2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009;
Pittsburgh, Pennsylvania
Session Y24: Focus Session: Hydrogen Storage: Atomic and Molecular Motions
8:00 AM–10:48 AM,
Friday, March 20, 2009
Room: 326
Sponsoring
Unit:
DMP
Chair: Eric Majzoub, University of Missouri--St. Louis
Abstract ID: BAPS.2009.MAR.Y24.1
Abstract: Y24.00001 : Atomic Motions in Ionic Hydrides: MgH$_{2}$, NaMg$_{3}$H$_{3}$, and LiBH$_{4}$
8:00 AM–8:36 AM
Preview Abstract
Abstract
Author:
Mark Conradi
(Washington University)
In hydrogen storage, rapid hydrogen diffusion is a key component
for rapid
reaction rates of dehydriding and rehydriding. In metallic
systems, the
light interstitial H atoms typically do display rapid diffusion.
However,
recent interest has focused on ionic and complex hydrides of light
metal-atoms because of their high weight fractions of reversible
hydrogen.
These ionic complex hydrides generally reveal slow hydrogen
diffusion and
resultingly slow reaction kinetics. We report here studies of H
diffusion
using NMR in several such hydrides.
In MgH$_{2}$, the rate $\omega _{H}$ of H hopping remains too
slow to
narrow the H NMR up to 400 $^{\circ}$C. T$_{1D}$ measurements,
however, can
detect the motion and find an activation energy of 1.72 eV, the
first
reported direct measurement of diffusion in MgH$_{2}$. In
ball-milled (bm)
material with Nb$_{2}$O$_{5}$ catalyst additive, a fraction of
the resonance
intensity is narrowed starting at 50 $^{\circ}$C, with the narrow
fraction
growing to 30{\%} by 400 $^{\circ}$C. A model for continuous
growth of the
narrow line, based on a wide distribution of motion rates, is
presented.
Ball-milling also greatly increases the laboratory-frame
relaxation rates,
T$_{1}^{-1}$, from paramagnetic defects created by the mechanical
process.
In bm NaMgH$_{3}$, an even larger fraction of the resonance is
motionally-narrowed, growing to nearly 100{\%} by 300
$^{\circ}$C. Clearly,
ball-milling has a much more profound effect on ionic hydrides
than the
simple reduction of grain sizes and diffusion distances.
In coarse-grain LiBH$_{4}$ (with 13.8 weight{\%} reversible
hydrogen), an
orientationally disordered solid phase occurs above 110
$^{\circ}$C. Above
the transition, the rate of Li ion diffusion increases remarkably. H
diffusion starts to narrow the H NMR line around 170 $^{\circ}$C,
continuing
to narrow up to the melt near 280 $^{\circ}$C. To distinguish
diffusion of
(already rapidly rotating) BH$_{4}$ units from H exchange between
neighboring BH$_{4}$, the $^{11}$B resonance was studied. The
boron line
central transition becomes much narrower (400 Hz) than the width
(1500 Hz)
expected from Van Vleck M$_{2}$ for the case of static boron
spins (with
rapid Li and H diffusion). Thus, intact BH$_{4}$ units are the
diffusing
species. Even in molten LiBH$_{4}$, the BH$_{4}$ lifetime is
found to be at
least 2 seconds from observations of the B-H J-coupling pattern,
so it is
probably much larger in the solid.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.MAR.Y24.1