Adam Pivovar* and Dan Neumann

NIST Center for Neutron Research, Materials Science and Engineering Laboratory

National Institute of Standards and Technology, Gaithersburg, MD 20899-8562
 
 

Reid Chesterfield and C Dan Frisbie
 
 

Department of Chemical Engineering and Materials Science

University of Minnesota, Minneapolis, MN 55455-0132
 
 

Bryan Pivovar
 
 

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.

Molecular Semiconductors

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.