Lithium-based ceramic breeder Li4SiO4 will suffer long-term high temperature during operation. That may lead to changes to its microstructure, chemical phase, composition elements, and further impacts its mechanical performance and hydrogen isotope release behavior. In the present work, long-term annealing at 900 °C in He incl. 0.1 vol% H2 flow atmosphere for designated times was conducted. Li4SiO4 pebbles with poor or excellent mechanical performance (abbr. "poor pebbles" and "excellent pebbles") have porous or dense microstructure, respectively. Coarsening process that grains and pores get larger as a function of annealing time was observed for Li4SiO4 poor pebbles. Li4SiO4 excellent pebbles can maintain its microstructure stability, and only slight grain growth was observed after 1000 h of annealing. Oxygen and lithium element loss was demonstrated over annealing time. Compression tests and Weibull analysis indicate that the distinct degradation of mechanical performance of Li4SiO4 pebbles mainly occur in the initial 100 h of annealing, and then the degradation process slows down. Thermal desorption spectroscopy reveals that 1000 h of annealing caused the disappearance of high temperature release peak (665 °C) and the shift of low temperature release peaks (around 300 °C) to lower temperature. The former is mainly attributed to decomposition of carbon impurities (Li2CO3) and the latter is attributed to carbon impurities and the transition from closed to open pores.
Preliminary investigation of Li4SiO4 pebbles structural performance