Lithium hydride structure, properties, production, uses

The lithium hydride it is a crystalline inorganic solid whose chemical formula is LiH. It is the lightest inorganic salt, its molecular weight is only 8 g / mol. It is formed by the union of a lithium ion Li⁺ and a hydride ion H^-. Both are linked by an ionic bond.

LiH has a high melting point. It reacts easily with water and hydrogen gas is produced in the reaction. It can be obtained by the reaction between molten lithium metal and hydrogen gas. It is widely used in chemical reactions to obtain other hydrides.

LiH has been used to protect against dangerous radiation such as those found in nuclear reactors, that is, ALPHA, BETA, GAMMA radiation, protons, X-rays and neutrons..

It has also been proposed for the protection of materials in space rockets powered by nuclear thermal propulsion. Studies are even being carried out to be used as protection of the human being against cosmic radiation during future trips to the planet Mars..

Article index

• 1 Structure
• 2 Nomenclature
• 3 Properties
  • 3.1 Physical state
  • 3.2 Molecular weight
  • 3.3 Melting point
  • 3.4 Boiling point
  • 3.5 Autoignition temperature
  • 3.6 Density
  • 3.7 Solubility
  • 3.8 Other properties
• 4 Obtaining
• 5 Use as a protective shield against dangerous radiation
  • 5.1 - In nuclear reactors
  • 5.2 - In space systems of nuclear thermal propulsion
  • 5.3 - In protection against cosmic radiation
• 6 Use as a means of safe storage and transport of hydrogen
• 7 Use in chemical reactions
• 8 References

Structure

Hydrogen in lithium hydride has a negative charge H^-, since it has subtracted an electron from the metal, which is in the form of a Li ion⁺.

The electron configuration of the Li cation⁺ is: [He] 1s^two which is very stable. And the electronic structure of the hydride anion H^- is: 1s^two, which is also very stable.

The cation and anion are joined by electrostatic forces.

The lithium hydride crystal has the same structure as sodium chloride NaCl, that is, a cubic crystal structure.

Nomenclature

- Lithium hydride

- LiH

Properties

Physical state

White or colorless crystalline solid. Commercial LiH may be blue-gray due to the presence of small amounts of lithium metal..

Molecular weight

8 g / mol

Melting point

688 ºC

Boiling point

It decomposes at 850 ºC.

Autoignition temperature

200 ºC

Density

0.78 g / cm³

Solubility

Reacts with water. It is insoluble in ethers and hydrocarbons.

Other properties

Lithium hydride is much more stable than hydrides of the other alkali metals and can be melted without decomposition..

It is not affected by oxygen if it is heated to temperatures below red. It is also unaffected by chlorine Cl_two and hydrochloric acid HCl.

The contact of LiH with heat and humidity causes an exothermic reaction (generates heat) and evolution of hydrogen H_two and lithium hydroxide LiOH.

It can form a fine dust that can explode in contact with flames, heat, or oxidizing materials. It must not come into contact with nitrous oxide or liquid oxygen, as it may explode or ignite.

It darkens when exposed to light.

Obtaining

Lithium hydride has been obtained in the laboratory by the reaction between molten lithium metal and hydrogen gas at a temperature of 973 K (700 ºC).

2 Li + H_two → 2 LiH

Good results are obtained when the exposed surface of the molten lithium is increased and the sedimentation time of LiH is decreased. It is an exothermic reaction.

Use as a protective shield against dangerous radiation

LiH has a number of characteristics that make it attractive for use as protection for humans in nuclear reactors and space systems. Here are some of these characteristics:

- It has a high content of hydrogen (12.68% by weight of H) and a high number of hydrogen atoms per unit volume (5.85 x 10²² H atoms / cm³).

- Its high melting point allows it to be used in high temperature environments without melting.

- It has a low dissociation pressure (~ 20 torr at its melting point) which allows the material to be melted and frozen without degrading under low hydrogen pressure..

- It has a low density which makes it attractive for use in space systems..

- However, its disadvantages are its low thermal conductivity and poor mechanical properties. But this has not diminished its applicability.

- The LiH parts that serve as shields are manufactured by hot or cold pressing and by melting and pouring into molds. Although this last form is preferred.

- At room temperature the parts are protected from water and water vapor and at high temperatures by a small overpressure of hydrogen in a sealed container.

- In nuclear reactors

In nuclear reactors there are two types of radiation:

Directly ionizing radiation

They are highly energetic particles that carry electrical charge, such as alpha (α) and beta (β) particles and protons. This type of radiation interacts very strongly with the materials of the shields, causing ionization by interacting with the electrons of the atoms of the materials through which they pass..

Indirectly ionizing radiation

They are neutrons, gamma rays (γ) and X rays, which are penetrating and require massive protection, since they involve the emission of secondary charged particles, which are the ones that cause ionization..

According to some sources, LiH is effective in protecting materials and people against these types of radiation.

- In space systems of nuclear thermal propulsion

LiH has recently been chosen as a potential nuclear radiation shielding and moderator for very long-voyage spacecraft nuclear thermal propulsion systems..

Its low density and high hydrogen content makes it possible to reduce the mass and volume of the nuclear powered reactor effectively..

- In protection against cosmic radiation

Exposure to space radiation is the most important risk to human health in future interplanetary exploration missions.

In deep space astronauts will be exposed to the full spectrum of galactic cosmic rays (high energy ions) and solar particle ejection events (protons)..

The danger of radiation exposure is compounded by the length of the missions. In addition, the protection of the places that explorers will inhabit must also be considered.

In this vein, a study carried out in 2018 indicated that LiH provides the greatest reduction in radiation per gram per cm among the materials tested.^two, thus being one of the best candidates to be used in the protection against cosmic radiation. However, these studies must be deepened.

Use as a safe storage and transport medium for hydrogen

Obtaining energy from H_two It is something that has been studied for several dozen years and has already found application to replace fossil fuels in transport vehicles.

The H_two can be used in fuel cells and contribute to the reduction of CO production_two and no_x, thus avoiding the greenhouse effect and pollution. However, an effective system for storing and transporting H has not yet been found_two securely, lightweight, compact, or small in size, quick to store and release the H_two equally fast.

Lithium hydride LiH is one of the alkali hydrides that has the highest storage capacity for H_two (12.7% by weight of H). Release H_two by hydrolysis according to the following reaction:

LiH + H_twoO → LiOH + H_two

LiH supplies 0.254 Kg of hydrogen for every Kg of LiH. In addition, it has a high storage capacity per unit volume, which means that it is lightweight and is a compact medium for storage of H_two.

Additionally, LiH forms more easily than other alkali metal hydrides and is chemically stable at ambient temperatures and pressures. The LiH can be transported from the manufacturer or supplier to the user. Then, by hydrolysis of LiH, H is generated_two and this is used safely.

The lithium hydroxide LiOH formed can be returned to the supplier that regenerates the lithium by electrolysis, and then produces LiH again..

LiH has also been successfully studied to be used together with borated hydrazine for the same purpose..

Use in chemical reactions

LiH allows the synthesis of complex hydrides.

It is used, for example, to prepare lithium triethylborohydride, which is a powerful nucleophile in organic halide displacement reactions..

References

1. Sato, Y. and Takeda, O. (2013). Hydrogen Storage and Transportation System through Lithium Hydride Using Molten Salt Technology. In Molten Salts Chemistry. Chapter 22, pages 451-470. Recovered from sciencedirect.com.
2. U.S. National Library of Medicine. (2019). Lithium Hydride. Recovered from: pubchem.ncbi.nlm.nih.gov.
3. Wang, L. et al. (2019). Investigation on the impact of thermal-kernel effect of lithium hydride on the reactivity of nuclear propulsion particle bed reactor. Annals of Nuclear Energy 128 (2019) 24-32. Recovered from sciencedirect.com.
4. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
5. Giraudo, M. et al. (2018). Accelerator-Based Tests of Shielding Effectiveness of Different Materials and Multilayers using High-Energy Light and Heavy Ions. Radiation Research 190; 526-537 (2018). Recovered from ncbi.nlm.nih.gov.
6. Welch, F.H. (1974). Lithium hydride: A space age shielding material. Nuclear Engineering and Design 26, 3, February 1974, pages 444-460. Recovered from sciencedirect.com.
7. Simnad, M.T. (2001). Nuclear Reactors: Shielding Materials. In Encyclopedia of Materials: Science and Technology (Second Edition). Pages 6377-6384. Recovered from sciencedirect.com.
8. Hügle, T. et al. (2009). Hydrazine Borane: A Promising Hydrogen Storage Material. J. Am. Chem. Soc. 2009, 131, 7444-7446. Recovered from pubs.acs.org.