Analysis of Breakdown and Decomposition Characteristics of C₄F₇N/N₂/O₂ Ternary Mixed Gas Based on Density Functional

Objective In the field of electrical equipment insulation, the application of gas insulation is becoming increasingly prevalent. SF₆ gas has been extensively utilised due to its stable chemical properties and its excellent insulation and arc-quenching capabilities. Nevertheless, SF₆ is a powerful greenhouse gas with the potential to persist in the atmosphere, underscoring the urgency of identifying a suitable replacement. C₄F₇N, with its superior insulation properties and environmental benefits, has the potential to be developed as an eco-friendly insulating gas that could replace SF₆. Owing to the elevated liquefaction temperature of C₄F₇N, it is frequently employed in engineering applications in conjunction with other gases including CO₂ and N₂. At present, CO₂ is predominantly employed as a buffer gas for C₄F₇N, typically constituting in excess of 90% of the mixture. Nevertheless, as CO₂ remains a greenhouse gas with a substantial greenhouse effect, the utilisation of N₂ as a buffer gas is egarded as a means of markedly enhancing the environmental benignity of the mixture. Nevertheless, the C₄F₇N/N₂ mixture may undergo decomposition during discharge, which could result in the formation of solid carbon deposits on metal electrodes. This phenomenon has the potential to significantly impair the normal operation of gas-insulated equipment. Some researchers have put forth the suggestion of incorporating oxygen as a second buffer gas into C₄F₇N mixtures with the aim of enhancing the arc-quenching performance of binary mixtures. The incorporation of O₂ can facilitate the breakdown characteristics of C₄F₇N/N₂ binary mixtures, while simultaneously inhibiting the formation of decomposition products. Nevertheless, the microscopic precise mechanism by which O₂ exerts its influence on the breakdown and decomposition characteristics of C₄F₇N/N₂ mixtures remains unclear. The objective of this study is to investigate the microscopic mechanism by which oxygen influences the breakdown and decomposition characteristics of C₄F₇N/N₂ binary mixtures.Methods This study employs density functional theory to determine the electrostatic potentials of the gas molecules present in C₄F₇N/N₂/O₂ mixtures. The electron affinity and electronegativity of O₂ are calculated, as are those of the C₄F₇N/N₂ mixture. Molecular dynamics methods based on the principle of energy optimisation are employed to derive the structural formulas of N₂/O₂ and C₄F₇N with varying molecular quantities (1, 2, 3, and 4 molecules). The interaction energy and charge transfer between varying quantities of N₂ and O₂ with C₄F₇N gas are calculated. The Fukui function of the C₄F₇N molecule is calculated based on Hirshfeld atomic charges, and the bond dissociation energies of C₄F₇N/N₂/O₂ gas molecules are determined. The quantum chemistry calculations employ the B3LYP functional and the 6–<italic>G</italic>(<italic>d</italic>,<italic>p</italic>) basis set, while the molecular dynamics simulations utilise the COMPASS force field. The results of the calculations are used to analyze the radical reaction processes involved in the discharge decomposition of C₄F₇N with O₂. This analysis investigates the impact of O₂ on the breakdown and decomposition characteristics of C₄F₇N/N₂ mixtures from a microscopic perspective.Results and Discussions The findings demonstrate that O₂ exhibits a greater electron affinity and electronegativity in comparison to N₂. The incorporation of O₂ results in a reduction of free electrons within the gaseous environment, thereby diminishing the likelihood of collision ionisation during the discharge process. This in turn, gives rise to an enhancement in the breakdown voltage of C₄F₇N/N₂ mixtures. In the case of N₂/O₂ gases, it can be observed that as the number of molecules present increases, the interaction energy also increases approximately in proportion. The interaction energy between O₂ and C₄F₇N is approximately five times that between N₂ and C₄F₇N. In the C₄F₇N/O₂ molecular system, the C₄F₇N molecule is observed to a negative charge, indicating that electrons are gained by C₄F₇N within the system. As the number of molecules increases, C₄F₇N gains more electrons, resulting in an overall increase in charge transfer. In the C₄F₇N/N₂ system, the C₄F₇N molecule is observed to carry a positive charge, indicating that electrons are lost by C₄F₇N within the system. As the number of molecules increases, C₄F₇N loses electrons in greater quantities. In the absence of discharge decomposition, the interactions between the buffer gas and C₄F₇N molecules are predominantly physical in nature. During the discharge process, O₂ provides a substantial number of oxygen radicals. Once the chemical bonds of C₄F₇N have been broken, forming small molecular radicals, these react with <inline-formula><tex-math id="Z-20240904174743">${\rm O}^{\,\cdot}$</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202301042_Z-20240904174743.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202301042_Z-20240904174743.png"/></alternatives></inline-formula> which has been produced by the homolytic cleavage of O₂. The addition of O₂ results in the consumption of the <inline-formula><tex-math id="Z-20240904174656">${\rm CF}_2^{\,\cdot} $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202301042_Z-20240904174656.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202301042_Z-20240904174656.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20240904174900">${\rm C}^{\,\cdot} $</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202301042_Z-20240904174900.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="202301042_Z-20240904174900.png"/></alternatives></inline-formula> radicals produced by the decomposition of C₄F₇N, thereby reducing the adherence of elemental carbon on the surface of metal electrodes.Conclusions In conclusion, the addition of oxygen can enhance the decomposition characteristics of the C₄F₇N/N₂/O₂ ternary gas mixture. Nevertheless, further research is required to ascertain the precise molecular ratio of the C₄F₇N/N₂/O₂ mixture.