Investigations of the thermal stability of lithium films on boron surfaces*

Boronization and lithiumization are used as wall conditioning processes to improve the impurity gettering and fuel recycling in current and future fusion devices including ITER. A combination of these processes is in the scope of experiments such as NSTX-U [1] and TJ-II [2]. Thus, interactions of lithium (Li) and boron (B) surfaces are expected, and an understanding of the Li-B system is necessary. Previous studies on Li-B systems have shown poor Li solubility in B, forming B-rich borides (LiB_x with x>10). Limited interactions at temperatures below 573 K were observed, while melting and decomposition were reported above 800 K and 1200 K, respectively [3,4]. In this preliminary study, we evaluate the characteristics of boron surfaces and the thermal stability of Li films on B surfaces using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). We used XPS to evaluate the surface chemistry and asses the impurity content of the B and Li-B surfaces, and SEM to evaluate the surface morphology. We studied the thermal stability of the Li films on B surfaces using TDS, as a function of Li surface coverage.

[1] NSTX-U Five Year Plan 2021-2025 [2] F. L. Tabarés, et al., Plasma Phys. Contr. F. 50, 124051 (2008). [3] H. Borgstedt and C. Guminski, Journal of Phase Equilibria. 24,572 (2003) [4] V. Milashius, et al., Inorganic Chemistry Frontiers, 5, 853 (2018)