Timothy D. Myles

Extension of anisotropic effective medium theory to account for an arbitrary number of inclusion types

The purpose of this work is to extend, to multi-components, a previously reported theory for calculating the effective conductivity of a two component mixture. The previously reported theory involved preferentially oriented spheroidal inclusions contained in a continuous matrix, with inclusions oriented relative to a principle axis. This approach was based on Bruggemans unsymmetrical theory, and is extended to account for an arbitrary number of different inclusion types. The development begins from two well-known starting points; the Maxwell approach and the Maxwell-Garnett approach for dilute mixtures. It is shown that despite these two different starting points, the final Bruggeman type equation is the same. As a means of validating the developed expression, comparisons are made to several existing effective medium theories. It is shown that these existing theories coincide with the developed equations for the appropriate parameter set. Finally, a few example mixtures are considered to demonstrate the ...

Effect of orientation anisotropy on calculating effective electrical conductivities

This paper develops an analytical effective medium theory (EMT) equation for calculating the effective conductivity of a mixture based on Maxwells and Maxwell-Garnett’s theories, extended to higher volume fractions using Bruggemans unsymmetrical treatment (BUT), with a long term goal of extending the treatment to mixtures more representative of real materials in order to calculate their effective electrical conductivity. The development accounts for spheroid shaped inclusions of varying degrees of anisotropic orientation. The orientation is described by the introduction of a distribution function. Two methodologies valid for the inclusion dilute limit were used to evaluate the effective conductivity: one based on Maxwells far field approach, and the other based on the Maxwell-Garnett in the matrix approach. It was found that while the dilute limit equations for the effective conductivity were different, the final EMT equations derived by applying BUT collapsed to the same formula which was generalized ...

Backing Out Diffusion Coefficients in Alkaline Anion Exchange Membranes

Water diffusion coefficients for electrolyte membranes are a subject of interest due to the relationship between water content and ionic conductivity. These diffusion coefficients are functionally dependent on water content. Many existing techniques for measuring the diffusion coefficient suffer from experimental complexity or a lack the diffusion coefficients detailed behavior. This paper uses a simple technique for determining the water diffusion coefficient involving the use of experimental data coupled with a numeric model. This back out algorithm was validated through the use of Nafion®, a proton exchange membrane, with the results comparing well with NMR data from the literature. The process has also been applied to SnowPure ExcellionTM, an alkaline exchange membrane, to obtain the diffusion coefficient as a function of water content. It was found that these membranes are much less permeable to water with the diffusion coefficient being almost an order of magnitude less than that of Nafion®.

Characterization of Solid Oxide Fuel Cell Materials Based on Microstructural Skeletonization

The understanding of the relationship between the microstructure of materials for energy applications and their transport and electrochemical properties is needed to optimize their long-term performance. The improvements of 3D imaging techniques such as x-ray nanotomography allow access to geometrical and elemental information with ever increasing accuracy and details. These advances warrant determining new relevant metrics for material characterization, the calculation of which will require adaptations of the methodologies for parameter extraction.This study presents the development of a tool for the characterization of porous, heterogenous materials that provides coherent geometrical and topological information. We illustrate the relevance of the methodology by discussing the differences between geometrical concepts for estimating phase size distributions of real heterogeneous materials investigated using x-ray nanotomography and how research between different scales and physics can be bridged. This is achieved by providing, on the one hand, inputs to classical continuum models and, on the other hand, by synergetic combination with discrete element methods.Copyright

Examination of Water Diffusion Process Within a Low Temperature Polymer Fuel Cell Membrane

Water transport in fuel cells is of interest since the hydration state of the electrolyte is strong related to its conductivity. This study focuses on one part of water transport in fuel cell membranes, namely diffusion. In order to study diffusion processes in a fuel cell membrane a computer model has been developed. It is validated using information reported for the electrolyte membrane material Nafion. When the model is compared to experimental data from the literature a maximum error of 24.7% is observed. Two effects in addition to molecular diffusion have been studied; interfacial absorption and desorption of water at the membrane surface, and convective mass transfer. The effect of convective mass transfer is shown to be negligible while the effects of absorption and desorption are significant. By completing this validation it allows for the additional studies in the future of diffusion in other types of proton exchange membranes and the improvement of fuel cell performance.Copyright