Ion-backscattering study of LiOH-to-Li2O conversion on a LiH substrate

Elastic proton backscattering has been used to study the mechanism by which LiOH corrosion layers on LiH are converted to Li2O. The concentration-vs-depth profile was measured during vacuum anneals in the temperature range 200–280°C, over which the conversion rate varied by two orders of magnitude. Conversion was found to proceed via two reaction fronts, one advancing from the LiH–LiOH interface, the other from the surface. Both H2 and H2O were evolved during the reaction. The results are interpreted in terms of H2O being formed at both reaction fronts, and then diffusing respectively to the LiH substrate and to the surface. Water reaching the LiH substrate then reacts to produce H2, while H2O at the surface is released. The conversion rate on the LiH substrate is related quantitatively to the static equilibrium pressure of H2O over a mixture of bulk LiOH and Li2O in the absence of LiH. Values for this thermodynamic quantity have been reported in the literature. From the analysis it is concluded that H2O formation at the reaction fronts is the rate-determining step in the conversion. The activation energy for the conversion process is 31 kcal/mole.