Deuterium retention and thermal stability of oxygen-containing boron films on polycrystalline tungsten

In the design considerations for ITER and other future fusion devices that employ boronization, serious operation constraints arise from tritium (T) retention properties and thermal stability of the boron (B) film, especially upon oxidation via reactions with commonly found oxygen-containing impurities in the reactor vessel. We investigated the deuterium (D) retention properties and thermal stability of oxygen-containing boron films deposited on polycrystalline tungsten (W) substrates utilizing temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and in-situ scanning electron microscopy (SEM). XPS showed the presence of B₂O₃ and other suboxides in the as-deposited B film, with little to no carbon impurity. TPD results for an O-containing B film (40 nm) irradiated with 170-eV D⁺ ions showed a constant desorption onset at 330 K and complete desorption of implanted D before 1000 K, with HD and D₂ being the major desorbed species. D uptake began to saturate at a fluence of 2.2 × 10²¹ D⁺ m^-2 when irradiated at 300 K. When the B film was irradiated at 600 K, no desorption peaks were present as identified in previous studies as the desorption of D after decomposition of B-D bonds. Annealing of the same O-containing B film sample within an environmental SEM revealed that surface microstructures were stable up to 1000-1100 K. Above 1100 K, the O-containing B film evaporated. The results suggest that a critical temperature of 600 K should be considered for the removal of D/T from boronized first walls.