Adam Pivovar* and Dan Neumann
NIST Center for Neutron Research, Materials Science and Engineering
National Institute of Standards and Technology, Gaithersburg, MD
Reid Chesterfield and C Dan Frisbie
Department of Chemical Engineering and Materials Science
University of Minnesota, Minneapolis, MN 55455-0132
Materials Science and Technology Division
Los Alamos National Laboratory, Los Alamos, N.M. 87545
The effective design and synthesis of materials with
optimized functionality requires a complete fundamental understanding of
the nanoscale physical and chemical material properties that control the
desired function. For some materials, traditional characterization techniques
are unable to provide sufficient information to allow one to form reliable
property-function relationships. Neutron scattering measurements, however,
can be an incredibly insightful tool for exploring both structural and
dynamic aspects of materials at the molecular/atomic scale. The information
obtained in neutron scattering measurements has the potential to provide
a crucial enhancement of our understanding of property-function correlations,
thereby (in some cases) allowing for a more directed approach to materials
design and synthesis.
Molecular semiconductors and fuel cell membranes are
two types of materials that have recently generated much interest as a
result of their future impacts on the computer and energy industries, respectively.
We have utilized neutron scattering as a method for characterizing specific
properties of these materials and have compared this with their function,
the specific details of this research is given below. We hope the results
of this research will allow us to elucidate which materials properties
should be considered paramount in the design and synthesis of these materials.
Aromatic molecules, one type of organic semiconductor,
tend to pack into solid state architectures having a herring-bone type,
layered arrangement. The herring-bone or ãedge on faceä arrangement
provides a significant overlap of the molecules p
electron cloud that, when doped as in a field effect transistor, permit
the conduction of charge carriers. It has been observed that as these materials
are put under pressure that, as a result of the increased overlap of the
electron cloud, the resistance to charge mobility is diminished. In order
to determine the structural and dynamic changes of these materials as a
function of pressure we have employed a high pressure apparatus and measured
the neutron powder diffraction and inelastic scattering respectively. We
have observed that compression of tetracene (4 fused benzene rings) as
a function of pressure occurs predominantly within the two-dimensional
herring-bone layers, with minimal constriction in the third dimension.
The vibrational character of these materials also has an asymmetric dependence
resulting from the structural strain in the layers.
Fuel Cell Membranes
Realization of the energy potential envisioned
by fuel cells requires the development of a diverse array of materials
that possess particular properties that positively contribute to the overall
performance of the system. In direct methanol fuel cells (DMFCs), protonic
conductivity by the proton exchange membrane is a primary factor affecting
overall fuel cell performance. We have employed quasielastic neutron scattering
to characterize the dynamics of hydrogen (in ionic form and water) within
swelled DMFC membranes. Particular emphasis was placed on the benchmark
standard material, Nafion, which has been thoroughly characterized as a
function of water loading from a dry membrane through saturation.
We use these results as a baseline for comparison with results from new,
easily modifiable membranes made from polysulfonated sulfones and relate
the hydrogen dynamics with actual performance in a fuel cell test station.
Overall we hope to understand how the material properties affect water
dynamics in these systems and how the desired performance can be incorporated
into the material properties.