Decomposition Mechanism of C5F10O: An Environmentally Friendly

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Decomposition mechanism of C5F10O: An environmental friendly insulation medium Xiaoxing Zhang, Yi Li, Song Xiao, Ju Tang, Shuangshuang Tian, and Zaitao Deng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02419 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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Environmental Science & Technology

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Decomposition mechanism of C5F10O: An environmental

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friendly insulation medium

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Xiaoxing Zhang1, Yi Li1, Song Xiao1,*, JuTang1, Shuangshuang Tian1 and Zaitao Deng1

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Abstract：：

School of Electrical Engineering, Wuhan University, Wuhan 430072, China

*E-mail: [email protected] Phone:+86 18986238962 Fax: +027 68775213 Address: School of Electrical Engineering, Wuhan University,BaYi Street No.299, Wuhan,Hubei province, P.R.China

SF6, the most widely used electrical-equipment-insulation gas, has serious greenhouse effects. C5F10O has attracted much attention as an alternative gas in recent two years, but the environmental impact of its decomposition products is unclear. In this work, the decomposition characteristics of C5F10O were studied based on gas chromatography–mass spectrometry and density functional theory. We found that the amount of decomposition products of C5F10O, namely, CF4, C2F6, C3F6, C3F8, C4F10, and C6F14, increased with increased number of discharges. Under a high-energy electric field, the C-C bond of C5F10O between carbonyl carbon and α-carbon atoms was most likely to break and generate CF3CO•, C3F7• or C3F7CO•, CF3• free radicals. CF3• and C3F7• free radicals produced by the breakage more easily recombined to form small molecular products. By analyzing the ionization parameters, toxicity, and environmental effects of C5F10O and its decomposition products, we found that C5F10O gas mixtures exhibit great decomposition and environmental characteristics with low toxicity, with great potential to replace SF6. Keywords: C5F10O decomposition characteristics; environment-friendly insulating medium; density functional theory; gas chromatography–mass spectrometry

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1 Introduction

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SF6 is widely used in insulation and arc-dielectric equipment because of its excellent insulation and arc-suppression performance. However, SF6 is one of the six most harmful greenhouse gases, with a global warming potential (GWP) of up to 23 500 and an atmospheric life of more than 3200 years [1-2]. Relevant statistics show that over the past five years, SF6 content in the atmosphere increased by 20% [3]. SF6 emissions are expected to reach 4270 tons in China by 2020. The growing SF6 use and the emissions brought about by its greenhouse effect cannot be ignored. According to a current estimate of greenhouse-gas emissions, the global average temperature may rise above 4°C by 2100[4]. With the increase in environmental protection requirements around the world, the carbon footprint of the power industry has also been severely limited. Therefore, identifying an environmentally friendly SF6 alternative gas as an insulating medium for use in electrical equipment is urgent. In the past two years, a new insulating media C5F10O has been the focus of alternative-gas 1

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research because of its nontoxicity, noncombustibility, and environmental friendliness with a Global Warming Potential (GWP) value of only 1. C5F10O also has excellent insulation properties; it has a dielectric strength twice that of SF6. However, the liquefaction temperature of C5F10O is 26.9°C under normal pressure and thus needs to be used with other buffer gases with lower liquefaction temperatures, such as N2 and CO2[5]. Some studies have focused on the environmental friendliness and high insulation performance of C5F10O. Mantilla et al. found that the power frequency breakdown voltage and 50% lightning breakdown voltage of 6.25% C5F10O with dry air mixture in 0.56 Mpa reached those of the pure SF6 in 0.4 Mpa [6]. Simka et al.found that the critical breakdown field strength 5.2% C5F10O/94.8% dry air gas mixture under 0.7 Mpa reaches 95% that of pure SF6 under 0.45 Mpa and 80% that of pure SF6 under 0.6 Mpa [7]. Hyrenbach et al.found that the heat transfer characteristics of C5F10O/dry air mixture is lower than those of SF6; the internal overpressure caused by arc initial stage is about 30% higher than that of pure SF6 [8]. Tatarinov et al. used dielectric barrier discharge (DBD) test to explore the decomposition products of C5F10O/dry-air gas mixture under10 kV voltage. They found that the main decomposition products of C5F10O were C4F10, C6F14, C3F6, and C5F12. [9]. Hyrenbach et al. tested the decomposition by-products of C5F10O/air mixture under normal operating conditions and defects. They found that the total decomposition of C5F10O over the lifetime of the (Gas Insulated Switchgear) GIS is at a low level of 300% compared with the 20th breakdown, and the peak intensity of C4F10, C3F8, and C6F14 increased by 267.23%, 92.51%, and 115.46%, respectively. The peak intensity of C3F6 also increased by107074. Therefore, with increased number of breakdowns, C5F10O in the gas mixture further decomposed. The growth rate of CF4, C2F6, and C4F10 was higher than that of C3F8, C6F14, indicating that CF3•, C3F7•, F•, and other free radicals more easily reunited to form small molecular products.

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3.2 Decomposition path and formation mechanism of the main products of C5F10O

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Electrons in the electric field were the main causes of the ionization and dissociation of C5F10O molecules. Under the action of a high electric field, the chemical bonds in the C5F10O molecules broke down to form free radicals and other fragments and thus destroy their own structures. Combined with the molecular structure of C5F10O shown in Figure. 7, the main decomposition pathways of C5F10O are shown in Table 3. The dissociation pathways A and B corresponded with the formation of free radicals under discharge by C-C bond broken between the carbonyl carbon atom and the α-position carbon atom in C5F10O. An energy of 397.322 kJ/mol was required in pathway A, which was higher than that in pathway B. Pathway C corresponded with the C-C bond broken between the carbonyl β-carbon atom and its connected carbon atom. This pathway required 293.258 kJ/mol. Paths D–F corresponded with C5F10O collision ionization to produce free radicals and positive ions. These paths considered most electron energy in low-temperature plasma from 416.3 kJ/mol to 1056.2 kJ/mol. Considering that the majority electron energy of low-temperature plasma ranged within 416.3–1056.2 kJ/mol [25], these process required the participation of high-energy particles. Considering the re-decomposition process of free radicals such as CF3CFCOCF3•, C3F7CO•, and CF3CO•[26], the energy that C3F7CO• and CF3CO• required for dissociation was 21.357 and 33.779 kJ/mol, which was much lower than CF3CFCOCF3• decomposition. Figure. 8 shows the relative energy change of the C5F10O major decomposition paths considering the secondary reaction, from which we can know that paths A and B were more likely to occur relative to path C. In the event of partial discharge or breakdown, the temperature near the 5



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center of the fault was much higher than that at room temperature, and temperature had a certain impact on reaction rate. Figure. 9 shows the Gibbs free energy curve of C5F10O decomposition paths within 300–1000 K. In addition to paths A1 and B1, the Gibbs free energy of the other paths within 300–1000 K were greater than zero, i.e., the corresponding reactions were not spontaneous. At above 625 and 825 K, the free energy of path A1 and B1 changed from positive to negative, and the reaction process changed from nonspontaneous to spontaneous. This finding showed that temperature greatly influenced the decomposition of C3F7CO• and CF3CO•, and the decomposition rate was higher than that at low temperatures. All kinds of free radicals such as CF3•, C3F7•, F•, and so on, which were produced by the ionization and dissociation of C5F10O molecules, had strong reactivity and can reunite to form CF4, C2F6, C3F8, C3F6, C4F10, C6F14, and other decomposition products(the structure is shown in Figure. 10). Table 4 shows the reaction heat of each reaction path. The formation of CF4, C2F6, C3F8, C4F10, C5F12, and C6F14 was exothermic, and the process of generating C3F6 was endothermic. From the thermodynamic point of view, CF4, C3F8, C4F10, and C2F6 were most likely to form, whereas the generation of C3F6 was more difficult. The effect of trace water on the decomposition of C5F10O is also considered. The cleavage process of H2O under discharge mainly produce H• and OH•, which can react with a variety of free radicals[27]. The reaction between C3F7• and H• may produce C3HF7 or C3F6 with HF, and the energy released by the two different paths is very close. We can also find that the reaction path I2 is exothermic, while the decomposition of C3F7• forming C3F6 and F• is endothermic, so the presence of trace water promotes the formation of C3F6 to a certain extent. From the kinetic point of view, paths G1-G2, H1, H3-H4, and I1 were a free-radical synthesis process that was spontaneous without activation energy. Paths H2 and I2 were also spontaneous process. Figure. 11 shows the free energy of these reaction paths within 300–1000 K. Except for path H2, the free energy of all reactions were negative, meaning they can proceed spontaneously. The free energy of G2, H3, H4 and I1 paths increased with increased temperature, so driving force decreased. The free energies of all other reaction paths was basically unchanged, indicating the limited formation of C4F10 and C6F14 at high temperatures. Therefore, the ease of formation of decomposition products followed the order CF4 > C3HF7> C2F6 > C3F8 > C4F10 > C6F14 > C3F6. We also found that the peak intensity growth of CF4, C2F6 and C4F10 was higher than that of C3F8 and C6F14 after the 50th breakdown. Although C3F8 was more likely to be produced relative to C4F10 based on the calculation results, its formation required the participation of F•, whose formation in turn required further ionization. Consequently, the production of C4F10 was easier in the actual decomposition process.

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3.3 Ionization parameters of C5F10O and its decomposition products

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The dielectric strength of the gas-insulating medium was related to the configuration of gas molecules, the collision of particles, and the electron-transport velocity. From the molecular point of view, excellent insulating medium should have better ionization parameters. Gas discharge mainly involves the first process of ionization. A larger ionization energy indicates that gas molecule has more difficulty in loosing electrons. Electron affinity can characterize the ability of molecules to adsorb electrons. The formation of negative ions can further inhibit discharge. The molecular-orbital energy gap reflects the molecule stability. The magnitude of this value reflects the ability of electrons to move from an occupied orbit to empty one. A larger energy gap indicates 6


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that electron transition is more difficult and a molecule is more stable. Figure. 12 compares the ionization energy, electron affinity energy, and molecular-orbital energy gap of SF6, C5F10O, and various decomposition products obtained in calculation. The ionization energy of SF6 was higher than that of C5F10O. Both electron affinity energy were positive, indicating that the formation of negative ions released energy. When molecules formed negative ions, ionization ability greatly decreased and hindered the development of streamer discharge. The electron affinity of C5F10O was higher than that of SF6, which indicated that C5F10O more easily adsorbed electrons to form negative ions. This finding proved that C5F10O had strong electronegativity. In fact, the strong electronegativity of C5F10O was due to the carbonyl group in its molecular structure. The carbonyl group was a strong electron-withdrawing group having electron-attracting and conjugation effects. The molecular-orbital energy gap of C5F10O was lower than that of SF6, which indicated weak molecular stability. The ionization energy of decomposition products such as CF4, C2F6, C3F8, C4F10, C3HF7 and C6F14 exceeded 13 eV, which was higher than that of C5F10O, indicating that the products more difficultly lost electrons. The ionization energy of C3F6 was close to C5F10O. The electron affinity of CF4, C2F6, C3F8, C4F10, C3HF7 and C6F14 was negative, meaning that the formation of negative ions of these molecular needed to absorb energy. With increased number of carbon atoms, electron affinity and electron adsorption ability increased gradually. In literature [28-29], the dielectric strengths of CF4, C2F6, C3F8, and C4F10 relative to SF6 were 0.6, 0.9, 1.12, and 1.32, respectively. In other words, the insulating properties of saturated fluorocarbon increased with increased number of carbon atoms, consistent with our analysis. The electron affinity of C3F6 was positive and higher than C5F10O, indicating strong electronegativity. The molecular-orbital gap values of the decomposition products were higher than those of C5F10O, except for C3F6, indicating that the molecular structure of decomposition products was stable. In fact, with increased breakdown times, the decomposition rate of C5F10O in gas mixture increased, and the formation rate of small molecular products such as CF4, C2F6, and C3F8 with relatively weak insulation performance was faster. This phenomenon led to decreased insulation performance of the entire system. The breakdown voltage of C5F10O/N2 gas mixture drops by 3.48% after 50 tests according to the results of this paper. The reduction of insulation properties of the system may lead to more serious discharge, exacerbating the further decomposition of C5F10O, forming very poor insulation products. With increased content of decomposition products, the normal operation of equipment, the health of maintenance personnel, and the potential environmental impact must be carefully considered.

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3.4 Toxicity and environmental effects of C5F10O and its decomposition products

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In engineering applications, electrical equipment operation and maintenance personnel will inevitably contact with gas insulation medium. The atmospheric emissions of C5F10O gas mixture should also be expected. From the perspective of environmental and human safety, clarifying the toxicity and environmental effects of C5F10O and its decomposition products is necessary before practical application. Table 5 shows the toxicity and GWP values of C5F10O and its decomposition products. As aforementioned, C5F10O is nontoxic and has an occupational exposure limit time average of 225 ppmv, which is lower than that of SF6 (1000 ppmv) [30]. However, considering that the content of C5F10O in gas mixture was below 20%, its safety was the same as the widely used SF6 gas. CF4, 7



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C2F6, C3F8, and C4F10 produced by C5F10O decomposition are perfluorocarbons and can be applied in the plasma industry, medicine, and other fields. At high concentrations, C2F6 has a fast choking feature, and C3F8 has an anesthetic effect [31-32]. C3F6 inflicts damage on the respiratory system and kidneys, and the inhalation of high concentrations of gas can cause toxic bronchitis, pneumonia, and even pulmonary edema [33]. C4F8,as also known as perfluoroisobutylene (PFIB) is the most toxic decomposition product, which LC50 is only 81 ppmv and can cause pneumonia or pulmonary edema[34]. CO, a common toxic gas, was one of C5F10O’s discharge-decomposition products. Literature [10] tested the decomposition products of C5F10O/air mixture under normal operating conditions and defects. They found that the maximum concentration of C3F6 was 50ppmv in the aged insulation gas mixture. Considering the allowable leakage limit of the gas mixture, only 6.5 ppbv of C3F6 may leak into the air, which is lower than its permitted occupational exposure limit values(0.1ppmv). The max concentration of CF4, C2F6, C3F8, C3F6 and C4F10 is below 100ppmv during internal arc tests. And the gas toxicity after an internal arc with the 13.6%C5F10O/86.4%air mixture is close to air. These tests confirmed that the concentration of toxic decomposition products produced by C5F10O is at ppmv level and the released gases do not pose a threat to health of maintenance personnel. Thus, using C5F10O gas mixture as an insulating medium is safe.

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The GWP of pure C5F10O is only 1, the same as CO2. In practical application, the content of C5F10O in gas mixture is generally below 20% [7], and we calculated the GWP value of C5F10O/N2 gas mixture according to the formula given in F-gas regulation[11]. The GWP value of gas mixture with the molar fraction of C5F10O lower than 20% is less than 0.7 (As shown in Figure 13 ). Meanwhile, the ozone depletion potential of C5F10O is zero, so it is not destructive to the ozone layer. Therefore, the emission of C5F10O gas mixture do not significantly increase the greenhouse effect and harm the atmospheric environment.

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The GWPs of CF4, C2F6, C3F8, C4F10, and C6F14 in the decomposition products of C5F10O were 7390, 12 200, 8830, 8860, and 9300, respectively, which were significantly higher than those of C5F10O. In engineering applications, the GWP value of the gas mixture is closely related to the decomposition degree of the gas mixture, which should be evaluated scientifically and rationally according to actual conditions.

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References

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(c) Chromatogram of C5F10O-N2 gas mixture after 50-times breakdown test Figure. 6 GC-MS chromatograms 14


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Optimized structure of C5F10O at at GGA-PBE level (bond lengths are in Å and angles are in °) 700

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Relative energy change of the main decomposition path of C5F10O

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Figure. 9 Relationship between the Gibbs free energy and temperature in the decomposition path of C5F10O

CF4

C2F6

C3F8

C3F6

C4F10

C6F14

C3HF7 Figure. 10 Optimized structure and parameters of C5F10O decomposition products (at GGA-PBE level) 16


Page 17 of 20


ReationG1 ReationG2 ReationH1 ReationH2 ReationH3 ReationH4 ReationI1 ReationI2

Gibbs free energy(kcal/mol )

200

100

-400

-500

200

300

400

500

600

700

800

900

1000

1100

468

Temperature(K)

469 470

Figure. 11 Relationship between the Gibbs free energy and temperature of decomposition-product formation 16

15.4

14.93

14.14

Ionization energy(eV)

14 12

13.67 13.83 13.43

13.38

11.89

11.76

10 8 6 4 2 0

471 472

SF6 C5F10O CF4

C2F6 C3F8

C3F6 C4F10 C6F14 C3HF7

a) Ionization energy

17



2.0

Page 18 of 20

1.76

Electron affinity(eV)

1.5

1.02

1.0

0.59 0.5 0.0

CF4

C2F6 C3F8

SF6 C5F10O

C4F10 C6F14 C3HF7 C3F6

-0.5

-0.67 -1.0

-0.98 -1.42 -1.37

-1.5

473 474

-1.02

-1.22

b) Electron affinity 0

-2.32

-2 LUMO

-1.94

-3.34

-2.49 -3.02

-3.05

-3.11 -4.27

Energy/eV

-4

-6

6.10

-11.41 -12.32

Figure. 12

8.06

-9.24

HUMO -12.70

SF6 C5F10O

-14

8.92

5.12

8.85

-9.15

-10

475 476 477 478

9.06

10.24

10.38

8.98

-8

-12

-4.12

CF4

-12.18

-12.08

C2F6

C3F8

-11.96

-12.34

C4F10 C6F14

C3F6 C3HF7

c) Molecular-orbital energy gap Ionization energy, electron affinity, and molecular-orbital energy gap of C5F10O decomposition products 0.8

GWP 0.7 0.6

GWP

0.5 0.4 0.3 0.2 0.1 0.0 -0.1 0

479

5

10

15

C5F10O (%)

18


20

Page 19 of 20


480

Figure. 13

481

482 483

Table 1 Power frequency breakdown voltage of C5F10O-N2 gas mixture

Time

Breakdown voltage (kV)

1 2 3 4 5 6 7 8 9 10

26.2 25.7 26 25.9 25.7 25.5 25.7 25.9 25.7 25.5 Table 2

484

Time


11 12 13 14 15 16 17 18 19 20

25.8 25.9 25.7 25.5 25.6 25.4 25.7 25.5 25.4 25.8

Time


21 22 23 24 25 26 27 28 29 30

25.5 25.4 25.5 25.3 25.5 25.3 25.4 25.2 25.6 25.4

Time


Time


31 32 33 34 35 36 37 38 39 40

25.3 25.3 25.5 25.3 25.4 25.4 25.5 25.4 25.2 25.2

41 42 43 44 45 46 47 48 49 50

25.4 25.5 25.4 25.2 25.3 25.1 25.4 25.3 25.4 25.2

Peak intensity of decomposition products of gas mixtures under different breakdown times a Product

Retention time (min)

20th

50th

Growth

Growth rate (%)

CF4 C2F6 C3F8 C3F6 C4F10 C6F14

4.310 4.400 4.650 5.215 5.080 7.290

97703 132342 81033 476487 23427 139336

478012 567530 155994 583561 86031 300218

380309 435188 74961 107074 62604 160882

389.25 328.84 92.51 267.23 115.46

a: base peak: 69

485

Table 3 Path

A B C D E F A1 B1 C1 C2 486

GWP values of C5F10O-N2 mixtures with different molar fraction of C5F10O

Discharge decomposition path of C5F10O

Chemical reaction equation

Ereactant (a.u.)a

Eproduct (a.u.)a

Reaction heat (kJ/mol)

(CF3)2CFCOCF3 → C3F7CO +CF3

-1264.3907 -1264.3907 -1264.3907 -1264.3907 -1264.3907 -1264.3907 -926.6406 -450.9723 -926.6453 -926.6453

-1264.2394 -1264.2658 -1264.2790 -1263.9626 -1264.0243 -1264.0351 -926.6325 -450.9594 -926.5712 -926.5053

397.322 327.989 293.258 1123.894 1166.567 1138.189 21.357 33.779 194.481 367.551

(CF3)2CFCOCF3 →CF3CO +C3F7 (CF3)2CFCOCF3 → CF3 +CF3CFCOCF3

(CF3)2CFCOCF3 → CF3CO +C3F7+ + e (CF3)2CFCOCF3 → C3F7CO +CF3+ + e (CF3)2CFCOCF3 → CF3+ + CF3CFCOCF3 +e

C3F7CO•→C3F7•+CO CF3CO•→CF3•+CO CF3CFCOCF3•→CF3•+CF3CFCO• CF3CFCOCF3•→CF3CF•+COCF3•

a: T=298.15 K at GGA-PBE level with zero-point energy correction and enthalpy correction[19] 19



487

488 489

490 491 492

493 494

Table 4

Page 20 of 20

Reaction equation and energy change of C5F10O decomposition products

Path

Chemical reaction equation

Ereactant (a.u.)a

Eproduct (a.u.)a

Reaction heat (kJ·mol-1)

G1 G2 H1 H2 H3 H4 I1 I2

CF3•+F•→CF4 2CF3•→C2F6 C3F7•+F•→C3F8 C3F7•→C3F6+F• CF3•+C3F7•→C4F10 2C3F7•→C6F14 C3F7•+H•→C3HF7 C3F7•+H•→C3F6+HF

-437.3757 -675.2580 -913.0599 -813.2922 -1150.903 -1626.5798 -813.787467 -813.787467

-437.5768 -675.3992 -913.2367 -813.2062 -1151.066 -1626.7197 -813.9631013 -813.9739351

-527.993 -370.584 -464.008 230.810 -423.638 -367.16 -461.127 466.006

a: T=298.15 K at GGA-PBE level with zero-point energy correction and enthalpy correction[19] Table 5

Toxicity and GWP of C5F10O decomposition products

Molecular

CAS

LC50a

GWPb

SF6 C5F10O CF4 C2F6 C3F6 C3F8 C4F10 C6F14 C4F8 C3HF7

2551-62-4 756-12-7 75-73-0 76-16-4 116-15-4 76-19-7 354-92-7 354-96-1 360-89-4 431-89-0

>500 000 ppm/4H >20 000 ppm/4H 895 000 ppm/15M >20 pph/2H 750 ppm/4H 81 ppm/4H -

23500 1 7390 12200 2 8830 8860 9300 3220

a：Lethal Concentration at 50% mortality, rat b：Reference [35-36]

For Table of Contents Only

20


Decomposition Mechanism of C5F10O: An Environmentally Friendly

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