Theoretical study of the chemical decomposition mechanism and model of Sulfur hexafluorid (SF6) under corona discharge

Journal of Fluorine Chemistry 220 (2019) 61–68

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Theoretical study of the chemical decomposition mechanism and model of Sulfur hexafluorid (SF6) under corona discharge

T

Lipeng Zhonga,b, Shengchang Jib, Feng Wanga, Qiuqin Suna, , She Chena, Jie Liua, Bin Haia, Lu Tanga ⁎

a b

College of Electrical and Information Engineering, Hunan University, Changsha, 410082, China State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, 710049, China

ARTICLE INFO

ABSTRACT

Keywords: SF6 Decomposition mechanism Decomposition model Decomposition products Density functional theory Formation pathway

SF6 will decompose to several byproducts (including SOF2, SO2F2, SO2, etc.) under corona discharge. Macro features of SF6 decomposition are closely linked with the micro decomposition process. A quantum chemistry analysis combining density functional theory (DFT) was used in this paper to study the complex decomposition process of SF6 in detail. A decomposition model of SF6 with two typical zones was then established. Electron distribution of low fluoride sulfides shows that (SF2)* and (SF4)* are relatively stable. Formation pathways, reaction rate constants, and reaction region of SOF4, SOF2, SO2F2, SO2 were derived. SOF4 mainly comes from dissociation of SF5OH generating by oxidation between (SF5)* and OH*. SO2F2 is mainly generated through the oxidation reaction between (SF2)* and O* in the high-energy region. SOF2 and SO2 are both generated through hydrolysis reaction in the low-energy region. SOF2 and SO2F2 are the most effective species to monitor the decay of SF6 under corona discharge.

1. Introduction Sulfur hexafluoride (SF6) is widely employed as the gaseous dielectric by the electric power industry for gas insulated equipment owing to its good insulating properties and excellent arc-extinguishing performance [1,2]. Despite its high reliability, multiple insulation defects may be introduced into SF6 gas insulated electric equipment during the process of manufacture, transportation, installation and maintenance. Insulation defects may cause partial discharge which lead to grave threat to the equipment safety [3–5]. Hence, it is necessary to study the detection method of partial discharge. SF6 may dissociate into low fluorine sulfides (such as (SF5)*, (SF4)*, (SF3)*) and F atoms under discharge. Most of lower fluoride sulfides recombine quickly with F atoms to reform SF6 [6]. However, there exist unavoidable impurities such as H2O and O2 with trace levels in gasinsulated power equipment [7]. Some sub-fluorides will react with these impurities to generate decomposition products, among which SOF2, SO2F2, SO2 are the most common ones [8,9]. Previous works have proved that insulation condition diagnosis can be realized by detecting SF6 decomposition products [10,11]. Macro features of SF6 decomposition such as type of by-products, content ratios between products will be different under different insulation defects, because the different

⁎

pathway and energy of the micro decomposition process of different byproducts. As a result, it is necessary to study the decomposition reaction mechanism of SF6. The decomposition mechanism and process have been vastly investigated. Wang et al found that collision between electronegative SF6 and electrons may form (SF6)*. The metastable state species contain sufficient internal energy to make dissociation energetically possible to generate SF6 , SF5 , F , SF6, (SF5)*, and F* [12]. Fehsenfeld found that the predominant products of the attachment of electrons to SF6 are SF6 and SF5 . And the ratio of SF5 and SF6 increases with discharge energy [13]. Mcgeehan et al investigated the dissociation process of SF6 and found several low fluorine sulfides, among which SF5 and SF4 are the predominant ones [14]. Tang et al explored the main stable decomposition products of SF6 under different defects and found that SO2F2, SOF2, CO2 and CF4 can be selected as feature components [15]. Wang et al found that the main products are SO2F2, SO2, S2OF10 under pointplane corona, and CF4, CS2, SO2 under creeping discharge [16,17]. Most of the previous studies were focused on the application in defect diagnosis. Although the key products has been confirmed by experimental results, the generation mechanism of main byproducts are not well understood. In recent years, some researchers carried out the theoretical research with the quantum chemistry analysis. Chen et al

Corresponding author. E-mail address: [email protected] (Q. Sun).

https://doi.org/10.1016/j.jfluchem.2019.01.005 Received 29 October 2018; Received in revised form 14 January 2019; Accepted 15 January 2019 Available online 17 January 2019 0022-1139/ © 2019 Elsevier B.V. All rights reserved.

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investigated the mechanism of CS2 development [18]. Fu et al estimated the rate constants of four chemical reactions:, (SF5)*+ SO2 SOF3+ (SF3)* (SF4)*+ SOF2 , F*+ SO2 F SO2 F2 , F SF6+ SO2 , F2+ SO2 SO2 F2 . They also studied the neutral decomposition of SF6 in the presence of H2O and O2 [19,20]. However, the complete chemical reactions of SF6 and decomposition model of SF6 under discharge have not been well studied.

3.1. Collision induced dissociation

2. Computational methods

3.1.1. Dissociation of SF6 Dissociation of SF6 is the most important reaction in the high energy region. Tsang and Herron have studied the kinetics and thermo(SF5)*+ F [25]. They found that SF6 dynamics of the reaction SF6 could be easily generated by collision between SF6 and electrons, because SF6 has a low electronic affinity energy. Parts of SF6 might be cracked into (SF5)* and F then.

Electrons may be accelerated in the region with concentrated electric field. Once these high-velocity electrons collide with neutral molecules including SF6, H2O, O2, highly active free radicals such as (SF5)*, (SF4)*, (SF2)*, OH*, O* may be generated. Collision induced dissociation of molecules have a great influence on the decomposition characteristic of SF6.

DFT is an efficient quantum chemical methods to provide approximate solutions to chemical reactions. The DFT with the use of B3LYP method is widely chosen in a variety of chemical system because of its high accuracy and low computational cost. Several previous works show that theoretical studies based on DFT with the use of B3LYP functional are available for sulphur-containing or fluorine-containing substances. Lobring studied the thermochemistry of SF5 and SF6 . It shows that the results of B3LYP and MP2 calculations are both in good agreement with the experimental results [21]. Cristian studied the reaction of SF5 radical with F2, Cl2 and SF5. The results are satisfactorily estimated by B3LYP [22]. Previous study by Grubbs shows that B3LYP method is well adapted for the calculation of accurate energies for C/O/ F systems [23]. The chemical reactions involving the main byproducts including SO2F, SOF2, SO2F2 of SF6 decomposition in power equipment has been calculated by Y.W. Fu with B3LYP method and get some satisfactory results [19]. Firstly, optimizations of SF6 and SF5 have been fully examined with B3LYP/6-311G(d,p), B3LYP/6-31G(d,p), PBE/6-311G(d,p), MPW1PW91/6-311G(d,p) and MP2/6-311G(d,p) levels. Results show that the largest deviations of bond lengths of SF6 under different levels is 0.03, and the largest deviations of bond lengths and bond angles of SF5 are 0.03 Å and 0.65°. The results obtained with B3LYP/6-311G(d,p) method is closed to that obtained with PBE/6-311G(d,p). Therefore, the optimized structure at B3LYP/6-311G(d,p) level is acceptable. Gaussian09 package was used to investigate the decomposition mechanism of SF6 in detail with the DFT-B3LYP/6-311G(d,p) quantum chemistry method. Choice of basis set usually depends on the accuracy required of the calculation. In order to obtain high quality results, 6311+G(2d,p) was used for further geometry optimizations and frequency calculations for all minimum compounds. Then, 6-311+ +(2df,p) was used to obtain single point calculations to improve the energies. The possible generated pathways of SF6 and reaction rate constants are determined with the introduced method and the results will be discussed later in this paper. The coordinates and harmonic vibrational frequencies of all the minima and transition states (TS) are listed in supplementary information, Tables S1 and S2. It shows that all TS have only 1 imaginary frequency and all minima have no imaginary frequencies.

SF 6

(1)

(SF5)*+F

(2)

SF6+e

SF6 F

(3)

F*+e

In summary, the bond dissociation energy of SF5-F lies between about 383 and 410 ( ± 12) kJ/mol. The average energy of active electron under corona discharge is 5–10 eV (1eV = 96.48 kJ/mol) which is enough to dissociate SF6. Besides, parts of SF5 come from the collision between SF6 and SF6 .

SF6 + SF6

e + SF6 + SF6

(4)

SF6 + SF6

F + (SF5)*+ SF6

(5)

SF6 + SF6

F*+ SF5 + SF6

(6)

Olthoff measured the above collisional electron detachment and collisional-induced dissociation cross sections for SF6 , SF5 , and F [26]. It shows that the threshold for collisional electron detachment from SF6 is near 90 eV. By contrast, the thresholds for productions of F from SF6 and SF5 are determined to be 2.0 and 1.35 eV, respectively. The threshold of reaction (6) is lower than that of reaction (5) because (SF5)* has a high electronegativity. Ziegler has concluded that the SFn . topmost dissociation channel of. should be SFn + M (SFn 1)*+F + M, followed by SFn + M SFn 1+F*+ M [27]. Therefore, it can be inferred that most of SF6 will dissociated into (SF5)* and F by collision with neutral molecules. SF6 might collide with electrons and generate metastable SF6 species when the discharge energy is relatively high [12]. (SF 6 )* has been excited and contains sufficient internal energy to make dissociation energetically possible [28].

(SF5)*+F + SF6 SF5 +F*+ SF6

(SF 6 )*+ SF6

(7) *

It can be concluded that (SF5) is the main product of the collision induced dissociation of SF6. It accounts for more than 90 percent of the low fluorine sulfides. Most of (SF5)* will react with F* to regenerate SF6. Besides, other parts of (SF5)* might collide with electrons and then form SF5 . McGeehan found that SF5 might participate in collision induced dissociation with SF6. (SF4)* which is a product with relatively high stability will then be generated as follow.

3. Decomposition mechanism of typical products Previous researches indicated that the electron density in the discharge region will drop by a factor of 10, when the value of E/N is reduced from 1.03(E/N)c to 0.97(E/N)c [24], where (E/N)c is the critical value at which the attachment coefficient equals to the ionization coefficient. Parts of reactions in the decomposition process can only happen within the region with the E/N larger than (E/N)c. The distribution of the electric field of a point-to-plane defect is shown in Fig. 1. It can be seen that the electric field strength drops dramatically from the location of discharge to the shell of chamber. And the volume of region with high intensity electric fields is quite small. Actually, reactions within the region of high electric fields are quite different from that within the low field region.

SF5 + SF6

(SF4)*+F + SF6

*

*

(8)

*

(SF3) , (SF2) , (SF) might then generate though the channel similar with SF4.

SFn + SF6

(SFn)*+F + (SF5)*

(SFn)*+e

SFn

(9) (10)

The collision induced dissociation of SF6 can be described as the 62

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Fig. 1. Electric field distribution of a point-to-plane defect.

required to form the further bond in (SF3)*. Additional energy should be supplied to form tetravalent configuration. Besides, it is necessary to promote one electron in an s orbital to a d orbital, which yields sp3d hybrids. The electron rearrangement might form a two-center threeelectron bond F (SF2)* as shown in Fig. 3(b) [29]. This bond is weak so that BDE(F2S-F)＞BDE(FS-F). (SF4)* has an molecular structure with two types of SeF bonds including equatorial bonds and axial bonds. The valence angle and length of equatorial SeF bonds made up of s-p hybrids are 100.92° and 1.581 Å, respectively. The valence angle and length of axial SeF bonds made up of p-d hybrids are 173.32° and 1.691 Å. The electrons of S atom will be rearranged, which will consume partial energy and forms a strong three-center four-electron bond. As a result, BDE(F3S-F)＞BDE (F2S-F). The generation process of (SF5)* and SF6 are similar with those of (SF3)* and (SF4)*, respectively. One electron of the last lone pair may be promoted from s orbital to d orbital to form a weak two-center threeelectron F (SF4)* bond when (SF5)* with sp3d2 hybrid configuration is built up from (SF4)* and F*. The molecular configuration change from a seesaw to tetragonal pyramid. SF6 has a configuration of regular octahedron. All SeF bonds in SF6 are equivalent and symmetrically distributed. Addition energy must be supplied to form (SF5)* with a hexavalent configuration from F* and (SF4)* with tetravalent configuration,

following chemical formula.

SF6+e

(SF6 n)*+nF*+e (n

6)

(11)

The bond dissociation energy of A-B→A + B can be calculated as follow.

BDE(A-B) =

r H298 (A) + r H298 (B)- r H298 (AB)

(12)

The bond dissociation energies of different low fluorine sulfides are obtained though quantum chemistry method and showed in Fig. 2. The coordinates of all species are exhibited in supplementary information, Table S2. The harmonic vibrational frequencies of all minima are listed in supplementary information, Table S1. The step-wise bond dissociation energies in SF6 are as follows: BDE BDE(SF4-F) = 154.44 kJ/mol, BDE(SF3(SF5-F) = 389.34 kJ/mol, F) = 365.33 kJ/mol, BDE(SF2-F) = 242.71 kJ/mol, BDE(SFF) = 361.53 kJ/mol, BDE(S-F) = 312.51 kJ/mol。 Sulfur is a chemically active element with a 3s23p4 valence structure. In addition, two unoccupied d orbitals can accept more extra electrons. The difference in electronegativity between sulfur atoms and fluorine atoms is 1.4 so that it may form polar covalent bond. Some valence excitation is required for the formation of the first SeF bond. The bonds in (SF2)* is partial sp3 hybrids with a relatively small amount of s character. The valence angle in (SF2)* is near 100°as shown in Fig. 2. The formation of (SF2)* requires no additional excitation because SF* and (SF2)* are both divalent. Therefore, BDE(FS-F) ＞BDE(S-F) as observed in Fig. 2. A major excitation from the divalent to the tetravalent state is

Fig. 2. SF6 stepwise bond dissociation energy.

Fig. 3. Electron distribution of low fluoride sulfides. 63

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making BDE(F5S-F)＞BDE(F3S-F)＞BDE(F4S-F). It can be seen that SF*, (SF3)*, and (SF5)* have relatively low bond dissociation energy among all low fluorine sulfides, which suggests that these three substances are high in chemical reactivity. It is reasonable to assume that most of these substance would be confined to high energy region. (SF2)* and (SF4)* have relatively high bond dissociation energy and chemical stability. Some of (SF2)* and (SF4)* might diffuse from the high energy region to the low energy region. 3.1.2. Dissociation of H2O and O2 Claydon has studied the dissociation of H2O by photochemical method [30]. H2O may split into OH* and H* when the energy is higher than 6.99 eV.

H2 O +hv

OH*+O*

(13)

H2O might be excited firstly by electron collision or optical radiation at an energy higher than 9.13 eV. The excited state is metastable and easy to dissociate.

H2 O +e

H2 O*

Fig. 4. Electron distribution of low fluoride sulfides.

(14)

H2 O +hv

H2 O*

(15)

H2 O*+hv

OH*+O*

(16)

H2O can also be decomposed into two hydrogen atoms and one oxygen atom. However, this process is probably not important under relatively low energy discharge since the cross section for this process is quite small.

H2 O +hv *

2H*+O*

(17)

*

OH and H could recombine in to H2O with a low reaction energy barrier. The activation energy for this reaction is only 0.21 eV [31]

OH +H*

H2 O

(18)

Another possible process is the disproportionation reaction between H2O and radical fragments including OH* and O*.

H2 O +O*

H2 O +2OH

2OH*

3OH+H

Fig. 5. Energy as a function of F5SeOH distance for generating SF5OH.

(19) (20)

3.2. Recombination and oxidation reaction of active particles (SF5)* is the main product produced through dissociation reaction of SF6. High-energy O* atoms or OH* are likely to be attracted by (SF5)*. SF5OH and SOF5 will be generated then.

O*+ (SF5)*

OH*+ (SF5)*

SOF5

SF5OH

(21) (22)

Fig. 4 shows the energy curve as a function of SeO distance for generating SOF5. Fig. 5 shows the energy curve as a function of F5SeOH distance for generating SF5OH. Reaction heat of these two exothermic reactions are −282.98 kJ/mol and −523.85 kJ/mol, respectively. It can be seen that the heat of the latter reaction is almost twice that of the former reaction. As a result, SF5 are more likely to react with OH* to form SF5OH. Besides, most of the unstable SOF5 will dissociate quickly to produce SOF4 and F*. SF5OH can dissociate through three different pathway as shown in Fig. 6. Reactions T1 and T3 have transient state (TS) alone the reaction paths. SOF4+HF can be obtained via TS with a barrier height of 139.67 kJ/mol. Fu has studied the reaction T1 with quantum chemistry method [20]. The barrier height is 643.19 kJ/mol. It can be seen that the barrier height of reaction T3 is 259.91 kJ/mol and 503.52 kJ/mol lower than those of reaction T2 and T1, respectively. As a result, due to its lower activation energy, reaction T3 is the most likely dissociation channel of SF5OH. Besides, reaction T3 is the main pathway to form SOF4 which is an important intermediate product for generating SOF2

Fig. 6. The relative energy of the dissociation channels of SF5OH.

and SO2F2. SF3 and SF, low fluorine sulfide with high activity, can react with O* easily to form unstable SOF3 and SOF. The reaction pathway of SOF3 include dissociation reaction and recombination reaction. SOF is quite unstable, which allows it to participate in recombination reaction with F* or oxidation reactions with O*.

O*+ (SF3)* OH*+ (SF3)* 64

SOF3 SF3 OH

(23) (24)

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SF*+O* SOF

(25)

SOF3+F*

SOF4

(26)

SOF2+F*

(27)

SOF3

SOF +F*

SOF2

(28)

SOF +O*

SO2 F

(29)

Table 1 Rate constants and ΔG of different reactions. Reaction

ΔG(a.u.)

k(cm3 mol−1s−1)

SF4+H2O→SF3OH+HF SF3OH→SOF2+HF

0.0457 0.0110

7.27 × 10−15 56.77

3.3. Generation process of typical sulfur-containing by-products 3.3.1. The generation pathway of SOF2 SOF2 can be generated in the high energy region from three different channels. (1) SOF3 can be formed from SF4OH, SOF4 and (SF3)*. It can dissociate to form SOF2 by absorbing little energy. (2) SF3OH being formed from (SF3)* and OH* is quite unstable. It will dissociate to form SOF2 easily. (3) Recombination reaction between SOF and F* will generate SOF2. SOF2 can be easily generated from dissociation of SOF3 or SF3OH. However, SOF3 and SF3OH come from second-order reaction. SOF is more likely to react with O* rather than F*. Therefore, the content of SOF2 generating in the high energy region is low. (SF4)* is a low fluorine sulfide that has relatively high chemical stability. Parts of (SF4)* can transfer from the high energy region to the low energy region. SOF2 can also be generated from hydrolysis reaction of (SF4)*. The reaction is conducted in two steps. Fig. 7 depicts the structure and relative energy of reactants, products and TS of the reaction (SF4)*+H2 O SF3OH + HF for the B3LYP/6-311(d,p) level. Firstly, a H2O molecule moves close to a (SF4)* molecule. One SeF stretches from 1.691 Å to 2.134 Å. Meanwhile, an FeH bond is formed in TS with barrier height 61.72 kJ/mol. Afterwards, a OeH bond and an SeF bond break, and the OH* continue to move towards the S atom to form products SF3OH + HF. Unstable SF3OH will dissociate to SOF2 and HF as shown in Fig. 7. Firstly, the F2S−OH bond shortens from 1.596 Å to 1.551 Å, while the F atom nearest to the OH* separates from the SF3OH molecule and moves towards the H atom. The barrier height of TS is 39.64 kJ/mol. Afterwards, OeH bond in the TS breaks. HeF bond and SeO bond shortens to 0.924 Å and 1.429 Å, respectively, and form products SOF2+HF. Due to the low activation energy of reaction shown in Fig. 7. Hydrolysis reaction of (SF4)* can occur in the low energy region. Actually, it is the main pathway to generate SOF2 under corona discharge. Rate constants and G 0, of reactions shown in Fig. 7 are depicted in Table 1. The total reaction rate is decided by the reaction with lowest

Fig. 8. Relative energy profile of the reaction of SO2F + SF5→SF6+ SO2.

rate. As a result, the rate constant of (SF4)*+H2 O 7.27 × 10−15 cm3 mol-1s-1.

SOF2 + 2HF is about

3.3.2. The generation pathway of SOF2 SO2 can be generated in the high energy region from SO2F and SF5. The relative energy profile of the reaction is shown in Fig. 8. Calculation result shows that the rate constant is about 7.77 × 10−53 cm3 mol-1s-1, which is extremely low. SO2 can also be generated from hydrolysis reaction of SOF2 whose energy profiles are shown in Fig. 9. The reaction is conducted in two steps. Firstly, intermediate product SOF−OH can be obtained via TS with the barrier heights of 117.62 kJ/mol. SeO bonds in TS are 1.569 Å and 1.543 Å respectively, which are longer than the normal SeO bond length of 1.429 Å in SOF2. Besides, an FeS bond in SOF2 is broken and an FeH bond is formed. In the second step, SO2 and HF will be formed from dissociation of unstable SOFeOH. The SeF bond stretches from 1.651 Å to 2.031 Å. The F atom is attracted by the H atom then and form TS with the barrier height of 109.95 kJ/mol. Afterwards, OeH bond in the TS breaks. HeF bond and SeO bond shortens to 0.925 Å and 1.446 Å, respectively, and

Fig. 7. Relative energy profile of SF4+H2O→SOF2+2HF. 65

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Fig. 9. Relative energy profile of SOF2+H2O→SO2+2HF.

Other than the above process, SO2F2 can also be generated from hydrolysis reaction of SOF4. This process is carried out in three steps as the relative energy profiles shown in Fig. 10. In the first step, the reactants SOF4 and H2O can react with each other to form SF4(OH)2 via TS1 with a barrier height of 122.73 kJ/mol. Subsequently, intermediate product SF4(OH)2 decomposes into SOF3eOH and HF with a barrier height of 111.18 kJ/mol. In the last step, SOF3−OH will further decompose into SO2F2 and HF via TS3. The barrier height of TS3 is 50.15 kJ/mol lower than that of TS1 and TS2. Table 4 displays the rate constants and G 0, of reactions shown Fig. 10. The rate constant of hydrolysis reaction of SOF4 is about 5.25 × 10−21 cm3 mol-1s-1. It can be concluded that SO2F2 is mainly generated from oxidation reaction of (SF2)*, when compared with data in Tables 3 and 4.

Table 2 Rate constants and ΔG of different reactions. Reaction

ΔG(a.u.)

k(cm3 mol−1s−1)

SOF2+H2O→SOF-OH*+HF SOF-OH*→SO2+HF

0.0603 0.0372

1.75 × 10−21 6.12 × 10−11

form products SO2+HF. Rate constants and G 0, of SOF2+H2 O SO2 + 2HF are shown in Table 2. The rate constant of (SF4)*+H2 O SOF2 + 2HF is about 1.75 × 10−21 cm3 mol-1s-1 because the total reaction rate is decided by the reaction with lowest rate. Comparison of Tables 1 and 2 indicates that the rate constant of formation reaction of SO2 is much lower than that of SOF2. As a result, the content of SOF2 will much higher than that of SO2 under the same condition.

4. Decomposition model Two typical zones of different chemical activity are defined in the decomposition model as shown in Fig. 11. Zone-1 corresponds to the small volume near the needle tip. It is within this region that molecules dissociate and then the dissociation products (such as (SF5)*, OH*, O*) react fast with each other with high probability. Zone-2 corresponds to the vast majority of the gas volume between the needle and the chamber wall in which slow gas-phase reactions involving relatively stable substance (such as (SF4)*, SOF4, SOF2) occur. The high energy region is supposed to have a radius less than quadruple that of tip curvature. A series of complicated chemical reactions take place in this region. Dissociation of neutral molecules such as SF6, H2O, and O2 is one of the most important reactions which may generate a variety of energetic substance including (SF5)*, (SF4)*, (SF3)*, (SF2)*, OH*, O*, etc. Most of the low fluorine (SFn) * sulfides will recombined with F* to reform SF6. Besides, parts of (SFn)* will quickly react with OH* and O* to produce SOFn or SFnOH, which may subsequently undergo dissociation reaction. (SF2)* may be oxidized step by step to generate SO2F2 which is a typical decomposition product. E/N will rapidly decrease from (E/N)c to zero near the boundary of the high energy region. As a result, enegetic electrons are quickly consumed by colliding with molecules and subsequently attach to form negative ions. Charge-transfer reaction between negative ions and molecules may then occur, among which the reaction between SF6 and SOF4 is supposed to be one of the most important one. Other chargetransfer reactions have little influence on the overall decomposition process. The highly active species are quite unstable, therefore it can be assumed that the reaction times in zone-1 are short compared to the times required for diffusion of the active species out of this region. Diffusion of some neutral by-products from zone-1 to zone-2 contribute to most of the products in the low energy region. It is reasonable to assume that neutral by-products than escape into zone-2 never return to zone-1. Parts of by-products such as SOF2, SO2F2 will remain and then diffuses uniformly as stable components of the gas in zone-2. The

3.3.3. The generation pathway of SO2F2 SO2F2 can be generated from oxidation reaction of (SF2)* and hydrolysis reaction of SOF4. The oxidation process of (SF2)* are expressed as the following chemical formulas.

(SF2)*+O*

(30)

SOF +F*

SOF +O*

SO2 F

(31)

SOF +F*

SOF2

(32)

SO2 F +F*

(33)

SO2 F2

*

*

(SF2) can easily react with O atom to form SOF, which is an exothermic reaction. Unstable SOF will further react quickly with O* or F* to form SO2F and SOF2, respectively. These two exothermic reaction can occur easily without transition states. SOF is more likely to react with O* atom. Lastly, SO2F2 can be generated by recombination reaction between SO2F and F* atom. Rate constants and G 0, of the oxidation reaction of (SF2)* are shown in Table 3. It can be seen that the rate constant is about 2.50 × 10−14 cm3 mol-1s-1 which is close to the result which is 1.0 × 10−14 cm3 mol-1s-1 in reference [24]. It is worth to note that this paper suggests that the oxidation reaction of (SF2)* occurs in the high energy region, which is different from the decomposition model in reference [21]. As a result, the content of SO2F2 is closely related to the discharge energy. Table 3 Rate constants and ΔG of different reactions. Reaction

ΔG(a.u.)

k(cm3 mol−1s−1)

SF2+O→SOF + F SO2F + F→SO2F2

0.03489 0.04024

7.12 × 10−12 3.26 × 10−14

66

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Fig. 10. Relative energy profile of the first step SOF4+H2O→SO2F2+2HF.

other parts such as (SF4)*, SOF2, SOF4 will undergo hydrolysis reactions with trace water in SF6. Reactions in the low energy region are necessarily much slower than those in the high energy region, because the activity of reactant and value of E/N is low. Fig. 12 shows the pathways for generation of sulfur-containing products. The bold lines display the main pathways. SOF2 and SO2 are both mainly generated from hydrolysis reaction, while SO2F2 is mainly generated from oxidation reaction. Besides, SOF2 and SO2 are both mainly generated in the low energy region, while SO2F2 is mainly generated in the high energy region.

Table 4 Rate constants and ΔG of different reactions. Reaction

ΔG(a.u.)

k(cm3 mol−1s−1)

SOF4+H2O→SF4(OH)2 SF4(OH)2→SOF3-OH+HF SOF3-OH→SO2F2+HF

0.0592 0.0433 0.0143

5.25 × 10−21 9.69 × 10−14 1.84

5. Conclusion In this paper, the DFT-B3LYP/6-311(d,p) quantum chemical method has been employed to study the decomposition mechanism of SF6. The SF6 decomposition model is divided into two regions. It is within the high energy region that molecules molecules (SF6, H2O, O2) dissociate and then the dissociation products (such as (SF5)*, OH*, O*, etc.) react fast with each other with high probability. Reactions within the low energy region are mainly hydrolysis reaction involving relatively stable substance (such as (SF4)*, SOF4, SOF2). Collision induced dissociation of molecules have a great influence on the decomposition process of SF6. H2O and O2 dissociate into radical fragments including OH* and O*. SF6 dissociate into low fluoride

Fig. 11. Decomposition model of SF6 under corona discharge.

Fig. 12. Pathways for generation of sulfur-containing products (Bold pathways display the main ones). 67

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sulfides, among which (SF5)* is the main product. Besides, (SF2)* and (SF4)* have relatively high bond dissociation energy and chemical stability. These two sulfides are important reactants for generating stable products. SF5OH , The main formation pathway of SOF4 is: (SF5)*+ OH* SF5OH SOF4 + HF . SOF2 and SO2 are both mainly generated through hydrolysis reaction in the low-energy region. The content of SOF2 will much higher than that of SO2 under the same condition because the rate constant of (SF4)*+H2 O SOF2 + 2HF is much higher than that of SOF2+H2 O SO2 + 2HF . SO2F2 can be generated from oxidation reaction of SF2 and hydrolysis reaction of SOF4, and the former is the main SO2 F2 is close to that of pathway. The rate constant of (SF2)*+O2 (SF4)*+H2 O SOF2 + 2HF . Consequently, SOF2 and SO2F2 are the most effective species to monitor the decay of SF6 under corona discharge.

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[16] [17]

Acknowledgement

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The research work has been supported by Chinese universities scientific fund.

[19]

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jfluchem.2019.01.005.

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