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AC susceptibilities (real χ ′ and imaginary χ ″) of Sr 0.6 K 0.4 Fe 2 As 2 (122 type) polycrystalline with Ag addition are analysed by the grained Bean model. A variety of characteristics, double peak in χ ″ and shoulder transition in χ ′, appear in the model simulation. Comparing the measured χ ′ and χ ″ with the model allows more clear insight on the polycrystalline structure. Estimated critical current densities J cg and J cl of the grain and the link in the iron-based pnictides show that the addition of 20 wt.% Ag increases J cl 9 times larger. Improvement of intergrain characteristics with Ag addition is clearly indicated.

It is important to investigate condensation energy density of superconducting material, since it determines its pinning property. According to the heavy-ion irradiation, the columnar defects are introduced to the superconductor which act as the strong pinning centers, and the critical current density is enhanced by the new pinning centers. Since the number density and the size of the columnar defects can be observed, the condensation energy density is estimated by using the pinning summation theory. In the present study, we prepared Ba(Fe0.93Co0.07)2As2 (Ba-122) pnictide superconductors by self-flux method. 200 MeV Au ions were irradiated into the specimens along c-axis and the matching field was 2 T. After the irradiation, the critical current density was 6 times larger than that before the irradiation. The estimated condensation energy density is in the order of 104 J/m3 and is slightly smaller than those of cuprate superconductors. The temperature dependence is similar to that of YBa2Cu3Ox oxide superconductors, since the anisotropy parameter is small in Ba-122. Therefore, it is expected to use Ba-122 at high temperatures near the critical temperature., Superconductivity Centennial Conference 2011 - EUCAS–ISEC–ICMC, Sep 18 - 23, 2011, Hague, Netherlands

A practical Bi-2223 superconducting magnet, working in liquid nitrogen (L.N 2 ), was designed and fabricated. Bi-2223 tape with a critical current of 147 A was prepared by a controlled overpressure (CT-OP) process at 77.3 K in self-field. Ten double-pancake coils were resistively connected by copper terminals. The bore diameter was 54 mm ϕ , the magnet outer diameter was 122 mm ϕ , the height of the magnet was 124 mm, and the weight of the magnet was about 3 kg. The maximum magnetic field at the center of the bore was 0.48 T with an operating current of 50 A. The experimental results agree well with design predictions calculated by finite element method. AC operation was also performed, and no distortion of the voltage waveform was observed. Therefore, this Bi-2223 superconducting magnet is a suitable replacement for copper magnets designed for applications in science and technology.

The irreversibility line was obtained in terms of the imaginary a.c. susceptibility measurement for a melt-processed superconducting Bi-2212 single grain with fine non-superconducting particles. This line showed different temperature dependences in high and low temperature regions. The one in the low temperature region agrees approximately with the prediction from the flux creep theory with experimentally observed pinning parameters. On the other hand, the one in the high temperature region is close to the well known characteristics in Y-based superconductors, suggesting flux pinning by boundaries such as the surfaces of non-superconducting particles. A quantitative discussion is also given on flux bundle size.

The irreversibility line was measured for three silver-sheathed Bi-2223 tape wires with different critical current densities. It was found that the irreversibility line was shifted to a higher temperature region according to an increasing critical current density. The temperature dependence of the irreversibility line was divided into two regions. In the lower temperature region the irreversibility line strongly depends on the temperature, while the dependence at higher temperatures is weaker and is similar to that observed for Y-Ba-Cu-O. Such a difference is considered to be due to different pinning mechanisms. Nonsuperconducting particles observed microscopically are expected to be responsible for a pinning interaction, contributing to the enhancement of the irreversibility line at high temperatures. This possibility is clarified by comparison of the observed irreversibility line with theoretical prediction.

The imaginary part of the AC susceptibility was measured as a function of temperature under various AC field amplitudes for a powder of sintered Y-Ba-Cu-O and melt-processed bulk Y-B-Cu-O with and without cracks. Its peak value was found to reduce from the prediction of the critical state model with a shift of the position to higher temperature for powder and cracked specimens, in agreement with the AC field amplitude being decreased. This deviation from the model is attributed to reversible fluxoid motion. This motion is prominent for superconductors with an effective size comparable to or smaller than Campbell's Ac penetration depth. The experimental results agree with the theoretical analysis based on Campbell's model for the reversible fluxoid motion. This analysis is useful to estimate the effective size of superconducting specimens and hence, to infer causative defects for percolative superconducting currents.

Critical current density and its history effect in Bi-based high Tc superconducting tapes were measured using an a.c. inductive method with various magnetic fields and temperatures. A rapid decrease in the critical current density with increasing temperature and the history effect were observed in the same low temperature region between 4.2 and 30 K. This suggests that the two phenomena are caused by low temperature weak links in the specimens. A weak link region is considered to lose superconductivity at temperatures higher than 30 K. Above 30 K, a strongly bonded area retains superconductivity and the Jc changes slowly with increasing temperature, showing no history effect.

It has been found in measurements of AC susceptibility versus temperature for high temperature superconductors that, with decreasing AC magnetic field amplitude, the peak value of the imaginary part, χ decreases remarkably accompanied by a shift of its position to higher temperatures, while the height of the transition of the real part, χ does not change appreciably. This behavior contradicts the prediction by the critical state model in which the flux pinning phenomenon is assumed to be completely irreversible. It is shown that this anomalous magnetic behavior originates from reversible fluxoid motion in pinning potentials. The AC susceptibility is calculated on the basis of the Campbell model for the reversible fluxoid motion. The result obtainained warns us against incorrect use of the critical state model for analysis of experimental data.

Significantly broad resistive transition has been observed in high temperature superconductors under a magnetic field. A similar broad transition was observed by French et al. in resistance versus magnetic field characteristics in low temperature superconductors for various current densities. In this case, the critical current density and the flow resistivity completely determine the overall resistive characteristics. This suggests that the resistive characteristics in high temperature superconductors may also be determined by these two quantities alone. In fact, the procedure outlined reproduces the commonly observed resistive characteristics. A notable feature determined from this procedure is that a knee exists in the resistance versus temperature curve. This projection corresponds to the irreversibility point, i.e. the temperature at which the critical current density is reduced to zero.

A significantly broad resistive transition has been observed in high-temperature superconductors in magnetic fields, and its mechanism has been extensively discussed. A similar broad transition was observed by French et al. in 1967 even for low-temperature superconductors, where a variation in the magnetic field dependence of the resistance was measured for various current densities. The resistive properties in low-temperature superconductors are mainly determined by the fluxoid motion. Hence, this suggests that the resistive properties of high-temperature superconductors are similarly determined. In this paper, the broad resistive transition in high-temperature superconductors expected from the simple flux flow is estimated. The fluxoid motion in the longitudinal field geometry where the Lorentz force does not work is also discussed.

It has been observed in measurements of imaginary ac susceptibility, χ″, of oxide superconductors vs temperature that a peak is shifted to higher temperature with a reduction in its value according to decreasing ac field amplitude. The anomalous reduction in the peak value contradicts the theoretical prediction by the critical state model and is caused by a reversible motion of fluxoids in pinning potential. In this paper χ″ is calculated on the basis of the Campbell model.

The intergrain critical current densities, Jc, parallel and perpendicular to c-axis were measured as a function of the magnetic field at 4.2 and 77.3K by an ac inductive method for the fairly oriented Bi1.6Pb0.4Sr2Ca2Cu3Oy specimens. The Jc's perpendicular to c-axis were higher than those parallel c-axis, about ten times within 5T at 4.2K, and about three times within 0.1T at 77.3K. Our results suggect the orientation of each grain to be especially worthwhile for performing the high Jc at the low temperature.

Percolative critical transport current densities in polycrystalline high Tc oxide superconductors with orientated and random textures are theoretically estimated at the liquid nitrogen temperature. The effect of depression of the current through grain boundaries recently found by Dimos et al. was taken into account in the estimation. The critical current density obtained in polycrystalline materials with orientated textures is of a comparable order of magnitude with the recent observation by Jin et al. and the result suggests a possibility of further improvement up to 109 A m−2 by increasing the pinning strength. As for randomly textured materials, the theoretical estimate of critical current density is much larger than values commonly observed. The depressing effect of the current by grain boundaries in randomly textured materials is deduced to be much more significant than that observed by Dimos et al.

Following the measurements of anisotropy and hysteresis in transport critical currents with changing temperature and polarity of magnetic field, a.c. magnetic measurements by the Campbell method were performed for sintered Ln-Y-Ba-Cu-O superconductors. The information derived from them indicated that hysteresis does not occur in the intragrain current but in the intergrain one. The results are interpreted in terms of flux pinning, where the anisotropy originates from the texture structure and hysteresis is associated with the superconducting weak links of the sintered oxide pellets.