Multifilamentary Tapes Research Articles

Persistent Current of Bi2223/Ag Closed Coil in 77 K

Closed superconducting coils made from Bi2223/Ag multi-filamentary tapes were successfully fabricated. By measuring the magnetic flux density at the center of the coil, we were able to observe the persistent current mode of superconducting closed coil in liquid nitrogen. The persistent current measured was 1.39 A and remained unchanging for more than 500 hours after several hours decay. It was estimated from the decay curve that the upper limit resistance of the closed-loop coil was in the order 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> Omega of . Current-voltage characteristic was derived form the decay curve and different models were applied in fitting the experiment data..

Test Results on ${\rm MgB}_{2}$ Windings for AC Applications

In the frame of a research and development project named LIMSAT several MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tapes and windings were fabricated and tested in DC and AC current regime. All technical aspects related to the choice of the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tape layout as well as the winding technology and insulation will be reported. Different tape layout and insulating technique were used for the different windings, in particular several lengths of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> multifilamentary Cu-stabilized Ni-sheathed tape. Critical current measurements on short samples of the tape used, performed in a cryogenic-free test facility, showed currents over 250 A at 20 K, 1 T. Five non inductive windings were made using lengths from about 35 to 130 m and tested in liquid He and liquid Ne. The results from electrical DC and AC tests are reported and discussed.

Numerical Evaluation of AC Loss Property in Bi-2223 Tapes Affected by Multifilament Structure

This paper describes the numerical results of AC losses in various types of non-twisted multifilamentary tapes that actual Bi-2223 Ag-sheathed wires are idealized. The penetration process of magnetic flux into the superconductor filaments subject to Bean's critical state model is simulated for the application of an external transverse magnetic field perpendicular to the broadest face of tape wire by means of energy minimization. The AC losses under cyclic fields are calculated from the distribution of a vector potential in an initial state. The influences of arrangements and shapes of the filaments on the losses are investigated as compared with those for a wire that the entire filamentary region is regarded as a homogeneous superconductor.

The Current Transport Properties of a 200 A-Class Bi-2223 Superconducting Wire at Various Temperatures and Magnetic Fields

Silver-sheathed Bi-2223 multi-filamentary tapes using controlled-overpressure sintering have attained the highest critical current of 200 A-class at 77 K, self field. The 200 A-class Bi-2223 tape has been evaluated on the critical currents at various temperatures from 4.2 K to 90 K and magnetic fields up to 12 T applied perpendicularly to the tape plane. The critical currents are 500 A at 4.2 K, 5 T, 370 A at 20 K, 3 T and 200 A at 20 K, 10 T typically. This is an improvement by a factor of 1.3-1.5 compared to current commercial tapes produced using controlled-overpressure sintering.

Stability Measurements in Multifilamentary ${\rm MgB}_{2}$ Tapes

We have analyzed the behavior of a 10 cm long straight piece of copper stabilized multifilamentary tape, manufactured by Columbus Superconductors, in a nearly adiabatic condition. The tape was biased at different current values as a function of either the temperature or the induced heat disturbances at fixed temperatures. From the analyses of the voltage increase, recorded from voltage taps placed at known positions, we estimated the highest temperatures which give rise to a quench of the tape at constant values of the current bias. We also measured the minimum energy triggering the quench at fixed values of current bias and temperature. The voltage taps have also been used for homogeneity check of the physical properties which rule the thermal runaway in the tape. The voltage-versus-time curves in different sections of the tape also provide information about the quench propagation speed as a function of the hot spot energy and the temperature. Due to the composite nature of the tape, made of nickel, iron, copper, and , to perform simple FEA computation we modeled the tape with a one dimensional single wire with averaged value of the physical quantities involved in the Joule heat generation and thermal conductivity.

Current sharing among filaments for Bi-2223 Ag-sheathed tapes with inter-filament barrier

Current distribution in Bi-2223 multifilamentary tapes with inter-filament barrier is numerically evaluated for the application of an alternating transport current. In the case of barrier materials with an infinite resistivity, the current distribution can be determined with a lumped parameter circuit model including the inductances and resistances of all the filaments. The mutual inductance between a pair of filaments is obtained by estimating their geometric mean distance. The power-law model is also assumed to take into account the effect of resistance in each filament. The influences of the silver ratio and filament number in the tape wire on the current distribution are discussed systematically as well as the power index and frequency.

Fabrication and properties of Ag–Bi2223 tapes with resistive barriers for filament decoupling

In this paper, we prepared the Bi2223 multifilamentary tapes with Ca2CuO3+Bi2212 as interfilamentary resistive barriers to suppress the electromagnetic coupling among the filaments under AC external magnetic field. The tapes with thin barrier layers of Ca2CuO3+30wt% Bi2212 around the filaments were prepared by using a standard powder-in-tube (PIT) method. The outside surface of monocore Ag-sheathed rods was coated by barrier materials. Then, the several coated monocore wires were stacked and packed into another Ag or Ag–Mg alloy tube. The packed tube was drawn and rolled into tape shape. The tape was subsequently sintered to form Bi2223 phase inside filaments. For the characterization of tapes, X-ray diffraction measurements were performed to investigate the phase formation inside the filaments. The uniformity of transport properties (Jc) for barrier tapes were evaluated on the order of several metre lengths and compared with the result for the tapes without barriers. Finally, AC loss characteristics under AC parallel transverse magnetic field were investigated to examine the effect of introducing the barriers on the filament decoupling.

Local and overall critical current of Bi2223-composite tape under applied tensile and bending strains

Local and overall transport critical current IC and n-value at 77K of multifilamentary Bi2223-composite tape were studied under applied tensile and bending strains. The IC and n-value of the local elements constituting of the overall sample and those of the overall sample were measured for the voltage probe distances 10 and 60mm, respectively. The local IC as well as n-value varied along the sample length under both tensile and bending strains, while the critical current reduction process under the tensile strain was quite different from that under bending one; a big difference in damage among the local elements was found under tensile strain but not under bending one. While the damage process was different between the tensile and bending strains, the relation of overall IC and n-value to the local ones was described comprehensively by the voltage summation model. From the analysis of the experimentally observed critical current–applied strain relation of the local elements, the variation of the critical current determining factor “fracture strain–residual strain” along the sample length was revealed.

The effect of post-annealing on the critical current recovery of bending-deformed (Bi,Pb)-2223/Ag tapes

(Bi,Pb)-2223/Ag tape fabricated by PIT methods is a promising candidate for electrical power application. In these applications, the tape often undergoes a winding process, which leads to the degradation of the critical current. In this paper, we report the effect of post-annealing on the critical current (Ic) recovery of bent-deformed (Bi,Pb)-2223/Ag multi-filamentary tapes. The tapes were bent at room temperature with different curvatures, then straightened and post-annealed with different processing parameters, including post-annealing temperature, dwelling time and oxygen partial pressure. The experiment results show that rather high Ic value could be obtained by post-annealing even for the tapes whose bending strain was 0.8%. SEM observation shows that the decreasing in Ic value is caused by the formation and propagation of transverse micro-cracks in the (Bi,Pb)-2223 filaments and the effect of post-annealing could be related to the healing of micro-cracks caused by bending strain as well as the improving of grain connectivity.

Critical current properties in multifilamentary Bi-2223 tape produced by the over pressure processing

Investigation is done on the effect of ConTrolled Over Pressure (CT-OP) processes on the critical current properties in multifilamentary Bi-2223 tapes. It is found that the critical current density at low magnetic fields is remarkably improved by the new CT-OP process. This result with those on irreversibility field and n-value are compared with the numerical calculation based on the flux creep and flow model. The analysis reveals that the most probable value is increased, although the statistical distribution width is increased for the flux pinning strength. This result indicates that the critical current density at high magnetic fields is deteriorated by the widened distribution. If such an inhomogeneity can be reduced to the level of previous tapes by improving the CT-OP process, the high field performance is shown to be significantly improved.

Influence of Sheath Materials on the Strain Tolerance of Bi-2223 Superconducting Tapes

The critical current versus tensile strain characteristics of mono- and multi-filament Bi-2223 tapes with Ag and AgMn and AgMg sheath has been studied. The strain tolerance ε0.9 of naked mono- and 37-filament Bi-2223 tapes were 0.13% and 0.20%, respectively. Ag sheath increased the strain tolerance ε0.9 of critical current by 0.08% and 0.16% for mono-filament and 37-filament Bi-2223/Ag tapes. The ε0.9 of Bi-2223 with Ag and AgMn and AgMg sheath increased with filament number. AgMn and AgMg sheath, increasing ε0.9 of Bi-2223 tapes by 0.07-0.09%, are effective to improve strain tolerance of critical current for multifilamentary Bi-2223 tapes.

Study on Minimum Propagation Current of Bi-2223/Ag Superconducting Multifilament Tape

Minimum propagation current of Bi-2223/Ag superconducting multifilament tape was studied in detail, including experimental investigation and numerical calculation using finite element method (FEM). A set of experimental system has been established, quench propagation velocities with various transport current have been measured and minimum propagation current has been obtained in the condition of N2 gas cooling. The effect of parallel background magnetic field on minimum propagation current has also been studied experimentally. In addition, minimum propagation current at different operating temperature has been gained by the simulated calculation using FEM.It has been found that the high temperature superconductor is stable if only transport current is smaller than minimum propagation current of the time no matter what condition it is in. Minimum propagation current can be considered as an important stability criterion of high temperature superconductor.

Design and analysis of a 2 T practical HTS magnet

A 2 T practical magnet is constructed using high- T c superconducting (HTS) Ag-clad (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ x multifilament tapes. Pancake units made by the HTS tapes are prepared for stacking assembly to form the HTS magnet having a closed cycle cooling system for its practical operation. The work presented is mainly focused to analyze the magnetic field of the magnet and to achieve high field uniformity as practically required. Design parameters, magnetic fields, and fabrication techniques of the HTS magnet will be introduced with its analysis result and performance, and its magnetic field characteristics will be mainly presented in the paper.

Controller design and test results for a four axis HTS coil based Maglev system

Controller design and experimental results are reported in this paper for a four axis high temperature superconductivity (HTS) coil based electromagnetic levitation (Maglev) system. The HTS coils are made of Bi2223/Ag multifilamentary tapes. It has been experimentally proved that the designed controller works satisfactorily, although the physical parameters of a HTS coil based electromagnet (HTSEM) vary significantly with the frequency of the input voltage. A performance comparison has also been made between the classical lead-lag compensator and the modern H ∞ loop-shaping controller. It becomes clear that robust control theories are capable of providing a controller with better performances, which is in a good agreement with numerical simulations. Moreover, it implies that the particular parameter variation characteristics can be simply dealt with by the available robust control theories that are naturally existent in a HTSEM.

Simulation of ferromagnetic shielding to the critical current of Bi2223/Ag tape underexternal fields

Ferromagnetic material was electroplated onto the surface of theBi2223/Ag multi-filamentary tape and as a result changed theIc(B) characteristic of the tape correspondingly. A numerical simulation was used to investigate theinfluence of ferromagnetic shielding (FS) on the performance of the tape, in particular theIc behaviour under perpendicular external fields. Using finite element analysis, we are able tounderstand how FS alters the flux distribution within the superconductor region under anyarbitrary shielding structure. The shielding width, thickness and nonlinear property ofthe ferromagnetic material were taken into account for the simulation. Finally,optimized shielding parameters in association with different operating fields weresuggested and these values could be considered for the next run of experimental work.

Current–voltage characteristic of Bi2223 multifilamentary tapes in parallel connection

The direct current (DC) voltage–current characteristic of parallel multifilamentaryBi2223/Ag tapes, the simplest case of two parallel tapes, is numerically simulated andmeasured by the conventional four-probe method at 77 K and in self-field. The circuitof the parallel tapes is equivalent to a concentrated constant-circuit model bywhich the effective resistance and then the voltage are calculated with differenttransport current. The results show that there is an anomalous voltage peak in thevoltage–current relation before the transporting current reaches its critical current; thevalues are in good agreement with those obtained from measurements. Therefore,the concentrated constant-circuit models can be used to explain the observedanomalous voltage peak due to current redistribution between the two parallel tapes.

The influence of bending strains on AC power losses in multifilamentary BSCCO-2223/Agtapes

Recent advances in the manufacturing of multifilamentary high temperature superconductingcomposite wires allow for wider practical applications of the conductors, e.g. inpower transmission cables, transformers and motors. The wires, based mainly onBSCCO-2223 and YBCO-123 cuprates, are used in the forms of variously shapedcoils; therefore they are subjected to different kinds of mechanical stresses andstrains. These, in turn, lead to some changes in the physical parameters of thesuperconducting material, mainly in the critical current density, and thus in the dissipatedelectromagnetic energy, when subjected to changing magnetic fields and transportcurrents. In this work we report some experimental results related to the ACloss characteristics of Bi-2223/Ag multifilamentary tapes and their dependenceson bending strains. These losses are compared to the losses of virgin, straighttapes. The total AC losses, i.e. transport current and magnetization losses, in theBi-2223/Ag tapes, were measured by means of the electrical and calorimetricmethods. The experimental data obtained are compared with the critical state modelpredictions for AC loss behaviour in the experimental conditions presented here.

Thermal stability analysis of high temperature superconducting magnet coils under overcurrent pulses

This paper presents some numerical simulation results of thermal behavior on the high temperature superconducting (HTS) magnet coils wounded by Bi2223/Ag multifilamentary tapes subjected to an overcurrent pulse using the finite element method (FEM). The analytical results show that the time-varying spatial temperature distribution of the HTS magnet coils under the condition of different overcurrent pulses has an important influence on its stability estimation, and the temperature dependence of minimum propagation current plays a key role in the stability analysis of the HTS magnet. Through the FEM analysis, the permitted duration time of 5-multiple overcurrent pulse has been obtained for the HTS magnet under the various operating conditions and cooling conditions. In this paper, an analytical method for determining the stability of the HTS magnet coils subjected to an overcurrent pulse has been proposed and a stability criterion has been presented. The critical current and minimum propagation current can be chosen as the restrictive condition of operating current in the HTS magnet coils subjected to an overcurrent pulse.

AC losses in Ag-sheathed Bi2223 tapes with Ca2CuO3 as interfilamentary resistive barriers

In this study, we prepared the Bi2223 multifilamentary tapes with Ca2CuO3 as interfilamentary resistive barriers and evaluated their AC magnetization loss properties at 77K. The Bi2223 tapes with thin barrier layers of Ca2CuO3 around the filaments were prepared by using a standard powder-in-tube (PIT) method. To fabricate the Ca2CuO3 layers around each filament, the outside surface of monocore Ag-sheathed wires was coated by Ca2CuO3 with the slurry. After the heat treatment to decompose and evaporate the organic binder in the slurry, the several coated monocore wires were stacked and packed into another Ag-tube. Then, the packed tube was drawn and rolled into tape shape. The tape was subsequently sintered to form Bi2223 phase inside filaments. The AC magnetization losses in an AC transverse magnetic field were measured by a pick-up coil method. The loss properties in the barrier tape were compared with those in the tape without barriers. The results indicated that introducing Ca2CuO3 barriers is very effective to suppress the electromagnetic coupling among the filaments and also to reduce the magnetization losses under parallel transverse field.

A Comparative Study on Pre-Standardization of Total AC Loss Measurements for Oxide-Superconducting Tapes in Japan

We have been considering about standard methods to estimate total AC loss in oxide-superconducting tapes under practical electromagnetic conditions in future power devices. We adopt 4 candidates of the standard methods to measure the AC loss in superconducting wires exposed to AC transverse magnetic field and alternating transport current. We also introduce two pickup coil methods for comparative references of external magnetic field loss. The specimen is a Bi-2223 Ag-alloy-sheathed multifilamentary tape without twisting. 60-100 mm short pieces of specimen were cut from a terminal of a 50 m long specimen for 4 in 6 methods. A double-layer non-inductive coiled specimen was also prepared from the long specimen for the other 2 methods. The scattering in AC loss property among the short specimens prepared is less than 1% in the perpendicular field with the amplitude from 0.01 to 0.1 T at 10 Hz. The scattering in repeated measurements of the loss during 3 heat cycles is within 2-3%, which almost corresponds to that of the critical current in the long specimen. We processed the results measured by each method for the external field loss and total AC loss, summarized dispersion among the observed results of all methods and discussed about the candidates of the standard methods.