| Budget Amount *help |
¥2,300,000 (Direct Cost: ¥2,300,000)
Fiscal Year 1993: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 1992: ¥1,500,000 (Direct Cost: ¥1,500,000)
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| Research Abstract |
The development of new superconductors capable of generating fields over 20 T at 4.2 K is useful for NMR magnets, fusion magnets, ets. A15 Nb_3Al and Nb_3(Al, Ge) whose upper critical fields H_<c2> (4.2 K) exceed 30 T are attractive for matching these requirements. However, it is difficult to obtain the stoichiometric A15 composition of Nb_3Al and Nb_3(Al, Ge), which is stable at elevated temperatures above -1800ﾟC.Moreover, the direct diffusion reaction between Nb and Al at ordinary temperatures produces only intermediate compounds richer in Al. In the present study, A15 Nb_3(Al, Ge) tapes have been prepared by a new reaction process between the sigma-phase Nb_2(Al, Ge) and Nb. Fabrication of a tape specimen using the sigma/Nb powder composite encased in a Nb sheath has been successfully performed without intermediate annealing. Heat treatment at 1300ﾟC produces almost single phase A15 Nb_3(Al, Ge). The T_c transition width is much reduced in the tape specimen compared to the bulk specimen. The highest onset T_c obtained in the tape specimen is 18.0 K.The finer sigma-phase and Nb starting powders yield pronounced improvement in J_c at magnetic fields throughout from 10 to 25 T.A J_c of over 2.5X10^4 A/cm^2 has been obtained at 21.5 T and 4.2 K.The addition of Mgo powder is newly found to be effective for enhancing J_c and reducing the peak effect. The Nb_3(Al, Ge) tape prepared in the present study may be promising for generating fields over 20 T at 4.2 K.The mechanical alloying between the sigma-phase and Nb powders will be studied in future for reducing the reaction temperature.
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