We investigated the effect of SiC doping on the critical current density (Jc) in MgB2 thick films using amorphous SiC impurity layers of various thicknesses: 7, 14, 35, and 70 nm. SiC impurity layers were first deposited on the Al2O3(0001) substrates at room temperature by using a pulsed laser deposition system, after which MgB2 films were grown on the SiC deposited precursor substrates by using a hybrid physical–chemical vapor deposition technique at a low growth temperature of 480 °C. All samples showed a high transition temperature of ∼40 K irrespective of the thickness of the impurity layer. The grain sizes of the MgB2 films slightly increased from 400 to 488 nm with increasing thickness of the impurity layer. The MgB2 thick film with a 35 nm thick SiC impurity layer exhibited the highest Jc, while all SiC doped samples showed a higher Jc than a pure MgB2 thick film throughout the whole magnetic field region. These results suggest that the SiC particles of the impurity layer diffused into the MgB2 films during film growth, and the SiC particles, along with the columnar grain boundaries in the MgB2 thick films, act as strong pinning centers.

We report on the fabrication of superconducting MgB 2 thin films on textured Cu(100) tape under low pressure and temperature by using a hybrid physical-chemical vapor deposition (HPCVD) technique to explore the possible broad range of deposition. Thermodynamic calculations for the Mg-B system have been carried out and the pressure-temperature phase diagram was obtained. Our results demonstrate that the deposition of superconducting MgB 2 films is possible beyond the theoretically calculated growth window, where the sublimation of Mg is taking place. The structural and microstructural investigations reveal that MgB 2 films are c-axis-oriented normal to the substrate. The highest J c of ∼1.34 × 10 5 A cm −2 at 5 K under 3 T is obtained for the film grown at 460 • C. The critical current density (J c ) and flux pinning force density (F p ) of MgB 2 films are enhanced with decreasing growth temperature. This could be attributed to the high density of grain boundaries which may act as effective flux pinning centers. These findings suggest an alternative route to fabricate MgB 2 tapes at low temperature for large scale applications.

A comparative study of the structural and magnetic properties of polymer coated ͑pc-͒ and uncoated ͑uc-͒Ni nanoparticles prepared by chemical reduction method are reported. pc-Ni nanoparticles have been synthesized by reducing Ni 2+ cations to Ni particles by sodium borohydride in aqueous solution in the presence of polyvinyl alcohol ͑PVA͒ solution. Both samples have been identified to have a tetragonal crystal structure, different from its usual fcc structure, though some traces of fcc-Ni phase have been observed in x-ray diffraction and selected area electron diffractogram patterns for pc-Ni. This structural modification of fcc-Ni occurs due to the presence of interstitial oxygen atoms in the Ni lattice and results in appreciably modified magnetic properties in this new phase of Ni, as, for example, a nonhysteretic magnetization response with applied field at 300 K. A comparative magnetic study of the uc-and pc-Ni particles exemplifies the role played by the PVA matrix in modifying the magnetic properties of the uc-Ni sample. The M-H and M-T curves have been analyzed using superparamagnetic/ferromagnetic/paramagnetic component models and anomalies observed were explained using two-phase model.

The MgB 2 coated superconducting tapes have been fabricated on textured Cu (0 0 1) and polycrystalline Hastelloy tapes using coated conductor technique, which has been developed for the second generation high temperature superconducting wires. The MgB 2 /Cu tapes were fabricated over a wide temperature range of 460-520 °C by using hybrid physical-chemical vapor deposition (HPCVD) technique. The tapes exhibited the critical temperatures (T c ) ranging between 36 and 38 K with superconducting transition width (∆T c ) of about 0.3-0.6 K. The highest critical current density (J c ) of 1.34 × 10 5 A/cm 2 at 5 K under 3 T is obtained for the MgB 2 /Cu tape grown at 460 ºC. To further improve the flux pinning property of MgB 2 tapes, SiC is coated as an impurity layer on the Cu tape. In contrast to pure MgB 2 /Cu tapes, the MgB 2 on SiC-coated Cu tapes exhibited opposite trend in the dependence of J c with growth temperature. The improved flux pinning by the additional defects created by SiC-impurity layer along with the MgB 2 grain boundaries lead to strong improvement in J c for the MgB 2 /SiC/Cu tapes. The MgB 2 /Hastelloy superconducting tapes fabricated at a temperature of 520 °C showed the critical temperatures ranging between 38.5 and 39.6 K. We obtained much higher J c values over the wide field range for MgB 2 /Hastelloy tapes than the previously reported data on other metallic substrates, such as Cu, SS, and Nb. The J c values of J c (20 K, 0 T) ~5.8 × 10 6 A/cm 2 and J c (20 K, 1.5 T) ~2.4 × 10 5 A/cm 2 is obtained for the 2-µm-thick MgB 2 /Hastelloy tape. This paper will review the merits of coated conductor approach along with the HPCVD technique to fabricate MgB 2 conductors with high T c and J c values which are useful for large scale applications.

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