Effect of irradiation on the mechanical properties of commercially produced Nb3Sn tapes

The effect of neutron irradiation on the mechanical properties of commercially produced Nb₃Sn tapes has been investigated using scanning electron microscopy, sound velocity measurements, tensile and three-point bend tests. The elastic moduli show an increase on irradiation which is independent of dose in the range 4×10²¹ to 4×10²³ neutrons m^–2. The majority of the Nb₃Sn tapes show no evidence of significant amounts of plastic deformation prior to failure, which occurs by the intergranular fracture of the Nb₃Sn layers followed immediately by ductile overload failures of the niobium core. The latter changes to a more brittle failure on irradiation and in tapes containing ZrO₂ particles. The fracture stress decreases for doses up to 10²³ neutrons m^–2 but increases at higher doses. Irradiation reduces the critical stress intensity factorK _c, butK _c and the fracture stress are increased in tapes containing ZrO₂. These results are discussed in terms of various micro-structural features and previously determined radiation damage.     

Microstructure of Ag/Bi(Pb)-2223 Tapes Prepared by Solid-State Reaction

The processing of Ag/Bi(Pb)-2223 tapes via the prevailing solid state reaction was investigated. A precursor powder of the composition Bi_1.86Pb_0.26Sr_1.96Ca_1.95Cu_2.97O₁₀₊ containing mainly Bi-2223 phase was employed. Particular attention was devoted to the microstructural properties of the tapes subjected to rolling and subsequent heat treatment in one, two, and three steps, respectively. A sharp texturing gradient exists in the superconducting cores, and the repeated treatment leads to deterioration of the superconducting properties. The latter effect is ascribed to the formation of periodic defects arising during the repeated rolling of the sintered superconducting core.     

Development of high-Tc superconducting Bi-2223 tape joints

High-T _c superconducting joints between Ag-clad Bi-2223 tapes have been developed for persistent current applications. Two presintered tapes with one side of the silver stripped were lapped and then wrapped by a silver foil. The complex was uniaxially pressed followed by appropriate sintering to form a high-T _c superconducting tape joint. It was found that the ratio of critical currents through the joint to that of the tape,I _cj/I_c, depended on the uniaxial pressure and the sintering conduction. At liquid-nitrogen temperature 77 K,I _cj/I_c=99% has been achieved. Persistent current loops formed by Bi-2223 tapes have also been fabricated and tested. Joint resistance of a loop was determined to be 4×10^–13 between the decay time of 120 and 3600 sec.     

Long multifilament Bi-2223 Ag-sheathed superconducting tapes and solenoids

Multifilament Ag-sheathed BiPbSrCaCuO (2223) superconducting tapes containing 49 filaments were fabricated by the powder-in-tube route and the roll-anneal process. The transport critical current densityJ _c was 1.3×10⁴ A cm^–2 at 77 K and 7×10⁴ A cm^–2 at 4.2 K in self-field. A 12-m-long tape was used to construct superconducting solenoids (50, 28, and 14 mm internal diameters) generating dc fields 380–1070 G at 4.2 K. Measurements of the variation ofJ _c with field (0–1.6 T) and bend strain (0–5%) are used to explain the performance of the solenoids. The critical bend strain of tapes was about 1.5%.     

The Jc Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

Effects of Rolling Passes on the Transport Properties of 37-Filamentary AgAu-Sheathed Bi-2223 Tapes

Thirty-seven-filamentary AgAu-sheathed Bi-2223 tapes were fabricated by a powder-in-tube (PIT) process. And, the round wires (? 1.86 mm) were rolled to 0.35-mm tapes with 12, 7, 5, and 4 rolling passes through flat rolling, respectively. The influences of different rolling passes on the core density, deformation, and transport properties of Bi-2223/AgAu tapes were systematically investigated. It was noticed that after rolling, the Vickers microhardness of the superconducting core and deform homogeneity along both the horizontal and vertical directions on the cross section of seven-pass rolled tape were better than those on the tapes with other passes, which proved the larger core density and uniform deformation with the seven-pass rolling process. Meanwhile for the wires with 12 and 7 passes, the AgAu/superconducting core interfaces were much flatter. With the rolling passes decreasing from 12 to 4, the critical current density (J_c) first increased and then decreased. Due to the better homogeneity and flatter interfaces, J_c reached the maximum value of 17.3 kA/cm² on the seven-pass sample. Meanwhile, the enhancement of current capacities in magnetic field applied parallel to the Bi-2223/AgAu tape surface could also be recognized as the evidence of improving intergrain connections due to the higher density in seven-pass rolled tapes.     

Fabrication and Properties of Ag–Ni Bimetallic Sheathed (Bi,Pb)-2223 Tapes

10-meter-long Ag?CNi bimetallic sheathed (Bi,Pb)-2223 tapes with outer nickel sheath and inner silver sheath have been successfully fabricated by the ??Powder in tube?? technique. Microstructure and phase evolution studies by means of SEM and XRD, as well as critical current density (J _c ) measurements have been performed. It is found that the nickel sheath and dwell time in the first sintering process have great influences on the texture evolution, phase transformation and J _c of the Bi-2223/Ag/Ni tapes. Mono-filament (Bi,Pb)-2223 tape with a J _c of 6656?A?cm^?2 and 61-filament tape with a J _c of 12420?A?cm^?2 are obtained. Although using composite bimetallic sheaths can reduce production costs and improve mechanical properties of the Bi-2223 tapes, the Bi-2223 content and J _c of Bi-2223/Ag/Ni tapes are relatively lower than that of traditional Bi-2223/Ag tapes. Meanwhile, due to higher Bi-2223 content and better alignment of Bi-2223 grains, tapes with 61-filament have higher J _c than mono-filament tapes.     

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I-V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I-V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm⁻² during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I-V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.     

Pressing Processing Diminishes Outgrowth and Bridging for AgMg/Bi-2223 Tapes

Multifilamentary Bi-2223 tapes were fabricated by PIT, using a silver alloy sheath with 2.5% magnesium metal. Outgrowth and bridging are major disadvantages for some silver alloy sheathed Bi-2223 tapes. To solve the outgrowth problem, green tapes were pressed with different reduction rates and then sintered at high temperatures. Critical current I _c of the sintered tapes was measured at nitrogen temperature using the four-probe method. The morphology of the filament core was observed with scanning electron microscope (SEM) to investigate outgrowth of sintered tapes with different reduction rates. Outgrowth and bridging on cross and longitudinal sections of filaments were studied using TEM images. Experimental results suggest that pressing processing may diminish outgrowth and bridging. Number of outgrowths and cases of bridging are reduced when the reduction rate increases, but the slope of the reduction falls at large reduction rates. The I _c curve indicates that there is an optimum reduction rate at which I _c reaches a maximum. At the best reduction rate the amount of outgrowth and bridging is close to the lowest. Experimental results show that pressing processing can diminish outgrowth and bridging by as much as 50%. Therefore, proper pressing is an effective method for both diminishing outgrowth and bridging and enhancement of I _c.     

Fabrication and Electromechanical Characterization of Silver-Clad BSCCO Tapes

The powder-in-tube technique consisting of industrial processes such as wire drawing and rolling has been widely used to fabricate superconducting tapes. In the present investigation a novel technique was adopted to fabricate BSCCO 2223 tapes. Instead of wire drawing, the silver billet was reduced in size by groove rolling. Stress conditions during groove rolling were analyzed and appropriate changes were incorporated to optimize the deformation process. Subsequent thermomechanical treatment resulted in tapes with average critical current densities of 18,000 A/cm². Phase development and microstructural evolution during the thermomechanical treatment were studied using XRD, SEM, and TEM. The electromechanical properties of monofilament and composite BSCCO tapes were evaluated by subjecting them to in situ tensile tests. The strain tolerance of the composite was found to be better than that of the monofilament BSCCO tape.     

Transport critical current density of Ag-sheathed Pr-doped (Tl,Cr)Sr2CaCu2O7 superconductor tapes in applied magnetic field

Powder-in-tube Tl-1212 (Tl,Cr)-Sr-(Ca,Pr)-Cu-O/Ag tapes were prepared and processed with different thermomechanical treatments. The processing conditions caused the variation of the transport critical current density (J_c) in the tapes. Tapes subjected to rolling showed lower J_c compared to the pressed tapes. Uniaxial pressing is more effective in densifying the tape cores by forcing the grains into closer contact and enhances the connectivity between grains. All the tapes showed a majority phase of 1212. The existence of 1201 phase considerably reduced J_c in the rolled tapes. Intermediate rolling is not favorable in improving J_c for the irregular grains structure, in contrast to Bi-2223 superconductors with plate-like grains. The in-field behavior of the tapes indicates that they are dominated by weak links as J_c falls rapidly in low applied field (H<0.06 T). No anisotropic transport properties were observed for all the tapes. SEM images showed randomly oriented irregular grains making texturing difficult to achieve. Neither the grain size nor the morphology was changed significantly by the thermomechanical treatments. The observed irregular grains imply that high-angle tilt boundaries were formed with intergranular weak links. Grain morphology is suggested to be the key factor that limits the J_c in these tapes.     

Effect of Various Intermediate Deformation Techniques on Jc and Grain Texture for Ag/Bi-2223 Tapes

Three Ag sheathed Bi-2223 multifilamentary tapes were produced by a processing method that consists of two sintering treatments with an intermediate deformation, involving sandwich rolling (SR), pressing (P), or normal rolling (NR). The magnetic field dependence of the critical current density J _c was measured with the magnetic field H applied parallel to both the ab plane (H ab) and the c-axis (H c) of the Bi-2223 grains. Experimental results show that J _c of the pressed (P) tape (J _cP) for both H ab and H c is about 1.5–1.8 times higher than that for the NR tape (J _cNR) and the SR tape, although J _cSR is always larger than J _cP. The ratio of J _cSR/J _cNR for H c increases rapidly with the applied magnetic field and reaches a maximum of about 12 at ₀ H 900 T. The calculated density of the pinning force F as a function of magnetic field shows that curves of F for SR, NR, and P tapes all have their maximum F _max at different magnetic fields and the magnitudes of F _max are also different from each other. The SR tape has the largest value of F _max, while NR has the smallest. XRD analysis shows that an intermediate deformation can destroy the grain alignment, and the larger the deformation, the worse the grain texture will be. Our experimental results, however, clearly show that J _c for Bi-2223 multifilamentary tapes is independent of grain alignment. The significant differences in J _c for tapes processed using the three different intermediate deformation procedures are dependent on the density of the pinning force and cannot be attributed to the grain alignment. Our experimental results support the view that SR processing is the best method for fabricating Ag/Bi-2223 tapes of high quality.     

The J c Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

The stability of high-T c phase in Bi1.6Pb0.4Sr2Ca2Cu3Oz compounds prepared by hot isostatic pressing

By using a sequence of hot isostatic pressing (HIP) and intermediate cold pressing, it has been possible to produce high-density Bi-based superconductors containing almost 100% high-T_c 2223 phase, without any post-fabrication heat treatment. The X-ray data showed no decomposition of high-T_c phase in hot isostatically pressed Bi-based superconductors. By using this technique, Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_z (where Z≈10) superconductors could be densified to 6.06 g cm^?3 at an HIP temperature of 850 °C, and at an HIP temperature of 870 °C the density was close to 6.15 g cm^?3. These densities could be compared with about 3 to 4 g cm^?3 obtained for conventionally sintered compounds. X-ray diffraction data at various stages of fabrication, microstructure and electrical resistivity data are presented.     

Development of high-T c superconducting Bi-2223 tape joints

High-T _c superconducting joints between Ag-clad Bi-2223 tapes have been developed for persistent current applications. Two presintered tapes with one side of the silver stripped were lapped and then wrapped by a silver foil. The complex was uniaxially pressed followed by appropriate sintering to form a high-T _c superconducting tape joint. It was found that the ratio of critical currents through the joint to that of the tape,I _cj/I_c, depended on the uniaxial pressure and the sintering conduction. At liquid-nitrogen temperature 77 K,I _cj/I_c=99% has been achieved. Persistent current loops formed by Bi-2223 tapes have also been fabricated and tested. Joint resistance of a loop was determined to be ～4×10^?13 Ω between the decay time of 120 and 3600 sec.     

The influence of electroplastic rolling on the mechanical deformation and phase evolution of Bi-2223/Ag tapes

Electroplastic rolling (EPR) of Bi-2223/Ag superconducting wires was performed, where pulse currents were applied during rolling to introduce an electroplastic effect. It was found that the rolling force decreased significantly compared with the traditional rolling process. Furthermore, EPR favorably minimized the sausage effect. It is revealed that the electroplastic effect can facilitate the mechanical deformation of Bi-2223/Ag composites. Segments of the Bi-2223/Ag tapes were heat treated at 830 °C for different time periods. The phase assemblies of these samples suggest that current pulses contribute to faster transformation kinetics from the Bi-2212 phase to the Bi-2223 phase. In addition, a preliminary improvement of 28% of critical current density has been achieved in a fully processed tape with EPR.     

Transport critical current density of Bi–Sr–Ca–Cu–O/Ag superconductor tapes with addition of magnetic nanopowder γ-Fe2O3

The effect of sintering temperature on the transport properties of Ag-sheathed-(Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O₁₀–(γ-Fe₂O₃)_0.01 superconductor tapes prepared by the powder-in-tube technique with sintering time fixed at 50 h has been investigated. The maximum transport critical current density, J_c of 6490 A/cm² at 77 K, was observed at sintering temperature of 845 °C. A further single intermediate rolling step increases J_c to 9560 A/cm². Sintering temperature from 830 to 845 °C increases the 2223 phase content and resulted in improved J_c. At 850 °C, the content of 2223 phase decreased resulting in a corresponding decrease in J_c. X-ray diffraction patterns suggest that the 2212 phase reacts with non-superconducting phase such as CaCuO₂, (SrCa)₂CuO₃, CaO, and CuO to form the 2223 phase. Samples without γ-Fe₂O₃ prepared under the same condition showed a lower J_c with maximum at 1560 A/cm². Our results show that nanomagnetic γ-Fe₂O₃ addition improved J_c which supports previous calculations on the possibility of frozen flux superconductor with nanomagnetic addition in this class of materials.     

Fabrication and Properties of Powder-in-Tube Processed MgB2 Tapes and Wires

MgB₂ tapes were fabricated with MgB₂ powder and several sheath materials such as Cu, Cu-Ni, Fe, carbon steel (Fe-C) and stainless steel. High-density MgB₂ cores were obtained for these sheath materials. J _c of the as-cold rolled (non heat treated) tape significantly increased with increasing the cross sectional area reduction by the cold working. Hard sheath materials (Fe-C and stainless steel) are effective to enhance J _c values. These results can be explained by the densification of MgB₂ core. Non heat treated MgB₂/(stainless steel) and MgB₂/(Fe-C) tapes showed extrapolated J _c values of 300–450 kA/cm² at 4.2 K and zero field. MgB₂ tapes show anisotropy in J _c with respect to field orientation. This anisotropy can be explained by the MgB₂ grain orientation. Heat treatment after the cold rolling is effective to enhance J _c values. An order of magnitude higher J _c values were obtained for Fe-C and stainless steel sheathed tapes by the heat treatment. J _c values extrapolated to zero field of MgB₂/(SUS 316) and MgB₂/(Fe-C) tapes reached 1,000 kA/cm² at 4.2 K.     

Effect of starting precursor powders on microstructural development and critical current density properties of BSCCO 2223 tapes

Bi-2223/Ag tapes with different lead content (Pb=0.2–0.4) powders were fabricated. The microstructural development and J_c properties were studied with starting precursor powder prepared in different conditions. The experimental results indicate that the variations of lead content extremely influence the reactivity of precursor powders, which is closely related to the formation rate of 2223 phase, microstructure and J_c values of Bi-2223/Ag tapes. In addition, the particle size distribution of precursor powders has a large effect on the transport properties. By optimizing these powder parameters, J_c values above 60,000 A/cm² (77 K, 0 T) in short tapes were achieved.     

Effect of high valency cations on the (BiPb)2Sr2Ca3Cu4O12+δ compounds

We have produced the (BiPb)₂V _x Sr₂Ca₃Cu_4−y Ti _y O_12+δ compounds by a melt-quenching method. For two different sintering times (185 and 192 hours), the effects of vanadium adding and Ti doping on the structure have been investigated by electrical resistance, scanning electron micrographs (SEM), XRD patterns and magnetic hysteresis loop measurements. It has been found that the high-T _c superconducting phase, (2223), is formed in the samples annealed at 845 ^○C for 185 and 192 h, with concentration x=0.2 and y=0.05. However, with increasing Ti doping the (2223) phase gradually transforms into the (2212) phase. The hysteresis loop areas decrease with increasing Ti concentration and sintering time. Our data have indicated that the critical current, J _c , decreases with increasing magnetic field.