We continue to review some of the most important materials in heat treatment and metallurgy.
Fluorine (chemical symbol: F)
Fluorine is a pale-yellow, highly reactive gas with a very strong odor. Elemental fluorine, F2, is the most reactive chemical element in existence. Special equipment and training are necessary to handle it safely. Fluorine is so reactive that it only exists in combination with other elements. When exposed to water it reacts violently to produce oxygen and the hydrofluoric acid, which is so corrosive it can etch glass.
The most common source of fluorine is fluorite (Fig. 1), or fluorspar (CaF2), a stable, nonreactive rock, which is plentiful worldwide, although China and Mexico are the major suppliers. In its purest form it is clear, but it exists in a wide range of colors, depending on what impurities in contains. It can also be tinted by heating or irradiation or structural defects within the lattice
Fluorine was discovered in 1886 by French chemist Henri Moissan, who collected the gas by passing an electric current through one of its compounds – hydrogen fluoride. His research was interrupted four times by fluorine poisoning. Since chemists had been trying to isolate the dangerous element for over 70 years, this was considered a great accomplishment; research on fluorine was so hazardous that several 19th-century experimenters were deemed "fluorine martyrs" after being killed or blinded.
Moissan received the Nobel prize for his efforts, with the following citation: “In recognition of the great services rendered by him in his investigation and isolation of the element fluorine ... The whole world has admired the great experimental skill with which you have studied that savage beast among the elements.” A very talented and important figure in the field of chemistry, Moissan is also credited for contributions to the manufacture of artificial diamonds and the invention of the arc furnace.
Fluorine is so volatile that it causes finely ground metals, glass, ceramics, carbon and even water to burn with a bright flame. It is this reactivity that lends its use in rocket fuel. Before World War II, there was no commercial market for elemental fluorine. The Manhattan project and nuclear-energy applications, however, made it necessary to produce large quantities. Fluorine is also used in producing uranium from hexafluoride. This continues to be the largest use for fluorine, comprising 75% of world demand.
Fluorine takes many forms and has a wide range of uses in industry, science and consumer goods (Fig. 2). It is used in the manufacture of steel, as well as the production of computer chips, microelectronic sensors, infrared lenses and television screens. High-performance plastics, such as Teflon, are manufactured using fluorinated compounds. To help prevent tooth decay, fluorine is added to mouthwashes, toothpaste and municipal water supplies.
Here are some interesting facts about fluorine.
- Melting point: 53.48 K (−219.67°C, −363.41°F)
- Boiling point: 85.03 K (−188.11°C, −306.60°F)
- Density (at STP): 1.696 g/L
- Density when liquid (at b.p.): 1.505 g/cm3
- Triple point: 53.48 K, 90 kPa
- Critical point: 144.41 K, 5.1724 MPa
- Heat of vaporization: 6.51 kJ/mole
- Molar heat capacity: Cp: 31 J/(mole·K) (at 21.1°C)
- Cv: 23 J/(mole·K) (at 21.1°C)
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