N2 gas mixture: An

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General Research

Decomposition properties of C4F7N/N2 gas mixture: An environmentally friendly gas to replace SF6 Yi Li, Xiaoxing Zhang, Song Xiao, Qi Chen, Ju Tang, Dachang Chen, and Dibo Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00010 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Industrial & Engineering Chemistry Research

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Decomposition properties of C4F7N/N2 gas mixture: An

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environmentally friendly gas to replace SF6

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Yi Li1, Xiaoxing Zhang1,*, Song Xiao1, Qi Chen1, Ju Tang1, Dachang Chen1 and Dibo Wang2

5 6 7 8 9 10 11 12

1

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Abstract

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

The insulation characteristics and decomposition components of C4F7N/N2 gas mixture, a potential substitute for SF6, were firstly explored by breakdown experiments/gas chromatography-mass spectrometer (GC-MS). The structural properties of C4F7N molecule and the decomposition mechanism of C4F7N/N2 gas mixture were analyzed based on the density functional theory (DFT) calculation and ReaxFF molecular dynamics (ReaxFF-MD) simulation. We found that C4F7N/N2 mixture has great self-recovery performance. The decomposition of C4F7N in a discharge mainly produces CF4, C2F6, C3F8, CF3CN, C2F4, C3F6 and C2F5CN, among which the relative content of C2F6, CF4 and CF3CN is higher. ReaxFF-MD simulations show that CF3, CN, F and C3F7 are the four main free radicals produced by C4F7N. The decomposition characteristics of N2 is better than that of C4F7N. The addition of N2 has a certain buffering effect to avoid the massive decomposition of C4F7N. The GWP value of gas mixture containing 20% C4F7N decreased by 94.32 % compared with SF6. Relevant results not only reveal the decomposition characteristics of C4F7N/N2 mixture in a discharge comprehensively, but also provide a reference for engineering application and emission of C4F7N/N2 gas mixture.

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1. INTRODUCTION

31 32 33 34 35 36 37 38 39

Reducing the emission of greenhouse gases has been an area of immense concern due to the abnormal climate changes arising from global warming [1]. The Paris climate agreement aims at holding global warming to well below 2 ℃ and to “pursue efforts” to limit it to 1.5 ℃ [2-3]. In order to achieve this, in addition to continuing reducing carbon dioxide emissions, efforts should also be made to reduce emissions of other greenhouse gases [4]. SF6, which is widely used in industry, is one of the six greenhouse gases with a Global Warming Potential (GWP100) of 22800 and an atmospheric lifetime about 3200 years [5]. The atmospheric content of SF6 has increased by 20% over the past five years, and its atmospheric mole fraction reaches 7.28 ppq (parts per quadrillion) corresponding to a radiative forcing of 0.0041 w/m2 [6-7]. Actually, about 80% of the

2

School of Electrical Engineering, Wuhan University, Wuhan 430072, China Electric Power Research Institute, China Southern Power Grid, Guangzhou 510623, China

*E-mail: [email protected] Phone: +86 13627275072 Address: School of Electrical Engineering, Wuhan University, BaYi Street No.299, Wuhan, Hubei province, People’s Republic of China ORCID: 0000-0001-5872-2039

Keywords: SF6 alternative gas, C4F7N/N2, decomposition, ReaxFF molecular dynamics

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ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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SF6 gas produced worldwide is used in power industry [8]. In order to realize the green development of the power industry and gradually reduce the use of SF6, it is imperative to search for an environmentally friendly insulation gas.

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Heptafluoroisobutyronitrile, C4F7N (also known as 3M™Novec™ 4710 dielectric fluid), is a potential alternative gas to SF6 with GWP value of 2100 and ozone depletion potential (ODP) value of 0 [6]. C4F7N is not Carcinogens Mutagenic Reprotoxic (CMR) substance and its Threshold Limit Values Time Weighted Average (TLV TWA) is equal to 65 ppmv (parts per million by volume) [9]. The insulation performance of C4F7N is 2 times that of pure SF6, but its liquefaction temperature reaches -4.7 °C [6]. Thus it is necessary to mix C4F7N with low liquefaction temperature gas such as CO2, N2 or dry air in engineering application. At present, many scholars have carried out experimental studies and found that the insulation properties of C4F7N gas mixture containing 20% C4F7N can reach that of pure SF6. C4F7N gas mixture is suitable for various types of High-voltage (HV) and Medium-voltage (MV) gas insulated equipment [4, 9-11].

54 55 56 57 58 59 60 61 62 63 64 65 66 67 68

In addition, all kinds of insulation defects caused by long-term operation or aging in the electrical equipment can cause partial discharge (PD) or flashover, accompanied by the decomposition of insulating medium. For example, the decomposition of SF6 under long-term operation condition produces SO2, SOF2, SO2F2 and many other toxic products [12]. The generation of toxic components poses a threat to the maintenance personnel safety and the atmospheric environment. Since the C4F7N contains cyano group (CN) and many compounds containing this functional groups are highly toxic, it is necessary to investigate the decomposition characteristics of C4F7N gas mixture in a discharge before large-scale engineering application. In particular, the decomposition components, the relative content and the toxicity of the C4F7N gas mixture should be clarified. Kieffel et al. tested the decomposition products from thermal degradation of C4F7N/CO2 gas mixture based on the Fourier transform infrared spectroscopy (FTIR). CO, COF2, CF3CN, C2F5CN and C2F6 were detected at temperatures above 800°C [11]. Our team studied the decomposition characteristics of C4F7N/CO2 gas mixture theoretically and found that the main decomposition species are CF3, F, CF, CNF and CN [13]. The possible decomposition paths of C4F7N and the effects of trace water were also evaluated theoretically [14].

69 70 71 72 73 74 75 76 77 78 79 80

In this paper, the insulation and decomposition components of C4F7N/N2 mixture in a discharge were investigated for the first time. We tested the power frequency breakdown characteristics of C4F7N/N2 gas mixture for several times and the decomposition components after discharges were detected using gas chromatography-mass spectrometer (GC-MS). In addition, the decomposition process of C4F7N/N2 gas mixture was studied theoretically using the reactive molecular dynamics (ReaxFF-MD) method and density functional theory (DFT). The composition and content of various free radicals produced by C4F7N were revealed microscopically. The influence of temperature and N2 content on the decomposition of gas mixture was explored. The GWP value and toxicity of the decomposition products were also evaluated. Relevant results not only reveal the decomposition characteristics of C4F7N/N2 gas mixture in a discharge comprehensively, but also provide guidance for engineering application and emission of C4F7N/N2 gas mixture.

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2. METHODS

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2.1 Test method

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The gas-insulated test platform is composed of power frequency test transformer, protective resistor, capacitive voltage divider and gas chamber. Figure 1 shows the experimental circuit of the test platform. The power frequency test transformer provides a high voltage (0-100kV) and the capacitive voltage divider is used to measure the actual voltage applied to the electrodes. The gas chamber is made of 304L stainless steel with the volume about 10L and polytetrafluoroethylene (PTFE) is used as the sealing material. The ball electrodes made of brass is used to simulate a quasi-uniform electric field [15]. The radius of the ball electrode is 25mm and the electrode interval is 5 mm. Resistor

Gas chamber

Electrode

NNNN2222

Capacitive Voltage Divider

Voltage Regulator

C4F7N

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


92 93

Figure1 Experimental circuit

94 95 96 97 98 99 100 101 102 103

Before the test, anhydrous alcohol was used to clean the inner wall of gas chamber to eliminate impurities. Then the gas chamber was vacuumed, filled with buffer gas (N2, purity of 99.999%), and vacuumed again. We did this process three times to remove impurities and its influences on the GC-MS detection. The gas chamber is injected with 7.5kPa C4F7N (purity of 99.99%, provided by 3M™ Company) and 142.5kpa N2, subsequently. The power frequency high voltage was applied to the electrodes until breakdown and the instantaneous breakdown voltage value was recorded. The test was repeated 30 times with the interval of 3mintues. The gas mixture in the chamber was sampled and detected by GC-MS (Shimadzu Ultra 2010plus with CP-Sil 5 CB column) to determine the type and relative content of the decomposition products after the 15th and 30th breakdown test, respectively.

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2.2 First-principles calculation

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The Mayer bond order and Fukui functions of C4F7N were calculated based on the DMol3 module of Materials Studio 7.0 to reveal the structure properties of C4F7N molecule [16]. Geometry optimization uses the generalized approximation method treated by the Perdew-Burke-Ernzerhof (GGA-PBE) functions to handle the exchange-correlation energy [17]. The double numerical atomic orbital augmented by d-polarization (DNP) is used as the basis set. The threshold of 1.0 × 10−5 Ha on energy, 0.005 Å on displacement and 0.002 Ha/Å on gradients is set to get accurate results. 3



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2.3 ReaxFF molecular simulation

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ReaxFF is a reactive force field based on the relationship of bond order and bond distance and has been widely used to study the physicochemical properties of large scale systems [18-20]. The bond cleavage and formation during chemical reactions can be described in ReaxFF. The total potential energy can be described as follows [21],

117

Esystem = Ebond + Eover + Eunder + Eval + Epen + Etors + Econj + EvdWaals + ECoulomb

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where Ebond is the bond energy, Eover and Eunder denote the over and under coordinated atom in

119

the energy contribution. Eval , Epen , Etors , Econj , EvdWaals and ECoulomb correspond to the

120 121

valence angle term, penalty energy, torsion energy, conjugation effects to energy, nonbonded van der Waals interaction and Coulomb interaction, respectively.

(1)

122 123 124 125 126 127 128 129 130 131 132

Figure2 Structure of C4F7N/N2 system Owens et al. tested the insulation properties of C4F7N/N2 gas mixture and found that gas mixture with about 20% C4F7N displays dielectric strengths comparable to SF6 [10]. In order to study the decomposition mechanism of C4F7N/N2 gas mixture in this scale, we built a periodic cube with the side length about 255 Å (as shown in Figure 2) which contains 100 C4F7N molecules and 400 N2 molecules. The initial density of the system is 0.0033125g/cm3. This structure corresponds to the actual parameters of C4F7N/N2 gas mixture at 25℃, 0.1Mpa. In addition, we also built several C4F7N/N2 systems containing 5%-25% C4F7N to explore the influence of N2 content on the decomposition of gas mixture. The key parameters are given in Table 1. The number of C4F7N molecules is set to 100 considering the computational cost.

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Table 1. Parameters of C4F7N/N2 system

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No

N2 content

Number of C4F7N molecules

Number of N2 molecules

1

95%

100

1900

2 3 4 5

90% 85% 80% 75%

100 100 100 100

900 566 400 300

Density(g/cm3) a

0.001623 0.001996 0.002368 0.002741 0.003114 a: Corresponds to the actual density of gas mixture at 25 ℃, 0.1Mpa 4


Box length(Å)

420.6 333.9 291.6 265 246

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135 136 137 138 139 140 141

The system was firstly optimized using NVE (keep the number of atoms, volume, and potential energy constant) ensemble for 5ps at 5K and then equilibrated with the NVT (keep the number of atoms, volume, and temperature constant) ensemble for 10 ps at 1000 K [22]. The NVT-MD simulations were carried out subsequently for 1000ps with the time step of 0.1fs. The Berendsen thermostat method with a 0.1 ps damping constant was used to control the temperature [23]. All the ReaxFF-MD simulations in this paper were performed based on the ReaxFF module of ADF (Amsterdam Density Functional) [24].

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3 RESULTS AND DISCUSSION

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3.1 Insulation and decomposition characteristics

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Figure 3 shows the power frequency breakdown voltage of 5% C4F7N/95% N2 gas mixture at 0.15Mpa, 25℃. We also tested the breakdown voltage of pure SF6 under the same conditions and found that the average breakdown voltage of the C4F7N/N2 gas mixture and pure SF6 is 33.59kV and 40.3 kV, respectively. The insulation performance of 5% C4F7N/95% N2 gas mixture reaches 83.34 % than that of SF6. The breakdown voltage of C4F7N/N2 gas mixture kept at about 33.6 kV with the increase number of tests, which indicates that its self-recovery characteristics are excellent.

151 152

Figure 3 Breakdown voltage of the gas mixture

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The chemical reactions initiated by high-energy discharges can alter the particles composition of the gas-insulated medium and then changes its microscopic physical and chemical properties. In order to reveal the components change of C4F7N/N2 gas mixture after discharges, we detected the gas sample in the chamber using GC-MS. Figure 4 gives the gas chromatograms of the gas mixture after the 15th and 30th breakdown tests. It can be found that the decomposition of C4F7N/N2 gas mixture produces several by-products such as CF4, C2F6, C3F8, CF3CN, C2F4, C3F6 and C2F5CN. The characteristic peak value of C2F6 is the highest, indicating that its content is the highest of all the decomposition products. The content of CF4, CF3CN is very close, followed by C2F4, C3F6, C3F8 and C2F5CN. The main decomposition products of C4F7N/N2 gas mixture in a discharge are C2F6, CF4 and CF3CN.

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Figure 4 Gas chromatogram of the C4F7N/N2 gas mixture after discharge

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With the increase number of breakdown tests, the characteristic peak height of various decomposition products increased obviously (as shown in Figure 5). The characteristic peak height of C2F6, CF3CN and C2F4 after the 30th tests increased by more than 110% than that of the 15th tests, and the relative growth rate of CF4，C3F8, C3F6 and C2F5CN is in the range of 80~91%. Therefore, with the increase number of discharges, the decomposition of C4F7N is aggravated and the yield of decomposition products increased. In addition, the generation of C2F6, CF3CN, C2F4 is easier than the other decomposition products.

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Figure 5 Relative change of the characteristic peak value of the decomposition components after 15th and 30th breakdown tests

3.2 Molecular structure properties of C4F7N

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Figure 6 Molecule structure of C4F7N

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The decomposition properties of the gas-insulated medium are related to the configuration of gas molecules. The optimized structure and calculated Mayer bond order of C4F7N molecule is shown in Figure 6 and Figure 7, respectively. The bond length of C atom and N atom in the CN group is 1.165Å, which is the shortest bond in C4F7N. And the bond length of C1-C2 and C2-C3 bond is 1.569Å, which is the longest bond in C4F7N. The Mayer bond order is widely used to describe the relative strength of chemical bonds. The small value of bond order indicates the weak interaction between two atoms [25]. We found that the bond order of C1-C2 and C2-C3 bond is 0.926, which value is the smallest of all the chemical bonds in C4F7N molecule, indicating the interaction between C1 (C3) and C2 atom is weak. And these chemical bonds are the most susceptible to cleavage under high temperatures or high-energy discharges, producing CF3 and other particles. The molecule structures of the main decomposition products such as C2F6, CF4, CF3CN, contain CF3 group, which is consistent with the bonder order analysis results.

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Figure 7 Mayer bond order (MBO) of C4F7N

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Fukui function can be used to predict molecular reactivity and reveal the sensitivity of molecules to gain or loss of electrons to a certain extent [26]. Figure 8 demonstrates the values of Fukui functions mapped onto the surface of the electron density image. For the f+ function, the positive (orange) region corresponds to the sites where electron density increased. It can be seen that these regions mainly concentrate in the cyano group in the C4F7N molecule, that is, the above region is easy to obtain electrons. For the f - function, the negative (blue) region corresponds to the 7



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sites where electron density decreased after the charge deduction. It can be seen that these regions mainly concentrate in the F atom in C4F7N molecule, that is, the above region is easy to lose electrons.

a) f+

b) f -

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Figure 8 Fukui functions of C4F7N

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Overall, the bond strength between central C atom and its linked C atoms in the C4F7N molecule is weak, and the CN group has strong reactivity. The decomposition of C4F7N may occur mainly between the carbon atoms.

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3.3 Decomposition mechanism of C4F7N/N2

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3.3.1 Effect of temperature on decomposition

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Arc discharge and partial discharge occurs inside the electrical equipment can lead to local overheating of the insulation medium. The temperature at the core region of arc discharge and partial discharge is in the range of 3000-12000K and 700-1200K, respectively [27-28]. It is noteworthy that the time scale for ReaxFF-MD simulation is normally limited to several dozens of nanoseconds due to the computational cost [29]. And the difference in temperature scales may certainly affect product distributions, reaction rates but not reaction mechanisms [30]. Thus we carried out ReaxFF-MD simulations at 2000K-3000K to reveal the decomposition process of C4F7N/N2 mixture.

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a) C4F7N 8


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b) N2 Figure 9 Time evolution of C4F7N and N2 decomposition at 2000-3000K

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Figure 9 shows time evolution of C4F7N and N2 decomposition at different temperature. It can be found that C4F7N and N2 began to decompose at 2000K. With the increase of temperature, both the decomposition amount and decomposition rate of C4F7N and N2 increased. For example, only 2 C4F7N molecules decomposed at 2000K, while 76 C4F7N molecules decomposed at 3000K at the end of simulation. The decomposition properties of N2 is better than that of C4F7N, and only 14.75% N2 molecules decomposed at 3000K.

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Figure 10 Time evolution of total potential energy of C4F7N/N2 system at 2000-3000K

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Figure 10 shows time evolution of total potential energy of C4F7N/N2 system at 2000-3000K. The potential energy increases during the whole reaction process at different temperature, indicating that the decomposition of gas mixture is endothermic, which is consistent with the actual situation. With the increase of ambient temperature, the growth rate of potential energy increases. The reaction process can be divided into three stages at 2600-3000K as follows. In the period of 0-150ps, the potential energy remains almost unchanged. Then the total potential energy increases rapidly at 150ps-650ps, indicating that the reaction rate in the system is high. The potential energy growth rate slows down and tends to saturation at 650-1000ps.

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a) CF3

b) C3F7

c) CF2

d) CF

e) F

f) CN

g) CNF

h) CF4 10


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i) C

g) N

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Figure 11 Time evolution of C4F7N/N2 decomposition products at 2000-3000K

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The distribution of various decomposition products at different temperature is given in Figure 11. The main decomposition products of C4F7N/N2 gas mixture are CF3, F, CN, CNF, CF2, CF, C3F7，C, N, CF4, C3NF4 and C4NF6. Among which CF3, CF, CN and F were produced at 2000K, while other products were produced at higher temperature. The yield of CF2, CN, CNF, F, C3F7 and N increase linearly with the reaction time at different temperatures. And the yield of CF3 shows a saturation trend in the period of 700ps-1000ps at temperatures above 2600K, which is related to the rapid generation of CF4. The content of CF decreases in the range of 600ps-1000ps at temperatures above 2600K, which is related to its decomposition or the formation of CF2.

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Figure 12 Maximum number of produced decomposition products of C4F7N/N2 at 2000-3000K

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Figure 12 shows the maximum number of produced decomposition products at different temperatures. It can be found that the content of CF3, F, CN and CNF is the highest. Thus CF3, F, CN and CNF are the main particles produced by the decomposition of C4F7N/N2 gas mixture. According to the experimental results, the main decomposition products of C4F7N/N2 gas mixture 11



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are C2F6, CF4 and CF3CN. The formation of these substances is inseparable from the participation of CF3 and CN. Thus the ReaxFF-MD simulation results is consistent well with the test results. In addition, the yield of CNF and CN exceeds the total decomposition amount of C4F7N molecules at the temperature above 2200K, indicating that the C atom and N atom generated by the decomposition of C4F7N and N2 are involved in the generation of the above two radicals.

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3.3.2 Effect of N2 content on decomposition

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In order to further investigate the influence of nitrogen content on the decomposition of gas mixture, we carried out ReaxFF-MD simulations for C4F7N/N2 mixture with different mixing ratios at 2700K and the results is shown in Figure 13. It can be found that the decomposition rates of C4F7N with different mixing ratios are basically the same, indicating that the decomposition rate of C4F7N is independent of N2 content. With the increase of N2 content, the decomposition amount of C4F7N molecules in the system decreased. For example, there are 51 C4F7N molecules decomposed in the 5%C4F7N/95%N2 system and 64 C4F7N molecules decomposed in the 25%C4F7N/75%N2 system.

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Figure 13 Time evolution of decomposed C4F7N at 2700K with different mixing ratio

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Figure 14 gives time evolution of main decomposition products at 2700K with different mixing ratios. It can be seen that the yield of the main decomposition products of gas mixture decreases with the increase of N2 content. CF3, F, CN and CNF are the main particles produced by the decomposition of C4F7N/N2 gas mixture. As mentioned above, the decomposition characteristics of N2 is better than that of C4F7N. Thus the addition of N2 has a certain buffering effect to avoid the massive decomposition of C4F7N.

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a) CF3

b) CN

c) F

d) CNF

e) CF2

f) CF

Figure 14 Time evolution of main decomposition products at 2700K with different mixing ratio

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3.3.3 Proposed decomposition mechanism

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Figure 15 Proposed decomposition paths of C4F7N/N2 mixture

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According to the experimental and simulation results, Figure 13 shows the proposed decomposition paths of C4F7N/N2 gas mixture. There are three main decomposition paths of C4F7N (marked as A, B and C as shown in Figure 15). The ReaxFF-MD simulation results and molecule structure properties analysis indicate that the pathway which produces C3F4N and CF3 is most likely to occur. We have calculated the possible decomposition enthalpy of C4F7N based on DFT in our early studies and found that the reaction enthalpy of path B is 73.24kcal/mol, which is the lowest one of all possible decomposition paths [14]. The CF3 produced by C4F7N combining with CN, F and other free radicals can generate CF4, C2F6, CF3CN and other decomposition products. And CF2 decomposed by CF3 can combine to form C2F4. In addition, C3F7 generated through path C can combine with F to produce C3F8 or dissociate to form C3F6. Thus, CF3, CN, F and C3F7 are the four important free radicals which combine with each other can form various decomposition products. The decomposition characteristics of N2 is superior to that of C4F7N. And N2 plays a buffering role to avoid the massive decomposition of C4F7N under high-energy electric field or local overheating conditions, which is favorable for the insulation performance of the system.

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3.4 Environmental effects and toxicity of C4F7N/N2 mixture

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The assessment of environmental and safety of C4F7N/N2 mixture is necessary. We firstly calculated the GWP value of C4F7N/N2 gas mixture according to the formula given in F-gas regulation (as shown in Figure 16) [31]. The GWP value of gas mixture containing 10% and 20% C4F7N are 916 and 1334, respectively, which decreased by 96.10 % and 94.32 % compared with SF6. Therefore, the use of C4F7N/N2 gas mixture as a gas-insulated medium will effectively reduce the greenhouse effect.

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Figure 16 GWP of C4F7N/N2 mixture with different mixing ratio

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As mentioned above, C4F7N is not CMR substance with the VLEP (Occupational Exposure Limits) value about 65ppmv (lower than that of SF6, 1000 ppmv), and its LC50 value is higher than 10000 ppmv. Literature [11] points out that the toxicity of gas mixture with C4F7N content below 10% is lower than that of SF6, so its engineering application is safe and reliable.

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Table 2. Toxicity of some byproducts of C4F7N/N2 gas mixture and SF6

311

Gas

LC50a

Gas

LC50a

C2F5CN

-

SOF2

-

CF3CN CF4 C2F6 C3F8 C3F6 C2F4

500ppm/H [8] 895 000 ppm/15M [5] >20 pph/2H [5] 750 ppm/4H [5] 40000 ppm/4H [5]

SO2F2 SO2 HF S2F10 SOF4 SiF4

991ppm/4H [32] 2520 ppm/1H [32] 1276 ppm/1H [32] 193 ppm/1Min [32] 922 ppm/1H [32]

a: Lethal concentration at 50% mortality, rat.

312 313 314 315 316 317 318 319 320 321 322 323

Table 2 shows the toxicity of some byproducts of C4F7N/N2 gas mixture and SF6. CF4, C2F6, C3F8 and other fluorocarbon compounds produced by C4F7N/N2 gas mixture belong to inert gases, which have been widely used in industrial field at present. The LC50 value of Perfluoropropylene, C3F6, is 750 ppm (4h, rat), and a large number of short-term inhalation will produce dizziness, weakness, poor sleep and other symptoms [5]. Both CF3CN and C2F5CN are toxic gases, in which the LC50 value of CF3CN is 500ppm (1h, rat) [8]. In addition, we can find that the toxicity of several SF6 decomposition products is relatively high, such as S2F10. Considering the content of C4F7N in gas mixture is lower than 20% and the decomposition products in the discharge is at the ppm level, the toxicity of C4F7N/N2 gas mixture is on a comparable or lower level as SF6. Kieffel et al. made toxicity measurements of C4F7N/CO2 gas mixture after current interruption and found that the toxicity of gas is less than SF6 under the same conditions [33]. Thus, using C4F7N/N2 gas as an insulating medium is safe.

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4 CONCLUSION 15



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We tested the insulation properties and decomposition characteristics of C4F7N/N2 gas mixture using gas-insulated test platform/GC-MS. The molecular structural propetries and decomposition mechanism of C4F7N/N2 gas mixture were explored based on the ReaxFF-MD and DFT methods. The feasibility of using C4F7N/N2 gas mixture in engineering was also evaluated in combination with its environmental and safety properties. We found that C4F7N/N2 mixture has great self-recovery performance. The decomposition of C4F7N in a discharge mainly produces CF4, C2F6, C3F8, CF3CN, C2F4, C3F6 and C2F5CN, among which the relative content of C2F6, CF4 and CF3CN is higher. ReaxFF-MD simulations show that CF3, CN, F and C3F7 are the four main free radicals produced by C4F7N, and they are the basis for the formation of other decomposition products. The decomposition characteristics of N2 is better than that of C4F7N. The addition of N2 has a certain buffering effect to avoid the massive decomposition of C4F7N. The GWP value of gas mixture containing 20% C4F7N decreased by 94.32 % compared with SF6. The C4F7N/N2 gas mixture has great environmental and safety performance, which has the potential to replace SF6 using in medium-voltage and high-voltage electrical equipment. Relevant results reveal the decomposition characteristics of C4F7N/N2 mixture in a discharge comprehensively, and provide a reference for subsequent engineering application and emission safety of C4F7N/N2 gas mixture.

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The current work is supported by the science and technology project of China Southern Power Grid (No. ZBKJXM20170090). And we thank 3M (Minnesota Mining and Manufacturing) for providing the sample of C4F7N.

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N2 gas mixture: An

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