Development of gas pressure melting (GPM) method for Ag-sheathed Bi-2212 wires

Abstract

A serious problem for Ag-sheathed Bi-2212 wires produced by a partial melt process is an oxygen release which causes effluence of the oxide materials from the Ag sheath and/or formation of voids inside the sheath. To solve it, the oxygen release from the powders should be minimized and any voids should be small and homogeneously distributed. Using formed powders with minimized oxygen content, we investigated melting atmospheric conditions on the properties of the Bi-2212 wires and established a Gas Pressure Melting (GPM) method which is characterized by a higher total gas pressure than the atmospheric pressure. We found that the combined conditions of appropriate oxygen partial pressures of 1.0 to 303.9 kPa and total gas pressure of over 303.9 kPa are very effective for obtaining small voids with a homogeneous dispersion and good superconductivity over a wide melting temperature range. We obtained reproducibly highJ _c round wires withJ _c of 140000 Acm⁻² (OT) and 40000Acm⁻² (23T) at 4.2 K. A solenoid coil which has several advantages in its fabrication compared with a pancake coil was successfully fabricated, using a 9 m round wire, based on the findings. The coil generated a maximum magnetic field of 0.7 T.