H. Hatakeyama

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.

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.

High temperature performance of MgB2 powder-in-tube composite tapes

The high temperature performance of monocore MgB2 powder-in-tube?(PIT) composite tapes has been investigated. MgB2/stainless steel?(SUS316) tapes are prepared by using an ex?situ process. In?situ processed MgB2/iron tapes are prepared by putting a mixture of MgH2 and amorphous boron in iron tubes and heating at 873?K. Critical current (Ic) and electrical resistance are measured in magnetic fields in the range 0?7?T and/or the temperature range 4.2?40?K by using a dc four-probe method. Irreversible fields (Birr) are determined by using R?T curves. Birr values of the in?situ processed tapes are higher than those of the ex?situ processed tapes in the whole temperature range. For example, Birr values of in?situ and ex?situ processed tapes are?2.5 and 0.5?T at 30?K, and 5.7 and 2.2?T at 25?K, respectively. Reflecting the better performance in Birr, the in?situ processed tapes show higher Ic performance than the ex?situ processed tapes. For example, Ic of 100?A can be obtained for the in?situ tapes at 32?K?0?T, 25?K?2?T, 16?K?5?T and 6?K?7?T, while Ic of the ex?situ processed tape reaches 100?A at 19?K?0?T and 7?K?2?T. In the relations between Ic and temperature, Ic of the in?situ processed tapes is less sensitive to temperature than that of the ex?situ processed tapes.

Critical current of magnesium diboride/stainless steel composite tapes under tensile or compressive strains

We investigate the influence of mechanical strain on the critical current (Ic) for magnesium diboride/stainless steel (SUS316) superconducting tapes. The tensile axial strain along the tape length is successfully induced in the sample by using a U-shaped holder made of stainless steel (SUS304). Two samples are examined at 4.2 K and 5 T (B is applied perpendicular to the tape surface). The Ic–strain relation can be divided into two regions. Rapid and large degradation occurs at a strain exceeding ~0.5%. On the other hand, Ic change is reversible against strain change between −0.4% (compressive strain) and 0.4%. Ic increases and decreases with increasing and decreasing external strain in this region, respectively. The critical temperature (Tc) also changes with the change in the external strain; Tc increases with increasing external strain. The reversible change in Ic originates in the change of Tc.

Fabrication and Properties of Powder-in-Tube Processed MgB2 Tapes and Wires

MgB2 tapes were fabricated with MgB2 powder and several sheath materials such as Cu, Cu-Ni, Fe, carbon steel (Fe-C) and stainless steel. High-density MgB2 cores were obtained for these sheath materials. Jc of the as-cold rolled (non heat treated) tape significantly increased with increasing the cross sectional area reduction by the cold working. Hard sheath materials (Fe-C and stainless steel) are effective to enhance Jc values. These results can be explained by the densification of MgB2 core. Non heat treated MgB2/(stainless steel) and MgB2/(Fe-C) tapes showed extrapolated Jc values of 300–450 kA/cm2 at 4.2 K and zero field. MgB2 tapes show anisotropy in Jc with respect to field orientation. This anisotropy can be explained by the MgB2 grain orientation. Heat treatment after the cold rolling is effective to enhance Jc values. An order of magnitude higher Jc values were obtained for Fe-C and stainless steel sheathed tapes by the heat treatment. Jc values extrapolated to zero field of MgB2/(SUS 316) and MgB2/(Fe-C) tapes reached 1,000 kA/cm2 at 4.2 K.