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Volume 61, Issue 9
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[in Japanese]1997 Volume 61 Issue 9 Pages 783-784
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSDownload PDF (166K) -
Kaname Matsumoto, Yasuzo Tanaka, Osuke Miura, Masataka Iwakuma, Kazuo ...1997 Volume 61 Issue 9 Pages 785-791
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSCritical current densities (Jc) due to artificially designed pinning structure composed of randomly oriented Nb/Nb-Ti or Ti/Nb-Ti multilayer clusters in Nb-Ti superconducting wires are investigated in terms of recent works on magnetic phase diagrams of the metallic superconducting multilayer system. Upper critical field (Bc2)-temperature curves of the Nb-Ti superconducting wires with artificial pinning structures have an anomalous upturn of Bc2. This behavior is similar to the dimensional crossover which occurs in the superconducting multilayer system. From Jc and anomalous Bc2 properties in the Nb-Ti superconducting wires with artificial pinning strutures, we propose the flux pinning mechanism of these systems, especially in the high field region at 4.2 K.View full abstractDownload PDF (723K) -
Naoya Iwasaki, Masaki Hojo, Shojiro Ochiai, Masayoshi Ono, Shuji Sakai ...1997 Volume 61 Issue 9 Pages 792-800
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSNeutron diffraction measurements were carried out on multifilamentary Nb-Ti/Cu superconducting composite wire in order to clarify the structural anisotropy and the residual stresses arising from the fabrication process. Highly oriented structure was observed both for copper and Nb-Ti components. The residual stress of copper in the composite wire was estimated to be 40 MPa by means of PSD method, which was close to the yield stress of matrix, 30 MPa. The reason why the measured elastic modulus is lower than the predicted one based on the simple rule of mixtures was accounted for from the viewpoint of the formation of the texture. While the failure strain of filaments tested separately was nearly 2%, that of the composite with the copper ratio 7.83 was about 20%. The high failure strain of the composite was achieved through the formation of multiple necking of the embedded filaments. The average strength of the extracted filaments was 1240 MPa, which was about 10 times higher than that of the oxides and comparable to that of A15 compounds. The scatter of strength was much smaller than that of the oxides and A15 compounds. The critical current decreased with increasing applied strain, but nearly 80% of the critical current for unloaded samples was retained even after deformation up to 20% strain due to the enhanced failure strain of the filaments through the multiple necking.View full abstractDownload PDF (760K) -
Satoru Murase, Shigeo Nakayama, Tamaki Masegi, Kei Koyanagi, Shunji No ...1997 Volume 61 Issue 9 Pages 801-806
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSIn order to fabricate a large-bore, high-field magnet which achieves a low coil weight and volume, a high strength compound superconducting wire is required. For those demands we have developed the reinforced Nb3Sn wire using alumina dispersion strengthened copper (alumina-Cu) as a reinforcement material and the tube process of the Nb3Sn wire fabrication.
The ductility study of the composites which consisted of the reinforcement, Nb tube, Cu, and Cu clad Sn brought a 1 km long alumina-Cu reinforced Nb3Sn wire successfully. Using fabricated wires measurements and evaluations of critical current density as parameters of magnetic field, tensile stress, tensile strain, and transverse compressive stress, and those of stress-strain curves at 4.2 K were performed.
They showed superior performance such as high 0.3% proof stress (240 MPa at 0.3% strain) and high maximum tolerance stress (320 MPa) which were two times as large as those of conventional Cu matrix Nb3Sn wire. The strain sensitivity parameters were obtained for the reinforced Nb3Sn wire and the Cu matrix one using the scaling law. Residual stress of the component materials caused by cooling down to 4.2 K from heat-treatment temperature was calculated using equivalent Young’s modulus, equivalent yield strength, thermal expansion coefficient and other mechanical parameters. Calculated stress-strain curves at 4.2 K for the reinforced Nb3Sn wire and the Cu matrix one based on calculation of residual stress, had good agreement with the experimental values.View full abstractDownload PDF (607K) -
Norikazu Matsukura, Takayoshi Miyazaki, Takayuki Miyatake, Masao Shima ...1997 Volume 61 Issue 9 Pages 807-813
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSWe have developed Nb3Sn superconducting wires for AC use fabricated by a powder-metallurgy process. Wires with niobium concentrations from 5 to 22.5% were produced and the effects of niobium concentration on the critical current density (Jc) and the hysteresis losses of these wires were investigated. Assuming that Jc is proportional to both the volume fraction of the superconducting filaments and the frequency of contact among superconducting filaments, Jc should be proportional to the square of the niobium concentration. However, it was proportional in the power of 1.5 for the present wires. Jc of the central region of high niobium concentration wire was relatively low, resulting from less tin diffusion to the central region due to the blocking by niobium filaments distributed near the surface. This effect may be responsible for reducing the power from 2 to 1.5.
The hysteresis loss was proportional to the niobium concentration in the power of 2. This power relation can be explained by the fact that the hysteresis loss is proportional to the product of Jc and the effective filament diameter, which are proportional to the niobium concentration in the powers of 1.5 and 0.5, respectively.View full abstractDownload PDF (1519K) -
Yuichi Yamada, Naoki Ayai, Akira Mikumo, Ken-ichi Takahashi, Ken-ichi ...1997 Volume 61 Issue 9 Pages 814-821
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSThe Nb3Al superconductor has been expected to the application under the high field, such as the one for International Thermo-nuclear Experimental Reactor (ITER), from the standpoint of superior characteristics of Ic under mechanical stress. We have established the manufactuing technology for long length Nb3Al wire by the Jelly-roll process, which is the most suitable method for the practical superconductor.
The microstructure of Nb-Al compound has been studied and a non-copper critical current density Jc of more than 800 A/mm2 at 12 T, 4.2 K was achieved. Further, a higher upper critical field Bc2 of 23.1 T and a higher Jc of 1130 A/mm2 at 12 T, 4.2 K were achieved by the 2-stage heat treatment.
The residual resistivity ratio of the wire was over 400, and even with Chrome plating, it was over 100.
For the field perpendicular to the wire length, the effective filament diameter was almost equal to the actual filament diameter. On the other hand, the hysteresis loss for the parallel field was around one-fourth as large as that for the perpendicular one.
For studying the applicability of Nb3Al to practical use, the stress-strain effect, mechanical properties and Jc-B-T characteristics were measured.
In order to demonstrate the applicability to the large current conductor, the 40 kA-class Cable-in-Conduit conductor and the 12 T-10 kA-100 m class long length Cable-in-Conduit conductor, of which conduit is titanium, were fabricated. Recently, the conductor for the ITER TF coil was fabricated and the Jc-B-T characteristics were measured at the SULTAN facility in Switzerland.
We are now fabricating more than 250,000 m wires for ITER Nb3Al Insert. In the prototype billet for ITER, the yield of the piece length over than 1500 m was greater than 90%.View full abstractDownload PDF (990K) -
Takehiro Sawada, Shojiro Ochiai, Masaki Hojo, Yuichi Yamada, Kenichi T ...1997 Volume 61 Issue 9 Pages 822-828
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSThe fatigue behavior at room temperature and its influence on the superconducting properties of Nb3Al composite wire with a copper ratio of 1.55, containing 241 Nb3Al filaments with an average diameter of 34 μm, were studied. The main results are summarized as follows.
(1) Fatigue cracks nucleated and grew in the matrix (Cu), and thereby caused the fracture of Nb3Al filaments. The Nb3Al filaments themselves were not fractured by fatigue. (2) The growth pattern of the fatigue cracks depends on the maximum-applied-stress (σc, max) level. (a) At high σc, max, overall fracture of the composite occurs before the growth of fatigue cracks. The filaments are merely broken except in the fracture surface as in the case of the static test. (b) At intermediate σc, max, fatigue cracks can grow at several points before the breakage of the composite. Thus the breakages of the filaments are accumulated not only in the fracture surface, but also in the whole body. (c) At low σc, max, one fatigue crack grows in advance of others, causing the breakage of the composite. Thus the filaments are broken only in the fracture surface. (3) Corresponding to the growth patterns of the fatigue cracks mentioned in (2), the result of the measurement of critical current (Ic) using the rest part of the broken composite after the fatigue test shows the reduction in Ic at intermediate σc, max, but not at high and low σc, max.View full abstractDownload PDF (942K) -
Yasuo Iijima, Takao Takeuchi, Kiyoshi Inoue1997 Volume 61 Issue 9 Pages 829-835
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSSince Nb3Al shows better strain-tolerances of superconducting properties than those of Nb3Sn, the Nb3Al multifilamentary wire is one of the most promising candidate superconductors for the large-scaled high-field superconducting applications. Although the Nb3Al conductors with off-stoichiometric compositions fabricated by low-temperature diffusion reaction show higher Jc values in fields below 14 T, their Jc values rapidly decrease in high fields above 14 T due to their relatively low Hc2 (4.2 K) caused by their off-stoichiometric compositions. On the other hand the Nb3Al fabricated at high temperatures has a stoichiometric composition and improved Hc2 but shows relatively low Jc. Recently, we developed a new fabrication process of Nb3Al multifilamentary wire, in which Nb/Al-5 at%Mg multifilamentary composite wires were rapidly heated up to 2300 K and rapidly quenched to obtain composite wires including many Nb-Al supersaturated bcc filaments. Then, the resulting composites were annealed to form Nb3Al filaments through the deposition from the metastable Nb-Al bcc filaments. When the Al-5 at%Mg alloy core diameters were about 0.56 μm, the wire showed the highest Tc and the highest Jc (20 T, 4.2 K) of 17.4 K and 2.5×108 A/m2, respectively. The optimized Jc values are 2∼3 times higher than those of the commercially available (Nb, Ti)3Sn multifilamentary wires. Small mechanical deformation (bending, twisting, and stranding) of the as-quenched composite wires did not degrade the superconducting properties of the final Nb3Al multifilamentary wires, which proves the good ductility of the supersaturated bcc filaments. Reducing temperature from 4.2 to 2.1 K shifted the Jc-B curves to the higher magnetic field by 2.1∼2.5 T and increased the n-index in the voltage-current characteristics from 14.3 to 31.7 at 20 T. All the test results indicate that the new wire is one of the most promising materials having a hopeful future in the various practical superconducting applications in high fields.View full abstractDownload PDF (660K) -
Yutaka Yamada, Fumiyoshi Yamashita, Katsuji Wada, Kyoji Tachikawa1997 Volume 61 Issue 9 Pages 836-841
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSBi-2212 oxide superconductors have been synthesized by the diffusion reaction between a high melting point Sr-Ca-Cu oxide substrate and a low melting point Bi-Cu oxide coating layer. The substrate with Sr : Ca : Cu atomic ratio of 2 : 1 : 2 was pressed into a cylindrical rod by cold isostatic pressing, and then sintered. The coating layer with Bi : Cu ratio of 2 : 1 and Ag2O addition was coated around the surface of the substrate. The heat treatment was performed to produce the Bi-2212 diffusion layer at 820∼860°C. The Ag added to the coating layer enhances the diffusion reaction and precipitates on the surface of the specimen. The Bi-2212 diffusion phase, formed through the Bi-2201 phase, showed a dense and oriented structure with plate-like grains. The transport Ic and Jc of the diffusion layer, about 150 μm in thickness, exceeded 300 A and 2×108 A/m2 up to 25 K under self-field, respectively. The transport Jc decreased with temperature, being about 1×108 A/m2 at 35 K under 0.5 T. Post-annealing in the inert gas atmosphere improved Tc and Jc at higher temperatures. The contact resistivity of bulk ends of the specimen was in the order of 10−7 μΩ·m2 between 4.2 K and 65 K up to 3 T. The present Bi-2212 oxide cylinders with large transport Ic and Jc as well as low contact resistance may be attractive as current leads for cryocooler-cooled and conventional cryogen-cooled superconducting magnets.View full abstractDownload PDF (714K) -
Takashi Hase, Kazuyuki Shibutani, Seiji Hayashi, Masao Shimada, Rikuo ...1997 Volume 61 Issue 9 Pages 842-848
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSDouble-sheated Bi2Sr2CaCu2Ox (Bi-2212) multifilamentary wires have been prepared in order to fabricate an oxide superconducting solenoidal coil for insertion in ultrahigh magnetic fields. The multifilamentary wires have double sheaths which are composed of the outer Ag-alloy layer and the inner Ag layer surrounding the Bi-2212 filament. The octagonal wire which was deformed by a groove roller shows a 0.2% proof strength over 200 MPa at 20°C, and the same critical current density as the Ag-sheathed Bi-2212 multifilamentary wire in which the Ag-alloy is replaced by Ag. By adjusting the total pressure and the oxygen partial pressure during the heat treatment in the mixture atmosphere of O2 and N2 gages to 0.6 and 0.24 MPa, respectively, a critical current density of 4.1×104 A/cm2 has been realized in the Bi-2212 filament at 4.2 K and 0 T. Meanwhile, the double-sheathed Bi-2212 multifilamentary round wire which was deformed by die-drawing shows a critical current density of 7.9×104 A/cm2 under the same condition. A small solenoidal coil fabricated employing the double-sheated octagonal wire has stably generated magnetic flux densities up to 0.18 T in a backup one of 21 T at 4.2 K.View full abstractDownload PDF (1044K) -
Michiya Okada, Keiji Fukushima, Kazuhide Tanaka, Tatsumi Hirano, Junic ...1997 Volume 61 Issue 9 Pages 849-855
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSProcessing and the superconducting properties of the butt-joint for Bi-2212/Ag multifilamentary tapes were studied in relation to the crystal orientation. The multifilamentary Bi-2212/Ag tapes with 19 and 55 filaments were prepared by a powder-in-tube technique. The tapes were butt-jointed, followed by partially melting in a flowing oxygen atomosphere. The typical dimensions of the tapes used in the experiments were 5 mm in width, 0.15 mm in thickness, and 30 mm in length. The critical current Ic of the joint reached around 500 A, showing a great possibility for practical applications. The crystal alignment of the butt-jointed tape was analyzed by means of a transmissive X-ray diffraction method, and found the preferred orientation was realized in the joint part. The marked angular dependence of Jc of the joint on angle between a tape and field at 4.2 K and 8∼10 T also proved the c-axis alignment.View full abstractDownload PDF (806K) -
Hiroki Fujii, Hiroaki Kumakura, Kazumasa Togano1997 Volume 61 Issue 9 Pages 856-861
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSGrain boundary structure of Bi-2212/Ag tapes processed in various oxgen partial pressures (PO2) was investigated by high-resolution transmission electron microscopy (HRTEM). Most frequently observed boundaries for each tape were (001) twist boundaries. Two kinds of twist boundaries were mainly observed. One has an amorphous region at the interface and the other has no amorphous region (clean boundary). The population of clean boundary increased with increasing PO2. The real part of AC susceptibility showed smaller amplitude dependence for the tapes processed in higher PO2, suggesting improved coupling of grains.View full abstractDownload PDF (836K) -
Akinobu Irie, Gin-ichiro Oya1997 Volume 61 Issue 9 Pages 862-868
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSWe have systematically studied the structural change and properties of intrinsic Josephson junctions in (Bi1−xPbx)2Sr2CaCu2Oy single crystals with x between 0 and 0.25. The single crystals were grown by a self flux method and characterized by using X-ray diffraction, electron diffraction and electron microscopy. The critical temperatures of the as-grown single crystals were 80∼90 K. It was successfully confirmed that the single crystals with x<0.2 have the orthorhombic structure with an incommensurate modulation, and the periodicity of the modulation increases and the c-lattice parameter decreases with increasing Pb content, while those with x>0.2 have the pseudotetragonal structure without any modulation structure. For the c-axis transport measurements, mesas were fabricated on the single crystals using photolithography and ion milling techniques. The results clearly showed that mesas for all x between 0 and 0.25 behave like stacks of superconductor-insulator-superconductor Josephson tunnel junctions. From the area dependence of critical current, it was found that the Josephson penetration depth of the intrinsic Josephson junctions in Bi2Sr2CaCu2Oy single crystals is ∼15 μm. Moreover, the critical current densities of the intrinsic Josephson junctions increased with x. This was interpreted by the variation of barrier property and decrease in structural modulation in (Bi1−xPbx)2Sr2CaCu2Oy single crystals depending on x.View full abstractDownload PDF (767K) -
Hiroyuki Shiibashi, Takayo Hasegawa, Kaoru Kimura, Hiromitsu Ino1997 Volume 61 Issue 9 Pages 869-872
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSThe 2212 and 4334 phases of Bi-based oxide superconductors have been studied mainly with respect to the 57Fe-probe Mössbauer effect using the substitution of Fe for Cu in them. Their Mössbauer spectra at room temperature are paramagnetic, while the spectra at 4.2 K without external magnetic field are composed of the paramagnetic doublet and magnetic component with a hyperfine field of about 40 T except for that of a sample containing 1 at%Fe. With an external field of 5 T the spectra of all samples at 4.2 K have only the magnetic component, suggesting that iron atoms at the Cu sites in the samples have a large magnetic moment of about 2.7 Bohr magneton. The appearance of the magnetic hyperfine structure of iron in the phases is probably due to the formation of antiferromagnetism in the CuO2 plane. On the other hand, the appearance of paramagnetic doublet at 4.2 K indicates the spins of iron atoms fluctuate faster than the life time of the 57Fe excited state(10−7 s) and is considered to be arising from the iron in the superconductive CuO2 plane.View full abstractDownload PDF (358K) -
Masatsugu Kaise, Kazuo Saito1997 Volume 61 Issue 9 Pages 873-878
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSSuperconducting BiSrCaCuO ultrathin films of thicknesses below 10 nm were synthesized by using modified single-target magnetron sputtering and by optimizing the heat-treatment conditions. The observed values of the superconducting zero-resistance temperature, Tc,0, were 106, 88 and 84 K for 7-, 4- and 2-nm-thick films, respectively. The surface of 7-nm-thick ultrathin films was terraced with very fine steps and became flat with decreasing the film thickness down to 2 nm. Cross-sectional HRTEM observations of ultrathin films revealed that a 4-nm-thick ultrathin film with Tc,0 of 88 K consisted of a set of half-unit-cell layers of both the low-Tc phase and the high-Tc phase just on the (001) MgO substrate. Instead of the repeated use of thermal treatment at high temperatures, irradiation of 100 keV Ar ions followed by annealing at relatively low temperatures was useful to improve the superconducting properties of our ultrathin films and modified a 4-nm-thick film with a somewhat low Tc,0 of 78 K to exhibit a Tc,0 of 88 K.View full abstractDownload PDF (903K) -
Akira Ishii, Takeshi Hatano, Keikichi Nakamura1997 Volume 61 Issue 9 Pages 879-884
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSIn order to keep better surface morphology and in-plane orientation, thin films of Bi2Sr2CaCu2Oy(2212) and Bi2Sr2Ca2Cu3Oy(2223) were deposited by pulsed laser ablation onto MgO(100) and LaAlO3(100) single crystal substrates at temperatures about 50∼100 K lower than the decomposition point of 2212 or 2223 phases. To get the effect of ex-situ annealing on the in-plane orientation and Tc, the films were accurately post-annealed in oxygen potential controlled furnace. The results showed that the Tc of the annealing films reached to 75∼80 K depending on the in-situ substrate temperature before annealing. The annealed 2223 films showed homogeneous intergrowth mixing of 2212 phase by 10∼20% and showed the Tc at 100∼105 K.
We found that the in-plane orientation of the 2212 films on MgO(100) substrates depends on the substrate temperature during deposition. The film deposited at the higher substrate temperatures have either 2212[100]\varparallelMgO[100]±11° and 2212[100]\varparallelMgO[110] relation or 2212[100]\varparallelMgO[100]±11°. While the film deposited at the lower substrate temperatures showed only the 2212[100]\varparallelMgO[100]±11° epitaxial relation. After annealing, the in-plane direction completely rotated by 45°, and only the 2212[110]\varparallelMgO[100] epitaxial relation became observable. On the other hand, from the films deposited on LaAlO3 substrate, only the strong 2212[100]\varparallelLaAlO3[100] in-plane epitaxial relation was observed irrespective of the substrate temperature and post-annealing.View full abstractDownload PDF (559K) -
Mamoru Satoh, Atsushi Murata, Shuji Yoshizawa, Kazuhiro Tobiishi, Tada ...1997 Volume 61 Issue 9 Pages 885-891
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSThe superconducting properties of sintered high-Tc superconductor bulk deteriorate gradually with time when the calcined powder for fabrication is exposed to the atmosphere. We examined the conditions of moisture and carbon in degraded samples by means of thermal desorption spectroscopy (TDS). In TDS spectra of the calcined powder of Bi-2223, a peak at m⁄z=44(CO2+) appeared in the temperature range between 773 and 873 K when the powder adsorbed moisture and carbon dioxide. When the calcined powder was kept in a carbon dioxide atmosphere, the peak at m⁄z=44(CO2+) was not observed, which was the same as in the TDS spectra measured after calcination. This means that the deterioration in quality of the calcined powder was caused by the adsorption of moisture and carbon dioxide. Microscopic Raman images and EPMA maps showed that carbon was segregated in the grain boundary of the polycrystalline Bi-2223 sintered bulk.View full abstractDownload PDF (592K) -
Xiaoye Lu, Akihiko Nagata, Minoru Yasuda, Kazuhisa Sugawara, Shin-ichi ...1997 Volume 61 Issue 9 Pages 892-899
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSThe influence of Bi, Sr, Ca and Cu contents on the formation and microstructure of the (Bi, Pb)-2223 phase in Bi1.6+bPb0.4Sr2−aCa2+aCu3+cOy, in the sintering process at 1113 K after partial melting at 1148 K for 1 h, was studied. The compositions which are slightly deficient in Sr and richer in Bi, Ca and Cu are favorable for the formation of the 2223 phase, and the optimum composition for the preparation of (Bi, Pb)-2223 phase is Bi1.8Pb0.4Sr1.9Ca2.1Cu3.5Oy. The high-temperature microstructure held at 1148 K for 1 h mainly consisted of (Sr, Ca)2CuO3, (Sr, Ca)CuO2 and a liquid phase. However, with increasing Cu content, the relative amount of (Sr, Ca)CuO2 increased and that of (Sr, Ca)2CuO3 decreased. The microstructure with equal amounts of (Sr, Ca)CuO2 and (Sr, Ca)2CuO3 prior to sintering is favorable for the formation of the (Bi, Pb)-2223 phase. A lot of the particle-like inclusions Sr3PbOx precipitated in the samples with the Sr-rich and Ca-deficient composition during the sintering process after the partial-melting. It is suggested that the precipitation of Sr3PbOx in large amounts in the Sr-rich and Ca-deficient samples results in a decrease of Pb content in (Bi, Pb)-2212 phase, thus slowing the formation-rate of the 2223 phase from the 2212 phase.View full abstractDownload PDF (1518K) -
Mamoru Satoh, Atsushi Murata, Masahiro Kojima, Shuetu Haseyama, Shuji ...1997 Volume 61 Issue 9 Pages 900-905
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSOxide superconducting calcined powder at first adsorbs atmospheric moisture, then reacts with carbon dioxide, which deteriorate a superconducting property of oxide superconductor. We reconsidered a calcined powder preparing process. It was found that moisture is easily absorbed by the solvent removing method after the wet pulverizing method. By removing the solvent under reduced pressure, it was possible to reduce the carbon content 0.03 mass% or less. The calcined powder was pressed to a pellet 20 mm in diameter and approximately 1 mm in thickness and then sintered at 1093∼1123 K in air with intermediate pressing by a CIP method. After the first CIP of the bulk, the orientation of the polycrystalline greatly improved during the sintering resulted in the increase in density. As the number of CIP increased, the Jc increased. The maximum value of Jc was obtained when the number of CIP was three times. The Jc of the bismuth-2223 sintered bulk of 0.2 mm2 cross-section was increased to 8500 A/cm2, with good reproducibility. After the second CIP, the orientation of the polycrystalline and the density of the bulk were not changed. Weak links were evaluated by a DC susceptibility measurement of the sintered bulk. In the high Jc sample, the peak sharpness of the differential curve of the susceptibility reveals that the weak link among grains is improved.View full abstractDownload PDF (640K) -
Kenji Kawano, Akio Oota1997 Volume 61 Issue 9 Pages 906-912
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSThe self-field distributions caused by the trapped currents were measured on the surfaces of the Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox monofilamentary tapes using a scanning Hall sensor with an active area 50 μm× 50 μm, after the applied magnetic fields perpendicular to the sample surface were turned off. The samples were fabricated by a powder-in-tube method combining the cold rolling process with an intermediate sintering step. The measured field profiles were nearly symmetric, and explained by the intergranular current transport that flowed through grain boundaries. The weak links induced by cold rolling on the tape caused a change in the field distributions, and the results were ascribed to the intragranular currents restricted within the grains themselves. In the tape after cold rolling, there existed macro-cracks along the width direction that are likely to disturb the current flow along the length direction. The experimental behavior was qualitatively explained by numerical calculations on the assumption of simplified current paths using the Biot-Savart law.View full abstractDownload PDF (813K) -
Kozo Osamura, Ken-ichi Matsuno, Hideki Itoh, Akira Sakai1997 Volume 61 Issue 9 Pages 913-919
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSIn order to investigate the inhomogeneous current distribution in the silver sheathed Bi2223 tapes, a scanning Hall probe measurement was carried out. The z component of self magnetic field was converted to the local current density, of which the spatial change comprised two characteristic features. The inhomogeneity of longitudinal current distribution along the current flow was suggested to be attributed to the positional change of oxide layer thickness due to the work instability. Transverse current distribution is caused from the degree of misalignment of Bi2223 layered grains as well as the distribution of non-superconducting phases.View full abstractDownload PDF (751K) -
Naomichi Sakai, Masaya Deguchi, Sang-Im Yoo, Masato Murakami1997 Volume 61 Issue 9 Pages 920-924
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSDecomposition temperature of Y123 decreased about 34 K by doping 10 mass% of silver. We could produce highly oriented Y123 superconductor with silver doping by using a Y123 bulk seed crystal. The obtained bulk shows no degradation in Tc and exhibits a uniform trapped field.View full abstractDownload PDF (676K) -
Masahiro Egami, Yuh Shiohara1997 Volume 61 Issue 9 Pages 925-930
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSContinuous growth of YBa2Cu3O6+x(Y123) single crystal was achieved by the modified pulling method. The largest size of the Y123 single crystal obtained reaches 20 mm square under 1-atmosphere oxygen partial pressure and 24 mm square for the NdBa2Cu3O6+x(Nd123) single crystal in air. Clear spiral growth patterns were observed on the crystal/liquid interface by AFM and STM. The growth model of the crystal was explained by the BCF model. The grown crystal was usually covered with a slow growth rate face such as a low index face, and the obtained crystal shape was cubic. Recently, we fabricated a round cornered Y123 single crystal by keeping the growth temperature around the peritectic temperature. A seed crystal was at first remelted at its corner, followed by growth of the round cornered crystal. Along the vertical axis, the crystal corner became wavy. The remelting process was explained by the free energy difference between the flat and square interfaces. Growth shapes of the round corner were explained by temperature-dependent growth rate anisotropy.View full abstractDownload PDF (618K) -
Tetsuo Oka, Yoshitaka Itoh, Yousuke Yanagi, Masaaki Yoshikawa, Tsutomu ...1997 Volume 61 Issue 9 Pages 931-936
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSWe succeeded in obtaining Y-Ba-Cu-O superconducting bulk magnets (abbreviated as Supermagnets) which can trap the magnetic fields exceeding those of ordinary permanent magnets. The flux motion during the pulsed field magnetization (PFM) was evaluated at 77 K. The flux penetration into the sample is sluggish when the field is low, but it suddenly penetrates when the external field exceeds 1.5 T. The PFM technique was applied at temperatures below 77 K to magnetize Supermagnets. The highest flux density of 2.1 T was achieved at the surface of the sample when magnetized by PFM at 30 K by using a cryocooler.
A superconducting motor was constructed for the first time in the world by using Supermagnets as field magnets. The motor performances at 77 K and 65 K reached the values of 2 kW and 3.5 kW, respectively. We have technically established that Supermagnets can be used in various practical applications.View full abstractDownload PDF (627K) -
Yasuji Yamada, Izumi Hirabayashi1997 Volume 61 Issue 9 Pages 937-941
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSLiquid phase epitaxy (LPE), which is performed under ambient pressure with a simple growth apparatus, allows us to fabricate high-JcYBa2Cu3O7−δ (Y123) crystalline thick films at high growth rate. We propose the LPE growth of Y123 films on single crystalline oxide fibers as a possible way to synthesize Y123-coated conductors for power application. In the conventional case of using a BaO-CuO mixture as the solvent, with which the Y123 crystals grow at as high as about 1000°C, Y123 coating on YSZ or SrTiO3 single crystalline fibers has not been successfully achieved because of a reaction between the solvent and the substrates. Recently we have discovered that the BaF2-added solution decreases the crystallization temperature of Y123 down to 900°C and suppresses the reactivity with no degradation of superconductivity of the Y123 crystal. The Y123-coated fibers are successfully prepared by using this solution. The characteristic of the critical currents under magnetic fields showed that this Y123 coated fibers have a potential to use under a high magnetic field at the liquid nitrogen temperature.View full abstractDownload PDF (588K) -
Yoshihiro Sugawara, Yuichi Ikuhara, Tasuku Kitamura, Izumi Hirabayashi1997 Volume 61 Issue 9 Pages 942-950
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSElectronic and magnetic properties of oxide superconducting films grown on a substrate strongly depend on the film/substrate interface. In this study, the lattice mismatch effects on the interface structure of YBCO films grown by liquid phase epitaxy were investigated by characterizing two model systems. These two systems, YBCO/NdGaO3(f : 0.84∼0.90%) and YBCO/MgO(f : 9.1∼25%), differ by their small and large lattice mismatches, respectively. In the case of the YBCO/NdGaO3 interface, misfit dislocation network is introduced to accomodate lattice strain along the interface. The misfit dislocations are composed of [\bar110]NdGaO3, [002]NdGaO3 and [100]YBCO edge type dislocations depending on the crystal orientation, and hence extra half planes exist both in NdGaO3 and YBCO crystals. The orthogonal sets of misfit dislocations are dissociated to form nodes which also have an edge component. The critical current density Jci in the YBCO film is as low as 9.0×102 A/cm2 at 77 K in 1 T. In the case of the YBCO/MgO interface, geometrical misfit dislocations are formed at the interface. The geometrical misfit dislocations accompany local lattice bending in the vicinity of the interface. In the YBCO film, a high density of edge dislocations with their Burgers vector perpendicular to the interface exist, which result in the formation of stacking faults on the ab plane in the YBCO crystals. Such defects may be introduced during crystal growth by following the local lattice bending due to the geometrical misfit dislocation. The Jci in the YBCO film is as high as 5.4×105 A/cm2 at 77 K in 1 T. The difference in critical current densities between two specimens presumably originates from the density of pinning centers for magnetic flux in YBCO crystals.View full abstractDownload PDF (1942K) -
Masanobu Kusunoki, Akira Osuka, Katsufumi Ehata, Sigetoshi Ohshima, Hi ...1997 Volume 61 Issue 9 Pages 951-955
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSYBa2Cu3Oy (YBCO) superconducting thin films were deposited on MgO single crystals by induction-coil-coupled sputtering. The effects of the induction coil on the qualities of YBCO thin films are examined. Uniformity of film thicknesses below ±5% was obtained over an area of 40 mm diameter using a 50 mm diameter target by the condition of the substrate-target distance of 90 mm. Even in the area of 60 mm diameter which was wider than that of the target, the uniformity of thickness within ±17% was obtained.
In order to clarify the effects of the induction-coil, we studied the dependence of the critical temperature (Tc) and the full width at half maximum (FWHM) of the films on the applying power of coil. Tc and FWHM showed optimum values at the applying coil power of 70W. It is confirmed from these results that the induction-coil operated effectively to fabricate high quality films. The epitaxial relations of YBCO films on MgO single crystal substrates were examined by X-ray pole figure measurements. Basically film of YBCO was aligned with the c-axis perpendicular to the substrate surface. The film deposited at the coil power of 70W exhibited well single-oriented grains with the a- or b-axis parallel to the a-axis of MgO. However, films deposited at the applying coil power of 0W contained grains with two different orientations which were rotated by 45°.
Scanning electron microscopy (SEM) of the film surface showed the films to consist of a smooth background surface and a distribution of particles. Energy dispersive X-ray analysis (EDAX) measurements showed that the Ba concentration was larger than that of exact stoichiometric composition. Auger electron spectroscopy (AES) revealed that the elements contained in the particles were richer in Ba than in the smooth background surface. The BaCuO2 particles seemed to precipitate on the surface of the film.View full abstractDownload PDF (455K) -
Masaki Sumida, Yuh Shiohara, Takateru Umeda1997 Volume 61 Issue 9 Pages 956-962
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSIn this paper, solidification microstructure control of a partial molten mixture is discussed. Since finely dispersed non-superconductive high temperature stable phase (Sm2Ba1Cu1O5, Sm211) particles in the superconductive Sm1Ba2Cu3O7−d (Sm123) can act as effective pinning centers and grain boundaries play as weak-links of electric transport, single crystalline Sm123 with finely dispersed Sm211 particles is required to improve the critical current density (Jc). To obtain the bulk single crystalline Sm123 with fine Sm211 particles by floating-zone partial melting and the solidification method, the microstructure of the partial molten mixture (Sm211 particles and liquid) is to be controlled, because the solidification microstructure of Sm123/211 is mainly determined by that of the mixture. During unidirectional melting, the mean diameter, d211, (or spacing, λ) of rod-likely aligned Sm211 particles after decomposition of the precursor Sm123 decreased with increasing growth rate, R. The mean diameter d211 became larger under PO2=1 kPa atomsphere than that at atomspheric pressure. Based upon the experimental results, a simple model for the formation of aligned Sm211 particles via peritectic decomposition was proposed, and prediction from the model was compared with the experimental results. Effects of melting conditions on the microstructure formation of the partial molten mixture were discussed.View full abstractDownload PDF (956K) -
Akihiko Endo, Yasuo Watanabe, Kouichi Miyake, Takateru Umeda, Kiyoshi ...1997 Volume 61 Issue 9 Pages 963-971
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSPushing/entrapment phenomenon of the high temperature stable phase, RE2BaCuO5 (RE211; RE=Y, Sm) during peritectic solidification of REBa2Cu3O7−δ (RE123) oxide superconductors was investigated. The RE123 crystals were grown at different undercoolings by isothermal solidification processing combined with the seeding method. In the Y-system , distribution and preferred orientation of trapped Y211 particles in solidified Y123 crystals depended on the growth direction and the growth rate as a function of undercooling. These phenomena could be at least qualitatively explained by the prevalent trapping/pushing theory considering the peritectic reaction as well as the faceted materials. In the Sm-system, a similar behavior of Sm211 particles was observed only in the Ba-enriched composition displaced from the Sm211-Sm123 tie-line. The results suggested that the variation of liquid composition altered the solid/liquid interfacial energy, which is one of the determining factors of particle pushing.View full abstractDownload PDF (1548K) -
Makoto Kambara, Masaru Nakamura, Xin Yao, Yuh Shiohara, Takateru Umeda1997 Volume 61 Issue 9 Pages 972-977
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSSingle crystals of Nd1+xBa2−xCu3O6+δ (Nd123) have been successfully grown by the top-seeded solution-growth (TSSG) method using BaO-CuO solvent in a Nd2O3 crucible as a solute source. Furthermore, compositions of 123 compounds could be controlled applying two different methods which were the oxygen partial pressure control of the atmosphere during growth and/or controlling liquid composition. Using the BaO-CuO solvent with the Ba/Cu ratio of 3/5, near-stoichiometric Nd123 single crystals were grown under a reduced oxygen partial pressure atmosphere (P(O2)=1.013×103 Pa). Even in the air atmosphere, the Ba-enriched liquid with the Ba/Cu ratio of 4/5 made it possible to obtain the almost stoichiometric Nd123 single crystal as well.
Moreover, to investigate whether the peak effect in Jc-H curve is an intrinsic property for Nd123 or not, different series of heat treatment were performed using near-stoichiometric Nd123 single crystals grown by the above-mentioned TSSG method.View full abstractDownload PDF (524K) -
Atsushi Takagi, Izumi Hirabayashi, Uichiro Mizutani1997 Volume 61 Issue 9 Pages 978-984
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSThe c-axis oriented Nd1+xBa2−xCu3Oy superconducting thick films were grown, by using the liquid phase epitaxy (LPE) technique, onto MgO(100) single crystal substrates in the Ar gas atmosphere containing 0.5 at%O2. The growth time dependences of the film thickness were found to be similar to those of semiconductors, when the films were grown in the transient mode LPE (TMLE) and the step-cooled growth mode. A superconducting transition temperature Tc was very broad, when the films were grown in the TMLE but became sharp when grown in the step-cooled growth mode. The Tc value could be increased up to 91 K by annealing at 340°C for 504 h in O2 atmosphere, when the film was grown in the step-cooled growth mode following the TMLE. Moreover the peak effect , which is characterized by an increase in magnetization with increasing magnetic fields, was observed only in the film grown in the step-cooled growth mode following the TMLE. These unique growth-mode dependent superconducting properties can be attributed to the formation of the composition-fluctuating region caused by the initial TMLE and that of uniform composition region by the subsequent step-cooled growth mechanism.View full abstractDownload PDF (584K) -
Naomi Sugiyama, Toshiya Doi, Toyotaka Yuasa, Hiroyuki Akata, Takeshi O ...1997 Volume 61 Issue 9 Pages 985-991
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSTo obtain Tl-1223 wires with a high critical current density (Jc), Tl-1223 layers are necessary to orient each other biaxially with the a-axis. The cube-textured silver tapes can be successfully prepared as the substrate for realizing the well-oriented Tl-1223 layer.
The cube texture of silver tape was formed by heat treatment at temperatures higher than 600°C after rolling at an intermediate temperature. The full width at half maximum of X-ray diffraction φ-scan of the tape was typically 6°. The homogeneously textured structure was obtained over the 15 m long tape.
The Tl-1223 thin layer was prepared on the cube-textured silver tape by spray pyrolysis method. The degree of the a-axis orientation of Tl-1223 layer was improved by the substrate effect. Due to this biaxial texturing, the grain boundary characteristics were improved, that is, the Jc was as high as 1.5×104 A/cm2 at 77 K under an external field of 1 T.
The present study demonstrated that the Tl-1223 long scale wires with improved biaxial orientations can be manufactured by a combination of the simple heat treatment process for the cube-textured silver tape and the mass-productive spray pyrolysis method for the biaxially oriented Tl-1223 thin layer.View full abstractDownload PDF (1006K) -
Tomoko Goto1997 Volume 61 Issue 9 Pages 992-997
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSA partial melting of the solution-spun filamentary HgxBa2Ca2Cu3Re0.2Ox superconductor is examined to improve the Jc. Long precursor Ba2Ca2Cu3Re0.2Ox filaments were prepared by dry spinning through a homogeneous aqueous solution. The as-drawn filaments were pyrolyzed to remove volatile components and calcined in flowing Ar. The calcined specimens were reacted in an evacuated fused quartz tube with a pellet of HgBa1.8Ca2Cu3Re0.2(BaF2)0.2Ox. It was found that the filamentary specimens were partially melted by heating up to 1183 K, whereas the co-reacted pellet was decomposed from the Hg1223 phase to the Hg1234 phase. The partial melting of filamentary specimens at a melting temperature ranging from 1183 to 1203 K was examined and a nearly single Hg1223 phase with Tc=120 K was obtained by controlling the heating condition. The Jc was enhanced by post-annealing up to 623 K in flowing O2, while the Tc lowered by 10 K. A maximum Jc value of 7.1×107 A/m2 at 77 K and 0 T was attained. The grains did not align along the fiber length.View full abstractDownload PDF (839K) -
Hirofumi Yamasaki, Yoshihiko Nakagawa, Yasunori Mawatari, Bisong Cao1997 Volume 61 Issue 9 Pages 998-1005
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSMercury-based oxide superconductors, HgBa2Ca2Cu3O8+δ (Hg-1223), show the highest critical temperature, Tc≈135 K. Such high Tc has drawn keen interest both in physics and in applications, and recently it was found that Re doping brings about chemical and structural stabilization of the superconducting phase. We have investigated the preparation and magnetic properties of Re-doped Hg-1223 specimens to clarify their potential for power applications.
Nearly phase-pure (Hg1−xRe0.1) Ba2Ca2Cu3O8+δ superconducting specimens were prepared by a conventional powder-mixing and encapsulation technique. The “BaCuO2” phase present in precursor powders seemed to hinder the formation of the Hg-1223 phase, and we prepared precursors at a partial melting temperature to reduce this phase. Auger and EPMA analyses showed that the doped Re atoms are actually incorporated in the Hg-1223 grains. Magnetization M was measured for grain-aligned powders and for the sintered bulk samples. The |M| decreased sharply just above the penetration field Hp in the field-increasing branch, and the M-H curves were completely asymmetric about the central line of M=0. This clearly shows that the magnetization originates from the Bean-Livingston surface barrier. The magnetic hysteresis curves of Re-doped Hg-1223 samples also exhibited paramagnetism, in which a Curie-Weiss behavior occurred above the critical temperature Tc.View full abstractDownload PDF (864K) -
Yoichi Horibe, Yasuhide Inoue, Yasumasa Koyama1997 Volume 61 Issue 9 Pages 1006-1011
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSIn order to elucidate features of low-temperature structural phase transitions in La1.5Sr0.1Nd0.4CuO4, in-situ observation was performed by transmission electron microscopy. A series of dark field images using the 100 forbidden spot reveal that there are two types of sites for the nucleation of the LTO/LTT phase; that is , the twin boundary between neighboring LTO I domains and the interior of the domain. When the temperature is lowered from the LTO I phase, the LTO/LTT phase is first nucleated along the twin boundary around 120 K and exhibits a lateral growth, which is relatively suppressed. As a result of the suppression, the LTO/LTT region nucleated along the boundary is never extended in a whole area. On further cooling to 12 K, the LTO/LTT phase with a spotty-shape also appears in the interior of the LTO I domain. The most interesting feature of the spotty-shape phase is that on aging at 12 K some LTO/LTT regions nucleated inside the domain are annihilated and then appear again. That is, the growth of the LTO/LTT phase in the interior of the domain exhibits a very unique behavior. The present data suggest that the unique behavior originates from the interaction between the order parameter and the spontaneous strain accompanied by the transition.View full abstractDownload PDF (1291K) -
Kazuo Watanabe, Yosuke Watanabe, Satoshi Awaji, Kazunori Jikihara, Tsu ...1997 Volume 61 Issue 9 Pages 1012-1017
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSA cryocooler-cooled split-pair superconducting magnet using high-temperature Bi2Sr2Ca2Cu3O10 current leads was successfully constructed. The magnet generated 5.0 T at the center of a vertical clear bore of 68 mm and a horizontal gap of 58 mm, and was combined with an X-ray diffraction apparatus. The performance test of the high field low temperature X-ray diffraction system was carried out in fields up to 5.0 T and at temperatures ranging from room temperature to 7.5 K. We measured the lattice parameters of Nd0.5Sr0.5MnO3 at 8.9 K and 5.0 T. It is demonstrated that the newly developed X-ray diffraction system in fields is very useful for the investigation of field-induced phase transformation phenomena.View full abstractDownload PDF (579K) -
Hiroyuki Shiibashi, Takayo Hasegawa, Kaoru Kimura, Hiromitsu Ino1997 Volume 61 Issue 9 Pages 1018-1019
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSDownload PDF (215K) -
1997 Volume 61 Issue 9 Pages 1020
Published: 1997
Released on J-STAGE: April 04, 2008
JOURNAL FREE ACCESSPlease see pdf. Wrong:Fig. 2 The relationship among the jet behavior, Vd and Vj. Vd; circumferential velocity of the drum. Vj; mean velocity of the jet. Right:Fig. 2 The relationship among the jet behavior, Vd and Vj. Vd; circumferential velocity of the drum. Vj; mean velocity of the jet. ○ Penetrating △ Off and on bouncing × Bouncing.View full abstractDownload PDF (25K)
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