Influence of thermal treatment at various temperatures on structure and radiation-induced effects in advanced ceramic breeder pebbles

Advanced ceramic breeder (ACB) pebbles consisting of 65 mol% lithium orthosilicate (Li₄SiO₄) and 35 mol% lithium metatitanate (Li₂TiO₃) are currently being developed as the European Union’s reference material for tritium breeding in future thermonuclear fusion reactors. In the present work, the influence of thermal treatment at various temperatures on structure and radiation-induced effects in the ACB pebbles is investigated. The produced ACB pebbles were thermally treated at selected temperatures between 500 and 1000 °C in different atmospheres (air, argon, and vacuum) and afterwards irradiated using X-rays with energies up to 45 keV. The formed and accumulated paramagnetic radiation-induced defect centres in the irradiated ACB pebbles before and after thermal treatment were analysed using electron paramagnetic resonance (EPR) spectroscopy. To determine the exact temperatures of the phase transitions and evaluate the influence of thermal treatment on the crystal structure, chemical bond vibrations, and microstructure of the ACB pebbles before irradiation, several other relevant physico-chemical analytical methods were applied: differential scanning calorimetry/thermogravimetry (DSC/TG), powder X-ray diffraction (PXRD) technique, attenuated total reflectance – Fourier transform infrared (ATR-FTIR) spectroscopy, and scanning electron microscopy (SEM). On the basis of the obtained results, it can be concluded that thermal treatment at temperatures higher than 700 °C influences the distribution of paramagnetic centres generated by exposure to X-rays. This is probably due to the second-order phase transition from “low-temperature” to “high-temperature” structure of the Li₄SiO₄ phase between 600 and 750 °C. The obtained results highlight the importance of considering the thermal treatment temperature of the produced ACB pebbles prior to irradiation when analysing these ceramic materials using EPR spectroscopy techniques.