Sintering of Co2MnO4 spinel for protective coatings in SOFC

Protective coatings are often deposited on SOFC interconnectors to avoid poisoning of cathode from chromium species that evaporate from stainless steel interconnects or supports. Co2MnO4 spinel compounds are usually considered as the main constituent of protection barriers. Nevertheless, such ceramic sinters at high temperatures (>1200°C) and this can be problematic for the properties of the stainless steel components. One of the major issues is, in fact, the creation of a compact and impermeable coating at relatively low temperature in order to preserve the metal substrate. In the present research work, Co2MnO4 spinel was synthesized by various methods (solid-state, gel-combustion, co-precipitation and reverse micelle) and the obtained specific surface area, structure and particle size were correlated with thermal behaviour, sintering temperature and achieved density. It was found that regardless the synthesis process the only obtained phase is Co2MnO4. Specific surface area from 0,8 to 65 m2g-1 was obtained, depending on the synthesis method. Sintering aids such as Nb2O5 and LiF were used to obtain dense microstructure at relatively low temperature. Considerable changes in sintering temperature were observed this being even 100-200ºC lower than that necessary for the consolidation of pure spinel though microstructure with only close pores was achieved. A novel sintering method based on Field Assisted Techniques (FAST) that promoted flash-sintering phenomenon was finally applied to Co2MnO4. Small electric field (<7,5 V cm-1) applied to the spinel decreases the sintering temperature down to 600°C. The application of an electric field above 7,5 V cm-1 flash-sintering phenomenon takes place and sintering temperature drops to about 300°C, the sintering time being less than 1 second.