Influence of Oxygen on the Thermal Decomposition Properties of C4F7N-N2-O2 as an Eco-Friendly Gas Insulating Medium.

The C4F7N (fluorinated nitrile) gas mixture has been recognized as the most potential substitute gas to SF6 used in gas-insulated equipment. In this paper, we explored the thermal stability and decomposition properties of the C4F7N-N2-O2 gas mixture. The influence mechanism of oxygen content and temperature on the byproduct generation was obtained and analyzed. It was found that thermal decomposition of the C4F7N-N2-O2 gas mixture mainly produces CO, C3F6, C3F8, CF3CN, (CN)2, and COF2. The addition of oxygen could accelerate the decomposition of C4F7N. The content of C3F6 and (CN)2 decreases, while the yield of CF4, CO, C3F8, and COF2 increases with the oxygen content. Thermal decomposition of the C4F7N-N2-O2 gas mixture at temperatures lower than 425 °C results from the interaction between C4F7N and the metal heating element, while the bond cleavage reactions occur at higher temperature. As for engineering application, the oxygen added in the 6%C4F7N-94%N2 gas mixture should not exceed 6% to avoid the negative effect of oxygen on the thermal stability of C4F7N.

Introduction

Sulfur hexafluoride (SF6) has been extensively used in high-voltage (HV) gas-insulated equipment (GIE) including gas-insulated switchgear (GIS) and gas-insulated lines as the gas insulating medium and switching (interrupting) medium since the early 1960s.[1−4] However, SF6 shows a high global warming potential (GWP) value (23500 times higher than that of CO2) and is listed as one of the most greenhouse gases by Kyoto Protocol.[5−8] The global SF6 mean has reached to 9.8 ppt in 2019, while this value was only 7.4 in 2012.[9] About 80% of the SF6 gas produced worldwide is used in HV GIS, indicating that the power industry is the main consumer.[10] In addition, policy makers have strengthened their position on high GWP gases and manufacturers have reacted accordingly in recent years.[11] Therefore, searching for potential low-GWP SF6 alternatives used in GIE has become a hot topic.

More recently, fluorinated nitrile (C4F7N) has been proposed as the most promising solution in HV applications. The pure C4F7N shows a dielectric strength of approximately twice that of SF6, a GWP of only 2090,[12,13] while its liquefaction temperature at normal condition is −4.7 °C, which can only be used as an additive to CO2 and N2 to avoid liquefaction of the gas at low ambient temperature in engineering application.[14] In addition, the dielectric strength of the C4F7N gas mixture with 18–20% C4F7N can be achieved similar to that of pure SF6.[15] The insulation properties,[16−19] decomposition characteristics,[20−25] and material compatibility[26−28] of the C4F7N gas mixture have been deeply investigated over the past 3 years, confirming that the C4F7N–CO2 or C4F7N–N2 gas mixture has the potential to be used in all kinds of GIE.

Furthermore, it has been pointed out that oxygen (O2) should be added to the C4F7N–CO2 or C4F7N–N2 gas mixture as a second additive gas to improve the switching performance, as well as to reduce the generation of harmful solid byproducts such as soot (carbon particles).[29] Although the addition of O2 could achieve the above advantages for engineering application, the influence of oxygen on the stability of the C4F7N gas mixture must be re-estimated considering the strong oxidation of O2. The gas insulating medium should be chemically stable under engineering application conditions. On one hand, the equipment has the intrinsic temperature rise of about 70 °C, resulting in the temperature of a metal conductor which when contacted with the gas reaches to 105–120 °C under normal operating conditions. On the other hand, the local overheating fault generated in the equipment could cause the decomposition of the gas insulating medium, producing several byproducts. For example, the thermal decomposition of SF6 could produce characteristic components such as H2S, SO2, SOF2, and SO2F2, which can be used to evaluate the working condition of GIE.[32,33] Thus, it is necessary to investigate the thermal stability and decomposition properties of SF6 alternative gas.

At present, the thermal stability of the C4F7N–CO2 and C4F7N–N2 gas mixture has been explored.[13,34,35] Relevant results indicate that the C4F7N gas mixture shows great thermal stability at temperatures lower than 350 °C with little C3F6 produced first. Moreover, CF4, C2F6, C3F8, COF2, CF3CN, (CN)2, and CO are the main thermal decomposition products of the C4F7N gas mixture. However, there are few reports on the thermal stability and decomposition properties of the C4F7N–N2–O2 gas mixture. In addition, the influence mechanism of oxygen on the thermal stability properties of the C4F7N–N2 gas mixture as well as the optimum oxygen addition content is still unclear. In this paper, we carried out long-term thermal stability and decomposition tests for the C4F7N–N2–O2 gas mixture with several O2 contents under different temperature conditions. The composition and content of the main decomposition products were detected and analyzed using gas chromatography–mass spectrometry (GC–MS). The influence of oxygen and temperature on the stability of the C4F7N–N2–O2 gas mixture was revealed first. Relevant results provide important reference for the engineering application of the C4F7N–N2–O2 gas mixture.

Results and Discussion

Influence of Oxygen Content on the Thermal Decomposition of the C4F7N–N2–O2 Gas Mixture

Variation Feature of CO, CF4, C2F6, C3F6, and C3F8

Figure reveals the content of CO, CF4, C2F6, C3F6, and C3F8 of the C4F7N–N2–O2 gas mixture with different oxygen ratios after local overheating aging tests. For the C4F7N–N2 gas mixture, the thermal decomposition results in the generation of C3F6 with the highest content, followed by CO. The content of CF4, C2F6, and C3F8 at the end of the test is lower than 10 μL/L. Relevant results coincide well with our previous thermal tests for the C4F7N–CO2 gas mixture.[34] As for the C4F7N–N2–O2 gas mixture, the content of CO after thermal decomposition tests ranks the highest among all products. In addition, the content of CF4 and C3F8 also increases to a certain extent.

Figure 1

Content of characteristic decomposition products of the C4F7N–N2–O2 gas mixture with different oxygen ratios after local overheating decomposition tests at 0.15 MPa, 450 °C. (a) 0% O2, (b) 2% O2, (c) 4% O2, (d) 6% O2, (e) 8% O2.

The influence of oxygen content on the generation of C3F6 is shown in Figure a. We can find that the yield of C3F6 decreases with the increase of oxygen content. There exists a sharp decrease when the oxygen content changes from 0 to 2%, indicating that the addition of O2 hinders the production of C3F6. We also introduced the effective formation rate (RRMS) defined in refs,[32,33] which could reflect the property, severity, and development trend of a potential fault in engineering to explore the relationship between the oxygen content and thermal decomposition properties of the C4F7N–N2–O2 gas mixture. The definition formula of the RRMS is given as followswhere C and C are the content (peak area) of component i in the first and second detection, respectively. Ra represents the absolute formation rate of component i in j h. Δt is the time interval between two detection periods, which is set to 2 in this paper. The thermal decomposition test for each group lasts 12 h, thus j is set to 6.

Figure 2

Effect of oxygen content on the generation of C3F6, CO, CF4, and C3F8 after local overheating decomposition tests at 0.15 MPa, 450 °C. (a) C3F6, (b) CO, (c) CF4, (d) C3F8.

The effective formation rate of C3F6 also decreases with the increase of oxygen content. Actually, the generation of C3F6 originates from the reaction between C4F7N and the metal heating element. Earlier studies have pointed out that the interaction between the C4F7N–N2 gas mixture and aluminum or copper could generate C3F6 when the temperature of the metal surface reaches to 220 °C.[26] The CN group has strong reaction reactivity, which could adsorb and dissociate on the metal surface, resulting in the decomposition of C4F7N to generate CN and C3F7 particles. Our studies on the thermal decomposition characteristic of the C4F7N–CO2 gas mixture also reveal that the generation of C3F6 begins at 350 °C and its yield increases with temperature lower than 450 °C.[34] Relevant results can also be verified in the C4F7N–N2 gas mixture in this paper. In addition, reactions between O2, O, and the metal heating element or C4F7N occur when oxygen is added. The oxidation process of C3F6 may also occur, causing the decrease of C3F6.

Figure b describes the variation feature of CO under different oxygen content conditions. The yield and effective formation rate of CO increase with the content of O2. The addition of O2 provides O particle, which could participate in the decomposition process and cause the oxidation of C4F7N. The yield of CO for the C4F7N–N2–O2 gas mixture with 8% oxygen reaches to 450 μL/L at the end of 12 h thermal decomposition test, which is 4 times higher than that of the C4F7N–N2 gas mixture.

According to the results given in Figure c, CF4 is not detected for the gas mixture with the oxygen content lower than 4%. The sharp increase for the yield and effective formation rate of CF4 occurs when the oxygen content reaches to 8%, which is higher than 45 μL/L at the end of the test. Earlier studies on the thermal decomposition of the C4F7N–CO2 gas mixture show that the generation of CF4 occurs at temperatures around 550 °C, which could be recognized as the occurrence of severe overheating faults in GIE.[34] As for the C4F7N–N2–O2 gas mixture, the detection of CF4 indicates that the decomposition of C4F7N is accelerated. Because of the reason that the formation of CF4 requires CF3 and F particles and the temperature of the heating element is fixed, the high content of oxygen has a negative effect on the thermal stability of C4F7N.

As for C3F8, its yield shows an increasing trend with the oxygen content lower than 6% and decreases when the oxygen content reaches to 8% (see Figure d). The formation of C3F8 originates from the recombination of C3F7 and F, so the changing trend indicates that the decomposition amount of C4F7N molecules increased with the oxygen content. The content of C3F8 in the 6%C4F7N–86%N2–8%O2 gas mixture is lower than that of the 6%C4F7N–88N2–6%O2 gas mixture, which may be due to the reason that the generated C3F7 is further oxidized to other particles such as CF3.

Variation Feature of CF3CN, (CN)2, and COF2

Figure gives the variation feature of CF3CN, (CN)2, and COF2 under different oxygen conditions after local overheating decomposition tests. It can be seen from Figure a that the peak area (content) of CF3CN increases with the oxygen content first and then decreases, which can be explained as follows. When the O2 content in the gas mixture is lower than 4%, CF4 is not detected during the thermal decomposition test according to Figure c. The reaction between generated CF3 and the CN group which could recombine to form CF3CN occurs. As the yield of CN and CF3 particles increases with the oxygen content, the yield of CF3CN increases when the oxygen content is lower than 4%. When the oxygen content in the gas mixture reaches to higher value (6%), the formation of CF4 occurs because of the generation of F particles. According to the reaction enthalpy for the recombination of CF4 and CF3CN given in eqs and 4, we can find that the formation of CF4 and CF3CN has the negative reaction enthalpies of −126.19 and −106.31 kcal/mol, respectively. Therefore, the generation of CF4 is more likely to happen than that of CF3CN. Considering that the generated CF3 particles are relatively fixed, the yield of CF3CN shows an increasing trend when the oxygen content in the gas mixture is higher than 6%.

Figure 3

Effect of oxygen content on the generation of CF3CN, (CN)2, and COF2 after local overheating decomposition tests (0.15 MPa, 450 °C). (a) CF3CN, (b) (CN)2, (c) COF2.

According to the variation feature of (CN)2 shown in Figure b, the content of (CN)2 has a sharp decrease when the oxygen is added. The changing trend of (CN)2 with the oxygen content is similar to that of C3F6. This is because the generation of both C3F6 and (CN)2 comes from the following decomposition path of C4F7N[20]

The generation of COF2 under different oxygen conditions is given in Figure c, and we can find that the yield of COF2 increases with the oxygen content. The sharp increase occurs when the oxygen content is higher than 2%, indicating that the formation of these byproducts is accelerated. The possible formation paths of COF2 are given as follows[35]

Overall, the addition of oxygen has a negative effect on the thermal stability of the C4F7N–N2 gas mixture, which could accelerate the decomposition of C4F7N. The content of C3F6 and (CN)2 decreases, while the yield of CF4, CO, C3F8, and COF2 increases with the oxygen content. The formation of CF4, which could be recognized as the characteristic component for the occurrence of severe overheating fault, starts when the oxygen content in the gas mixture is higher than 6%. Generally, the addition of oxygen in the C4F7N–N2 gas mixture is suggested not to exceed 6% to avoid the negative effect of oxygen on the stability of C4F7N.

Influence of Temperature on the Thermal Decomposition of the C4F7N–N2–O2 Gas Mixture

Variation Feature of CO, CF4, C2F6, C3F6, and C3F8

In order to further evaluate the thermal stability properties of the C4F7N–N2–O2 gas mixture under different temperature conditions, we conducted thermal decomposition tests for the 6%C4F7N–88%N2–6%O2 gas mixture at 400–500 °C (Figure ).

Figure 4

Content of characteristic decomposition products of the 6%C4F7N–88%N2–6%O2 gas mixture at different temperatures after local overheating decomposition tests at 0.15 MPa. (a) 400, (b) 435, (c) 450, (d) 475, (e) 500 °C.

As we can see from Figure , the content of CO is the highest among all byproducts at 400–475 °C, followed by C3F6 and C3F8. The yield of CF4 and C2F6 is quite lower than that of the other products. Figure a demonstrates the content of C3F6 at different temperature conditions. We can find that the yield and effective formation rate of C3F6 increase with temperature in the range of 400–475 °C. Then, there was a sharp decrease when the temperature reaches to 500 °C. The relevant changing trend indicates that the decomposition of 6%C4F7N–88%N2–6%O2 occurs at temperatures around 400 °C. The sharp decrease indicates that the decomposition mechanism of the C4F7N–N2–O2 gas mixture has changed. The high temperature could directly result in the decomposition of C4F7N to produce CF3 and other small particles.

Figure 5

Effect of temperature on the generation of C3F6, CO, CF4, and C3F8 in the 6%C4F7N–88%N2–6%O2 gas mixture after local overheating decomposition tests (0.15 MPa). (a) C3F6, (b) CO, (c) CF4, (d) C3F8.

The effect of temperature on the yield of CO, CF4, and C3F8 shown in Figure b–d indicates that higher temperature could promote the decomposition of the 6%C4F7N–88%N2–6%O2 gas mixture. In detail, the yield of CO and C3F8 increased dramatically at temperatures higher than 450 °C. In addition, the formation of CF4 is accelerated at temperatures higher than 450 °C. The contents of CO, CF4, and C3F8 after thermal decomposition tests at 400 °C are 111, 4.25, and 35.21 μL/L, respectively, which increased to 522.75, 21.27, and 105.32 μL/L at 500 °C. In addition, the effective formation rate of CO, CF4, and C3F8 also has a positive relationship with fault temperature.

Variation Feature of CF3CN, (CN)2, and COF2

Figure gives the variation feature of CF3CN, (CN)2, and COF2 in the 6%C4F7N–88%N2–6%O2 gas mixture after local overheating decomposition tests. The content of CF3CN presents an increasing trend with temperature at 400–475 °C (see Figure a). In detail, the peak area of CF3CN increases slowly at temperatures lower than 450 °C, and a sharp increasing trend occurs when the fault temperature reaches to higher than 475 °C. In addition, the yield and formation rate of CF3CN slow down at 500 °C. According to the variation feature of (CN)2 shown in Figure b, the peak area and effective formation rate of (CN)2 have a similar changing trend to that of C3F6. The yield of (CN)2 has a positive relationship with temperature in the range of 400–475 °C and reaches the summit value at 475 °C. The content of (CN)2 presents a decreasing trend at temperatures higher than 475 °C, indicating that the generation of the CN particle slows down. Thus, the formation amount and rate of CF3CN also decreased at temperatures higher than 475 °C.

Figure 6

Effect of temperature on the generation of CF3CN, (CN)2, and COF2 in the 6%C4F7N–88%N2–6%O2 gas mixture after local overheating decomposition tests (0.15 MPa). (a) CF3CN, (b) (CN)2, (c) COF2.

Figure 8

Gas chromatogram of the 6%C4F7N–90%CO2–4%O2 gas mixture after the local overheating decomposition test (at 0.15 MPa, 450 °C). (a) total chromatogram; (b) high-resolution chromatogram.

The peak area and formation rate of COF2 also show an increasing trend with temperature. The growth rate is comparatively slow below 425 °C and then increases dramatically at 425–500 °C. Considering that the formation of COF2 needs the participation of CF3, CF2, and O particles, we can conclude that the generation of these particles is accelerated at temperatures higher than 425 °C.

Discussion

The above results show that the fault temperature has a slight influence on the thermal stability and decomposition characteristics of the C4F7N–N2–O2 gas mixture. The thermal decomposition mechanism at different temperatures results in some differences.

The thermal decomposition of the C4F7N–N2–O2 gas mixture starts at temperature around 400 °C. At this stage, C3F6 and CO are the main decomposition byproducts. Their concentration after 12 h aging test is 21.62 and 111 μL/L, respectively. The content of other byproducts such as CF4 and C3F8 is lower than 5.5 μL/L. In addition, the peak area of CF3CN, (CN)2, and COF2 at 400 °C also has the lowest value. The thermal decomposition of the C4F7N–N2–O2 gas mixture at 400 °C is mainly attributed to the reaction between C4F7N and the metal heating element. The fault source may not provide enough energy to result in the direct decomposition of the C4F7N molecule. The CN group in C4F7N could adsorb on the metal surface, and then the dissociation of C4F7N occurs, generating C3F7 and CN.

With the increase of fault temperature, the amount and formation rate of all kinds of products show a positive relationship with temperature at 425–475 °C. C3F6, C3F8, and CO are the main thermal decomposition products at this stage, whose content went up to 556, 143, and 133 μL/L at 475 °C, respectively. The yield of CF4 reaches to 19.81 μL/L at 475 °C. In addition, the peak area of CF3CN, (CN)2, and COF2 also increases with temperature at this stage. The yields of CF3CN and (CN)2 are in their peak value at 475 °C. The decomposition of the C4F7N–N2–O2 gas mixture at this stage results from the interaction between C4F7N and the metal heating element partly. The produced C3F7 and CN could recombine to form amounts of C3F6, C3F8, and (CN)2. In addition, the direct decomposition of C4F7N occurs at temperatures higher than 425 °C considering the quick increase of CF4, CF3CN, and C3F8. Small particles such as CF3, F, and CF2 start to generate at this stage. Actually, the decomposition paths and reaction enthalpy of C4F7N have been calculated based on the density functional theory by Fu et al. and Zhang et al.[20,21] They both pointed out that the bond cleavage reaction of C4F7N generating C3F4N and CF3 has the lowest reaction enthalpy among all possible paths, which is most likely to occur. Kieffel et al. also tested the thermal decomposition properties of the C4F7N–CO2 gas mixture using the tube furnace and found that the decomposition of the gas mixture occurs at temperature higher than 650 °C. In addition, C3F6 was not detected.[13] Thus, we conclude that the thermal decomposition mechanism of the C4F7N–N2–O2 gas mixture at lower temperature (<425 °C) is due to the reaction between C4F7N and metal the heating element, and the bond cleavage process occurs at temperatures higher than 425 °C.

When the temperature reaches higher than 475 °C, the yield of C3F6, CF3CN, and (CN)2 slows down and the content of CO, CF4, and C3F8 is still increasing. The bond cleavage process becomes the dominant decomposition reactions at this stage.

As for engineering application, considering that the temperature of the metal conductor is at 105–120 °C, the C4F7N–N2–O2 gas mixture has great thermal stability at this temperature range. Thus, oxygen has few impacts on the thermal stability properties of the C4F7N–N2–O2 gas mixture under normal application conditions. In addition, it is necessary to monitor the temperature of GIE to avoid the appearance of overheating faults, which could cause the decomposition of the C4F7N–N2–O2 gas mixture. The existence of C3F6 indicates that thermal faults at early stage occur, and the detection of CF4 indicates that severe faults occur in GIE.

Conclusions

In this paper, we investigated the thermal stability and decomposition properties of the C4F7N–N2–O2 gas mixture used as the alternative gas for SF6 in HV GIE. The components of the gas mixture under different test conditions were detected using GC–MS. The variation feature and influence mechanism of the oxygen content and temperature on the thermal decomposition characteristics of the C4F7N–N2–O2 gas mixture were obtained and analyzed. The following conclusions can be obtained:

The decomposition of the C4F7N–N2–O2 gas mixture mainly produces CO, C3F6, C3F8, CF4, CF3CN, (CN)2, and COF2. The addition of oxygen has a negative effect on the thermal stability of the C4F7N–N2 gas mixture, which could accelerate the decomposition of C4F7N. The content of C3F6, (CN)2 decreases, while the yield of CF4, CO, C3F8, and COF2 increases with the oxygen content.

Thermal decomposition of the C4F7N–N2–O2 gas mixture at temperatures lower than 425 °C results from the interaction between C4F7N and the metal heating element. The bond cleavage process of C4F7N occurs at temperatures higher 425 °C, generating CF3, F, and other particles. In addition, the thermal decomposition of the gas mixture at higher temperature (>475 °C) is mainly attributed to the bond cleavage reactions.

As for engineering application, the recommended oxygen added in the C4F7N–N2–O2 gas mixture should not exceed 6% to avoid the negative effect on the gas mixture. The existence of C3F6 indicates that thermal faults at early stage occur and the detection of CF4 indicates that severe faults happen in GIE.

Methods

Test Platform

Figure shows the schematic diagram of the local overheating decomposition test platform, which mainly consists of the gas chamber, the heating system, and the temperature control system. The stainless-steel gas chamber could withstand the high pressure of 0.7 MPa. The K-type heating element is set in the center of the gas chamber to simulate the overheating faults in GIE.[32,33] The temperature control system includes the electric relay, the PID control device, and temperature sensor. The temperature sensor embedded in the heating element tests the actual temperature and sends the signal to the PID control device. Then, the PID control device compares the actual temperature with the set temperature and sends switching signal to the electric relay. The no-deviating regulation of the operating temperature and target temperature can be achieved by this system.

Figure 7

Schematic diagram of the local overheating decomposition test platform.

The gas samples were analyzed by GC–MS (Shimadzu QP2010). The CP-Sil5CB (60 m × 0.32 mm) capillary column was chosen to separate the characteristic decomposition products. The heating procedure of the column is given as follows: (1) the column should be kept at 32 °C for 7 min. (2) The column should be heated to 150 °C with the rate of 60 °C/min. (3) The column should be kept at 150 °C for 2 min. We used single ion monitoring method to detect the possible decomposition products, and Table gives the mass-to-charge ratios (m/z) of characteristic decomposition products of the C4F7N gas mixture.

Table 1

Mass-to-Charge Ratios of Characteristic Decomposition Products of the C4F7N Gas Mixture

no	mass-to-charge ratios (m/z)
CF4	69
C2F6	69, 119
C3F8	119, 69, 169
C3F6	69, 131, 100
CO	12
CF3CN	76, 69
(CN)2	52
COF2	66, 47

In order to confirm the separation performance, we conducted the thermal aging test for the 6%C4F7N–90%CO2–4%O2 gas mixture for 12 h. Figure gives the gas chromatogram obtained using the above method. We can find that the characteristic peak of CO, COF2, C3F8, CF3CN, C3F6, and (CN)2 appears, indicating that the chosen capillary column is applicable.

As for quantitative analysis of the characteristic decomposition products, we established an external standard method for CF4, C2F6, C3F8, C3F6, and CO. In addition, the peak area integral method was used for relative quantitative analysis for COF2, CF3CN, C3F6, and (CN)2 because of the reason that the standard gas for these byproducts is not accessible at present. GC–MS has the detection limit of 0.1 μL/L, which guarantees the accuracy of the detection results.

Test Method

In order to explore the thermal decomposition properties of the C4F7N–N2–O2 gas mixture, we carried out aging tests for each group of gas mixture for 12 h. The content of C4F7N in the gas mixture is set to 6% considering that the content of C4F7N in the gas mixture should be less than 6% at 0.6 MPa for the minimum operating temperature of −25 °C in engineering application.[34] We totally conducted nine groups of tests to explore the influence of temperature and oxygen content on the thermal decomposition characteristics of the C4F7N–N2–O2 gas mixture, as shown in Table . The content of C4F7N is fixed to 6% for all groups. In addition, the content of oxygen is set to 0, 2, 4, 6, and 8%. For the 6%C4F7N–84%N2–8%O2 gas mixture, the content of O2 is higher than that of C4F7N. Considering the strong oxidized characteristic of O2, we think that using 8% as the upper limit of oxygen is enough.

Table 2

Thermal Decomposition Test Conditions of the C4F7N–N2–O2 Gas Mixture

no	gas mixture composition	test temperature (°C)	pressure (Mpa)
1	6%C4F7N–94%N2–0%O2	450	0.15
2	6%C4F7N–92%N2–2%O2	450	0.15
3	6%C4F7N–90%N2–4%O2	450	0.15
4	6%C4F7N–88%N2–6%O2	450	0.15
5	6%C4F7N–86%N2–8%O2	450	0.15
6	6%C4F7N–88%N2–6%O2	400	0.15
7	6%C4F7N–88%N2–6%O2	425	0.15
8	6%C4F7N–88%N2–6%O2	475	0.15
9	6%C4F7N–88%N2–6%O2	500	0.15

The gas chamber was cleaned with anhydrous alcohol to remove the impurities first. Then, pure CO2 was injected into the chamber to 0.3 MPa and pumped to 0 MPa. This procedure was repeated three times to ensure that there is no impurity gas in the chamber. Finally, the C4F7N–N2–O2 gas mixture with different O2 contents was filled. The C4F7N (purity of 99.2%) used in this paper was provided by 3M company. In addition, the He (purity of 99.999%) and N2 (purity of 99.999%) were supplied by Wuhan Newred Special Gas Co., Ltd.