 decomposes into Bi:3221 and other cuprates, which degrade J_c further. Magnetization, AC susceptibility and resistivity measurement results confirm that the Bi:2201 and Bi:3221 act as weak links. (author)

[en] Critical current density J_c of an Ag-clad tape Bi₂Sr₂CaCu₂O_8+x has been studied in the temperature range 7-77 K and magnetic field B ≤ 6.5 T. For T ≥ 21 K, J_c(B) and the pinning force density F_p=J_cB show scaling behaviour with the scaling field B* corresponding to the field of maximum F_p. At intermediate fields (0.5≤B/B*≤2.5) J_c ∼ exp(-B^3/2), whereas well below and above B*, J_c shows the asymptotic B^-α variations with α approx. 0.2 and 3 respectively. The thermally activated flux motion and the thermal smearing of the pinning potential describe equally well the temperature dependence of B* for T ≤ 70 K. These findings show that in finite fields well-prepared tape behaves as homogeneous high-T_c superconductor (HTS). The implications of these results for the understanding of J_c in HTSs and the applications of tapes are briefly discussed. (author)

[en] With high upper critical fields, high temperature superconductors (HTSC) have been recognised as candidate materials for coil and magnet applications. High field devices, at one time or another, when operated close to their rated limits are often faced with instability problems which normally are electrical, magnetic or mechanical in nature. The determination of stability parameters, therefore, is of interest to the conductor designer which very much assists in the processing aspect of long length production of wires and tapes. Due to the morphology of the superconductor most HTSC devices made with Bi(Pb)-2223 precursor exist in tape form. Here the determination of stability parameters (for flux-jump and cryogenic stability) for multifilamentary Bi(Pb)-2223/Ag tapes are presented. Processing parameters such as intermediate deformation and filling factors have been found to play a crucial role, not only on the critical current density of the tapes but on the stability aspect in conductor design as well. (orig.)

[en] V-I characteristics of Ag-clad BPSCCO tape have been studied in the temperature (T) range 80-106K and magnetic fields (B) up to 60 mT. The zero-field J_c at 77K is 10100 A/cm². J_c showed power law variations with T and B, characteristic for the intragranular flux creep. The magnitude and variations of the slope resistance R_f with T and B are consistent with the flux flow in low fields. Very broad and featureless critical current distribution function (CCD) is ascribed to the variation in the flux pinning energies within BPSCCO. (orig.)

[en] A ''sandwich rolling'' process was developed to prevent the formation of sausaging and cracks in the longitudinal direction since the stress-strain state of the tape in ''sandwich'' rolling is the same as that of uniaxial pressed tape. Critical current densities of 3.2 x 10⁴ A/cm² at 77 K and 2.7 x 10⁵ A/cm² at 4.2 K and zero field Ag-sheathed Bi-based 2212 tapes have been achieved using a melt and atmosphere-controlled process. The comparison of pinning potential U₀(B)=U(T=0, B) for Bi-2212 tape and Bi-2223 tapes consisting of different fractions of the 2212 phase as well as Bi-2212 and Bi-2223 thin films shows that, for the same fields, the U₀ for good quality 2223 tapes is at least 1.3 times that for the best 2212 tape and epitaxial thin films, indicating that in BSCCO compounds, in addition to anisotropy, the specific pinning centres such as dislocations, introduced during processing, affect the flux motion at lower B. (orig.)

[en] The irreversibility fields B_irr and critical current densities J_c of undoped and Si and SiC nanoparticle doped (5, 10 and 20 wt%) MgB₂ tapes were measured in the temperature (T) range 2-38 K and in magnetic fields B ≤ 16 T. Whereas B_irr of undoped tapes varies smoothly with T, those of doped tapes show a change in slope around a crossover field B_cr which increases with nanoparticle content and also depends on their type. This indicates matching effect in vortex pinning, probably associated with Mg₂Si nanoprecipitates formed during heat treatment. Indeed, B_irr of doped tapes was enhanced in respect to that of undoped one with the highest enhancement for B_irr ∼ B_cr, but the enhancement remained high ∼1.4 even for B_irr >> B_cr (low temperatures). The variations of J_c and the pinning force density F_p = J_cB with B and T support the above findings