Mechanisms and kinetics of MgB2 synthesis from boron fibers

Abstract Superconducting MgB 2 fibers were synthesized through the reaction of liquid magnesium with 140 μm 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 °C.