A significant enhancement of Jc, Hirr, and Hc2 in MgB2 tapes has been achieved by using maleic anhydride (C4H2O3) as dopants. MgB2 tape samples with 0 up to 30wt% C4H2O3 added were prepared by an in situ processed powder-in-tube method. Compared to the pure tapes, Jc for the 10wt% C4H2O3 doped samples was improved by more than an order of magnitude in high fields up to 18T. At 4.2K, the transport Jc for the 10wt% doped samples reached 1.08×104, 5.42×103, and 2.18×103A∕cm2 at 12, 14, and 16T, respectively, and were better than those of the best MgB2∕Fe tapes reported by far. Furthermore, the Hirr for the 10wt% doped sample reached 9.6T at 20K, which was comparable to that of NbTi at 4.2K. The improvement of the superconducting properties in doped tapes can be attributed to the increase of Hc2 and the grain size refinement by C4H2O3 doping.

Fe-sheathed MgB 2 tapes with cheap stearic acid, Mg stearate and Zn stearate doping were prepared through the in situ powder-in-tube method. It is found that J c , H irr , and H c2 were significantly enhanced by doping. Compared to the pure tapes, J c for all the doped samples was improved by more than an order of magnitude in a high-field regime. At 4.2 K, the transport J c reached 2.02×10 4 A/cm 2 at 10 T for the Zn stearate doped tapes and 3.72×10 3 A/cm 2 at 14 T for the stearic acid doped samples, respectively. Moreover, at 20 K, H irr for the Zn stearate doped tape achieved 10 T, which is comparable to that of the commercial NbTi at 4.2K. The improvement of J c -H properties in doped samples can be attributed to the increase of H c2 resulting from the incorporation of C atoms into the MgBB 2 crystal lattice as well as a high density of flux-pinning centers.

A series of C doped and undoped MgB2 tapes sintered at 600–950 °C was fabricated. The relationships between the critical current properties, crystallinity and microstructures were systematically studied as functions of the C doping level and heating conditions. A decrease in the a-axis length due to carbon substitution was observed in the C doped samples. The full width at half maximum (FWHM) of the MgB2 (101) peak increased with increasing C doping level, suggesting that the introduction of lattice defects and/or the decrease of the grain size of MgB2 occurred on C doping. Due to the substitution of C for B, the upper critical field was improved in C doped MgB2 samples. Substantially improved critical current density, Jc, was found in all of the C doped samples. At 4.2 K, 10 T, Jc reached 2.2 × 104 A cm−2 in the 8% C doped samples.

Abstract:MgB 2 /Fe tapes with 2.5-15 at.% ZrBB 2 additions were prepared through the in situ powder-in-tube method. Compared to the pure tape, a significant improvement in the in-field critical current density Jc was observed, most notably for 10 at.% doping, while the critical temperature decreased slightly. At 4.2 K, the transport Jc for the 10 at.% doped sample increased by more than an order of magnitude than the undoped one in magnetic fields above 9 T. Nanoscale segregates or defects caused by the ZrB 2 additions which act as effective flux pinning centers are proposed to be the main reason for the improved Jc field performance. a) Author to whom correspondence should be addressed, E-mail: ywma@mail.iee.ac.cn 1 MgB 2 is considered as a possible substitute for Nb-Ti or high-T C oxide superconductors operated at around 20 K because of its high transition temperature (T C ) and weak-link free character. The low material costs, simple structure and small anisotropy are additional advantages of MgB 2 . Therefore, it is thought as the most promising candidate for engineering application, especially for magnetic resonance imaging (MRI) magnet which prefers to work at cryogen-free circumstances.However, critical current density (J C ) of MgB 2 decreases rapidly under magnetic fields compared to those for the Nb-based superconductors. MgB 2 is a two-gap structure material, and phonon scattering induced by the lattice distortions in this material is very pronounced. This suggests that the upper critical field (H C2 ) of MgB 2 can be enhanced by introducing electron-scattering defects. It has been demonstrated that C doped thin films have H C2 (0K) about 50 T [1].Various dopants [2-8] have been investigated, leading to a wide variation in the reported values of H irr and J C . In the case of ZrB 2 , we reported earlier that the improved J C value was observed in 5% ZrB 2 -doped MgB 2 tapes when sintered at 600°C [9]. Furthermore, an increase in HC2 from 20.5 T to 28.6 T and enhancement of H irr from 16 T to 24 T were obtained in the ZrB 2 doped bulk MgB 2 samples as compared to the binary sample at 4.2 K [10]. This means that ZrB 2 doping is a very promising method for MgB 2 tapes to get higher J C in high magnetic fields. Therefore, it is very necessary to carry out further investigation on the doping effect of this material due to the lack of systematic study. In this work, a series of ZrB 2 doped MgB 2 tapes were prepared using an in situ powder-in-tube (PIT) method. The highest J C values were achieved in samples with 10% ZrB 2 doping level, more than 11-fold improvement compared to the undoped tapes. The field dependence of J C decreased by the ZrB 2 doping, suggesting that pinning centers effective in a high-field region were possibly introduced.The detailed procedure for preparation of MgB 2 /Fe tapes has been reported elsewhere [11]. Mg (325 mesh, 99.8%), B (amorphous, 99.99%), and ZrB 2 (2-5 μm, 99%) powders were used as the starting materials. Mg, B powders were mixed with the nominal composition of 1:2, th...