Phase analysis of superconducting polycrystalline MgB(2)

Superconducting MgB(2) ceramics were prepared and yield superconducting transition temperatures of about 39 K. For covering the various length scales on which inhomogeneities appear in MgB(2), electron-probe micro-analysis (EPMA) and analytical transmission electron microscopy (TEM) were applied for a phase analysis. Particularly useful were the preliminary electron spectroscopic imaging (ESI) results in the TEM. It could be shown by EPMA that the microstructure consists of a Mg-B-O matrix and boron-rich secondary phases of composition close to MgB(12). It was unclear in which form oxygen was present in the superconducting matrix. By combining the acquisition of B-K and O-K edge jump ratio images and energy-dispersive X-ray spectroscopy in the TEM, we could prove that the matrix consists of superconducting MgB(2) and MgO. Most of the MgO precipitates and grains appear with diameters between 20 and 300 nm. The size distribution of MgO was inhomogeneous and oxygen-rich areas of dimensions >1 microm were also observed. Edge jump ratio images obtained by ESI were analysed for determining the signal values and effects of multiple inelastic scattering.