Mechanisms and kinetics of MgB2 synthesis from boron fibers

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 degrees C. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.