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Abstract
C4F7N is one of the most remarkable replacements for SF6, and its decomposition mechanism has a great influence on insulating performance and environmental properties. It is noteworthy that discharges or high temperature also evaporates metal electrodes (e.g., Cu) in the equipment, and the generated metal gases interact with C4F7N and thus affect the C4F7N decomposition mechanism, but the decomposition mechanism is still not clear. In this paper, therefore, the B3LYP method in conjunction with 6-311G(d, p) basis set (for C, N, and F atoms) and Lanl2DZ basis set (for Cu atom) combining transition state theory is used to study the decomposition mechanism of C4F7N-Cu mixtures. 31 reactions are determined in decomposition pathways of C4F7N-Cu mixtures, and their potential energy surface as well as reaction mechanisms are obtained. The rate constants over 300 K–3500 K relevant to the insulation breakdown temperature are calculated based on the above calculations, and dominant reactions in different temperature regions are selected. The results show that (R14) C4F7N + Cu → CF3CFCN + CF2CuF plays a major role in the decomposition of C4F7N-Cu mixtures below 1500 K, while (R1) C4F7N + Cu → TSa1 → CuCN + C3F7 and (R21) C4F7N + Cu → TSc1 → CF3CF(CN)CF2 + CuF are dominant above 1500 K; (R23) CF3CF(CN)CF2 + Cu → CF2(Cu)C(F)CN + CF3 is the most important reaction leading to the generation of CF3 below 1500 K with the overwhelming rate constant, but other reactions also generating CF3 are dominant above 1500 K.
Highlights
SF6 is widely used as an insulating and arc-quenching medium in power equipment, but its global warming potential (GWP) is 23 900 times as much as CO2 and its decomposition products (e.g., SO2F2, SO2, and SOF2) under discharges or high temperature are poisonous
Our previous work10 studied the decomposition mechanism of C4F7N, and the results indicated that 16 reactions are included in its decomposition pathway and the dominant reactions and species are selected by analysis of the rate constant
Basic atoms (C, N, Cu, and F) and charged species will be theoretically generated after multiple chemical reactions, but the overall rate constants are usually very low and their contents are much less than other decomposition products under the rapid decreasing rate of temperature in electric breakdown
Summary
SF6 is widely used as an insulating and arc-quenching medium in power equipment, but its global warming potential (GWP) is 23 900 times as much as CO2 and its decomposition products (e.g., SO2F2, SO2, and SOF2) under discharges or high temperature are poisonous. Great efforts have been taken to explore environmental-friendly alternative gases for SF6 in consideration of environment and safety issues.. As one of the most promising candidates for SF6 gas, C4F7N with excellent insulation properties and low GWP (i.e., 1/6 of SF6) has attracted widespread attention.. A small proportion of C4F7N is usually mixed with buffer gases to decrease the condensing point, but its insulating capacity is still comparable to SF6. The decomposition products of C4F7N and its mixtures meet safety requirements. Environmental-friendly power equipment filled with C4F7N have been developed with excellent insulating behavior
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