Alternating Current Losses in Ag-Sheathed (Bi, Pb)-2223 Multifilamentary Tape at Liquid-Nitrogen Temperature

The alternating current losses have been investigated at 77 K on the silver-sheathed (Bi, Pb)-2223 multifilamentary tape having 37 filaments using an inductive technique under ac magnetic field with frequencies between 100 and 510 Hz. The measured data on the multifilamentary tape have been discussed vis-à-vis that reported for monofilamentary tapes. The results obtained could be accounted fairly well by considering contributions from the eddy current losses in the silver matrix along with the hysteresis losses in the superconducting filaments.     

Transport and Magnetic Properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)y Superconductors

We investigated the superconducting critical temperature, the intra- and intergranular critical current density, and the thermopower properties of Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiCl) _y samples. All these properties have been compared with those of Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiF) _y specimens. It was found that the critical temperature determined from resistive and AC complex susceptibility measurements show a maximum and the transition width shows a minimum for the intermediate values of y. Powder X-ray diffraction studies and the AC complex susceptibility measurements reveal that in our samples the amount of Bi₂Sr₂Ca₂Cu₃O_{10 +} high-temperature superconducting phase is maximum for y 0.02. The amount of LiCl in Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiCl) _y changes the superconducting properties of the grains as well as of the intergrain matrix. The splitting of the peak in the temperature dependence of the imaginary part of the complex susceptibility, corresponding to the dissipation inside the grains, was also observed.     

Bi（2223）超导体的磁屏蔽效应

研究了Ｂｉ（２２２３）超导体磁屏蔽筒的磁屏蔽效应，从而得出磁屏蔽效果与超本的屏蔽空间大小有关，以及将０．３ｍｍ厚的软铁筒放于两超导体屏蔽筒之间，可使磁屏蔽效果提高为单超导体蔽筒的２１倍的结论。     

Preparation of high performance (Bi,Pb)-2223 superconductor using a sol–gel synthesized amorphous precursor through controlled gelation

A controlled gelation method through an acrylate route is identified for the synthesis of high quality precursors for the production of bulk (Bi,Pb)-2223 superconductors. The method overcomes the problem of undesirable precipitation of crystallites during gelation by controlling the vapour loss from the sol and the temperature and leads to an amorphous gel which on subsequent processing results in a highly reactive and homogenous precursor. (Bi,Pb)-2223 superconductors prepared from the precursor have shown highly enhanced properties with respect to final phase purity, sintered density, microstructure and critical current density compared to those prepared from conventional solid state precursors.     

Intergranular Critical Current Density in Polycrystalline (Bi-Pb)-2223 High Temperature Superconductor by ac Susceptibility Measurements

The field dependence of real () and imaginary () component of ac susceptibility of superconductors within the critical state model can conveniently be used for evaluating the critical current when field amplitude is larger than the penetration field. A method to analyze the real () and imaginary () component of fundamental ac susceptibility with the objective of extracting the temperature dependence of critical current density J _c(T) is reported. The procedure makes use of the ac susceptibility data of two polycrystalline (Bi-Pb)-2223 samples measured with different excitation amplitudes below and close to the critical temperature. The temperature dependence of J _c is extracted using the isothermal scan over and data. Results obtained from this procedure are found to be in fair agreement with J _c(T) calculated from traditional loss-maximum data.     

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.     

Magnetization and Susceptibility Studies on BaZrO3-Doped YBa2Cu3O7 ? x Bulk Superconductors

We studied the YBa₂Cu₃O_{7 – x} bulk superconductor doped with BaZrO₃ up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO₃ doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO₃ does not lead to degradation of the superconducting properties of bulk YBa₂Cu₃O_{7 – x} .     

Primary Phase Field of the Pb-Doped 2223 High-Tc Superconductor in the (Bi,Pb)-Sr-Ca-Cu-O System

Both liquidus and subsolidus phase equilibrium data are of central importance for applications of high temperature superconductors in the (Bi, Pb)-Sr-Ca-Cu-O system, including material synthesis, melt processing and single crystal growth. The subsolidus equilibria of the 110 K high-T_c Pb-doped 2223 ([Bi, Pb], Sr, Ca, Cu) phase and the location of the primary phase field (crystallization field) have been determined in this study. For the quantitative determination of liquidus data, a wicking technique was developed to capture the melt for quantitative microchemical analysis. A total of 29 five-phase volumes that include the 2223 phase as a component was obtained. The initial melt compositions of these volumes range from a mole fraction of 7.3 % to 28.0 % for Bi, 11.3 % to 27.8 % for Sr, 1.2 % to 19.4 % for Pb, 9.8 % to 30.8 % for Ca, and 17.1 % to 47.0 % for Cu. Based on these data, the crystallization field for the 2223 phase was constructed using the convex hull technique. A section of this “volume” was obtained by holding two components of the composition at the median value, allowing projection on the other three axes to show the extent of the field.     

Metastable states of the 2223 phase in the Bi(Pb)SrCaCuO system

By the magnetically modulated microwave absorption method (MAMMA) we observed the modifications induced by different calcination temperatures (between 830°C and 870°C) on the 2223 phase formed in a system sintered at (855±5)°C with the starting composition Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _y . The presence of the 2223 phase in almost distinct states (consequently, multiple 2223 phases) in the same sample was observed. As the calcination temperature was increased up to 850°C, the highest temperature state of the 2223 phase intensified. Higher calcination temperatures resulted in the enhancement of other lower-temperature states. The homogeneity of the 2223 phase was greatly improved by annealing the samples at, 800°C for 5 min in a flowing nitrogen atmosphere. We labeled as metastable the lower-temperature states having excess oxygen, which, by easily losing the supplementary oxygen under the above annealing procedure, were shifted to higher temperatures.     

Sc Addition to (Bi,Pb)2Sr2Ca2Cu3Ox Freeze-Dried Ceramic

In a freeze-dried nitrate precursor powder with cation composition Bi:Pb:Sr:Ca:Cu = 1.7:0.3:2.0:2.5:3.5, Sc₂O₃ was added. The stoichiometry for the mixed precursor powder was Bi:Pb:Sr:Sc:Ca:Cu = 1.7:0.3:2.0:0.25:2.5:3.5. The two nitrate precursor powders with and without Sc were thermally decomposed, pressed, and sintered in the same conditions. They show different behavior, as revealed by thermal analysis, X-ray diffraction (XRD), Scanning electron microscopy (SEM), and (T) measurements. Addition of Sc slows down synthesis processes, including 2223-phase during reactive sintering. After 70 h of sintering, 43% of 2223-phase formed in the pellet with Sc, whereas in the Sc-free pellet, 51% of the 2223-phase was attained. Moreover, addition of Sc leads to a certain morphology, resulting in lower superconducting characteristics.     

Development of Ag-Sheathed Bi(2223) Tapes with Improved Microstructure and Homogeneity

The development of the fabrication process of Ag-sheathed Bi(2223) tapes has been carried out in order to improve their transport and mechanical properties, as required by the power applications which are so far under study. Critical current density values of 28 kA/cm² at 77 K have been achieved on long multifilamentary Bi(2223) tapes, with a fabrication process that has been successfully employed in the fabrication of samples longer than 50 m. The microstructure and homogeneity of Ag-sheathed multifilamentary Bi(2223) tapes has been markedly improved by employing an alternative deformation technique. In a substantial part of the fabrication process, swaging, drawing, and rolling have been replaced by deformation with an active turks-head machine, which allows the deformation of rectangular shaped wires. At present, critical current densities in excess of 25 kA/cm² at 77 K have been achieved on long samples prepared with this technique. Moreover, innovative filament configurations have been employed for the fabrication of square-shaped Bi(2223) wires with reduced anisotropy and with critical current densities exceeding 20 kA/cm² at 77 K.     

Preparation and characterization of glassy and superconducting (Bi,Pb, Sb)-Sr-Ca-Cu-O systems

Superconducting compounds with nominal compositions Bi_1.7Pb_0.3Sr_1.6Ca₂ Cu_3.4O _x and Bi_1.9Sb_0.1Sr₂Ca₂Cu₃O _y have been synthesized by ceramic and glass routes and characterized by X-ray diffraction, electrical resistivity and dielectric constant measurements. The zero-electrical resistance temperatures are about 70K. The dielectric constant of the glasses at room temperature is around 30.     

The Pb–Pb and O–Pb Nuclear Spin Coupling in Ba(Pb,Bi)O3 Oxides

We report the ¹⁷O, ²⁰⁷Pb NMR and ¹⁷O–²⁰⁷Pb SEDOR study of superconducting BaPb_1-x Bi _x O₃ (x 0.35) oxides. Inhomogeneous The results points to the formation of microscopically inhomogeneous distribution of the carrier density in metallic phase. The local spin susceptibility is increased around Bi cation within an area, which is delimited by its two first cation shells.     

Hydrothermal synthesis and characterization of (Bi,K)TiO3 ferroelectrics

A lead-free ferroelectric (Bi,K)TiO₃ (BKT) was synthesized by a hydrothermal process and characterized systematically at various temperatures. Well-crystallized BKT in the tetragonal phase was identified at a hydrothermal temperature over 220 °C. Small cubic particles were observed, regardless of hydrothermal temperature. The BKT sintered at 1050 °C was observed to be a typical relaxor behavior and very stable against frequency and temperatures, respectively. The sintered-BKT ceramics exhibited a high temperature of maximum dielectric permittivity (T_max = 356 °C at 10⁶ Hz) with piezoelectric constant (d₃₃ = 65 pC/N) and electromechanical coupling factors (k_p = 0.22, k_t = 0.43). Thus, the sintered-BKT showed excellent temperature stability with a high-T_max and piezoelectric properties.     

Thermal stability in high-Tc coil and magnet design by process control of Bi(Pb)-2223/Ag multifilamentary tapes

It has been found that the degree of thermal stability of Bi(Pb)-2223/Ag pancake-shaped coils at 77 K can be determined by controlling the amount of matrix and superconducting materials during processing. The intermediate deformation step between sintering stages has been found to be crucial in optimizing the performance of the processed composite tapes as well as governing the thermal stability of the subsequently made pancake-shaped coils. Results obtained from numerical analysis of the finite element method has shown that monolayer coils produced from Bi(Pb)-2223/Ag composite tapes are thermally very stable with high values of the fill factor. However, increasing the number of co-wound tapes would require either a reduction in the fill factor or an increase in cooling rate for thermal stability to be sustained as would otherwise be achieved with the metallurgically same single tape.     

Magnetic field hysteresis of critical currents in Bi(2223)/Ag tapes and a way to overcome it

Experiments showing hysteresis of critical currents versus the external magnetic field I_c(B_e) were performed with two multifilamentary Bi(2223)/Ag tapes. The I_c(B_e) hysteresis is observable in the transversal as well as in the longitudinal orientation of the long axis of the tape with respect to the magnetic field. Based on the idea that the hysteresis is the effect of trapped flux in a network of well-connected current paths, a way to overcome this effect has been proposed and experimentally verified. The induced frozen-in screening currents are split into several parallel current patterns by cycling the external magnetic field around the adjusted value. Using the proposed method, the ‘neutral’ I_c(B_e) characteristics have been found. Approximate calculations of the penetration depth of the trapped flux show that the network of well-connected current paths could be formed by several disk-shaped grains (≈ Φ8 × 0.4 μ) stacked into more or less axially ordered (quasi cylindrical) colonies of average dimensions estimated to ≈ Φ8 × 4 μm.     

热压处理对室温轧制Ag/Bi(2223)超导带材显微组织的影响

采用SEM和TEM观测了经热压处理后的Ag/Bi(2223) 带材(Jc＝56000A/cm2,在77K、自场)和只经过室温轧制处理的带材.与室温轧制的带材比较发现:即使在临近第二相的区域,热压处理后的带材结构也非常密实;热压处理后,晶体缺陷诸如裂缝、亚晶界、残余无定形态和中间相等,易阻碍或中断电流传输的缺陷显著减少;热压处理后带材的位错密度基本相同,但在某些区域分布不均匀或呈网状结构;在一定方向上晶界能捕获途经它的位错;热压过程促使第二相部分转变为超导相Bi(2223),进而提高带材的载流能力.     

Effects of local characteristics on the performance of full length Bi2223 multifilamentary tapes

The effects of local characteristics on the performance of full length Bi2223 multifilamentary tapes are investigated computationally and experimentally at 77 K and self-field. Generally the current-voltage characteristics of superconductors are described by the standard power law model with parameters such as critical current I_c and index n. By measuring the critical current {I_ci} and index {n_i} values of local tapes, we can get the critical current I_c and n value of full length tapes by means of statistical method. The results show that the distribution of local critical currents are non-uniform, and local critical currents have important effect on the performance of the entire tapes. The critical current of the entire tape is different from the mean value of local critical current based on Gaussian statistical distribution along the long tape.     

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.