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Title: Kinetic measurement and prediction of the hydrogen outgassing from the polycrystalline LiH/LiOH system

Abstract

In this report, we present the use of temperature programmed reaction/decomposition (TPR) in the isoconversion mode to measure outgassing kinetics and to make kinetic prediction concerning hydrogen release from the polycrystalline LiH/LiOH system in the absence of any external H{sub 2}O source.

Authors:
; ; ; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab., Livermore, CA (US)
Sponsoring Org.:
US Department of Energy (US)
OSTI Identifier:
15015944
Report Number(s):
UCRL-PROC-210786
TRN: US200509%%457
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Conference
Resource Relation:
Conference: Presented at: 26th Compatibility, Aging, and Stockpile Stewardship, Aiken, SC (US), 04/26/2005--04/28/2005; Other Information: PBD: 9 Mar 2005
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; 36 MATERIALS SCIENCE; AGING; COMPATIBILITY; DEGASSING; FORECASTING; HYDROGEN; KINETICS; STOCKPILES

Citation Formats

Dinh, L N, Grant, D M, Schildbach, M A, Smith, R A, Leckey, J H, Siekhaus, W J, Balazs, B, and McLean II, W. Kinetic measurement and prediction of the hydrogen outgassing from the polycrystalline LiH/LiOH system. United States: N. p., 2005. Web.
Dinh, L N, Grant, D M, Schildbach, M A, Smith, R A, Leckey, J H, Siekhaus, W J, Balazs, B, & McLean II, W. Kinetic measurement and prediction of the hydrogen outgassing from the polycrystalline LiH/LiOH system. United States.
Dinh, L N, Grant, D M, Schildbach, M A, Smith, R A, Leckey, J H, Siekhaus, W J, Balazs, B, and McLean II, W. 2005. "Kinetic measurement and prediction of the hydrogen outgassing from the polycrystalline LiH/LiOH system". United States. doi:. https://www.osti.gov/servlets/purl/15015944.
@article{osti_15015944,
title = {Kinetic measurement and prediction of the hydrogen outgassing from the polycrystalline LiH/LiOH system},
author = {Dinh, L N and Grant, D M and Schildbach, M A and Smith, R A and Leckey, J H and Siekhaus, W J and Balazs, B and McLean II, W},
abstractNote = {In this report, we present the use of temperature programmed reaction/decomposition (TPR) in the isoconversion mode to measure outgassing kinetics and to make kinetic prediction concerning hydrogen release from the polycrystalline LiH/LiOH system in the absence of any external H{sub 2}O source.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2005,
month = 3
}

Conference:
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  • Due to the exothermic reaction of lithium hydride (LiH) salt with water during transportation and handling, there is always a thin film of lithium hydroxide (LiOH) present on the LiH surface. In dry or vacuum storage, this thin LiOH film slowly decomposes. We have used temperature-programmed reaction/decomposition (TPR) in combination with the isoconversion method of thermal analysis to determine the outgassing kinetics of H{sub 2}O from pure LiOH and H{sub 2} and H{sub 2}O from this thin LiOH film. H{sub 2} production via the reaction of LiH with LiOH, forming a lithium oxide (Li{sub 2}O) interlayer, is thermodynamically favored, withmore » the rate of further reaction limited by diffusion through the Li{sub 2}O and the stability of the decomposing LiOH. Lithium hydroxide at the LiOH/vacuum interface also decomposes easily to Li{sub 2}O, releasing H{sub 2}O which subsequently reacts with LiH in a closed system to form H{sub 2}. At the onset of dry decomposition, where H{sub 2} is the predominant product, the activation energy for outgassing from a thin LiOH film is lower than that for bulk LiOH. However, as the reactions at the LiH/Li{sub 2}O/LiOH and at the LiOH/vacuum interfaces proceed, the overall activation energy barrier for the outgassing approaches that of bulk LiOH decomposition. The kinetics developed here predicts a hydrogen evolution profile in good agreement with hydrogen release observed during long term isothermal storage.« less
  • 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 H{sub 2}O can be reasonably approximated by the rate at which H{sub 2}O is released from the moisture containing materials. In a vacuum or dry environment, LiOH decomposes slowly with time into Li{sub 2}O even at room temperature according to: 2LiOH(s) {yields} Li{sub 2}O(s) + H{sub 2}O(g) (1). The kinetics of the decompositionmore » 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 Li{sub 2}O 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, H{sub 2}O vapor is continuously removed and LiOH decomposes into Li{sub 2}O from the LiOH/vacuum interface (where it is thermally less stable) inward according to reaction (1) in an effort to maintain the equilibrium H{sub 2}O vapor pressure at the sample/vacuum interface. In a closed system containing both LiH and LiOH, the H{sub 2}O released from the decomposition of LiOH reacts with LiH to form hydrogen gas according to the following reaction: 2LiH(s) + H{sub 2}O(g) {yields} Li{sub 2}O(s) +2H{sub 2}(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 H{sub 2}O concentration gradient across the Li{sub 2}O 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/Li{sub 2}O/LiOH interface. As a function of increasing time and temperature, the Li{sub 2}O layer in between LiH and LiOH gets thicker, causing the energy barrier for the LiOH decomposition at the LiOH/Li{sub 2}O/LiH interface to increase, and eventually LiOH at the LiOH/vacuum interface also starts to decompose into Li{sub 2}O for reasons described in the previous paragraph. Thereafter, the Li{sub 2}O 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.« less
  • 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 dimensionalmore » 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.« less
  • The objectives of this report are to measure the H{sub 2}O outgassing kinetics of TR55 silicon after a few hours of vacuum pumping; and to make H{sub 2}O outgassing kinetic predictions for TR55 at low temperatures in a vacuum/dry environment.
  • Moisture outgassing rates from materials are of interest and importance to a variety of different fields. Because water can attack and accelerate decomposition, aging, or rusting of various parts, the assembly of an apparatus with 'wet' materials can shorten the lifetime of the apparatus. Outgassing of moisture from materials can be quite slow and a material that is seemingly dry at the time of assembly may slowly release water over years. This slow release of water will compromise the other constituents of the apparatus (e.g. electrical components, metals, organic materials) and shorten the lifetime of the apparatus. For apparatuses thatmore » are expensive or laborious to construct, it is especially important to understand and be able to predict the mechanisms and rates of water release from various materials. Such an understanding can support the development of accurate estimates of the apparatus's serviceable age and may allow for mitigation strategies in order to protect other parts from water. Energetic materials such as TATB based PBX-9502 (95% TATB, 5% Kel-F 800) and LX-17 (92.5% TATB and 7.5% Kel-F) pose a particularly challenging problem because they are heterogeneous materials with potentially many different sources and mechanisms of water release. Water molecules could be adsorbed into the polymeric binder matrix, trapped in occlusions within the polymer and the TATB crystals/particles, or trapped within defect sites in the TATB crystal. Finally, many studies indicate that water is a decomposition product under rapid heating conditions, at high temperatures and/or high pressure. Previous studies have measured the water release rate(s) from LX-17 or PBX-9502 prill/powder in order to establish oven drying times prior to use. These studies limited their time frame to a few days or a week of drying. Other studies have looked at the rate of water release of large pressed parts contained in sealed containers. Finally, some studies have looked at the rate of water diffusion through pressed parts, or the effects of wet vs. dry machining, or the influence of the synthesis methods in the amount of water present. There are a few different models that have been developed to predict the rate of water release from LX-17 or PBX-9502. These models are, to some extent, limited by the limitations of the experiments. Because all these experiments looked at water release over a relatively short period of time and left the samples relatively undamaged, they serve as a lower bound. In this work, we perform experiments and develop models that can serve as an upper bound on the rate and amount of water that can be released. Our experimental approach is to use temperature programmed desorption (TPD) and monitor the rate and amount of water release as a function of temperature. We analyzed our experimental data using two different kinetic analysis methods (isoconversional analysis and nth-order Arrhenius kinetic fits) and used the results to make predictions. The suitability of these kinetic analysis methods as well as the applicability of these experiments to long term aging (e.g. years) issues are discussed. Using the kinetics from our experiments, we predict the water release at temperature and timescales relevant to the existing literature. Based on our analysis and comparison with older data, the kinetic model(s) developed in this work serve as a relatively accurate (i.e. order of magnitude) method for predicting the water release under a variety of thermal histories.« less