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Engineering materials
Published in William Bolton, R.A. Higgins, Materials for Engineers and Technicians, 2020
An atom of lithium contains three electrons in orbit around its nucleus, two of these completing the first shell and leaving a lone electron in the second shell. The atom of fluorine contains nine electrons in orbit, two of these filling the first shell with seven in the second shell. The force of attraction between the positive nucleus of lithium and its outer lone electron is comparatively weak and it is easily snatched away so that it joins the outer electron shell of the fluorine atom. Because the lithium atom has lost an electron, it now has a resultant positive charge, whilst the fluorine atom has gained an electron and so has a resultant negative charge. Charged atoms of this type are called ions (Figure 1.3). Metals always form positively charged ions, because they are always able to easily lose electrons, whilst non-metals form negatively charged ions. As these lithium and fluorine ions carry opposite charges, they will attract each other. In a solid, the lithium and fluorine ions arrange themselves in a geometrical pattern in which each fluorine ion is surrounded by six lithium ions as its nearest neighbours, whilst each lithium ion is surrounded by six fluorine ions (Figure 1.4). The compound lithium fluoride forms a relatively simple cubic type of crystal structure. Other such salts may form more complex crystal patterns, depending upon the relative sizes of the ions involved and the electrical charges carried by each type of ion. Here we have dealt with atoms which lose, or gain, only one electron; those which lose, or gain, two electrons will produce ions carrying twice the electrical charge.
Manufacture of Carbon Articles
Published in R. Robert Paxton, Manufactured Carbon: A Self-Lubricating Material for Mechanical Devices, 1979
The three impregnants described so far were chosen for their ability to decrease the permeability of carbon and strengthen its structure. In these grades, the carbon body itself normally generates the transfer film so necessary for a low friction coefficient and low wear rate. Carbon, however, cannot generate a transfer film in the absence of polar fluids, a situation that is found in vacuum systems, dry gas systems, and most cryogenic applications. These applications require the aid of a supplementary film former such as the lithium fluoride used in P-5N. In addition to its value as a supplementary film former, lithium fluoride is substantially insoluble in water, thermodynamically stable, and has a high melting point.
Physical Properties of Crystalline Infrared Optical Materials
Published in Paul Klocek, Handbook of Infrared Optical Materials, 2017
James Steve Browder, Stanley S. Ballard, Paul Klocek
Notes: When lithium fluoride is grown in a vacuum, the absorption at 2.8 µm attributed to the H–F band disappears. Cylindrical castings of diameter 6 in. are available. Lithium fluoride is only slightly soluble in water but can be dissolved in acids.
Heat-Pipe Heat Exchangers for Salt-Cooled Fission and Fusion Reactors to Avoid Salt Freezing and Control Tritium: A Review
Published in Nuclear Technology, 2020
Bahman Zohuri, Stephen Lam, Charles Forsberg
Heat transfer from the primary salt coolant in a fluoride-salt-cooled high-temperature reactor (FHR), molten salt reactor (MSR), or salt-cooled fusion reactor presents two challenges not seen in water-, sodium-, and helium-cooled reactors: (1) liquid salts have melting points above 400°C and must not freeze to avoid blocking of primary coolant flow and (2) the salt coolant generates tritium that must not be allowed to escape to the environment. Lithium fluoride is used in coolant salts to lower the melting point of the coolant salt; however, neutron irradiation of lithium generates tritium. Isotopically separated 7Li is used in salt fission systems to minimize neutron adsorption and tritium production. Isotopically separated 6Li is used in fusion systems to maximize tritium production because tritium is the fuel for a fusion reactor. Tritium production rates in salt-cooled fusion machines are about three orders of magnitude larger than in fission machines.