Structural evolution of La 0.9 Sr 0.1 Cr 0.9 Co 0.1 O 3 thin films for SOFCs and catalysis
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Structural evolution of La0.9Sr0.1Cr0.9Co0.1O3 thin films for SOFCs and catalysis Anthony Coratolo, Nina Orlovskaya Department of Materials Science and Engineering, Drexel University Philadelphia, PA 19104 Christopher Johnson, Randal Gemmen National Energy Technology Laboratory, Department of Energy Morgantown, WV 26507 ABSTRACT The deposition of La0.9Sr0.1Cr0.9Co0.1O3 thin films by RF magnetron sputtering and annealing leads to the formation of a conducting perovskite film for the protection of metallic interconnects in SOFCs. The nanoporous structure, formed by a two-step phase transition, may also be a promising candidate for catalytic applications. INTRODUCTION Metallic alloys containing Cr have recently become viable options for interconnects in IT-SOFCs [1]. Chromium containing alloys meet most requirements for an interconnect material such as strength, high conductivity, and reasonable price. Even at high temperatures when the metals can oxidize easily, Cr alloys are practical because chromia (Cr2O3), which forms on the alloy surface as a result of oxidation, is still a reasonably good conductor. However, over time oxidation layers grow thicker, reducing conductivity. It is also possible for the Cr3+ ions of Cr2O3 alloy to migrate, contaminating other parts of the stack and in turn, decreasing performance. One way of overcoming this problem is application of a protective coating to the alloy. Recently it has been shown that La-Cr-O based perovskites may be an effective coating for metallic interconnects [2, 3]. It is important for this coating to be as dense as possible to provide maximum protection, to be electrically conductive and to have a thermal expansion coefficient similar to that of the alloy. The goal of this research is to show that a LaCrO3 based coating will provide protection for Cr containing alloy substrates at temperatures of 700-800°C. Initial testing was performed with stainless steel 446, which was coated with a thin layer, approximately 800nm thick, of pure LaCrO3 and with La0.9Ca0.1Cr03 [4]. It was discovered that during heating the film’s transformation to the LaCrO3 perovskite structure occurred in two steps. There is an initial transition from an X-ray amorphous as deposited phase to a monoclinic monazite structure at around 500°C and then a second transition to the orthorhombic perovskite phase at around 800°C. During this transition there is both a decrease in the molecular volume and an increase in the theoretical density from 82.3 Å3 molecule and 5.15 g/cm3 to 58.58 Å3 molecule and 6.77 g/cm3 respectively, which lead to the formation of a nanoporous film [2]. In our previous work, the SS446 alloy was chosen as a substrate material, but it was found to be unsuitable because of high Si and Al impurity content. After 100 hours at 800°C, in both air and forming gas, only the substrate coated with La0.9Ca0.1Cr03 and annealed in air was below an acceptable resistance limit of 0.1 Ω•cm2 [4]. Testing showed that a thin SiO2 based non-conducting oxide layer was forming betwe
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