The Impact of Sintering Conditions on the Inner Microstructure of Composites: LSM:YSZ Electrode Case Study

Abstract

High-temperature solid oxide cells (SOCs) is in the center of scientific attention for a while as a promising technology related to sustainable society. In this case, SOCs consist of two closely interlinked technologies: solid oxide fuel cells and solid oxide electrolysis cells. Both technologies share several similar traits such as high electrical efficiency and fast reaction kinetics due to the high operating temperature (600–900 °C) without the need for PGM catalysts. These cells basically consist of two porous composite electrodes and a dense ion conducting electrolyte. In particular, the inner microstructure of the electrodes predetermines the SOCs performance owing to its governing effect on the electrical conductivity and/or electrocatalytic activity. Despite its importance, contemporary understanding of the development of the emerging microstructure during the fabrication of these composite materials is far from sufficient.Lanthanum-strontium-manganite (LSM, La1-xSrxMnO3-δ) paired with yttria stabilized zirconia (YSZ, ZrO2:Y2O3) is the typical example of the state-of-the art porous oxygen electrode exhibiting a mixed ion-electron conductivity suitable for SOCs. The fabrication process of LSM:YSZ electrodes involves a crucial step – the sintering, which principle lies in the thermal treatment of the precursor particles for a defined period of time. The LSM:YSZ composite is usually sintered at 1150 °C. At this temperature, exclusively LSM particles tend to soften and form an interlinked solid framework between individual particles, while the YSZ particles remain unaffected by the temperature treatment due to their high melting point (around 2500 °C).Despite the sintering process belongs among the oldest known fabrication techniques, there is no hard evidence on how the sintering proceeds over time at different temperatures while employing materials with significantly different melting points, such as LSM and YSZ. In addition, the sintering temperature along with the sintering time directly influences the resulting inner microstructure and consequently the electrical conductivity of such electrode. Prediction of the resulting inner microstructure is frequently performed by postmortem analyses of multiple samples sintered at different temperatures. However, such practice results only in indirect proofs of the impact of the sintering temperature and is significantly time consuming. The goal of this study is, therefore, to investigate the sintering process of LSM:YSZ composite in situ and to set into relation the resulting inner microstructure with the applied thermal treatment.To answer the present goal, a novel combination of SEM paired with inbuilt MEMS heating chip was developed. This setup allows for online monitoring of the LSM:YSZ sintering process. The results show that at 900 °C the LSM particle agglomerates begin to shrink as the individual LSM particles start to adhere to each other. At this point, significant porosity emerges in the composite, which is considered essential to ensure a sufficient mass transfer during the SOCs operation. On the other hand, the degree of the emerged porosity can be beyond the threshold of mechanical stability of the electrode body, rendering such an electrode unusable. At 1150 °C the densification of the composite occurs, which is essentially a self-healing process resulting in a well-interlinked and mechanically stable composite. The densification is related to the adhesion of the LSM particles to the YSZ particles, thus forming a solid framework, ensuring ample mixed ionic-electronic conductivity of the fabricated electrode. Based on the results, a slight variation in sintering temperature and/or time leads to a significant difference in the electric conductivity of the electrode due to a different sintering degree or, in other words, due to the different void fraction in the body of the electrode. Within the literature, this outcome is reflected by a major inconsistency in reported electric conductivity of the commonly utilized 50:50 wt.% ratio of LSM:YSZ.An interesting insight into the sintering process of the LSM:YSZ composite has provided the SEM paired with inbuilt MEMS heating chip. This experimental technique is very promising for in situ studies of sintering and/or synthesis processes of various materials related to numerous technologies.This work was supported by the Technology Agency of the Czech Republic under project no. TK04030143.