Enhanced Hydrogenation Kinetics of Magnesium Diboride through Carbon Substitution into the Boride Layers

Abstract

S/TEM, synchrotron X-ray and neutron diffraction analyses, and high pressure hydrogenation studies have established that carbon substitution into the bulk lattice MgB2 lattice results in enhanced hydrogenation kinetics. S/TEM analyses of mechanical milled MgB2/graphene show no reduction in grain size, indicating that the enhanced hydrogenation kinetics previously observed for this material cannot be attributed to increased surface area. In agreement with previous findings, comparison of the lattice parameters obtained by Rietveld refinements for directly synthesized Mg1+x/2B2-xCx and undoped MgB2 show that the a lattice parameter (a = 3.05915(8) Å) is about 0.8% (Δa = 0.022 Å) smaller than that of undoped material (a = 3.08158(5) but the c lattice parameter is invariant. Reliable refinement of the synchrotron X-ray data obtained for mechanical milled MgB2/graphene was not possible due to pronounced peak broadening. However, refinements of powder neutron diffraction show similar reduction of the a parameter in both the materials prepared by ball-milling and direct synthesis (~3.0476(7) Å) compared to the undoped sample (a = 3.0758(2)) while the c lattice parameters remain constant. The finding of significant increase in the c/a ratio for these material indicates that the mechanical milling of MgB2 with graphene represents an alternative method to preparing Mg1+x/2B2-xCx. This conclusion is supported by the finding that both materials undergo major conversion to Mg(BH4)2 upon hydrogenation under the identical conditions under which the mechanically milled material was found to undergo hydrogenation, establishing that the correlation between carbon substitution into the bulk lattice MgB2 lattice and enhanced hydrogenation kinetics.