Correlation of superconducting properties and microstructure of MgB2 wires and tapes

Within the 'Hipermag' project, MgB2 wires and tapes were prepared by the powder in tube method using different precursor powders and processing technologies. Either pre-reacted MgB2 (ex-situ), mixture of MgH2 + 2B (in-situ) or mechanically alloyed (MA) MgB2 is used. In this work superconducting properties of a long length 14-filament tape with standard ex-situ powder, MA tapes, and hydrostatically extruded wires with in-situ MgB2 powder with SiC nanoinclusions, are correlated with their microstructure investigated using advanced electron microscopy techniques. Apart from the conventional diffraction imaging, EDX elemental maps in SEM and TEM and electron spectroscopic imaging (ESI) in TEM have been used to study the MgB2 phase formation and granularity. The critical current densities of the wires and tapes were measured at 4.2 and 20 K for different magnetic fields. Wires and tapes prepared by different technologies show large differences by orders of magnitude in their critical current densities. MgB2 colony formation is found as a universal phenomenon with several grains forming a dense colony. Such colonies are embedded in a matrix of reduced density introducing granularity in MgB2 wires and tapes. Wires and tapes that do not show colony formation contain a large fraction of B-rich phases and their critical current density is limited by the MgB2 phase formation