Microstructure and interfaces in directionally solidified eutectics for Solid Oxide Fuel Cell anodes

Abstract

Directionally solidified eutectics (DSE) such as NiO-YSZ, CoO-YSZ, NiO-Ce02, NiO-GDC, CoO-Ceo2 and CoO-GDC, generally display a self-organized lamellar microstructure characterized by a strong adhesion between their components. After reduction of the transition metal oxide, the resulting are cermets formed by alternate lameilae of an ionic conductor and a porous metal. Those cermets present high electric conductivity and thermal, chemical and mechanical stability at high temperatures, and thus they could be used as anodes in Solid Oxide Fuel Cells (SOFC). The good adhesion through low-energy interfaces that is obtained in DSE cermets induces a great stability of the metallic particles against coarsening during the high operation temperatures and long exposure times of SOFCs. In general, cerrnets produced from DSE precursors withstand metal coarsening longer than traditionally processed cermets. Understanding the microstructure of these materials, before and after reduction, is essential to assess their suitability and the expected performance in a SOFC. Eutectics of NiO-YSZ, CoO-YSZ, NiO-Ce02, NiO-GDC, CoO-Ceo2 and CoO-GDC were produced by the Laser Floating Zone (LFZ) technique with growth rates ranging from 10 to 200 mmlh. The miscrostructure and the orientation reiationships between the components in the eutectics were analyzed by means of Electron Backscatter Diffraction (EBSD) in a Scanning Electron Microscope (SEM). This is an Orientation lmaging Microscopy (OIM) technique that enables us to characterize each phase's growth direction, as well as the orientation relationship and the interphase plane, which ultimately determines the strength of the bonding between phases. We have observed that, as a general rule, the directional eutectic growth imposes the formation of faceted interfaces between the {IOO} plane of the fluorite structure of the ionic conducting phase, GDC and YSZ, and the (111) plane of the rock-salt structure of Ni0 and CoO phases. The formation of these interfaces, and its implication on the metal-ceramic interfaces formed after reduction, is discussed according to the eutectic growth laws and related to the ionic charge balance at the interface.