Influence of surface chemisorption and release processes of water vapour and carbon dioxide on radiation-induced effects in lithium-based ceramic materials

Advanced ceramic breeder (ACB) pebbles, composed of lithium orthosilicate (Li₄SiO₄) with additions of lithium metatitanate (Li₂TiO₃) as the second phase, are currently being developed and extensively tested as the European Union’s reference material for tritium breeding in future thermonuclear fusion reactors. In the present work, the influence of sample preparation, storage conditions, and thermal treatment on the surface microstructure and chemical composition of pellets prepared from the biphasic ACB pebbles was systematically investigated. Surface characterisation was performed using scanning electron microscopy – energy dispersive X-ray spectroscopy (SEM-EDS), attenuated total reflectance – Fourier transform infrared (ATR-FTIR) spectroscopy, and secondary ion mass spectrometry (SIMS). The gravimetric measurements and thermogravimetry/differential scanning calorimetry (TG/DSC) were applied to study the chemisorption and release processes of water (H₂O) vapour and carbon dioxide (CO₂) during storage up to 1050 h at room temperature in air atmosphere of varying humidity and subsequent step-wise thermal treatment up to 900 °C. The formation and accumulation of paramagnetic radiation-induced defect centres under exposure to X-rays with energies up to 45 keV were analysed using electron paramagnetic resonance (EPR) spectroscopy. On the basis of the obtained results, it can be concluded that storage in high humidity significantly affects the surface microstructure and chemical composition of the pellets, which leads to the formation of a dense layer of chemisorption products of H₂O vapour and CO₂ characterised by highly asymmetric structures, primarily consisting of lithium carbonate (Li₂CO₃). Although thermal treatment can largely reverse these high humidity-induced changes, it does not fully restore the original state of intrinsic and extrinsic defects, which can subsequently influence the formation and accumulation of paramagnetic centres during irradiation.