Abstract
As a widely used gas insulator, sulfur hexafluoride (SF6) has a large cross section for electron absorption, which may make the molecule ionized to the -1 charge state in the high-voltage environment. Using ab initio calculations, we show that the absorbed electron is located averagely on the six F atoms, occupying the antibonding level of the s-p σ bonds and increasing the S-F bond length. The ionized SF6- molecule decreases its decomposition energy to only 1.5 eV, much lower than that of the neutral molecule (4.8 eV), which can be understood according to the occupying of the antibonding orbital and thus weakening of the s-p σ bonds. The weakening of the bonds results in an obvious red shift in the vibrational modes of the ionized SF6- molecule by 120-270 cm-1, compared to those of the neutral molecule. The detailed origin of these vibrational modes is analyzed. Since the appearance of the ionized SF6- molecules is before the decomposition reaction of the SF6- molecule into low-fluoride sulfides, this method may improve the sensitivity of the defection of the partial discharge and save more time for the prevention of the insulation failure in advance.
Highlights
IntroductionAs a hypervalent molecule with the high octahedral Oh symmetry, the sulfur hexafluoride (SF6) has attracted wide attention for both the fundamental scientific interest and practical industrial application.[1,2,3,4,5,6] Firstly, it is widely used as the gaseous dielectric medium in the high-voltage electrical equipment such as the gas insulated switchgear (GIS) and circuit breaker because of a series of advantages in its properties, including the excellent insulating (very high electric strength, almost two times higher than that of air) and arc-extinguishing properties, non-corrosivity, non-toxicity, non-polarity, and non-flammability.[1,2,3,4] Secondly, SF6 is an important dopant in the polymer science because the electrical conductivity of the polymers doped by SF6 can be increased by several orders of magnitude after the electron-beam irradiation,[5,6] e.g., from 10-8 to 10-3 S/cm for the polythiophene doped by SF6.7 Thirdly, SF6 is the most potent greenhouse gas with a global warming potential of 23,900 times that of CO2 according to the Intergovernmental Panel on Climate Change
Structural relaxation shows that the S-F bond length in the neutral SF6 molecule is 1.60 Å with the generalized gradient density approximation (GGA) functional and 1.57 Å with the local density approximation (LDA) functional
When an electron is absorbed on the molecule, the structural relaxation shows that the bond length is increased, no matter using the GGA or LDA functional
Summary
As a hypervalent molecule with the high octahedral Oh symmetry, the sulfur hexafluoride (SF6) has attracted wide attention for both the fundamental scientific interest and practical industrial application.[1,2,3,4,5,6] Firstly, it is widely used as the gaseous dielectric medium in the high-voltage electrical equipment such as the gas insulated switchgear (GIS) and circuit breaker because of a series of advantages in its properties, including the excellent insulating (very high electric strength, almost two times higher than that of air) and arc-extinguishing properties, non-corrosivity, non-toxicity, non-polarity, and non-flammability.[1,2,3,4] Secondly, SF6 is an important dopant in the polymer science because the electrical conductivity of the polymers doped by SF6 can be increased by several orders of magnitude after the electron-beam irradiation,[5,6] e.g., from 10-8 to 10-3 S/cm for the polythiophene doped by SF6.7 Thirdly, SF6 is the most potent greenhouse gas with a global warming potential of 23,900 times that of CO2 according to the Intergovernmental Panel on Climate Change It is very stable and inert in the troposphere and stratosphere with an estimated atmospheric lifetime of 800–3200 years.[8] the emission and linking of SF6 gas are strictly controlled nowadays, and its formation and decomposition are attracting intensive research interests.[2,5,9,10,11,12].
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