Polycrystalline Lithium Hydride Hydrolysis Behavior Of Low-temperature Conditions

Recent studies on hydrolysis products and kinetics of lithium hydride (LiH) have proved that LiH hydrolysis products are composed of Li2O, LiOH and LiOH·H2O. The products make up a layered structure of LiOH·H2O/LiOH/Li2O/LiH from the surface to the inside of the sample. The hydrolysis kinetics of LiH generally showed a'paralinear'behavior, and affected by H2O concentration, temperature, pressure and etc. The proposed model of "Layer Diffusion Control" can be used to successfully explain the hydrolysis behavior of LiH. However, there are still some exceptions, for instance, it can not explain the opposite effect of temperature and gas pressure to hydrolysis kinetics, which were observed in some experiments. No experimental evidence by far can prove the suitability of the model for LiH hydrolysis under low humid argon atmosphere, and elucidate the relationship between the key product of Li2O with the moisture concentrations and its effect to the hydrolysis kinetics of LiH.In this study, the ambient hydrolysis kinetics of poly crystalline LiH in argon with water concentration (cw) ranging from 200μL·L-1 to 1200μL·L-1 were measured by gravimetry and Raman spectroscopy. The results showed that the LiH hydrolysis curve revealed a paralinear shape, which was a combination of a parabolic section and a linear section. This was attributed to two different reaction stages as explained by the'Layer Diffusion Control' model. Based on the model, a novel two-stage rate equation in weight gain for LiH hydrolysis reactions was developed as following: The rate equation could be used to fit the experimental data for determination the steady thickness (Hs) of Li2O and the ultimate hydrolysis rate (vs). The fitted data presented that the Hs would increase with the increasing of water concentration, which was also confirmed by Raman spectra. However, in spite of the negative effect imposed by Hs increasing, the upward trend of vs remained, which implied that the water concentration, rather than Li2O thickness, played a predominant role in LiH hydrolysis kinetics. In addition, the proportional relationship between vsHs and cw predicted by rate equation was confirmed by gravimetric data, which validated the model for LiH hydrolysis in such conditions.Additionally, a DFT simulation of the adsorption and dissociation behavior of H2O to the surface of LiH was also carried out. The result showed that the dissociation path ofâ…¢ H2Oâ†'O+H+H was thermodynamically favorable rather than the path of H2Oâ†'OH+H, which implied that the H2O molecule was more likely to dissociate by the former path and to produce Li2O instead of LiOH. This was consistent with the experimental observation.