Controlling flux pinning precipitates during MgB₂ synthesis

MgB₂ samples prepared by three different sets of synthesis parameters were investigated using transmission electron microscopy. Results suggest that oxygen dissolved in bulk MgB₂ at high synthesis temperatures when the MgB₂ samples were exposed to trace amount of oxygen from flowing ultrahigh purity Ar gas. The lower solubility of oxygen in MgB₂ at a lower temperature led to the precipitation of nanometer-sized coherent Mg(B,O) in the interior of MgB₂ grains during subsequent cooling. The precipitates, which act as effective flux pinning centers, are of composition-modulated structures with the same basic crystal lattice and orientation as the MgB₂ matrix. This study has demonstrated the potential of tailoring the size and distribution of Mg(B,O) precipitates through optimizing synthesis parameters for optimum flux pinning. © 2002 American Institute of Physics.