Qualitative comparative analysis of MgB2 powder-in-tube wires: superconductivity and X-ray cone-bean microtomography

摘要

Commercial wires of MgB2 with different architectures and two different heat treatments were characterized with respect to their superconducting properties (T-c, J(c), H-irr, pinning force, macro flux jumps behaviour) through magnetic measurements and were visualized by 3D x-ray micro-tomography (XRT). For a particular architecture, heat treatment conditions of 625 degrees C/3 hrs or 700 degrees C/30 min produced relatively small differences, whereas the architecture of the wires showed a strong influence on superconducting characteristics. XRT checks the integrity of the wires easily detecting in a non-invasive way 3D macro defects and shows their hidden extended shape. XRT also allows a comparative geometry analysis between similar elements (e.g. filaments or filaments-matrix interfaces) from a particular wire or from wires with different architectures. Namely, XRT shows that the geometrical perfection (defined as the degree of departure of the geometry from the designed one) of the inner MgB2 filaments from the wires with 18 elements was lower than for the outermost ones from the same wires and was also lower than for the filaments from the wires with 7 elements. It is proposed that these results of geometrical perfection correlate with better overall superconducting quality of the wires with 7 filaments (except for the stronger presence of macro flux jumps in the wires with 7 filaments).