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Title: Measurement and Prediction of Water Outgassing from TR55 Silicone by the Isoconversional Technique

Abstract

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.

Authors:
; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
897952
Report Number(s):
UCRL-CONF-221282
TRN: US200706%%146
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Conference
Resource Relation:
Conference: Presented at: North American Thermal Analysis Society 2006 conference, Bowling Green, KY, United States, Aug 05 - Aug 10, 2006
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; DEGASSING; FORECASTING; KINETICS; PUMPING; SILICON; SILICONES; THERMAL ANALYSIS; WATER

Citation Formats

Dinh, L N, Schildbach, M A, Burnham, A K, Maxwell, R S, and Balazs, B. Measurement and Prediction of Water Outgassing from TR55 Silicone by the Isoconversional Technique. United States: N. p., 2006. Web.
Dinh, L N, Schildbach, M A, Burnham, A K, Maxwell, R S, & Balazs, B. Measurement and Prediction of Water Outgassing from TR55 Silicone by the Isoconversional Technique. United States.
Dinh, L N, Schildbach, M A, Burnham, A K, Maxwell, R S, and Balazs, B. Mon . "Measurement and Prediction of Water Outgassing from TR55 Silicone by the Isoconversional Technique". United States. doi:. https://www.osti.gov/servlets/purl/897952.
@article{osti_897952,
title = {Measurement and Prediction of Water Outgassing from TR55 Silicone by the Isoconversional Technique},
author = {Dinh, L N and Schildbach, M A and Burnham, A K and Maxwell, R S and Balazs, B},
abstractNote = {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.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon May 01 00:00:00 EDT 2006},
month = {Mon May 01 00:00:00 EDT 2006}
}

Conference:
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  • 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.
  • 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
  • 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
  • The isoconversional technique was employed for the measurement and prediction of H2O outgassing kinetics from silica-filled polydimethylsiloxane TR55 and S5370 in a vacuum or dry environment. Isoconversional analysis indicates that the energy barrier for H2O release from TR55 and S5370 is an increasing function of the fractional H2O release. This can be interpreted as the release of H2O from physisorbed water and then chemisorbed water with decreasing OH density from the surfaces of the embedded silica particles. Model independent predictions of H2O outgassing based on the measured kinetics follow the trend of actual isothermal outgassing at elevated temperatures, and suggestmore » gradual outgassing in dry storage over many decades at low temperatures for both TR55 and S5370.« less
  • The amount of water transported below the root-zone and available for drainage (recharge) must be known in order to quantify the potential for leaching at low-level waste sites. Under arid site conditions, we quantified drainage by using weighing lysimeters containing sandy soil and measured 6 and 11 cm of drainage for a 1-yr period (June 1983-May 1984) from grass-covered and bare-soil surfaces, respectively. Precipitation during this period at our test site near Richland, Washington, was 25 cm. Similar drainage values were estimated from neutron probe measurements of water content profile changes in an adjacent grass-covered site. These data suggest thatmore » significant amounts of drainage can occur at arid sites when soils are coarse textured and precipitation occurs during fall and winter months. Model simulations predicted drainage values comparable to those measured with our weighing lysimeters. Long-term, 500- to 1000-yr predictions of leaching are possible with our model simulations. However, additional studies are needed to evaluate the effect of soil variability and stochastic rainfall inputs on drainage estimates, particularly for arid sites. 15 references, 9 figures, 1 table.« less