Lithium orthosilicate (Li₄SiO₄) is considered as the most promising tritium breeder because of its high lithium density, good chemical stability and promising tritium release property. During the operation of a fusion reactor, the tritium breeders can be exposed to harsh condition for a long time, such as high temperature, thermal gradient and irradiation, which causes the degradation of the structure and property. In this work, Li₄SiO₄ pebbles were fabricated by a melt spraying method. The effect of annealing temperature on the microstructure and mechanical property of ceramic pebbles was investigated. The evolution of crush load at high temperatures was analyzed. In addition, the thermal stability of Li₄SiO₄ pebbles was also examined by long- time annealing experiments at high temperatures.

Li₄SiO₄ pebbles were fabricated by a melt spraying process. Firstly, Li₄SiO₄ powder was synthesized by a solid-state reaction. Lithium carbonate and silicon dioxide were mixed by grinding, and the ground powders were calcined at 850 ℃ for 4 h. Li₄SiO₄ ceramic powder was added into a platinum crucible, and heated until the powder melted to liquid. The melt liquid flowed out of the crucible, the droplet size was adjusted via spraying gas flowrate. During flying, the liquid droplets cooled down and formed spherical pebbles due to the surface tension. Finally, Li₄SiO₄ ceramic pebbles could be obtained by thermal annealing.

The microstructure of Li₄SiO₄ pebbles was determined by scanning electron microscopy (SEM). The crush load was measured by a universal testing machine, the pebble size was (1.00±0.05) mm. The chemical analysis was performed by inductively coupled plasma-optical emission spectrometry (ICP-OES).

Li₄SiO₄ pebbles with uniform microstructure and excellent properties are fabricated by a melt spraying process. The suitable annealing temperature is 600 ℃, and the grains grow up and the mechanical properties deteriorate significantly with further increasing the temperature. The fabricated pebbles annealed at 600 ℃ for 2 h display a dense and homogeneous structure with an average grain size of 3.88 μm, and the crush load is 20.7 N. The crush load at a high temperature decreases with the increase of temperature due to the thermal stress and plastic deformation, and the crush load at 700 ℃ is 8.7 N. According to the results of the long-time annealing experiments, the temperature is a key factor affecting the thermal stability of Li₄SiO₄ pebbles. The pebbles show a good mechanical property stability with moderate grain growth by annealing at 500-600 ℃. However, significant structure and property degradation occur due to rapid grain growth by annealing at 700 ℃. The microstructure degradation can be an inherent microstructural instability related to the competition between the densification and the grain growth kinetics, which is enhanced via increasing the annealing temperature.

Li₄SiO₄ pebbles with a fine structure and impressive property were fabricated by a melt spraying process. The pebbles displayed a dense and homogeneous structure with an average grain size of 3.88 μm, and the crush load was 20.7 N by annealing at 600 ℃ for 2 h. The crush load at high temperatures decreased with the increase of temperature. Besides, the Li₄SiO₄ pebbles could become unstable and deteriorate during the long-time annealing at high temperatures, accompanied by significant grain growth and pore evolution. The original isolated spherical pores could deteriorate to produce a crack-like interconnected pore network. Furthermore, the lithium evaporation was enhanced until the temperature reached 900 ℃. The degeneration of structure and properties of Li₄SiO₄ pebbles at > 700 ℃ could be considered in the design and operation of TBM.