Advanced electron microscopy methods for the analysis of MgB2 superconductor

Advanced electron microscopy methods used for the analysis of superconducting MgB(2) wires and tapes are described. The 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. Microstructure analysis on mu m to nm length scales is necessary to understand the superconducting properties of MgB(2). For the MgB(2) phase analysis on mu m scale an analytical SEM, and for the analysis on nm scale a energy- filtered STEM is used. Both the microscopes were equipped with EDX detector and field emission gun. Electron microscopy and spectroscopy of MgB(2) is challenging because of the boron analysis, carbon and oxygen contamination, and the presence of large number of secondary phases. Advanced electron microscopy involves, combined SEM, EPMA and TEM analysis with artefact free sample preparation, elemental mapping and chemical quantification of point spectra. Details of the acquisition conditions and achieved accuracy are presented. Exsitu wires show oxygen- free MgB(2) colonies ( a colony is a dense arrangement of several MgB(2) grains) embedded in a porous and oxygen- rich matrix, introducing structural granularity. In comparison, in- situ wires are generally more dense, but show inhibited MgB(2) phase formation with significantly higher fraction of B- rich secondary phases. SiC additives in the in- situ wires forms Mg2Si secondary phases. The advanced electron microscopy has been used to extract the microstructure parameters like colony size, B- rich secondary phase fraction, O mole fraction and MgB(2) grain size, and establish a microstructure- critical current density model [ 1]. In summary, conventional secondary electron imaging in SEM and diffraction contrast imaging in the TEM are by far not sufficient and advanced electron microscopy methods are essential for the analysis of superconducting MgB(2) wires and tapes.