Monocore Tapes Research Articles

磁場中熱処理したＢｉ２Ｓｒ２ＣａＣｕ２Ｏ８線材の電流輸送特性

The transport properties for in-field heat-treated monocore and multicore Bi2Sr2CaCu2O8 (Bi2212) tapes and multicore Bi2212 round wires were measured in detail. We found that the in-field heat-treatment enhances not only the critical current density Jc but also the n-value for all samples used in this study. These enhancements for the monocore tapes were more remarkable, compared in those of the multicore tapes and wires. Therefore the in-field heat-treatment is more effective for the Bi2212 tapes with a large core size. Microstructure observation shows that the grain size of the monocore tapes decreases as the result of in-field heat treatment. The analysis based on the percolation model related to the local Jc distribution suggests that the in-field heat-treatment increases the m-value, which determines the shape of the local Jc distribution and is related to the n-values. The two Jc distribution model can explain that the increases in n- and m-values are due to the decreases in the aspect ratio of the grains during in-field heat treatment.

Reactions Between Bi-2212 and Silver-Nickel Composite Sheath

The use of a Ag-Ni composite sheath has been proposed as a means to improve the mechanical performance and reduce the cost of Bi-2212 conductor. However, Bi-2212 is highly reactive with metals other than Ag and the precious metals. Previous work has also shown that the Cu-oxide in Bi-2212 will react with oxides applied to the exterior of the Ag sheath. To investigate the feasibility of using a Ni reinforced composite sheath, a monocore tape was fabricated using a Ag sheathing tube reinforced with 44 volume percent Ni filaments. The tape was melt processed in oxygen and argon atmospheres and the structure of the Ni filaments and oxide core was examined. The Ni filaments were found to completely oxidize and incorporate Cu from the core. The oxide in the core was depleted in Cu as a result, which affected 2212 phase formation.

Superconducting Properties and Structure of In-Situ MgB$_{2}$ Tapes With SiC and TiC Addition Prepared by Hot Pressing

In-situ powder-in-tube MgB2 tapes have been prepared through a hot pressing process. Magnesium hydride MgH2 and amorphous B powders were mixed to synthesize MgB2 superconductor. The mixed powder was encased in a steel tube and rolled into a monocore tape of about 4 mm in width and 0.5 mm in thickness. Nano-sized SiC and TiC powders were added to the MgB2 core. The tapes were hot-pressed under 100 MPa during the heat treatment in Ar gas atmosphere. The Ic for the hot-pressed tapes with 5 mass% SiC addition exceeds 300 A at 4.2 K and 5 T, which corresponds to a core Jc of nearly 1,000 A/mm2. The core Jc is enhanced by more than a factor of two compared to that of tapes without pressing during the heat treatment. The higher Jc in hot pressed tapes results from the improvement of the denser MgB2 structures with less voids and better linkage among MgB2 grains.

Distribution of the critical current density in a mono-core Bi-2223 tape prepared by the over pressure processing

Effect of the over pressure processing on the distribution of the critical current density in mono-core tapes was measured in detail using Campbell's method. It was found that the magnetic flux profile is more straight in the over-pressured specimen, suggesting that the critical current density is more uniform. This seems to be partly caused by the improved uniformity of the thickness due to the disappearance of voids and the reduction in sausaging of superconducting layer by application of the over pressure. The possibility of the further improvement of the critical current density is discussed

Effect of initial composition of precursor powder on transport properties of Ag–Cu alloy sheathed Bi2223 tapes

In this study, we fabricated mono, 7, 19, 37 and 61 filamentary Bi2223 tapes sheathed with the Ag–Cu alloy, and several compositions with different Cu content were selected for precursor powders. The effect of the filament thickness and the initial composition of the precursor powder on the microstructure as well as the Jc properties was investigated. It was found that the multifilamentary tapes with the thin filament had the high sensitivity by controlling the Cu content in precursor powder compared to the monocore tapes with the thick filament. The improvement of the Jc values by controlling the Cu content was achieved in the multifilamentary tapes using the Cu deficient precursor powder comparing to that using the normal composition precursor. However, the formation and growth of large 14:24 particles were observed after the 1st sintering process. These large impurity particles caused the low Jc compared with the tape sheathed with pure Ag. Hence the suppression of the growth in the early stage of sintering should be required for further improvement of Jc of Ag–Cu alloy tape.

ニッケルシースＰＩＴ法ＭｇＢ２テープの応力／ひずみ特性

The stress/strain dependence of critical current and the mechanical properties of nickel sheathed PIT MgB2 mono-core tapes with various kinds of metal powders addition to the core were evaluated at 4.2 K in the magnetic field of 2 T. No marked change in the stress-strain curve was found by the powder addition. The critical current was markedly enhanced by In and Sn powder addition. The critical current was almost constant irrespective of strain up to the irreversible strain where a steep degradation of critical current begins. The irreversible strain for the tape increased from 0.25 to 0.50% by the addition of In and Sn powder. The In addition also suppressed a slight increase in the critical current with strain. The effects of Ag and Cu addition are less as compared with those of the low melting point metal powder such as In and Sn. SEM observations on the longitudinal section of In added tape after the tensile test clarified that the cracks were few and rather large as compared with those in the tapes with no addition or high melting point metal powder addition. The strain characteristics were compared with those of MgB2 with other sheath materials and those of a conventional Nb3Sn wire.

Effect of post-annealing at extensively varied temperature on phase formation and superconductive properties in Bi-2223/Ag tapes

The phase formation and superconductive properties of the high Tc superconductive Bi-2223/Ag tapes were studied by different post-annealing conditions. Monocore tapes were prepared for the thermo-mechanical process in air, in which a final heat treatment with isothermal post-annealing was performed. After an isotherm at 836 °C for 48 h in this process, successive isotherms were applied at extensively varied temperatures in a range of 820–400 °C at a fixed time of 12 h. Two peaks were observed in the critical current density (Jc) as well as in the critical temperature (Tc) against this temperature. The phase formation behaviour and microstructure development were identified from XRD and SEM–EDS. The results could be correlated with each other, and a qualitative understanding was obtained with the aid of the reported equilibrium phase relationships. Furthermore, a stepwise post-annealing process was developed and applied to the same tapes. Post-annealing of two sequential isotherms was designed by adopting the two peak temperatures found above. The Jc was enhanced relative to that by normal post-annealing.

Stress/strain characteristics of PIT MgB2 tapes with nickel sheath––effect of indium addition to the core

Abstract The electro-mechanical characteristics of the nickel sheathed powder-in-tube MgB 2 mono-core tapes with 10 vol% indium powder were evaluated. No marked change in the stress–strain curve was found by indium addition. On the other hand, the irreversible strain for the tape where a drastic degradation of critical current I c begins increased from 0.25% to 0.50% by the addition of indium. The indium addition also suppressed the slight increase in I c with strain. These characteristics of I c with strain were compared with those reported by other researchers. Scanning electron microscope observations of the longitudinal section of degraded samples revealed that the number of micro-cracks was appreciably smaller in the indium added tapes.

Hysteresis losses in YBCO coated conductors on textured metallic substrates

Hysteresis losses of YBCO coated conductors on two different textured metallic substrates - (NiFe and NiCrW)- were measured. The measurements were performed in a dc SQUID magnetometer at 5 K in applied magnetic fields up to 6 T. An YBCO layer of the sample on NiCrW substrate was cut to form 2 filaments and measured again in a perpendicular magnetic field as well as in a field at an angle of 45/spl deg/ with respect to the tape face. In a perpendicular magnetic field the hysteresis losses of the 2 filaments were a factor of about 0.6 lower than the losses of the original single filament. The influence on the magnetization of dividing a monocore tape into 2 and 4 filaments was numerically modeled using the critical state model. For full penetration and a perpendicular magnetic field the magnetization decreases proportionally with increasing number of filaments. Hysteresis losses in metallic substrates were substantially lower than the total losses of the composite tapes.

Critical current density and microstructures of thick monocore Bi2212 tapes grown in high magnetic fields

Ag-sheathed Bi2212 monocore tapes with core thickness between 80 and 700 /spl mu/m were heat-treated in flowing O/sub 2/ in magnetic fields up to 15 T. A uniform high-degree of texture is achieved throughout the thickness of the tapes when heat-treated in a magnetic field (H/sub a/) whereas a large portion of nontextured region exists without magnetic field. The critical current density J/sub c/ of the tapes increases with increasing H/sub a/ due to enhancement in texture. The self field critical I/sub c/>1000 A was achieved for tapes with a core thickness of 180 /spl mu/m. However, for tapes with larger core thicknesses, I/sub c/ decreases due to inhomogeneous melting. The magnetic field is more effective in enhancing texture in the early stages of crystal growth.

Effects of metal powder addition on the structure and critical current of Ni-sheathed PIT MgB/sub 2/ tapes

MgB/sub 2/ commercial powder was encased in a Ni tube and rolled into a monocore tape of about 5 mm in width and 0.3 mm in thickness. Different kinds of metal powder, i.e., In, Sn, Ag, Cu and Ni, were added to the MgB/sub 2/ core. Addition of the low melting point metal powder, e.g., In and Sn, has been found to enhance significantly the J/sub c/ value in MgB/sub 2/ tapes. Annealing at a low temperature yields further increase in the J/sub c/ of tapes with In and Sn addition. The enhancement in J/sub c/ by the In addition and the low temperature annealing reaches a factor of 6-7. The J/sub c/ of the tapes with 10vol% In addition, annealed at 200/spl deg/C for 10 h is on the order of 10/sup 5/ A/cm/sup 2/ at 4.2 K and 0.5 T. The TEM observation shows that In fills up the gaps among MgB/sub 2/ grains. The additional annealing produces better impregnation of In in the core. The improvement in grain linkage seems to be the probable origin for the enhancement in J/sub c/ by the In addition. The optimum amount of In addition to the MgB/sub 2/ core is likely 10-15vol%. Ag, Cu and Ni powder addition yields no apparent effect on J/sub c/ of PIT MgB/sub 2/ tapes.

ＰＩＴ法で作製したＭｇＢ２超電導線材及びコイルの臨界電流特性

This paper reports on the successful fabrication and testing of a MgB2 coil. We have fabricated a 15 m-long MgB2/stainless steel mono-core tape using a mixture of commercial MgB2 powder and tin powder (10wt% of MgB2 powder) using the PIT method. The tape is made applying an ex-situ process without any heat treatment during the processing procedure. The MgB2 superconducting tapes show good uniformity, with a high Jc value (180-290 A/mm2 at 4.2 K and 3 T) along the tape's length, as well as good bending tolerance. Ic degradation of the tape occurred at a bending strain of as high as 1.3%. Using wire over 12 m-long, we made a small solenoid coil, 48.5 mm in outer diameter and 40 mm height, and tested it at 4.2 K and self-field. The highest Ic value we obtained was 255 A, which generated a central field of 0.5 T, and a maximum magnetic field of 0.575 T.

Effects of metal powder addition on the critical current in MgB 2 tapes

MgB 2 commercial powder was encased in a Ni sheath and fabricated into a monocore tape. Different kinds of metal powder, i.e., Ag, Ni, Cu, In and Sn, were mixed with the MgB 2 powder. The addition of low melting point metal powder, especially In, has been found to enhance appreciably the J c in MgB 2 tapes. The annealing at a low temperature yields further increase in the J c of tapes with In addition. The enhancement in J c by the In addition and the low temperature annealing reaches a factor of five. The In addition may improve the linkage among MgB 2 grains, which provides the improvement in J c and avoids a sintering treatment at high temperatures. The optimum amount of In addition to the MgB 2 core seems to be about 10 vol%. The effect of other metal powder addition is also described.

Microstructure and critical current density of Bi2212 tapes grown by magnetic melt-processing

Vertical magnetic fields up to 9 T were applied to the magnetic melt-processing (MMP) of dip-coated and Ag-sheathed Bi2212 mono-core tapes with a core thickness of about 150 μm, which were set horizontally. SEM observation shows that a uniform high-degree of texture is formed throughout the thickness of the tapes by the magnetic field H a during MMP, whereas a large portion of the area is non-textured without H a. The critical current density J c of the tapes increases with increasing H a up to about 6 T due to the texture development. However, when H a>6 T some disturbance in J c appears; one is increased and the other is decreased. This may be due to the change of O 2 atmosphere around the tapes during MMP, which comes from the change of O 2 gas flowing by the magnetic field. The results suggest that a high magnetic field is very effective to enhance the texture development and to fabricate tapes with high critical current I c.

Effect of solidification conditions on microstructure of melt processed Bi2212/Ag conductors

The effect of cooling conditions on microstructure and electromagnetic properties of Bi2212/Ag monocore tapes and multifilamentary round wires was studied. We found that using isothermal solidification at temperatures 5–8 °C below the peritectic decomposition temperature of the 2212 phase it is possible to consume almost all second-phase particles present in the melt and reduce the density of 2201 intergrowths below 1%, but this occurred at the expense of forming larger amount of 2201 as a separate phase. Different optimum cooling rates were found for tape and wire samples.

Magnetic hysteresis loss in Bi-2223/Ag tapes with different filament arrangement

Hysteresis losses in perpendicular AC magnetic field were investigated experimentally on a set of six Bi-2223/Ag tapes with different filament architecture. Along with a monocore tape and a tape with overlapping filaments, tapes with filament arranged in less or more regular stacks, as well as a tape with filaments perpendicular to the tape wide face were tested. AC susceptibility technique was used, allowing to present the loss behavior in terms of two multiplicative parameters: the external susceptibility at low amplitudes, χ 0, and the imaginary part of internal susceptibility, χ″. We show that both the filament stacking and its parallel orientation with respect to the applied field suppress χ 0, leading to hysteresis loss reduction. However, the unconventional filament architecture was accompanied by imperfections of filament microstructure that broadened the susceptibility peak. Then, the loss reduction in these tapes was less than expected. In the temperature interval 77–95 K, excellent scaling of the susceptibility data was found, indicating the validity of the critical state model for these systems.

Material for resistive barriers in Bi-2223/Ag tapes

The reactivities of several oxide materials (OM) in direct contactwith BSCCO powder has been tested at a temperature of approximately845 °C in air. The OM such as BaZrO3, SrCO3, MgO and ZrO2showing little or no reactivity with BSCCO were mixed (10 wt%) with a BSCCOprecursor powder and used for monocore tapes made by a standard powder-in-tubetechnique. The microstructure of the BSCCO+OM cores was analysed by SEM andXRD and the transport current properties - critical current, pinning force andresistance up to 16 T - were measured as a function of the magnetic field forvarious orientations with respect to the ab plane. The OM used influencedthe electrical properties of the Bi-2223 phase in different ways. This isbecause the oxides react with BSCCO during the heat treatment andsimultaneously affect the 2212→2223 phase transformation as well as theBi-2223 grain growth and grain connectivity. Submicrometre commercial SrCO3powder was evaluated as the best material from all those tested, for resistivebarriers in Bi-2223/Ag tapes.

Competitiveness of porosity and phase purity in melt processed Bi2212/Ag conductors

The porosity remaining in Bi2212/Ag monocore tapes after melt processing is shown to be a function of the conductor geometry. The effect of other processing variables on the final microstructure was studied. It was found that increasing the length of the soak time, t m, at the maximum processing temperature increases the size of second-phase particles and the fraction of 2201 intergrowth layers within the 2212 grains.

High critical current density BSCCO-2212 tapes formed by a modified powder-in-tube method

In this study, we fabricated monocore tapes of Bi-2212 imbedded in a silver sheath. We then fabricated multi-filamentary conductors by laminating several monocore tapes using a turkshead roller and a heat treatment. The conductors exhibit satisfactory structural integrity, and for short lengths, the critical current density at 4.2 K was as high as 93000 A/cm/sup 2/ for a 7-filament conductor in a 5 T field perpendicular to the conductor surface.

Current distribution and flow in magnetic fields for Ag-sheathed Bi2223 monocore tapes

Using a micro Hall-sensor, magnetic-field distributions are measured at 77 K on a surface of zero-field-cooled samples of Bi2223 monocore tapes, in magnetic fields B/sub ex/ up to 800 mT perpendicular to a tape surface. The one-dimensional magnetic profile along a width direction of a sample shows a marked concave due to shielding currents, which shape changes as B/sub ex/ increases. In low fields up to 10 mT, there is a sharp peak positioning at the tape center in the profile, while it disappears and converts to a broad concave with an increase of B/sub ex/. From numerical calculations on magnetic fields, such a change in the profile is related to a change in current flow with B/sub ex/. An increase in B/sub ex/ limits and narrows the region of current flow in the Ag-sheathed Bi2223 tapes, from the whole part of superconductor to the Ag/Bi2223 interface.