Critical Current Density Behaviors for Sinter-Forged Bi-2223 Bulks

Bi-2223 bulks were fabricated by a technique, a combination of cold-isostatic-press (CIP) and sinter-forging. Two batches of samples were prepared as presintering was conducted after (Route 1) and before CIP (Route 2). The effects of the deformation rate during sinter-forging, and presintering to critical current density J _c from exterior and interior sections were investigated. The results revealed that the critical current density of bulks fabricated by Route 1 was much higher than for bulks made by Route 2. The interior section of a sample with higher deformation rate has more contribution to J _c, whereas that with low and too high deformation rate has more equal contribution from exterior and interior sections. Field dependence of critical current density revealed that optimized sinter-forging could improve the critical current density J _c behavior in external fields, particularly in the low field region below 50 mT, for the samples where CIP was conducted before presintering. For the samples initially pressed with CIP, a higher sinter-forging deformation rate can improve the flux pinning force when the external field is parallel to the sample surface, but degrades the pinning force density when the external fields is perpendicular to the sample surface (H//c, 77 K).     

Variance of Critical Current Density with Microstructure by a Special Process

A promising method —“powder in tube”technique was used to fabricate Ag-sheathed Bi-2223superconductive tapes with high critical current density.After a combination processing of pressingand subsequent heat treatment,we obtained tapes with high degree of texture,good compactionand uniform properties.At 77 K in zero field,J_C was higher than 1×10~4 A/cm~2 while the highestJ_C was 1.69×10~4 A/cm~2.SEM and XRD was used to detect the tapes texture,and the relationship be-tween J_C and the degree of texture is discussed.In addition,the reason for obstacling the improvementof J_C is also investigated.     

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.     

The Influence of BaZrO3 on the Magnetic Response of (Bi,Pb) : 2223 High-Temperature Superconductors

DC magnetization and AC complex susceptibility measurements on (Bi,Pb) : 2223 high-temperature superconductors impurified with various amounts of BaZrO₃ are presented. The results are discussed in the frame of the critical state model, and the values of the inter- and intragranular critical current density as well as of the field for full penetration are estimated. The values of the intergranular critical current density are consistent with those obtained from transport measurements. The intragranular critical current density and the field for full penetration have similar values from both DC magnetization and AC susceptibility measurements. It was shown that, in the (Bi,Pb) : 2223 system, BaZrO₃ impurification changes only the properties of the intergrain matrix, while the superconducting properties of the grains are not modified.     

Extraction of Critical Current Density and Flux Creep Exponent in the Magnetic Superconductor Ru-1212 Using the ac Magnetic Susceptibility Measurements

The ac susceptibility data was employed to extract the temperature dependence of the critical current density, J _c(T), as well as the variation of flux-creep exponent n(T,H _ac) with temperature and ac field amplitude in bulk samples of polycrystalline magnetic superconductor RuSr₂GdCu₂O₈ (Ru-1212). The critical state models and the collective flux-creep approximation model were successfully accounted to describe such behavior below the transition temperature. The calculated values of n(T,H) are well fitted to a power law of the following form: n(T,H)=n ₀(H)T ^−s(H), where s is field dependent exponent whose values varied from −2.4, −1.01 for field amplitudes ranging from 0.5 G and 3.8 G. The power law describing the frequency dependence of χ′ is found to be consistent with the results of the current-dependent effective activation energy of the form U(J)=U ₀ln (J _c/J). Additionally, the dependence of the current density is found to scale according as: J _c(T)=J _c0(1−T/T _c) ⁿ , where the exponent n values varied from 1.05 to 1.25. Such dependence is an indication of intergrain coupling that could be ascribed in terms of superconductor–insulator–superconductor junctions. The derived temperature dependence of J _c(T) is in good agreement with the data obtained from the measurements using the traditional “loss-maximum” approach. Furthermore, the flux-creep effect increased with increasing both ac fields and temperatures except at about 15–25 K below the onset of T _c, where a slowing down of the flux creep was observed.     

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.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.     

Structure and superconductivity of Bi-(Pb)-Ca-Sr-Cu-O ceramics processed by arc melting

The Bi-(Pb)-Ca-Sr-Cu-O ceramics of typical cation composition 2 (0.4) 223, presintered at 800°C, are formed by arc melting and rapidly cooling the 2021 superconducting phase, CaO, and Cu₂O. The arc-melted samples sintered in air at 840°C exhibit a solid-state structural transformation of the components and a mixture of 2122 and 2223 superconducting phases, and small amounts of Ca₂CuO₃, Ca₂PbO₄, and CuO appear. When the arc melting is used as an intermediate stage in the preparation of the high-T _c superconductors in this system, a significant increase in density (from 3.7 to 5.7 g/cm³) and in critical current density (from 28 to 60 A/cm² in zero field and at liquid-nitrogen temperature) is observed, while the critical temperature remains practically unchanged (–104 K).     

Enhancement of the Critical Transport Current Density in a Bulk High-Temperature Superconducting Ceramics by Means of Thermomagnetic Treatment

An optimal mode of thermomagnetic treatment of the materials and articles being in paramagnetic state is presented. The results given show the efficiency for application of this mode of thermomagnetic treatment and its combination with the technique of growing crystals from the melt to increase the density of the critical transport current in a bulk high-temperature superconducting ceramics.     

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.     

The Effect of Post Annealing in Reduced Oxygen Atmosphere on Critical Current Density of (Bi, Pb)2Sr2Ca2Cu3Oy Long Bulks

Sinter forging has been employed to improve critical current density of Bi-2223 bulks. After that, optimal post annealing is also very important to get good performance. Different annealing schedules have been used to produce Bi-2223 sinter-forged bulks in different atmospheres. In this paper, Bi-2223 long bulks were prepared by a combination of cold isostatic pressing (CIP) and sinter-forging techniques. The samples were undergone deformation rates from 50 to 90% totally. The effect of post annealing in atmosphere of 7.5% O₂/N₂ on critical current density J_c for such (Bi, Pb)₂Sr₂Ca₂Cu₃O_y long bulks was investigated. Their microstructure features and phase changes were also studied.     

Oxygen chain disorder and compensation by Ca in Y(1-2-3) cuprates

Several M dopants such as Al, Fe, and Co at the Cu site destroy the superconductivity of YBa₂Cu₃O_6+z . However, superconductivity is restored by substituting Ca at the Y site. Arguments are developed to show that the oxygen chain disorder is not the only cause for destroying the superconductivity. A universal relation seems to exist between the net hole density as a result of Ca substitution andT _c . To stabilize the perovskite structure of YSr₂Cu₃O_6+z , it is necessary to substitute Cu by certain elements. Examples are given on Ti and Re substitution. Again, Ca cosubstitution increasesT _c . Further, the irreversibility line is enhanced by Ca, indicating improved pinning in these materials in spite of the oxygen disorder.