The Effect of Sheath Materials on the Superconductivity of MgB 2
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The Effect of Sheath Materials on the Superconductivity of MgB2 W. Zhu, D.R. Watson and J.R. Cave IREQ, Hydro Québec, 1800 Lionel Boulet Varennes, Québec, J3X 1S1, Canada ABSTRACT Inconel, tantalum and a silver-palladium alloy were used to fabricate MgB2 tapes. The tapes, made by the Powder In Tube (PIT) method, were heat-treated at 800oC and 900oC in 1 atmosphere of flowing argon. The microstructure and phase composition of the tapes were examined by Scanning Electron Microscopy (SEM) and X-Ray Diffractometry (XRD). Critical temperatures (Tc) and magnetization-applied magnetic field (M-H) curves were measured by SQUID magnetometry. It is found that between 800-900oC inconel and tantalum sheaths have no effect on the Tc,inductive of the material. However, under certain processing conditions, the silverpalladium sheath can decrease the Tc or destroy the superconductivity completely. The inconel sheathed tape was found to have better superconducting properties than the tantalum and silverpalladium sheathed tapes. Consistency checks have been applied to verify the validity of the Bean Model in obtaining the critical current density (Jc) from magnetization data. The inconel sheathed tape is estimated to have a magnetization Jc (5K, ∼0.5 T) of 1.4 × 105 A/cm2. INTRODUCTION The superconductor MgB2 has a relatively high critical temperature at ∼40K compared to the other inter-metallic superconductors. Its simple composition, simple structure and high Tc has generated a great interest in the properties of this material. Superconducting tape is a preferred conductor geometry in many applications, however, brittle materials, such as MgB2, require mechanical reinforcement. The PIT method is a suitable technique for fabricating MgB2 tape. One of the main difficulties in tape fabrication is the selection of the sheath material due to the reaction of the sheath with Mg vapor from the decomposition of MgB2. Jin et al [1] have studied the effects of Fe, Mo, Nb, V, Ta, Ti, Hf and W on the superconducting properties and on the fabrication of MgB2 wire. They found that iron was the best candidate as the sheath material due to its chemical compatibility with MgB2 and its ductility during mechanical deformation. Their MgB2/Fe wire has transport Jc > 85,000 A/cm2 at 4.2K. Glowacki and co-workers [2] have fabricated MgB2 wires sheathed with Ag, Cu, Ag/stainless steel and Cu/stainless steel. Their magnetization measurement results show that the Cu sheathed wire made from Mg and B powder has the highest Jc value compared to the other three sheath materials. A transport Jc of ∼ 21,000 A/cm2 (self field) has been obtained for this Cu sheathed wire. Goldacker et al.[3] have made mono-filamentary MgB2 wires with composite sheaths of Ta/Cu/steel and Nb/Cu/steel. A transport Jc up to 100, 000 A/cm2 at 4.2K under self field has been achieved for the Nb/Cu/steel reinforced wire. The present work is focused on the effect of three different sheath materials and the processing temperature on the superconducting properties of MgB2. E5.7.1
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