RELATIONSHIP OF MICROMOPHOLOGY TO CHARGE STORAGE AND TRANSFER PROPERTIES IN HETEROGENEOUS FUNCTIONAL MATERIALS

Abstract

The present paper is concerned with heterogeneous materials in which the mor- phology is specifically designed to achieve functional properties. These materials, or mate- rial systems, are found in energy systems such as fuel cells and batteries, but also in aeronau- tical structures such as the conductive skins of commercial airplanes which have polymer composite primary structures. Many of these heterogeneous materials are dielectric and, under certain conditions, conductive. Charge transport and storage in such materials is typi- cally discussed in terms of equivalent (electrical) circuits, without recourse to first principles physics or mechanics. The present paper discusses the transport and storage properties of several examples of such materials from a computational standpoint, and compares some critical predictions with experimental data. Opportunities and needs for improving our un- derstandings and computational capabilities for this class of problems will also be discussed. The foundation for our present discussion is heterogeneous functional materials, i.e., heterogeneous material systems which actively interact with and often alter or transform the fields applied to them. Examples are found in many common man-made devices such as the electrodes and membranes that make fuel cells, electolyzers, batteries, and separation mem- branes work, and in a host of natural materials and organisms such as the lungs of animals, living bone and wood, and the cornea of the eye of mammals and other animals. We will re- fer to such material systems as HeteroFoaMs.(1) The applied fields may be mechanical, thermal, electrical, or electrochemical, applied singularly or in combination. In the present paper we address the computational concepts commonly used to address the calculation of the properties of such functional materials, and more particularly, to design them for specific functional performance. For expedience, we will limit the scope of our discussion to what we will call the generalized compliance of the material system with the applied fields. For me- chanical fields, that compliance may be the classical material stiffness (associated with dis- placement of material points); for electrical fields the compliance may be discussed as permit- tivity (associated with the dielectric or conductive displacement of charge), etc.