Mechanisms and kinetics of MgB2 synthesis from boron fibers

Superconducting MgB2 fibers were synthesized through the reaction of liquid magnesium with 140 μm diameter boron fibers. Fiber reaction occurs by two concurrent mechanisms. First, concentric MgB2, MgB4 and MgB7 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 MgB4 or MgB7 shells, allowing for Mg ingress into the fibers and diffusional growth of MgB2 wedges in the radial and circumferential directions. Combining both shell and wedge MgB2 growth mechanisms into a single model provides predictions of overall MgB2 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 °C.