Bi-2223 Silver-sheathed Tape Research Articles

Effect of number of filaments on the critical current density in multifilamentary Bi-2223 tape

The critical current property of Bi2Sr2Ca2Cu3Ox(Bi-2223) silver-sheathed tape has been remarkably improved by application of ConTrolled Over Pressure (CT-OP) process with adjusting temperature and oxygen partial pressure. For further improvement of the property of Bi-2223 tapes, the effect of number of filaments on the critical current density Jc was investigated. It was found that Jc was higher and its decrease with increasing magnetic field was weaker in the tape with larger number of filaments. In addition, the irreversibility field Bi increased with increasing filament number. The field-angle anisotropy Jc(0°)/Jc(90°) of the tape with the largest filament number was smallest, where the field-angle was measured from the a–b plane. However, the out-of-plane misorientation angle is fairly small in this tape. Hence, the low field-angle anisotropy in this tape seems to be attributed to improved superconductivity in the block layers in the superconducting region near the interface with silver matrix.

Measurement of Losses of Current Leads With Bi2223/Ag Tape Conductors Above Liquid Nitrogen Temperature

We measured the heat flow of current leads used for high-Tc superconducting apparatuses operating at liquid nitrogen temperature by the nitrogen boil-off method. To reduce the heat flow at the cold end of the current leads, the use of commercial Bi2223 silver-sheathed (Bi2223/Ag) tape conductors for current leads above liquid nitrogen temperature was also studied. We derived average Lorenz numbers between room temperature and liquid nitrogen temperature from the measurement results to assess the performance of the current leads. We tested Bi2223/Ag-soldered oxygen-free copper leads and obtained slightly lower heat flows and average Lorenz numbers than those of solid copper leads. The average Lorenz numbers of copper alloys were measured as well.

DC Magnetization System for a 35 kV/90 MVA Superconducting Saturated Iron-Core Fault Current Limiter

A dc magnetization system, comprises a HTS dc bias coil, a current source, a high-speed switch, and an energy release circuit, was built for a 35 kV/90 MVA saturated iron-core fault current limiter to enhance its functional capability and reliability and to reduce its size and weight. The dc bias coil is made of high temperature superconducting silver sheathed Bi-2223 tapes with a magnetization capacity of 141,000 ampere ldr turns. The current source is able to supply a maximum of 350 A constant current. The energy release circuit was designed to quickly discharge the magnetic energy of the saturated iron cores and to suppress the voltage surge across the dc coil when the magnetization circuit was switched to open in a short-circuit event. This controllable magnetization system enables the fault current limiter to respond to a short-circuit fault in one millisecond, to reach full current limiting function in 5 milliseconds, and to re-saturate the iron cores in 800 milliseconds after the clearance of a fault. In this paper, we introduce the configuration and specifications of the system. Some experimental results are also reported.

Evaluation of critical current density in multifilamentary Bi-2223 tapes with different lead compositions

The critical current property of Bi-2223 silver-sheathed tape has been remarkably improved by application of ConTrolled Over Pressure (CT-OP) process with adjusted temperature and oxygen partial pressure. For further improvement of the property of Bi-2223 tapes, the effect of variation in the lead composition was investigated. It was found that the critical current density J c was higher and its decrease with increasing magnetic field was weaker in the tape with higher lead composition than other tapes in a parallel magnetic field. On the other hand, although J c of this tape was higher, its decrease with increasing magnetic field was stronger in a normal magnetic field. This seems to be attributed to the larger condensation energy density due to the carrier doping by lead and to the better crystal axis alignment.

DI-BSCCO wire with Ic over 200 A at 77 K

The highest Ic of a latest DI-BSCCO® tape, which is the high performance silver sheathed Bi2223 tapes produced with CT-OP® technique, has reached 210 A (500A/cm-width) at 77K, self-field. The wide-range measurement of Ic using temperature variable system also has proved the highest performance at each temperature and external magnetic field. The combination of CT-OP® and stainless steel lamination techniques has concurrently obtained Ic higher than 200 A, rated tensile stress more than 300 MPa and rated tensile strain more than 0.4 % at 77 K. A texture analysis using synchrotron radiation has revealed explicit correlation between Jc and c-axis misalignment angle in CT-OP® processed tapes.

Improvements of Current Decay Behavior of HTS Coils in Persistent Current Operations

In order to improve the current decay behavior of high-temperature superconducting (HTS) coils using silver-sheathed Bi-2223 tapes in persistent current operations, the effect of the shielding current on the current decay behavior has been studied for magnetically levitated superconducting coils in internal ring devices. Generally, the current decay behavior of HTS coils is evaluated by the inductance of the HTS coils, the joint resistance and electric field versus current density relationship of the HTS tapes. In this paper, the effect of shielding currents in HTS tapes is discussed. It is shown that improvements of their properties of HTS tapes, which are the n-value and the critical current, are not so effective to decrease the shielding current in HTS tapes. Numerical electromagnetic analysis using finite element method and experiments have been carried out. Furthermore, improvements of the current decay behavior using excitation techniques are discussed.

The Bi-2223 Superconducting Wires With 200A-Class Critical Current

Critical current of a silver-sheathed Bi-2223 tape using controlled-overpressure sintering has attained 201 A, which is equivalent to 493 A per 10 mm-width. The engineering current density is 21kA/cm2 , while the critical current density on the total filaments area is 56 . Critical currents are also improved noticeably at low temperatures and high magnetic fields normal to the tape plane. Typical critical currents have reached 500 A at 4.2 K in 5 T and 200 A at 20 K in 10 T. The highest critical currents of long wires currently have reached 195 A for 70 m and 191 A for 178 m. The wires with 200 A-class critical current are similar in mechanical properties to 150 A-class long wires commercially produced by Sumitomo Electric using the controlled-overpressure sintering. The microstructure study has predicted far higher potential of critical current, according to the confirmed remaining issues on the 200 A-class tapes; incomplete orientation order, aggregation of alkaline earth cuprates and inter-granular Bi-2212.

Effects of over pressure sintering for Bi-2223 silver-sheathed tape

The critical current properties of a superconducting Bi-2223 tape produced by the over pressure processing at the final heat treatment were investigated. The critical current density increased more than the factor of densification of the superconducting region. In addition, the irreversibility field also increased. This means that the quality of flux pinning is improved by the new process. From the numerical calculation based on the flux creep and flow model, it is speculated that the fraction of weakly pinning regions decreases due to the improved connectivity of grains.

Characterization of Critical Current Density in Silver-Sheathed Bi-2223 Tape

The critical current property of a superconducting Bi-2223 tape produced by the over pressure processing at the final heat treatment was investigated. The critical current density increased by a factor of 1.6-1.8 at 77.3 K in comparison with the tape produced by the usual process. Thus, the critical current increased in spite of a reduction in the filament thickness. It was also found that the irreversibility field increased by a factor of 1.34 at 77.3 K by the new process. As a result, the enhancement of the critical current density at higher fields is remarkable. These results suggest that the over pressure process not only enriches the mass concentration of the superconducting phase but also contributes to the c-axis alignment. This is supported directly by the reduction of FWHM of rocking curve and indirectly by an enhancement the n value. The critical current property at lower temperatures is also investigated and the possibility of the further improvement of the property is discussed.

Influence of Modification of Annealing and Sheath Materials on Critical Current Density of Bi-2223/Ag Tapes

It is important to improve critical current density of silver-sheathed Bi-2223 tapes for practical applications of material. Bi-2223/Ag superconducting wires with various sheath materials were fabricated by stacking, drawing process with different pre-annealing schedules. Two kinds of powders were prepared. One was pre-annealed at 760/spl deg/C and low oxygen partial pressure, and the other was only calcined state. Before rolling process, round wires were pre-annealed at 760/spl deg/C and in a low oxygen partial pressure. Bi-2223/Ag conductor was needed softening annealing at low temperature during drawing process because pre-annealed precursor powder brought about decrease in workability. Pre-annealing let tetragonal structure of Bi-2212 phase of precursor transform to orthorhombic structure and reduce the fraction of second phases at superconducting wire, which could increase critical current density. However the breakages occurred at Ag-alloy sheathed long Bi-2223/Ag tapes during rolling after pre-annealing. Several pre-annealing scenarios were introduced to reduce the breakages during rolling process. We could achieve proper pre-annealing conditions for Ag-alloy clad Bi-2223 tapes by the analysis of mechanical properties. We could achieve highest Je of 8800 A/cm/sup 2/ at Bi-2223 tape by two-step pre-annealing.

Ｍｉｎｉ−ＲＴ Ｍｉｎｉ−ＲＴ装置用高温超伝導磁気浮上コイルの開発

A magnetically-levitated superconducting coil device, Mini-RT, has been constructed using a high temperature superconductor (HTS) for the purpose of examining a new magnetic confinement scheme of high-beta non-neutral plasmas. The floating coil is wound with silver-sheathed Bi-2223 tapes, and it is operated in the temperature range of 20-40 K. A number of studies and experiments were carried out in order to realize the necessary system. One of them was to demonstrate magnetic levitation using a miniature HTS floating coil having a diameter of 80 mm. The coil was fabricated using Bi-2223/Ag tapes of 12 m and excited by field cooling with liquid nitrogen. The magnetic levitation was examined using a real-time feedback control system with laser displacement gauges. Additionally, a persistent current switch (PCS) has been developed using Bi-2223/Ag tapes of 21 m, and a prototype HTS-PCS was tested in a cryostat. After construction of the floating coil and HTS-PCS for the Mini-RT device was completed, excitation tests were carried out in the cryostat and the basic properties up to the nominal operation condition were examined.

Ｍｉｎｉ−ＲＴ装置用高温超伝導磁気浮上コイルの冷却・励磁試験

A magnetically levitated superconducting coil device, Mini-RT, has been constructed using a high temperature superconductor (HTS) for the purpose of examining a new magnetic confinement scheme of high-beta non-neutral plasmas. The floating coil and persistent current switch (PCS) are wound with silver-sheathed Bi-2223 tapes, and they are operated in the temperature range of 20-40 K. After the basic properties of the main coil and HTS-PCS were examined in a liquid helium cryostat, the coil was installed into the vacuum chamber of the Mini-RT device. The HTS floating coil was then cooled by helium gas using GM cryo-coolers equipped with detachable transfer tubes. The excitation tests of the coil were carried out by supplying current from an external DC power supply through the detachable current feeder terminals as the HTS-PCS was turned off. The nominal operation condition was examined by overcoming many difficulties and the persistent current mode was tried by turning on the HTS-PCS. The decay time-constant was evaluated by maintaining the coil temperature for up to four days and the results were compared with calculations. The first magnetic levitation was also examined.

The evaluation of E–J characteristics of Bi-2223 silver-sheathed tape in a wide range of electric field

The E–J characteristics of Bi-2223 silver-sheathed tape in a DC field normal to and parallel to thetape surface were measured in a wide range of electric field by a combination of the fourprobe method, Campbell’s method for AC magnetic field with various frequencies and therelaxation of magnetization. These characteristics were different depending on the kind ofmeasurements, and the difference was reasonably explained by the anisotropy ofJc with respect to the field angle and the filament sausaging. It was found that theexperimental results when compensated for these effects agree with the theoretical analysisbased on the flux creep-flow model.

Theoretical analysis of E– J characteristics in a Bi-2223 silver-sheathed tape

From an analysis of E– J curves with the aid of the flux creep-flow model, it is found that most of the electric field is caused by the flux creep even in the range of electric field in usual resistive measurement. This result is consistent with the fact that the mechanism of flux creep generally explains various phenomena such as the critical current density, the irreversibility field and so on. On the other hand, it is much easier to use the parameters used in the percolation flow model to describe the E– J characteristics phenomenologically. The two models are compared and it is found that the two models are consistent. This gives a theoretical proof for the parameters of the percolation flow model.

Thermal and phase characterization of Bi-2223 superconductors

Advances in processing and fabrication of high critical current density ( J c) long length conductors, silver sheathed superconducting magnets and coils from silver sheathed Bi-2223 tapes, pancake coil magnets from 1 to 10 m length and mono-multifilament Bi-2223/2212 high critical temperature superconductors by powder in tube technique continue to bring these materials closer to commercial applications. In our laboratory Bi-2223 conductors doped with Sm, Nb, Ag and Gd were prepared by the heat treatment of rapidly quenched glass precursors. Activation energies and frequency factors, employing different models were evaluated. It was observed that both peritectic transition and reaction rates were dependent on ambient atmosphere. The resistivity measurements revealed that the critical temperature decreases with increasing Gd, Sm, Nb, and Ag concentration. The critical current density J c measured at 77 K from I–V data shows an increase with silver addition. The critical current densities of silver sheathed tapes using the powder in tube were found to be in the range 7×10 8 A/m 2. X-ray diffraction results showed that the volume fraction of the high- T c (2223) phase decreases and that of low- T c (2212) phase increases with increase of these rare-earth ion contents. These results are explained on the basis of possible variation of hole concentration with trivalent rare-earth ion substitution and also by considering the magnetic nature of the substituted ion in the composition.

Testing of a cryocooler-cooled HTS magnet with silver-sheathed Bi2223 tapes for silicon single-crystal growth applications

A project to develop a high-temperature superconducting split magnet for 8 inch silicon single-crystal growth applications began in October 1997. This split coil system is composed of 2 coils, each consisting of 18 pancakes. Each coil has an outer diameter of 1.2 m and a thickness of 0.1 m. Moreover, we developed a highly efficient GM-type cryocooler to cool a HTS coil to below 20 K. This paper presents the test results for a full-scale magnet. Each coil was cooled by the GM-cryocooler with a 3.3 kW compressor. The cool-down of the coil was completed within 480 hours with temperature difference among pancakes maintained at less than 18 K. In April 2001 this split coil was successfully energized up to rated current of 210 A at 20 K without a quench. The stored energy of this split coil reached 1.1 MJ at 216 A, 20 K, and the fastest charging time in our experiments was 1 minute.

Irreversibility field of Bi-2223 silver-sheathed tape determined with different electric field criteria

Dependence of the irreversibility field ( B i) on the electric field criterion ( E c) was measured for a Bi-2223 silver-sheathed tape wire using various measuring methods. It was found that B i increased monotonically with the electric field criterion. A good agreement was obtained for B i between experiments and the theoretical analysis using the flux creep-flow model. This implies that the resistive property of superconducting wires can be well described by the mechanism of flux creep and flow. The vortex glass–liquid transition field ( B g) was also determined from E– J curves which measured in the low and high electric field regions. It was found that B g also depends on the ranges of electric field. This is incompatible with the prediction of the vortex glass–liquid transition theory.

E– J characteristics in a wide range of electric field for a Bi-2223 silver-sheathed tape wire

The E– J characteristics are measured for a Bi-2223 silver-sheathed tape wire using the four probe method and a relaxation method of DC magnetization in the high and low electric field regions, respectively. The obtained results in the two regions are approximately explained by a theoretical analysis based on the flux creep-flow model in which the distribution of flux pinning strength is taken into account. It was found that the value of the pinning force density and its peak magnetic field are largely different between the two measurements.

Effect of Pb content variation on microstructure and critical current density of Bi-2223/Ag superconducting tapes

The effect of Pb content cariation on microstructure, phase evolution, Bi-2223 grain growth and critical current density (J c) of silver sheathed Bi-2223 tapes has been studied. The results indicate that high Pb content samples have higher conversion rate of Bi-2212 to Bi-2223 than low Pb content samples. However, too high Bi-2223 formation rate is not beneficial to Bi-2223 grain growth and thus the achieved critical current density in the tapes. The influence of coarse particles in precursor powder on the phase formation has also been investigated.

Inhomogeneous transport properties in Ag/Bi2223 tapes

In order to elucidate the influence of weak link on the E– J characteristics in the silver sheathed Bi2223 tape, the weak link path model has been applied by using three different stochastic regimes. In general, the growth of non-SC chains proceeds at the low current density region and at the high current density region, the coagulation among non-SC chains takes place and the fast growth rate of percolation network is realized. Comparing three cases, the generating rate of the percolation path becomes lowest for the Markov process. When the correlation becomes stronger, the initial rate of generation of percolated columns becomes faster. The theoretical E– J curves can reproduce the experimental results qualitatively. It is clear that the power-law relationship, E∝ J n , does not hold over the whole range of current. The n value becomes smaller in sequence when the I c decreases for three cases.