Kinetics of MgB2 formation studied by in-situ synchrotron x-ray powder diffraction

We have studied the kinetics of the formation of MgB2 from a stoichiometric mixture of Mg and 2B in Fe tubes using in-situ synchrotron powder diffraction at photon energies between E = 76-100 keV. Reaction with the sheath was studied in Fe tubes holding only Mg or B. We here demonstrate the use of a Pb absorber ring or aperture to damp the signals from the sheath material and thereby increase the quality of the diffraction pattern from the superconductor considerably. The new method has revealed a slow reaction between Mg and B when the temperature is below the melting point of Mg and a dramatic increase in the reaction kinetics as the Mg is melted. The MgB2 reaction is complete on a timescale of the order 15-20 minutes after the Mg is melted.