Critical Current Anisotropy of Tl-1223 Superconductors

A comparative study of the critical current anisotropy of Tl-1223 conductors manufactured by different techniques, prepared from different compositions and originating from different research laboratories is presented and discussed. For highly oriented layers, with the magnetic field (B) parallel to the ab plane, the normalised critical current density (j) versus magnetic field characteristic shows very little drop of j value due to the intrinsic properties of the Tl-1223 phase, while all powder-in-tube conductors show a rapid drop of j value in fields as low as 10 mT. For all powder-in-tube conductors the anisotropy coefficient k shows a very pronounced minima at fields up to 30 mT followed by a monotonic decrease of anisotropy with increase of the magnetic field. This is due to poor grain alignment and a demagnetising effect in the powder-in-tube Tl-1223 tapes.     

Study on cross section of high temperature superconducting coil

It is in particular of importance for HTS coils to secure a larger central magnetic field and/or a large stored energy with shorter length of HTS tapes. The critical current of an HTS tape depends on both the flux density and the flux angle against tapes. From this point, the performance improvement of HTS coils is taken into account with an analytical model. The minimum volume coil derived from the Fabry Factor constant curve is taken concerning the original coil shape, which is often employed in low temperature superconducting coils. The coil critical current was analyzed in consideration of the anisotropic properties of the tape.The electric field of HTS tapes in the coil was calculated at the coil critical current and the high electric field portion were cut out. The optimal coil cross section is obtained by iterating this calculation process. As a result, the critical current and the stored energy density of the coil were improved. The stored energy density increased about 17% and the central magnetic field was almost kept constant regardless of 19% reduction of HTS tapes, as compared with the original coil with the rectangular cross section.     

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.     

Development of Ag-Sheathed Bi(2223) Tapes with Improved Microstructure and Homogeneity

The development of the fabrication process of Ag-sheathed Bi(2223) tapes has been carried out in order to improve their transport and mechanical properties, as required by the power applications which are so far under study. Critical current density values of 28 kA/cm² at 77 K have been achieved on long multifilamentary Bi(2223) tapes, with a fabrication process that has been successfully employed in the fabrication of samples longer than 50 m. The microstructure and homogeneity of Ag-sheathed multifilamentary Bi(2223) tapes has been markedly improved by employing an alternative deformation technique. In a substantial part of the fabrication process, swaging, drawing, and rolling have been replaced by deformation with an active turks-head machine, which allows the deformation of rectangular shaped wires. At present, critical current densities in excess of 25 kA/cm² at 77 K have been achieved on long samples prepared with this technique. Moreover, innovative filament configurations have been employed for the fabrication of square-shaped Bi(2223) wires with reduced anisotropy and with critical current densities exceeding 20 kA/cm² at 77 K.     

Doped Tl-1223 Superconductors

Doped Tl-1223 superconductors of the composition Tl_aBi_bPb_cBa_nSr_2?nCa₂Cu₃O_y (a = 0.50 ? 0.76, b = 0, 0.16 and 0.3, c = 0.5, 0.24 and 0, n = 0.15 ? 0.4) were prepared from oxidic Ba-Sr-Ca-Cu precursor material. Upon calcination of the tartrate gels or the nitrates, the respective amounts of Tl₂O₃, PbO and Bi₂O₃ were added to the oxidic powders by milling. The samples were sintered in silver foil. The specimens consisted predominantely of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. Tc₍₀₎-values ranged between 115 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.4 kA cm^?2 at 77 K were measured for the assintered samples. Texturing was obtained by repressing and resintering of the superconducting material followed by annealing at 750°C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 7 kA cm^?2 by the thermomechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.     

The Study of Purity Improvement on Tl-1223 Thin Films by DC Sputtering and Post-annealing Method

Un-substituted TlBa₂Ca₂Cu₃O _x (Tl-1223)-superconducting thin films have been fabricated on a LaAlO₃ (001) substrate in oxygen by using a two-step method, which includes direct current (DC) magnetron sputtering and post-annealing process. Thallium (Tl) content in amorphous precursor films is found to be important to the crystallization of Tl-superconducting phase. Using the nominal composition of Tl_1+δ Ba₂Ca₂Cu₃ O ₈ (δ = 0.1～0.2) precursor films, the formation is promoted to Tl-1223 and Tl-2223 phase rather than Tl-1212 and Tl-1223 phase with accompanying Tl-rich source pellets. When the annealing process continues, Tl-2223 phase will be converted to Tl-1223 phase at a suitable annealing time and temperature. From the X-ray diffraction pattern, only Tl-1223 (00l) peaks are observed, which shows that the purity of Tl-1223 film is improved significantly by this method. The critical temperature T _c of Tl-1223 film is characterized at 110 K, and the critical current density J _c (77 K, T = 0) is up to 1.5 MA/cm².     

Doped Tl-1223 Superconductors

Doped Tl-1223 superconductors of the composition Tl_aBi_bPb_cBa_nSr_2–nCa₂Cu₃O_y (a = 0.50 – 0.76, b = 0, 0.16 and 0.3, c = 0.5, 0.24 and 0, n = 0.15 – 0.4) were prepared from oxidic Ba-Sr-Ca-Cu precursor material. Upon calcination of the tartrate gels or the nitrates, the respective amounts of Tl₂O₃, PbO and Bi₂O₃ were added to the oxidic powders by milling. The samples were sintered in silver foil. The specimens consisted predominantely of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. Tc₍₀₎-values ranged between 115 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.4 kA cm^–2 at 77 K were measured for the assintered samples. Texturing was obtained by repressing and resintering of the superconducting material followed by annealing at 750°C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 7 kA cm^–2 by the thermomechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.     

Transport and Magnetic AC Losses in Ag/Tl-1223 Tape and the Effect of Mechanical Damage

In this paper we present the results of AC loss measurements on silver clad monocore tapes of Tl-1223 prepared by the powder-in-tube method. Losses arising from an external AC magnetic field are compared with those generated by AC transport currents (self-field losses). Critical current densities are derived from magnetic loss data and compared with those measured by the four point transport method. It was found that mechanically induced stresses (introduced by bending) only have a significant effect on the transport losses, via the critical current degradation. Such changes are not seen in measured magnetic losses, which are independent of mechanical stress, and remain unchanged, at least in the stress range used in our experiment.     

Factors determining the magnetic field generated by a solenoid made with a superconductor having critical current anisotropy

The magnetic field generation in a simple solenoid is reconsidered for the case where the magnetic field is generated by a superconductor with anisotropy in its critical current density. In this case the influence of the radial magnetic field at the solenoid ends on the weak direction of the conductor has to be taken into account. Instead of the usual load line which stems from the maximum axial field at the inner turns, two load lines must be considered: one as usual, and the second one representing the radial field at the coil end. The maximum field generated by the solenoid is determined by which load line meets its respective j_c-H curve first. For tapes of (Bi,Pb)₂Sr₂Ca₂Cu₃O_x it is the radial field at the solenoid ends which determines the central field which can be generated by the solenoid. This is also the case for most other anisotropic superconductors even with a moderate j_c anisotropy of, for example, two. Insert coils in a background field can significantly raise the maximum central field as the ratio between axial and radial fields is different. This gain for a magnet made from Bi(2223) tapes is of the order of 30% (at T= 77 K). Some alternatives for maximum field generation using anisotropic tapes are discussed.     

Development of 3kA conduction cooled HTS current lead system

The research and development of superconducting magnet energy storage (SMES) system, a national project, began in 1999. One of the purposes of this project is investigation concerning the application of high-temperature superconducting (HTS) SMES. As a part of this project, the 3 kA class HTS small model coil was manufactured in order to verify the possibility of realizing conduction cooled HTS SMES. Therefore, it is important to develop the conduction cooled current lead system for applying this coil. We developed a kA class conduction cooled HTS current lead system. This current lead system consists of the copper current lead and the YBaCuO (YBCO) HTS current lead. The YBCO bulk manufactured by Nippon Steel Corporation was applied to the HTS current lead. The YBCO bulk keeps high critical current density (J_c > 10,000 A/cm²) in the magnetic field (1 T) at 77 K compared with Bi2223 superconductor. The experiment of this HTS current lead system was carried out, and rated current of 3000 A was achieved successfully.     

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.     

Critical current test results of 13 T-46 kA Nb3Al cable-in-conduit conductor

In the framework of ITER-EDA, a 13 T-46 kA Nb₃Al conductor with stainless steel jacket has been developed in order to demonstrate applicability of an Nb₃Al conductor with react-and-wind technique to ITER-TF coils. Using a 3.5 m sample consisting of a pair of conductors with 0% and 0.4% bending strain, the critical current performances of the Nb₃Al conductors were studied to verify that the conductor achieves the expected performance and the bending strain of 0.4% does not originate degradation. The critical currents were measured at background magnetic fields of 7, 9, 10 and 11 T at temperatures from 6 to 9 K. The expected critical currents were evaluated taking into account the variation of the strain in the cross-section due to the bending strain as well as self-field and non-uniform current distribution as results of an imbalance in the joint resistance and inductances. The calculation results indicate that the current distribution is almost uniform and the experimental results showed good agreement with the expected critical currents. Accordingly, we can conclude that the fabrication process of this conductor is appropriate and the react-and-wind technique using the Nb₃Al conductor is applicable to ITER-TF coils. In addition, the critical current of the Nb₃Al conductor is expected to be 108 kA at 13 T and 4.5 K, resulting in a sufficient margin against the nominal current of 46 kA. Furthermore, it was found that the decrease in the critical current by thermal strain can be made small by applying the bending strain to the conductor so as to reduce the compressive strain at higher fields, i.e. inner side of the coil, in the conductor cross-section.     

Synthesis and properties of magnetoresistive (La,Sr)MnO<Subscript>3</Subscript>-based glass-ceramic borate-matrix composites

Magnetic glass-ceramic borate-matrix composites containing micron-sized lanthanum strontium manganite grains have been prepared by ceramming amorphous La₂O₃-SrO-MnO _x -B₂O₃ materials at 800 and 900°C. The glass-ceramics had a magnetization of up to 48.7 A m²/kg in a magnetic field of 400 kA/m. Their relative magnetoresistance reached 6.2% at 290 K in a magnetic field of 80 kA/m and 16% at 77 K in a field of 160 kA/m.     

Low-temperature tensile strength of the ITER-TF model coil insulation system after reactor irradiation

The windings of the superconducting magnet coils for the ITER-FEAT fusion device are affected by high mechanical stresses at cryogenic temperatures and by a radiation environment, which impose certain constraints especially on the insulating materials. A glass fiber reinforced plastic (GFRP) laminate, which consists of Kapton/R-glass-fiber reinforcement tapes, vacuum-impregnated in a DGEBA epoxy system, was used for the European toroidal field model coil turn insulation of ITER. In order to assess its mechanical properties under the actual operating conditions of ITER-FEAT, cryogenic (77 K) static tensile tests and tension-tension fatigue measurements were done before and after irradiation to a fast neutron fluence of 1×10²² m⁻² (E>0.1 MeV), i.e. the ITER-FEAT design fluence level. We find that the mechanical strength and the fracture behavior of this GFRP are strongly influenced by the winding direction of the tape and by the radiation induced delamination process. In addition, the composite swells by 3%, forming bubbles inside the laminate, and loses weight (1.4%) at the design fluence.     

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.     

The cryogenic system for ITER CC superconducting conductor test facility

This paper describes the cryogenic system of the International Thermonuclear Experimental Reactor (ITER) Correction Coils (CC) test facility, which consists of a 500 W/4.5 K helium refrigerator, a 50 kA superconducting transformer cryostat (STC) and a background field magnet cryostat (BFMC). The 500 W/4.5 K helium refrigerator synchronously produces both the liquid helium (LHe) and supercritical helium (SHe). The background field magnet and the primary coil of the superconducting transformer (PCST) are cooled down by immersing into 4.2 K LHe. The secondary Cable-In-Conduit Conductor (CICC) coil of the superconducting transformer (SCST), superconducting joints and the testing sample of ITER CC are cooled down by forced-flow supercritical helium. During the commissioning experiment, all the superconducting coils were successfully translated into superconducting state. The background field magnet was fully cooled by immersing it into 4.2 K LHe and generated a maximal background magnetic field of 6.96 T; the temperature of transformer coils and current leads was reduced to 4.3 K; the inlet temperature of SHe loop was 5.6 K, which can meet the cooling requirements of CIC-Conductor and joint boxes. It is noted that a novel heat cut-off device for High Temperature Superconducting (HTS) binary current leads was introduced to reduce the heat losses of transformer cryostat.     

Development of Bi-2223 HTS tape and its application to coil and current leads

We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I_c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.     

Development of Bi-oxide superconducting tapes and coils

We have developed Bi-2212 and 2223 tapes. For Bi-2212, two double stacked pancake type coils were fabricated using Bi-2212/Ag tapes prepared by a combination of the continuous dip-coating process and melt-solidification. A small coil (13 mm inner bore, 46.5 mm outer diameter) was inserted in a conventional superconducting magnet system. In a bias field of 20.9 T, the generated field of the coil was 0.9 T, at an I_c of 310 A (criterion 10⁻¹³ Ωm) at 1.8 K. Thus, the superconducting magnet system achieved the generation of a field of 21.8 T in the full superconducting state. A large coil (20 mm inner bore, 94 mm outer diameter) generated a field of 2.6 T (I_c = 385 A (10⁻¹³ Ωm)) at 4.2 K and 1.53 T (I_c = 225 A (10⁻¹³Ωm)) at 20 K in self-field. For Bi-2223, tapes were prepared by the powder-in-tube technique using Ag-10% Cu-x%M (x = 0–1.0, M = Ti, Zr, Hf or Au) alloy sheaths. The high J_c values of 5–7 × 10⁴ A cm⁻² at 4.2 K and 14 T were obtained for the tapes doped with x = 0.03–0.1 at.% Ti, 0.1 at.% Zr, 0.1 at.% Hf or 0.3% Au. These tapes have a modified Bi-2223 grain structure at the sheath/core interface and also a dense and more aligned microstructure, resulting in higher J_c values.     

Effect of magnetic field, thermal cycling and bending strain on critical current density of multifilamentary Bi-2223/Ag tape and fabrication of a pancake coil

Multifilamentary HTSC tapes are important for their applications in various electrical devices. Powder-in-tube technique with improved optimized synthesis parameters is regarded as one of the most promising ways to prepare long-length multifilamentary Bi-2223/Ag tapes. Nevertheless, usefulness of such tapes depends on their electrical and mechanical properties. Critical current density of a Bi-2223/Ag tape with 37 filaments has been studied at 77 K with field, field orientation, thermal cycling and bending strain as parameters. Results have been discussed in light of various mechanisms and models. A small pancake coil has been fabricated out of the same tape and the test results presented.     

Magnetic Field Angular Dependence of Magnetization Loss in ReBCO Superconducting Tapes

In the applications of high-temperature superconductors (HTS), the HTS tapes are usually exposed to the external magnetic field with different orientations. The critical current and AC loss are affected by both the field amplitude and field angles due to the anisotropy of HTS tapes. In this work, we first introduce the experimental system to measure the magnetization loss in HTS tapes based on the calibration-free method. Then, we present the magnetization loss results in 4.8-mm-wide AMSC wire, 4-mm-wide SuperPower wire, 4-mm-wide SuNam wire, and 10-mm-wide Fujikura wire in a perpendicular applied field at 77 K. The field amplitude is up to 100 mT, and the frequency varies from 44.2 to 87.1 Hz. We also present the magnetization loss in AMSC wire, SuperPower wire, SuNam wire, and Fujikura wire at different field angles. The field angle varies from 10 to 90 ^° in 10 ^° steps. The loss reduction with the decreasing of the field angle shows the anisotropic property of HTS tapes. We finally plot the measured magnetization loss of the samples as a function of the magnetic field amplitude normalized by the field angle to verify an empirical formula.