Preparation and characterization of silver clad (Bi,Pb)-Sr-Ca-Cu-O 2223 superconducting tapes with high critical current density

Silver clad Bi-2223 tapes with consistently high critical current densities of over 30,000 A/cm² at 77 K and zero field were prepared by powder-in-tube (PIT) technique. Powder XRD, electron microscopy, a.c. susceptibility and critical current measurements were used to study the phase assemblage, microstructure and transport properties of these tapes at various stages of processing. The precursor powder for PIT process was prepared by a sol-gel route by acrylate method using freshly prepared nitrates of Bi, Pb, Sr, Ca and Cu. The carbon content in the powder was minimized by subjecting it under dynamic vacuum calcination followed by heating in free flow of oxygen for long durations with intermittent grindings. The choice of initial stoichiometry, high reactivity of the precursor, effective removal of carbon, choice of phase assemblage at the filling stage and the multistage thermomechanical processing at optimized conditions were found to be responsible for the high critical current density. The work was done under the National Superconductivity Programme funded by the Department of Science and Technology (DST), New Delhi.     

Electric,magnetic, and transport critical current density behavior of the 2201, 2212, and 2223 phases of ‘BSCCO’

We have studied the electrical properties of polycrystalline pellets of the high-T _c superconducting phases occurring in the Bi₂O₃-SrO-CaO-CuO (BSCCO) system, having nominal compositions Bi₂Sr₂Cu₁O_6+y, (2201 phase, withT _c = 10 K.) Bi₂Sr₂CaCu₂O_8+y (2212 phase, withT _c = 85 K), and Bi₂Sr₂Ca₂Cu₃O_10+y (2223 phase, withT _c = 110 K). Pellets containing the 2223 phase having zero resistance below 105 K were obtained. For the 2212 and 2223 phases, the transport critical current density was measured as a function of temperature and of the externally applied magnetic field. As previously found for YBa₂Cu₃O_7-x (YBCO), results are consistent with the occurrence of a weak superconducting coupling among the grains. Such coupling was, however, noticeably stronger for the 2223 phase than for the 2212 phase.     

The influence of the first heat-treatment on the critical current density of (Bi,Pb)-2223/Ag superconducting tapes

Optimization heat-treatments have been performed on multi-filament Bi-2223/Ag superconducting tapes under 1 bar total gas pressure, the oxygen partial pressure being 8.5%. In a first heat-treatment (HT1), the tapes were sintered within 822-838 °C for 1-50 h. After intermediate deformation, all the samples underwent the second heat-treatment (HT2) at 825 °C and 830 °C for 20 h followed by a thermal sliding procedure. The relative content of the phases present in HT1 samples was measured by XRD. It was found that the Bi-2212 phase content after HT1 strongly influences the values of J_c after HT2. There is a correlation between the amount of Bi-2212 phase after HT1 and the final J_c values after HT2. A maximum of J_c was found for a ratio of 0.15 between Bi-2212 and Bi-2223.     

Effect of Bi deficiency on grain alignment of BiPb-2223 thin film fabricated using rf sputtering process and on critical current density properties

Abstract

In the present study, thin films prepared as a function of the Bi concentration in the BiPbSrCaCuO system were synthesised. Thin films were fabricated using radio frequency sputtering method. Crystal structure of the films fabricated was determined from X-ray diffraction measurements. The crystal orientation was analysed by X-ray pole figure and in-plane alignment. Both X-ray diffraction and pole figure analysis revealed that crystallinity in the films decreased significantly with decreasing Bi concentration in the system. A systematic decrease in the superconducting transition temperature and hole concentration per CuO plane was obtained with decreasing Bi concentration. The J_c values of the films were calculated using Bean formula. It exhibits a significant dependence on the magnetic field and temperature. It was found that J_c decreased sharply with increasing applied magnetic field. The highest J_c value was found to be 1·06×10⁶ A cm^?2 at 10 K, which corresponded to the best flux pinning among the films fabricated.     

A study of microstructural variations in BSCCO Ag-clad tapes and a method to improve grain alignment

The microstructural variations in BSCCO Ag-clad tapes obtained by a conventional fabrication procedure were studied by SEM and microhardness tester. A method to improve grain alignment in the tape by treatment of the magnetic field with mechanical vibration was developed and investigated.     

Bi系2223高温超导带材力学性质

对4种Bi系2223高温超导带材77 K下的力学性质进行了实验研究.通过选择合适的合金包套替代传统的银包套,带材的机械性能能有所提高,但电学性质有所降低,尤其是内包套采用合金、外包套采用银,其机械性能反而低于纯银包套带材.通过X射线衍射图谱发现,当包套采用合金材料,尤其是内包套采用合金、外包套仍采用纯银,会大大降低样品中2223相的体积含量.相比较临界电流密度,n值对材料的破坏更为敏感.利用X光同步辐射技术初步探讨了银包套带材在室温和77 K下的损伤破坏过程.     

Granular and intergranular properties of hot pressed BSCCO (2223) superconductors

The effect of hot pressing conditions (sintering temperature and time) on the superconducting properties of (2223) Bi(Pb)SrCaCuO were investigated by transport and magnetic measurements. The advancement of the densification process leads first to an improvement of the electrical connectivity between grains (J_c transport increases) and then to a deterioration of both intragranular and intergranular properties (T_c, J_c magnetic and J_c transportdecrease) because of the induced loss of oxygen, 2212 continuous intergrain network and other defect formation.     

Processing of BSCCO 2212 and 2223 Superconductor Compounds

High-T_c BSCCO superconductor tapes were prepared by the conventional powder-in-tube method. Some tapes involved partial melting, while the others involved solid-state processing only. Bulk samples were prepared by powder metallurgy. X-ray diffraction (XRD) showed the presence of both BSCCO 2223 (T_c = 105 K) and BSCCO 2212 (T_c = 80 K) phases in all the materials. For the mostly BSCCO 2212 phase samples, x-ray studies indicated that tape #1 which involved partial melting at 850^°C for 0.3 h had a higher degree of basal orientation than either a tape #2 sample which involved partial melting at 855^°C for 0.5 h, or a bulk (#2) sample. For the mostly BSCCO 2223 phase samples, however, a comparison of bulk (#1.), tape #3 (solid state processing at 840^°C) and tape #4 (partial melting at 865^°C for 0.5 h) samples, showed that the solid-state processed tape (#3) had the highest degree of basal orientation. Direct current magnetic susceptibility measurements were used to follow the transition at T_c. Critical current density, J_c, values were estimated from DC magnetic hysteresis loops for all bulk and tape samples.     

Sintering and microstructural development of ceria-gadolinia dispersed powders

Well-dispersed ceria-gadolinia oxide powders were obtained from thoroughly isopropanol-washed coprecipitated oxalates, followed by calcination at 800 °C. The characteristics of the calcined powders and the microstructure of the green compacts were found to be of great importance in the sintering behaviour. Those green bodies in which some agglomerate survived after compaction reached a lower final density, while those having soft agglomerates were almost fully densified at a sintering temperature as low as 1400 °C. The densification process was studied by isothermal and constant heating rate dilatometry, and microstructural development at each stage in the processing was followed by SEM. By controlling the processing variables it was possible to obtain low-temperature near fully dense (better than 99%) and tough CeO₂-Gd₂O₃ bodies with homogeneous microstructure.     

Effect ofin Situ cooling on superconductivity of artificially layered BSCCO films with 2223 structure

Thin BSCCO films with the 2223 structure were deposited onto MgO substrate kept at 665°C by applying a sequential sputtering method using metallic Bi and Sr-Ca-Cu-O targets. The resultant films were then cooled under various oxygen partial pressures and cooling rates. It was found that the oxygen partial pressure and substrate temperature during cooling affect significantly the development of superconductivity of the as-deposited films. When the films were cooled stepwise under a critical oxygen partial pressure around 0.3 torr, the as-deposited films showed either superconductivity or nonsuperconductivity behavior. HRTEM images of the films showed no essential difference in the modulated structure, suggesting that the excess oxygen in the Bi-O double layers is the same and not the cause of the different superconducting behavior in the as-deposited films.     

Bi2223/Ag高温超导带材失超传播特性研究

对高温超导Bi2223/Ag带材在自场、气冷环境条件下的失超传播速度进行了实验研究,并比较分析了不同传输电流条件下带材失超的最小触发能量。实验结果表明:Bi2223/Ag高温超导带材的失超传播速度范围在0.1—0.6 cm/s,其失超传播速度与传输电流大小有关,与触发能量无关,带材载流越大,失超传播越快;存在最小传输电流48.5 A,带材失超传播在该电流值以下被截止。     

AC loss measurements of twisted and untwisted BSCCO multifilamentary tapes

AC losses in twisted and untwisted BSCCO multifilamentary superconducting tapes with Ag matrix developed in DAPAS program were measured by an electrical method. Magnetization and transport losses were measured by a pick-up coil and by a voltage taps. Total AC loss during simultaneous application of AC transport current and an AC transverse magnetic field was given by the sum of the magnetization and transport losses measured during this simultaneous application. The magnetization loss without transport current of untwisted and twisted tapes was measured first to evaluate the effect of twisting to decouple filaments. Then, the total AC loss of the twisted tape was measured in transverse magnetic fields with various amplitudes and orientations, while the amplitude of the transport current was fixed. The measured total AC loss in a parallel transverse magnetic field was compared with some theoretical models to study the detailed characteristics of the measured total AC loss of the sample.     

Effect ofin Situ cooling on superconductivity of artificially layered BSCCO films with 2223 structure

Thin BSCCO films with the 2223 structure were deposited onto MgO substrate kept at 665°C by applying a sequential sputtering method using metallic Bi and Sr-Ca-Cu-O targets. The resultant films were then cooled under various oxygen partial pressures and cooling rates. It was found that the oxygen partial pressure and substrate temperature during cooling affect significantly the development of superconductivity of the as-deposited films. When the films were cooled stepwise under a critical oxygen partial pressure around 0.3 torr, the as-deposited films showed either superconductivity or nonsuperconductivity behavior. HRTEM images of the films showed no essential difference in the modulated structure, suggesting that the excess oxygen in the Bi-O double layers is the same and not the cause of the different superconducting behavior in the as-deposited films.     

Effect of magnetic field, thermal cycling and bending strain on critical current density of multifilamentary Bi-2223/Ag tape and fabrication of a pancake coil

Multifilamentary HTSC tapes are important for their applications in various electrical devices. Powder-in-tube technique with improved optimized synthesis parameters is regarded as one of the most promising ways to prepare long-length multifilamentary Bi-2223/Ag tapes. Nevertheless, usefulness of such tapes depends on their electrical and mechanical properties. Critical current density of a Bi-2223/Ag tape with 37 filaments has been studied at 77 K with field, field orientation, thermal cycling and bending strain as parameters. Results have been discussed in light of various mechanisms and models. A small pancake coil has been fabricated out of the same tape and the test results presented.     

薄膜厚度对YBCO超导薄膜微结构及临界电流密度的影响

用MOD方法在LAO基片上沉积不同厚度的YBCO薄膜.用X射线衍射仪对这一系列样品进行表征分析,研究随着薄膜厚度的增加微结构的变化及临界电流密度的变化.X射线衍射分析发现YBCO薄膜取向度随薄膜厚度的增加,在800nm时取得最佳值之后又随厚度增加由强变弱.晶格常数c随着膜厚的增加而增大.本研究对这一系列样品进行了φ扫描分析和摇摆曲线半高宽的计算,并对800nm的YBCO薄膜做了倒易空间图谱分析,实验结果发现800nm的YBCO薄膜晶化比较好.本研究还分析了薄膜厚度对YBCO薄膜临界电流密度的影响.     

Preparation and critical current density of hot pressed Bi2Sr2Ca2Cu3Ox/Ag tapes

Bi2Sr2Ca2Cu3Ox(Bi-2223)/Ag tapes have been prepared by hot pressing performed at 800–840 °C for 12–96 h under 6 or 12 MPa in air. The highest transport critical current density, Jc, is 3900 A cm–2 at 77 K and 8800 A cm–2 at 65 K under zero magnetic field, which is observed in tape hot pressed at 820 °C under 12 MPa for 24 h twice. The tape has undergone a cold pressing under 260 MPa between hot pressings. Jc is limited to 120 A cm–2 in tape hot pressed for 48 h continuously, in spite of total hot pressing time, temperature and pressure all being the same as for tape hot pressed for 24 h twice. It is found that alternate hot pressing and cold pressing is effective in the preparation of Bi-2223 tapes with high Jc, which is determined by the strength of grain coupling. Grain coupling is strengthened in tape hot pressed with an intermediate cold pressing. © 1998 Chapman & Hall