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

The compositions of SF₆ arc 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 Lanl2DZ 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 → IM_x → TS_y (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.