Decomposition characteristics of C₄F₇N-based SF₆-alternative gas mixtures

C₄F₇N [2,3,3,3-tetrafluoro-2-(trifluoromethyl)propanenitrile]/CO₂ gas mixtures are being developed as an eco-friendly electrical insulator to replace SF₆, the most potent greenhouse industrial gaseous dielectric. However, recent studies have reported complicated and often conflicting decomposition pathways for C₄F₇N/CO₂ gas mixtures, which has raised concerns. In this work, the decomposition characteristics of C₄F₇N/CO₂ gas mixtures were studied comprehensively by both designed computations and experiments. Computations were performed starting from fundamental propositions of C₄F₇N/CO₂ decompositions, which were further experimentally verified by pyrolysis, long-term thermal aging with/without catalytic materials (industrial-grade molecular sieves 4A), and electrical decomposition by spark discharge. The results of both computations and experiments suggest that in an ideal thermal decomposition, C₄F₇N is likely to decompose into C₂F₆ and small fluoronitriles first at high temperatures. The generation of C₃F₆ and C₂N₂ from C₄F₇N thermal decomposition at lower temperatures appears because of the catalytic effect of incompatible materials, for example, the industrial-grade molecular sieves 4A that we tested. The electron impact dissociation of C₄F₇N plays an important role in C₄F₇N electrical decomposition, leading to additional formation of distinctive small molecules of CF₄ and C₂N₂ of low concentrations. It was pointed out based on a real arcing test in a load disconnector that the decomposition of C₄F₇N gas mixtures in real applications will be at a much moderate and manageable rate than what was obtained from the highly accelerated laboratory tests presented in this work. The signatures of decomposition products extracted in this study provide invaluable guidance for developing decomposition-based diagnosis and fixation of decomposition byproducts toward SF₆-free power grids.