Mechanism of vacuum-annealing defects and its effect on release behavior of hydrogen isotopes in Li2TiO3

Li₂TiO₃ is one of the most promising candidates among solid breeder materials. However, defects introduced into Li₂TiO₃ will act as the strong trapping sites for tritium. In the present study, mechanism of vacuum-annealing defects and its effect on release behavior of hydrogen isotopes in Li₂TiO₃ were investigated by means of X-ray diffraction, Raman spectroscopy, electron spin resonance and thermal desorption spectroscopy. The color of samples becomes dark blue and the defects were found to be introduced into Li₂TiO₃ when annealed in vacuum. This color change suggests the change from Ti⁴⁺ to Ti³⁺ due to decrease in oxygen content. The color recovers to white again after annealing in air. X-ray diffraction and Raman spectroscopy results indicate that there are no modifications on Li₂TiO₃ crystal phases, but on crystallinity. The main vacuum-annealing defects are E-centers and no other obvious types of defects were observed from electron spin resonance. Based on the experimental results, the production of defects by annealing in vacuum should be satisfied to the following conditions: (1) Li₂TiO₃ has been exposed in air more than 1 day; (2) Li₂TiO₃ must be annealed at the temperature higher than 300 °C; (3) Li₂TiO₃ should be annealed in vacuum lower than 10 Pa. E-centers formed under vacuum-annealing processes have considerable effects on release behavior of hydrogen isotopes investigated by thermal desorption spectroscopy and further should be considered in future fusion reactor. The present work gives some suggestions for future fusion reactors: (1) Li₂TiO₃ should be preserved in vacuum or kept from water vapor; (2) Li₂TiO₃ should be annealed at high temperature to remove the adsorbed water before loading into the facility, and must be finished within two days to avoid defects coming from reduction; (3) Li₂TiO₃ should be improved by adding more oxygen or other elements to refrain from defects introduced by reduction reaction.