Kinetic and Prediction of Hydrogen Outgassing from Lithium Hydride [electronic resource].
- Washington, D.C. : United States. Dept. of Energy, 2006. and Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy.
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- Additional Creators:
- Lawrence Berkeley National Laboratory, United States. Department of Energy, and United States. Department of Energy. Office of Scientific and Technical Information
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- Free-to-read Unrestricted online access
- In most industrial or device applications, LiH is placed in either an initially dry or a vacuum environment with other materials that may release moisture slowly over many months, years, or even decades. In such instances, the rate of hydrogen outgassing from the reaction of LiH with H₂O can be reasonably approximated by the rate at which H₂O is released from the moisture containing materials. In a vacuum or dry environment, LiOH decomposes slowly with time into Li₂O even at room temperature according to: 2LiOH(s) → Li₂O(s) + H₂O(g) (1). The kinetics of the decomposition of LiOH depends on the dryness/vacuum level and temperature. It was discovered by different workers that vacuum thermal decomposition of bulk LiOH powder (grain sizes on the order of tens to hundreds of micrometers) into Li₂O follows a reaction front moving from the surface inward. Due to stress at the LiOH/vacuum interface and defective and missing crystalline bonding at surface sites, lattice vibrations at the surfaces/interfaces of most materials are at frequencies different than those in the bulk, a phenomenon observed in most solids. The chemical reactivity and electronic properties at surfaces and interfaces of materials are also different than those in the bulk. It is, therefore, expected that the amount of energy required to break bonds at the LiOH/vacuum interface is not as large as in the bulk. In addition, in an environment where there is a moisture sink or in the case of a continuously pumped vacuum chamber, H₂O vapor is continuously removed and LiOH decomposes into Li₂O from the LiOH/vacuum interface (where it is thermally less stable) inward according to reaction (1) in an effort to maintain the equilibrium H₂O vapor pressure at the sample/vacuum interface. In a closed system containing both LiH and LiOH, the H₂O released from the decomposition of LiOH reacts with LiH to form hydrogen gas according to the following reaction: 2LiH(s) + H₂O(g) → Li₂O(s) +2H₂(g) + heat (2). Such is the case of vacuum thermal decomposition of a corrosion layer previously grown on top of a LiH substrate. Here, the huge H₂O concentration gradient across the Li₂O buffer layer in between the hydrophilic LiH substrate and LiOH, coupled with the defective nature of LiOH at surfaces/interfaces as discussed above, effectively lowers the energy barrier for LiOH decomposition here in comparison with bulk LiOH and turns the LiH substrate into an effective moisture pump. As a result, in the case of vacuum thermal decomposition of LiOH on top of a LiH substrate, the LiOH decomposition front starts at the LiH/Li₂O/LiOH interface. As a function of increasing time and temperature, the Li₂O layer in between LiH and LiOH gets thicker, causing the energy barrier for the LiOH decomposition at the LiOH/Li₂O/LiH interface to increase, and eventually LiOH at the LiOH/vacuum interface also starts to decompose into Li₂O for reasons described in the previous paragraph. Thereafter, the Li₂O fronts keep moving inward from all directions until all the LiOH is gone. This vacuum thermal decomposition process of LiOH previously grown on top of a LiH substrate is illustrated in the cartoon of figure 1.
- Published through SciTech Connect., 08/31/2006., "ucrl-proc-224237", Presented at: 27th Aging, Compatibility and Stockpile Stewardship Conference, Los Alamos, NM, United States, Sep 26 - Sep 28, 2006., and Dinh, L N; Schildbach, M A; Smith, R A; Balazs, B; McLean II, W.
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