Hideyuki Yamada

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.

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 molu200a% 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 molu200a% 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.

The excellent superconducting properties of in situ powder-in-tube processed MgB2 tapes with both ethyltoluene and SiC powder added

We found that the addition of ethyltoluene to the starting powder of in situ processed powder-in-tube (PIT) MgB2/Fe tapes is more effective in enhancing Jc values than other hydrocarbon additions such as benzene, naphthalene and thiophene, in spite of the smaller amount of carbon substitution for the boron site. This suggests that the dominant mechanism of Jc enhancement for ethyltoluene-added tape is different from carbon substitution for boron. The addition of both ethyltoluene and SiC nanopowder to the starting powder is much more effective in increasing Jc values. This is because both mechanisms of Jc improvement—one comes from the addition of ethyltoluene and the other comes from the carbon substitution for boron by the SiC addition—work together. The highest Jc values at 4.2 K reached 320 A mm−2 in 10 T and 140 A mm−2 in 12 T for 10 mol% ethyltoluene and 10 mol% SiC-added tape.

Upper critical field, irreversibility field, and critical current density of powder-in-tube-processed MgB2/Fe tapes

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.

Improvement of the critical current properties of in situ powder-in-tube-processed MgB2 tapes by hot pressing

We applied hot pressing to in situ powder-in-tube-processed (PIT-processed) MgB2 tapes. We enhanced the superconducting properties by adding nanometer-sized SiC powder and ethyltoluene (C9H12) to the mixed powder. Hot pressing was performed at 100xa0MPa and 630u2009°C in an Ar gas atmosphere for 2–10xa0h. Hot pressing reduced the cross-sectional area of the MgB2 tapes from ~ 0.55xa0mm2 for conventionally heat-treated tapes to ~ 0.44xa0mm2. This increased the MgB2 core density from 50% to 70%. Undoped MgB2 tapes hot pressed for 5xa0h exhibited a transport Jc of 90xa0Axa0mm − 2 at 4.2xa0K and 10xa0T, which is about three times greater than that for a tape heat treated without hot pressing. The value of Jc for codoped tape hot pressed for 5xa0h was 450xa0Axa0mm − 2 at 10xa0T, which is larger than that for codoped tape heat treated without hot pressing (280xa0Axa0mm − 2). These results clearly demonstrate that hot pressing is effective in increasing the MgB2 core density, and hence in enhancing the Jc values of PIT-processed MgB2 tapes.

Fabrication and transport properties of an MgB2 solenoid coil

This paper reports on the fabrication and testing of an MgB2 coil made by a wind and react method. We made a 130xa0m-long Fe/Cu-composite sheathed MgB2-superconducting round wire applying an in situ PIT method. Using a 58xa0m-long wire, we fabricated a small solenoid coil with 459 turns. The Ic of the coil were measured under different external fields and temperatures. Excellent correspondence of Ic between the short sample and the coil was obtained at 4.2xa0K and in external fields above 4xa0T. This indicates that the 58xa0m-long wire has a very homogeneous Ic distribution. At 25xa0K this coil generated a maximum magnetic field of 1xa0T (Ic = 100xa0A, overall-Jc = 200xa0Axa0mm−2) in an external field of 0.5xa0T. At 4.2xa0K this coil generated 1.9xa0T (Ic = 184xa0A, overall-Jc = 366xa0Axa0mm−2) in the external field of 0.5xa0T. These results demonstrate the possible use of an MgB2 superconducting coil for superconducting magnet applications such as in MRI.

Superconducting properties of in situ powder-in-tube-processed MgB2 tapes fabricated with sub-micrometre Mg powder prepared by an arc-plasma method

We fabricated in situ powder-in-tube-processed MgB2/Fe tapes using sub-micrometre Mg powder prepared by applying an arc-plasma method. We found that the use of this sub-micrometre Mg powder was very effective in increasing the Jc values. The transport Jc value of 10xa0mol% SiC-added tapes fabricated with this sub-micrometre Mg powder reached 275xa0Axa0mm−2 at 4.2xa0K and 10xa0T. This value was about six times that of 5xa0mol% SiC-added tapes fabricated with commercial Mg powder. Microstructure analyses suggest that this Jc enhancement is primarily due to the smaller MgB2 grain size.

Fabrication of A15-type superconducting tape conductors by applying the ex situ powder-in-tube method

It is well known that MgB2 tape conductors can be fabricated by the ex situ powder-in-tube (PIT) method with MgB2 powder and without any heat treatment. We fabricated different A15-type superconducting tapes by applying this ex situ PIT method. Nb3Sn, Nb3Al, or V3Si superconducting powder was tightly packed into a stainless steel (SUS316) tube or a Cu–10 wt% Ni tube. These tubes were cold rolled into tapes by using groove rolling and flat rolling. Rigid A15 superconducting cores were obtained after cold rolling. All the tapes showed substantial Jc values at 4.2 K without any heat treatment. The superior mechanical hardness of the sheath material was effective for obtaining a superconductor core of high density and realizing high Jc values. Jc increased with increasing degree of cold rolling. This Jc increase was also due to the increase in the density of the superconducting core. The highest Jc of at 6 T and 4.2 K was obtained for the Nb3Al/(SUS316) tape. This Jc value was much higher than that of the ex situ-processed MgB2/(SUS316) tapes. The temperature dependence of the Bc2 of the Nb3Sn/(SUS316) tape, d Bc2/d T, was almost equal to that of the bronze-processed Nb3Sn conductor. Annealing at after cold rolling considerably enhanced Jc values of all tapes.

Development of 2T‐Class MgB2 Solenoid Coil

This paper reports on the fabrication and testing of a MgB2 coil made using a wind & react method. We made a 130m‐long Fe/Cu‐composite sheathed, SiC‐doped MgB2‐superconducting round wire using an in‐situ PIT method. Using a 92m‐long wire, we fabricated a solenoid coil with 722 turns. In the coil test, we measured the Ic of the coil under various external fields and temperatures. The Ic of the coil reached 162 A (Je = 340 A/mm2) at 4.2 K and in the external field of 2 T. The coil generated 2.2 T (the center magnetic field, Bo); thus the total field reached 4.2 T. The Je exceeded 100 A/mm2 in the external fields of 4 T, 2.5 T, and 1 T at 15 K, 20 K, and 25 K, respectively. The Jc of the coil was almost equal to that of the short sample. This indicates that the 92m‐long wire has a very homogeneous Jc distribution.

Pulsed field magnetization of large-grain Gd–Ba–Cu–O bulk superconductor

Abstract We measured the field distribution and the variation of temperature in a large-grain Gd–Ba–Cu–O bulk superconductor during pulse field magnetization. When the applied field was below 3.1 T, the temperature rise due to flux motion was small because the magnetic flux penetrated only the surface thin layer. The trapped field increased with increasing field, although the sample temperature was raised and the trapped field reached 1.6 T for the applied pulse field of 3.1 T. However, when the applied field exceeded 3.1 T, the trapped field suddenly decreased due to an excessive temperature rise in the sample. Thus there is an optimum peak field for the pulse field magnetization process.