Silver-sheathed Tapes Research Articles

Superconducting joints for the 1.3 GHz persistent NMR magnet under JST-Mirai Program

Superconducting joint technologies connecting high-T c superconducting (HTS) tapes, such as REBCO-coated conductors and Bi2223 silver-sheathed tapes, have been developed for the fabrication of superconducting magnets generating 30.5 T with persistent current operation at 4.2 K under the JST-Mirai Program since 2017. These are indirect joints using a superconducting intermediate layer to join highly crystallized HTS layers of tapes. Thus far, high I c values of above 100 A at 77 K in the self-field have been achieved by both joints. These performances are sufficient for installation to the 30.5 T magnet, while efforts to further improve the reproducibility of high I c for joints connecting long-length HTS tapes have been continuously made. The current status of HTS joint technologies is summarized in this paper partly from the view point of material science.

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.

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.

Critical current properties in high-temperature superconducting wires and tapes

The hot topics of the critical current properties in YBCO coated conductors and Bi-2223 silver-sheathed tapes are discussed. The dependence of the critical current density in YBCO coated conductors on the superconducting layer thickness is partly associated with the defects nucleated in thicker films and is also caused by the thickness dependent influence of the flux creep. Experimental results on PLD processed conductors with different thicknesses are compared with the theoretical analysis based on the flux creep-flow model. A discussion will also be given for the optimum layer thickness for applications.In Bi-2223 tapes introduction of CT-OP (ConTrolled Over Pressure) process has significantly improved the critical current properties. This progress can be attributed to the improvement of the mean value of the flux pinning strength and the decrease in its distribution width. The possibility of further improvement of the critical current properties is discussed using the flux creep-flow model.

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.

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.

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.

Phase relations study on the melting and crystallization regions of the Bi-2223 high temperature superconductor

The melting and solidification behavior of Bi2Sr2Ca2Cu3 O10 (Bi-2223) precursors has been studied. Nominal compositions corresponding to excess of liquid, Ca2CuO3 and CuO have been investigated. Each sample was made by packing a precursor powder into a silver crucible, in order to approximately simulate the situation found in 2223 silver-sheathed tapes. The samples were partially melted and then slow-cooled, being quenched from different temperatures and analyzed through X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDS). The precursors decomposed peritectically during melting, forming liquid and solid phases. Very long plates with compositions falling in the vicinity of the 2223 primary phase field formed upon slow-cooling. The 2223 phase may have been formed and the results suggest that long grains of this phase might be obtained by melting and crystallization if the exact peritectic region and the optimum processing conditions are found.

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.

The Impact of Fe Doping on Jc Behavior of Bi2223/Ag Tapes under Magnetic Field

Bi1.8Pb0.4Sr2Ca2.2Cu3—xFexOy silver-sheathed (Bi2223/Ag) tapes with different Fe-doping amounts, x = 0.001, 0.002, 0.003, 0.004, were fabricated by powder-in-tube (PIT) technique. It is found that the magnetic field dependence of critical current density Jc(H) is improved for samples with Fe-doping amounts of x = 0.001, 0.002, and 0.003 at high magnetic fields, while the Jc(H) behavior deteriorates with Fe-doping level increasing further. X-ray diffraction analyses manifest that a small amount of Fe-doping has little effect on the phase composition of Bi2223/Ag tapes. Scanning electron microscopy (SEM) observations do not show an obvious difference between un-doped and Fe-doped samples. It is concluded that the enhanced flux pinning originates from the introduction of effective pinning centers due to Fe ions partial substitution for Cu ions.

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.

Effects of residual carbon on phase formation of TlBa 2Ca n−1 Cu nO y ( n=3 and 4) superconductors

The effects of residual carbon in precursors on phase formation of single Tl–O layer superconductors of TlBa 2Ca n−1 Cu n O y ( n=3 and 4) have been investigated. The samples were prepared from Tl 2O 3 and precursors of Ba 2Ca n−1 Cu n O 2 n+1+ δ (C x ) ( n=3 and 4) with various residual carbon contents ( x=0.02–0.19). Even a small amount of carbon promotes the formation of double Tl–O layer phases of Tl 2Ba 2Ca n−1 Cu n O y ( n=3 and 4). Using the carbon reduced precursor ( x=0.02), a TlBa 2Ca 2Cu 3O y phase is synthesized directly in a silver-sheathed tape without such elements as Pb, Bi and Sr.

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.

Jc distribution in the Bi-2223/Ag–AgMg tapes

Bismuth-based, silver-sheathed tapes are considered to have great possibilities for electric power applications. Remarkable progress in Bi-2223/Ag tapes has been made in the world. We also have been developing Bi-2223/Ag tape since 1989, and we can fabricate 1 km length tape now. One of the important properties for J c of Bi-2223/Ag tape is the distribution in the cross-section of tape, as well as that of longitudinal distribution. We measured J c distribution in the cross-section of tapes that were cut in the constant period from the edge of tape by using laser beam. It was cleared that J c increased periodically toward the center of the tape but J c slightly decreased at the center. We consider that J c distribution in the tape has good correlation with the density of filament. By this result, we succeeded in manufacturing the long Bi-2223/Ag–AgMg tape of J c =31 000 A/cm 2 and J e=8000 A/cm 2. Furthermore, we achieved I c=50 A in Bi-2223/Ag–AgMg twisted long tape.

Scaling behavior of higher harmonic responses in textured (Bi,Pb)2Sr2Ca2Cu3Oy/Ag tapes

A silver-sheathed (Bi,Pb)2Sr2Ca2Cu3Oy tape was studied in terms of harmonic responses χn=χn′−iχn″ (n=3, 5, 7) to reveal the angular dependence of its physical properties. In the screening method higher harmonic responses are directly related to the nonlinear voltage–current [E(J)] relation of the sample. Therefore, the effect of the silver sheath on the harmonic measurements can be neglected. We performed measurements of the harmonic responses as a function of applied magnetic field or angle between the magnetic field and the tape plane at various temperatures. The measured data could be scaled onto a single curve by using the three-dimensional scaling relation. The derived anisotropic parameter γ ranged from 5.1 to 5.6 at the measured temperature region. Such a small γ value was attributed to the misalignment of the grains in the tape.

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.

Vortex dynamics of (Bi,Pb) 2Sr 2Ca 2Cu 3O y silver-sheathed tape by ac susceptibility measurements

The complex ac susceptibility ( χ n = χ ′ n −i χ ′′ n ) measurements were performed on a (Bi,Pb) 2Sr 2Ca 2Cu 3O y silver-sheathed tape to study the vortex dynamics in a time window of 10 −4–10 −2 s . The fundamental ac susceptibility of the tape as a function of temperature, ac field amplitudes and frequencies were obtained. The current dependent activation energy was derived as U( j)∝ j −0.42, characteristic of the vortex glass/collective pinning model. By using such U( j) relation, higher harmonic susceptibilities χ n ( n=2–7) were obtained based on the numerical resolution of the flux diffusion equation. We found that theoretical results agree well with experimental data.

Homogeneous current distribution in a coaxial superconductor with and without return current path

The authors have developed a theoretical method based on magnetic flux conservation between adjacent layers. One of the advantages of this method is that one can directly obtain twisting pitch and radius for realizing homogeneous current distribution in coaxial multi-layer superconductor. A set of the obtained twisting pitch and radius was employed in a sample three-layer conductor comprised of silver-sheathed multi-filamentary BSCCO-2223 tapes and the current distribution was measured by a Rogowski coil. Agreement between the experiment and the theory on current distribution is quite remarkable. Using this theory, the authors analytically investigated the influence of the manufacturing error of twisting pitch and radius on current distribution. The results revealed that the manufacturing errors of twisting pitch and radius have large effect on current distribution and a suitable set of twisting pitch and radius against manufacturing error can be found. They also investigated the relationship between twisting pitch and current distribution in coaxial six-layer conductor with return current path. The characteristics of twisting pitch in the conductor with return current path are different from those of the conductor without return current path.