Desenvolvimento da metodologia de fabricação de um fio supercondutor de MgB₂

Abstract

The discovery of superconductivity in magnesium diboride (MgB 2) in 2001 with critical temperatute (T c) of 39 K and upper critical magnetic field (H c2) (H ⊥ c2 (0)) ∼ 40 (T), caused a frenetic search for lightweight materials with similar crystalline structures and constituded by simple elements, and with potential to replace the already traditional Nb-Ti and Nb 3 Sn in the manufacturing of superconducting wires and tapes. However, despite the MgB 2 has high T c , the H c2 values decay rapidly when an external magnetic field is applied, mainly due to the poor pinning of the magnetic flux lines in the material. Furthermore, the plastic deformation process of the set containing the granular powder, constituded of metals used as diffusion barrier thermal and electric stabilizer, is a challenge, because the hardening of the metals by cold working and the accommodation of the superconducting powder. The objective of this work is to develop a process for the manufacture of multifilamentary MgB 2 wires, doped with carbon in the form of graphite and addition of diborides with crystalline structure similar to MgB 2 , as well as the optimization of the mechanical deformation processes, of the intermediate heat treatments and the characterization of the crystallographic, microstructural and superconducting properties of the wire. The methodology used for the production of the wire was the Powder-In-Tube (PIT) ex-situ with: milling of the powder in a high energy ball mill, chemical doping with graphite, addition of vanadium diboride (VB 2), magnesium in excess and use of stearic acid (C 18 H 36 O 2) as a process controlling agent. The mechanical deformation was done by means of rotary swaging. The final results suggest that the route used for the fabrication process of the graphite-doped MgB 2 multifilamentary wire with introduction of VB 2 , should be changed to use MgB 2 produced in laboratory from pure magnesium and boron, with a certificate of purity, using the in-situ methodology in a glove-box with controlled and monitored atmosphere with respect to the oxygen and humidity contents. The process described in this work improves methodologies shown in literature and guarantees the integrity of the wire during the entire fabrication process. In addition, it is suggested the use of an external reinforcing material, such as: iron, Glidcop (Cu-Al 15) or stainless steel (SUS 316L), so that the wire has a higher mechanical tensile strength and a lower cost.