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Author: R. A. Smith
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ISBN:
Category :
Languages : en
Pages : 57
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Book Description
Lithium hydride is a nuclear material with a great affinity for moisture. As a result of exposure to water vapor during machining, transportation, storage and assembly, a corrosion layer (oxide and/or hydroxide) always forms on the surface of lithium hydride resulting in the release of hydrogen gas. Thermodynamically, lithium hydride, lithium oxide and lithium hydroxide are all stable. However, lithium hydroxides formed near the lithium hydride substrate (interface hydroxide) and near the sample/vacuum interface (surface hydroxide) are much less thermally stable than their bulk counterpart. In a dry environment, the interface/surface hydroxides slowly degenerate over many years/decades at room temperature into lithium oxide, releasing water vapor and ultimately hydrogen gas through reaction of the water vapor with the lithium hydride substrate. This outgassing can potentially cause metal hydriding and/or compatibility issues elsewhere in the device. In this chapter, the morphology and the chemistry of the corrosion layer grown on lithium hydride (and in some cases, its isotopic cousin, lithium deuteride) as a result of exposure to moisture are investigated. The hydrogen outgassing processes associated with the formation and subsequent degeneration of this corrosion layer are described. Experimental techniques to measure the hydrogen outgassing kinetics from lithium hydride and methods employing the measured kinetics to predict hydrogen outgassing as a function of time and temperature are presented. Finally, practical procedures to mitigate the problem of hydrogen outgassing from lithium hydride are discussed.
Author: R. A. Smith
Publisher:
ISBN:
Category :
Languages : en
Pages : 57
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Book Description
Lithium hydride is a nuclear material with a great affinity for moisture. As a result of exposure to water vapor during machining, transportation, storage and assembly, a corrosion layer (oxide and/or hydroxide) always forms on the surface of lithium hydride resulting in the release of hydrogen gas. Thermodynamically, lithium hydride, lithium oxide and lithium hydroxide are all stable. However, lithium hydroxides formed near the lithium hydride substrate (interface hydroxide) and near the sample/vacuum interface (surface hydroxide) are much less thermally stable than their bulk counterpart. In a dry environment, the interface/surface hydroxides slowly degenerate over many years/decades at room temperature into lithium oxide, releasing water vapor and ultimately hydrogen gas through reaction of the water vapor with the lithium hydride substrate. This outgassing can potentially cause metal hydriding and/or compatibility issues elsewhere in the device. In this chapter, the morphology and the chemistry of the corrosion layer grown on lithium hydride (and in some cases, its isotopic cousin, lithium deuteride) as a result of exposure to moisture are investigated. The hydrogen outgassing processes associated with the formation and subsequent degeneration of this corrosion layer are described. Experimental techniques to measure the hydrogen outgassing kinetics from lithium hydride and methods employing the measured kinetics to predict hydrogen outgassing as a function of time and temperature are presented. Finally, practical procedures to mitigate the problem of hydrogen outgassing from lithium hydride are discussed.
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Category :
Languages : en
Pages : 6
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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.
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Category :
Languages : en
Pages : 33
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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 H2O from pure LiOH and H2 and H2O from this thin LiOH film. H2 production via the reaction of LiH with LiOH, forming a lithium oxide (Li2O) interlayer, is thermodynamically favored, with the rate of further reaction limited by diffusion through the Li2O and the stability of the decomposing LiOH. Lithium hydroxide at the LiOH/vacuum interface also decomposes easily to Li2O, releasing H2O which subsequently reacts with LiH in a closed system to form H2. At the onset of dry decomposition, where H2 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/Li2O/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.
Author:
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ISBN:
Category :
Languages : en
Pages : 33
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Book Description
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 H2O from pure LiOH and H2 and H2O from this thin LiOH film. H2 production via the reaction of LiH with LiOH, forming a lithium oxide (Li2O) interlayer, is thermodynamically favored, with the rate of further reaction limited by diffusion through the Li2O and the stability of the decomposing LiOH. Lithium hydroxide at the LiOH/vacuum interface also decomposes easily to Li2O, releasing H2O which subsequently reacts with LiH in a closed system to form H2. At the onset of dry decomposition, where H2 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/Li2O/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.
Author: Jerry L. Modisette
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Category : Aerodynamic heating
Languages : en
Pages : 14
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Author: Mina Rea Jones Perlow
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Category :
Languages : en
Pages :
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Author: Les Mallinson
Publisher: Springer Science & Business Media
ISBN: 1461512158
Category : Technology & Engineering
Languages : en
Pages : 659
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The first International Conference on Ageing Studies and Lifetime Extension of Materials was held on th July 12-14 , 1999 at St. Catherine's College, Oxford, United Kingdom. Over 230 delegates attended during the three days and heard nearly ninety papers, together with over thirty poster presentations. Sixteen of these papers were keynotes from invited speakers eminent in their field of research. The proceedings were organised into six separate sessions: observation and understanding of real-time and accelerated ageing; experimental techniques; modelling and theoretical studies; lifetime prediction and validation; lifetime extension; and material design for ageing. In doing this, it was hoped to cover most issues of scientific concern inthefield ofmaterials ageing. One important aspect was that the conference did not concentrateon any particular group or type ofmaterial; rather the aim was to attract contributions from workers engaged in ageing studies with as wide a range of materials as possible. In this way, it was hoped that delegates could interactwith and learnfrom those whom they perhapswould not normally come across and that metallurgists could learn from polymer scientists, ceramicists could talk to modellers, and so on, in this important field. A read through the diverse papers contained within these proceedings will confirm that this aim was happily satisfied. Why hold such a meeting? In the modem world, engineered systems are expected to last longer.
Author: Swain, Jr. (Edwin E.)
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Category : Chemical reactions
Languages : en
Pages : 22
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Author: E.E. Shpil'rain
Publisher: Springer Science & Business Media
ISBN: 9780883185322
Category : Science
Languages : en
Pages : 242
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Author: Lyle O. Hill
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Category :
Languages : en
Pages :
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