Fabrication of Chemically Doped, High Upper Critical Field Magnesium Diboride Superconducting Wires

Abstract

SummaryControlled chemical doping of magnesium diboride (MgB 2 ) has been shown to substantially improve its superconducting properties to the levels required for high field magnets, but the doping is difficult to accomplish through the usual route of solid state reaction and diffusion. Further, superconducting cables of MgB 2 are difficult to fabricate because of the friable nature of the material.In this Phase I STTR project, doped and undoped boron fibers were made by chemical vapor deposition (CVD). Several >100m long batches of doped and undoped fiber were made by CVD codeposition of boron plus dopants. Bundles of these fibers infiltrated with liquid magnesium and subsequently converted to MgB 2 to form Mg-MgB 2 metal matrix composites. In a parallel path, doped boron nano-sized powder was produced by a plasma synthesis technique, reacted with magnesium to produce doped MgB 2 superconducting ceramic bodies. The doped powder was also fabricated into superconducting wires several meters long.The doped boron fibers and powders made in this program were fabricated into fibermetal composites and powder-metal composites by a liquid metal infiltration technique. The kinetics of the reaction between boron fiber and magnesium metal was investigated in fibermetal composites. It was found that the presence of dopants had significantly slowed the reaction between magnesium and boron. The superconducting properties were measured for MgB 2 fibers and MgB 2 powders made by liquid metal infiltration. Properties of MgB 2 products (J c , H c2 ) from Phase I are among the highest reported to date for MgB 2 bulk superconductors.Chemically doped MgB 2 superconducting magnets can perform at least as well as NbTi and NbSn3 in high magnetic fields and still offer an improvement over the latter two in terms of operating temperature. These characteristics make doped MgB 2 an effective material for high magnetic field applications, such as magnetic confined fusion, and medical MRI devices. Developing fusion as an energy source will dramatically reduce energy costs, global warming, and radioactive waste. Cheaper and more efficient medical MRI devices could lower examination costs, find potential health problems earlier, and thus also benefit society as a whole. Other potential commercial applications for this material are devices for the generation and storage of electical power, thus lowering the cost of delivered electricity.