Hitoshi Kitaguchi

Effect of SiO2 and SiC doping on the powder-in-tube processed MgB2 tapes

Effect of SiO2 and SiC nano-powder doping was investigated for the powder-in-tube processed MgB2/Fe tapes. Mg or MgH2 powder was used as the Mg source of starting materials, and heat treatment was carried out at 600 °C for 1 h. These heat treatment conditions of lower temperature and shorter heating time are advantageous from the aspect of practical production processes. MgH2 powder improved the connection of MgB2 grains and prevented oxidation of MgB2. SiC and SiO2 doping greatly enhanced the critical current density (JC) values of the tapes prepared with Mg + B powder. However, only the SiC doping was effective in enhancing JC values for MgH2 + B powder. SiC doping decreased magnetic field sensitivity of JC, while SiO2 doping did not change the field dependence of JC. The SiC doped tape showed transport JC value of about 6 500 A cm−2 at 4.2 K and in the magnetic field of 12 T. The irreversibility field increased from 17 T to 23 T by the SiC doping.

Effect of aromatic hydrocarbon addition on in situ powder-in-tube processed MgB2 tapes

We fabricated in situ powder-in-tube processed MgB2/Fe tapes using the aromatic hydrocarbons benzene, naphthalene, and thiophene as additives, and investigated the superconducting properties. We found that these aromatic hydrocarbons were very effective for increasing the Jc values. The Jc values of 20 mol% benzene-added tapes reached 130 A mm−2 at 4.2 K and 10 T. This value was almost comparable to that of 10 mol% SiC-added tapes and about four times higher than that of tapes with no additions. Microstructural analyses suggest that this Jc enhancement is due to both the substitution of carbon for boron in MgB2 and the smaller MgB2 grain size.

Evaluation of connectivity, flux pinning, and upper critical field contributions to the critical current density of bulk pure and SiC-alloyed MgB2

Measurement of critical current density Jc, normal state resistivity ρn, and upper critical field Hc2 on pure and 10% SiC-doped MgB2 bulks show systematic enhancement of Hc2 by SiC addition and by lowering reaction temperature. Hc2(10K) exceeds 33T, while the extrapolated zero temperature value exceeds 40T. The Rowell [Supercond. Sci. Technol. 16, R17 (2003)] analysis suggests that only 8%–17% of the MgB2 cross section actually carries current. Higher reaction temperature enhances the connectivity but degrades Hc2 and flux pinning, making the measured Jc a complex balance between connectivity, Hc2, and flux pinning induced by grain boundaries and precipitates.

MgB2 films with very high critical current densities due to strong grain boundary pinning

MgB2 superconductor has a great potential for applications because of its high Tc and Bc2, exceeding those of any Nb-base superconductors at any temperature. It is now important to understand its flux pinning so as to raise Jc to high values over a wide field range. We show that nanometer-sized columnar-grain structure can produce Jc exceeding 5×106A∕cm2. The angular dependence of Jc indicates that the strongest pinning occurs when the field is aligned parallel to the grain boundaries. Our results confirm earlier deductions that grain boundaries in MgB2 act as effective pinning centers like those in Nb3Sn.

Upper critical fields of powder-in-tube-processed MgB2/Fe tape conductors

We measured the upper critical field, Bc2, of pure and SiC-added MgB2/Fe tapes prepared by the powder-in-tube process. We found that the Bc2 of the MgB2 tapes was much higher than the Bc2 of MgB2 single crystals. At 4.2 K, the Bc2 of the 10 mol % SiC-added MgB2 tape reached 22.5 T. This Bc2 was almost equal to the Bc2 of a conventional bronze-processed Nb3Sn conductor. At 20 K, the Bc2 of the 5 mol % SiC-added tape was around 10 T, which was comparable to the Bc2 of commercial Nb–Ti at 4.2 K. These results indicate that powder-in-tube-processed MgB2 tape is promising not only for high-field applications but also for applications at 20 K with a convenient cryo-cooler.

Critical current densities of powder-in-tube MgB2 tapes fabricated with nanometer-size Mg powder

We fabricated powder-in-tube MgB2/Fe tapes using a powder mixture of nanometer-size Mg and commercial amorphous B and investigated the transport properties. High-purity nanometer-size Mg powder was fabricated by applying the thermal plasma method. 5–10 mol % SiC powder doping was tried to enhance the Jc properties. We found that the use of nanometer-size Mg powder was effective to increase the Jc values. The transport Jc values of the nondoped and 10 mol % SiC-doped tapes prepared with nanometer-size Mg powder reached 90 and 250 A/mm2 at 4.2 K and 10 T, respectively. These values were about five times higher than those of the tapes prepared with commercial Mg powder.

Fabrication conditions and superconducting properties of Ag‐sheathed Bi‐Sr‐Ca‐Cu‐O tapes prepared by partial melting and slow cooling process

Ag‐sheathed Bi2Sr2Ca1Cu2Ox(Bi‐2212) tapes with various oxide superconductor core (SC‐core) thicknesses, SC‐core ratios (SC‐core area to overall cross‐sectional area), and tape configurations were prepared by a partial melting and slow cooling process. For single‐core tapes the critical current density, Jc(core), for SC‐core, increases with decreasing SC‐core thickness, due to the highly aligned Bi‐2212 grains, and the highest Jc(core) of 1.4×105 A/cm2 at 4.2 K in 10 T is achieved at a core thickness of 15 μm. Furthermore, the Jc(core) increases with increasing SC‐core ratio (i.e., with decreasing Ag‐sheath thickness). From the practical point of view, high Jc(overall) as well as high critical current (Ic) is more important than Jc(core). This requirement is attained by the double‐tube method, which shows the highest Jc(overall) value of 3.0×104 A/cm2 at 4.2 K in 10 T. In the tape fabrication process, the slow heating rate just below the partial melting becomes important, because oxygen released from the S...

Strain effect in Bi-based oxide/Ag superconducting tapes

The influence of mechanical strain on the critical current (I/sub c/) is investigated for (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/(Bi-2223)/ Ag-0.2wt%Mg-0.3wt%Sb superconducting tapes at 77 K. The tensile axial strain along tape length is successfully induced to the sample by using a U-shape holder. Continuous change of the axial strain can be obtained by changing the distance between both ends of the holder. The U-shape holders made of Ti, SUS304, or brass are used to examine the effect of thermal strain due to the contraction caused by the cooling. For example, the results for the samples glued to Ti or brass holder are as follows. Strain dependence of normalized I/sub c/ (I/sub c//I/sub c/(at as-cooled state)) is affected by holder material. A steep decrease of I/sub c/ occurs when we apply 0.1% and 0.3% strain for Ti and brass holder, respectively. The different thermal expansion (-0.15% for Ti and -0.37% for brass from 300 K to 77 K) explains it. All the results for normalized I/sub c/ vs. strain relation fall on a master curve with taking into account of the effect of the thermal strain. Thermal expansion of the tape from 300 K to 77 K is measured to be -0.35% by using a strain gage. The critical strain ( c) where a steep decrease of I/sub c/ occurs is evaluated to be 0.27% from self-contracted state of the tape.

Bi2Sr2CaCu2Ox/Ag multilayer tapes with Jc>500 000 A/cm2 at 4.2 K and 10 T by using pre-annealing and intermediate rolling process

Abstract The high transport critical current density ( J c )>500 000 A/cm 2 at 4.2 K, 10 T (critical current>700 A) is obtained in the Bi-2212/Ag multilayer tape fabricated by using the PAIR ( P re- A nnealing and I ntermediate R olling) and melt–solidification process. The combination of the improvement in geometric configuration of the tape and PAIR process make it possible to obtain the most preferable microstructure and enormous J c . Effect of processing and geometry of the tape on J c and Bi-2212 grain alignment has been studied. The enormous transport J c is attributed to highly textured grain alignment and uniformity in the tapes, which are achieved by PAIR process. After post annealing in flowing argon at 450°C for 4 h, transition temperature T c is increased from 83 K to 92 K and transport J c >25 000 A/cm 2 at 77 K is obtained. PAIR process is applicable for fabricating long Bi-2212 tapes with large transport J c for practical applications.

Effect of high‐energy ion irradiation and electron irradiation on textured Bi2Sr2CaCu2Ox—180‐MeV Cu11+ and Br11+ irradiations and 3‐MeV electron irradiation

180‐MeV Cu11+ and Br11+ irradiations and 3‐MeV electron irradiation were carried out on textured Bi‐2212 tapes and the effects of irradiation on the microstructure and superconducting properties were investigated. Ion irradiation, which creates linear tracks, is very effective in increasing both intragrain Jc and irreversibility field at temperatures below 60 K when field is parallel to the tracks. At 60 K, an order of magnitude higher Birr is obtained for both Cu11+ and Br11+ irradiations. Electron irradiation which creates point defects, on the other hand, is not as effective as ion irradiation except for low temperatures of ∼4.2 K. Apparent pinning potential energy U0 was increased by ion irradiation from ∼15 to ∼40 meV at 41.8 K, while U0 was unchanged by the electron irradiation. However, even U0 of the ion‐irradiated sample is still small. This small U0 increases flux creep at temperatures above 60 K and makes ion irradiation less effective. A small increase of transport Jc was obtained by both ion ...