Superconductivity of powder-in-tube MgB2 wires

A new class of 'powder-in-tube' Mg-B superconducting conductors has been prepared using two different methods: an in situ technique where an Mg + 2B mixture was used as a central conductor core and reacted in situ to form MgB2, and an ex situ technique where fully reacted MgB2 powder was used to fill the metal tube. Conductors were prepared using silver, copper and bimetallic silver/stainless steel tubes. Wires manufactured by the in situ technique, diffusing Mg to B particles experienced ∼25.5% decrease in density from the initial value after cold deformation, due to the phase transformation from Mg + 2(β-B)→MgB2 all with hexagonal structure. A comparative study of the intergranular current and grain connectivity in wires was conducted by AC susceptibility measurements and direct four point transport measurements. Using a SQUID magnetometer, magnetization versus magnetic field (M-H) curves of the round wires before and after sintering and reactive diffusion were measured at 5 K and in magnetic fields up to 5 T to define the Jcmag. The direct current measurements were performed in self field at 4.2 K. A comparison between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility measurements for sintered Ag/MgB2, and reacted Cu/Mg + 2B conductors revealed systematic differences in the flux pinning in the wires which is in very good agreement with direct high transport current measurements. (author)