Development of Ag Enhanced Textured Powder Bi-2212/Ag Superconducting Wire Technology

Abstract

The current development technique for Bi-2212 multifilament superconducting wire is the oxide powder in tube process. For this process, phase pure Bi-2212 fine powder is loaded into a silver tube then drawn, stacked, and redrawn until the desired filament size and number is achieved. The wire is then cabled and wound onto its final form where it undergoes a partial melt heat treatment in an overpressure environment of 5 -10 MPa. The entire form is cooled in a controlled way to allow the liquid to recrystallize into phase pure Bi-2212. However, the partial melt heat treatment process leads to several difficulties. Small void spaces within the initial powder can coalesce into large bubbles after the heat treatment. These large bubbles can occlude any supercurrent from traveling along the filament. The melt liquid is also chemically aggressive and will etch and therefore weaken the silver matrix. Lastly, the recrystallized Bi-2212 is not textured and there is limited connectivity between individual crystal formations. Each of these difficulties need to be addressed simultaneously to optimize the performance of future Bi-2212 superconducting wires. A new method for preparing Bi-2212 multifilament wires is being developed to minimize the effects of the above mentioned difficulties. Fine Bi-2212 powder is homogenously mixed with nano-scale silver powder and uniaxially compressed into square cross section bars. The bars are loaded into a silver tube then drawn, stacked, and redrawn until the desired filament size and number is achieved. To avoid the complications of the partial melt heat treatment a non-melt heat treatment was developed. The non-melt heat treatment takes advantage of the material properties of the Bi-2212/silver interface. The bulk of the Bi-2212 is never melted, but interconnectivity and grain growth is still achieved. The cores used to fabricate the multifilament wires display significant superconductivity in liquid nitrogen, but the multifilament wires show no signs of superconductivity. Separate optimization of the wires in liquid helium is required to maximum the supercurrent carrying potential of the multifilament wires. Additional study on oxygen doping during the non-melt heat treatment may also be necessary.