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Lithium hydride is an inorganic compound with the formula LithiumHydride. 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 corresponding lithium-6 deuterium ( or 6LiD) is the primary nuclear fusion fuel in thermonuclear weapons. In hydrogen warheads of the Teller–Ulam design, a nuclear fission trigger explodes to heat and compress the lithium-6 deuteride, and to bombard the 6LiD with to produce tritium in an exothermic reaction:
Before the Castle Bravo nuclear weapons test in 1954, it was thought that only the less common isotope 6Li would breed tritium when struck with fast neutrons. The Castle Bravo test showed (accidentally) that the more plentiful 7Li also does so under extreme conditions, albeit by an endothermic reaction.
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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