Microstructure – critical current density model for MgB2 wires and tapes

Abstract

MgB2 wires and tapes were prepared by the powder in tube method using different processing technologies and thoroughly characterised for their superconducting properties within the HIPERMAG project. Either pre-reacted MgB2 (ex-situ) or a mixture of Mg + 2B (in-situ) were used as the precursor powders. In some wires the precursor powders were mixed with SiC. The critical current density (Jc) of these wires was found to differ by orders of magnitude, the highest Jc's being 104Acm-2 at 10.5 T and 4.2 K. A detailed understanding of the thermodynamics in Mg-B-O and Mg-B-Si-C-O system is necessary to control the phase and microstructure formation in these systems. Therefore, thermodynamical parameters like annealing temperature and annealing time used for the synthesis of the wires were systematically varied. The microstructure of these wires was investigated using advanced electron microscopy methods [1]: combined SEM, EPMA and TEM analysis with artefact-free sample preparation, elemental mapping and chemical quantification. Ex-situ wires show oxygen-free MgB2 colonies (a colony is a dense arrangement of several MgB2 grains) embedded in a porous matrix introducing structural granularity. The MgB2 grains in the porous matrix are surrounded by MgSixOy layers yielding poor connectivity. In-situ wires are generally more dense, but show inhibited MgB2 phase formation with significantly higher fraction of B-rich secondary phases in comparison to the ex-situ wires. SiC in the in-situ wires results in the formation of Mg2Si secondary phases. The size of the MgB2 grains varies between 20-1000 nm. A microstructure- critical current density model is proposed to explain the large, order of magnitude, differences in the Jc's of MgB2 wires and tapes. The model contains the following microstructure parameters: 1) colony size, 2) volume fraction of B-rich secondary phases, 3) oxygen mole fraction and 4) MgB2 grain size.