Superconductivity of powder-in-tube MgB2 wires

A new class of 'powder-in-tube' Mg-B superconducting conductors has been pr epared using two different methods: an in situ technique where an Mg + 2B m ixture was used as a central conductor core and reacted in situ to form MgB 2, and an ex situ technique where fully reacted MgB2 powder was used to fil l the metal tube. Conductors were prepared using silver, copper and bimetal lic silver/stainless steel tubes. Wires manufactured by the in situ techniq ue, diffusing Mg to B particles experienced similar to 25.5% decrease in de nsity from the initial value after cold deformation, due to the phase trans formation from Mg + 2(beta - beta) --> 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 m agnetic field (M-H) curves of the round wires before and after sintering an d reactive diffusion were measured at 5 K and in magnetic fields up to 5 T to define the J(cmag). The direct current measurements were performed in se lf field at 4.2 K. A comparison between zero-field-cooled (ZFC) and field-c ooled (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 curren t measurements.