Development of suspension plasma sprayed lanthanum perovskite ceramic layers for clean energy technologies.

摘要

This thesis is devoted to developing the suspension plasma spraying process for fabricating ceramic layers applicable to green energy technologies, i.e., solid oxide fuel cell cathodes and oxygen separation membranes. The materials of interest for this study include the mixed ionic and electronic conducting perovskite lanthanum strontium cobalt ferrite (LSCF), its composite with yttria-stabilized zirconia (YSZ), and the excellent electronically, but less ionically, conductive perovskite lanthanum nickel ferrite (LNF). The objective of this thesis is to investigate the processing-property relationships between the plasma spray parameters and resulting properties of coatings fabricated with them, and to use the resulting understanding to fabricate dense coatings for oxygen separation membranes and porous coatings for fuel cell cathodes on stainless steel substrates.;It was observed that fairly dense LSCF coatings can be fabricated by suspension plasma spraying, but the gas-tightness was dictated by the coating thickness required to bridge the surface pores on the supporting metal substrate. Higher porosity of the cathode layers was obtained by adding carbon black pore former into the feedstock suspension, and single-phase LSCF cathodes made in this way exhibited excellent symmetrical fuel cell performance with a polarization resistance of 0.062 Ocm2 at 744°C. Fabricating an LSCF/YSZ composite by plasma spray processing was more difficult, because the plasma power must be sufficient to melt the YSZ phase without decomposing the LSCF phase. The composite cathodes fabricated had lower performances than those of the single-phase cathodes. The suspension plasma sprayed LNF cathodes had higher polarization resistances and lower durability than those of suspension plasma sprayed LSCF cathodes due to severe material decomposition. Therefore, single-phase LSCF cathodes were chosen for short-term and medium-term durability testing in full fuel cells. It was demonstrated that suspension plasma sprayed single-phase LSCF cathodes with the fine microstructures, and thus, large surface area for reactions, performed comparably to more-expensive dry powder plasma sprayed LSCF/SDC composite cathodes in a complete fuel cell, even though the latter cathodes have a higher overall ionic conductivity.