Akio Oota

AC losses of Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox monofilamentary and multifilamentary tapes

Abstract The AC losses are investigated at 77 K on the Ag-sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x monofilamentary tape and multifilamentary tapes with filament numbers equal to 7, 19, 37 and 61, through an AC transport method under self-fields and an AC magnetic method under longitudinal fields. Although the main contribution to the AC losses around a commercial frequency of 50 Hz comes from the hysteresis loss independent of the measuring methods, the AC transport losses for the monofilamentary tape are larger than the AC magnetic losses by about one order of magnitude. Such a discrepancy is explained by a large aspect ratio of the superconductor core, which lowers only the magnetic losses. The superconductor subdivision in the multifilamentary tapes causes no notable influence on the AC transport losses ascribed to the hysteresis loss. This is compared with the result for the AC magnetic losses that the subdivision suppresses the contribution of the hysteresis loss while enhancing that of the eddy current loss. The density distribution of the AC transport losses obtained from numerical calculations based on a critical state model shows a resemblance between the monofilamentary tape and the multifilamentary tapes, which conforms to the experimental results for the AC transport losses.

The effect of Ag doping on superconducting properties of the Bi‐Pb‐Sr‐Ca‐Cu‐O tape fabricated by an Ag sheath

The powder‐in‐tube method using an Ag sheath is applied to achieve high current capacity in the tapes of Bi‐Pb‐Sr‐Ca‐Cu‐O (BPSCCO) with the high‐Tc phase (Tc=110 K), by a combination process of cold working and sintering. Two kinds of the BPSCCO powder with and without Ag doping are used as the materials for tape fabrication. The morphology, structure, and critical current density Jc are investigated on the tapes by x‐ray diffraction, differential thermal analysis, scanning electron microscope, magnetic susceptibility, and I‐V curves. The optimum sintering conditions to obtain a maximum in the value of Jc at 77 K and 0 T are presented in the Ag‐doped tapes and the nondoped ones. An improvement in the Jc vs B properties up to 1 T at 77 K is presented in the BPSCCO tape by Ag doping. The Ag doping causes neither change in the value of Tc (=110 K) nor decomposition of the high‐Tc phase.

The Effect of Pb Addition on Superconductivity in Bi–Sr–Ca–Cu–O

The effect of Pb addition on superconductivity was studied on the systems with nominal compositions (BiPbx)2Sr2Ca2Cu3Oy, (Bi1-xPbx)2Sr2Ca2Cu3Oy and (Bi1-xPbx)2Sr2Ca2Cu3.6Oy. The melting point decreases monotonically from ~880 to ~860°C as x increases from 0 to 0.45 in the systems, while the volume fraction of the high-Tc phase (HTP) and the offset value of Tc show a peak around x=0.2~0.3. Further increase in x produces the Ca2PbO4, leading to the suppression of the HTP. The amount of Pb suitable for the formation of the HTP is limited by the competition between the reaction to form the HTP by the catalytic effect of Pb and the one to form the Ca2PbO4.

Critical current density, morphology and microstructure of Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes

Abstract The critical current density Jc, morphology and microstructure have been investigated on the Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3O x superconducting tapes with overall tape thickness d (0.05mm≤d≤1.0 mm). The tapes were fabricated by a process of combined drawing and rolling with intermediate sintering steps, where no uniaxial pressing technique was applied. The thickness dependence of Jc at 77 K is characterized by three distinctive stages. At the first stage (0.2 mm≤d≤1.0 mm), Jc is increased gradually as d decreases, by the co-deformation between Ag sheath and ceramic core. At the second stage (0.1 mm≤d≤0.2 mm), it is raised sharply with decreasing d, by the densification of ceramic core itself in the tape. At the third stage (0.05 mm≤d≤0.1 mm), however, it is lowered by the distortion of the Ag/ceramics interface, which is caused by the extreme densification of ceramic core. A peak of Jc = 1.53×104 A/cm2 (77 K, 0 T) at d = 0.1 mm is explained by a competition between the improvement of grain connectivity and the disturbance in transport current path. The temperature dependence of Jc in zero external field is determined by a competition between the flux-creep-limited behavior and the weak-link-limited behavior, which are likely to be separated on a threshold value 1×104 A/cm2 (77 K, 0 T) in the sample.

Total AC loss of Ag-sheathed Bi2223 tapes with various filament arrangements carrying AC transport current in AC parallel transverse magnetic field

For the applications of Bi2223 tapes as the strands to power cables with multilayer structures, the tapes must carry the AC transport current in an AC parallel transverse field with various field amplitudes depending on the layer positions. In this study, we experimentally investigated the total AC loss characteristics under parallel transverse field for Bi2223 tapes with various filament arrangements in their cross sections. The tapes with different filament arrangements in the final tape sections were prepared by a conventional powder-in-tube (PIT) process with drawing using round dies and a rectangular deformation process using a two-axial rolling (TAR) technique. Total AC losses in the tapes carrying AC transport current in an AC parallel transverse field were measured by means of the electromagnetic method. The measured loss values in the tapes were compared with several analytical models. Based on the experimental results, the influence of filament arrangements on total AC loss behaviour is discussed, under the different operating conditions with various amplitudes of current and applied field.

Field-free core, current distribution, and alternating current losses in self fields for rectangular superconductor tapes

A method is proposed to calculate the shape of field-free core, the current distribution, and accordingly the alternating current (ac) losses in self fields as a parameter of current amplitude I0 for straight superconductor tapes with the arbitrary sectional geometry. The validity for this is justified by showing that magnetic fields inside the field-free core are negligibly small and by checking that calculation results of ac losses for elliptic tapes agree with the prediction by the theory of Norris. The ac losses for rectangular tapes behave as a thin strip superconductor in high I0 values near critical current, while showing a gradual deviation from this behavior as I0 is lowered. A decrease in the aspect ratio of width to thickness in the rectangular tape makes the deviation more remarkable.

A study on self-field distribution in Ag-sheathed (Bi,Pb) 2Sr2Ca2Cu3Ox monofilamentary tape using a scanning Hall sensor magnetometry

Abstract Using a scanning Hall sensor magnetometry with an active area 50 μm × 50 μm, two-dimensional self-field distributions due to trapping currents, induced by a removal of external fields perpendicular to a sample surface, have been measured with 0.2 mm steps on the Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Oxmonofilamentary tapes with Jc values equal to 71 and 110 A/mm2 (77 K, 0 T). The samples are fabricated by a rolling process with an intermediate sintering step. The measured profiles are nearly symmetric with respect to the tape center and mainly ascribed to the inter-grain current transport that flows in the whole part of superconductor through grain boundaries. The line distribution derived from the results suggests that the current transport along a length direction is higher than that along a width direction by about twice. The mechanical rolling for superconductor tape introduces the weak links and causes severe modulation in the magnetic profiles that is ascribed to the intra-grain current circulation restricted within the grains themselves. The experimental behavior is qualitatively explained by numerical calculations on the self-field distributions using the Biot-Savart law based on the electromagnetism.

Growth Process of the (2223) Phase in Pb-Added Bi-Sr-Ca-Cu-O

The growth process of the (2223) phase in Bi-Sr-Ca-Cu-O has been studied for oxides with nominal composition Bi2Pb0.6Sr2Ca2Cu3Ox prepared by sintering at 835~855°C for 5~160 h in air. The partial melting which starts to occur at Tpm842°C enhances the growth rate of the (2223) phase and produces a nearly single phase within 40 h. However, it causes a disturbance in current paths and suppresses the value of Tc(ρ=0) below 100 K. The sample which has the highest Tc(ρ=0) and the nearly single (2223) phase is obtained by sintering at a temperature just below Tpm for a long period of time.

Critical current density and microstructure of c-axis-oriented Bi2Sr2CaCu2Oχ and (Bi, Pb)2Sr2Ca2Cu3Oχ superconducting tapes

Abstract The critical current density J c is investigated as a function of both field direction and temperature in c -axis-oriented Bi 2 Sr 2 CaCu 2 O χ “2212” and (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O χ “2223” polycrystals: a screen-printed “2212” tape with J c (77 K, 0 T) = 1.4×10 4 A / cm 2 , a screen-printed “2223” tape with J c (77 K, 0 T) = 1.5×10 4 A / cm 2 and a Ag-sheathed “2223” tape with J c (77 K, 0 T) = 2.2×10 4 A / cm 2 . The J c versus θ data at 77 K with the angle θ between B and the c -axis show that J c in a field is influenced only by the field component parallel to the c -axis. There is a correlation between the anisotropy ratio γ=J c ( B ⊥ c )/J c ( B || c ) and the half-height angular width Δθ of a peak for B ⊥ c . An increase of B in a sample enhances γ and narrows Δθ. At fixed B , the screen-printed “2212” tape has the largest γ and the narrowest Δθ in all samples, which corresponds to the SEM result showing the highest c -axis alignment. The second highest is the screen-printed “2223” tape and the lowest is the Ag-sheathed “2223” tape. J c in zero field increases almost linearly with decreasing temperature below 70 K, which is explained by the flux creep model.

Alternating‐current losses in Ag‐sheathed (Bi,Pb)2Sr2Ca2Cu3Ox multifilamentary tapes

The alternating‐current (ac) losses in the Ag‐sheathed (Bi,Pb)2Sr2Ca2Cu3Ox multifilamentary tapes with the filament number equal to 7, 19, 37, and 61 are investigated through an inductive method at 77 K under ac magnetic fields with the frequency 50 Hz≤f ≤700 Hz and the field amplitude 2 mT≤B0≤50 mT, and the results are compared with those for a monofilamentary tape. The subdivision of superconductor into multifilaments suppresses the hysteresis loss within the filaments, which is the main contribution to the ac losses in the monofilamentary tape. However, the enhancement of the eddy current loss in the multifilamentary tapes makes the ac losses comparable to those in the monofilamentary tapes.