Effect of additives on sintering and conductivity of yttria-stabilized zirconia
Date of Issue2016
School of Materials Science and Engineering
The effect of additives on sintering and functional properties of ceramics is one of the on-going interests in the fabrication of high performance advanced components for novel applications. In the present work, the effect of additives on yttria-stabilized zirconia (YSZ) was studied. YSZ is a well accepted electrolyte material used in solid oxide fuel cell (SOFC) system. In the multi-layer SOFC system, YSZ is sandwiched between the anode (Ni/YSZ cermet) and cathode (Sr-doped LaMn03)' There are several practical concerns in the co-sintering of this system, such as the higher sintering temperature of YSZ than that of the electrodes, unavoidable interdiffusion of NiO from anode, and ubiquitous presence of Si02 impurity. The present work focuses on the understanding of the influence of the additives, either in the form of purposely added Ah03 or inevitably existed NiO, Si02 , on sintering and conductivity of YSZ. The study aims to clarify the sintering mechanisms, hence shed light to the lowering of the sintering temperature, and to improve the conductivity of YSZ. Through the study, we also established models that provide prediction on the microstructural evolution during sintering, which could support the design of new material system in co-sintering and other application. It has been reported that a small amount of Ah03 or NiO could enhance the densification of YSZ, however, it was found in the present work that in the heavily doped YSZ samples, there exists an optimal additive amount for the densification. Grain growth of YSZ was also found to be initially advantageous by Al203 or NiO addition, but showed deteriorating effect upon further addition. In the present work, densification and grain growth kinetics of YSZ with/without additives were studied via the proposed master curve approach, which consists of Master Densification Curve (MDC) and Master Grain Growth Curve (MGGC) approach. During the firststage sintering (p = 60%), the abnormally higher activation energy (620730 kJImol) was observed, which was attributed to the contribution from interface reaction and surface diffusion. For the second stage of sintering (60% ~ p ~ 95%), lattice diffusion was proposed as the dorninant rnechanism for undoped 8YSZ. However, upon 1.0 wt.% A1203, or 0.28 wt.% NiO addition, grain boundary diffusion was found to significantly contribute to the densification, where the activation energy was noted to decrease about 250 k.l /rnol. The proposed MDC as well as MGGC were found to be able to facilitate prediction of microstructural evolution with good accuracy for a wide range of sintering profiles. In order to further reduce the sintering temperature of YSZ, the approach of co-addition of Ab03 and Si02 was evaluated. The lowest temperature for maximum densification rate (Tm ax ) has been achieved at 1210°C in YSZ with 0.5 wt.% Ab03 and 0.05 wt.% Si02 co-additives. The improved densification behavior is ascribed to the enhanced grain boundary diffusivity and liquid phase sintering. Finally, the conductivity of YSZ with additives was examined, with respect to its application as electrolyte for SOFC. In contrary to the prevailing thought that Si02 contamination should be especially avoided, the present work demonstrated that co-addition of Al203 and Si02 could not only further enhance densification, but also could improve the conductivity of YSZ, provided a careful manipulation of additive content and suitable heat treatment temperatures. The optimum concentration of co-addition of Al203 and Si02 was identified via carefully designed experiments.
DRNTU::Engineering::Electrical and electronic engineering