# Mechanism of the Effect of Micro-Water on the Thermal Decomposition Characteristics of C 4 F 7 N/CO 2 /O 2 Gas Mixture

Fanchao Ye, Yitian Chu, Pascal Brault, Dunpin Hong, Shuangshuang Tian, Yi Li, Song Xiao, Xiaoxing Zhang

► To cite this version:

Fanchao Ye, Yitian Chu, Pascal Brault, Dunpin Hong, Shuangshuang Tian, et al.. Mechanism of the Effect of Micro-Water on the Thermal Decomposition Characteristics of C 4 F 7 N/CO 2 /O 2 Gas Mixture. IEEE Transactions on Dielectrics and Electrical Insulation, 2025, 32 (4), pp.2033-2042. 10.1109/TDEI.2024.3496839. hal-05230038

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# Mechanism of the Effect of Micro-Water on the Thermal Decomposition Characteristics of C₄F₇N/CO₂/O₂ Gas Mixture

Fanchao Ye, Yitian Chu, Pascal Brault, Dunpin Hong, Shuangshuang Tian, Yi Li, Song Xiao, Xiaoxing Zhang

**Abstract**—The C₄F₇N/CO₂/O₂ ternary gas mixture, with its excellent environmental and insulating properties, has emerged as one of the most promising alternatives to SF₆ and has seen initial applications in global power systems. Currently, there is limited research on the stability of C₄F₇N/CO₂/O₂ in equipment under long-term operation with micro-water content. This study uses reactive molecular dynamics simulations, using ReaxFF forcefield, for investigating the effect of water (up to 2%) on the thermal decomposition characteristics of C₄F₇N/CO₂/O₂ at the microscopic level. Additionally, thermal decomposition experiments of C₄F₇N/CO₂/O₂ under different micro-water concentrations were conducted to analyze the relationship between decomposition products and micro-water content. When the micro-water concentration is below 1.6%, H₂O promotes the generation of O radicals, increasing the decomposition of C₄F₇N. However, as the H₂O concentration increases further, the large amount of generated OH radicals consume O radicals, inhibiting the main decomposition reactions of C₄F₇N. Experimental results show that the main thermal decomposition products of the gas mixture are CF₄, C₃F₈, C₃F₆, C₂F₆, and CHF₃. The findings of this study provide theoretical support and technical foundation for enhancing the reliability of C₄F₇N/CO₂/O₂ insulation equipment in long-term engineering applications.

**Index Terms**—C₄F₇N/CO₂/O₂; Molecular dynamics; Micro-water; Decomposition characteristics; Eco-friendly insulation gas

## I. INTRODUCTION

Sulphur hexafluoride (SF₆) is widely used in various types of Gas Insulated Equipment (GIE) due to its excellent insulation and arc extinguishing properties [1]. However, SF₆ is a potent greenhouse gas with a global warming potential (GWP) of 23,500 times that of CO₂ and an atmospheric lifetime of 3,200 years [2][3]. Therefore, finding new environmentally friendly gas insulation media and widely applying them in the power industry are

fundamental strategies to mitigate SF₆ emissions [4][5]. Among the currently researched and applied environmentally friendly gas insulation media, C₄F₇N exhibits twice the insulation strength of SF₆, a GWP of 2090, and zero ozone depletion potential (ODP). The ternary gas mixture of C₄F₇N with CO₂ and O₂ is considered the most promising environmentally friendly insulation gas due to its excellent performance [6][7][8][9].

Research experiments on the insulation, arc extinguishing, and decomposition characteristics of C₄F₇N/CO₂/O₂ have achieved some results. Zhuo Ran et al. from the Electric Power Science Research Institute of Southern Power Grid in China studied the solid decomposition product generation characteristics of C₄F₇N/CO₂/O₂ under partial discharge (PD) faults. They found that adding 4% O₂ had little effect on the elemental composition and valence state of PD solid deposits but could reduce their production to some extent [10]. Our research team has conducted preliminary studies on the influence of O₂ on the insulation, electrical, and thermal decomposition characteristics of C₄F₇N/CO₂/O₂. We found that adding 2%-10% O₂ can enhance the dielectric strength of the mixture to varying degrees. Additionally, it can reduce the generation of C₃F₆ and C₂N₂ but increases the production of other by-products. When the O₂ content exceeds 8%, the production and effective generation rate of C₃F₈ and COF₂ accelerate. Therefore, in practical applications, adding 4%-6% O₂ to the C₄F₇N/CO₂ gas mixture is considered appropriate [11][12]. Currently, electrical equipment manufacturers both domestically and internationally have developed C₄F₇N/CO₂/O₂ insulated electrical equipment, which has been installed and successfully operated at voltage levels ranging from 10 kV to 420 kV.

Due to the inevitable presence of micro-water in gas-insulated electrical equipment, researchers domestically

This work was supported in part by the National Natural Science Foundation of China under Grant 52407170, in part by the Natura Science Foundation of Hubei Province under Grant 2024AFB306. (PI: Xiaoxing Zhang, corresponding author of this paper).

Fanchao Ye is with the Hubei Engineering Research Center for Safety Monitoring of New Energy and Power Grid Equipment, Hubei University of Technology, Wuhan, 430068 China (yefanchao@hbut.edu.cn).

Yitian Chu is with the Hubei Engineering Research Center for Safety Monitoring of New Energy and Power Grid Equipment, Hubei University of Technology, Wuhan, 430068 China 1500637903@qq.com).

Pascal Brault is with the GREMI Laboratory at the University of Orleans in France and the French National Centre for Scientific Research (pascal.brault@univ-orleans.fr).

Dunpin Hong is with the GREMI Laboratory at the University of Orleans in France and the French National Centre for Scientific Research (dunpin.hong@univ-orleans.fr).

Shuangshuang Tian is with the Hubei Engineering Research Center for Safety Monitoring of New Energy and Power Grid Equipment, Hubei University of Technology, Wuhan, 430068 China (tianss@hbut.edu.cn). Yi Li is with the School of Electricity and Automation, Wuhan University, 430068 China (liyi_whuee@163.com).

Song Xiao is with the School of Electricity and Automation, Wuhan University, 430068 China (xiaosongxs@gmail.com).

Xiaoxing Zhang is with the Hubei Engineering Research Center for Safety Monitoring of New Energy and Power Grid Equipment, Hubei University of Technology, Wuhan, 430068 China (xiaoxing.zhang@outlook.com).
have begun studying the effects of trace moisture on the insulation performance and decomposition characteristics of insulation gases. When SF₆ contains a certain amount of moisture, excessive moisture affects the generation of discharge decomposition products, resulting in the production of acidic substances such as HF, which corrode insulating materials. [13]. Yang Yuan et al. found that the AC breakdown voltage of C₄F₇N/CO₂ gas mixture is negatively correlated with the volume fraction of trace moisture, indicating that micro-water has a certain negative impact on the insulation performance of C₄F₇N/CO₂ gas mixture [14]. In summary, micro-water not only affects the decomposition characteristics of gases but also impacts their insulation performance. However, the specific influence of micro-water on the long-term operational reliability of C₄F₇N/CO₂/O₂ insulated equipment remains unclear at present.

To investigate the impact of micro-water on the thermal stability and decomposition characteristics of the C₄F₇N/CO₂/O₂, this study employs Reactive Force Field Molecular Dynamics (ReaxFF-MD) [15][16][17] to explore the influence of varying concentration of micro-water on the thermal decomposition of the mixture. The study constructs pathways for decomposition and product generation of the ternary gas mixture under conditions of trace moisture, aiming to reveal underlying reaction mechanisms. Additionally, using a local overheating fault decomposition simulation platform, experimental data on the generation characteristics of major products under different micro-water conditions were obtained. The theoretical and experimental findings provide insights into guiding the engineering application and operational strategies of C₄F₇N/CO₂/O₂.

## II. THEORETICAL RESEARCH AND METHODS

### A. Simulation Methods

ReaxFF molecular dynamics (ReaxFF-MD) can utilize changes in bond order during bond breaking and forming processes to describe chemical reactions in realistic structural models [18][19][20][21]. In the reaction field of ReaxFF-MD, the distribution of energy contributions to the system [22][23][24][25][26][27] is given by the following equation (1):

$$E_{system} = E_{bond} + E_{over} + E_{angle} + E_{torsion} + E_{vdwaals} + E_{coulomb} + E_{specific} \quad (1)$$

where Esystem is the total system energy; Ebond is the bond energy term between the atoms; Eangle and Etorsion represents the three-bond correction energy term and the four-body energy action term, respectively; Eover is an over-coordination energy correction term; Evdwaals and Ecoulomb are van der Waals and Coulomb terms, respectively, which describe the electrostatic and dispersive effects in the interaction, and are non-bonding terms; Especific is usually used for energy terms for specific systems, such as lone pair electrons, conjugated electrons, hydrogen bonding and C2 correction terms.

We first constructed a periodic simulation box of dimensions 260 Å × 260 Å × 260 Å. The entire reaction system comprises 500 molecules, including 30 C₄F₇N molecules (6%), 25 O₂ molecules (5%), and 445 CO₂ molecules. ReaxFF-MD simulations were conducted at a temperature of 3000 K by adding varying numbers of H₂O molecules (0, 2, 4, 6, 8, 10) to simulate different micro-water environments. The main focus of this paper is to investigate the impact of micro-water on the distribution patterns of decomposed radicals and reaction pathways in C₄F₇N/CO₂/O₂. Changes in pressure only affect reaction rates but not reaction types, therefore the simulation pressure is set to atmospheric pressure.

Firstly, based on the NVE system (i.e., the number of atoms, the size of the reaction system, and the energy value are fixed), the geometry of the system was optimized for 10 ps at 20 K (a closed system, with no heat exchange with the outside) to establish a more reasonable initial reaction structure. Subsequently, based on the NVT system (constant number of atoms, size of the reaction system, and temperature), the system was equilibrated at 1000 K for 10 ps. Finally, the C₄F₇N/CO₂/O₂ was simulated at 3000K with varying H₂O contents. Considering computational resources and simulation results, the total simulation duration was 500 ps with a sampling step of 0.25 fs. The simulations were performed using the NVT system, with the Berendsen thermostat method for temperature control. All ReaxFF-MD simulations in this study were based on the ReaxFF module in the cross-scale computational chemistry platform AMS (Amsterdam Density Functional), using the NiCH ff developed by MuEller et al. The force field file was optimized using the Monte Carlo method, taking into account the properties of the C₄F₇N molecule [28]. The optimized reaction system is shown in Figure 1.

<figure><img src="image_2.png" /><figcaption>Fig1 Snapshot of the initial configuration of C₄F₇N/CO₂/O₂ system</figcaption></figure>
## B. Experimental Methods

The experiments in this paper were conducted on the experimental platform shown in Figure 2 under different micro-water conditions for the thermal decomposition experiments. The experimental setup mainly consists of a gas chamber, heating rods, PID controller, pressure gauge, GC-MS, and exhaust gas recovery device.

<figure><img src="image_3.png" /><figcaption>Fig 2. Experimental platform for gas thermal decomposition</figcaption></figure>

In engineering applications, the operating pressure of gas-insulated electrical equipment such as switchgear, GIS, and GIL varies, but generally does not exceed 0.65 MPa, and the operating pressure of GIL is often higher than that of GIS. In this experiment, the C₄F₇N/CO₂/O₂ was subjected to thermal decomposition at 0.4 MPa (absolute pressure) to simulate the occurrence of local overheating faults during the operation of GIL. Research shows that due to the strong synergistic effect between CO₂ and C₄F₇N, gas mixtures containing only 4%-6% by volume of C₄F₇N can achieve dielectric strength similar to that of SF₆, and have a low boiling point suitable for low-temperature applications [29]. Therefore, the volume fraction of C₄F₇N was set to 6% in the experiments conducted in this paper. The addition of 2%-6% O₂ can improve the insulation strength and chemical stability of the C₄F₇N/CO₂ gas mixture, but when the volume fraction of O₂ exceeds 6%, these properties decline [12]. Additionally, adding 5% O₂ to the C₄F₇N/CO₂ gas mixture can suppress gas and solid decomposition products, thereby improving the breaking performance of the circuit breaker [8]. Therefore, the volume fraction of O₂ was set to 5% in the experiments conducted in this paper. The volume fractions of the gas components in the experimental gas mixture and the micro-water concentrations are shown in Table 1. To ensure that C₄F₇N decomposes fully and to better simulate local overheating faults in actual equipment, the heating duration for this experiment was set to 12 hours, with the heating temperature controlled at 500°C. The decomposition products generated from the overheating experiment were analyzed using GC-MS.

<table><caption>Table I</caption><caption>EXPERIMENTAL CONDITION SETTINGS FOR EACH GROUP</caption><thead><tr><th>Volume fraction of C₄F₇N</th><th>Volume fraction of O₂</th><th>Volume fraction of CO₂</th><th>Concentration of micro-water (%)</th></tr></thead><tbody><tr><td>6%</td><td>5%</td><td>89%</td><td>0.05</td></tr><tr><td>6%</td><td>5%</td><td>89%</td><td>0.13</td></tr><tr><td>6%</td><td>5%</td><td>89%</td><td>0.16</td></tr><tr><td>6%</td><td>5%</td><td>89%</td><td>0.19</td></tr><tr><td>6%</td><td>5%</td><td>89%</td><td>0.22</td></tr></tbody></table>

# III. RESULTS AND ANALYSIS

## A. Simulation Results Analysis

The system was heated at a temperature of 3000 K to simulate local overheating faults occurring in the equipment. The temperature will not affect the reaction sequence or the reaction mechanism. Different numbers of water molecules were added to the system to simulate the severity of faults in insulation equipment under actual operating conditions. In this process, C₄F₇N in the gas mixture undergoes decomposition, resulting in the production of radicals such as CF₃, F, CFN, CN, C₃F₄N, and C₃F₇. Figure 3 shows the variation of the main product quantities over time under conditions without added water and with micro-water (As an example, by adding 2 water molecules.). From Figure 3, it can be observed that in the absence of water conditions, C₄F₇N undergoes extensive decomposition, resulting in the following types and quantities of products: a large amount of CF₃ and F radicals, a small amount of CFN and CN radicals, and very small amounts of C₃F₄N and C₃F₇ radicals. When micro-water is introduced into the reaction system, in addition to the above-mentioned products, OH radicals are also generated, and their quantity is positively correlated with the amount of micro-water content. The quantities of radicals such as CF₃, F, CFN, and CN increase over time, while the number of C₃F₄N radicals shows an initial increase followed by a decrease trend. The number of C₃F₇ radicals remains very low throughout the simulation process with minimal variation. This indicates that during the decomposition of C₄F₇N, the initial breakdown occurs at the C-C bonds, resulting in the production of C₃F₄N and CF₃ radicals. Subsequently, C₃F₄N radicals undergo further decomposition. Additionally, the relatively low production of C₃F₇ radicals indicated that the C-C bonds connecting N radicals in C₄F₇N decomposition are difficult to break. The OH radicals partly come from the decomposition of H₂O and partly from the combination of O and H.
<figure><img src="image_4.png" /><figcaption>(a) 0 H₂O</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>(b) 2 H₂O</figcaption></figure>

<caption>**Fig. 3.** Evolution of main decomposition products and radicals counts in C₄F₇N/CO₂/O₂ under conditions without added water and with added water.</caption>

Based on the quantities of various radicals produced by different simulation models at 500 ps, Figure 4 illustrates the variation of primary reactants and products in the decomposition process of the gas mixture with respect to micro-water concentration, as well as the main decomposition reactions of C₄F₇N.

From Figure 4(a), it can be seen that the addition of micro-water promotes the decomposition of C₄F₇N in the gas mixture to varying degrees. As the micro-water concentration increases, this enhancement effect further strengthens. However, when the micro-water concentration becomes excessively high, the ability of micro-water to promote the decomposition of C₄F₇N weakens. This is because further increase of H₂O brings a large number of H radicals, which are more likely to combine with F and CN radicals compared to C₄F₇N. This reduces reactions like C₄F₇N + F → C₃F₇ + CFN and C₄F₇N + CN → C₄F₆N + CFN, resulting in a slight decrease in the decomposition of C₄F₇N, as shown in Figure (c). In the reaction system, the decomposition of C₄F₇N primarily involves the formation of intermediate product C₄F₇NO through the combination of C₄F₇N with O, followed by further decomposition; alternatively, C₄F₇N can directly decompose into C₃F₄N and CF₃. The O radicals in the reaction system originate from the decomposition of CO₂, O₂, and H₂O. When the concentration of H₂O in the system is low, H₂O has minimal impact on the generation of O radicals and

may even promote their production. However, as the concentration of H₂O increases further, OH and H radicals introduced by H₂O will consume O radicals in the system, leading to a reduction in reactions where C₄F₇N combines with O, as shown in Figures 4(d) and 4(e). Figure 4(f) shows the variation in the quantity of the reaction C₄F₇N → C₃F₄N + CF₃ in the reaction system under different H₂O concentrations. The addition of H₂O molecules promotes the occurrence of this reaction. Although increasing H₂O concentration slightly inhibits the main decomposition reactions of C₄F₇N, further increasing H₂O concentration leads to more pathways for C₄F₇N decomposition. As a result, the decomposition amount of C₄F₇N remains higher than that without added water, indicating that H₂O promotes the decomposition of C₄F₇N.

Further analysis combined with Figure 4(b) shows that as the H₂O concentration increases, the quantity of F radicals produced by the decomposition of the C₄F₇N/CO₂/O₂ initially rises and then decreases. This is because the addition of micro-water promotes the decomposition of C₄F₇N, but when the H₂O concentration increases further, a large number of free H radicals exist in the reaction system. These H radicals combine with F radicals to form HF, resulting in a decrease in the number of F radicals.

<figure><img src="image_6.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(b)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Changes in radical quantities and number of reactions (500 ps)</figcaption></figure>

## B. Experimental Results Analysis

Based on early experimental results on the decomposition of C₄F₇N/CO₂/O₂, the decomposition products and reaction mechanisms were identified. The initial decomposition temperature of the gas mixture is 500°C, with major products including CO, C₃F₈, C₃F₆, C₂F₆, C₂N₂, and CF₄[¹²,³⁰]. In subsequent frequency breakdown and corona discharge experiments under different micro-water conditions, CHF₃ was detected [³¹]. Therefore, qualitative and quantitative analysis of CF₄, C₃F₈, C₃F₆, C₂F₆, and CHF₃ was conducted using an external standard method in this experiment. The concentration variations of each product are shown in Figures 5-6.

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Concentration variation of decomposition products in C₄F₇N/CO₂/O₂ with H₂O concentration</figcaption></figure>

From Figure 6, it can be seen that the concentration relationship of the five thermal decomposition products of the C₄F₇N/CO₂/O₂ under different H₂O concentrations is consistent: C₃F₈ has the highest concentration, followed by C₃F₆, then CF₄ and CHF₃, with C₂F₆ having the lowest concentration. This indicates that under different micro-water concentrations, C₃F₈ and C₃F₆ are the main decomposition products of the gas mixture during thermal decomposition. Further observation of the effect of micro-water content on the concentration changes of the decomposition products of C₄F₇N/CO₂/O₂ at each sampling interval, as shown in Figure 6 indicates that the concentration of C₃F₆ does not change significantly with increasing heating time. This suggests that C₃F₆ is relatively stable and easily reaches saturation, and that micro-water concentration has little effect on the concentration of C₃F₆. The concentrations of other decomposition products generally increase with heating time. At the same time, the concentrations of all decomposition products increase with the rise in H₂O concentration, then decrease. Except for C₂F₆, the concentrations of the other products reach their peak when the H₂O concentration is 0.16%. As the H₂O concentration
continues to increase, the concentrations of the decomposition products begin to decrease to varying degrees.

<figure><img src="image_10.png" /><figcaption>(a)CF₄</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>(b)C₃F₈</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>(c)C₃F₆</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>(d)C₂F₆</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig.6.** Variation Curves of Decomposition Products under Different H₂O Concentrations</figcaption></figure>

### C. Discussion

To further investigate the influence of micro-water on the decomposition process of C₄F₇N/CO₂/O₂, it is necessary to understand the reactions involving H₂O molecules and the hydrogen (H) and hydroxyl (OH) radicals generated from their decomposition during the decomposition of the gas mixture. Based on the AMS software and using ChemTraYzer 2, the reaction trajectories involving H₂O, H, and OH in the gas mixture model with 6 H₂O molecules added were organized and analyzed, as presented in Table 2.

<table><caption>**Table II**
THE REACTION PATHWAYS INVOLVING H₂O, H AND OH</caption><thead><tr><th>The types of reactions.</th><th>Reaction number.</th><th>Reaction pathways</th></tr></thead><tbody><tr><td>Involvement of H₂O in the reaction.</td><td>(1)<br>(2)<br>(3)<br>(4)</td><td>H₂O→H+OH<br>F+H₂O→HF+OH<br>O+H₂O→OH+OH<br>CN+H₂O→HCN+OH</td></tr></tbody></table>
<table><tbody><tr><td></td><td>(5)</td><td>O+H<sub>2</sub>O→H<sub>2</sub>O<sub>2</sub></td></tr><tr><td></td><td>(6)</td><td>CF<sub>3</sub>+H<sub>2</sub>O→CHF<sub>3</sub>+OH</td></tr><tr><td></td><td>(7)</td><td>C<sub>4</sub>F<sub>7</sub>N+H<sub>2</sub>O→HF+C<sub>4</sub>F<sub>6</sub>NOH</td></tr><tr><td></td><td>(8)</td><td>C<sub>4</sub>F<sub>7</sub>N+H<sub>2</sub>O→HF+C<sub>3</sub>F<sub>6</sub>+HOCN</td></tr><tr><td></td><td>(9)</td><td>C<sub>4</sub>F<sub>7</sub>N+H<sub>2</sub>O→C<sub>4</sub>F<sub>7</sub>N·H<sub>2</sub>O</td></tr><tr><td rowspan="10">Involvement of H in the reaction.</td><td>(10)</td><td>H<sub>2</sub>O→H+OH</td></tr><tr><td>(11)</td><td>OH→O+H</td></tr><tr><td>(12)</td><td>H+CN→CHN</td></tr><tr><td>(13)</td><td>H+C<sub>3</sub>F<sub>7</sub>→C<sub>3</sub>F<sub>7</sub>H</td></tr><tr><td>(14)</td><td>H+CFN→HF+CN</td></tr><tr><td>(15)</td><td>H+F→HF</td></tr><tr><td>(16)</td><td>H+CF<sub>3</sub>→CHF<sub>3</sub></td></tr><tr><td>(17)</td><td>OH+CN→CHNO</td></tr><tr><td>(18)</td><td>HO+C<sub>4</sub>F<sub>7</sub>N→HOCN+C<sub>3</sub>F<sub>7</sub></td></tr><tr><td>(19)</td><td>HO+C<sub>4</sub>F<sub>7</sub>N→C<sub>4</sub>HF<sub>7</sub>NO</td></tr><tr><td rowspan="5">Involvement of OH in the reaction.</td><td>(20)</td><td>H<sub>2</sub>O→H+OH</td></tr><tr><td>(21)</td><td>OH→O+H</td></tr><tr><td>(22)</td><td>H<sub>2</sub>O<sub>2</sub>→OH+OH</td></tr><tr><td>(23)</td><td>F+H<sub>2</sub>O→HF+OH</td></tr><tr><td>(24)</td><td>O+OH→HO<sub>2</sub></td></tr><tr><td></td><td>(25)</td><td>O+H<sub>2</sub>O→OH+OH</td></tr></tbody></table>

From Table 2, it can be seen that reactions (1) to (9) are the main reactions involving H₂O in the decomposition process of the C₄F₇N/CO₂/O₂. Reactions (10) to (16) are the main reactions involving H radicals, primarily derived from the decomposition of H₂O and OH radicals. These radicals combine with free CN, F, CF₃, C₃F₇ radicals within the reaction system to generate stable products such as HCN, HF, CHF₃, and C₃F₇H. It is noteworthy that reactions (2) to (15) are the primary pathway reactions for generating HF, and reaction (16) is the primary pathway for generating CHF₃.

To reveal the differences in the participation of H₂O molecules in reactions under different micro-water contents, the reactions involving H₂O, H, and OH at different H₂O levels were organized and analyzed, as shown in Figure 7. Figures 7(a) and 7(b) show that with the increase in the number of H₂O molecules, the number of reactions H₂O → H + OH, H₂O + O → OH + OH, and HO₂ → H + O₂ also increases. This means that the number of H and OH radicals increase with the increase in H₂O molecules. Figure 7(c) shows the reactions that are unique when 10 H₂O molecules are added, especially the appearance of the reaction CF₃ + H → CHF₃. This indicates that as the H₂O concentration further increases, free H in the system will consume radicals such as CF₃, which are important components for the synthesis of products like CF₄ and C₂F₆. Therefore, when the H₂O concentration reaches a certain level, the concentrations of products like CF₄ and C₂F₆ may decrease. References indicates that there is no difference between the results of low-temperature long-term simulations and high-temperature short-term simulations. Therefore, to expedite the simulation process, the temperature for this simulation was set to 3000K. However, excessively high temperatures make it

difficult for reactions involving the combination of H with radicals like F and CN to occur. But in experiments, the combination of H with radicals like F and CN reduces the amount of free F and CN in the system, leading to a decrease in the concentrations of related products.

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>(c)</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Fig. 7. The number and type of main reactions with changing water concentration</figcaption></figure>

Using the simulation results of the C₄F₇N/CO₂/O₂ with 6 H₂O molecules added, a multi-stage decomposition diagram of C₄F₇N is depicted in Figure 8.

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /><figcaption>Fig. 8. Multi-stage decomposition process of C₄F₇N</figcaption></figure>

As shown in Figure 8, the decomposition of C₄F₇N first involves the breaking of C-C bonds, forming primary decomposition products such as CF₃, C₃F₄N, C₄F₆N, and C₃F₇, which is consistent with experimental results. At the same time, C₄F₇N undergoes decomposition reactions by combining with O, OH, and H₂O in the reaction system, forming primary decomposition products such as C₃F₇N, HOCN, C₃F₇, CF₃,

C₃F₄NO, C₄HF₇NO, NCO, and C₄F₆NOH, as well as stable products like HF and CO, and the C₄F₇N·H₂O compound. In addition, H radicals produced from the decomposition of trace amounts of water will combine with free CN, F, C₃F₇, and CF₃ radicals in the system to form stable products such as HCN, HF, C₃F₇H, and CHF₃. The aforementioned primary decomposition products CF₃ and F will further combine with free radicals in the system to form stable products such as C₃F₈, CF₄, C₃F₆, C₂F₄, C₂F₆, and CF₃CN.

Compared with the decomposition process of the C₄F₇N/CO₂/O₂ without micro water, the addition of micro-water first causes H₂O to combine with C₄F₇N, directly promoting the hydrolysis of C₄F₇N. Additionally, the OH radicals produced from the decomposition of a small amount of micro-water will decompose to produce O radicals, as seen in reactions (11) and (21), leading to an increase in the number of O radicals in the system. This promotes the combination of C₄F₇N with O radicals to form C₄F₇NO, thus facilitating the decomposition of C₄F₇N. However, as the H₂O concentration further increases, reactions (1), (3), (5), (24), and (25) become more frequent, consuming the free O radicals in the system. This results in a decrease in the combination of C₄F₇N with O radicals, thereby reducing the amount of C₄F₇N decomposition. On the other hand, the increased concentration of H₂O brings a significant amount of H radicals. Compared to C₄F₇N, H radicals more readily combine with F and CN radicals, reducing reactions like C₄F₇N + F → C₃F₇ + CFN and C₄F₇N + CN → C₄F₆N + CFN. This slight reduction in such reactions leads to a slight decrease in C₄F₇N decomposition. However, the addition of micro-water diversifies the decomposition reactions of C₄F₇N. Despite variations in the primary decomposition reactions of C₄F₇N at different H₂O concentrations, overall, the addition of micro-water promotes the decomposition of C₄F₇N.

## IV. CONCLUSION

In this paper, a combined theoretical and experimental approach was used to investigate the decomposition mechanism of the environmentally friendly gas-insulated medium C₄F₇N/CO₂/O₂ in the presence of micro water. Several conclusions can be summarized as follows,

(1) Simulation calculations reveal that the decomposition of C₄F₇N in C₄F₇N/CO₂/O₂ ternary gas mixture initially increases and then decreases with increasing H₂O concentration. Overall, the addition of H₂O enhances the decomposition of C₄F₇N/CO₂/O₂ mixture. The number of F radicals initially increases and then decreases with increasing H₂O concentration. However, the formation of H radicals tends to combine with F radicals, leading to a decrease in F molecule count as the micro-water concentration increases further.

(2) Thermal decomposition experiments detected major decomposition products such as CF₄, C₃F₈, C₃F₆, C₂F₆, and CHF₃. When the micro-water concentration is below 0.16%, it promotes the decomposition of C₄F₇N, resulting in an increase in the production of major decomposition products. As the micro-water concentration further increases, the concentration of these products shows a decreasing trend.
(3) The decomposition of C₄F₇N begins with the breaking of C-C bonds, forming carbon-fluorine groups such as CF₃ and C₃F₄N, and primary decomposition products like C₃F₄NO. These intermediates further generate radicals such as C₃F₇, CN, F, and C₄F₁₀, eventually yielding stable products like (CN)₂, HF, C₂F₄, C₂F₆, C₃F₆, and CF₄. The promotion of C₄F₇N decomposition by H₂O is mainly reflected in the increase in the number and types of primary decomposition reactions. As the H₂O concentration continues to increase, abundant H and OH radicals consume radicals like F and CF₃ in the system, leading to a decrease in the concentration of primary decomposition products.

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<figure><img src="image_21.png" /></figure>

Fanchao Ye was born in Honghu City, Hubei Province, China, in 1993. He received his Doctor's degree in electrical engineering from Wuhan University, Wuhan, China. Dr Ye is currently a lecturer with the School of Electrical and Electronic Engineering, Hubei University of Technology. His research interests include eco-friendly gas insulating media and fault diagnosis of high voltage electrical insulation equipment.
<figure><img src="image_22.png" /></figure>

**Yitian Chu** was born in Honghu City, Hubei Province, China in 2000. He majored in Electrical Engineering at the School of Electrical and Electronic Engineering, Hubei University of Technology in 2023. He is mainly engaged in eco-friendly gas insulating media and fault diagnosis of high voltage electrical insulation equipment.

<figure><img src="image_23.png" /></figure>

**Pascal Brault** is researcher at the French National Centre for Scientific Research. His research interests include modelling of plasma sputtering deposition, thin film growth and plasma chemistry using classical and ab-initio molecular dynamics as well as DFT/quantum chemistry methods.

<figure><img src="image_24.png" /></figure>

**Dunpin Hong** received his M.Sc degree from the University of Grenoble in France and his Ph. D degree in plasma spectroscopy from the University of Orléans in France. He is currently a Professor at the University of Orléans and works at the laboratory GREMI, a joint laboratory of C.N.R.S. and the University of

Orléans. His research interests include plasma production by electric discharge, optical diagnostics of transient plasmas and non-thermal plasmas for subsonic airflow control and for water pollution control. Dr. Hong is a member of the French associations AAE (Association des Arcs Electriques) and EEA (Electronique, Electrotechnique, Automatique). He is a former IEEE member.

<figure><img src="image_25.png" /></figure>

**Shuangshuang Tian** was born in Shandong, China in 1989. She is a Ph.D./postgraduate supervisor. She is mainly engaged in the online monitoring of power transmission and transformation equipment and application technology of eco-friendly gas insulating media.

<figure><img src="image_26.png" /></figure>

**Yi Li** was born in Shangluo City, Shanxi Province, China, in 1994. He received his Doctor's degree in electrical engineering from Wuhan University, Wuhan, China. Dr. Li is currently an associate researcher at Wuhan University. His research interests include eco-friendly gas insulating media and fault diagnosis of

high voltage electrical insulation equipment.

<figure><img src="image_27.png" /></figure>

**Song Xiao** was born in Zhangjiakou City, Hebei Province, China, in 1988. He received his Ph.D. degree in electrical engineering at Chongqing University, Chongqing, China, and his Ph.D. degree in plasma engineering at Université de Toulouse, Toulouse, France. Dr. Xiao is an associate professor at the School of Electric

Engineering, Wuhan University. His research interests include partial discharge online monitoring and eco-friendly gas insulating media.

<figure><img src="image_28.png" /></figure>

**Xiaoxing Zhang** was born in Qianjiang City, Hubei Province, China, in 1972. He received his Bachelor and Master degrees from Hubei University of Technology, and his Doctor's degree from Chongqing University. Prof. Zhang is currently the dean of the School of Electrical and Electronic Engineering at Hubei

University of Technology. His research interests include the online monitoring and fault diagnosis of high voltage electrical insulation equipment, eco-friendly gas insulating media, and new nano-sensors.