Ag-alloy Sheathed Bi-2223 Tapes Research Articles

Effect of Post-Annealing on the Phase Transformation and Critical Current Density of (Bi, Pb)-2223/AgAu Tapes

During the sintering process of Ag-alloy sheathed Bi-2223 tapes, post-annealing (PA) played an important role on the final current capacity by forming a Pb-rich phase named (Pb,Bi)3Sr2Ca2CuOx (3221) and healing the cracks. Due to the different oxygen penetration rate between AgAu alloy and pure Ag, which is traditionally used in other Bi-2223 tapes, the influence of PA parameters on the 3221 phase content, microstructures, and current capacity of AgAu tapes should be systematically studied. In this study, PA processes with different temperatures of 770–800 °C have been performed on fully reacted 37-filamentary (Bi, Pb)-2223/AgAu tapes in the atmosphere of 7.5% O2 balanced with Ar. It is found that the 3221 phase appeared after PA with its amount first increasing then decreasing with increasing PA temperature. The critical temperature Tc and peak temperature (Tp) obtained with AC susceptibility measurement both increased with increasing PA temperature, and reached the maximum value at 780 °C then decreased with increasing PA temperature. Due to the improvement of Tc and intergrain connectivity, the critical current density (Jc) increased by 24% after PA at 780 °C. Meanwhile, the 3221 phase formation mechanism has also been systematically discussed by varying the amount of remnant liquid in tapes in HT1 process.

Experimental Manufacture and Performance Evaluation of Linear Switched Reluctance Motor with HTS Excitation Windings

This paper presents an experimental manufacture and performance evaluation of prototype linear switched reluctance motor with HTS excitation windings (HTS-LSRM). The Ag-alloy sheathed Bi-2223 tapes are used for HTS coils. We first present a structure of the prototype HTS-LSRM. Next, current-carrying properties of the HTS coils are measured. Furthermore, current and voltage waveforms are measured and we evaluation a control performance of the current and voltage.

Microstructure Observations of Ag and Ag-Alloy Sheathed Bi2223 Tapes

37 filament Bi2223 tapes with different configurations of Ag, AgAu7wt% AgSb0.6wt% and AgMg0.2wt% as the precursor and restack sheaths were fabricated using commercial Bi2223 precursor material and powder-in-tube techniques. Short length samples were heat treated at a temperature in the range of 832degC to 846degC for the first stage (HT 1), followed by a second stage (HT 2) at 825degC for 40 h and slow cooled to 785degC. An intermediate roll pass was performed between the heat treatment stages. The critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) of the tapes was measured at 77 K in self-field and in fields up to 1 T. The microstructure of the alloy sheaths was examined using optical microscopy, and the Bi2223 filaments after HT 1 and HT 2 were examined using scanning electron microscopy (SEM). It was observed that the sequence of hardness, tensile strength, and critical bend strains from higher to lower levels is very much related to the grain sizes in the restack sheath. J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the tapes in zero field and in applied field was influenced by the phase composition, core density, grain connectivity, grain alignment and the interface between the Bi2223 filament and sheath.

Deformation and I&lt;tex&gt;$_rm c$&lt;/tex&gt;Degradation Behavior in Bent Bi-2223 Tapes Under Pressurized Liquid Nitrogen

The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> degradation behaviors of Bi-2223 superconducting tapes under pressurized liquid nitrogen were investigated using a newly developed sample holder which gives a series of bending strains to a sample. Four kinds of commercially available multifilamentary Bi-2223 superconducting tapes were used. At atmospheric pressure, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> degradation behavior depended upon the reinforcement adopted, the tapes externally reinforced or densified by over pressure showed better bending strain tolerance than the Ag alloy-sheathed Bi-2223 tapes. But these tapes showed a larger <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> degradation when pressurized to 1 MPa in liquid nitrogen. For all samples, after depressurization to atmospheric pressure from 1 MPa, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> was completely recovered to its initial values at atmospheric pressure. When the samples were warmed up to room temperature after being depressurized from 1 MPa, ballooning occurred at some parts of some samples. It was found that the larger degradation of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> occurred at the regions where significant ballooning occurred.

Bending strain effect on critical current of Bi-2223 superconductor tapes—report of international round-robin-test

An inter-comparison program (round-robin-test) aimed at the establishment of a standard test method for the bending strain effect on the critical current in Ag alloy-sheathed Bi-2223 tapes was implemented with participation of 12 laboratories around the world. Samples were bent at room temperature using bending devices that were specially designed for this RRT. The critical current was measured at 77 K and in the self-field. After critical current measurement of an unbent sample, bending of the sample and subsequent critical current measurements were done from 0.2% to 1.0% in 0.2% steps of strains on a single sample. Reported data from participants showed that the critical current values start to decrease from 0.4% strain and continue to decrease to 1.0% strain. Moreover, the data scatter starts to increase stepwise at 0.4% strain, being correspondent to the variation in the critical current values with bending strain, and it continues to increase gradually to 1.0% strain. The coefficient of variation values are 2.5%, 5.1% and 6.4% for 0%, 0.4% and 1.0% bending strain, respectively. Results were also compared in terms of the irreversible bending strain. Possible sources of error are discussed.

Influence of transverse compressive stress on Ic degradation of Ag alloy sheathed Bi-2223 tapes

In order to investigate the degradation of critical current (Ic) in the Ag alloy sheathed Bi-2223 tapes due to a transverse compressive stress introduced during manufacturing and operation of the HTS systems, a sample holder consisting of the upper block made of Ti alloy and the lower support plate made of glass fibre reinforced plastic was prepared. A shorter spacing of voltage taps caused large degradation of critical current with respect to compressive stress. It was found that the extent of the Ic degradation is proportional to the initial critical current density of the tapes. Through the experiment optimizing the shape of voltage terminals and the pressing load for the continuous contact type 4-probe Ic measurement system, it was found that the conical shape tip with large curvature radius was effective in suppressing the Ic degradation in Bi-2223 tapes during the Ic measurement. A hard alloy sheath of Ag–0.6wt%Mn was found to be quite tolerant to the Ic degradation against the pressing load of voltage terminals.

Complementary analysis of the results of a bending strain effect round robin-test on the critical currents of Ag alloy-sheathed Bi-2223 tapes

As a result of the first round robin-test (RRT) on the bending strain effect in Ag alloy-sheathed Bi-2223 superconducting tapes, it has been found that the scatter in the critical current data reported from 12 participant laboratories is not very large; the coefficient of variation (COV) is, for example, less than 6% at a bending strain of 0.6%. However, the bending strain limit, which is practically a more important parameter and is defined as a strain where the critical current degrades by 10%, has been found much scattered (17% in COV) in the first RRT. To clarify the origins of data scatter and finally reduce it, complementary analysis and tests on the effects of thermal stress relating to the sequences of the bending of the specimen and critical current measurements have been carried out. Additionally, it has been found that the sample inhomogeneity in mechanical properties contributes to some part of data scatter.

A comparison of ag and Ag-alloy sheathed Bi-2223 tapes

Ag and Ag-alloy sheathed Bi-2223 tapes were fabricated by a powder-in-tube technique with different configurations of the precursor and restack sheath materials: Ag, AgAu7 wt%, AgSb0.6 wt%, AgMg0.2 wt%. Analysis of the I/sub c/ and volume fractions of the Bi-2223, Bi-2212, Bi-2201 and Bi-3221 phases indicated that volume fractions of Bi-2223>90%, Bi-2212/spl sim/5%, Bi-2201/spl sim/0% and Bi-3221<2%, normally result in tapes with the highest I/sub c/. The mechanical properties of the tapes revealed consistent results. Generally, the harder the sheath material, the higher tolerance to the bending strain and higher the tensile strength of the tape. The sequence of the alloys' hardness from highest to lowest was AgMg0.2 wt%, AgSb0.6 wt%, AgAu7 wt% or Ag.

Test results of 60-kA HTS current lead for fusion application

A 60-kA high-temperature-superconductor (HTS) current lead has been fabricated and tested aiming at the application to a fusion magnet system. The HTS current lead consists of a forced flow cooled copper part and a conduction cooled HTS part. The HTS part is composed of 288 Ag-alloy sheathed Bi-2223 tapes, which are cylindrically arrayed on a stainless steel tube. For the Ag alloy, Ag–10at.%Au is used to obtain low thermal conductivity. Operation of a 60-kA current, which is the world record, was successfully achieved at a cooling condition of 20 K, 3.2 g/s for the copper part, and a low heat leak of 4.8 W at 4.2 K was demonstrated. This result indicates that the electric power of a refrigerator to cool the current lead can be reduced by 1/3 of that in a conventional current lead. In conclusion, technology of a large HTS current lead for fusion application is established.

Fabrication and characterisation of superconducting Bi-2223/Ag tapes with high critical current densities in km lengths

Long length Bi-2223/Ag tapes with high critical current densities are needed for large-scale applications. In this paper, recent results from NST are being presented, describing recent results reaching critical current densities of 23 kA/cm/sup 2/ and engineering critical current densities of 5.2 kA/cm/sup 2/ at 77 K over the whole length of a 1250 metre long Bi-2223/Ag tape. To our knowledge this is the highest critical current density reported for Bi-2223/Ag tapes longer than 1 km. The approach for improving the homogeneity and the considerations of processing and handling the long length tape are presented. Various Ag alloy sheathed Bi-2223 tapes have been made by our standard production line. The properties of these tapes are compared.

Development of reinforced Ag alloy sheathed Bi2223 tapes

We developed the Ag-Mg-Sb alloy which had a tensile strength of about 500 MPa and elongation of 5.5%. The tensile strength of 150 MPa was obtained at room temperature in the Bi-2223 tape using this Ag-Mg alloy for the outer sheath. The J/sub c/ value of the Ag-Mg alloy sheathed Bi-2223 tape has come to be above 10/sup 4/ A/cm/sup 2/ at 77 K in self field. And the J/sub c/ value was kept until a tensile stress of 120 MPa was applied. The critical bending strain of the tape was the same as that of the pure Ag sheathed tape. These results indicated that this tape was tolerant of the mechanical stress in the manufacturing process and hoop stress during the magnet operation.

Fabrication and properties of some Ag-alloy sheathed Bi-2223 tapes

Mono and multifilamentary powder-in-tube tapes with sheaths of Ag, AgCu/sub 0.02/, Ag(AgCu/sub 0.02/), AgAl/sub 0.25/, Ag(AgAl/sub 0.25/), AgNi/sub 0.25/Mg/sub 0.25/, AgTi/sub 0.25/Mg/sub 0.25/, and AgTi/sub 0.25/ alloys have been fabricated and their physio-chemical properties and effect on the phase formation of Bi-2223 determined. Alloying was found to have a significant effect on phase formation of the Bi-2223 phase formation linked to the alloying elements reactive to oxygen, Cu<Ni<Al<Mg<Ti. Multifilamentary tapes, composed of an inner Ag sheath and an outer Ag alloy sheath showed no such effects. The resistivity and mechanical properties of the various Ag alloys used as sheathing materials are also reported on and their possible use in improving the performance of PIT tapes.