Structural Properties and Reversible Deuterium Loading of MgD2–TiD2 Nanocomposites

Abstract

Structural properties and reversible deuterium uptake of MgD2–TiD2 nanocomposites have been studied by joint X-ray and neutron diffraction analyses to shed light on the extremely fast hydrogenation kinetics of these materials. (1 – x)MgD2–xTiD2 nanocomposites with compositions ranging between x = 0 and 0.5 have been prepared by reactive ball milling of Mg and Ti powders under deuterium pressure. They consist of mixtures of MgD2 (β-and γ-polymorphs) and ε-TiD2 phases homogenously distributed at the nanoscale with crystallite sizes below 15 nm. Minor phase miscibility is detected with Mg solubility in the TiD2 phase up to 8 at.% and Ti solubility in the β-MgD2 up to 7 at.% Ti. At moderate temperatures and pressures (T < 600 K, PD2 < 1 MPa) reversible deuterium loading in MgD2–TiD2 nanocomposites only occurs through the β-MgD2 to Mg transformation. Mg/MgD2 thermodynamics is not modified as γ-MgD2 and Ti solubility in β-MgD2 are metastable and do not operate during reversible deuterium loading. However, the TiD2 phase allows for outstanding D-sorption kinetics in the Mg/MgD2 system. This paper demonstrates that TiD2 inclusions limit the grain growth of Mg and MgD2 phases allowing for short D-diffusion paths. Furthermore, we provide evidence that the TiD2 phase also favors H-mobility through the existence of coherent coupling between TiD2 and Mg/MgD2 phases and the presence of sub-stoichiometric MgD2-η and TiD2-η phases.