Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg + 2B mixture was used as the central conductor core and reacted in-situ to form MgB 2 . The tapes were sintered in pure Ar at 800 o C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 µm. The Fe clad tape shows a sharp transition with transition width of ∆T c of 0.2 K and T c0 at 37.5 K. We have achieved the highest transport critical current reported so far at 1.6 × 10 4 A/cm 2 for both 29.5 K in 1 Tesla and 33 K in null field. R-T and critical current were also measured for fields perpendicular and parallel to the tape plane. The iron cladding shielded on the core from the applied external field, with the shielding being less effective for the field in the tape plane. Fe cladding may be advantageous for some applications as it could reduce the effects of both the self-field and external fields.

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The effect of the phase compositions after the final stage of heat treatment on the critical current density in Bi:2223/Ag tapes has been investigated. A high volume fraction of Bi:2223 has been considered the most critical factor in achieving a high ; however, it is extremely difficult to attain pure Bi:2223 phase via routine thermomechanical processing. Although a higher volume fraction of Bi:2223 phase can be attained using a higher processing temperature, it is always found to co-exist with Bi:2201 which resides at Bi:2223 colony boundaries. As these possess a plate-like morphology, the Bi:2201 joins weak links in a conductible chain. Bi:2201 can be eliminated using an annealing step at the end of the last thermal cycle, resulting in a significant improvement in . At annealing temperatures below , Bi:2223 decomposes into Bi:3221 and other cuprates, which degrade further. Magnetization, AC susceptibility and resistivity measurement results confirm that the Bi:2201 and Bi:3221 act as weak links.

The magnetoresistivity and critical current density of well characterized Si-nanoparticle doped and undoped Cu-sheathed MgB2 tapes have been measured at temperatures T ≥ 28 K in magnetic fields B ≤ 0.9 T. The irreversibility line Birr(T ) for doped tape shows a stepwise variation with a kink around 0.3 T. Such Birr(T ) variation is typical for high-temperature superconductors with columnar defects (a kink occurs near the matching field B φ ) and is very different from a smooth Birr(T ) variation in undoped MgB2 samples. The microstructure studies of nanoparticle doped MgB2 samples show uniformly dispersed nanoprecipitates, which probably act as a correlated disorder. The observed difference between the field variations of the critical current density and pinning force density of the doped and undoped tape supports the above findings.

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