出版者・発行元：Journal of Physics: Conference Series  

In this study, we focused on the longitudinal magnetic field effect in critical current densities and investigated the enhancement of the electrical conduction properties in a commercial Bi-2223 superconducting tape. Since the samples used in this study were polycrystalline and tape-shaped, the longitudinal magnetic field effect was expected to differ from that in single crystals or conventional superconducting alloys. Therefore, the critical current characteristics at magnetic fields of up to 5 T as well as at various angles between the magnetic field and current were measured in detail.

DOI： 10.1088/1742-6596/1054/1/012043

Scopus

記述言語：英語  

出版者・発行元：IEEE Transactions on Applied Superconductivity  

Since novel iron-based pnictide of Sr0.6K0.4Fe2As2 superconductor was discovered, the applications on high magnetic field and large current transportation are highly expected due to its low cost and unique superconductivity such as low anisotropy, relatively high critical current density, and critical temperature. Recently, we have successfully developed the Fe-clad superconducting Sr0.6K0.4Fe2As2 tape with the critical temperature of 32.5 K and fairly high critical current density of 1.5× 108 A/m2 at 10 T and 4.2 K. Since the critical current density is considered as one of the most important factors for applications of superconducting materials, detailed investigation on the critical current characteristic and flux pinning mechanism in the supposed operation temperature range is necessary. In this paper, the critical current densities in the temperature range of about 20 K-30 K and at the magnetic field of up to 7 T were measured by using an inductive method (Campbell's method). It turned out that the critical current density of the tape is fairly high around 20 K. In this paper, basing on the experimental results of the penetrating ac magnetic flux profile and the feature of ac magnetic field dependence of penetration depth, the flux pinning mechanism of the sample was discussed.

DOI： 10.1109/TASC.2016.2523800

Scopus

出版者・発行元：Physica C: Superconductivity and its Applications  

AC susceptibilities (real χ′ and imaginary χ″) of Sr0.6K0.4Fe2As2 (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 Jcg and Jcℓ of the grain and the link in the iron-based pnictides show that the addition of 20 wt.% Ag increases Jcℓ 9 times larger. Improvement of intergrain characteristics with Ag addition is clearly indicated.

DOI： 10.1016/j.physc.2012.03.018

Scopus

出版者・発行元：Xiyou Jinshu Cailiao Yu Gongcheng/Rare Metal Materials and Engineering  

We report the effect of fluorine doping on the fabrication and superconductivity of the iron-based arsenic oxide SmO 1-xF δFeAs (δ≥x) compound. The fluorine over-doping can greatly reduce the heat treatment temperature and the heat treatment time, meanwhile the superconductivity of the SmO 1-xF δFeAs samples does not reduce. The critical transition temperature (T c) is as high as 56.5 K for the SmO 0.7F 0.3FeAs sample prepared by the two-step solid state reaction method at 1120°C for 40 h, the J c of the sample is 2.4×10 5 A/cm 2 (10 K, 0 T). T c of the fluorine over-doped SmO 0.7F 0.35FeAs and SmO 0.7F 0.4FeAs samples prepared by the two-step solid state reaction method at 1100°C for 20 h are 56 and 55 K, respectively. J c of the both samples are 1.9×10 5 and 1.7×10 5 A/cm 2 (10 K, 0 T), respectively. Copyright © 2012, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

Scopus

出版者・発行元：Journal of the Korea Institute of Applied Superconductivity and Cryogenics  

In this review paper, we report the characteristics of the critical current density in FeAs based superconductors which is newly discovered by Hosono group of Tokyo Institute of Technology on 2008. Since the many specimens in present stage are not single crystals, there are two kinds of critical current density observed in the specimens which are so-called local and global critical current densities. Therefore, it is necessary to evaluate both kinds of critical current densities. The history effect in which the global critical current density shows different values in increasing and decreasing magnetic field is also observed when the specimens have the local and the global critical current densities. The wire which critical current is 180 A is successfully developed with using the knowledge of abovementioned characteristics of two kinds of critical current densities and the history effect.

DOI： 10.9714/psac.2012.14.2.001

Scopus

出版者・発行元：Physics Procedia  

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.

DOI： 10.1016/j.phpro.2012.06.269

Scopus

出版者・発行元：Physics Procedia  

The superconducting Ba1-xKxFe2As2 (x = 0.4) single crystals were prepared by the so-called FeAs self-flux method. The critical temperature by ac susceptibility measurement was estimated to be about 36 K. The magnetic field and temperature dependences of critical current densities were investigated by an ac inductive measurement (Campbell's method). Unlike the phenomenon of co-existence of the global and local critical current densities observed in many polycrystalline Fe-based superconducting pnictides, it was found that only a uniform critical current density (Jc) flows through the whole sample. The value of Jc at 20 K and 1 T was about 5×108 A/m2, which is much smaller than the local critical current density observed in polycrystalline samples. The force-displacement characteristic of fluxoids in sample was investigated. The Labusch parameter was found to increase monotonously with increasing magnetic field, while the interaction distance was proportional to the fluxoid spacing. These results are consistent with the prediction based on a simple flux pinning mechanism.

DOI： 10.1016/j.phpro.2012.06.271

Scopus

出版者・発行元：Physica C: Superconductivity and its Applications  

The effect of distribution of powder size on the estimation of intra-grain critical current density was investigated for polycrystalline samples of Sr 0.6K0.4Fe2As2 with and without addition of Ag 20 wt.%. It turned out that the value of intra-grain critical current density calculated using the average of powder size Jcl is more overestimated than that calculated taking account of the distribution of powder size JclP as the distribution of powder size is widened, since the magnetic moment drastically increases due to the larger contribution from larger particles. From the distribution of powder size obtained by the microscopic observation using SEM, the factor of overestimation of J cl/JclP is found to be larger than two in both samples. Moreover, the distribution of powder size in the Ag-added sample is found to be smaller than that of pure sample. Therefore, the factor of overestimation of Jcl is considered to be smaller in the Ag-added sample. It is recommended to take account of the distribution of powder size for precise estimation of intra-grain critical current density. © 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2011.05.087

Scopus

出版者・発行元：IEEE Transactions on Applied Superconductivity  

The superconducting Sr0.6K0.4Fe2As 2 (122 type) polycrystalline iron-based pnictides with Pb or Ag addition were prepared by a normal PIT method. The critical current densities were investigated by ac inductive (Campbell's) method. It was found that a fairly large critical current density over 1010 A/m2 even at the relatively high temperatures flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low global critical current density of 104106 A/m 2flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller critical current density in the increasing-field process than that in the decreasing-field process. Basing on the experimental results of the penetrating ac magnetic flux profile and the characteristic of ac magnetic field vs. penetration depth, the inter- and intra-granular super-current, the weak-link and the history effect in these materials were discussed. © 2010 IEEE.

DOI： 10.1109/TASC.2010.2088092

Scopus

出版者・発行元：Physica C: Superconductivity and its Applications  

The local and global critical current densities of polycrystalline bulks Sr0.6K0.4Fe2As2 were investigated by both the remnant magnetic moment and Campbell's methods. It was found that the global critical current density was improved by addition of Ag. It was speculated that the reason for the enhancement of the global critical current density was ascribed to the enlarged size of particle and the clean boundaries due to addition of Ag. It was also found that the critical current density estimated from usual magnetization measurement was a type of average value of both local and global critical current densities and can be explained by simple contributions from magnetic moments by local and global shielding currents. Therefore, the size of the grain was estimated by the present model and the agreement with the observation by SEM was obtained. © 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.01.033

Scopus

出版者・発行元：Physica C: Superconductivity and its Applications  

The recent discovery of superconductivity in iron arsenide compounds REFeAsO (RE = rare earth) or AFe2As2 (A = alkaline earth) has attracted great attention due to the high Tc that raised above 50 K. In this study, the effect of Ag addition (20 wt.%) on the critical current density Jc of polycrystalline Sr0.6K0.4Fe 2As2 was investigated for improvement of the weak links in polycrystalline iron-based superconductors. Although effect of Ag addition on the intra-grain current density Jclocal was not confirmed, the connectivity among grains is speculated to be improved by filling voids among grains by Ag. © 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2010.05.077

Scopus

記述言語：英語  

出版者・発行元：Journal of Physics: Conference Series  

Performance of a Bi-2223 superconducting magnet consisted by ten double-pancake coils was improved by using iron plates (SS400) for top and bottom flanges. The magnetic field dependence of the critical current density of Bi-2223 tape for magnetic field normal to the tape was determined the maximum magnetic field of the magnet. Hence, a reduction of the normal component of the magnetic field to the tape was achieved by using the iron plates. The magnetic field distribution in the magnet was calculated by a finite element method, and the reduction of the normal magnetic field component was confirmed. The maximum magnetic field at the center of the magnet bore reached 1.30 T at 65.0 K of subcooled liquid nitrogen and 0.70 T at 77.3 K. These values were about 1.6 times higher than the previous magnet and agreed well with the calculated result. © 2010 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/234/3/032046

Scopus

出版者・発行元：Journal of Physics: Conference Series  

The superconducting SmFeAsO1-xFx (x=0.2) polycrystalline bulks were prepared by the powder-in-tube (PIT) method. The magnetic field and temperature dependences of critical current densities in the samples were investigated by resistive and ac inductive (Campbell's) methods. It was found that a fairly large shielding current density over 109 A/m2, which is considered to correspond to the local critical current density, flows locally with the perimeter size similar to the average grain size of the bulk samples, while an extremely low transport current density of about 105 A/m2 corresponding to the global critical current density flows through the whole sample. Furthermore, a unique history dependence of global critical current density was observed, i.e., it shows a smaller value in the increasing-field process than that in the decreasing-field process. The history dependence of global critical current characteristic in our case can be ascribed to the existence of the weak-link property between the grains in SmFeAsO1-xFx bulk. © 2010 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/234/1/012028

Scopus

出版者・発行元：Physica C: Superconductivity and its Applications  

The critical current densities of polycrystalline bulk SmFeAsO1-xFx prepared by the powder-in-tube (PIT) method and by a conventional solid-state reaction were investigated using the remnant magnetic moment method and Campbell's method. Two types of shielding current, corresponding to global and local critical current densities Jc were observed using both measurement methods. The global and local Jc were on the order of 107 A/m2 and 1010 A/m2 at 5 K, respectively. The local Jc decreased slightly with increasing magnetic field. The global Jc was independent of the preparation method, while the local Jc was larger for samples prepared by PIT than for those prepared by solid-state reaction. © 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.06.013

Scopus

出版者・発行元：Cryogenics  

A practical Bi-2223 superconducting magnet, working in liquid nitrogen (L.N2), 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 mmφ{symbol}, the magnet outer diameter was 122 mmφ{symbol}, 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. © 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.cryogenics.2008.11.007

Scopus

出版者・発行元：IEEE Transactions on Applied Superconductivity  

In this study, we focused on the estimation of the global critical current density, super-current path, grain connectivity and their relationships with the faults volume fraction in the MgB2 bulks prepared by a modified PIT (powder in tube) method. Campbell's method was applied for the purpose of acquiring the penetrating AC magnetic flux profile and the characteristic of AC magnetic field vs. penetration depth from the specimen's surface. A computer simulation on the penetrating AC flux profile in MgB2 bulks with certain amount of randomly distributed voids, oxidized grains and other faults was also carried out. The global critical current density obtained by Campbell's method turned out to be smaller than that obtained from the SQUID measurement, implying that the global super-current was reduced by the existence of various kinds of faults and the lack of the electrical connectivity between grains. It was verified that the relationship between the global critical current characteristics and the faults contained in specimens can be quantitatively clarified by comparing the simulated critical current densities and other factors with the experimental results. ©2009 IEEE.

DOI： 10.1109/TASC.2009.2019553

Scopus

記述言語：中国語  

出版者・発行元：Physica C: Superconductivity and its Applications  

Many recent reports on the critical current density (Jc) in superconducting MgB2 bulks indicated that improving the grain connectivity is important, since the obtained Jc values were generally much lower than those in other metallic superconductors and it was ascribed to the poor connectivity between grains in polycrystalline MgB2. In this study, we focused on the estimation of the global critical current density, super-current path, grain connectivity and their relationships with the faults volume fraction in the MgB2 bulks prepared by a modified PIT (powder in tube) method. Campbell's method was applied for the purpose of obtaining the penetrating AC flux profile and the characteristic of AC magnetic field vs. penetration depth from the sample's surface. A computer simulation on the penetrating AC flux profile in MgB2 bulks with randomly distributed voids, oxidized grains and other faults was also carried out. Jc obtained by Campbell's method turned out to be smaller than that obtained from the SQUID measurement, implying that the global super-current was reduced by the existence of various faults and the lack of the electrical connectivity. It was verified that the relationship between the global critical current characteristics and the faults contained in MgB2 samples can be quantitatively clarified by comparing the simulated critical current densities and other factors with the experimental results. © 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2008.05.242

Scopus

出版者・発行元：Journal of Physics: Conference Series  

The purpose of this study is to generate a highly stable DC current by using a superconducting transformer. For the first trial stage, a 100 A class superconducting transformer was designed and fabricated for operation at the temperature of liquid nitrogen. The secondary coil of the transformer was wound with a superconducting Bi-2223 tape, and the turn ratio of the primary and secondary windings was about 11:1, which brought an output current of about 80 A by a small varying control current. The control system was constructed with a PC, an AD/DA conversion PCI board and a small current supply. The primary input current was controlled by a normal PID control program. The secondary output current was evaluated by measuring the voltage on a low resistivity shunt. As the result of this study, it was suggested that in the case of small impedance in secondary winding circuit, the DC output current with high stability can be obtained by applying an easy control algorithm to the program running on the controlling PC. © 2006 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/43/1/204

Scopus

出版者・発行元：IEEE Transactions on Applied Superconductivity  

In order to substitute cheaper sheath materials for Ag, Bi-2212 superconducting thick films grown on oxidized Ni substrates were prepared by using a normal partial melt process. 0-5 vol% of fine MgO particles were doped in Bi-2212 phase during the fabrication for the purpose of enhancement of the critical current density (Jc) in Bi-2212, The samples were analyzed with the assistances of X-ray diffractometer (XRD) and electron probe microanalyzer (EPMA). The critical temperature and Jc were measured by using the conventional resistive method (4-probe method). An apparent improvement in Jc characteristic was observed in the samples with fine MgO particles doped. The Jc value in the 5 vol% MgO doped sample reached to the level comparable with that in other Ag-sheathed samples. Furthermore, the irreversibility field was confirmed to be largely enhanced by the addition of MgO particles. The pin parameters derived from the scaling behavior of pinning force density turned out to be similar to those predicted in the case of normal precipitate flux pinning, indicating that MgO particles in Bi-2212 act as effective pinning centers.

DOI： 10.1109/TASC.2003.812518

Scopus

出版者・発行元：Physica C: Superconductivity and its Applications  

Several kinds of Bi-2212 superconducting thick films grown on the oxidized Ni substrates were prepared by using a normal partial melt process. 0-5 vol% of fine MgO particles were doped in Bi-2212 phase during the fabrication in order to enhance the critical current density (Jc) in Bi-2212. The samples were analyzed with the assistances of X-ray diffractometer and electron probe microanalyser. The critical temperature and Jc were measured by using the conventional resistive method (four-probe method). An apparent improvement in Jc values was observed in the samples with fine MgO particles doped. The Jc in the 5 vol% MgO doped sample reached to the level comparable with that in the Ag-sheathed samples. Furthermore, the irreversibility field (Birr) was confirmed to be largely enhanced by the addition of MgO particles. The pin parameters derived from the scaling behavior of pinning force density turned out to be similar to those predicted in the case of normal precipitate flux pinning, indicating that MgO particles in Bi-2212 act as effective pinning centers. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)02139-1

Scopus

出版者・発行元：Physica C: Superconductivity and its Applications  

In order to substitute cheaper metal for Ag, superconducting Bi-2212 thick films were prepared on an oxidized Ni substrate. As one of the approaches enhancing the critical current characteristic in Bi-2212 thick films, several percent of fine MgO particles were doped during the specimen fabrications. The detailed investigations on MgO particles doped in Bi-2212 and their effect on the critical current density (Jc) were carried out. The results of EPMA analysis show that MgO particles disperse uniformly in Bi-2212. Apparent enhancements in Jc were obtained in specimens with several volume percent of MgO doped, which is fairly similar to the results from the superconducting Bi-2212 bulk materials in our previous study. Since no apparent differences in physical and chemical properties could be recognized between the specimens with and without MgO particles doped, the enhancement of Jc implies that the MgO particles act as effective flux pinning centers. © 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4534(02)00998-X

Scopus

出版者・発行元：IEEE Transactions on Applied Superconductivity  

In order to introduce effective pinning centers into Bi-2212 superconducting bulk, the addition of small MgO particles to Bi-2212 bulk and its effect on flux pinning were studied. Bi-2212 bulk samples were prepared by employing a normal partial-melting process, with a 0-8 vol.% of MgO particles addition. The morphology of Bi2212 with MgO doping was investigated by means of XRD and EPMA. The magnetization and critical current densities (Jc's) in various magnetic fields and temperatures were measured by using a SQUID magnetometer. It was found that Jc in samples with relatively large MgO particles introduced shows a larger value than that in MgO-free sample, which implies that the MgO particles act as effective pinning centers and enhance the flux pinning characteristic in Bi-2212 bulk. A qualitative agreement between the experimental results and theoretical estimation was obtained. © 1999 IEEE.

DOI： 10.1109/77.784942

Scopus

出版者・発行元：IEEE Transactions on Applied Superconductivity  

The characteristics of electric field E vs. current density J at various magnetic fields and temperatures in superconducting PbBi samples with different pinning strength were measured by using a usual resistive method. The obtained E-J curves show a nonlinear behavior similar to that in high Tc superconductors(HTSC). With a scaling generally used in HTSC, E-J curves collapse onto one of the two universal branches, with the exponents of v and z similar to those reported in HTSC. It was found that the stronger flux pinning strength leads to the higher transition points of Tg and Bg in E-J characteristic. The nonlinear E-J curves and the enhancement of the transition points were explained and reproduced by using the flux creep and flux flow theory. © 1997 IEEE.

DOI： 10.1109/77.621010

Scopus

出版者・発行元：Superconductor Science and Technology  

Estimation of irreversibility fields in pinning-strength-controlled metallic superconductor is carried out for superconducting Pb-Bi alloys. The magnitude and temperature dependence of the irreversibility field depend only slightly on the flux pinning strength. The difference between the irreversibility field and the upper critical field remains finite in specimens with a strong pinning force. The experimental result is compared with the flux creep theory and found to be explained well by the theory.

DOI： 10.1088/0953-2048/9/6/006

Scopus

出版者・発行元：Physica C: Superconductivity and its applications  

Magnetic properties of n-type "infinite-layer superconductors", (Sr0.9La0.1)CuO2 prepared under high pressure were studied. From VSM and a.c. penetration measurements, the intra-grain critical current density was found to reach ∼1010A/m2 at 4.2K. Its temperature dependence as well as irreversibility field suggested that, in spite of its relatively low Tc (42K) value, the flux line lattice is rather rigid in this compound, reflecting its unique crystal structure composed only of CuO2 planes and alkali earth (rare earth) ions. © 1994.

DOI： 10.1016/0921-4534(94)90958-X

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

In iron-related deep levels in n-type silicon, the introduced concentrations of Ec—0.21 eV(B) and Ec -0.41 eV(C) levels decrease with increasing the diffusion time at 1160°C, indicating that the formation process includes the conversion process into other types of complexes. The concentration of the C level decays exponentially with the duration of isothermal annealing up to 200°C, the time constant activation energy being 0.65 eV. The results indicate that the observed defects are due to the intermediate states in consecutive reactions of iron-related complex formation. © 1993 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.32.L1645

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

Measurements of the pinning force density and the displacement of fluxoids in ac magnetic field were carried out for a melt -processed Y-Ba-Cu-O bulk superconductor using the Campbell method. The Labusch parameter, ॅL, and the interaction distance, di, were estimated from the obtained force-displacement profile. It was found that ॅL depended greatly on temperature and magnetic field, especially in the field parallel to the c-axcis, and di decreased gradually with increasing temperature. These variationss were deduced to be caused by flux creep. The depth of pinning potential, U0, for the trapped flux bundle was estimated using the values of ॅL and di. This was compared with the apparent pinning potential which is commonly observed from magnetic relaxation measurements. © 1993 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.32.L1215

Scopus

出版者・発行元：Cryogenics  

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. © 1993.

DOI： 10.1016/0011-2275(93)90040-U

Scopus

出版者・発行元：Superconductor Science and Technology  

The effect of flux creep is noticeable on the critical current density, Jc, of oxide superconductors especially at high temperatures. One of the remarkable features of the phenomenon is that the irreversibility line, i.e., the boundary of zero and nonzero Jc's, is far below the phase boundary, Bc2(T). The irreversibility line was observed for bulk oxide Bi-2223 and ordinary Nb-Ti tapes with different pinning strength. These results and results on Y-based superconductors given in literatures are compared with the collective flux pinning theory.

DOI： 10.1088/0953-2048/5/1S/012

Scopus

出版者・発行元：Superconductor Science and Technology  

A rapid decrease in Jc with elevating temperature was recently observed in melt-processed Y-Ba-Cu-O at low temperatures. This suggests an existence of weak links in this material. In order to investigate weak link property in this material, the history effect of Jc and Jc in the longitudinal field geometry were measured. These measurements suggest that weak link property in this material is quite different from those at grain boundaries in polycrystalline materials.

DOI： 10.1088/0953-2048/5/1S/003

Scopus

出版者・発行元：Physica C: Superconductivity and its applications  

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. © 1992.

DOI： 10.1016/0921-4534(92)90117-U

Scopus

出版者・発行元：Physica C: Superconductivity and its applications  

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. © 1992.

DOI： 10.1016/0921-4534(92)90144-2

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

Applicability of the ac inductive measuring method for estimation of the critical current density, Jc, in superconductors of relatively small size is theoretically investigated in terms of the Campbell model for pinning force density vs the displacement of fluxoids. It is shown that Jc is significantly overestimated by the conventional method due to reversible fluxoid motion for superconductors comparable to or smaller than Campbell’s ac penetration depth, [Equation found]. Only the recently proposed analysis of imaginary ac susceptibility yields relatively accurate values even for superconductors much smaller than [Equation found]. © 1992 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.31.L169

Scopus

出版者・発行元：Physica C: Superconductivity and its applications  

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. © 1991.

DOI： 10.1016/0921-4534(91)91291-B

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

Two kinds of YBaCuO bulk samples with different densities and sizes of 211 particles were prepared by the melt powder melt growth method. The magnetic field and temperature dependences of critical current density, 7C, weremeasured inductively. Strong dependence ofJcon size and density of 211 particles at higher temperatures was observed, while no relationship could be recognized between the two at lower temperatures. This result supports the expectationthat nonsuperconducting particles such as the 211 phase act as dominant pinning centers at higher temperatures. Ahistory effect was observed at 4.2 K, suggesting an existence of weak links. © 1991, IOP Publishing Ltd.

DOI： 10.1143/JJAP.30.L1861

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

The irreversibility line, i.e., a boundary between zero and nonzero critical current density in the magnetic held vs temperature plane, was measured for polycrystalline Bi-based superconductors of highTcphase with oriented c-axis, and its anisotropy was discussed. Both the anisotropy with respect to the direction of the current and that of the magnetic heldwere small, and the quasitwo-dimensionality of the crystal structure did not play an important role. Although theseresults are affected by the weak link of grain boundaries, it is suggested that the flux pinning strength is the main determinant of the irreversibility line. © 1991, IOP Publishing Ltd.

DOI： 10.1143/JJAP.30.L1857

Scopus

出版者・発行元：Cryogenics  

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. © 1991.

DOI： 10.1016/0011-2275(91)90218-L

Scopus

出版者・発行元：Physica C: Superconductivity and its applications  

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. © 1991.

DOI： 10.1016/0921-4534(91)90462-8

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

Critical current densities in superconducting Y-Ba-Cu-O specimens with different sizes of 211 particles prepared by the melt process were measured under various magnetic fields and temperatures. Discussion is given on the flux pinning properties by 211 particles and background pinning centers. At temperatures above 60 K, 211 particles are expected to be dominant pinning centers. A rapid degradation of the critical current density with elevating temperature at low temperatures below 25 K seems to be attributed to weak links in specimens. © 1991, IOP Publishing Ltd.

DOI： 10.1143/JJAP.30.L342

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

When SnO2 was added to Y-123 (YBa2Cu3O6+x) powder and sintered at 1223 K for 86.4 ks, the densification was enhanced remarkably, and the density reached 99% of the theoretical value for the 2-mol%-SnO2-added specimen, while the average grain size became smaller with Sn addition. Fine spherical BaSnO3 precipitates were observed to be distributed in the grains and at grain boundaries. The AC magnetization measurements indicated that Sn addition causes intragrain Jcm to increase at 77 K in the magnetic field between 0.02 and 0.12 T. The Jcm exceeded 106 A/cm2 at 0.04 T for the 3-mol%-SnO2-added specimen. © 1990 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.29.L1621

Scopus

出版者・発行元：Cryogenics  

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. © 1990.

DOI： 10.1016/0011-2275(90)90295-N

Scopus

出版者・発行元：Physica C: Superconductivity and its applications  

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. © 1990.

DOI： 10.1016/0921-4534(90)90039-H

Scopus

出版者・発行元：Physica B: Condensed Matter  

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. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.

DOI： 10.1016/S0921-4526(09)80291-3

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

The flux creep rate in sintered superconducting Y-Ba-Cu-O was measured at 77 K and in fields of up to 0.1 T. The pinning potential mainly caused by flux pinning inside grains was 0.25 eV at B=50 mT and decreased monotonically with increasing magnetic field. The obtained results can be explained by the theoretical model with observed intragrain current density. This theoretical model suggests that the flux creep at high fields is notable even if a superconducting wire with the critical current density of 2×1010A/m2at B=5 T is realized. © 1989 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.28.L1508

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

The critical current characteristic of a Y-Ba-Cu-O specimen prepared by the quench-and-melt growth (QMG) technique was investigated by the ac-inductive method. It was found that there were two kinds of currents. The bulk critical current density is 1.0×108A/m2at 77 K and 0.12 T. This value is comparable to a previous estimate from dc-magnetization measurements. The local current density amounts to 3.2×1010A/m2under the same conditions. Appearance of two kinds of currents seems to be caused by nonsuperconducting second-phase layers formed along the direction of grain growth, which disturb uniform current. © 1989 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.28.L1545

Scopus

出版者・発行元：Cryogenics  

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. © 1989.

DOI： 10.1016/0011-2275(89)90193-8

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

A simple measuring method to discriminate between inter- and intragrain current densities in oxide superconductors under given magnetic fields and temperatures is proposed. This is a measurement of the imaginary part of the ac permeability or susceptibility as a function of the ac field amplitude. In this method, the analysis used to derive the penetration depth of the ac field in Campbell’s method or the waveform analysis method is not necessary. This method is useful when the value of the penetration field into the grains and that into the bulk specimen are remarkably different. © 1989 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.28.L419

Scopus

出版者・発行元：Cryogenics  

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. © 1989.

DOI： 10.1016/0011-2275(89)90171-9

Scopus

出版者・発行元：IEEE Transactions on Magnetics  

Critical current densities are theoretically estimated for single-crystalline thin films, polycrystalline bulk materials with oriented and random textures of superconducting oxides. The percolation theory is used and the effect of depression of the transport current through grain boundaries is taken into account. A comparison is made with existing experimental results. © 1989 IEEE

DOI： 10.1109/20.92765

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

Bulk intergrain current and closed intragrain current in sintered Y-Ba-Cu-O superconductors were measured at 77 K by using an ac inductive method. These currents can be separated because of the large difference in penetrating rates of the magnetic flux into specimens with respect to the ac field amplitude. The obtained intergrain current density agreed approximately with critical current density measured resistively. The closed intragrain current density amounted to 4.3×108 A/m2 at B=0.5 T and decreased gradually with increasing magnetic field. The present results show that the ac inductive measurement is one of the available nondestructive methods to characterize sintered oxide specimens. © 1988 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.27.1658

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

Percolative critical transport current density in sintered Y–Ba–Cu–O superconductors is estimated by taking into account the effect of anisotropies in the upper critical field and the pinning structure. This value is much smaller than the estimate for the optimized situation, where misorientation between the magnetic field and anisotropic pinning structures does not occurs. As the volume fraction of the superconducting region becomes small, the critical transport current density decreases drastically due to a decrease in the percolation probability of the current. To improve the transport property, it is necessary to align the orientation of grains or to introduce isotropic pinning centers, in addition to increasing the superconducting volume fraction. © 1988 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.27.929

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

Attainable critical current density in oxide superconductor YBa2Cu3O7-ै is estimated by an ordinary analyzing method in flux pinning phenomena with superconducting parameters obtained from a magnetization measurement. The pinning mechanisms considered here are an anisotropy in the upper critical field and an electron scattering effect for grain boundaries and twinning planes. While the elementary pinning force of these planar defects is small due to a small thermodynamic critical field at 77 K, a critical current density comparable to that of Nb-Ti at 4.2 K is expected to be attained by increasing the pin concentration. © 1987 IOP Publishing Ltd.

DOI： 10.1143/JJAP.26.L1524

Scopus

出版者・発行元：Japanese Journal of Applied Physics  

We have measured magnetization of sintered YBa2Cu3O7-x plates in a parallel magnetic field at 77 K. It was found that the difference खM in the magnetization between increasing and decreasing processes of the external field depends little upon the thickness of the plate, and it is predicted by the `critical-state' model that the difference is proportional to the thickness in a uniform specimen. The present experimental results suggest that the sintered plates consist of small superconducting sub-structures and that the superconducting connections among these sub-structures are not strong. © 1987 IOP Publishing Ltd.

DOI： 10.1143/JJAP.26.L1445

Scopus

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