New phase of hydrogen-storage material could be missing link in
understanding dehydrogenation process
Sodium alanate (NaAlH4) has attracted a great deal of attention as a
template material for engineering practical hydrogen-storage devices
based on a class of materials known as complex light-metal hydrides.
Using molecular dynamics simulations derived from first principles
(i.e., a fully quantum-mechanical description), we have discovered and
characterized a hitherto unknown phase of this important material, which
we present as a vital step towards understanding the complex process of
hydrogen release in NaAlH4. Our calculations indicate that this new
phase, which is formed at surfaces and grain boundaries, should become
the favored structure once temperatures relevant for dehydrogenation are
approached. Structurally similar to the product of the complete hydrogen
release process in sodium alanate, the new phase could be a key missing
link in a comprehensive picture of the phase dynamics of hydrogen uptake
and release. Our findings offer a new interpretation of recent
experiments on NaAlH4 and provide an additional avenue for future
research and optimization efforts.
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LF12333
Energy Landscape of Social Balance
The shifting of alliances and rivalries in a social group
can be viewed as arising from an energy minimization
process. For example, suppose you have two friends who
happen to detest each other. The resulting awkwardness
often resolves itself in one of two ways: either you drop one
of your friends, or they find a way to reconcile. In such
scenarios, the overall social stress corresponds to a kind of
energy that relaxes over time as relationships switch from
hostility to friendship or vice versa.
We model a close-knit community of friends and enemies as a fully connected network with positive
and negative signs on its edges. Theories from social psychology suggest that certain sign patterns are
more stable than others. This notion of social ‘‘balance’’ allows us to define an energy landscape for such
networks. Its structure is complex: numerical experiments reveal a landscape dimpled with local minima
of widely varying energy levels. We derive rigorous bounds on the energies of these local minima and
prove that they have a modular structure that can be used to classify them.
