Effect of SiO 2 and SiC nano-powder doping was investigated for the powder-in-tube processed MgB 2 /Fe tapes. Mg or MgH 2 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. MgH 2 powder improved the connection of MgB 2 grains and prevented oxidation of MgB 2 . SiC and SiO 2 doping greatly enhanced the critical current density (J C ) values of the tapes prepared with Mg + B powder. However, only the SiC doping was effective in enhancing J C values for MgH 2 + B powder. SiC doping decreased magnetic field sensitivity of J C , while SiO 2 doping did not change the field dependence of J C . The SiC doped tape showed transport J C 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.

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.

Ќ transition is more appropriate for untextured samples than the bottom or 10% point on the small-current-density, resistive H c2 transition which corresponds to H c2 ʈ . However, the resistive H c2 transition is still useful for measuring the breadth of the parallel H c2 transition ⌬H, which may be indicative of inhomogeneity in composition in the sample. Hopes for expanding the useful range of MgB 2 are encouraged by earlier work that has shown that H c2 ʈ ͑0͒ can exceed 70 T in C-doped MgB 2 thin films, 2 but so far the highest H c2 ͑0͒ of C-or SiC-doped wires or bulks is ϳ35 T, 3,4,9,10 only half this value. Since H c2 and H irr enhancement is crucial for magnet applications, we have here systematically studied the H c2 transition and J c ͑H , T͒ behavior of pure and SiC-doped bulks. Irrespective of this high-field perspective on MgB 2 , we should also point out that J c ͑H͒ falls off only slowly in the 10-30 K range, making MgB 2 useful for lower field applications without liquid He.Our previous reports 6,7 showed that higher J c values were obtained in tapes using MgH 2 rather than Mg powder. Nano-SiC addition improved the high-field J c at low temperatures and produced a measured H c2 value of 23 T at 4.2 K. Here we present a more detailed study of MgB 2 samples cut from this same tape measured without any extraneous sheath material.MgB 2 bulk samples were prepared by conventional in situ powder-in-tube method with commercial MgH 2 and amorphous B powders which were mixed and packed into a pure Fe tube in air.7 5 or 10 mol % of ϳ30 nm SiC powder 5 was added for the doped samples. The filled tubes were groove rolled into 2 mm square rods and then flat rolled into 0.5 mm thick by 4 mm wide tapes. 50 mm long samples were heat treated at 600, 700, 800, and 900°C for 1 h under Ar atmosphere making the 12-sample set. 7 After peeling away the Fe sheath, resistivity curves were measured with 5 mA transport currents in a 9 T Quantum Design physical properties measurement system, the 33 T Bitter magnet at the National High Magnetic Field Laboratory ͑NHMFL͒ in Tallahassee, and the 60 T short pulse magnet at the NHMFL in Los Alamos National Laboratory. The 10% and 90% points on the resistive transition curves were used to define a transition breadth ⌬H and H c2 ʈ . Magnetization properties were measured in an Oxford Instruments vibrating sample magnetometer, from which the critical current density J c ͑H , T͒ was calculated assuming fully connected samples using the expression J c ͑H , T͒ = 0.5⌬M12b / ͑3bd − d 2 ͒, where b and d are the width and thickness of the rectangular section bar. Extrapolation of J c ͑H͒ to zero allowed extraction of H irr . However, following Rowell, 11 we believe that the connected cross section 1 / F of our samples is much less than unity, based on calculations of 1 / F using the relation ͑T͒ = F͓⌬ sc ͑T͒ + ͑0͔͒, where n is the measured normal state resistivity and ⌬ sc ͑300-50 K͒ = 7.3 ⍀ cm ͑Table I͒. Table I provides an overview of the properties of the four samples. SiC additions depres...

We fabricated in situ powder-in-tube processed MgB 2 /Fe tapes using aromatic hydrocarbon of 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 20mol% benzene-added tapes reached 130A/mm 2 at 4.2K and 10T. This value was almost comparable to that of 10mol% SiC -added tapes and about four times higher than that of tapes with no additions.Microstructure analyses suggest that this Jc enhancement is due to both the substitution of carbon for boron in MgB 2 and the smaller MgB 2 grain size.The MgB 2 superconductor is expected to be applied to practical superconducting wires because its transition temperature, 39K, is much higher than those of conventional metallic superconductors.The lower cost of the raw materials, Mg and B, than that of Nb, is an additional advantage of MgB 2 .Recently, small coils using MgB 2 wires have been produced. In order to evaluate the potential of MgB 2 for power applications, wire processing techniques are now being actively developed throughout the world. The most popular method is the so-called in situ powder-in-tube (PIT) method,

MgB 2 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.

Partial substitution of Pb for Bi in the Bi-Sr-Ca-Cu-O system has been found to sharply increase the volume fraction of the high-T c phase when both the starting material (coprecipitated oxalate being used in the present study) and the heating process are appropriate. The sharp powder X-ray diffraction pattern obtained from well-grown particles, 5∼10 µm wide and 0.5 µm thick typically, was assigned to an orthorhombic cell with a=0.537 nm, b=2.682 nm, and c=3.726 nm. The electrical resistance dropped to zero at 107 K within the experimental limit of 10-6 Ω. A large diamagnetic response in the ac susceptibility due to the Meissner effect was seen below 120 K. The dominance of the high-T c phase over the low-T c phase was roughly estimated at 9/1 in volume.

We fabricated pure and SiC-added MgB2/Fe composite tapes using a MgH2 starting powder and applying heat treatments at 600–900 °C and systematically investigated their superconducting properties. For both the pure and SiC-added tapes, the critical temperature (Tc) increased with increasing heat-treatment temperature due to the improved crystallinity of MgB2.The SiC addition decreased the Tc but increased the slope of the Bc2–T and Birr–T curves, d Bc2/d T and d Birr/d T, for all heat-treatment temperatures. The d Bc2/d T and d Birr/d T of the pure tape decreased with increasing heat-treatment temperature from 600 to 700 °C because of the longer coherence length associated with the improved crystallinity. However, the SiC addition significantly decreased the heat-treatment temperature dependences of d Bc2/d T and d Birr/d T. At a temperature of ∼20 K, which is easily obtained using a cryocooler, the Birr is governed by both the Tc and d Birr/d T. The Birr of a pure tape at 20 K decreased with increasing heat-treatment temperature from 600 to 700 °C, but the Birr of the 10 mol% SiC-added tape increased with the temperature. These behaviours can be explained by the heat-treatment temperature dependence of the Tc and d Birr/d T. At 20 K the highest Birr of 10 T was obtained under the conditions of a 10 mol% SiC addition and heat-treatment temperature of 900 °C. This Birr at 20 K is comparable to that of commercial Nb–Ti at 4.2 K. The 10 mol% SiC-added tape heat treated at 900 °C and the 5 at.% SiC-added tape heat treated at 800 °C showed Jc (MgB2 core) values higher than 104 A cm−2 at 20 K in 5 T.

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.