Thermomechanical processing and transport current density of Ag-sheathed (Tl,Cr)Sr2(Ca0.9,Pr0.1)Cu2O7 superconductor tapes

Powders with nominal composition (Tl,Cr_0.15)Sr₂(Ca_0.9,Pr_0.1)Cu₂O₇ (Tl-1212) and T_c90 K were used to fabricate Ag-sheathed superconducting tapes employing the powder-in-tube (PIT) method. The tapes were subjected to intermediate mechanical rolling or pressing. Conditions that enhance the transport critical current density (J_c) of the tapes were investigated. Optimum annealing temperature and period together with uniaxial pressing are necessary to increase J_c of the Tl-1212/Ag tapes. Annealing at 910 °C for 0.5–1 h enhanced the 1212 phase formation and improved intergranular connectivity between grains, as well as to provide healing for the fractured structure caused by deformation process. A relatively longer annealing time at higher temperature gave rise to secondary phases and resulted in the decrease of J_c. Mechanical uniaxial pressing greatly densified the tapes core and thus led to closer contact between grains. At liquid nitrogen temperature and zero field, J_c of the pressed tapes annealed at 910 °C for 1 h is 3060±127 A cm⁻². The initial drastic drop of J_c in low fields (<0.06 T) indicates the performance of the tapes is limited by weak links. No significant anisotropic transport properties were observed in applied magnetic field. This is due to the absence of texturing in the tapes as the grains are randomly oriented revealed through SEM micrographs.     

Magnetic-field angle dependent critical current densities and flux pinning in commercial YBCO tapes at liquid nitrogen temperatures

The magnetic-field angle dependence of critical current densities J_c(H,θ) in commercial YBCO tapes grown by MOCVD and MOD was examined at liquid nitrogen temperatures. We first measured J_c(H,θ) in MOCVD-YBCO tapes at 70 and 77.3 K in fields up to 2 T using both transport and inductive (the third harmonic voltage) methods and compared the results. It was observed that, in low magnetic fields, the transport measurements gave higher J_c than the inductive ones; however, in high fields they agreed well, which is well explained by the effects of weak links due to low-angle grain boundaries. We then investigated J_c(H,θ) in MOCVD- and MOD-YBCO tapes at 77.3 K in fields up to 2 T. All the tapes exhibited peaks at H//ab in the shape of Mount Fuji, which shows that small random pinning plays a major role. However, an anisotropic scaling analysis showed that the flux pinning mechanisms in those tapes were different, resulting in distinctive angular behaviors of J_c(H,θ). It is suggested that the difference in the flux pinning mechanisms of the two types of tapes came from the different sizes of point defects originating from the growing processes.     

Improvement of Jc and Grain Alignment of Bi-2212/Ag Multilayer Tapes by Pre-Annealing and Intermediate Rolling Process

The high transport critical current density (J _c ) > 500 kA/cm ² at 4.2 K, 10 T is obtained in the Bi-2212/Ag multilayer tapes fabricated by using PAIR (Pre-Annealing and Intermediate Rolling) and melt-solidification process. This J _c value is twice higher than existing high-quality Bi-2212/Ag tapes (250 kA/cm ² ). By applying PAIR process to Bi-2212/Ag tapes, Bi-2212 grain alignment is much improved and a large J _c enhancement is achieved. J _c has been increased strongly by performing pre-annealing at 840°C in oxygen (1 atm) and intermediate rolling with 25% deformation.     

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.     

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.     

Effect of processing history on the properties of Ag-based tapes

The Bi-system tapes and coils were prepared by the powder-in-tube method. The effect of processing and heat treatment on the properties of the tapes and coils was studied. It is found that the shearing stress produced by the rolling process may destroy the 2223 phase, and the destroyed 2223 phase cannot be recovered by heat treatment, whereas the pressing stress produced by the uniaxial pressing process only makes the 2223 phase grains break, and the broken 2223 phase can be closed by the heat treatment. TheJ _c values of the rolling samples, pressing sample, and the coil with a diameter of 35 mm are over 1.3×10⁴, 2.5×10⁴, and 4×10³ A/cm² (77 K, 0 T), respectively.     

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.     

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.     

Phase and Texture Evolution of Ag/Ni Composite-Sheathed Bi-2223 Tapes with Different Thickness

The influence of green tape thickness on the Bi-2223 phase formation and texture evolution in Ag/Ni composite-sheathed tapes fabricated by the “powder-in-tube” technique has been studied. Microstructural observations by SEM as well as critical current density (J _c) measurements at 77 K, 0 T have been performed to analyze the performance of the tapes. The results show an important influence of the green tape thickness on the critical current depending on the content and texture of Bi-2223 phase. The J _c increases with decreasing thickness. Moreover, texture measured by omega scans shows that the texture of the Bi-2223 phase is significantly influenced by the thickness of the green tape after the first and final sintering processes. Alignment of Bi-2223 grains in the thin tapes is much better. Higher performance of Ag/Ni composite-sheathed Bi-2223 tapes can be obtained by controlling the thickness of the green tapes.     

Weak Links and Phase Slip Centers in Superconducting MgB2 Wires

MgB₂ superconducting wires were produced by the Mg diffusion method. Scanning electron microscopy (SEM), optical microscopy, dispersive X-ray analysis (EDS), and XRD diffraction were used to study the physical structure and content of the wires. Magnetic properties (T _c ^m, H _c1, H _c2, J _c by the Bean model) were obtained with a SQUID magnetometer, and transport properties (T _c ^r , H _c2, resistivity and residual resistivity ratio) were measured using a standard four-lead configuration. The V-I characteristics of the wires close to the critical temperature showed a staircase response, which was attributed to the presence of weak links, creating phase slip centers. The origin of those weak links is discussed in relation to their formation and structure.     

Weak Links and Phase Slip Centers in Superconducting MgB2 Wires

MgB₂ superconducting wires were produced by the Mg diffusion method. Scanning electron microscopy (SEM), optical microscopy, dispersive X-ray analysis (EDS), and XRD diffraction were used to study the physical structure and content of the wires. Magnetic properties (T _c ^m, H _c1, H _c2, J _c by the Bean model) were obtained with a SQUID magnetometer, and transport properties (T _c ^r , H _c2, resistivity and residual resistivity ratio) were measured using a standard four-lead configuration. The V-I characteristics of the wires close to the critical temperature showed a staircase response, which was attributed to the presence of weak links, creating phase slip centers. The origin of those weak links is discussed in relation to their formation and structure.     

Influence of filament thickness, thermal process and magnetic field on misalignment angles in multifilament Bi2212 tapes

Misalignment angles in multifilament Bi2212 tapes have been studied by transport current density (J_c) measurements. Transversal misalignment angles depend on the filaments thickness due to the crystal growth along the Bi2212/silver interface. Therefore, the thinner the filaments, the higher the J_c.A dynamic thermal treatment improves grain alignment inside the tapes due to preferential crystal growth along the thermal gradient direction. Furthermore, melting in high magnetic field reduces the misalignment angle due to magnetic anisotropy effects. Therefore, good transport J_c performances can be expected for thicker multifilament Bi2212 tapes prepared using a combination of the dynamic process and melting in a high magnetic field.     

Optimization of fabrication conditions of MgB2/Fe superconducting tapes using response surface methodology

We have optimized fabrication conditions of MgB₂/Fe superconducting monofilament tapes fabricated using ex-situ powder-in-tube method without any intermediate annealing. Influences of three effective parameters (annealing temperature, annealing time and argon (Ar) gas atmosphere pressure during the heat treatments) on the transport critical current densities (J_c) were investigated. The samples were characterized using X-ray diffraction, scanning electron microscope, energy dispersive X-ray spectrometer, optical microscope, critical transition temperature and J_c measurements. Response surface methodology was used to optimize the conditions for the maximum J_c of the samples and to understand the significance and interaction of the factors affecting J_c. 3-level-3-factor central composite design was employed to evaluate the effects of the fabrication conditions parameters such as annealing temperatures (850–950?°C), annealing times (30–120?min.) and Ar pressures (1–5 bars) on J_c of the samples. Based on the analysis of ridge max, the optimum fabrication conditions were as follows: annealing temperature 948?°C, annealing time 58?min. and Ar pressure 3 bars.     

Superconducting Performance of Ex-situ SiC Doped MgB2 Monofilamentary Tapes

The superconducting performance of the ex-situ SiC doped MgB₂ monofilamentary tapes are reported. Polycrystalline powders of MgB₂ doped with 5 and 10 wt% SiC were synthesized by a conventional solid-state reaction route and characterized for their superconducting performances. It was found that the superconducting parameters viz., upper critical field (H _c2), irreversibility field (H _irr) and critical current density (J _c) were improved significantly with SiC addition. It was also found that relatively lower synthesis temperature resulted in further improved superconducting parameters in comparison to higher synthesis temperature. Thus, synthesized powders are used for the fabrication of ex-situ powder-in-tube (PIT) monofilamentary tapes. The superconducting performance in terms of critical current density (J _c), being determined from both magnetization (J _cm) and transport (J _ct) measurements, was improved significantly. In particular, the SiC doped MgB₂ tapes (fabricated using 700 °C heat treated bulk powder) exhibited the transport J _ct of nearly 10⁴ A/cm² under applied fields of as high as 7 Tesla. Further, it was found that the J _ct anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB₂ tapes was responsible for decreased anisotropy and improved J _c(H) performance.     

Process Optimization for Ag-Sheathed Bi-2212 Superconductors

The powder-in-tube (PIT) process has been widely used to fabricate long lengths of superconducting wires and tapes. However, it has been noted that the performance of long lengths of superconductor is variable and difficult to replace. To help pinpoint the possible sources of variation, a systematic study of the effect of processing variables, including deformation and heat treatment procedures, on the electrical properties of the Bi-2212 tapes at cryogenic temperatures was conducted. In addition, the effect of varying powder particle sizes was examined. For tapes fabricated by different thickness reduction schedules, significant variations in critical current density (J _c) were observed. It is concluded that a combination of small roll diameter and small reduction-per-pass produces tapes with highest J _c. Moreover, the maximum J _c occurred in a narrow temperature range when melt processing was done in pure oxygen. Microstructural examination was used to correlate J _c and both the volume fractions of a nonsuperconducting second phase and the Bi-2212 grain orientation.     

Epitaxial Film Growth of Tl0.78Bi0.22Sr1.6Ba0.4Ca2Cu3O9 on Rolling Assisted Biaxially Textured Nickel Substrates with YSZ And CeO2 Buffer Layers

Epitaxial film growth of Tl_0.78Bi_0.22Sr_1.6Ba_0.4Ca₂Cu₃O₉ ((Tl,Bi)-1223) on rolling assisted biaxially textured substrates with YSZ and CeO₂ buffer layers (RABiTS) has been successfully demonstrated by laser ablation and post-deposition annealing in flowing argon. X-ray diffraction (XRD) θ-2θ spectra showed that the films consisted mainly of c-axis aligned 1223 phase with some intergrown 1212 phase, while XRD Φ-scans of (102) pole figure revealed that the films are also a- and b-axes aligned, with an epitaxy of the «100» of (Tl,Bi)-1223 film on the «110» of the top YSZ buffer layer. Four-terminal electrical transport measurements showed that the zero-resistance transition temperature (T_c) was in the range of 106 - 110 K, and the critical current density (J_c) at 77 K and zero field was about 10⁵ A/cm² for the entire film width (3 mm) of a longer film (14 mm) which was processed differently from the shorter films (7 mm). For a shorter film (7 mm) that showed better ab-in-plane alignment, the magnetization J_c, at 77 K and extrapolated to zero field, calculated from Bean's model using the full film width (3.5 mm) as the appropriate lateral dimension, was 2 × 10⁵ A/cm².     

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.     

Improved critical current density in ex situ processed MgB2 tapes by the size reduction of grains and crystallites by high-energy ball milling

We have fabricated Fe-sheathed MgB₂ tapes through an ex situ process in a powder-in-tube (PIT) technique using powders ball milled under various conditions. Although the ex situ processed wires and tapes using the high-energy ball milled MgB₂ powders have been studied and the decrease of grain and crystallite sizes of MgB₂ and the critical current density (J_c) improvement of those conductors were reported so far, the use of filling powders milled at a higher rotation speed than previously reported further decreases the crystallite size and improves the J_c properties. The improved J_c values at 4.2 K and 10 T were nearly twice as large as those previously reported. Those milled powders and hence as-rolled tapes easily receive contamination in air. Thus, the transport J_c properties are easily deteriorated and scattered unless the samples are handled with care. The optimized heat treatment temperature (T_opt) of those tape samples at which best performance in the J_c property is obtained decreases by more than 100 °C, compared with that of tapes using the as-received MgB₂ powder.     

Low-temperature magnetic and microwave studies of Gd0.95Ba2Cu3.05O7−δ: Agx composites

Gd_0.95Ba₂Cu_3.05O_7?δ: Ag_x (x=0.0, 0.02, 0.04, 0.06) ceramic superconducting samples were prepared by a co-precipation technique using organic carbonates in the presence of stable polymers. The room-temperature normal-state resistivity decreases with Ag addition. However,T _cand crystallographic parameters were uneffected. Low-temperature (4.2 K) magnetic and electrical transport studies in high magnetic fields (5 T) reveal that the addition of Ag into the Gd_0.95Ba₂Cu_3.05O_7?δ (GBCO) enhances the magnetic critical current density (J _mc), volume pinning force (F _p), flux explusion, and transport critical current density (J _tc). Microwave-induced dc voltage studies show the reduction in the total number of weak links between superconducting GBCO grains with increase in Ag concentration. The flux creep rate was also increased with increase in Ag concentration in GBCO resulting in stronger pinning potential. The increase inJ _mc, J_tc, and, hence,F _pwith Ag addition in the GBCO suggests the creation of an SNS-type proximity junction at the intergrannular region and stronger Josephson current paths between the superconducting intergrains, which may be due to the physical densification and reduction of the total number of weak links by Ag addition into the GBCO superconducting system.     

Angular Dependence of Critical Currents in Silver-Sheathed Tl-1223 Tapes

Experimental results on the angular dependence of the critical current densities in silver-sheathed Tl-1223 tapes are presented.We show, that the improved texture of the Tl-based tapes leads to a better fit of the angular dependence of the critical current density J_c to the 2D-model. Deviations are found at very low magnetic fields, where the crystallographic misalignment of the tape structure becomes dominant. We observe a rapid decay of J_c with magnetic field, which implies weak-link dominated current transport. Both the angular and the magnetic field dependence of J_c are measured on fluorine-free and fluorine-containing samples.