Investigation of mechanical activation on Li-N-H systems using Li-6 magic angle spinning nuclear magnetic resonance at ultra-high field

Abstract

The significantly enhanced spectral resolution in the Li-6 MAS NMR spectra of Li-N-H systems at ultra-high field of 21.1 T (corresponding to a proton Larmor frequency of 900 MHz) is exploited, for the first time, to study the detailed electronic and chemical environmental changes associated with mechanical activation (MA) of the Li-N-H system using high-energy balling milling. Complementary to ultra-high field studies, the hydrogen discharge dynamics are investigated using variable temperature in situ H-1 MAS NMR at 7.05T field. It is shown that the changes in the Li-6 MAS spectra of LiH and LiNH2 induced by MA can be separated from those of the LiOH and LiOH center dot H2O impurities in the samples, and a new Li-6 peak induced by MA, which has never been reported before, is identified with the aid of the ultra-high field. The formation of this new peak and its associated upfield shift are attributed to the increased lattice defects induced by ball milling, which in turn enhances hydrogen release of the LiH + LiNH2 mixture observed in the in situ study of the hydrogen discharge dynamics. The study also clearly indicates that ball milling at liquid nitrogen temperature produces more mechanical activation than ball milling at room temperature. (C) 2008 Elsevier B.V. All rights reserved