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 20th 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
For examine chemical properties of SF6 gas, we first introduce structure of SF6 molecule. In this gas molecule, one sulphur 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 500oC. It is highly non-flammable. H2O and Cl cannot react with this gas. It also does not react with acid.
The SF6 gas is one of the heaviest gases. Density of this gas at 20oC at one atmospheric pressure, is about 6.139 kg/m3 which is about 5 times higher than air at same conditions. The molecular weight of this gas is 146.06. The variation of pressure with temperature is linear for sulfur hexafluoride and it is small within the service temperature, i.e. from – 25 to + 50oC. The volumetric specific heat of this gas is also high. It is around 3.7 times more than that of air, and that is why this gas also has a tremendous cooling effect in electrical equipment. The thermal conductivity of this gas is not very high it is even lower than air. Still it is quite suitable for cooling effect in circuit breaker. It is because, during dissociation of sulfur hexafluoride molecules around the electric arc, these molecules absorb a high amount of heat. This heat is then released when the molecules reform at the periphery of the arc. This process helps to transfer heat from a hot region to cool region very rapidly. That is why; this gas has an excellent cooling effect at high temperature although the thermal conductivity of SF6 is not very high.
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
|Density at 20oC||6.14 kg/m3|
|Color of Gas||colourless|
|Thermal Conductivity||0.0136 w/mK|
|Critical Density||730 Kg/m3|
|Critical Pressure||3.78 MPa|
|Sound Velocity in SF6||136 m/s. It is 3 times less than that in air|
|Formation Heat||-1221.66 Kg/mol|
|Specific Heat||96.6 j/mole K|
|Breakdown Field Relative to Pressure||89 V/m Pa|
|Relative Dielectric Constant at 25oC and 1 bar absolute||1.00204|
|Dissipation Factor or tanδ at 25oC and 1 bar absolute||<2 × 10-7|