Study of sintering mechanisms of Ca-doped yttrium aluminum garnet ceramics: From nanostructure to macroscopic behaviour

This paper aims at elucidate the role of calcium as sintering aid for YAG ceramics elaborated by a reactionsintering process. From structural and microstructural analyses, the solubility limit of calcium was found to be close to 0.065 +/- 0.015 at% at 1700 degrees C. For calcium content higher than this value up to 1 at%, CaAl12O19 crystallized precipitates with a melting temperature close to 1850 degrees C were observed. Whatever its content, calcium segregates at grain boundaries as evidenced by STEM and NanoSIMS chemical analyses at nanometric scale, and strongly decreases densification and grain growth kinetics of YAG ceramics. This result was explained by a solute-drag mechanism due to the segregation of complex Ca2+- V..O clusters with low mobility at grain boundaries. Finally, calcium appears as a very efficient sintering additive to limit grain growth in YAG, thus avoiding the formation of intragranular porosity that is a critical point for the manufacturing of highly transparent ceramics.

Automated summary

This paper aims to investigate the role of calcium as a sintering aid in the production of YAG ceramics through a reaction-sintering process. The study reveals that the solubility limit of calcium is approximately 0.065 +/- 0.015 at% at a temperature of 1700 degrees Celsius. When the calcium content exceeds this value, the formation of CaAl12O19 crystallized precipitates with a melting temperature close to 1850 degrees Celsius is observed. Regardless of its content, calcium segregates at grain boundaries, which significantly hinders the densification and grain growth kinetics of YAG ceramics. This phenomenon is explained by the solute-drag mechanism caused by the segregation of complex Ca2+- V..O clusters with low mobility at grain boundaries. Overall, calcium proves to be an effective sintering additive in limiting grain growth in YAG ceramics, thereby preventing the formation of intragranular porosity, which is crucial for the production of highly transparent ceramics.