Investigation of Deuterium Permeability of Sputtered Beryllium and Graded Copper-Doped Beryllium Shells

Graded copper-doped Be shells have been fabricated by sputter coating on spherical mandrels. While such coatings have consistent microstructure and acceptable void content and size, we have found that they suffer from sufficient interconnected porosity leading to relatively rapid gas leakage. In this paper, we present an extensive study of D₂ leakage out of Be shells made by sputter coating. The leakage appears to follow molecular flow dynamics as determined by examining the temperature dependence of the flow. Furthermore, the time dependence of the leakage suggests that the flow channels are nanometerish in diameter, propagating through the thickness of the coating, possibly brought about by residual stress in the coatings. We have investigated the D₂ leakage time constant as a function of a large number of coating parameters, including the effect of introducing boron-doped layers. Addition of thin 0.25 m amorphous boron-doped layers near the inside surface has been most effective in producing shells with long time constants (greater than 7 days to immeasurable) with yield of greater than 50%. There is still substantial scatter in the data, even within a given coating batch, suggesting a possible stochastic cracking process driven by residual stress in the coating.