Theoretical Investigation of C₄F₇N–CO₂ Mixture Decomposition Characteristics Under Extreme Conditions

Abstract

Due to their low greenhouse effect and exceptional insulating properties, C₄F₇N-CO₂ gas mixtures have garnered significant attention. In particular, understanding the decomposition characteristics of C₄F₇N-CO₂ is crucial for their practical use as an eco-friendly dielectric medium. At elevated temperatures, the pyrolysis of C₄F₇N produces high concentrations of CFN, CF₃, and C₂F₂, along with lower levels of C₃F₅, C₄F₆N, C₂F, and CN. A further increase in temperature may lead to the decomposition of CO₂ into CO and additional components such as C₂, C₂F₃, C₃F₄, C₄F₇ and C₃F₆, CF, CO, C₃F₇, C₃F₂, C₃F, C₃F₃, C₃F₃N, C₃, CF₂, and CF₂N. Under electrical discharge conditions, the decomposition of CO₂ becomes more pronounced, forming products like CO, C₂O, O₂, C₂O₂, and C₂O₄, with up to 25 decomposition components observed. These include products originated from both C₄F₇N and CO₂ and their combinations. In ultra-high electric field intensities, only small molecules such as O₂, C₂, C₃, and N₂ are detected among the decomposition products. This study aims to provide theoretical insights and valuable data to advance research into the decomposition behavior and practical engineering applications of C₄F₇N-CO₂ gas mixtures under extreme conditions.

1. Introduction

With the rapid development of energy worldwide, the safety and intelligence of electrical equipment have become increasingly critical [1]. High-voltage power transmission in complex environments has spurred extensive research into gas-insulted switchgear (GIS), gas-insulated transformers (GITs), and gas-insulated transmission lines (GILs) [2,3].

Insulating gas is a crucial material in GIS, GITs, and GIL within electrical engineering [4]. In recent years, perfluoroisobutyronitrile (C₄F₇N) has attracted significant attention due to its low global warming potential (GWP) value of just 2400 [5,6,7,8]. Under standard conditions, C₄F₇N demonstrates an insulation strength of over twice that of sulfur hexafluoride (SF₆), a widely used greenhouse gas (GHG). Furthermore, both C₄F₇N and SF₆ have an ozone-depletion potential (ODP) of 0, making C₄F₇N an ecofriendly alternative to SF₆. Compared to carbon capture, utilization, and storage (CCUS) as a post-treatment technology, the usage of environmentally friendly insulating gases can essentially weaken the greenhouse effect [9]. However, the relatively high liquefaction temperature (−4.7 °C) limits its applications below 0 °C.

To address this limitation, mixing C₄F₇N with CO₂ (liquefaction temperature of −37 °C) has been proposed as a viable option [10]. CO₂ as an insulating gas provides a new pathway to extend the utilization function in CCUS. The insulation performance, arc-extinguishing capabilities, and thermal stability of C₄F₇N-CO₂ mixtures have been extensively investigated [11,12]. Especially, the possible decomposition reaction pathways, intermediates, and products of C₄F₇N-CO₂ mixtures have been explored to optimize their composition [13,14,15,16,17]. Studies reveal that the decomposition temperature of the C₄F₇N-CO₂ mixture is 650 °C, with products such as C₂F₆, COF₂, CF₃CN, C₂F₅CN, and CO forming at 775 °C [13]. For a mixture containing 6% C₄F₇N and 94% CO₂, the primary decomposition products after 12 h at 0.15 MPa and 550 °C include CF₄, C₂F₆, COF₂, CF₃CN, C₃F₆, C₃F₈, (CN)₂, and CO [10,11].

The nature of the pyrolysis products depends on the temperatures. Below 550 °C, C₃F₆ is the main decomposition product of C₄F₇N-CO₂ mixed gas, while above this temperature, the main decomposition products include CO, CF₄, and C₂F₆. Besides temperature, partial discharge of the C₄F₇N-CO₂ mixture can produce a wide range of compounds, including CO, CF₄, CF₂O, C₂F₄, C₂F₄O, CF₃CN, C₂F₆, C₂N₂, C₂F₅CN, C₃F₆, C₃F₈, C₃HF₇, and HCN [11,13]. Among these, C₂F₄, CO, and C₃F₆ are the most abundant, with C₃F₆ being the main pyrolysis product. Despite these findings, experimental limitations have hindered the investigation of C₄F₇N-CO₂ decomposition under extremely high temperatures and voltages.

In this study, an atomic-scale simulation method was employed to investigate the decomposition of C₄F₇N-CO₂ mixtures across a broad range of temperatures (400–1800 °C) and electric fields (0.0025–100 V/Å). The decomposition components and their concentrations were systematically analyzed. The findings provide theoretical insights and data to deepen the understanding of the decomposition characteristics of C₄F₇N-CO₂ mixed gas, offering valuable guidance for practical applications.

2. Calculation Models and Methods

Figure 1 illustrates the simulation model, which consists of 364 CO₂ molecules and 36 C₄F₇N molecules, corresponding to a molar content of 9% C₄F₇N. The cubic simulation box has a side length of 18.9 nm and a density of 5.67 g/cm³, consistent with the previously reported experimental value [18].

Molecular dynamics (MD) simulations were performed using the LAMMPS software (7Aug19) package [19,20,21]. The ReaxFF force field was employed to simulate the decomposition of the C₄F₇N-CO₂ gas mixture, capturing the variation in chemical bonds [22]. In this study, a series of canonical ensemble (NVT) ReaxFF MD simulations were performed to investigate the decomposition of C₄F₇N-CO₂ mixtures under varying temperatures and electronic field intensities. The simulated system was equilibrated for 1 ns with a time step of 0.25 fs under fixed conditions, followed by a 5 ns NVT production run for analyzing the decomposition products.

3. Results and Discussion

3.1. Pyrolysis

Figure 2 shows that with increasing temperature from 400 to 800 °C, the two components of the C₄F₇N-CO₂ mixture remain unchanged, indicating its thermal stability. However, when the temperature rises to 1000 °C, the decomposition of the mixture produces nine distinct components. The computed decomposition temperature of the C₄F₇N-CO₂ mixture is higher than the experimental value (650 °C) [10,12]. The current calculations were performed to obtain the qualitative results.

Figure 3 illustrates the molecular structures of the pyrolysis products of the C₄F₇N-CO₂ gas mixture at 1000 °C. Among these, small molecules such as CFN, CF₃, C₂F, and CN, are difficult to detect under conventional conditions. Their highly reactive unsaturated bonds render them unstable and transient. At elevated temperatures, these small molecules gain enough energy to maintain their configuration temporarily. The experiments in [10,12] reported saturated small molecules, such as CF₄, C₂F₆, etc., as the pyrolysis products. The product molecules with unsaturated bonds in the current calculations are actually the transition state of those experimental products.

The pyrolysis products of the C₄F₇N-CO₂ mixture at 1000 °C have been quantitatively analyzed in Figure 4. CO₂ is absent in the statistics analysis of pyrolysis products due to its lack of observable decomposition. According to the content levels, the pyrolysis products of the C₄F₇N are divided into two parts. The first part includes CFN, CF₃, and C₂F₂, with their total molar content close to 1.85%. The second part refers to components with a molar content of less than 1%, such as C₃F₅, C₄F₆N, C₂F, and CN. CFN, the main pyrolysis product, comes from the combination of a cyano group and the adjacent F atom of the C₄F₇N. Additionally, the cyano group of the C₄F₇N molecule can decompose into other products, such as C₃F₅ via defluorination. Further pyrolysis of C₃F₅ molecules leads to the formation of CF₃, C₂F₂, and C₂F.

Further increasing the temperature from 1000 to 1800 °C can lead to the generation of more pyrolysis products, as shown in

Table 1. Percent of pyrolysis products of C₄F₇N-CO₂ mixture at elevated temperatures.

Product	Temperature			Product	Temperature
	1000 °C	1500 °C	1800 °C		1000 °C	1500 °C	1800 °C
CO2	90.68	90.35	86.54	C4F7	---	0.012	---
C4F7N	8.797	8.43	6.16	C3F6	---	0.002	0.003
CF3	0.17	0.24	1.51	CF	---	---	0.68
C2F	0.001	---	0.30	CO	---	---	0.48
C2F2	0.17	0.20	1.26	C3F7	---	---	0.07
C3F5	0.002	---	0.001	C3F2	---	---	0.05
C4F6N	0.001	0.003	0.01	C3F	---	---	0.02
CN	0.0005	---	0.47	C3F3	---	---	0.005
CFN	0.175	0.47	1.97	C3F3N	---	---	0.002
C2	---	0.25	0.48	C3	---	---	0.002
C2F3	---	0.02	0.002	CF2	---	---	0.0005
C3F4	---	0.02	0.002	CF2N	---	---	0.0005

. The number of decomposition components rises to 9, 11, and 24 at pyrolysis temperatures of 1000, 1500, and 1800 °C, respectively. At 1500 °C, newly formed products include C₂, C₂F₃, C₃F₄, C₄F₇, and C₃F₆. Notably, C₂, as a major product, readily reacts with other molecules due to high levels of unsaturation. In comparison to the products observed at 1000 °C, C₂F, C₃F₅, and CN are not observed at 1500 °C.

At 1800 °C, new decomposition products include CF, CO, C₃F₇, C₃F₂, C₃F, C₃F₃N, C₃, CF₂, and CF₂N. It can be seen that at this temperature, CO₂ undergoes decomposition, producing CO, while CF emerges as a primary decomposition product. Moreover, there are more C₃ molecules, instead of C₂, in the new pyrolysis products. This may be attributed to the competition between the thermal decomposition and recombination process.

Besides the original C₄F₇N and CO₂ molecules, the decomposition products common to the 1000–1800 °C include CF₃, C₂F₂, C₄F₆N, and CFN. Additionally, the concentrations of these four products increase with increasing temperature, highlighting their significance in studies of the pyrolysis of C₄F₇N-CO₂ gas mixtures.

3.2. Electrical Discharge

Figure 5 shows the number of components in the C₄F₇N-CO₂ mixture under varying electric field intensities (0.0025–15 V/Å) at room temperature. The system transitions from two components to twelve components at an electric field intensity of 15 V/Å, indicating the decomposition of mixed gas. Among the decomposition products, only five species—C₄F₆N, C₃F₆, C₃F₄, CF₃N and CN—are derived entirely from perfluoro molecules. In particular, C₃F₆, C₃F₄, and CF₃N are only observed in the electrical discharge decomposition products and are absent in the pyrolysis products. Figure 6 presents the content percentages of the decomposition products, revealing that the five species have the same content of 2.3%, suggesting a relatively simple decomposition process with consistent recombination probabilities. In the partial discharge testing of C₄F₇N-CO₂ mixture, C₃F₆ is one of the most abundant decomposition products [11,13], agreeing well with the current calculations.

For CO₂, five decomposition products—CO, C₂O, O₂, C₂O₂, and C₂O₄—are detected (Figure 7). Among these, CO is a common product of CO₂ decomposition, which is consistent with the experimental testing [11,13]. While the presence of O₂ suggests a high probability of recombination reactions, C₂O and C₂O₄ are identified as key intermediates, while C₂O₂ is an imaginary structure that has not been directly observed in the experiment. Figure 7 also illustrates the relative content of CO₂ decomposition products, showing that CO is the main product, followed by O₂ and C₂O₂. The contents of intermediates C₂O and C₂O₄ are relatively low. It is worth noting that the moderate content of the hypothetical C₂O₂ structure challenges the possibility of its existence.

As the electric field intensity increases from 10 to 30 V/Å, the number of decomposition product components shows a nearly linear growth trend, reaching a maximum of 25 components at an electric field intensity of 30 V/Å (Figure 8). However, with further strengthening of the electrical discharge, the number of components decreases, with only five decomposition products observed at the electric field intensity of 100 V/Å.

The percentage contents of 25 products at an electric field intensity of 30 V/Å are calculated and presented in

Table 2. Percent of decomposition products of C₄F₇N-CO₂ mixture at the electric field intensity of 30 V/Å.

Product	Content (%)	Product	Content (%)	Product	Content (%)
C4F7N	6.72	CF3	0.42	CO2	42.40
CF	3.36	CF2N	0.42	CO	22.27
C2	3.21	C3F4	0.42	O2	11.41
CF2	1.26	C3F	0.42	C2O4	0.57
C2F3	1.26	C3F7N	0.42
C3F4N	1.26	FN	0.42	CFO	0.42
C2F	0.84	C4F4	0.42	C4F5N2O	0.42
CN	0.79	C3F6	0.37	FO	0.02
C3	0.42	C4F6N	0.05

. Based on their origins, the decomposition products can be divided into three groups. The first group consists of 18 components (labeled green in Table 2) derived from the C₄F₇N molecule, with CF and C2 being the main products. The second group includes CO₂ decomposition products (CO, O₂, and C₂O₄, indicated by the gray region in Table 2). Compared to the decomposition products observed at 15 V/Å, C₂O and C₂O₂ are absent, indicating that the hypothetical C₂O₂ structure appears only under specific conditions and not in extremely strong electric fields. The third group comprises three decomposition products (CFO, C₄F₅N₂O, and FO, indicated by the bluish-brown region in Table 2) originating from C₄F₇N and CO₂. The oxygen content in these products is attributed to the CO₂ molecule. CFO and FO can be understood as the combination of –CF or –F from C₄F₇N and O from the CO₂, while C₄F₅N₂O is relatively a more complex and newly identified molecule.

At an electric field intensity of 50 V/Å, the system contains only eight components. The original molecules, C₄F₇N and CO₂, are no longer present, indicating complete decomposition. The main decomposition products include C₂, O₂, and C₃, while the long-chain molecule C₃F₂N is present in small amounts.

When the electric field intensity is further increased to 100 V/Å, only five decomposition products (O₂, C₂, C₃, N₂, and FO) are observed. All these products are small molecules, with the stable molecules O₂ and N₂ appearing, corresponding to complete decomposition of the C₄F₇N-CO₂ mixture. The main productions are C₂ and O₂, with FO present in low concentrations.

4. Conclusions

In conclusion, the decomposition behavior of the C₄F₇N-CO₂ gas mixture under varying temperatures and electric field intensities was investigated using the reactive force field molecular dynamics simulations. Detailed analyses of the resulting decomposition products yielded the following key findings:

(1)

The primary pyrolysis products of C₄F₇N at moderate temperatures include CFN, CF₃, and C₂F₂, while C₃F₅, C₄F₆N, C₂F, and CN are also formed in very low amounts. As the temperature increases, additional products such as C₂, C₂F₃, C₃F₄, C₄F₇, and C₃F₆ appear. At very high temperatures (1800 °C), the newly added products include CF, CO, C₃F₇, C₃F₂, C₃F, C₃F₃, C₃F₃N, C₃, CF₂, and CF₂N. Across different temperatures, CF₃, C₂F₂, C₄F₆N, and CFN are consistently observed, with their concentrations increasing with rising temperature.

(2)

The decomposition products of electrical discharge include C₄F₆N, C₃F₆, C₃F₄, CF₃N, and CN. Among them, C₃F₆, C₃F₄, and CF₃N were not observed in the pyrolysis, while the decomposition of CO₂ is significant under electrical discharge, and the decomposition products include CO, C₂O, O₂, C₂O₂, and C₂O₄. Additionally, increasing the electric field intensity initially increases the number of decomposition components, peaking at 25, primarily from the decomposition of the C₄F₇N molecule. However, at ultra-high electric field intensities, the number of decomposition products decreases, dominated by small molecules such as O₂, C₂, C₃, and N₂.

This study provides a comprehensive analysis of the decomposition products of the C₄F₇N-CO₂ gas mixture over a broad range of temperatures (400–1800 °C) and electric field intensities (0.0025–100 V/Å). Unstable and transient molecular fragments during decomposition, which are difficult to capture in experiments, are presented in this study. These findings offer valuable data to support future theoretical research and practical applications of C₄F₇N-CO₂ gas mixtures as an eco-friendly dielectric medium.