Ivica Kušević

High-transport critical current density above 30 K in pure Fe-clad MgB2 tape

Abstract Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg+2B mixture was used as the central conductor core and reacted in situ to form MgB 2 . The tapes were sintered in pure Ar at 800°C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 μm. The Fe-clad tape shows a sharp transition with transition width of ΔT c of 0.2 K and a T c0 of 37.5 K. We have achieved the highest transport critical current reported so far at 1.7×10 4 A/cm 2 for both 29.5 K in 1 T and 33 K in zero applied field. Resistivity temperature dependence and transport critical current were also measured in magnetic fields applied perpendicular and parallel to the tape plane. Not only is the use of an Fe sheath necessary for the successful processing of in situ reacted powder-in-tube MgB 2 , it confers on the finished wire the additional benefit of magnetic screening.

Effect of silver on the processing and properties of Bi-2223Ag tapes

The effect of Ag addition and Ag sheath on the processing and properties of Ag-clad Bi-2223 tapes has been studied. Ag addition in the Bi-2223 tapes lowers the annealing temperature and accelerates the formation of 2223 phase. Ag is inert to the oxide core and exists as an isolated phase within the tape. However, its undesirable shape and large size cause grain misorientation and hence leads to a decrease of Jc. For Ag-sheathed Bi-2223 tapes, the critical strains for bend tests show a dependence on the tape thickness, the number of filaments and Agoxide volume ratio, attributable to the improved strength and flexibility of the special interfacial layer between Ag and oxides. The results of magnetoresistance and the V-I curves of a well defined tape after correction for the parallel conduction through the Ag-sheathing and the core show a similar behaviour as single crystals and the epitaxial thin films. The effective activation energy for the flux creep is larger than those for the single crystals and thin films. Long length Ag-sheathed Bi-based superconducting multifilamentary tapes up to 45 m have been fabricated using powder-in-tube technique. A Ic up to 15 A and Jc of 8500 A/cm2 at 77 K over the entire length has been achieved. The fluctuation of Ic along the length is less than 10%.

Flux pinning and critical currents in polycrystalline MgB2

The results of accurate electrical resistivity and ac susceptibility measurements for a well-characterized, single phase MgB2 sample are reported. The onset of electrical resistivity in magnetic fields B less than or equal to I T is thermally activated R exp(-U(T, B)/k(B)T) with U-0(B) = U(T = 0, B) infinity 1/rootB for B greater than or equal to 0.2T. The pinning potential U-0(B less than or equal to I T) is similar to that for YBa2Cu3O7 compound, but the region of flux liquid is quite narrow and the uppercritical field Bc(2) fairly low. High-resolution ac susceptibility measurements show homogeneous superconducting behavior (no Josephson coupled grains) and yield true zerofield critical cur-rent density J(c) similar to (1 - T/T-c)(n) with n approximate to 1.2. The comparison of literature results for J(c) with our ones shows pronounced mass density dependence of J(c) in bulk untextured MgB2 samples.

Effect of silver on phase formation and superconducting properties of Bi-2223/Ag tapes

The effect of Ag on the phase formation and superconducting properties of Ag-clad Bi-2223 tapes has been studied. TEM studies reveal that the grains adjacent to the Ag-sheath are nearly single 2223 phase with their c-axis perpendicular to the Ag-sheath, but in the interior of the superconductor core, an intergrowth of the 2223/2212 polytypoids within the grain and misalignment become evident. Ag addition in the Bi-2223 tapes lowers the annealing temperature and accelerates the formation of 2223 phase. However, its undesirable shape and large size cause grain misorientation and hence leads to a decrease of J/sub c/. The improvement in bend strain is attributable to the interfacial layer adjacent Ag sheath. The results of magnetoresistance of a well defined tape after correction for the parallel conduction through the Ag-sheath and the core show that the effective activation energy for the flux creep is larger than those for the single crystals and thin films. But the dissipative behaviour is similar to single crystals and the epitaxial thin films.<<ETX>>

Vortex state and flux pinning in Bi2223/Ag tape

Abstract The resistivity and critical current density Jc of an almost single phase Ag-clad (Bi,Pb)2Sr2Ca2Cu3O10-y (hereafter Bi2223) tape have been measured in the temperature (T) range 77–120K and magnetic field (B) up to 1T. Within the explored T and B range, Jc(B) and the pinning energy U(B) obey the relation UJ c ∝ 1 B predicted by the Larkin-Ovchinnikov theory of the collective pinning. The observed, very low maximum pinning force densities Fpmax=(JcB)max in Bi2223 tapes are explained in terms of the percolative current flow in these materials. The implications of these findings on the study of vortex states in Bi2223 tapes are briefly discussed.

Flux pinning in Ag-clad Bi-2223 tapes containing different fractions of Bi2212 phase

Abstract Magnetoresistance, V-I curves and ac susceptibility of well-characterized Ag-clad (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+y (Bi2223) tapes containing different Bi2212 contents (2–25%) have been measured (T) range 60–110K and magnetic fields B≤1T. The transport critical current densities J c (77K,0T) ranged up to 29000A/cm 2 whereas those deduced from ac susceptibility (J s ) were up to 39000A/cm 2 . For T≥76K J c and J s of all tapes show (1−T/T c ) n and initial B -0.5 variations with T and B respectively. Since the exponent n and the suppression of J c with B increase with increasing Bi2212 content, this phase deteriorates the performance of tapes. The pinning potentials (U) corroborate the above conclusion. Since U 0 (B≤1T) are larger then those of Bi2223 epitaxial films, a low fraction of a well-connected material limits J c in tapes.

Differential resistance and critical current distribution in an Ag-clad (BiPb)2Sr2Ca2Cu3O10+y tape

Abstract V-I characteristics of well characterised Ag-clad BPSCCO tape have been studied in the temperature (T) range 80–106 K and magnetic fields (B) up to 60 mT. The zero field critical current density at 77 K was about 10 000 A cm−2. The variations of Jc with B (B−0.5) and T[( 1 − T T c ) 1.3 ] probably arise from the intragranular flux creep. The magnitude and the variation of the differential resistance (Rƒ) of BPSCCO core with T are consistent with those arising from the flux flow in low B. Very wide distribution of critical currents (CCD) within the core is tentatively ascribed to a broad distribution of the flux pinning energies in BSCCO.

Critical current density and pinning energy in Ag-clad BPSCCO tapes

Abstract 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 × 10 4 A/cm 2 at 77 K and 2.7 × 10 5 A/cm 2 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 U 0 ( 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 U 0 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 .

FLUX PINNING IN AG-CLAD (BI,PB)2SR2CA2CU3O10+Y TAPE

Abstract The resistance of a well-characterized Ag-clad (Bi,Pb)2Sr2Ca2Cu3O10+y (hereafter Bi2223) tape containing about 2% of Bi2212 phase has been measured in the temperature (T) range 77–120K and magnetic field (B) up to 1T. The measurements were performed on two sections of the same tape showing very different critical current densities (Jct(77K,0T)=6 100 (SA) and 23 000 A/cm2 (SB), respectively as well as on a bare core of sample SB. All resistances showed an initial exp(-U(T,B)/kBT) variation enabling the determination of the pinning potential U(T,B). The values of U0(B)≡U(T→0,B) were nearly the same for all samples and showed rather modest anisotropy for B⊥c and B∥c consistent with the imperfect grain alignment in the studied tape. Since U0(B) of the tape is larger than those of the best Bi2212 and Bi2223 epitaxial films the intragranular flux pinning is not limiting Jc of Bi2223 tapes in low fields.

Scaling of Jc-B curves in BSCCO/Ag tapes

Abstract Critical current density Jc of high-Jc, Ag-clad Bi2Sr2CaCu2O8+x and (Bi,Pb)2Sr2Ca2Cu3O10+y tapes has been studied in the temperature (T) range 4.5–92 K and magnetic field B ≤ 1 T. Both types of tapes exhibit clear scaling of Jc and the pinning force density Fp = JcB with B for T T c > 0.4 . The scaling field B ∗ (Fp reaches its maximum for B = B ∗ ) shows an exp (− T T 0 ) variation with T0 ≈ 11 K for T T c . The implications of these results for the understanding of Jc in high-Tc superconductors and the practical applications of BSCCO/Ag tapes are briefly discussed.