processes Article
The Influence of O2 on Decomposition Characteristics of c-C4F8/N2 Environmental Friendly Insulating Gas Song Xiao 1 , Shuangshuang Tian 1,2, *, Xiaoxing Zhang 1, * , Yann Cressault 2 , Ju Tang 1 , Zaitao Deng 1 and Yi Li 1 1
2
*
School of Electrical Engineering, Wuhan University, BaYi Street No. 299, Wuhan 430072, China;
[email protected] (S.X.);
[email protected] (J.T.);
[email protected] (Z.D.);
[email protected] (Y.L.) Laboratoire Plasma et Conversion d’Energie (LAPLACE), UPS, Université de Toulouse, 31062 Toulouse, France;
[email protected] Correspondence:
[email protected] (S.T.);
[email protected] (X.Z.); Tel.: +86-158-2757-4383 (S.T.); +86-136-2727-5072 (X.Z.)
Received: 5 August 2018; Accepted: 19 September 2018; Published: 29 September 2018
Abstract: The c-C4 F8 gas is considered to have great potential as a gaseous medium for gas-insulated equipment, due to its good insulation properties and its relatively low greenhouse gas potential (GWP) relative to SF6 . However, the decomposition is an important indicator of its use in equipment. In this paper, the decomposition characteristics of c-C4 F8 and the influence by oxygen have been explored through experiments and theoretical calculations. Firstly, the breakdown test of mixed gas was carried out and the precipitated elements of the electrodes and breakdown products of gas mixture were analyzed by X-ray photoelectron spectroscopy (XPS) and gas chromatography mass spectrometry (GC-MS). At the same time, the differences in decomposition products have also been studied when a small amount of O2 was present. The path and mechanism of c-C4 F8 decomposition is then discussed, based on density functional theory (DFT). The results show that the black powdery substance descends on the electrode surface after the breakdown of the mixture of c-C4 F8 /N2 gas containing O2 , and its main constituent elements are C, O and F. O2 can promote the decomposition of c-C4 F8 . The mixture with O2 produced a large number of additional toxic and corrosive COF2 in addition to generating more CF4 , C2 F4 , C2 F6 , C3 F6 and C3 F8 . The GWP values of the products are lower than SF6 . Comprehensive insulation properties and decomposition characteristics, c-C4 F8 should not be mixed with dry air for use, and the oxygen content should be strictly controlled in c-C4 F8 mixed gas. Keywords: SF6 alternative gas; c-C4 F8 /N2 gas mixture; breakdown characteristics; decomposed component; greenhouse effect
1. Introduction Sulfur hexafluoride (SF6 ) has been widely used in the power industry due to its excellent insulation and arc extinguishing properties. A total of 80% of the SF6 is used in high voltage circuit breakers and other high-pressure gas insulation equipment [1]. However, SF6 was listed as one of the six major greenhouse gases with a GWP (Global Warming Potential) of 23,500 and an atmospheric lifetime of 3200 years in the 1972 Kyoto Protocol [2,3]. Although the atmospheric concentration of SF6 is relatively low, contributing to 0.1% of the total anthropogenic radiative forcing, the concentration is growing continuously because of the compound’s long lifetime of ~3200 years [4,5]. Over the past five years, the content of SF6 in the global atmosphere has increased by 20% [6]. California proposed that the use of SF6 in the electrical field should be reduced annually from 2020 and the European Union
Processes 2018, 6, 174; doi:10.3390/pr6100174
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plans to reduce SF6 emissions to 2/3 between 2014 and 2030 [7]. In order to ensure the sustainable development of the power industry, reducing the use of SF6 has become an important task in the current production and use of power equipment. In addition, SF6 could generate a variety of harmful gases such as SO2 , SOF2 , SO2 F2 , S2 F10 , under partial discharge, spark discharge or arc discharge [8–10]. Therefore, looking for an environmentally friendly and safe gas as an alternative insulation medium in power equipment, has become a sought-after solution. In the current research, the ideal substitutes for SF6 include perfluorocarbons (PFCs), trifluoroiodomethane (CF3 I), and other gases. Among them, the C-I bond of CF3 I can easily be broken to produce iodine solid, which produces toxic gases such as CH3 I, COF2 after discharge, which is not conductive to the long-term safe operation of the equipment [11,12]. Of course, the adsorbent can adsorb by-products, but finding a suitable adsorbent and whether the adsorbent has an effect on the insulation is also a problem. PFCs mainly include C2 F6 , C3 F8 , c-C4 F8 , among which c-C4 F8 is the most excellent insulating property (about 1.18 times that of SF6 ). It is non-toxic to humans and the environment with the GWP of 8700 and the liquefaction temperature of −6 ◦ C [13]. Although c-C4 F8 also has greenhouse effect, its global warming potential is much lower than SF6 (23,000). Replacement of SF6 with c-C4 F8 in power equipment will significantly reduce the greenhouse effect. So far, there are still many scholars to study the possibility of replacement SF6 . Concerning the new environmentally friendly insulation gas produced by 3M, apart from the C5 F10 O (Novec 5110) mentioned by reviewers, there is also C4 F7 N (Novec 4710) [14,15]. Both of these gases are hot in the current stage of research. We do not deny the superior performance of these two gases. However, gas liquefaction temperatures such as C5 F10 O are too high to be suitable for all types of gas-insulated equipment. In recent years, many researchers have studied the insulation performance of c-C4 F8 with N2 , CO2 and other mixed gases under DC, AC and lightning impulse voltage. It is concluded that the electrical strength of c-C4 F8 -N2 mixtures can only reach approximately 0.57 times that of pure SF6 at 0.5 MPa with c-C4 F8 concentrations of 20% [16], but 15–20% c-C4 F8 gas mixture meets the requirements of electrical equipment, and mixtures can greatly reduce the impact of insulating gas on the environment [17–19]. Li et al. calculated the mixtures between c-C4 F8 and variety of gases in order to determine the mixed gas insulation. For example, the critical breakdown strength of c-C4 F8 -N2 and c-C4 F8 -air are similar and significantly higher than other mixed gases. When the content of c-C4 F8 exceeds 80%, the insulation property of mixture can reach the level of pure SF6 but there is a lack of experimental data for validation [20]. Some studies on the decomposition characteristics of c-C4 F8 have achieved some results. Li et al. [21] explored the decomposition products of c-C4 F8 under various typical faults based on the gas chromatograph-mass spectrometer test. It was found that the decomposition of c-C4 F8 mainly produced CF4 , C2 F6 and C2 F4 , C3 F8 , C3 F6 . Hayashi et al. [22] discussed the main pathways for the dissociation of c-C4 F8 to generate free radicals CF2 based on the density functional theory (DFT), providing guidance for further investigation of the decomposition mechanism of c-C4 F8 . As the core insulating gas, c-C4 F8 may need to be mixed with other gases due to the liquefaction temperature, and dry air is the common buffer gas. The theoretical calculations and experimental studies have shown that c-C4 F8 -N2 has good insulation properties. Potential alternative environmentally-friendly alternative gas and dry air often are mixed as an insulating medium. Additionally, in order to promote the recovery after the arc, O2 often appears in the arc medium. However, the related research is lacking, and the toxicity and environmental safety of the decomposition products after mixing with oxygen are urgent to be investigated. Oxygen introduction is divided into passive introduction and active introduction. Passive introduction mainly refers to the introduction of trace amounts of oxygen introduced by the equipment during operations such as transportation, assembly, maintenance, and ventilation. Generally, it will not exceed 1%, which is similar to the problems faced by SF6 gas insulation equipment [23]. The other is active introduction, because the environment-friendly insulation gas may be mixed with dry air to fill the equipment. In addition to nitrogen and CO2 , dry air mainly contains O2 . The safety of the other two gases has been verified in many studies [24]. Therefore, the influence of O2 on the insulation performance and decomposition properties of c-C4 F8 also needs to be studied, and it is very urgent.
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Processes 2018, 6, 174 The influence
3 of 14 of oxygen on the decomposition of mixed gas can provide the basis for exploring c-C4F8-air and the performance analysis of the decomposition products is also an important indicator of evaluation gas application. In order to obtain a new insulation formula for environmental safety The influence of oxygen on the decomposition of mixed gas can provide the basis for exploring and to study the possible causes of hazardous products, it is necessary to test and analyze the physical c-C4 F8 -air and the performance analysis of the decomposition products is also an important indicator and chemical processes of oxygen gas mixture discharge. of evaluation gas application. In order to obtain a new insulation formula for environmental safety In this paper, the decomposition products of pure c-C4F8 and the c-C4F8/N2 mixed gas are and to study the possible causes of hazardous products, it is necessary to test and analyze the physical explored experimentally and the influence of oxygen are also considered. N2 is just the buffer gas and chemical processes of oxygen gas mixture discharge. added in order to reduce the liquefaction temperature, because its physical and chemical properties In this paper, the decomposition products of pure c-C4 F8 and the c-C4 F8 /N2 mixed gas are are stable and not easy to decompose. After several frequency breakdown experiments, the explored experimentally and the influence of oxygen are also considered. N2 is just the buffer gas decomposition products and electrode precipitation elements were obtained by GC-MS (gas added in order to reduce the liquefaction temperature, because its physical and chemical properties are chromatography mass spectrometry) and XPS (X-ray photoelectron spectroscopy). Based on the stable and not easy to decompose. After several frequency breakdown experiments, the decomposition density functional theory, the structural characteristics of c-C4F8 were calculated firstly, the stability products and electrode precipitation elements were obtained by GC-MS (gas chromatography mass of the molecular structure and the reactive sites were analyzed. The possible dissociation pathways spectrometry) and XPS (X-ray photoelectron spectroscopy). Based on the density functional theory, of c-C4F8 and the formation mechanism of the decomposition products were discussed. The purpose the structural characteristics of c-C4 F8 were calculated firstly, the stability of the molecular structure of this paper is to study the changes of decomposition products with the participation of oxygen and and the reactive sites were analyzed. The possible dissociation pathways of c-C4 F8 and the formation explore the safety and environmental protection of products. The comprehensive evaluation of the mechanism of the decomposition products were discussed. The purpose of this paper is to study influence of oxygen on the decomposition characteristics of c-C4F8 provides a reference for the the changes of decomposition products with the participation of oxygen and explore the safety and research of SF6 alternative gas. environmental protection of products. The comprehensive evaluation of the influence of oxygen on the decomposition characteristics of c-C4 F8 provides a reference for the research of SF6 alternative gas. 2. Experimental Setup
2. Experimental The test wasSetup carried out at a 50 Hz AC voltage gas insulation performance test platform. In the experiment, 2, which is stable chemically gas, was used as the buffer gas. The testN was carried out at a 50 Hz AC voltage gas insulation performance test platform. In the Before the thechemically airtightness ofwas the used gas chamber wasgas. checked, and the sealed gas experiment, N2 , experiment, which is stable gas, as the buffer chamber was using vacuum pump (BECKER/VT4.16, BECKER, Wuppertal, Germany) Before theevacuated experiment, thea airtightness of the gas chamber was checked, and the sealed gas and was allowed to stand still for 60 min (less than 10 Pa). N 2 gas is used to clean the gas chamber. chamber was evacuated using a vacuum pump (BECKER/VT4.16, BECKER, Wuppertal, Germany) The was above steps are repeated to60 3 times in order to Pa). avoid ofclean impurity gases. After and allowed to stand still 2for min (less than 10 N2the gasinfluence is used to the gas chamber. cleaning, mixed is introduced into in theorder gas chamber. ball-ball electrode wasgases. used in the The abovethe steps are gas repeated 2 to 3 times to avoid The the influence of impurity After experiment. The diameter of the copper ball was 50 mm with the electrode spacing of 5 mm. The cleaning, the mixed gas is introduced into the gas chamber. The ball-ball electrode was used in the range of applications for mixed gases may devices suchThe as switch experiment. The diameter of the copper ball be wasmedium-voltage 50 mm with the gas-insulated electrode spacing of 5 mm. range cabinets. Because the key part of the conductivity in these devices is made of copper, it is used as the of applications for mixed gases may be medium-voltage gas-insulated devices such as switch cabinets. electrode material. The purity level of the c-C4F8 and N2 is 99.999%. Because the key part of the conductivity in these devices is made of copper, it is used as the electrode According to the literature [24], it and has N2 been shown that the breakdown behavior of c-C4F8/N2 material. The purity level of the c-C4F8 is 99.999%. mixture (the mixing ratio of c-C 4F8 is 5%~20%) under uniform electric field is similar to that of SF6/N2 According to the literature [24], it has been shown that the breakdown behavior of c-C4 F8 /N2 with the(the same mixing ratio. TheFCorona discharge performance of c-C4F8/N2 is better than the same mixture mixing ratio of c-C 4 8 is 5%~20%) under uniform electric field is similar to that of SF6 /N2 mixing 6/N2. If the c-C4F8/N2 gas mixture is not liquefied at −30 °C, the content of c-C4F8 in the with theratio sameSFmixing ratio. The Corona discharge performance of c-C4 F8 /N2 is better than the same gas mixture is at most about 15% [25]. Therefore, the pure c-C4F8, 15% c-C4◦F8/N2 and with 3% O2 were mixing ratio SF6 /N 2 . If the c-C4 F8 /N2 gas mixture is not liquefied at −30 C, the content of c-C4 F8 in selected as samples in thisabout paper. The test was carried out by the gas mixture is at most 15% [25]. Therefore, the pure c-Ca4step-and-step F8 , 15% c-C4 F8method, /N2 and and withthe 3%gas O2 mixture was broken down 50 times to detect decomposition products. The details are shown in Figure were selected as samples in this paper. The test was carried out by a step-and-step method, and the gas 1. mixture was broken down 50 times to detect decomposition products. The details are shown in Figure 1. 14 12
voltage(kV)
10 8 6 4 2 0
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Figure Figure 1. 1. The The step step method. method.
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GC-MS is used for detection of gas mixture components after the breakdown discharge. CP Sil 5CB GC-MS was selected asfor thedetection column. GC analysis, thecomponents high purity after He (the of gas discharge. is above 99.999%) is used of gas mixture thepurity breakdown CP Sil gas is used as a carrier gas, and carrier gas filters (including He filter and RP oil filter) are99.999%) used to 5CB was selected as the column. GC analysis, the high purity He (the purity of gas is above filter out the may affect the column and thefilter) experimental results. gas is used as aimpurities, carrier gas,which and carrier gas filters (including Heinterfere filter andwith RP oil are used to filter Inlet temperature is 220 °C and inlet pressure is 56.1 kPa. The injection mode uses a split flow method out the impurities, which may affect the column and interfere with the experimental results. Inlet with a split is ratio injection volume 1 mL. The mode column flow rateflow is 1.2method mL/min, and ◦ C10:1 temperature 220 of andand inletan pressure is 56.1 kPa. of The injection uses a split with a the purge flow rate is 3.0 mL/min. The heating rate of gas into the oven is shown in Figure 2. The split ratio of 10:1 and an injection volume of 1 mL. The column flow rate is 1.2 mL/min, and the purge ovenrate hadisan temperature of 35rate °C,of a gas finalinto temperature 150 °C,inand a temperature flow 3.0initial mL/min. The heating the oven isofshown Figure 2. The ovenincrease had an rate of 40 °C/min. The column temperature was maintained at 35 °C for 0~8 min, and after 8 min, the ◦ ◦ ◦ initial temperature of 35 C, a final temperature of 150 C, and a temperature increase rate of 40 C/min. temperature rose steadily to 150 °C, and the temperature rising rate was kept at 40 °C/min. MS ◦ The column temperature was maintained at 35 C for 0~8 min, and after 8 min, the temperature rose analysis,tothe temperature is 200 °C,rate the was chromatographic mass spectrometry ◦ C,source steadily 150ion and the temperature rising kept at 40 ◦ C/min. MS analysis,interface the ion temperature is 220 °C, ionization mode is Electronic ionization (EI). The solvent delay time ◦ source temperature is 200 C, the chromatographic mass spectrometry interface temperature is was 220 ◦0.1 C, min; the detector voltage threshold was 100 and the voltage was 0.1 kV. The specific steps can be ionization mode is Electronic ionization (EI). The solvent delay time was 0.1 min; the detector voltage found in [26–30]. threshold was 100 and the voltage was 0.1 kV. The specific steps can be found in [26–30]. 150℃
Oven Temperature（℃）
150 125 100 75 50
35℃ 25 0
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Figure2.2. Oven Oven temperature temperatureof ofGC-MS GC-MS(gas (gaschromatography chromatographymass massspectrometry). spectrometry). Figure
3.3. Experimental ExperimentalResults Results 3.1. Electrode Precipitate Component Analysis 3.1. Electrode Precipitate Component Analysis Figure 3 shows the surface of the ball electrode before and after breakdown of pure c-C4 F8 , Figure 3 shows the surface of the ball electrode before and after breakdown of pure c-C4F8, cc-C4 F8 /N2 gas without O2 and with 3% O2 . It can be seen that the surface of the copper ball electrode C4F8/N2 gas without O2 and with 3% O2. It can be seen that the surface of the copper ball electrode is is covered with a layer of black powdery solid. Compared to pure gas, the mixed gas deposits covered with a layer of black powdery solid. Compared to pure gas, the mixed gas deposits are are significantly less. However, the addition of oxygen increases the production of solid products significantly less. However, the addition of oxygen increases the production of solid products while while there only was a clear trace of ablation on the electrode surface at absence of O2 in mixture. there only was a clear trace of ablation on the electrode surface at absence of O2 in mixture. The The ESCALAB 250Xi photoelectron spectrometer (Thermo Scientific Corp., Waltham, MA, USA) was ESCALAB 250Xi photoelectron spectrometer (Thermo Scientific Corp., Waltham, MA, USA) was used to scan the black surface of the electrode by XPS. It was found that it consisted of C, F, O and N used to scan the black surface of the electrode by XPS. It was found that it consisted of C, F, O and N elements. Table 1 shows the content of each component in the black material, in which the C content is elements. Table 1 shows the content of each component in the black material, in which the C content 58.44%, which is the main constituent element, the content of O is 17.32%, the N element content is is 58.44%, which is the main constituent element, the content of O is 17.32%, the N element content is 0.75% and the F element content is 23.49%. Figure 4 shows the C photoelectron scanning. 0.75% and the F element content is 23.49%. Figure 4 shows the C photoelectron scanning. Table 1. Element content of precipitates on spherical electrode surface. Table 1. Element content of precipitates on spherical electrode surface. Element
Element C1s C1s O1s O1s N1s N1s F1s F1s
Concentration (%) (Precision: 0.01%)
Concentration (%) (Precision: 0.01%) 58.44 58.44 17.32 17.32 0.75 0.75 23.49 23.49
As 285 shown Figure 4, the maximum intensity of the corresponding C element energy is about eV,incorresponding to the chemical structure of the C–C single bondbinding structure, which about 285 eV, to the chemical theofC–C single the bond structure, which constitutes thecorresponding C elemental elements. With Cstructure element of loss electrons, binding energy can constitutes the C elemental elements. With C element loss of electrons, the binding energy can increase to 290 eV, corresponding to the chemical structure of O–C=O, which may constitute a variety increase to oxides. 290 eV, Through corresponding to the chemical structure of the O–C=O, which may constitute a variety of carbon XPS analysis, the black surface of electrode may show a single element of carbon oxides. Through XPS analysis, the black surface of the electrode may show a single element Processes 2018, 6, 174 of C and various oxyfluorides of carbon. The test results are consistent with the literature [21].5 of 14 of C and various oxyfluorides of carbon. The test results are consistent with the literature [21].
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Figure 3. The spherical electrode before and after breakdown of c-C4F8/N2 gas. (a) the ball electrode The spherical and gas. N (a) the ball ball electrode Figure electrode before andafter afterbreakdown breakdownofofc-C c-CF 4F /N 2 gas. (a) the 88/N 8/85% N2; (c) 100% c-C4F8; (d) 15%4c-C 4F82 /82% 2/3% O2. before3.breakdown; (b) electrode 15% c-C4Fbefore /85%NN2;2(c) ; (c)100% 100%c-C c-C ; (d) 15% c-C F8 /82% N2 /3% 4F48F ; 8(d) 15% c-C 4F84/82% N2/3% O2. O2 . before breakdown; (b) 15% c-C44F88/85%
20000 20000
C1s C1s
Counts(/s) Counts(/s)
15000 15000 10000 10000 5000 5000 0 0
Baseline Baseline
294 292 290 288 286 284 282 280 278 276 294 292 290 288Binding 286 Energy 284 (eV) 282 280 278 276 Binding Energy (eV)
Figure 4. C element photoelectron scanning. Figure 4. C element photoelectron scanning. Figure 4. C element photoelectron scanning. As shown in Figure 4, the maximum intensity of the corresponding C element binding energy
Experimental Analysis of the Decomposition Products of c-C 8 under Condition which is3.2. about 285 eV, corresponding to the chemical structure of4Fthe C–CMicro-Oxygen single bond structure, 3.2. Experimental Analysis ofelements. the Decomposition Productsloss of c-C 4F8 under Micro-Oxygen Condition constitutes the C elemental With C element of electrons, the binding energy In order to further verify the accuracy of the experimental results, the gas-insulated can test increase platform towas 290In eV, corresponding to the chemical structure of O–C=O, which may constitute a variety of order to further verify the accuracy of the experimental results, the gas-insulated test platform used to conduct continuous, 50 rounds of pure c-C4F8, c-C4F8/N2 mixed gas with 0% andcarbon 3% O2 oxides. XPStests analysis, the50 black of the electrode may single element oftest C and was used to conduct continuous, rounds of pure c-C 4F8, c-C4F 8/N2 show mixeda gas 0% and 3% O2 contentThrough breakdown and analysis ofsurface discharge breakdown components. Thewith breakdown was various oxyfluorides of carbon. The test results are consistent with the literature [21]. content breakdown tests and discharge breakdown components. The breakdown was conducted on the mixed gasanalysis and theofdecomposition components of the mixed gas were test detected. conducted on the mixed gas and the decomposition components of the mixed gas were detected. Figure 5a,b shows the GC-MS chromatograms(Varian Medical Systems, Inc., Palo Alto, CA, USA) of 3.2. Experimental Analysis of thechromatograms(Varian Decomposition Products of c-C4 F8 Systems, under Micro-Oxygen Condition Figure 5a,b shows theafter GC-MS Medical CA, USA) of the two mixed gases breakdown. Qualitative analysis is carried out Inc., usingPalo the Alto, combined methods the mixed after breakdown. Qualitative analysis and is carried outthe using the combined In order togases further verify the accuracy of experimental results, gas-insulated test methods platform of two the United States National Institute ofthe Standards Technology (Nist14.0) database, and ofstandard the United States National Institute of Standards and Technology (Nist14.0) database, was used to conduct continuous, 50 rounds of pure c-C F , c-C F /N mixed gas with 0% andand 3% 4 8 4 8 2 gas chromatography scanning. standard gas chromatography scanning. O2 content breakdown tests and analysis of discharge breakdown components. The breakdown test was conducted on the mixed gas and the decomposition components of the mixed gas were detected. Figure 5a,b shows the GC-MS chromatograms(Varian Medical Systems, Inc., Palo Alto, CA, USA) of the two mixed gases after breakdown. Qualitative analysis is carried out using the combined methods of the United States National Institute of Standards and Technology (Nist14.0) database, and standard gas chromatography scanning.
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C2F6
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(b) M/Z = 69 Figure Figure5.5.Chromatogram Chromatogramanalysis analysisofofGC-MS GC-MS(The (Theordinate ordinateindicates indicatesthe therelative relativepeak peakintensity). intensity).
The wasselected selectedasasthe thebase basepeak. peak.AsAs shown in Figure The mass-to-charge mass-to-charge ratio ratio M/Z M/Z ==11was shown in Figure 5a, 5a, the the chromatograms of the two mixed gases showed the characteristic peaks of CF , C F , 6 CC 23FF46. chromatograms of the two mixed gases showed the characteristic peaks of CF4, C2F6, 4C2F42 and and peak2 also of COF appears with COF highly with 3 F6 . The characteristic 2 also with 2 of 3%. 2 is atoxic TheCcharacteristic peak of COF appears O2 of 3%.OCOF 2 is a highly gastoxic withgas a strong airritating strong irritating effect on the skin and respiratory mucosa. It is also corrosive and can irreversibly effect on the skin and respiratory mucosa. It is also corrosive and can irreversibly damage damage equipment insulation. order toall display all products containing CF3 •the groups, results equipment insulation. In order In to display products containing CF3• groups, resultsthe of the scan of the scan wereusing analyzed = 69peak, as theasbase peak, as shown Figure 5b. Chromatogram were analyzed M/Z using = 69 asM/Z the base shown in Figure 5b. in Chromatogram in addition to in ,C ,C , C3 Fappeared appeared characteristic From the testthe results, 6 , C3 F8 also CFaddition 4, C2F6, to C2CF F4, 4C 3F26F , 6C 3F28F4 also characteristic peaks. Frompeaks. the test results, main the main decomposition c-C4 F8 molecules, N2 in the did not decomposition products products are from are the from c-C4F8the molecules, and N2 in and the mixed gasmixed did notgas participate participate in thereaction. chemical reaction. in the chemical Test results Test results show show that that pure pure gas gas isis more more likely likely toto decompose decompose and and the the intensity intensity ofof each each decomposition was higher higherthan thanthat thatofofc-C c-C F8 /N In order to investigate the 2 .order decomposition product product monitored monitored was 4F48/N 2. In to investigate the effect effect of O on the decomposition of c-C F /N gas mixture in more detail, Table 2 shows the peak 2 decomposition of c-C4F8/N2 gas 4 8 mixture 2 of O2 on the in more detail, Table 2 shows the peak intensities intensities of the decomposition components of c-C 0%Oand It canthat be seen 4 Fmixture 8 /N2 gasatmixture 2 . seen of the decomposition components of c-C4F8/N 2 gas 0% andat3% 2. It 3% canObe with that with the addition O2 ,intensities the peak intensities CF C2 F6C, 2C have large 8 and 2 F4 all the addition of O2, theof peak of CF4, C2Fof 6, C 3F48, and F43 Fall haveClarge changes, andchanges, the peak and the peak intensity of CF4by produced c-C4 F3%O with 3% Oby C2 F 8 /N22 increased 2 increased 6 relative intensity of CF 4 produced c-C4F8/Nby 2 with 275.34%,by C2275.34%, F6 relative increase of increase of about growth of about 140%. rate The of growth rateC2of 4 and Cgrowth 3 F8 relative 4 and about 200%, C2F4200%, and CC 3F28Frelative of about 140%. The growth CF4 and F6 CF is higher Cthan higher that of C3analysis F8 . The above shows that the of decomposition c-C4 Fto8 produce is more 2 F6 is that of C3than F8. The above shows analysis that the decomposition c-C4F8 is moreoflikely likely to produce small molecule products and the mixed gas containing O generates extra COF 2 2 .2 small molecule products and the mixed gas containing O2 generates extra COF 2. The presence of O The presence O2 promotes the decomposition of c-C4 F8 /N2 . promotes theofdecomposition of c-C 4F8/N2.
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Table 2. Peak strength of c-C4F8/N2 mixed gas decomposition products under different O2 contents. Table 2. Peak strength of c-C4 F8 /N2 mixed gas decomposition products under different O2 contents.
Products CF4 CF4 C2F6 C2 F6 3F 8 CC 3 F8 CC F 2 2F 44 CC 3F 66 3F
Products
0% O2 1,052,859 1,052,859 1,944,746 1,944,746 196,263 196,263 8,377,435 8,377,435 1,479,519 1,479,519 0% O2
3% O2 The Growth Rate (%) 3% O2 The Growth Rate (%) 2,898,954 275.34 2,898,954 275.34 3,808,650 195.84 3,808,650 195.84 275,746 140.50 275,746 140.50 11,928,574 142.39 11,928,574 142.39 1,627,829 110.02 1,627,829 110.02
4. Discussion Discussion In order to explore the discharge decomposition path of c-C c-C44F88 and and the the effect of O22 on its decomposition, aathermodynamic thermodynamic kinetic combination method was used totheanalyze the andand kinetic combination method was used to analyze orientation orientation and mechanism of the reaction based functional on density theory functional theory With regard to and mechanism of the reaction based on density (DFT). With(DFT). regard to theoretical theoretical of calculations, has beenindescribed in our previous work knowledgeknowledge of calculations, it has been it described detail in in ourdetail previous work [30,31]. The [30,31]. Dmol3 The Dmol3 package was used to process exchange-related interactions using a local density package was used to process exchange-related interactions using a local density approximation approximation (LDA-PWC) approachthe to structure optimize the structure of O c-C F8 andToO2model [25]. To the (LDA-PWC) approach to optimize of c-C themodel possible 4 F8 and 2 4[25]. possible reaction stable molecular configuration theenergy lowestis energy needed for reaction path, thepath, stablethe molecular configuration with the with lowest neededisfor selection. selection. By calculating energy ofand reactants andwe products, weenergy can getchange the energy change before By calculating the energythe of reactants products, can get the before and after the and after and the reaction, judge the difficulty of from the reaction from the thermodynamic point of view. reaction, judge theand difficulty of the reaction the thermodynamic point of view. In addition, In the constructed non-decomposing needs tofor be transition searched for transition state theaddition, constructed non-decomposing reaction pathreaction needs topath be searched state [32]. Because [32]. Because during the process chemical bond breaking and the intermediate during the process of chemical bondofbreaking and rearrangement, therearrangement, intermediate activated complexes activated complexes willreactants be absorbed by the areactants to produce a certain amount of energy, and the will be absorbed by the to produce certain amount of energy, and the process of generating process of generating activated complexes often determines the reaction rate. Therefore, the transition activated complexes often determines the reaction rate. Therefore, the transition state structure and state structure andactivation the corresponding arethe helpful to evaluate reaction the the corresponding energy areactivation helpful toenergy evaluate reaction from thethe kinetic pointfrom of view. kinetic of view. Finally, combined withdynamic the test equilibrium results, the dynamic equilibrium processthe of Finally,point combined with the test results, the process of particles during particles duringofthe decomposition of analyzed. c-C4F8 with O2 was analyzed. decomposition c-C 4 F8 with O2 was The Basic Basic Properties Properties of c-C44FF88 4.1. The Figure 6 shows length is is Å shows the the geometry geometryof ofoptimized optimizedc-C c-C 4F moleculestructure, structure,where wherethe thebond bond length 4F 8 8molecule ◦ and thethe bond angle is is. °.The Å and bond angle Thec-C c-C 4F moleculehas hasaa high high degree degree of of symmetry. symmetry. The C-C bond length 4F 8 8molecule ◦ is 1.583 Å and the C-F bond length is 1.350 Å. The F-C-F bond angle is 110.150 110.150° and the F-C-C bond 113.587◦ . angle is 113.587°.
Figure 6. Molecular structure of c-C44F88..
The bond bond level level is is a the bond bond strength adjacent atoms atoms The a physical physical quantity quantity that that describes describes the strength between between adjacent in the the molecule, molecule, indicating indicating the the relative relative strength strength of of the the bond. In the the collision collision of of electrons electrons and and other other in bond. In particles or under high temperature conditions, the chemical bond may be broken, wherein the strength particles or under high temperature conditions, the chemical bond may be broken, wherein the of the chemical bond relative to the strength of the smaller chemical bonds bonds more difficult to break. strength of the chemical bond relative to the strength of the smaller chemical more difficult to Figure 7 shows the calculated chemical bond levels for the c-C F and SF molecules. According to the 4 8 6 break. Figure 7 shows the calculated chemical bond levels for the c-C4F8 and SF6 molecules. According calculation results, the C-C in thein c-C has ahas bond levellevel value of 0.924 and and the C-F 4 Fc-C 8 molecule to the calculation results, thebond C-C bond the 4F8 molecule a bond value of 0.924 the C-F bond with a value of 0.896. The intensity of the C-F bond in the c-C4F8 molecule is weaker than
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Processes 2018,C-C 6, x FOR PEER REVIEW the C-F bond is more likely to dissociate than the C-C bond. In 8 of 15 that of the bond. Therefore, the bond with a value of 0.896. The intensity of the C-F bond in Fthe c-C4 Fand is weaker SF6 molecule, the S-F bond level value of the four co-planar atoms the S atom is 0.826,than andthat the 8 molecule that of the C-C bond. Therefore, the C-F bond is more likely to dissociate than the C-C bond. In the of the C-C bond. Therefore, the C-F bond is more likely to dissociate than the C-C bond. In the SF other the S-F bond level value is 0.829. Overall, the bond level of the chemical bonds in the SF 66 SF 6 molecule, the S-F bond level value of the four co-planar F atoms and the S atom is 0.826, and the molecule, the S-F bond level the of four atomsin and is 0.826,indicating and the other molecule is smaller than the value bond of order theco-planar chemicalFbonds thethe c-CS4Fatom 8 molecule, that other the S-Fof bond level is 0.829.ofthe Overall, bond of the chemical bonds in the SF the S-F bond level is value 0.829. Overall, bond level of thelevel chemical in4Fthe SF6 molecule is6 stability thevalue molecular structure the SF6 the is inferior to that of bonds the c-C 8 molecule, which molecule is smaller than theofbond order ofisthe chemical bonds in the c-C4F8 molecule, indicating that smaller than the 4bond order thestrength chemical bonds in the c-C explains the c-C F8 gas dielectric superior to SF 6 gas. 4 F8 molecule, indicating that the stability the stability of the molecular structure of the SF 6 is inferior to of the c-C4F8 molecule, which of the molecular structure of the SF6 is inferior to that of the c-Cthat 4 F8 molecule, which explains the explains the c-C4F8 gas dielectric strength is superior to SF6 gas. c-C 4 F8 gas dielectric strength is superior to SF6 gas.
(a) c-C4F8
(b) SF6
Figure 7. c-C4 8 and SF66 bond-level Figure bond-level distribution. distribution. (a) c-C 4F8 7. c-C4 F8 (b) SF6 Figure 7. c-C4F8 and SF6 bond-level distribution. The distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital orbital (LUMO) (LUMO) of of c-C c-C4F calculatedfrom fromthe the orbital distribution molecules, 8 8isis calculated orbital distribution of of thethe molecules, as 4F The ofcorresponding the highest occupied molecular orbital and the unoccupied as shown in Figure 8. The corresponding energies −0.298645 Ha and − 0.086768 Ha, respectively. shown in distribution Figure 8. The energies areare −0.298645 Ha(HOMO) and −0.086768 Ha,lowest respectively. The molecular orbital (LUMO) of LUMO c-C4Fand 8 is calculated from the orbital distribution of the molecules, as The energy difference between and HOMO characterizes thestability stabilityofofthe thegas gasinvolved involved in the energy difference between LUMO HOMO characterizes the shown in Figure 8. The corresponding energies are −0.298645 Ha and −0.086768 Ha, respectively. The chemical thethe difference is, the the energy requiredrequired for the electron chemical reaction. reaction.The Thelarger larger difference is,larger the larger the energy for thetransition electron energy difference between LUMO and HOMO characterizes the stability of the gas involved in the in the molecule and the more the molecule transition in theismolecule is andstable the more stable theis. molecule is. chemical reaction. The larger the difference is, the larger the energy required for the electron transition in the molecule is and the more stable the molecule is.
HOMO (highest occupied molecular orbital)
LUMO (lowest unoccupied molecular orbital)
Figure 8. c-C c-C44orbital) F88 molecular orbital distribution. Figure 8. F molecular orbital(lowest distribution. HOMO (highest occupied molecular LUMO unoccupied molecular orbital) Figure 8. c-C 4FMain 8Main molecular orbital distribution. 4.2. F88 and and the Product Generation Mechanism 4F 4.2. The The Decomposition Decomposition Path Path of of c-C c-C the Product Generation Mechanism
Electrons are factors that cause cause collision collision ionization ionization and and dissociation dissociation in in the the electric electric field. field. Electrons are the the main main factors 4.2. The Decomposition Path of c-C4Fthat 8 and the Main Product Generation Mechanism Under the high-energy electric field or local overheating, the chemical bonds in the molecular structure Under the high-energy electric field or local overheating, the chemical bonds in the molecular are the maincleaved, factors that cause types collision ionization and dissociation in the electric field. of c-CElectrons be cleaved, various of free radicals, thereby damaging the insulation 4 F8 will structure of c-C 4F 8 will begenerating generating various types of free radicals, thereby damaging the Under the high-energy electric field or local overheating, the chemical bonds in the molecular structure. combination with the molecular c-C4 F8 , Table the major pathways insulation In structure. In combination with the structure molecularofstructure of c-C34Fshows 8, Table 3 shows the major structure of 8 will be cleaved, generating variousprocess. types of free radicals, thereby damaging the for the decomposition during theduring discharge pathways forc-C the4Fdecomposition the process. discharge insulation structure. In combination with the molecular structure of c-C4F8, Table 3 shows the major pathways for the decomposition during the discharge process.
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Table 3. c-C4F8 discharge decomposition path. Table 3. c-C4 F8 discharge decomposition path.
Pathway (1) Pathway (2) (1) (3)
Chemical Equation
Reaction Energy (kJ/mol)
Equation c-C 4 FChemical 8 → C 2 F 4 +C 2 F 4
Reaction Energy (kJ/mol) 173.8539
: : · C2 F4 ·· c -Cc-C F 4 F→ CC F2:F+CF 8→ 4· + : · · 4
8
3
6
173.8539 342.3815 342.3815 434.8436 434.8436
2
c-C4 F8 → C3 F6 · + CF2 · c-C 4c-C F 8 4→ 4FC⋅ F +F ⋅ F8 C→ 7 4 7 · + F·
(2) (3)
The Thedecomposition decomposition pathway pathway (1) (1) refers refers to to the the process process of of two two C-C C-Cbonds bondsthat thatlocated locatedat atthe therelative relative positions of the c-C 4F8 molecule being disconnected to form free radicals. The pathway (2) refers to positions of the c-C4 F8 molecule being disconnected to form free radicals. The pathway (2) refers to the adjacent in in the the C-C C-C4F 8 molecule. The absorption the disconnecting disconnecting process process of of C-C C-C bonds bonds which which are are adjacent 4 F8 molecule. The absorption energy required for pathway (1) is 173.8539 kJ/mol, which is lower than the energy required for pathway (1) is 173.8539 kJ/mol, which is lower than the 342.3815 342.3815 kJ/mol kJ/molrequired required for pathway (2). The pathway (3) refers to the process of c-C 4F8 which cleaves the C-F bond to produce for pathway (2). The pathway (3) refers to the process of c-C4 F8 which cleaves the C-F bond to produce CC4FF7••and F• free radicals, and the process need to absorb energy of 434.8436 kJ/mol, higher than 4 7 and F• free radicals, and the process need to absorb energy of 434.8436 kJ/mol, higher than pathway and pathway pathway (2). (2). pathway (1) (1) and Figure 9 shows the change trendofofreaction reaction energy three decomposition paths (in Table 3) Figure 9 shows the change trend energy of of three decomposition paths (in Table 3) with with temperature. As the temperature increases, the energy required for molecular decomposition temperature. As the temperature increases, the energy required for molecular decomposition decreases decreases and the of temperature in reaction. favor of The the effect reaction. The effect of gradually,gradually, and the increase of increase temperature is in favor ofisthe of temperature is temperature is considered mainly because the temperature changes during the discharge. considered mainly because the temperature changes during the discharge. In particular, there will beIna particular, willinbe significant increase in the temperature at and the moment of breakdown, and significant there increase thea temperature at the moment of breakdown, the temperature at the center the temperature at arc themay center of the breakdown arc may reach 1000 k. of the breakdown reach 1000 k. (1) (2) (3)
450 400
Enthalpy(kJ/mol)
350 300 250 200 150
0
200
400
600
800
1000
Temperature (K)
Figure9.9.The Thereaction reactionenergy energyof ofdecomposition decompositionpaths pathswith with temperature. temperature. Figure
All kinds of free radicals generated by ionization or dissociation of c-C F molecules can produce All kinds of free radicals generated by ionization or dissociation of c-C44F88molecules can produce a series of new products by secondary reactions, mainly CF , C F , C F , C F and C F . Figure 10 a series of new products by secondary reactions, mainly CF4 4, C22F66, C33F88, C22F44 and C33F66. Figure 10 shows the molecular model of the above decomposition product after optimization, and the bond shows the molecular model of the above decomposition product after optimization, and the bond length bond angle parameter is basically the same as that given in [30]. length bond angle parameter is basically the same as that given in [30]. Table 4 shows the chemical equations of decomposition products, energy changes and activation energy. Among them, the processes of free radical recombination are exothermic reactions to generate CF4 , C3 F8 , C2 F4 , C3 F6 and C2 F6 . From a thermodynamic point of view, CF4 , C2 F6 , C2 F4 are relatively easy to form, and C3 F8 generation is more difficult.
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(b) C2F6
(a) CF4
(c) C3F8
(d) C2F4
(e) C3F6
Figure Figure 10. 10. Molecular Molecular model model of of decomposition decomposition products products and structural parameters. Table 4. Decomposition products and energy change.
Table 4 shows the chemical equations of decomposition products, energy changes and activation −1 ) energy. them, Equation the processes ofEnergy free radical recombination exothermic reactions to generate No Among Chemical Changes (kJ·mol−1 ) areActivation Energy (kJ·mol CF4, C 3F8, C2F4, C3·F6 and C2F6. From a thermodynamic point of view, CF4, C2F6, C2F4 are relatively easy B1 CF2 · + F· → CF3 · −307.37 — to form, more difficult. B2 andCC33FF68·· generation + 2F· → C3 Fis − 83.7318 — 8 B3 B4 B5 B6
No
CF3 · + F· → CF4 −190.262 — 2CF2 ·· → C2 FTable −332.313 — 4. Decomposition products and energy change. 4 2CF3 · → C2 F6 −373.811 — Activation Energy Energy Changes C3 F6 ·· → C3 F6 −81.21444 80.97188
Chemical Equation
(kJ·mol−1) (kJ·mol−1) 2 : +F → C F 3 B1 — barrier, are the According to theCFkinetic analysis, the path B1−307.37 to B5 reaction without energy C 3F6:+2F → C3F8 B2 −83.7318 — processes of free radical complex into molecules, without activation spontaneously. The formation CF 3 +F → CF4 B3 −190.262 — process of C2 F6 release more energy, and the reaction is more prone to occur. The path B2 needs to F2: → isC2the F4 most difficult to−332.313 B4 release 83.7318 kJ/mol,2C which occur with the least energy.— Figure 11 shows the 2CF 3 → C 2F6 B5 −373.811 energy changes of reactions with temperature. With the increase of temperature, — the absolute value of C3F6: → a C3different F6 B6 enthalpy showed 80.97188is conducive to the reaction degree of−81.21444 decline. That is, the temperature
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Enthalpy(kJ/mol)
the reaction. According to the kinetic analysis, the path B1 to B5 reaction without energy barrier, are the processes of free radical complex into molecules, without activation spontaneously. The formation -100 process of C2F6 release more energy, and the reaction is more prone to occur. The path B2 needs to B1 least energy. Figure 11 shows the release 83.7318 kJ/mol, which is the most difficult to occur with the B2 energy changes of reactions with temperature. With the increase B3 of temperature, the absolute value -200 B4 B5 B6 is, the temperature is conducive of the reaction enthalpy showed a different degree of decline. That to the reaction. -300
0
200
400
600
800
1000
Temperature(K)
Figure 11. 11. The The energy energy changes changes of reactions with temperature. Figure
Path B6 B6 reaction reaction requires requires the the formation formation of of an an activated activated complex complex transition transition state state (TS). (TS). The The reactant reactant Path absorbs energy of 80.97188 kJ/mol, and then the activated complex TS releases energy to form the absorbs energy of 80.97188 kJ/mol, and then the activated complex TS releases energy to formfinal the product. The progress of the of reaction is shown in Figure 12. Small productproduct formation process final product. The progress the reaction is shown in Figure 12.molecule Small molecule formation
process releases more energy, so it can be concluded that free radicals such as F•, CF2:, CF3• generated by the decomposition of c-C4F8 molecules tend to recombine into small molecules, resulting in a large content of small molecule products, which is in good agreement with the previous experimental results.
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releases more energy, so it can be concluded that free radicals such as F•, CF2 :, CF3 • generated by the decomposition of c-C4 F8 molecules tend to recombine into small molecules, resulting in a large content of small molecule products, which is in good agreement with the previous experimental results. 100
TS 80.98188
Energy (kJ)
50
C3 F6 : 0
-50
C3F6 -81.21444
-100
Reaction process
Figure 12. Reaction process of pathway B6 (TS: transition state).
In order to study the effect of O2 on the decomposition of c-C4 F8 , the discharge decomposition path of O2 must be studied first. In [20], the main pathways for generating O• are given by O2 →2O• and O2 + e→O• + O. It is also pointed out that the free radicals CF2 : and C2 F4 are oxidized to COF2 under O2 conditions. Figure 12 shows the geometrically optimized COF2 molecular structure model. Table 5 shows the reaction equations and energy changes for the free radical binding to COF2 produced by c-C4 F8 [20]. The reaction is an exothermic reaction. O2 can oxidize CF2 : to form COF2 and O• oxidize C2 F4 : to generate COF2 and CF2 :. The structure of COF2 is shown in Figure 13. Table 5. Reaction path and energy change. No
Chemical Equation
C1
CF2 ·· + O2 →COF2 + O•
Processes PEER C2 2018, C F6,· x+FOR O•→ COFREVIEW + CF · 2 4·
2
2·
Energy Changes (kJ·mol−1 )
Activation Energy (kJ·mol−1 )
−138.52 −112.64
180.18 184.61
12 of 15
Figure 13. COF2 molecular structure model. Figure 13. COF2 molecular structure model.
4.3. The Basic Properties of the Main Products 4.3. The Basic Properties of the Main Products According to the experimental measurement and calculation results, it can be seen that the main According to the experimental and calculation results, it can be seen that the main decomposition products are CF4 , Cmeasurement 2 F4 , C2 F6 , C3 F6 and C3 F8 , and COF2 is produced when O2 is decomposition are CFmolecular 4, C2F4, C2F6, C3F6 and C3F8, and COF2 is produced when O2 is involved. involved. Figureproducts 14 shows the orbital energies of the main product. Based on the molecular Figure 14 shows the molecular orbital energies of thehave maingood product. Based on CtheF molecular orbital orbital gap value, the main decomposition products stability. The 2 4 , C3 F6 and COF2 gap value, the main decomposition products have good stability. The C 2F4, C3F6 and COF2 containing containing double bonds are poorly stable in the chemical reaction. double aretime, poorly in insulation the chemical reaction. At bonds the same thestable relative properties of the decomposition products were compared. The environmental values andLUMO ecological toxicity are shown in Table 6.
4.3. The Basic Properties of the Main Products According to the experimental measurement and calculation results, it can be seen that the main decomposition products are CF4, C2F4, C2F6, C3F6 and C3F8, and COF2 is produced when O2 is involved. Figure 14 shows the molecular orbital energies of the main product. Based on the molecular orbital Processes 2018, 6, 174 12 of 14 gap value, the main decomposition products have good stability. The C2F4, C3F6 and COF2 containing double bonds are poorly stable in the chemical reaction. LUMO
C2F4
C3F6
C-C4F8
COF2 HOMO
CF4
C2F6
C3F8
Figure 14. Molecular orbital gap values of decomposition products. Figure 14. Molecular orbital gap values of decomposition products.
Table 6. Parameters of decomposition products.
At the Dielectric same time, the relative properties ofLifetime the decomposition products were Strength Boilinginsulation Point GWP (Year) Acute Toxicity Gas to SF6 [33–36] (◦ C) [29–37] (100-Year) [5,36] are shown [36–39] in Table 6. compared. Relative The environmental values and ecological toxicity −63 23,900 3200 >500,000 ppm/4 h, LC50 −6 8700 2600 78 pph/2 h, LCLo Table 6.−Parameters of decomposition products. 186.8 6300 50,000 895,000 ppm/15 min, LCLo −78 9200 10,000 >20 pph/2 h, LC Boiling − Point GWP (100-Year) Lifetime 37 (°C) 7000 2600 (Year) 750 ppm/4 h, LC50 Acute Toxicity [29–37] [5,36] [36–39] −28 — 500,000 ppm/4 h, LC50 − 76.3 0 1.9 days 40,000 ppm/4 h, LC50 3 , 4h, −684 78 pph/2 − 08700 — 2600 270 mg/m h,LCLo LC50 895,000 ppm/15 min, Note: warming potential; LCLo—Lowest published CFGWP—global 4 0.39 −186.8 6300lethal concentration; 50,000 LC50—Lethal concentration, LCLo 50 percent kill; LC—Lethal concentration; The measurement of acute toxicity in the table is the test value of C2F6 0.78–0.79 −78 9200 10,000 >20 pph/2 h, LC rat inhalation. C3F8 0.96–0.97 −37 7000 2600 750 ppm/4 h, LC50 — −28 —
