A new class of powder-in-tube Mg-B superconductors was prepared by using 2 different methods: an in situ technique where a 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 Ag, Cu and bimetallic Ag/stainless-steel tubes. Wires manufactured by using the in situ technique (diffusing Mg to B particles) exhibited an approximately 25.5% decrease in density from the initial value after cold deformation. This was due to the phase transformation, Mg+2(β-B) → MgB2; all with hexagonal structures. A comparative study of the intergranular current and grain connectivity in wires was conducted by means of alternating-current susceptibility measurements and direct 4-point transport measurements. Using a SQUID magnetometer, the M-H curves of the round wires before and after sintering and reactive diffusion were measured at 5K and in magnetic fields of up to 5T in order to define JCmag. Direct-current measurements were performed in the self field at 4.2K. A comparison of zero-field cooled and field-cooled susceptibility measurements for sintered Ag/MgB2, and reacted Cu/Mg+2B conductors revealed systematic differences, in the flux-pinning in the wires, which were in very good agreement with direct high transport current measurements.

Superconductivity of Powder-in-Tube MgB2 Wires. B.A.Glowacki, M.Majoros, M.Vickers, J.E.Evetts, Y.Shi, I.McDougall: Superconductor Science and Technology, 2001, 14, 193-9