Keywords

1 Introduction

SF6 is recognized as a greenhouse gas that has a great harm to the atmospheric environment. Its greenhouse warming potential (GWP) is 23,900 times than that of CO2, and its survival life in the atmosphere is 3400 years [1]. In the “Kyoto Protocol” signed in 1997, SF6 was listed as one of the six restricted-use greenhouse gases, and the use of SF6 was required to be restricted. Looking for alternative gas for SF6 to tackle with environmental issues, has become a hot topic in recent decades [2]. Among the existing SF6 replacements, perfluoroisobutyronitrile (C4F7N) and its mixtures have excellent insulation ability and arc extinguishing ability, and its global warming potential is relatively lower than SF6 [3,4,5,6]. At present, most of the researches on C4F7N gas are from the perspective of insulation performance. Literature [7, 8] studied the power frequency breakdown voltage characteristics of C4F7N/CO2 and C4F7N/N2 mixed gas by building an insulation characteristic experimental platform, and the results show that the C4F7N mixed gas has the potential to be applied to gas insulation equipment. GE company do arc extinguishing test by used C4F7N/CO2 and SF6 on 420 kV disconnector. Comparing the test results we can see The arcing time is slightly shorter than that of SF6, which indicates that gas mixture’s arc extinguishing performance is close to SF6 [9]. Kieffel et al. observed there are many kinds of species in the decomposition products of C4F7N such as CF3CN [10]. Zhang Xiaoxing team studied the decomposition characteristics of C4F7N/CO2 gas mixture theoretically, then found that the main decomposition species are CF3, F, CF, CNF and CN [11]. Literature [12,13,14] show C4F7N gas mixture is suitable for various types of High-voltage (HV) and Medium-voltage (MV) gas insulated equipment.

In this paper,the main decomposition pathways and formed species of C4F7N/N2 mixture were investigated, and we used high level quantum chemistry calculations with density functional theory (DFT). There are five main pathways in the decomposition, two derefent reaction types are shown in this paper to explore the feasibility of C4F7N/N2 mixture's application in insulation equipment.

2 Calculation Method

B3LYP functional form in the density functional theory [15] combined with the 6-311G (d, p) basis set [16,17,18,19,20] is used in this paper to study decomposition reactions of C4F7N/N2 mixtures, and calculate the optimized geometric configuration and energy of the product. Based on above calculations, the rate constants of the reactions are calculated with transition state theory according to Formula (1).

$$ K(T) = \frac{{k_{B} T}}{h}\frac{{Q^{ \ne } (T)}}{{\varphi^{R} (T)}}e^{{ - V^{ \ne } /k_{B} T}} $$
(1)

where T is temperature; is Boltzmann's constant; h is the Planck's constant; and are the transition state structure and the internal partition functions of the reactants; is the potential energy difference between the transition state and the reactant.

3 Results and Discussion

The reactions involved in this paper include the combine of C4F7N molecules with N atoms and the decomposition reaction of C4F7N2 molecules. The decomposition reaction channels mainly study the breaking reactions of C–C bonds, C–F bonds, and C–N bonds. First, we optimize the geometric configuration of C4F7N and N atoms to combine them. Optimized structure of some molecules, such as C4F7N2, as shown in Fig. 1

Fig. 1
figure 1

Molecular geometry of C4F7N2

Full size image

.

Table 1 lists the main decomposition reactions of C4F7N2 molecules. The decomposition pathways of C4F7N2 molecules include three barrierless reactions: B, D, and E, and two reactions A and C with transition states.

Table 1 Main decomposition reactions of C4F7NO molecules
Full size table

3.1 Optimization of Geometric Configuration of Reaction Particles

This article shows reaction E without a transition state and reaction A with transition state TS1. Figures 2 and 3 show the optimized molecular structures and key geometric parameters in the reactions E and A, respectively.

Fig. 2
figure 2

The geometry of the particles involved in reaction E

Full size image
Fig. 3
figure 3

The geometry of the particles involved in reaction A

Full size image

3.2 Particle Energy and Frequency Calculation

At the B3LYP/6-311G(d,p) level, the stable point energies of the decomposition reaction channels of the C–C, C–N and C–F of the C4F7N2 molecular center carbon bonds were calculated, including reactants, transition states, intermediate products and the zero-point vibration energy (ZPE) and total energy of the product (including electronic energy and zero-point energy) E total show in Table 2.

Table 2 Energy information of reactants, transition states and products
Full size table

The vibration frequencies of the particles involved in the reactions E and A calculated at the same calculation level are listed in Table 3.

Table 3 Vibration frequencies of reactants, transition states and products calculated
Full size table

3.3 Decomposition of C4F7N/N2 Mixtures

Figure 4 shows the energy changes of all reaction pathways. Pathway A corresponds to the break of the C–C bond between the core carbon atom and the adjacent carbon atom in the C4F7N2 molecule. This process forms the CN2 and F3C(F)CF3. The energy difference between the reaction product and the reactant is 85.341 kcal \(\cdot\) mol−1, and in reaction A, there is a transition state TS1, and the reactant C4F7N2 needs to cross the energy barrier of 108.559 kcal \(\cdot\) mol−1 to occur. Pathway B corresponds to the breaking process of C-F bond between carbon atom and F atom, and F2NC(FC≡F)CF3 molecule and F atom are formed. The reaction process needs to absorb 42.043 kcal \(\cdot\) mol−1 energy. For pathway C also corresponds to the process of breaking the C–C bond between the central carbon atom of C4F7N2 and the adjacent carbon atom. This process has the transition state TS2, and the energy potential barrier is 15.688 kcal \(\cdot\) mol−1. After crossing the barrier, the products F3CNC(F)CN and CF3 were obtained, and the energy difference between the product and the reactant C4F7N2 is only 6.275 kcal \(\cdot\) mol−1. Similarly, reaction D is a process in which the C–C bond between the central carbon atom and the adjacent carbon atom CN in C4F7N2 is broken. This process absorbs 39.533 kcal \(\cdot\) mol−1 energy to generate F3CC(F)NCF3 and CN. Pathway E shows the C-N bond breaking process between the central carbon atom of C4F7N2 and the adjacent nitrogen atom, absorbing 57.731 kcal \(\cdot\) mol−1 energy and generating F3CC(F)CN and F3CN. Among the five reaction pathways, pathway A needs to absorb the energy of 85.341 kcal \(\cdot\) mol−1, which is the most difficult to occur, while pathway C only needs to absorb the energy of 6.275 kcal \(\cdot\) mol−1, which is the most prone process.

Fig. 4
figure 4

The energy changes of all reaction pathways

Full size image

4 Conclusion

In this paper, the DFT-B3LYP/6-311G (d, p) method is used to calculate the decomposition reactions and main decomposition components of the C4F7N2 molecule.There are five decomposition pathways shown in this paper such as A-E, and the main decomposition species are CxFxNx, CxNx. Microscopic data such as the molecular structure, key geometric parameters, zero-point vibration energy and frequency are important to compute the composition of C4F7N plasma when C4F7N used in power equipment as an insulator. Therefore these parameters are the basis in any attempt to test the dielectric properties of C4F7N used in power equipments for insulating.