Effects of long-term high temperature annealing on Li2TiO3 and advanced core-shell Li2TiO3-Li4SiO4 tritium breeders

The present work investigates the evolution of mechanical performance and provides a comprehensive study of the changes in microstructure, phase, and compositional elements of Li2TiO3 and core-shell Li2TiO3-Li4SiO4 pebbles resulting from annealing at 900 degrees C in He incl. 0.1 vol % H-2 flow atmosphere for specific durations. Compression tests show that long-term annealing improves the mechanical performance of Li2TiO3 instead of degrading it. The mean crush load of Li2TiO3 increases from 41 N to 45 N after 1000 h of annealing. Li2TiO3 pebbles can maintain their microstructure stability, with only limited grain growth and a decrease in open porosity identified. These changes are believed to be responsible for the improvement in mechanical performance after 1000 h. The mechanical performance of core-shell Li2TiO3-Li4SiO4 pebbles primarily degrades during the initial 100 h, with a decrease in mean crush load from 118 N to 91 N. Subsequently, it gradually stabilizes with a mean crush load of 91 N over the remaining annealing period. An increase in open pores and slight Li4SiO4 grain growth was observed after 1000 h of annealing, which is the primary cause of the mechanical performance degradation in core-shell Li2TiO3-Li4SiO4 pebbles. The loss of lithium in core-shell Li2TiO3-Li4SiO4 pebbles is a combined effect of the Li2TiO3 phase and Li4SiO4 phase, with a stronger association to the Li4SiO4 phase.