Effect of anode atmosphere on the mechanical properties of doped lanthanum chromite

Abstract

In the planar design of the solid oxide fuel cell (SOFC), the interconnect, or bipolar plate, is an important component which allows single cells to be stacked in multi-cellular units [1]. The interconncect must possess high chemical stability in both reducing and oxidizing environments, high electronic conductivity, good stability, a thermal expansion coefficient similar to the adjacent components, and significant strength to support the other cell components, particularly in the planar design [2~4]. The alkaline-earth-doped lanthanum chromites are the most promising ceramic candidate materials [5,6], although there is some recent work that suggest that metallic or metal/ceramic composites may replace these systems in the near future [7,8]. In the ceramic systems, all but a few have the required properties for consideration. Strontium-doped lanthanum chromite (LSC) has been chosen as the interconnect material in this study, because of its superior electronic conductivity and high temperature stability [9].We have shown that a high sintered density (96% theoretical) can be achieved for LSC, when sintered at 1700°C [10]. For practical applications, under such extreme temperatures, fabrication is uneconomical and detrimental to other cell components which may have to be co-sintered with the interconnect. Our recent investigation [11] on lanthanum chromites, showed that doping onto the B-site,. with Co, significantly reduces the sintering temperature. For example, 96% theoretical density was obtained for La0.sSr0.2Cr0.9Co0.103 (LSCC) when sintered at 1500 °C.This work investigates the effect of anode (fuel) atmosphere on the mechanical properties of LSC and LSCC. Non-agglomerated LSC and LSCC powders were synthesized using a modified Pechini method, described elsewhere [12]. The powders were then calcined at 900 °C for 5 h. Powder X-ray diffraction, using a Philips X-ray diffractometer, revealed the materials to have a perovskite structure, with no other discernible peaks present. The powders were pressed into 30 mm × 10 mm × 2 mm bars, compacted using a die press at 400 kg cm -2, and sintered for 5 h at 1700 °C for the LSC sample and at 1500 °C for the LSCC sample, see reference [11]. Bulk density measurements indicated that both the LSC and the LSCC samples had sintered densities of greater than 96% theoretical. The bars were then exposed to the anode (H2) atmosphere using the following regime. The bars were initially placed into a tube furnace and heated to 1000 °C at a rate of 5 °C/min. On reaching 0 2 6 1 -8 0 2 8 © 1996 Chapman & Hall 1000 °C, the samples were purged with argon for 15 min and then exposed to the hydrogen atmosphere. After the desired time, argon was reintroduced and the sample cooled to room temperature. The samples were surface ground and polished, using SiC and synthetic diamond compound, prior to testing, to remove any surface defects that may affect the results. The fracture strength was then measured using a three-point bend test, with a distance of 24 mm between the points of contact, us...

Lanthanum Chromite‐Based Interconnects as Key Materials for SOFC Stack Development

“…A survey of mechanical strength in reducing atmosphere shows a much more complicated situation. The in‐situ mechanical strength of La 1‐x Ca x CrO3 25 and La 1‐x Sr x Cr 1‐y CoO 3 26 is drastically degraded in reducing atmosphere at high temperature, whereas La 1‐x Sr x CrO 3 held sufficient strength. This fact strongly suggests that the amount and the distribution state of secondary phases at grain boundary or triple point in the polycrystals will affect the strength in reducing atmospheres.…”