D.W. Ha

Distribution of normalized critical current of bent multifilamentary Bi2223 composite tape

The distribution of the normalized critical transport current (critical current normalized with respect to the original value) of Bi2223/Ag/Ag alloy composite tape under bending strain of 0%–0.833% was studied experimentally and analytically. The experimental results were analyzed by a modeling approach based on the relation of the heterogeneous damage evolution to the distribution of the critical current. The main results are summarized as follows. (1) The measured distribution of the critical current values varying with bending strain was described well by the present approach. (2) When all specimens were damaged at high bending strains (0.338%–0.833% in the present work), the distribution of the critical current of the bent-damaged specimens was expressed by the three-parameter Weibull distribution function, the reason for which was revealed. (3) The distribution of the irreversible strain was estimated, with which the influence of the increase in the fraction of damaged specimens on the variation of c...

Ic degradation behavior in Bi-2223 superconducting tapes under torsional deformation

Abstract In this study, the I c degradation behavior in HTS tapes due to torsional deformation was investigated. Three kinds of commercially available Bi-2223 tapes were used. The sample holder for torsion test of HTS tapes was newly devised. Utilizing it, the influence of the torsion angle on the critical current ( I c ) and n -value in Ag alloy/Bi-2223 tapes was investigated at 77 K. The I c began to degrade when the torsion angle exceeded 150°, after which, it showed a gradual decrease with an increase in torsion angle. Superconducting tapes with larger number of filaments showed more significant I c degradation compared to tapes with smaller number of filaments. The n -value also showed a similar behavior to the I c ( θ ) behavior. Eventually, it was found that the torsion strain did not significantly reduce the critical currents of Bi-2223 tapes, when compared with cases of other loading types such as bending or tension.

Simulation on the relation of distribution of overall critical transport current to that of local one in bent-damaged Bi2223 superconducting composite tape

The superconducting Bi2223/Ag/Ag alloy composite tape specimens with a length 6 cm, composed of a series circuit of six local elements with a length 1 cm, were bent by 0.37% and 1.0% for measurement of distributions of the overall (6 cm) and local (1 cm) critical transport current and n-value. When the damage amount was small (0.37% bending strain), the distributions of the measured local and overall critical currents were described by the three parameter Weibull distribution function, while when the damage amount was large (1.0% bending strain), those were described by the bimodal Weibull distribution function. The distributions of n-value in the diagram of n-value versus critical current for both local elements and overall specimens were expressed by the regression curve of n-value as a function of critical current and the normal distribution function for the deviation of the measured n-values from the regression curve. The experimentally measured critical current distribution and the diagram of the n-v...

Single-filament composite MgB2/stainless-steel ribbons by powder-in-tube process

Abstract We report the successful fabrication of single-filament composite MgB 2 /stainless-steel ribbons, as an ultrarobust conductor type. The transport critical current of the non-sintered MgB 2 /stainless-steel ribbons is remarkably high, as an unexpected result. Transport critical currents I c ∼316 A at T =4.2 K and I c ∼82 A at T =20 K were observed at self-field for the non-sintered composite MgB 2 /stainless-steel ribbons. The I c value of 316 A is the higher than any other value reported so far. In addition, the persistent current density, that was estimated by Bean formula, of the sintered composite MgB 2 /stainless-steel ribbons was more than J p ∼6×10 5 A/cm 2 at T =5 K with H =0 G, and J p ∼1.1×10 5 A/cm 2 at T =30 K with H =0 G.

Analysis of the residual strain change of Bi2212, Ag alloy and Ag during the heating and cooling process in Bi2212/Ag/Ag alloy composite wire

The residual strain change of Bi2212 and Ag during the cooling and heating process in the Bi2212/Ag/Ag alloy composite superconductor was studied. First, the residual strain of Bi2212 filaments at room temperature was measured by the x-ray diffraction method. Then, the Youngs moduli of the constituents (Bi2212 filaments, Ag and Ag alloy) and yield strains of Ag and Ag alloy were estimated from the analysis of the measured stress–strain curve, based on the rule of mixtures. Also, the coefficient of thermal expansion of the Bi2212 filaments was estimated from the analysis of the measured thermal expansion curve of the composite wire. From the modeling analysis using the estimated property values and the residual strain of Bi2212 filaments, the changes of residual strain of Bi2212, Ag alloy and Ag with temperature during the cooling and heating process were revealed.

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

Abstract 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.

Characteristics of critical current and index n of Bi-2223/Ag tape up to 30 T at 4.2 K

We fabricated 55-filament Bi-2223/Ag tape and evaluated the dependence of critical current (I/sub c/) and index n in an external magnetic field up to 30 T at 4.2 K, in order to explore the possible application for a magnet in persistent current mode. I/sub c/ and index n were characterized as a function of temperature and the magnitude and orientation of the magnetic field by increasing and decreasing field. It was observed that I/sub c/ was dependent on the magnitude and orientation of the magnetic field. The I/sub c/ decreased exponentially as the field increased; in a parallel and increasing field, I/sub c/ was 128 A at 30 T, which is approximately 40% of the I/sub c/ in self-field. On the other hand, index n was not significantly dependent on the field up to 30 T, nor did it vary significantly on increasing or decreasing field; the n value in the range of 0.1/spl sim/1 /spl mu/V/cm was 23.0/spl plusmn/5.2 and 27.8/spl plusmn/8.0 on increasing and decreasing field, respectively, in the range of 0/spl sim/30 T. In addition, we observed that local I/sub c/ and index n of a long length of tape (4.2 m) were dispersed over a wide range and that the variation of each of these two values with the position of the tape was similar, suggesting that these properties depend on the microstructural uniformity.

Deformation and I

The Ic degradation behaviors of Bi-2223 superconducting tapes under pressurized liquid nitrogen were investigated using a newly developed sample holder which gives a series of bending strains to a sample. Four kinds of commercially available multifilamentary Bi-2223 superconducting tapes were used. At atmospheric pressure, the Ic degradation behavior depended upon the reinforcement adopted, the tapes externally reinforced or densified by over pressure showed better bending strain tolerance than the Ag alloy-sheathed Bi-2223 tapes. But these tapes showed a larger Ic degradation when pressurized to 1 MPa in liquid nitrogen. For all samples, after depressurization to atmospheric pressure from 1 MPa, the Ic was completely recovered to its initial values at atmospheric pressure. When the samples were warmed up to room temperature after being depressurized from 1 MPa, ballooning occurred at some parts of some samples. It was found that the larger degradation of Ic occurred at the regions where significant ballooning occurred

_rm c

The endurance evaluation of HTS conductors for practical applications is necessary. The mechanical properties and the critical current, I/sub c/, of multifilamentary Bi-2223 superconducting tapes, externally reinforced with stainless steel foils, subjected to high cycle fatigue loading in the longitudinal direction were investigated at 77 K. The relations between the applied stress amplitude and the fatigue life (S-N curves) were obtained using the externally reinforced Bi-2223 tapes, and its transport property was evaluated with the increase of repeated cycles at different stress amplitudes. The effect of the stress ratio, R, on the I/sub c/ degradation behavior under fatigue loading was also examined considering the practical application situation of HTS tapes. Microstructure observation was performed in order to understand the I/sub c/ degradation mechanism in fatigued Bi-2223 tapes.

Degradation Behavior in Bent Bi-2223 Tapes Under Pressurized Liquid Nitrogen

In order to investigate the effect of Ge addition to the Cu matrix on the microstructure and the critical current density, four kinds of internal tin processed Nb/sub 3/Sn strands with pure Cu and Cu-0.2, 0.4, 0.6 wt% Ge alloys were drawn to 0.8 mm diameter. The microstructure and critical current of internal tin processed Nb/sub 3/Sn wires that were heat treated at temperatures ranging from 680/spl deg/C to 740/spl deg/C for 240 h were investigated. The Ge addition to the matrix did not make workability worse. A Ge rich layer in the Cu-Ge matrix suppressed the growth of the Nb/sub 3/Sn layer and promoted grain coarsening. The greater the Ge content in the matrix, the fewer the net J/sub c/ result after Nb/sub 3/Sn reaction heat treatment. There was no significant variation in J/sub c/ observed with heat treatment temperature ranging from 680/spl deg/C to 740/spl deg/C.