Outgassing in the LiD/LiOH System
Temperature programmed decomposition (TPD), scanning electron microscopy (SEM) and x-ray diffraction (XRD) were performed on lithium hydroxide (LiOH) polycrystallites and LiD/LiOH composite nanocrystals. Our studies revealed that LiOH grains are thermally decomposed into Li{sub 2}O, releasing water, following a three dimensional phase boundary movement from the surface inward. The rate of H{sub 2}O released is controlled by a rate constant that is expressed as: d{alpha}/dt ={upsilon}.e {sup -E/RT}.f({alpha}) where t is time; {alpha} is the reacted fraction (0 to 1); {upsilon} is the pre-exponential factor which includes many constants describing the initial state of the sample such as three dimensional shape factors of initial particles, molecular mass, density, stoichiometric factors of chemical reaction, active surface and number of lattice imperfections, and so forth; E is the activation energy for the rate controlling process, R is the gas molar constant, and f({alpha}) is an analytical function which is determined by the rate-limiting reaction mechanism (random nucleation, diffusion, phase boundary motion, etc.). Due to fewer neighboring bonds at the surface, surface lithium hydroxide decomposes at low activation energies of {approx} 86-92 kJ/mol with corresponding pre-exponential factors of {approx} 2.7 x 10{sup 6}-1.2 x 10{sup 7} s{sup -1}. Near-surface hydroxide, having bonding much like bulk hydroxide but experiencing more stress/strain, decomposes at activation energies of {approx} 89-108 kJ/mol with corresponding pre-exponential factors of {approx} 9.5 x 10{sup 5}-9.3 x 10{sup 7}s{sup -1}. Bulk lithium hydroxide, however, decomposes at higher activation energies of {approx} 115-142 kJ/mol with corresponding pre-exponential factors of {approx} 4.8 x 10{sup 6}-1.2 x 10{sup 9} s{sup -1}. Bulk lithium hydroxide is very stable if stored at room temperature. However, lithium hydroxide molecules at or near the surface of the grains slowly decompose, in a vacuum or dry environment at room temperature, into Li{sub 2}O releasing water over many decades. These surface and near-surface molecules account for a few percent of the total lithium hydroxide with micrometer grain size. Experimental data also confirm that the conversion of Li{sub 2}O grains back to lithium hydroxide during moisture exposure proceeds from the surface inward such that surface states are always filled before bulk states. In a different set of experiments, nanometer scale composite grains composed of LiD inner cores and LiOH outer layers were observed to form on top of pressed polycrystalline LiD upon moisture exposure. Experimental data suggest that the long-term outgassing effects of surface states in these composite hydroxide nanocrystals, which have very high ratios of surface molecules to bulk molecules, are much more significant than in the case of micrometer grain (bulk) hydroxide. The measured kinetics in this work enable the construction of the evolution steps in these composite nanocrystals in a dry environment as a function of time. Our investigation also confirmed that vacuum heating of the LiD/LiOH composite nanocrystals converts most of LiOH into Li{sub 2}O. Subsequent moisture re-exposure at up to 50 ppm for many days converts only surface and near surface Li{sub 2}O to surface and near surface LiOH.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15013651
- Report Number(s):
- UCRL-CONF-200583; TRN: US200803%%1059
- Resource Relation:
- Conference: Presented at: The Minerals, Metals & Materials Society 2004 Meeting, Charlotte, NC, United States, Mar 14 - Mar 18, 2004
- Country of Publication:
- United States
- Language:
- English
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