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Lithium hydride ( LiH) is an inorganic compound composed of lithium and hydrogen. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not soluble but reactive with all Polar solvent organic solvents. It is soluble and nonreactive with certain such as lithium fluoride, lithium borohydride, and sodium hydride. With a molar mass of 7.95 g/mol, it is the lightest ionic compound.
The thermal conductivity of LiH decreases with temperature and depends on morphology: the corresponding values are 0.125 W/(cm·K) for crystals and 0.0695 W/(cm·K) for compacts at 50 °C, and 0.036 W/(cm·K) for crystals and 0.0432 W/(cm·K) for compacts at 500 °C. The linear thermal expansion coefficient is 4.2/°C at room temperature.
This reaction is especially rapid at temperatures above 600 °C. Addition of 0.001–0.003% carbon, and/or increasing temperature/pressure, increases the yield up to 98% at 2-hour residence time. However, the reaction proceeds at temperatures as low as 29 °C. The yield is 60% at 99 °C and 85% at 125 °C, and the rate depends significantly on the surface condition of LiH.
Less common ways of LiH synthesis include thermal decomposition of lithium aluminium hydride (200 °C), lithium borohydride (300 °C), n-Butyllithium (150 °C), or ethyllithium (120 °C), as well as several reactions involving lithium compounds of low stability and available hydrogen content.
Chemical reactions yield LiH in the form of lumped powder, which can be compressed into Pelletizing without a binder. More complex shapes can be produced by casting from the Melting. Large single crystals (about 80 mm long and 16 mm in diameter) can be then grown from molten LiH powder in hydrogen atmosphere by the Bridgman–Stockbarger technique. They often have bluish color owing to the presence of colloidal Li. This color can be removed by post-growth annealing at lower temperatures (~550 °C) and lower thermal gradients. Major impurities in these crystals are Sodium (20–200 ppm), Oxygen (10–100 ppm), Magnesium (0.5–6 ppm), Iron (0.5-2 ppm) and Copper (0.5-2 ppm).
Bulk cold-pressed LiH parts can be easily machined using standard techniques and tools to micrometre precision. However, Casting LiH is brittle and easily cracks during processing.
A more energy efficient route to form lithium hydride powder is by ball milling lithium metal under high hydrogen pressure. To prevent cold welding of lithium metal (due to its high ductility), small amounts of lithium hydride powder are added during this process.Solvent-and catalyst-free mechanochemical synthesis of alkali metal monohydrides IZ Hlova, A Castle, JF Goldston, S Gupta, T Prost… - Journal of Materials Chemistry A, 2016
LiH is highly reactive towards water and other protic reagents:
LiH is less reactive with water than Li and thus is a much less powerful reducing agent for water, alcohols, and other media containing reducible solutes. This is true for all the binary saline hydrides.
LiH pellets slowly expand in moist air, forming LiOH; however, the expansion rate is below 10% within 24 hours in a pressure of 2 Torr of water vapor. If moist air contains carbon dioxide, then the product is lithium carbonate. LiH reacts with ammonia, slowly at room temperature, but the reaction accelerates significantly above 300 °C. LiH reacts slowly with higher alcohols and phenols, but vigorously with lower alcohols.
LiH reacts with sulfur dioxide to give the dithionite:
LiH reacts with acetylene to form lithium carbide and hydrogen. With anhydrous organic acids, phenols and acid anhydrides, LiH reacts slowly, producing hydrogen gas and the lithium salt of the acid. With water-containing acids, LiH reacts faster than with water. Many reactions of LiH with oxygen-containing species yield LiOH, which in turn irreversibly reacts with LiH at temperatures above 300 °C:
Lithium hydride is used in the production of a variety of reagents for organic synthesis, such as lithium aluminium hydride () and lithium borohydride (). Triethylborane reacts to give superhydride ().Peter Rittmeyer, Ulrich Wietelmann "Hydrides" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim.
The common isotope also can undergo a tritium and neutron-producing reaction under the influence of high energy neutrons:
Both reactions leave the deuterium () from the LiD capable of undergoing fusion with the tritium that has just been produced:
The high-energy neutron produced by the deuterium-tritium reaction can then go on to react with more LiD, or go on to induce nuclear fission in fissionable materials (which can include uranium-238 in this context, because the fusion neutron is of sufficiently high energy), which can produce more neutrons, to continue the process. In the 1960s and 1970s, the U.S. Department of Energy declassified the following statements about lithium hydrides as thermonuclear fuels: "The fact that lithium, deuterium (Li6D, LiD) are used in unspecified thermonuclear weapons." "The mere fact that normal lithium deuteride (LinD) is used in unspecified TN weapons." "The fact that compounds of Li6 containing tritium are used in the design of weapons as TN fuel." The maximum explosive yield of lithium deuteride fusion is around 50 per kilogram of material reacted, making it about 3 times more energy dense than nuclear fission.
Using lithium deuteride as a thermonuclear fuel source simplifies thermonuclear weapons design over attempts to use pure deuterium (which must be kept cryogenically cold to stay in a liquid form, as was done as part of the Ivy Mike experiment in 1952), or to produce large amounts of gaseous tritium, which is prohibitively expensive. Lithium deuteride allows the tritium to be formed in situ as part of the reaction. The ignition and burn rate of LiD is much higher than pure deuterium, and the burn time is shorter than a pure deuterium-tritium reaction, and it requires a strong source of neutrons to sustain the tritium production cycle. It is also necessarily heavier than pure deuterium. But the practical advantages outweigh these deficits.
Before the Castle Bravo nuclear weapons test in 1954, it was thought that only the less common isotope would breed tritium when struck with fast neutrons. The Castle Bravo test showed (accidentally) that the more plentiful also does so in significant quantities under the extreme conditions of an exploding thermonuclear weapon, and the test yield was 2.5 times larger than predicted as a result of this additional tritium production.
Some lithium salts, which can be produced in LiH reactions, are toxic. LiH fire should not be extinguished using carbon dioxide, carbon tetrachloride, or aqueous fire extinguishers; it should be smothered by covering with a metal object or graphite or dolomite powder. Sand is less suitable, as it can explode when mixed with burning LiH, especially if not dry. LiH is normally transported in oil, using containers made of ceramic, certain plastics or steel, and is handled in an atmosphere of dry argon or helium. Whilst nitrogen can be used, it will react with lithium at elevated temperatures. LiH normally contains some metallic lithium, which corrodes steel or silica containers at elevated temperatures.
target="_blank" rel="nofollow"> University of Southampton, Mountbatten Centre for International Studies, Nuclear History Working Paper No5.
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