Durability Methodologies for Material Systems

In the present work, we are concerned with heterogeneous material systems that have multiple distinct materials and void phases, and associated interfaces, for which the geometric scale and specific morphology at the micro/nano-level play an essential role in the global properties, functional behavior, and material system performance. Such heterogeneous combinations of materials that are engineered to function together to create unique properties and performance that is a function of their interaction are called material systems. Engineered heterogeneous material systems are at the heart of revolutionary advances in devices that convert and store energy (e.g., batteries, fuel cells, solar cells, capacitors, and many electro-optical devices), and other membrane-based devices used in chemical and fuel processing, sequestration, and extraction. In recent years, advances in a variety of additive manufacturing methods and techniques have made it possible to design, control, and fabricate specific micro- or nano-structures to achieve prescriptive functional performance of the material systems and devices in which they appear. However, systematic multiphysics analysis methods properly set on field equations that represent the local details are not available, so that first-principles understandings and designs of those materials are not properly founded. Recently, the DoE established an Energy Frontiers Research Center for Physics Based Nanostructure Design and Fabrication of Heterogeneous Functional Materials, called the HeteroFoaM Center, to address this and related questions. The present paper presents some initial findings of part of that effort related specifically to durability. For the present study, our approach to the question of durability of engineered material systems will be to construct a damage model, wherein damage is defined as (and by) changes in material state as a function of some generalized time variable that defines the intensity and history of the applied conditions that drive those changes. To bring the reality of engineering practice to our discussion, the durability concepts will be related to the damage and functional degradation observed in solid oxide fuel cells (SOFCs) during service. SOFCs convert the chemical energy of fuel to electrical power. This focus will serve to define the scope of our discussion, which will be the durability of complex, heterogeneous material systems as measured by degradation of their functional performance defined by mechanical, thermal, and electrical behavior.