Lian Zhou

Effects of axial tensile and bending strains on the critical current of Bi-2223 superconducting tapes

Abstract The effects of axial tensile and bending strains on the critical current of Ag and AgMn/Bi-based tapes produced by the powder-in-tube method have been studied at 77 K. The observed degradation of critical current due to tensile strain effects is irreversible. AgMn sheath and multifilament are beneficial to improve the tensile and bending strain tolerance of Bi-2223 superconducting tapes. In the case of Ag/Bi-based tapes, the e irr ≈0.15%, while that of AgMn/Bi-based tapes is 0.34%. The larger tensile and bending strain tolerance of AgMn/Bi-2223 superconducting tape may result from better mechanical properties.

The AC loss of transport current in (Bi, Pb)-2223 superconducting tapes

Abstract The self-field AC loss of the untwisted (Bi,Pb)-2223 tapes made with different matrices, number of filaments, critical currents and process techniques was measured in the frequency range from 20.8 to 363 Hz at 77 K. The losses of different matrix (Bi,Pb)-2223 tapes under different tensile strain were investigated at 50 Hz, 77 K. We find that: (1) in the case of I m / I c I m / I c >0.4, the eddy current loss must be taken into consideration; (2) above the critical current, the loss, which is shown resistive, is independent of current frequency, and differences in processing have a major impact; (3) below the critical current, the losses, P , increase in proportion with I m / I c with slopes less than 3, which is not in accordance with the Bean model; (4) during the elastic stage the self-field AC losses of AgMn and Ag matrix (Bi,Pb)-2223 tapes do not change, while in yielding stage the unstable relations of AC loss vs. axial tensile strain maybe caused by the serrated curve of stress vs. tensile strain relation, and the losses of both AgMn and Ag matrix tapes increase more in the plastic stage.

Critical current of Bi2223 superconducting tape and extended exponential model

Abstract We have measured J c ( B , T ) mainly at 4.2 and 77 K under perpendicular external fields and parallel ones. The results make it possible to elucidate the critical state model theoretically and experimentally. In the present work, an attempt has been made to elucidate the critical state model of high- T c superconductors theoretically according the measured J c ( B , T ) characteristics. Possible explanations of hysteretic J c versus H e characteristic were presented and discussed.

Phase Transition Mechanism of Fe1.1Se0.5Te0.5 Fabricated by High-Energy Ball Milling Aided Sintering Method

We report the phase transition mechanism of Fe_1.1Se_0.5Te_0.5 fabricated by the high-energy ball milling (HEBM) aided sintering method, which was achieved by multistep quenching heat treatment process. The precursor powders, composed of Fe and (Se, Te)_ss, were synthesized via an HEBM method for 2xa0h using Fe, Se, and Te mixture powders. Under the sintering temperature of 400xa0°C, the two compositions began to diffuse into each other and Fe(Se, Te)₂ phase with uniform Se and Te distribution was formed directly. With further increasing temperature to 500xa0°C, tetragonal β-Fe(Se, Te) phase was obtained owing to the elemental diffusion of Fe into Fe(Se, Te)₂. Due to the effective shortened length of diffusion paths, the superconducting phase formation temperature was ∼100xa0°C lower during our HEBM aided sintering method than that in the traditional solid state sintering method. Finally, a critical temperature of 14.2xa0K was observed in the final treatment sample, which was sintered for 12xa0h at 700xa0°C with a slow cooling rate followed by an annealing process under oxygen atmosphere for 24xa0h. This <italic>T_c </italic> value could demonstrate a good superconducting phase obtained by the HEBM aided sintering method.

Effect of axial tensile strain on self-field loss of Bi2223/Ag tapes

Abstract The effect of axial tensile strain on self-field loss of Bi2223/Ag tape is studied. The axial irreversible tensile strain limit ( e irr ) is about 0.35%. When the applied axial strain is less than e irr , the self-field loss curve with certain axial tensile strain is almost coincided with the curve of no axial tensile strain. When the applied axial strain is larger than e irr , the self-field loss increases sharply. These experimental results can be explained by the relation between the loss and I c : when e e irr , with the increase of e the I c has little decrease, so loss has little increase; when e > e irr , the large strain causes the superconducting filaments delaminating from the silver matrix and the superconducting filament begins to disconnect, such structural damage causes a localized depression of critical current and gives rise to flux flow and ohmic loss contribution. Experimental results were compared with the Norris equation on the self-field loss, and the loss of no axial tensile strain has good agreement with the Norris elliptical geometry equation.

The Effects of Heat Treatments on Critical Current Density and Microstructure of Bi-2223/Ag Tapes

The Bi-2223/Ag superconducting tapes were made by the two cold rolling/heat treatment cycles. The temperature of middle heat treatment was 835°C for 30 to 100 hours. The temperature of final heat treatment was as same as that of previous heat treatment, but for 100 to 200 hours. Our results indicate that the middle heat treatment affects strongly on the Jc of tapes. The microstructure of Bi-2223/Ag tape with different Jc values has been studied by SEM. The chemical composition of each phase has been determined by EDX in the SEM. The SEM analysis reveals larger size of 2223 grains and better connection between 2223 grains in the best tape, and more inclusions and more defects in the tapes with lower Jc value. The relationships among the heat treatment conditions, the microstructure and the transport properties of the tapes have been discussed in this work.

A Temperature Control System for Measuring J c of Bi-system HTS Tapes in LN 2 Temperature Range

In order to study practical applications and to investigate flux pinning characteristics of Bi-system HTS tapes, a temperature control system was established to measure Jc in a LN2 temperature range. The sample to be measured was immersed in LN2 and the desired temperature of the sample was reached by means of changing its saturated vapor pressure with a PID technique. The accuracy of the system can reach 0.1K for a long time. With this simple and effective system, Jc of Bi(2223)/Ag tapes was measured as a function of B(0–0.8T), T(77–90K), and θ(0–90o).

Investigation of flux pinning and magnetic relaxation behavior in PMP YBa2Cu2.95Sn0.05Oy sample

Abstract The magnetic relaxation has been measured using a SQUID magnetometer for the YBa 2 Cu 2.95 Sn 0.05 O y superconductor, which was prepared by the powder melting process (PMP) method. It is found that the magnetization shows a good linear dependence on lnt in a very large range of temperature (4.5–85K). Moreover, the pinning potential increases with increasing temperature in the Sn-doped sample.

AC losses of Bi2223/Ag tapes prepared by different process

Abstract The ac losses of Bi2223/Ag tapes were measured at 77K and under alternating magnetic field(50Hz) with the peak magnitude ranges fron 0.6 to 150mT. The samples were prepared by different process. It is found that the appropriate decreasing of temperature and time of the heat treatment can reduce ac losses. The subdivision of Bi2223/Ag tapes and the reduce of the thickness can also reduce ac losses. All of these belongs to the decrease of hysteresis loss.

An optimum heating program for fabricating multifilamentary Bi-2223/Ag tapes

Abstract Ag-sheathed multifilamentary Bi-2223 tapes have been fabricated by a conventional powder-in-tube method with an optimum final heating program. Many heating programs were done by varying sintering time (t s ), post annealing time (t p ) and cooling rate (R) from sintering temperature to post annealing temperature. It is found that Jc has been increased by a factor 1.5 to 2 at 0T over the normally processed tapes, with the ΔT=(Ts-Tp)=10°C and R=0.3°C per hour conditions. XRD indicate that these tapes have a high degree of phase purity, and well-aligned grains. The typical value of Jc is 2.0–2.5×10 4 A/cm 2 .