Theoretical study of the decomposition mechanism of SF6/Cu gas mixtures

The compositions of SF6 are plasma are contaminated by Cu vapor through electrode erosion and thus affect the performance of the high-voltage SF6 circuit breaker. But the essential data for chemical kinetic models to study the non-equilibrium compositions of SF6/Cu mixtures, namely the decomposition mechanism of SF6/Cu mixtures, is still not clear. Besides, SF6 decomposition products can be used to diagnose insulation faults of GIS and to evaluate the electric life of the SF6 circuit breaker, but the uncertainty of the decomposition mechanism of SF6/Cu mixtures hinders the application of the analysis method of SF6 decomposition products. Therefore, this work is devoted to investigating the decomposition mechanism of SF6/Cu mixtures by means of density functional theory in conjunction with 6-311G(d,p) basis set (for S and F atoms) and Lan12DZ basis set (for the Cu atom). The optimized molecular structures, harmonic vibrational frequencies and energetic information of reactants, intermediate products (IM), transition states (TS) and products are thoroughly studied. And the complete decomposition pathways of SF6/Cu mixtures involving 15 reactions are derived. Detailed potential energy surfaces of SF6 + Cu decomposition reactions are also depicted as a result. Among all the decomposition pathways, the favorable decomposition reactions of SF6 + Cu mixtures are found: reaction R1 contributes more than reaction R2 in the decomposition of SF6 + Cu mixtures due to its lower energy released; reaction R3 is dominant in the decomposition of SF5 + Cu other than multi-step reactions; the chemical processes of SF4 + Cu -> IMx -> TSy (where x stands for 9, 10 and 13, and y denotes 7, 8 and 9) may play the same role in SF4 + Cu decomposition, but the reaction rate of R6 is higher than other reactions; reaction R9 and R11 are relative favorable among SF3 + Cu decomposition reactions; reaction R13 is dominant in the decomposition of SF2 + Cu because of its high potential well. This work hopes to outline a theoretical basis to investigate the non-equilibrium compositions of SF6/Cu arc plasma and the SF6 decomposition products analysis method.