SF6 Gas or Sulfur Hexafluoride Gas Properties

History of SF₆

SF₆ or sulfur hexafluoride gas molecules are combined by one sulfur and six fluorine atoms. This gas was first realized in the year 1900 in the laboratories of the Faculte de Pharmacie de, in Paris. In the year of 1937, General Electrical Company first realized that SF₆ gas could be used as gaseous insulating material. After the Second World War, i.e. in the middle of 20^th century, the popularity of using sulfur hexafluoride gas as an insulating material in electrical system was rising very rapidly. Allied Chemical Corporation and Pennsalt were the first American industries, who began to produce this gas commercially in 1948. During 1960, using of sulfur hexafluoride gas in high voltage switchgear became popular. As the demand for this gas was increasing, many manufacturers in Europe and America started producing SF₆ gas in large scale, during that time. In the beginning, SF6 gas was only used for the insulating purpose in the electrical system. But soon it was realized that this gas has tremendous arc quenching property. Hence, this gas also began to be used in circuit breaker as arc quenching medium. World’s first SF₆ gas insulated substation was established in Paris in the year 1966. Sulphur hexafluoride medium voltage circuit breakers launched into the market from 1971.

Manufacturing of SF6 Gas

SF6 gas is commercially manufactured by the reaction of fluorine (obtained by electrolysis) with sulfur.
During process of producing of this gas, other byproducts like SF₄, SF₂, S₂F₂, S₂F₁₀ are also produced in small percentages. Not only these byproduct, impurities like air, moisture, and CO₂ are also present in the gas, during production. All these byproducts and impurities are filtered at different stages of purification to get the pure and refined final product.

Chemical Properties of SF6 Gas

To understand the chemical properties of SF6 gas, we first look at its molecular structure. In an SF6 molecule, one sulfur atom is surrounded by six fluorine atoms.

The sulfur has atomic number of 16. Electronic configuration of sulphur atom is 2, 8, 6 i.e. 1S² 2S² 2P⁶ 3S² 3P⁴. The fluorine atom has atomic number 9. The electronic configuration of fluorine is 1S² 2S² 2P⁵. Each sulphur atom in SF₆ molecule creates a covalent bond with 6 fluorine atoms. In this way, sulfur atom gets total 6 covalent bonds, i.e. 6 pairs of electrons at its outer shell, and each fluorine atom gets 8 electrons in its outer most shell.

NB: – Here we can observe that, in sulfur hexafluoride external shell of sulphur atom has 12 electrons instead of 8 electrons. That means here sulfur does not obey general octal rule of atomic structure which states that, a stable atom requires 8 electrons at its outermost shell. This is not an exceptional case. Some elements in the 3rd period and below can form a compound that exceeds 8 electrons in its outer most shell. The molecular structure of this gas is shown below,
In this way, SF6 fully satisfies a stable structural condition. The effective radius of a sulfur hexafluoride molecule is 2.385 A. This electronic configuration and structure of this gas make SF₆ extremely stable. The gas can be stable without any decomposition in its molecular structure up to 500^oC. It is highly non-flammable. H₂O and Cl cannot react with this gas. It also does not react with acid.

SF6 gas is one of the heaviest gases, with a density of 6.139 kg/m³ at 20°C and one atmospheric pressure, about five times denser than air. Its molecular weight is 146.06. The pressure-temperature variation is linear within the service range of -25 to +50°C. SF₆ has a high volumetric specific heat, about 3.7 times that of air, giving it excellent cooling properties in electrical equipment. Despite its low thermal conductivity, SF₆ is effective for cooling in circuit breakers because the gas absorbs and releases heat during molecular dissociation and reformation around an electric arc, quickly transferring heat from hot to cool areas.

Electrical Properties of SF₆ Gas

SF₆ gas is highly electronegative. Due to high electronegativity, it absorbs free electrons which are produced due to arcing between contacts of circuit breaker. Combination of free electrons with molecules produces heavy and big ions, which have very low mobility. Because of the absorption of free electrons and low mobility of ions SF6 has very excellent dielectric property. The dielectric strength of SF6 gas is about 2.5 times more than that of air.

List of Sulphur Hexafluoride Gas Properties