MgB2 Wires and Bulks With High Superconducting Performance

The critical current density at 4.2 and 20 K up to 15 T and the temperature dependence of the irreversible field have been studied in magnesium diboride wires and bulks prepared from boron without and with carbon addition. Structural investigations using X-ray microprobe analysis have been performed. A monofilamentary wire with a round-shaped core manufactured from boron containing 2 mol.% carbon (approximate composition of its matrix MgB_1.8-2.4O_0.04-0.71C_0.12-0.16) demonstrated J_c(20 K, 2 T) = 10⁵ A/cm², J_c(4.2 K, 15 T) = 2 × 10³ A/cm² and B_irr (20 K) = 8 T, B_irr(12.5 K) = 15 T. Bulk material synthesized at 2 GPa, 600 °C, 1 h from carbon doped boron demonstrated J_c(10 K, 10 T) = 4 × 10³ A/cm ² and J_c(20 K, 6 T) = 4 × 10³ A/cm ².

I. Introduction

Magnesium diboride wires and bulks are promising for many different applications, for example, fault current limiters, magnetic resonance imaging, superconducting magnetic energy storage devices, transformers, electrical motors and generators, cryogenic pumps, adiabatic demagnetization refrigerators, magnetic separators, magnetic levitation transport and bearings, magnets for high-energy physics [1]–[9]. Lightweight superconducting wires can be used for aviation and space applications and for powerful offshore wind generators [10], [11], etc. Magnesium diboride wires and bulks compete to some extend with YBCO coated conductors and melt-textured ceramics [12]–[16]. The working temperature of magnesium diboride can be chosen near liquid hydrogen or neon (20–30 K). It is easily produced and comparatively cheap, but prone to quenching during pulsed magnetization. This problem has to be solved in order not to restrict the material´s wide spread application. The working temperature of YBCO-based materials can be higher (e.g., around the temperature of liquid nitrogen, 77 K, or somewhat lower), but their production is much more expensive and complicated and the problems with ac losses [17] and appropriate optimal twisting are not solved. A big risk of quenching and damaging powerful magnets is existing for coated conductors as well.