基于密度泛函的C_(4)F_(7)N/N_(2)/O_(2)三元混合气体击穿及分解特性分析

Analysis of Breakdown and Decomposition Characteristics of C_(4)F_(7)N/N_(2)/O_(2)Ternary Mixed Gas Based on Density Functional

C_(4)F_(7)N具有优异的绝缘性能和环保特性,是有潜力替代SF_(6)的环保型绝缘气体。目前,主要采用CO_(2)作为C_(4)F_(7)N的缓冲气体,其含量一般大于90%,由于CO_(2)为温室气体,将CO_(2)替换为N_(2)后与C_(4)F_(7)N混合,会使混合气体环保性大大提升。虽然,C_(4)F_(7)N/N_(2)混合气体在放电时可能有固体碳析出情况,严重影响气体绝缘设备正常运行,但是,O_(2)的加入能够改善C_(4)F_(7)N/N_(2)二元混合气体的击穿特性,抑制分解产物的产生。为此,本文基于密度泛函理论计算了O_(2)及C_(4)F_(7)N/N_(2)混合气体的电子亲和能、电负性、不同分子数量的O_(2)和N_(2)与C_(4)F_(7)N气体之间的相互作用能及电荷转移量,得出了C_(4)F_(7)N分子的福井函数、C_(4)F_(7)N/N_(2)/O_(2)各气体分子化学键的解离能,分析了C_(4)F_(7)N与O_(2)放电分解涉及的自由基反应过程,从微观角度研究O_(2)对C_(4)F_(7)N/N_(2)混合气体击穿特性及分解特性的影响。结果表明:相较于N_(2),O_(2)的电子亲和能及电负性更强,加入O_(2)有助于提高C_(4)F_(7)N/N_(2)混合气体的击穿电压。O_(2)-C_(4)F_(7)N的相互作用能约为N_(2)-C_(4)F_(7)N相互作用能的5倍;随着缓冲气体分子数量的增加,缓冲气体与C_(4)F_(7)N之间电荷转移量逐渐增加,使得C_(4)F_(7)N/O_(2)体系中C_(4)F_(7)N得到电子,而C_(4)F_(7)N/N_(2)体系中C_(4)F_(7)N失去电子。C_(4)F_(7)N未发生放电分解时,缓冲气体与C_(4)F_(7)N分子间主要以物理作用为主,O_(2)的加入消耗C_(4)F_(7)N分解产生的CF_(2)·与C·自由基,减少金属电极表面碳单质的附着。综上,O_(2)的加入能够减少碰撞电离,消耗C_(4)F_(7)N分解产生的自由基,抑制分解产物的产生,改善三元混合气体C_(4)F_(7)N/N_(2)/O_(2)的击穿特性及分解特性。

Objective In the field of electrical equipment insulation,the application of gas insulation is becoming increasingly prevalent.SF_(6)gas has been extensively utilised due to its stable chemical properties and its excellent insulation and arc-quenching capabilities.Nevertheless,SF_(6)is a powerful greenhouse gas with the potential to persist in the atmosphere,underscoring the urgency of identifying a suitable replacement.C_(4)F_(7)N,with its superior insulation properties and environmental benefits,has the potential to be developed as an eco-friendly insulating gas that could replace SF_(6).Owing to the elevated liquefaction temperature of C_(4)F_(7)N,it is frequently employed in engineering applications in conjunction with other gases including CO_(2)and N_(2).At present,CO_(2)is predominantly employed as a buffer gas for C_(4)F_(7)N,typically constituting in excess of 90%of the mixture.Nevertheless,as CO_(2)remains a greenhouse gas with a substantial greenhouse effect,the utilisation of N_(2)as a buffer gas is egarded as a means of markedly enhancing the environmental benignity of the mixture.Nevertheless,the C_(4)F_(7)N/N_(2)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 C_(4)F_(7)N mixtures with the aim of enhancing the arc-quenching performance of binary mixtures.The incorporation of O_(2)can facilitate the breakdown characteristics of C_(4)F_(7)N/N_(2)binary mixtures,while simultaneously inhibiting the formation of decomposition products.Nevertheless,the microscopic precise mechanism by which O_(2)exerts its influence on the breakdown and decomposition characteristics of C_(4)F_(7)N/N_(2)mixtures remains unclear.The objective of this study is to investigate the microscopic mechanism by which oxygen influences the breakdown and decomposition characteristics of C_(4)F_(7)N/N_(2)binary mixtures.Methods This study employs density functional theory to determine the electrostatic potentials of the gas molecules present in C_(4)F_(7)N/N_(2)/O_(2)mixtures.The electron affinity and electronegativity of O_(2)are calculated,as are those of the C_(4)F_(7)N/N_(2)mixture.Molecular dynamics methods based on the principle of energy optimisation are employed to derive the structural formulas of N_(2)/O_(2)and C_(4)F_(7)N with varying molecular quantities(1,2,3,and 4 molecules).The interaction energy and charge transfer between varying quantities of N_(2)and O_(2)with C_(4)F_(7)N gas are calculated.The Fukui function of the C_(4)F_(7)N molecule is calculated based on Hirshfeld atomic charges,and the bond dissociation energies of C_(4)F_(7)N/N_(2)/O_(2)gas molecules are determined.The quantum chemistry calculations employ the B3LYP functional and the 6-G(d,p)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 C_(4)F_(7)N with O_(2).This analysis investigates the impact of O_(2)on the breakdown and decomposition characteristics of C_(4)F_(7)N/N_(2)mixtures from a microscopic perspective.Results and Discussions The findings demonstrate that O_(2)exhibits a greater electron affinity and electronegativity in comparison to N_(2).The incorporation of O_(2)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 C_(4)F_(7)N/N_(2)mixtures.In the case of N_(2)/O_(2)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 O_(2)and C_(4)F_(7)N is approximately five times that between N_(2)and C_(4)F_(7)N.In the C_(4)F_(7)N/O_(2)molecular system,the C_(4)F_(7)N molecule is observed to a negative charge,indicating that electrons are gained by C_(4)F_(7)N within the system.As the number of molecules increases,C_(4)F_(7)N gains more electrons,resulting in an overall increase in charge transfer.In the C_(4)F_(7)N/N_(2)system,the C_(4)F_(7)N molecule is observed to carry a positive charge,indicating that electrons are lost by C_(4)F_(7)N within the system.As the number of molecules increases,C_(4)F_(7)N loses electrons in greater quantities.In the absence of discharge decomposition,the interactions between the buffer gas and C_(4)F_(7)N molecules are predominantly physical in nature.During the discharge process,O_(2)provides a substantial number of oxygen radicals.Once the chemical bonds of C_(4)F_(7)N have been broken,forming small molecular radicals,these react with O^(·)which has been produced by the homolytic cleavage of O_(2).The addition of O_(2)results in the consumption of the CF_(2)^(·)and C^(·)radicals produced by the decomposition of C_(4)F_(7)N,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 C_(4)F_(7)N/N_(2)/O_(2)ternary gas mixture.Nevertheless,further research is required to ascertain the precise molecular ratio of the C_(4)F_(7)N/N_(2)/O_(2)mixture.