Characterization of material behavior during the manufacturing process of a co-extruded solid oxide fuel cell

Abstract

Recent developments in powder metal oxide processing have enabled co-extrusion of a honeycomb structure with alternating layers of metal and ceramic. Such a structure is envisioned for use as a Solid Oxide Fuel Cell (SOFC) if defects can be minimized during the manufacturing process. The two dissimilar materials tend to shrink at different rates during hydrogen reduction and sintering, inducing internal stresses that lead to structural defects such as cracks, or high residual stresses. The objective of this thesis is to characterize the shrinkage and relaxation mechanisms inherent in both the metal and ceramic so that internal stresses developed during manufacturing can be estimated and ultimately minimized. Constitutive models are adapted from the literature to simulate the sintering and viscoelastic behaviors of the ceramic. Likewise, existing models in the literature are used to characterize the viscoplastic relaxation of the porous powder metal phase and its sintering behavior. Empirical models are developed for the reduction behavior of the metal oxides, based on a series of experiments conducted that measure water vapor (hygrometry) and dimensional change (dilatometry) during reduction and sintering. Similarly, the necessary parameters for the sintering model and viscoplastic model were determined through a series of experiments. The constructed system of constitutive equations appears to have the essential elements for modeling dimensional change, porosity/strength and development of internal (residual) stresses in co-extruded SOFC structures.