Mechanisms and kinetics of MgB_2 synthesis from boron fibers

摘要

Superconducting MgB_2 fibers were synthesized through the reaction of liquid magnesium with 140 mu m diameter boron fibers. Fiber reaction occurs by two concurrent mechanisms. First, concentric MgB_2, MgB_4 and MgB_7 shells grow radially into the B fibers. Diffusion modeling provides rate constants and effective diffusion coefficients for the borides and their activation energies. Second, radial cracks form in the MgB_4 or MgB_7 shells, allowing for Mg ingress into the fibers and diffusional growth of MgB_2 wedges in the radial and circumferential directions. Combining both shell and wedge MgB_2 growth mechanisms into a single model provides predictions of overall MgB_2 reaction kinetics as a function of time and temperature, which are in good agreement with in situ X-ray diffraction measurements performed at 885-1025 deg C.