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.